Advertisement

Unusual Molecular Regulation of Dorsolateral Prefrontal Cortex Layer III Synapses Increases Vulnerability to Genetic and Environmental Insults in Schizophrenia

Open AccessPublished:February 12, 2022DOI:https://doi.org/10.1016/j.biopsych.2022.02.003

      Abstract

      Schizophrenia is associated with reduced numbers of spines and dendrites from layer III of the dorsolateral prefrontal cortex (dlPFC), the layer that houses the recurrent excitatory microcircuits that subserve working memory and abstract thought. Why are these synapses so vulnerable, while synapses in deeper or more superficial layers are little affected? This review describes the special molecular properties that govern layer III neurotransmission and neuromodulation in the primate dlPFC and how they may render these circuits particularly vulnerable to genetic and environmental insults. These properties include a reliance on NMDA receptor rather than AMPA receptor neurotransmission; cAMP (cyclic adenosine monophosphate) magnification of calcium signaling near the glutamatergic synapse of dendritic spines; and potassium channels opened by cAMP/PKA (protein kinase A) signaling that dynamically alter network strength, with built-in mechanisms to take dlPFC “offline” during stress. A variety of genetic and/or environmental insults can lead to the same phenotype of weakened layer III connectivity, in which mechanisms that normally strengthen connectivity are impaired and those that normally weaken connectivity are intensified. Inflammatory mechanisms, such as increased kynurenic acid and glutamate carboxypeptidase II expression, are especially detrimental to layer III dlPFC neurotransmission and modulation, mimicking genetic insults. The combination of genetic and inflammatory insults may cross the threshold into pathology.

      Keywords

      By diverse means we arrive at the same end—Michel de Montaigne
      The etiology of schizophrenia remains a puzzle, with seemingly divergent mechanisms among the many risk factors for this complex cognitive disorder. A large number of genetic factors as well as environmental factors, such as perinatal inflammatory events and psychological stress in adolescence, confer risk. Although this landscape remains challenging, accumulating data are beginning to provide a foothold, suggesting how risk factors may weaken the recurrent excitatory circuits in the dorsolateral prefrontal cortex (dlPFC) that subserve higher cognition.
      There is consistent evidence that schizophrenia involves impaired functioning of the dlPFC, with deficits in working memory and dlPFC blood oxygen level–dependent response relating strongly to symptoms of thought disorder [e.g. (
      • Weinberger D.R.
      • Berman K.F.
      • Zec R.F.
      Physiologic dysfunction of dorsolateral prefrontal cortex in schizophrenia. I. Regional cerebral blood flow evidence.
      ,
      • Docherty N.M.
      • Hawkins K.A.
      • Hoffman R.E.
      • Quinlan D.M.
      • Rakfeldt J.
      • Sledge W.H.
      Working memory, attention, and communication disturbances in schizophrenia.
      ,
      • Perlstein W.M.
      • Carter C.S.
      • Noll D.C.
      • Cohen J.D.
      Relation of prefrontal cortex dysfunction to working memory and symptoms in schizophrenia.
      ,
      • Barch D.M.
      The cognitive neuroscience of schizophrenia.
      ,
      • Keefe R.S.
      • Harvey P.D.
      Cognitive impairment in schizophrenia.
      )], including worsening with stress exposure (
      • Docherty N.M.
      • Evans I.M.
      • Sledge W.H.
      • Seibyl J.P.
      • Krystal J.H.
      Affective reactivity of language in schizophrenia.
      ). Risk genes for schizophrenia are enriched in dlPFC compared with other brain areas (
      • Ripke S.
      • Walters J.T.
      • O’Donovan M.C.
      The Schizophrenia Working Group of the Psychiatric Genomics Consortium
      Mapping genomic loci prioritises genes and implicates synaptic biology in schizophrenia.
      ); structural imaging studies show waves of PFC gray matter loss heralding disease onset (
      • Cannon T.D.
      • Thompson P.M.
      • van Erp T.G.
      • Toga A.W.
      • Poutanen V.P.
      • Huttunen M.
      • et al.
      Cortex mapping reveals regionally specific patterns of genetic and disease-specific gray-matter deficits in twins discordant for schizophrenia.
      ,
      • Cannon T.D.
      • Chung Y.
      • He G.
      • Sun D.
      • Jacobson A.
      • van Erp T.G.
      • et al.
      Progressive reduction in cortical thickness as psychosis develops: A multisite longitudinal neuroimaging study of youth at elevated clinical risk.
      ), accompanied by elevated inflammation (
      • Cannon T.D.
      • Chung Y.
      • He G.
      • Sun D.
      • Jacobson A.
      • van Erp T.G.
      • et al.
      Progressive reduction in cortical thickness as psychosis develops: A multisite longitudinal neuroimaging study of youth at elevated clinical risk.
      ,
      • Föcking M.
      • Sabherwal S.
      • Cates H.M.
      • Scaife C.
      • Dicker P.
      • Hryniewiecka M.
      • et al.
      Complement pathway changes at age 12 are associated with psychotic experiences at age 18 in a longitudinal population-based study: Evidence for a role of stress.
      ); and positron emission tomography markers show reduced presynaptic labeling (
      • Radhakrishnan R.
      • Skosnik P.D.
      • Ranganathan M.
      • Naganawa M.
      • Toyonaga T.
      • Finnema S.
      • et al.
      In vivo evidence of lower synaptic vesicle density in schizophrenia.
      ). Most pertinent to the current review, postmortem neuropathological studies show consistent reduction in the numbers of spines and dendrites in deep layer III dlPFC (
      • Glantz L.A.
      • Lewis D.A.
      Decreased dendritic spine density on prefrontal cortical pyramidal neurons in schizophrenia.
      ,
      • Berdenis van Berlekom A.
      • Muflihah C.H.
      • Snijders G.J.L.J.
      • MacGillavry H.D.
      • Middeldorp J.
      • Hol E.M.
      • et al.
      Synapse pathology in schizophrenia: A meta-analysis of postsynaptic elements in postmortem brain studies.
      ), the sublayer that contains the recurrent excitatory microcircuits that subserve working memory (
      • Goldman-Rakic P.
      Cellular basis of working memory.
      ). The reduction in spines shows striking laminar specificity (
      • Glausier J.R.
      • Lewis D.A.
      Dendritic spine pathology in schizophrenia.
      ), with normal levels in superficial III or deep layers V and VI (
      • Glantz L.A.
      • Lewis D.A.
      Decreased dendritic spine density on prefrontal cortical pyramidal neurons in schizophrenia.
      ,
      • Kolluri N.
      • Sun Z.
      • Sampson A.R.
      • Lewis D.A.
      Lamina-specific reductions in dendritic spine density in the prefrontal cortex of subjects with schizophrenia.
      ), and regional specificity, with relatively preserved spine numbers in the primary visual cortex (
      • Glantz L.A.
      • Lewis D.A.
      Decreased dendritic spine density on prefrontal cortical pyramidal neurons in schizophrenia.
      ). Thus, clues to the etiology of schizophrenia may be found in trying to understand this selective pathology, determining why spine numbers are particularly reduced in deep layer III dlPFC. It is possible that the distorted thinking of schizophrenia is related to this selective change in layer III, while more global insults to all layers of dlPFC would produce a simpler syndrome of cognitive impairment. Note that the reduction in spine number (here termed atrophy) may reflect impaired spine formation and/or increased removal of existing spines and that there are decreases in spine/synapse numbers in other cortical areas as well, e.g., in anterior cingulate cortex (
      • Roberts R.C.
      • Barksdale K.A.
      • Roche J.K.
      • Lahti A.C.
      Decreased synaptic and mitochondrial density in the postmortem anterior cingulate cortex in schizophrenia.
      ), temporal association cortex (
      • Garey L.J.
      • Ong W.Y.
      • Patel T.S.
      • Kanani M.
      • Davis A.
      • Mortimer A.M.
      • et al.
      Reduced dendritic spine density on cerebral cortical pyramidal neurons in schizophrenia.
      ), and primary auditory cortex (
      • Moyer C.E.
      • Delevich K.M.
      • Fish K.N.
      • Asafu-Adjei J.K.
      • Sampson A.R.
      • Dorph-Petersen K.A.
      • et al.
      Intracortical excitatory and thalamocortical boutons are intact in primary auditory cortex in schizophrenia.
      ,
      • McKinney B.C.
      • MacDonald M.L.
      • Newman J.T.
      • Shelton M.A.
      • DeGiosio R.A.
      • Kelly R.M.
      • et al.
      Density of small dendritic spines and microtubule-associated-protein-2 immunoreactivity in the primary auditory cortex of subjects with schizophrenia.
      ) [although excitatory synapse numbers remain constant in auditory cortex despite loss of spines (
      • Moyer C.E.
      • Delevich K.M.
      • Fish K.N.
      • Asafu-Adjei J.K.
      • Sampson A.R.
      • Dorph-Petersen K.A.
      • et al.
      Intracortical excitatory and thalamocortical boutons are intact in primary auditory cortex in schizophrenia.
      )]. However, as little is known about the molecular regulation of these areas in primates, they will not be considered here.
      The current review discusses how a variety of genetic and environmental risk factors may lead to loss of spines on layer III dlPFC, based on studies of the rhesus monkey dlPFC. Layer III dlPFC spines in the rhesus macaque express a concentration of proteins that are risk factors for disease, with many related to the special molecular properties needed for working memory. These properties include a reliance on NMDA receptor (NMDAR) rather than AMPA receptor (AMPAR) neurotransmission, cAMP (cyclic adenosine monophosphate) magnification of Ca2+ signaling near the synapse, and K+ channels opened by cAMP/PKA (protein kinase A) signaling that dynamically alter network strength. These mechanisms render layer III spines especially vulnerable to atrophy when they are dysregulated owing to genetic and/or inflammatory insults, weakening network connectivity.

      Summary of dlPFC Neurotransmission and Neuromodulation

      The dlPFC is essential to working memory, i.e., our mental sketch pad—the ability to generate, maintain, and manipulate mental representations without sensory stimulation, the foundation of abstract thought and the executive functions, including cognitive control (
      • Goldman-Rakic P.
      Cellular basis of working memory.
      ,
      • Jacobsen C.
      Studies of cerebral functions in primates.
      ,
      • Fuster J.
      The Prefrontal Cortex.
      ,
      • Robbins T.W.
      Dissociating executive functions of the prefrontal cortex.
      ,
      • Szczepanski S.M.
      • Knight R.T.
      Insights into human behavior from lesions to the prefrontal cortex.
      ). The dlPFC accomplishes these functions through widespread connections; e.g., it has reciprocal connections with the sensory association cortices, mediodorsal thalamus, and hippocampus (
      • Goldman-Rakic P.S.
      • Selemon L.D.
      • Schwartz M.L.
      Dual pathways connecting the dorsolateral prefrontal cortex with the hippocampal formation and parahippocampal cortex in the rhesus monkey.
      ,
      • Giguere M.
      • Goldman-Rakic P.S.
      Mediodorsal nucleus: Areal, laminar, and tangential distribution of afferents and efferents in the frontal lobe of rhesus monkeys.
      ,
      • Selemon L.D.
      • Goldman-Rakic P.S.
      Common cortical and subcortical targets of the dorsolateral prefrontal and posterior parietal cortices in the rhesus monkey: Evidence for a distributed neural network subserving spatially guided behavior.
      ) as well as outputs to basal ganglia, premotor cortices, and pons-cerebellum to influence motor response (
      • Goldman-Rakic P.S.
      • Selemon L.D.
      • Schwartz M.L.
      Dual pathways connecting the dorsolateral prefrontal cortex with the hippocampal formation and parahippocampal cortex in the rhesus monkey.
      ,
      • Giguere M.
      • Goldman-Rakic P.S.
      Mediodorsal nucleus: Areal, laminar, and tangential distribution of afferents and efferents in the frontal lobe of rhesus monkeys.
      ,
      • Selemon L.D.
      • Goldman-Rakic P.S.
      Common cortical and subcortical targets of the dorsolateral prefrontal and posterior parietal cortices in the rhesus monkey: Evidence for a distributed neural network subserving spatially guided behavior.
      ,
      • Goldman-Rakic P.S.
      Circuitry of the primate prefrontal cortex and the regulation of behavior by representational memory.
      ). Fuster and Goldman-Rakic found Delay cells in macaque dlPFC with tuned persistent firing that represent information (e.g., a location) over a delay period of many seconds without sensory stimulation (
      • Fuster J.
      • Alexander G.
      Neuron activity related to short-term memory.
      ,
      • Funahashi S.
      • Bruce C.J.
      • Goldman-Rakic P.S.
      Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex.
      ,
      • Li D.
      • Constantinidis C.
      • Murray J.D.
      Trial-to-trial variability of spiking delay activity in prefrontal cortex constrains burst-coding models of working memory.
      ). Goldman-Rakic (
      • Goldman-Rakic P.
      Cellular basis of working memory.
      ) showed that persistent firing arises from recurrent excitatory circuits concentrated in deep layer III dlPFC (Figure 1A), with information refined by lateral inhibition, a model that has been affirmed by in vitro recordings (
      • González-Burgos G.
      • Barrionuevo G.
      • Lewis D.A.
      Horizontal synaptic connections in monkey prefrontal cortex: An in vitro electrophysiological study.
      ). Thus, pyramidal cells excite each other through glutamatergic synapses on spines to keep information “in mind” (
      • González-Burgos G.
      • Barrionuevo G.
      • Lewis D.A.
      Horizontal synaptic connections in monkey prefrontal cortex: An in vitro electrophysiological study.
      ).
      Figure thumbnail gr1
      Figure 1Schematic diagram of layer III dlPFC recurrent excitatory microcircuits subserving working memory. (A) Pyramidal cells with shared properties excite each other to keep information “in mind” through glutamatergic synapses on spines. Note: There is also lateral inhibition by GABA interneurons to refine the contents of working memory; this is not shown. (B) A glutamatergic NMDAR synapse on a dendritic spine in the young adult, healthy dlPFC with tightly regulated, feedforward Ca2+-cAMP-K+ channel signaling. In these microcircuits, neurotransmission depends on NMDAR with GluN2A and GluN2B subunits, with permissive activation by cholinergic Nic-α7R and M1R (M1R via closure of KCNQ5 channels, not shown). These spines also contain the molecular machinery for cAMP-PKA to magnify Ca2+ signaling needed to sustain persistent firing, including internal Ca2+ release from the SER spine apparatus, which, in turn, increases cAMP production, leading to feedforward cAMP-Ca2+ signaling. A variety of receptors are localized on spines that drive cAMP-Ca2+ signaling, including the dopamine D1R, the vasoactive intestinal peptide and PACAP receptor, VIPR2, and the norepinephrine receptor α1-AR. Layer III spines also express K+ channels that are opened by cAMP-PKA signaling to provide negative feedback and for dynamic changes in network connectivity. Under healthy conditions, these intracellular signaling pathways are tightly regulated by receptors that inhibit cAMP production, PDE4s that are anchored to the SER by DISC1 and that catabolize cAMP once it is generated, and calbindin to bind cytosolic Ca2+. PDE4s are also found in dendrites near mitochondria, positioned to regulate cAMP drive on Ca2+ release from the SER into mitochondria. AC, adenylyl cyclase; AR, adrenoceptor; calb, calbindin; cAMP, cyclic adenosine monophosphate; D1R, D1 receptor; dlPFC, dorsolateral prefrontal cortex; GABA, gamma aminobutyric acid; HCN, hyperpolarization activated cyclic nucleotide gated cation; M1R, M1 receptor; Nic-α7R, nicotinic-α7 receptor; NMDAR, NMDA receptor; PDE4, phosphodiesterase type 4; PKA, protein kinase A; PKC, protein kinase C; PSD, postsynaptic density; SER, smooth endoplasmic reticulum.
      Recent research shows that Delay cells have unusual neurotransmission and neuromodulation, likely related to their need to sustain dynamic, ever-changing mental representations. Delay cell neurotransmission relies heavily on glutamate stimulation of NMDARs (GluN2B and GluN2A), but not AMPARs (Figure 1B) (
      • Wang M.
      • Yang Y.
      • Wang C.J.
      • Gamo N.J.
      • Jin L.E.
      • Mazer J.A.
      • et al.
      NMDA receptors subserve working memory persistent neuronal firing in dorsolateral prefrontal cortex.
      ). GluN2B subunits close slowly and flux high levels of Ca2+ and reside exclusively within the postsynaptic density (PSD) in layer III dlPFC (
      • Wang M.
      • Yang Y.
      • Wang C.J.
      • Gamo N.J.
      • Jin L.E.
      • Mazer J.A.
      • et al.
      NMDA receptors subserve working memory persistent neuronal firing in dorsolateral prefrontal cortex.
      ). This dependence on GluN2B channels had been predicted by computational models (
      • Wang X.J.
      Synaptic basis of cortical persistent activity: The importance of NMDA receptors to working memory.
      ). In classic circuits, such as rodent hippocampus, AMPARs normally serve to depolarize the postsynaptic membrane, ejecting the Mg2+ block from the NMDAR channel and permitting NMDAR neurotransmission. However, dlPFC Delay cells have surprisingly little reliance on AMPARs, and instead this critical permissive function is performed by acetylcholine, through actions at nicotinic-α7 receptors (Nic-α7Rs) (
      • Yang Y.
      • Paspalas C.D.
      • Jin L.E.
      • Picciotto M.R.
      • Arnsten A.F.T.
      • Wang M.
      Nicotinic α7 receptors enhance NMDA cognitive circuits in dorsolateral prefrontal cortex.
      ) and/or muscarinic M1 receptors (
      • Galvin V.C.
      • Yang S.T.
      • Paspalas C.D.
      • Yang Y.
      • Jin L.E.
      • Datta D.
      • et al.
      Muscarinic M1 receptors modulate working memory performance and activity via KCNQ potassium channels in primate prefrontal cortex.
      ) that reside within the PSD (Figure 1B). M1 receptors act in part by closing KCNQ5 K+ channels (
      • Galvin V.C.
      • Yang S.T.
      • Paspalas C.D.
      • Yang Y.
      • Jin L.E.
      • Datta D.
      • et al.
      Muscarinic M1 receptors modulate working memory performance and activity via KCNQ potassium channels in primate prefrontal cortex.
      ), while Nic-α7Rs directly flux Na+ and Ca2+. The heavy reliance of these synapses on Nic-α7Rs may help explain why most patients with schizophrenia smoke cigarettes or vape nicotine.
      Layer III Delay cells also have unique neuromodulation needed to sustain neuronal firing without sensory stimulation and to dynamically alter network strength, e.g., according to arousal state (Figure 1B). Layer III spines express the molecular machinery to magnify Ca2+ signaling near the PSD through multiple mechanisms, including Ca2+ entry through NMDAR and Nic-α7R and through voltage-gated Ca2+ channels as well as internal Ca2+ release from the smooth endoplasmic reticulum (called the spine apparatus in spines). These Ca2+ actions are increased by cAMP/PKA signaling, and Ca2+ can also increase internal Ca2+ release; e.g., Cav1.2 calcium channels drive Ca2+ release through ryanodine receptors (
      • Vierra N.C.
      • Kirmiz M.
      • van der List D.
      • Santana L.F.
      • Trimmer J.S.
      Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons.
      ). Cytosolic Ca2+ can, in turn, increase cAMP production via adenylyl cyclase, thus driving feedforward signaling (Figure 1B). Layer III dlPFC spines express a large number of cAMP-signaling proteins, especially near the spine apparatus and the PSD (
      • Paspalas C.D.
      • Wang M.
      • Arnsten A.F.T.
      Constellation of HCN channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex—potential substrate for working memory deficits in schizophrenia.
      ,
      • Arnsten A.F.T.
      • Datta D.
      • Wang M.
      The genie in the bottle—magnified calcium signaling in dorsolateral prefrontal cortex.
      ). There is also a concentration of cAMP/Ca2+ signaling near mitochondria in nearby dendrites (
      • Paspalas C.D.
      • Wang M.
      • Arnsten A.F.T.
      Constellation of HCN channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex—potential substrate for working memory deficits in schizophrenia.
      ,
      • Datta D.
      • Enwright J.F.
      • Arion D.
      • Paspalas C.D.
      • Morozov Y.M.
      • Lewis D.A.
      • et al.
      Mapping phosphodiesterase 4D (PDE4D) in macaque dorsolateral prefrontal cortex: Postsynaptic compartmentalization in higher-order layer III pyramidal cell circuits.
      ), coordinating energy demands.
      Layer III spines also express ion channels that are opened by cAMP/PKA signaling that weaken connectivity. The open state of HCN (hyperpolarization activated cyclic nucleotide gated cation) channels is increased by cAMP, and these channels are concentrated on layer III spines in dlPFC (
      • Paspalas C.D.
      • Wang M.
      • Arnsten A.F.T.
      Constellation of HCN channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex—potential substrate for working memory deficits in schizophrenia.
      ,
      • Wang M.
      • Ramos B.
      • Paspalas C.
      • Shu Y.
      • Simen A.
      • Duque A.
      • et al.
      Alpha2A-adrenoceptor stimulation strengthens working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex.
      ), but not V1 spines (
      • Yang S.T.
      • Wang M.
      • Paspalas C.P.
      • Crimins J.L.
      • Altman M.T.
      • Mazer J.A.
      • et al.
      Core differences in synaptic signaling between primary visual and dorsolateral prefrontal cortex.
      ). Opening HCN channels with cAMP reduces Delay cell firing, while HCN channel blockade enhances firing (
      • Wang M.
      • Ramos B.
      • Paspalas C.
      • Shu Y.
      • Simen A.
      • Duque A.
      • et al.
      Alpha2A-adrenoceptor stimulation strengthens working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex.
      ). Recent data suggest a likely partnership with Slack K+ channels (
      • El-Hassar L.
      • Datta D.
      • Chatterjee M.
      • Arnsten A.F.T.
      • Kaczmarek L.K.
      Interaction between HCN and Slack channels regulates mPFC pyramidal cell excitability and working memory function.
      ), where cAMP opens HCN channels, and the entry of sodium opens neighboring Slack channels to have a net outflow of K+. Another key channel localized in layer III spines is the KCNQ2 K+ channel, whose open state is increased by PKA signaling (
      • Galvin V.C.
      • Yang S.T.
      • Paspalas C.D.
      • Yang Y.
      • Jin L.E.
      • Datta D.
      • et al.
      Muscarinic M1 receptors modulate working memory performance and activity via KCNQ potassium channels in primate prefrontal cortex.
      ). Opening these channels reduces, while blocking them increases, Delay cell firing (
      • Galvin V.C.
      • Yang S.T.
      • Paspalas C.D.
      • Yang Y.
      • Jin L.E.
      • Datta D.
      • et al.
      Muscarinic M1 receptors modulate working memory performance and activity via KCNQ potassium channels in primate prefrontal cortex.
      ). The capability to rapidly open and close these ion channels allows very rapid, dynamic changes in network strength (termed dynamic network connectivity), needed for a constantly changing mental sketch pad (
      • Arnsten A.F.T.
      • Wang M.
      • Paspalas C.D.
      Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses.
      ), including taking dlPFC “offline” during stress (see below).
      In the young, healthy dlPFC, feedforward Ca2+/cAMP/K+ channel signaling is tightly regulated by mGluR3 and α2A adrenoceptor (AR) inhibition of cAMP production and by phosphodiesterase type 4 (PDE4) catabolism of cAMP once it is formed (
      • Arnsten A.F.T.
      • Datta D.
      • Wang M.
      The genie in the bottle—magnified calcium signaling in dorsolateral prefrontal cortex.
      ). Layer III dendritic spines are a focus of these proteins, e.g., with PDE4A anchored to the spine apparatus by DISC1 (
      • Arnsten A.F.T.
      • Wang M.
      • Paspalas C.D.
      Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses.
      ). Layer III dlPFC pyramidal cells are also enriched in the Ca2+-binding protein calbindin, which regulates Ca2+ in the cytosol (
      • Datta D.
      • Leslie S.N.
      • Wang M.
      • Yang S.
      • Morozov Y.
      • Mentone S.
      • et al.
      Age-related calcium dysregulation linked with tau pathology and impaired cognition in non-human primates.
      ). Loss of these regulatory proteins, e.g., owing to inflammation or advancing age, leads to toxic Ca2+ dysregulation, atrophy of dendritic spines, and reduced Delay cell firing (
      • Arnsten A.F.T.
      • Datta D.
      • Wang M.
      The genie in the bottle—magnified calcium signaling in dorsolateral prefrontal cortex.
      ).

      Stress Signaling in dlPFC Weakens Connectivity and Takes dlPFC “OFFLINE”

      Layer III dlPFC circuits have built-in molecular mechanisms to rapidly take dlPFC “offline” during uncontrollable stress exposure. This has survival value under some dangerous conditions (e.g., being cut off on the highway, where it is helpful to rapidly switch control to circuits mediating habitual/instinctive behaviors), but it is detrimental when one needs higher cognition to deal with the threat (e.g., an invisible virus). As summarized in Figure 2, high levels of catecholamine release during uncontrollable stress activate large numbers of dopamine D1 receptors (D1Rs) and norepinephrine α1-ARs to drive high levels of Ca2+/cAMP/K+ channel signaling and disconnect dlPFC recurrent networks. For example, high levels of D1R signaling markedly reduce Delay cell firing via increased cAMP-PKA signaling, and this is prevented by HCN channel blockade (
      • Vijayraghavan S.
      • Wang M.
      • Birnbaum S.G.
      • Bruce C.J.
      • Williams G.V.
      • Arnsten A.F.T.
      Inverted-U dopamine D1 receptor actions on prefrontal neurons engaged in working memory.
      ,
      • Gamo N.J.
      • Lur G.
      • Higley M.J.
      • Wang M.
      • Paspalas C.D.
      • Vijayraghavan S.
      • et al.
      Stress impairs prefrontal cortical function via D1 dopamine receptor interactions with HCN channels.
      ). Other Gs-coupled receptors likely contribute as well: β-ARs are currently under study, and transcriptomics show very high levels of ADCYAP1 (encoding PACAP) in layer III dlPFC (
      • González-Burgos G.
      • Miyamae T.
      • Krimer Y.
      • Gulchina Y.
      • Pafundo D.E.
      • Krimer O.
      • et al.
      Distinct properties of layer 3 pyramidal neurons from prefrontal and parietal areas of the monkey neocortex.
      ), the master stress activating peptide that drives cAMP signaling (
      • Mustafa T.
      Pituitary adenylate cyclase-activating polypeptide (PACAP): A master regulator in central and peripheral stress responses.
      ), e.g., via VPAC2 receptors (see below). High levels of α1-AR stimulation also reduce Delay cell firing via IP3-mediated Ca2+/protein kinase C signaling (
      • Birnbaum S.B.
      • Yuan P.
      • Wang M.
      • Vijayraghavan S.
      • Bloom A.
      • Davis D.
      • et al.
      Protein kinase C overactivity impairs prefrontal cortical regulation of working memory.
      ,
      • Datta D.
      • Yang S.T.
      • Galvin V.C.
      • Solder J.
      • Luo F.
      • Morozov Y.M.
      • et al.
      Noradrenergic α1-adrenoceptor actions in the primate dorsolateral prefrontal cortex.
      ). Interestingly, rodent studies have shown that protein kinase C can cause internalization and/or decoupling of α2A-ARs (
      • Liang M.
      • Eason M.G.
      • Jewell-Motz E.A.
      • Williams M.A.
      • Theiss C.T.
      • Dorn 2nd, G.W.
      • et al.
      Phosphorylation and functional desensitization of the alpha2A-adrenergic receptor by protein kinase C.
      ,
      • Zhu Q.
      • Qi L.J.
      • Shi A.
      • Abou-Samra A.
      • Deth R.C.
      Protein kinase C regulates alpha(2A/D)-adrenoceptor constitutive activity.
      ) and mGluR3 (
      • Macek T.A.
      • Schaffhauser H.
      • Conn P.J.
      Protein kinase C and A3 adenosine receptor activation inhibit presynaptic metabotropic glutamate receptor (mGluR) function and uncouple mGluRs from GTP-binding proteins.
      ), which, if true in dlPFC as well, would reduce the inhibitory regulation of stress signaling pathways. Detrimental effects of high levels of D1R or α1-AR stimulation can also be seen at the behavioral level, where D1R or α1-AR stimulation in dlPFC impairs working memory performance and, conversely, D1R or α1-AR blockade reduces stress-induced working memory deficits [reviewed in (
      • Arnsten A.F.T.
      Stress signaling pathways that impair prefrontal cortex structure and function.
      ,
      • Arnsten A.F.
      Stress weakens prefrontal networks: Molecular insults to higher cognition.
      )]. It is noteworthy that atypical antipsychotics have α1-AR blocking properties that may be beneficial in blocking stress signaling in the dlPFC (
      • Baldessarini R.J.
      • Huston-Lyons D.
      • Campbell A.
      • Marsh E.
      • Cohen B.M.
      Do central antiadrenergic actions contribute to the atypical properties of clozapine?.
      ).
      Figure thumbnail gr2
      Figure 2The effects of uncontrollable stress exposure on layer III dorsolateral PFC spines. Acute exposure to an uncontrollable stressor increases catecholamine release in the PFC, driving feedforward cAMP-Ca2+-K+ channel signaling, to rapidly weaken synaptic efficacy, reduce persistent firing, and take dorsolateral PFC “offline.” Cortisol release exacerbates (or, on its own, mimics) these actions, likely by blocking the extraneuronal catecholamine transporters on glia that take up catecholamines from the extrasynaptic space. With chronic stress exposure, there are additional architectural changes, with loss of spines and dendrites that correlate with cognitive deficits. Phagocytosis of spines and dendrites likely involves Ca2+ overload of mitochondria, initiating an inflammatory response. Calbindin expression is decreased by chronic stress exposure, which may further elevate cytosolic Ca2+ levels. AC, adenylyl cyclase; AR, adrenoceptor; calb, calbindin; cAMP, cyclic adenosine monophosphate; D1R, D1 receptor; M1R, M1 receptor; MAOS, mitochondria-on-a-string; Nic-α7R, nicotinic-α7 receptor; NMDAR, NMDA receptor; PFC, prefrontal cortex; PKC, protein kinase C.
      During stress, glucocorticoids (e.g., cortisol) are also released by the adrenal cortex and cross into the brain. High levels of glucocorticoids can mimic and/or exacerbate the effects of catecholamines in PFC, e.g., impairing working memory in rats (
      • Barsegyan A.
      • Mackenzie S.M.
      • Kurose B.D.
      • McGaugh J.L.
      • Roozendaal B.
      Glucocorticoids in the prefrontal cortex enhance memory consolidation and impair working memory by a common neural mechanism.
      ) and deactivating PFC in humans (
      • van Stegeren A.H.
      • Roozendaal B.
      • Kindt M.
      • Wolf O.T.
      • Joëls M.
      Interacting noradrenergic and corticosteroid systems shift human brain activation patterns during encoding.
      ). Glucocorticoids are known to block the extraneuronal catecholamine transporters on glia that normally serve to remove catecholamines from the extracellular space (
      • Grundemann D.
      • Schechinger B.
      • Rappold G.A.
      • Schomig E.
      Molecular identification of the cortisone-sensitive extraneuronal catecholamine transporter.
      ), which would exacerbate the stress response.
      With chronic stress exposure, there continues to be elevated catecholamine and glucocorticoid release, but there are additional architectural changes, with loss of layer III dendrites and spines [(
      • Liston C.
      • Miller M.M.
      • Goldwater D.S.
      • Radley J.J.
      • Rocher A.B.
      • Hof P.R.
      • et al.
      Stress-induced alterations in prefrontal cortical dendritic morphology predict selective impairments in perceptual attentional set-shifting.
      ,
      • Radley J.J.
      • Rocher A.B.
      • Miller M.
      • Janssen W.G.
      • Liston C.
      • Hof P.R.
      • et al.
      Repeated stress induces dendritic spine loss in the rat medial prefrontal cortex.
      ,
      • Ota K.T.
      • Liu R.J.
      • Voleti B.
      • Maldonado-Aviles J.G.
      • Duric V.
      • Iwata M.
      • et al.
      REDD1 is essential for stress-induced synaptic loss and depressive behavior.
      ), reviewed in (
      • Woo E.
      • Sansing L.H.
      • Arnsten A.F.T.
      • Datta D.
      Chronic stress weakens connectivity in the prefrontal cortex: architectural and molecular changes.
      )]. Much of this research has been done in rodent models, largely for ethical reasons. These studies show that elevated cAMP/PKA and Ca2+/protein kinase C signaling contributes to layer III spine loss and that spine loss correlates with working memory deficits (
      • Hains A.B.
      • Vu M.A.
      • Maciejewski P.K.
      • van Dyck C.H.
      • Gottron M.
      • Arnsten A.F.
      Inhibition of protein kinase C signaling protects prefrontal cortex dendritic spines and cognition from the effects of chronic stress.
      ,
      • Hains A.B.
      • Yabe Y.
      • Arnsten A.F.T.
      Chronic stimulation of alpha-2A-adrenoceptors with guanfacine protects rodent prefrontal cortex dendritic spines and cognition from the effects of chronic stress.
      ). In particular, dysregulated Ca2+ signaling can initiate a number of toxic actions, including Ca2+ overload of mitochondria, leading to inflammatory signals such as complement that initiate spine removal (Figures 2, 3) [reviewed in (
      • Woo E.
      • Sansing L.H.
      • Arnsten A.F.T.
      • Datta D.
      Chronic stress weakens connectivity in the prefrontal cortex: architectural and molecular changes.
      )]. In layer III dlPFC, stress may simultaneously elevate cytosolic Ca2+ and weaken synaptic efficacy through opening of nearby K+ channels. However, either of these conditions may be sufficient to initiate spine removal. It is possible that related mechanisms contribute to the loss of spines and dendrites from layer III dlPFC in schizophrenia when intracellular stress signaling pathways are dysregulated by genetic and/or environmental insults. In this regard, it is of interest that there is an increased DA innervation of macaque layer III dlPFC in adolescence (
      • Rosenberg D.R.
      • Lewis D.A.
      Changes in the dopaminergic innervation of monkey prefrontal cortex during late postnatal development: A tyosine hydroxylase immunohistochemical study.
      ), which can drive stress signaling (
      • Gamo N.J.
      • Lur G.
      • Higley M.J.
      • Wang M.
      • Paspalas C.D.
      • Vijayraghavan S.
      • et al.
      Stress impairs prefrontal cortical function via D1 dopamine receptor interactions with HCN channels.
      ,
      • Arnsten A.F.
      Stress weakens prefrontal networks: Molecular insults to higher cognition.
      ). As there are increased DA D1Rs in the dlPFC in the earliest stages of schizophrenia (
      • Abi-Dargham A.
      • Xu X.
      • Thompson J.L.
      • Gil R.
      • Kegeles L.S.
      • Urban N.B.
      • et al.
      Increased prefrontal cortical D1 receptors in drug naive patients with schizophrenia: A PET study with [11C]NNC112.
      ), magnified stress signaling in layer III may contribute to the distinctive spine loss in this layer at the onset of disease, especially if proteins that regulate the stress response are impaired by genetic and/or inflammatory insults.
      Figure thumbnail gr3
      Figure 3Speculations regarding the sequence of events underlying spine removal. Initial events involve reduced growth factors and/or elevated, dysregulated Ca2+ signaling followed by actin destabilization, Ca2+ overload of the mitochondria, and inflammatory signaling, which engages glial removal of the spine. Complement C1q signaling is shown for illustrative purposes. Note that genetic alterations in schizophrenia may propel spine loss at multiple stages of this sequence. For more details on the mechanisms underlying spine removal, see (
      • Woo E.
      • Sansing L.H.
      • Arnsten A.F.T.
      • Datta D.
      Chronic stress weakens connectivity in the prefrontal cortex: architectural and molecular changes.
      ). BDNF, brain-derived neurotrophic factor; MOAS, mitochondria-on-a-string; ROS, reactive oxygen species.

      Genetic Risk Factors Weaken Layer III dlPFC Connectivity

      The genetics of schizophrenia are complex, with a large number of factors of (mostly) very small effect size (
      • Ripke S.
      • Walters J.T.
      • O’Donovan M.C.
      The Schizophrenia Working Group of the Psychiatric Genomics Consortium
      Mapping genomic loci prioritises genes and implicates synaptic biology in schizophrenia.
      ,
      Schizophrenia Working Group of the Psychiatric Genomics Consortium
      Biological insights from 108 schizophrenia-associated genetic loci.
      ). As summarized in Figure 4, many of the genetic insults that increase risk of schizophrenia would weaken the connectivity of layer III dlPFC synapses. A general pattern emerges whereby there are 1) loss-of-function alterations in proteins that are necessary to strengthen connectivity, including those needed for synapse creation or structure, for NMDAR neurotransmission, for mitochondrial energy production, or for inhibitory regulation of cAMP/Ca2+/K+ channel signaling, and 2) gain-of-function alterations in proteins that weaken synaptic connectivity, including those that increase cAMP/Ca2+/K+ channel signaling or that mediate phagocytic removal of spines such as complement C4a (
      • Sekar A.
      • Bialas A.R.
      • de Rivera H.
      • Davis A.
      • Hammond T.R.
      • Kamitaki N.
      • et al.
      Schizophrenia risk from complex variation of complement component 4.
      ). As a thorough review of this extensive field is beyond the scope of this review, this section focuses on genetic alterations that directly relate to the neurotransmission and neuromodulation of layer III dlPFC circuits that may help to explain their exceptional vulnerability.
      Figure thumbnail gr4
      Figure 4There are multiple genetic risk factors for schizophrenia that would weaken layer III dorsolateral prefrontal cortex network connectivity by either reducing beneficial actions or increasing detrimental actions. The loss-of-function alterations in gene products that normally strengthen connectivity are shown in gray; the gain-of-function alterations in gene products that normally weaken connectivity are shown in red. Thus, multiple different genotypes can lead to the same phenotype of weakened layer III dorsolateral prefrontal cortex connectivity. AC, adenylyl cyclase; cAMP, cyclic adenosine monophosphate; MAOS, mitochondria-on-a-string; NMDAR, NMDA receptor; PDE4, phosphodiesterase type 4; PKA, protein kinase A; SER, smooth endoplasmic reticulum.

      Loss-of-Function Alterations in Gene Products That Normally Strengthen Layer III dlPFC Network Connectivity

      There are multiple genes where loss-of-function genetic mutations would weaken synapses, including those involved with synaptic development and adherence [e.g., ZNF804A (
      • Zhou Y.
      • Dong F.
      • Lanz T.A.
      • Reinhart V.
      • Li M.
      • Liu L.
      • et al.
      Interactome analysis reveals ZNF804A, a schizophrenia risk gene, as a novel component of protein translational machinery critical for embryonic neurodevelopment.
      ), NRXN1 (
      • Kirov G.
      • Rujescu D.
      • Ingason A.
      • Collier D.A.
      • O’Donovan M.C.
      • Owen M.J.
      Neurexin 1 (NRXN1) deletions in schizophrenia.
      )], actin cytoskeletal dynamics [e.g., CDC42, ARP2/3 (
      • Datta D.
      • Arion D.
      • Corradi J.P.
      • Lewis D.A.
      Altered expression of CDC42 signaling pathway components in cortical layer 3 pyramidal cells in schizophrenia.
      ,
      • Datta D.
      • Arion D.
      • Roman K.M.
      • Volk D.W.
      • Lewis D.A.
      Altered expression of ARP2/3 complex signaling pathway genes in prefrontal layer 3 pyramidal cells in schizophrenia.
      )], and mitochondrial energy production [e.g. (
      • Hjelm B.E.
      • Rollins B.
      • Mamdani F.
      • Lauterborn J.C.
      • Kirov G.
      • Lynch G.
      • et al.
      Evidence of mitochondrial dysfunction within the complex genetic etiology of schizophrenia.
      ,
      • Arion D.
      • Corradi J.P.
      • Tang S.
      • Datta D.
      • Boothe F.
      • He A.
      • et al.
      Distinctive transcriptome alterations of prefrontal pyramidal neurons in schizophrenia and schizoaffective disorder.
      ,
      • Schulmann A.
      • Ryu E.
      • Goncalves V.
      • Rollins B.
      • Christiansen M.
      • Frye M.A.
      • et al.
      Novel complex interactions between mitochondrial and nuclear DNA in schizophrenia and bipolar disorder.
      )]. However, these genetic insults should affect all synapses, and thus a key question is why they would particularly affect synapses in deep layer III more than others, e.g., those in layer V. The recurrent excitatory circuits in deep layer III are thought to reside on the basal dendrites of deep layer III pyramidal cells, which have remarkably high levels of dendritic branching and spine density (
      • Elston G.N.
      Specialization of the neocortical pyramidal cell during primate evolution.
      ,
      • Elston G.N.
      • Benavides-Piccione R.
      • Elston A.
      • Zietsch B.
      • Defelipe J.
      • Manger P.
      • et al.
      Specializations of the granular prefrontal cortex of primates: Implications for cognitive processing.
      ,
      • Elston G.N.
      • Benavides-Piccione R.
      • Elston A.
      • Manger P.R.
      • Defelipe J.
      Pyramidal cells in prefrontal cortex of primates: marked differences in neuronal structure among species.
      ), and thus these insults may be more evident against a background of high connectivity. It is also possible that synaptic errors could be magnified in a recurrent excitatory circuit, including interactions with additional factors that preferentially weaken connectivity in layer III dlPFC, as discussed below.
      Several genetic insults linked to schizophrenia target glutamate neurotransmission in ways that may be particularly detrimental to layer III dlPFC recurrent circuits. Proteins that normally strengthen dlPFC network connectivity and are associated with loss-of-function mutations are shown in gray in Figure 4. As discussed above, layer III dlPFC Delay cell circuits heavily rely on NMDARs with either GluN2B or GluN2A subunits (
      • Wang M.
      • Yang Y.
      • Wang C.J.
      • Gamo N.J.
      • Jin L.E.
      • Mazer J.A.
      • et al.
      NMDA receptors subserve working memory persistent neuronal firing in dorsolateral prefrontal cortex.
      ). GRIN2A, which encodes NMDAR/GluN2A, is a replicated risk factor for schizophrenia (
      Schizophrenia Working Group of the Psychiatric Genomics Consortium
      Biological insights from 108 schizophrenia-associated genetic loci.
      ,
      • Lencz T.
      • Malhotra A.
      Targeting the schizophrenia genome: A fast track strategy from GWAS to clinic.
      ). Inadequate NMDAR/GluN2A neurotransmission may lead to spine pruning, as weak synapses are generally removed, e.g., in developing circuits (
      • Stephan A.H.
      • Barres B.A.
      • Stevens B.
      The complement system: An unexpected role in synaptic pruning during development and disease.
      ). Interestingly, insults to GRIN2B are associated with even more profound intellectual disability, which may relate to the key role this receptor plays in cortical development (
      • Myers S.J.
      • Yuan H.
      • Kang J.Q.
      • Tan F.C.K.
      • Traynelis S.F.
      • Low C.M.
      Distinct roles of GRIN2A and GRIN2B variants in neurological conditions.
      ).
      Perhaps most distinct to layer III dlPFC would be genes that regulate feedforward, Ca2+/cAMP-K+ channel signaling in layer III spines, where loss of regulation would weaken synaptic connectivity. A key gene in this regard is GRM3, which encodes mGluR3. As mentioned above, mGluR3 are concentrated on layer III dlPFC spines [rather than axon terminals, where they reside in classic circuits, e.g., rodent spinal cord (
      • Di Prisco S.
      • Merega E.
      • Bonfiglio T.
      • Olivero G.
      • Cervetto C.
      • Grilli M.
      • et al.
      Presynaptic, release-regulating mGlu2-preferring and mGlu3-preferring autoreceptors in CNS: Pharmacological profiles and functional roles in demyelinating disease.
      )]. In layer III dlPFC, mGluR3 inhibits cAMP/K+ channel signaling, strengthening connectivity and enhancing Delay cell firing (
      • Jin L.E.
      • Wang M.
      • Galvin V.C.
      • Lightbourne T.C.
      • Conn P.J.
      • Arnsten A.F.T.
      • et al.
      mGluR2 vs. mGluR3 in primate prefrontal cortex: Postsynaptic mGluR3 strengthen cognitive networks.
      ). GRM3 is a consistent risk factor for schizophrenia (
      • Saini S.M.
      • Mancuso S.G.
      • Mostaid M.S.
      • Liu C.
      • Pantelis C.
      • Everall I.P.
      • et al.
      Meta-analysis supports GWAS-implicated link between GRM3 and schizophrenia risk.
      ), with reduced mGluR3 protein in the dlPFC of patients with schizophrenia (
      • Ghose S.
      • Gleason K.A.
      • Potts B.W.
      • Lewis-Amezcua K.
      • Tamminga C.A.
      Differential expression of metabotropic glutamate receptors 2 and 3 in schizophrenia: A mechanism for antipsychotic drug action?.
      ). Our data suggest that loss-of-function mutations in GRM3 would be especially detrimental to layer III dlPFC connectivity (
      • Jin L.E.
      • Wang M.
      • Galvin V.C.
      • Lightbourne T.C.
      • Conn P.J.
      • Arnsten A.F.T.
      • et al.
      mGluR2 vs. mGluR3 in primate prefrontal cortex: Postsynaptic mGluR3 strengthen cognitive networks.
      ,
      • Arnsten A.F.T.
      • Wang M.
      The evolutionary expansion of mGluR3-NAAG-GCPII signaling: Relevance to human intelligence and cognitive disorders.
      ), consistent with findings that genetic alterations in mGluR3 are associated with impaired dlPFC function in patients with schizophrenia (
      • Egan M.F.
      • Straub R.E.
      • Goldberg T.E.
      • Yakub I.
      • Callicott J.H.
      • Hariri A.R.
      • et al.
      Variation in GRM3 affects cognition, prefrontal glutamate, and risk for schizophrenia.
      ) and in healthy individuals (
      • Zink C.
      • Barker P.
      • Sawa A.
      • Weinberger D.
      • Wang A.
      • Quillian H.
      • et al.
      Missense mutation in FOLH1 is associated with decreased NAAG levels and impaired working memory circuitry and cognition.
      ).
      A rare loss-of-function translocation in DISC1 is associated with high rates of mental disorders (
      • Millar J.K.
      • Wilson-Annan J.C.
      • Anderson S.L.
      • Christie S.
      • Taylor M.S.
      • Semple C.A.
      • et al.
      Disruption of two novel genes by a translocation co-segregating with schizophrenia.
      ). DISC1 has multiple functions, including anchoring PDE4s, the enzymes that catabolize cAMP (
      • Millar J.K.
      • Pickard B.S.
      • Mackie S.
      • James R.S.
      • Christie S.
      • Buchanan S.R.
      • et al.
      DISC1 and PDE4B are interacting genetic factors in schizophrenia that regulate cAMP signaling.
      ). We have documented DISC1 anchoring PDE4A to the spine apparatus in layer III dlPFC spines (
      • Paspalas C.D.
      • Wang M.
      • Arnsten A.F.T.
      Constellation of HCN channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex—potential substrate for working memory deficits in schizophrenia.
      ,
      • Arnsten A.F.
      • Jin L.E.
      Guanfacine for the treatment of cognitive disorders: A century of discoveries at Yale.
      ), positioned to regulate feedforward, cAMP/Ca2+/K+ channel signaling (
      • Arnsten A.F.T.
      • Datta D.
      • Wang M.
      The genie in the bottle—magnified calcium signaling in dorsolateral prefrontal cortex.
      ,
      • Arnsten A.F.
      • Jin L.E.
      Guanfacine for the treatment of cognitive disorders: A century of discoveries at Yale.
      ). Thus, a loss-of-function mutation in DISC1 would result in high levels of K+ efflux and elevated cytosolic Ca2+, reducing Delay cell firing and increasing risk of dendritic atrophy. DISC1 knockdown in rat medial PFC lowered the threshold for stress-induced deficits in working memory, with no effect under nonstress conditions (
      • Gamo N.J.
      • Duque A.
      • Paspalas C.D.
      • Kata A.
      • Fine R.
      • Boven L.
      • et al.
      Role of disrupted in schizophrenia 1 (DISC1) in stress-induced prefrontal cognitive dysfunction.
      ). These data suggest that phenotypic expression of reduced DISC1 may be seen only under stressful conditions when there is elevated cAMP signaling in layer III dlPFC. Similar effects in humans may help to explain the heterogeneity of the DISC1 phenotype.

      Gain-of-Function Alterations in Gene Products That Normally Weaken Layer III dlPFC Network Connectivity

      There are also multiple gain-of-function alterations in proteins that weaken layer III dlPFC connectivity by increasing Ca2+/cAMP/K+ channel signaling (Figure 4, left side). For example, a rare microduplication of the Gs-coupled receptor VIPR2 (i.e., VPAC2) increases risk of schizophrenia (
      • Vacic V.
      • McCarthy S.
      • Malhotra D.
      • Murray F.
      • Chou H.H.
      • Peoples A.
      • et al.
      Duplications of the neuropeptide receptor gene VIPR2 confer significant risk for schizophrenia.
      ). VIPR2s are stimulated by either vasoactive intestinal peptide or the master stress peptide pituitary adenylate cyclase-activating polypeptide, increasing cAMP signaling. Studies in patients with schizophrenia that express this microduplication show increased VIPR2 transcription and increased cAMP signaling in cultured lymphocytes (
      • Vacic V.
      • McCarthy S.
      • Malhotra D.
      • Murray F.
      • Chou H.H.
      • Peoples A.
      • et al.
      Duplications of the neuropeptide receptor gene VIPR2 confer significant risk for schizophrenia.
      ). VIPR2s are concentrated on layer III dlPFC spines, positioned near the smooth endoplasmic reticulum spine apparatus where they can drive feedforward cAMP/Ca2+/K+ channel signaling [(
      • Datta D.
      • Mentone S.A.
      • Morozov Y.
      • Arnsten A.
      Subcellular localization of schizophrenia risk genes encoding Cav1. 2 (CACNA1C) and VIPR2 in rhesus macaque dorsolateral prefrontal cortex.
      ); D. Datta, Ph.D., et al., unpublished data, 2021). Thus, microduplications that increase VIPR2/cAMP signaling would weaken layer III dlPFC connectivity.
      Genetic alterations in voltage-gated Ca2+ channels are also a replicated risk factor for schizophrenia (
      • Andrade A.
      • Brennecke A.
      • Mallat S.
      • Brown J.
      • Gomez-Rivadeneira J.
      • Czepiel N.
      • et al.
      Genetic associations between voltage-gated calcium channels and psychiatric disorders.
      ). In particular, a gain-of-function mutation (
      • Yoshimizu T.
      • Pan J.Q.
      • Mungenast A.E.
      • Madison J.M.
      • Su S.
      • Ketterman J.
      • et al.
      Functional implications of a psychiatric risk variant within CACNA1C in induced human neurons.
      ) in CACNA1C, which encodes for the α subunit of the L-type Ca2+ channel, Cav1.2, is consistently linked to increased risk of schizophrenia and bipolar disorder (
      Schizophrenia Working Group of the Psychiatric Genomics Consortium
      Biological insights from 108 schizophrenia-associated genetic loci.
      ,
      • Lencz T.
      • Malhotra A.
      Targeting the schizophrenia genome: A fast track strategy from GWAS to clinic.
      ,
      • Bhat S.
      • Dao D.T.
      • Terrillion C.E.
      • Arad M.
      • Smith R.J.
      • Soldatov N.M.
      • et al.
      CACNA1C (Cav1.2) in the pathophysiology of psychiatric disease.
      ,
      • Ripke S.
      • O’Dushlaine C.
      • Chambert K.
      • Moran J.L.
      • Kähler A.K.
      • et al.
      Genome-wide association analysis identifies 13 new risk loci for schizophrenia.
      ,
      • Gordovez F.J.A.
      • McMahon F.J.
      The genetics of bipolar disorder.
      ). This gain-of-function in Cav1.2 is associated with inefficient dlPFC function (
      • Zink C.F.
      • Giegerich M.
      • Prettyman G.E.
      • Carta K.E.
      • van Ginkel M.
      • O’Rourke M.P.
      • et al.
      Nimodipine improves cortical efficiency during working memory in healthy subjects.
      ) and poor working memory and executive control in healthy control subjects and especially in patients with schizophrenia (
      • Thimm M.
      • Kircher T.
      • Kellermann T.
      • Markov V.
      • Krach S.
      • Jansen A.
      • et al.
      Effects of a CACNA1C genotype on attention networks in healthy individuals.
      ,
      • Cosgrove D.
      • Mothersill O.
      • Kendall K.
      • Konte B.
      • Harold D.
      • Giegling I.
      • et al.
      Cognitive characterization of schizophrenia risk variants involved in synaptic transmission: Evidence of CACNA1C’s role in working memory.
      ). In the heart, Cav1.2 is central to the fight-or-flight stress response, where its activation by noradrenergic β-AR/PKA signaling drives internal Ca2+ release from the sarcoplasmic reticulum, increasing muscle contraction (
      • Catterall W.A.
      Regulation of cardiac calcium channels in the fight-or-flight response.
      ). Our immuno-electron microscopy data found parallel localization on layer III dlPFC spines near the smooth endoplasmic reticulum spine apparatus (
      • Datta D.
      • Mentone S.A.
      • Morozov Y.
      • Arnsten A.
      Subcellular localization of schizophrenia risk genes encoding Cav1. 2 (CACNA1C) and VIPR2 in rhesus macaque dorsolateral prefrontal cortex.
      ), positioned to drive Ca2+/cAMP/K+ signaling, which can weaken connectivity and increase toxic Ca2+ actions that lead to spine removal.
      There also appear to be gain-of-function insults to the gene that encodes for one of the key channels that weakens synaptic connectivity in layer III spines: HCN1 (
      • Lencz T.
      • Malhotra A.
      Targeting the schizophrenia genome: A fast track strategy from GWAS to clinic.
      ,
      • Pardiñas A.F.
      • Holmans P.A.
      • Pocklington A.J.
      • Escott-Price V.
      • Ripke S.
      • Carrera N.
      • et al.
      Common schizophrenia alleles are enriched in mutation-intolerant genes and in regions under strong background selection.
      ). The HCN1 risk allele is associated with impaired spatial memory (
      • Greenwood T.A.
      • Lazzeroni L.C.
      • Maihofer A.X.
      • Swerdlow N.R.
      • Calkins M.E.
      • Freedman R.
      • et al.
      Genome-wide association of endophenotypes for schizophrenia from the Consortium on the Genetics of Schizophrenia (COGS) study.
      ), and physiological data are consistent with the risk allele conferring a gain of function (
      • Refisch A.
      • Chung H.Y.
      • Komatsuzaki S.
      • Schumann A.
      • Mühleisen T.W.
      • Nöthen M.M.
      • et al.
      A common variation in HCN1 is associated with heart rate variability in schizophrenia.
      ).
      All of these gain-of-function alterations would weaken the efficacy of dlPFC synapses, and increased expression of VIPR2 or Cav1.2 would additionally increase cytosolic Ca2+, which can cause Ca2+ overload of mitochondria, leading to an inflammatory response, e.g., complement activation (
      • Stephan A.H.
      • Barres B.A.
      • Stevens B.
      The complement system: An unexpected role in synaptic pruning during development and disease.
      ), to initiate phagocytosis and the removal of spines and dendrites (Figure 4). Alternatively, over-pruning of spines and dendrites could occur through a gain-of-function genetic alteration in complement C4a, which has the largest likelihood of being a genetic risk factor for schizophrenia (
      • Sekar A.
      • Bialas A.R.
      • de Rivera H.
      • Davis A.
      • Hammond T.R.
      • Kamitaki N.
      • et al.
      Schizophrenia risk from complex variation of complement component 4.
      ). The C4a gain-of-function risk allele has been shown to increase phagocytosis in mouse models (
      • Sekar A.
      • Bialas A.R.
      • de Rivera H.
      • Davis A.
      • Hammond T.R.
      • Kamitaki N.
      • et al.
      Schizophrenia risk from complex variation of complement component 4.
      ) and is associated with reduced neuropil in patients (
      • Prasad K.M.
      • Chowdari K.V.
      • D’Aiuto L.A.
      • Iyengar S.
      • Stanley J.A.
      • Nimgaonkar V.L.
      Neuropil contraction in relation to complement C4 gene copy numbers in independent cohorts of adolescent-onset and young adult-onset schizophrenia patients—a pilot study.
      ), consistent with excessive spine pruning contributing to the etiology of schizophrenia. The current discussion highlights how multiple genetic insults, indirectly or directly, can lead to weakening and removal of layer III dlPFC excitatory synapses on spines.

      Inflammatory Insults Can Mimic Genetic Insults to Weaken dlPFC Connectivity

      Environmental insults are also major risk factors for schizophrenia, including perinatal inflammation (e.g., influenza during the second trimester, hypoxia at birth), and psychological stressors during adolescence/early adulthood are associated with symptom onset (e.g., leaving home for college) (
      • Breier A.
      • Wolkowitz O.
      • Pickar D.
      Stress and schizophrenia: Advances in neuropsychiatry and psychopharmacology.
      ). It is hypothesized that perinatal inflammation may sensitize the inflammatory response and contribute to greater spine removal during adolescence. This section describes three inflammatory responses that may be especially detrimental to layer III dlPFC connections, as summarized in Figure 5, mimicking several of the genetic insults shown in Figure 4.
      Figure thumbnail gr5
      Figure 5Inflammation weakens layer III dlPFC network connectivity in ways that mimic genetic insults. Inflammation increases the expression and release of KYNA and GCPII by astrocytes and increases p38-MK2 signaling within neurons. KYNA blocks NMDAR and Nic-α7R, which would mimic loss-of-function genetic alterations to GRIN2A; GCPII reduces NAAG stimulation of mGluR3, which would mimic loss-of-function genetic alterations to GRM3; and MK2 unanchors and disinhibits PDE4s, which would mimic loss-of-function genetic alterations to DISC1. Calbindin expression may also be decreased by chronic stress exposure, which would increase cytosolic Ca2+ levels, similar to gain-of-function mutations in CACNA1C. Ca2+ overload of mitochondria can lead to complement activation, which may mimic gain-of-function genetic alterations in complement C4A. Thus, the combination of genetic and environmental insults may interact to cross the threshold into pathology. AC, adenylyl cyclase; calb, calbindin; cAMP, cyclic adenosine monophosphate; dlPFC, dorsolateral prefrontal cortex; GCPII, glutamate carboxypeptidase II; KYNA, kynurenic acid; MAOS, mitochondria-on-a-string; MK2, mitogen-activated protein kinase–activated protein kinase 2; NAAG, N-acetylaspartylglutamate; Nic-α7R, nicotinic-α7 receptor; NMDAR, NMDA receptor; PDE4, phosphodiesterase type 4; ROS, reactive oxygen species.

      Kynurenic Acid

      Inflammation increases tryptophan metabolism to kynurenine, which can then be further processed to kynurenic acid (KYNA), especially in astrocytes (
      • Parrott J.M.
      • O’Connor J.C.
      Kynurenine 3-monooxygenase: An influential mediator of neuropathology.
      ). Kynurenine is actively taken up into brain, so it can also originate from peripheral sources (
      • Gál E.M.
      • Sherman A.D.
      L-kynurenine: Its synthesis and possible regulatory function in brain.
      ). Most pertinent to the current discussion, KYNA is known to block both NMDAR and Nic-α7R (
      • Pullan L.M.
      • Cler J.A.
      Schild plot analysis of glycine and kynurenic acid at the N-methyl-D-aspartate excitatory amino acid receptor.
      ,
      • Albuquerque E.X.
      • Schwarcz R.
      Kynurenic acid as an antagonist of α7 nicotinic acetylcholine receptors in the brain: Facts and challenges.
      ), the receptors most essential to dlPFC neurotransmission. Thus, KYNA inflammatory signaling would mimic loss-of-function alterations in GRIN2A as well as reduce factors permissive for NMDAR neurotransmission in layer III dlPFC and thus may be particularly detrimental to the circuits mediating higher cognition. Schizophrenia is associated with higher KYNA levels (
      • Plitman E.
      • Iwata Y.
      • Caravaggio F.
      • Nakajima S.
      • Chung J.K.
      • Gerretsen P.
      • et al.
      Kynurenic acid in schizophrenia: A systematic review and meta-analysis.
      ), including in the dlPFC (
      • Sathyasaikumar K.V.
      • Stachowski E.K.
      • Wonodi I.
      • Roberts R.C.
      • Rassoulpour A.
      • McMahon R.P.
      • et al.
      Impaired kynurenine pathway metabolism in the prefrontal cortex of individuals with schizophrenia.
      ,
      • Kindler J.
      • Lim C.K.
      • Weickert C.S.
      • Boerrigter D.
      • Galletly C.
      • Liu D.
      • et al.
      Dysregulation of kynurenine metabolism is related to proinflammatory cytokines, attention, and prefrontal cortex volume in schizophrenia.
      ), and KYNA is especially evident in cases of schizophrenia with signatures of inflammation, where it is associated with reduced dlPFC volume and impaired attention regulation (
      • Kindler J.
      • Lim C.K.
      • Weickert C.S.
      • Boerrigter D.
      • Galletly C.
      • Liu D.
      • et al.
      Dysregulation of kynurenine metabolism is related to proinflammatory cytokines, attention, and prefrontal cortex volume in schizophrenia.
      ). Current treatment strategies aim to inhibit KYNA production and restore dlPFC function (
      • Schwarcz R.
      • Pellicciari R.
      Manipulation of brain kynurenines: Glial targets, neuronal effects, and clinical opportunities.
      ,
      • Blanco-Ayala T.
      • Sathyasaikumar K.V.
      • Uys J.D.
      • Pérez-de-la-Cruz V.
      • Pidugu L.S.
      • Schwarcz R.
      N-acetylcysteine inhibits kynurenine aminotransferase II.
      ).

      Glutamate Carboxypeptidase II

      As shown in Figure 5, glutamate carboxypeptidase II (GCPII) reduces mGluR3 signaling by catabolizing NAAG (N-acetylaspartylglutamate), the endogenous ligand for mGluR3 that is co-released with glutamate (Figure 5) (
      • Vornov J.J.
      • Hollinger K.R.
      • Jackson P.F.
      • Wozniak K.M.
      • Farah M.H.
      • Majer P.
      • et al.
      Still NAAG’ing after all these years: The continuing pursuit of GCPII inhibitors.
      ). GCPII expression is increased by inflammation, e.g., in the aged rat medial PFC (
      • Datta D.
      • Leslie S.N.
      • Woo E.
      • Amancharla N.
      • Elmansy A.
      • Lepe M.
      • et al.
      Glutamate carboxypeptidase II in aging rat prefrontal cortex impairs working memory performance.
      ), and, especially relevant to schizophrenia, by perinatal inflammation (
      • Zhang Z.
      • Bassam B.
      • Thomas A.G.
      • Williams M.
      • Liu J.
      • Nance E.
      • et al.
      Maternal inflammation leads to impaired glutamate homeostasis and up-regulation of glutamate carboxypeptidase II in activated microglia in the fetal/newborn rabbit brain.
      ). Thus, inflammation can mimic loss-of-function mutations in GRM3 by reducing mGluR3 signaling. As mGluR3s are concentrated on layer III spines where they enhance connectivity, loss of beneficial mGluR3 actions would be particularly deleterious to dlPFC working memory function. Elevated GCPII levels have been documented in the dlPFC of patients with schizophrenia (
      • Ghose S.
      • Gleason K.A.
      • Potts B.W.
      • Lewis-Amezcua K.
      • Tamminga C.A.
      Differential expression of metabotropic glutamate receptors 2 and 3 in schizophrenia: A mechanism for antipsychotic drug action?.
      ), consistent with increased inflammation in this disorder. Interestingly, in healthy individuals, a gain-of-function variant in the FOLH1 gene that encodes for GCPII is associated with decreased NAAG levels, inefficient dlPFC activity, and impaired cognition (
      • Zink C.
      • Barker P.
      • Sawa A.
      • Weinberger D.
      • Wang A.
      • Quillian H.
      • et al.
      Missense mutation in FOLH1 is associated with decreased NAAG levels and impaired working memory circuitry and cognition.
      ), highlighting the importance of this signaling mechanism to human intelligence.

      Mitogen-Activated Protein Kinase–Activated Protein Kinase 2

      Mitogen-activated protein kinase–activated protein kinase 2 (MAPKAPK2 or MK2) is a downstream substrate of p38MAPK signaling that is activated under conditions of stress and/or inflammation (
      • Duraisamy S.
      • Bajpai M.
      • Bughani U.
      • Dastidar S.G.
      • Ray A.
      • Chopra P.
      MK2: A novel molecular target for anti-inflammatory therapy.
      ). MK2 phosphorylates the PDE4s such that they can no longer be anchored by DISC1 to the correct location (
      • MacKenzie K.F.
      • Wallace D.A.
      • Hill E.V.
      • Anthony D.F.
      • Henderson D.J.
      • Houslay D.M.
      • et al.
      Phosphorylation of cAMP-specific PDE4A5 (phosphodiesterase-4A5) by MK2 (MAPKAPK2) attenuates its activation through protein kinase A phosphorylation.
      ,
      • Houslay K.F.
      • Christian F.
      • MacLeod R.
      • Adams D.R.
      • Houslay M.D.
      • Baillie G.S.
      Identification of a multifunctional docking site on the catalytic unit of phosphodiesterase-4 (PDE4) that is utilised by multiple interaction partners.
      ). In this way, MK2 inflammatory signaling would mimic loss-of-function translocation in DISC1. MK2 also prevents PKA from activating PDE4s, thus taking away negative feedback that would normally regulate stress signaling that, once initiated, can “run wild.”

      Psychological Stressors

      Although most studies of inflammation use infectious agents or hypoxia to induce an inflammatory state, it is important to remember that psychological stress also impacts many of these same signaling pathways (Figure 2). As described above, psychological stress drives Ca2+/cAMP/K+ signaling, and chronic stress exposure leads to loss of spines. There is some evidence from rodents that psychological stress exposure can reduce the expression of the Ca2+ binding protein calbindin (
      • Li J.T.
      • Xie X.M.
      • Yu J.Y.
      • Sun Y.X.
      • Liao X.M.
      • Wang X.X.
      • et al.
      Suppressed calbindin levels in hippocampal excitatory neurons mediate stress-induced memory loss.
      ), which would further increase cytosolic Ca2+ levels, leading to inflammation and spine removal (
      • Woo E.
      • Sansing L.H.
      • Arnsten A.F.T.
      • Datta D.
      Chronic stress weakens connectivity in the prefrontal cortex: architectural and molecular changes.
      ).
      Taken together, it is evident that there are multiple ways in which inflammatory/stress signaling can weaken dlPFC network connectivity and, in several cases, increase cytosolic Ca2+ signaling, mimicking genetic insults. Thus, the interaction between environmental and genetic insults may cross the threshold into pathology, leading to atrophy of layer III dlPFC dendrites and spines.

      Limitations, Future Directions, and Summary

      There is much that is not known about the primate cortex that limits our current hypotheses and interpretations. For example, almost nothing is known about the molecular regulation of synapses in primate cortices beyond the dlPFC and V1. Thus, it is not possible to relate spine differences in schizophrenia in other cortical areas to vulnerabilities in their underlying molecular regulation. There is also little known about the mechanisms governing gray matter thinning in the primate cortex, whether this process targets ineffective connections, and how these processes may be altered in schizophrenia. The current review is an initial attempt to begin to relate synaptic and inflammatory mechanisms under study in primate dlPFC to the striking pathology of these circuits in schizophrenia.
      In summary, a unique feature of layer III dlPFC circuits is their built-in mechanisms to rapidly take them “offline” during psychological and/or physiological stress. These include cAMP-PKA magnification of Ca2+ signaling in spines and the concentration of cAMP-PKA opened K+ channels on spines that rapidly weaken synaptic efficacy. These properties may interact with genetic insults (e.g., gain-of-function expression of Cav1.2), and/or environmental insults (e.g., GCPII-mediated dysregulation of cAMP signaling) to cross the threshold into pathology. Flaws in neuronal signaling may also be exaggerated owing to the recurrent nature of layer III microcircuits, e.g., where the reduction in neurotransmission caused by genetic insults to GRIN2A and/or blockade of NMDAR by KYNA would be amplified across a large number of layer III recurrent NMDAR synapses on spines. Understanding the unique needs of these circuits may help in the design of preventive treatments to protect these recently evolved circuits that are so critical to cognitive function.

      Acknowledgments and Disclosures

      This work was supported by the National Institutes of Health (Grant Nos. MH108643 and AG061190 [to AFTA] and Grant No. MH093354 [to MW]).
      AFTA and Yale University receive royalties from the U.S. sales of Intuniv (extended-release guanfacine) from Shire/Takeda Pharmaceuticals, but do not receive royalties from generic or international sales. All other authors report no biomedical financial interests or potential conflicts of interest.

      References

        • Weinberger D.R.
        • Berman K.F.
        • Zec R.F.
        Physiologic dysfunction of dorsolateral prefrontal cortex in schizophrenia. I. Regional cerebral blood flow evidence.
        Arch Gen Psychiatry. 1986; 43: 114-124
        • Docherty N.M.
        • Hawkins K.A.
        • Hoffman R.E.
        • Quinlan D.M.
        • Rakfeldt J.
        • Sledge W.H.
        Working memory, attention, and communication disturbances in schizophrenia.
        J Abnorm Psychol. 1996; 105: 212-219
        • Perlstein W.M.
        • Carter C.S.
        • Noll D.C.
        • Cohen J.D.
        Relation of prefrontal cortex dysfunction to working memory and symptoms in schizophrenia.
        Am J Psychiatry. 2001; 158: 1105-1113
        • Barch D.M.
        The cognitive neuroscience of schizophrenia.
        Annu Rev Clin Psychol. 2005; 1: 321-353
        • Keefe R.S.
        • Harvey P.D.
        Cognitive impairment in schizophrenia.
        Handb Exp Pharmacol. 2012; 213: 11-37
        • Docherty N.M.
        • Evans I.M.
        • Sledge W.H.
        • Seibyl J.P.
        • Krystal J.H.
        Affective reactivity of language in schizophrenia.
        J Nerv Ment Dis. 1994; 182: 98-102
        • Ripke S.
        • Walters J.T.
        • O’Donovan M.C.
        • The Schizophrenia Working Group of the Psychiatric Genomics Consortium
        Mapping genomic loci prioritises genes and implicates synaptic biology in schizophrenia.
        medRxiv. 2020; https://doi.org/10.1101/2020.09.12.20192922
        • Cannon T.D.
        • Thompson P.M.
        • van Erp T.G.
        • Toga A.W.
        • Poutanen V.P.
        • Huttunen M.
        • et al.
        Cortex mapping reveals regionally specific patterns of genetic and disease-specific gray-matter deficits in twins discordant for schizophrenia.
        Proc Natl Acad Sci U S A. 2002; 99: 3228-3233
        • Cannon T.D.
        • Chung Y.
        • He G.
        • Sun D.
        • Jacobson A.
        • van Erp T.G.
        • et al.
        Progressive reduction in cortical thickness as psychosis develops: A multisite longitudinal neuroimaging study of youth at elevated clinical risk.
        Biol Psychiatry. 2014; 77: 147-157
        • Föcking M.
        • Sabherwal S.
        • Cates H.M.
        • Scaife C.
        • Dicker P.
        • Hryniewiecka M.
        • et al.
        Complement pathway changes at age 12 are associated with psychotic experiences at age 18 in a longitudinal population-based study: Evidence for a role of stress.
        Mol Psychiatry. 2021; 26: 524-533
        • Radhakrishnan R.
        • Skosnik P.D.
        • Ranganathan M.
        • Naganawa M.
        • Toyonaga T.
        • Finnema S.
        • et al.
        In vivo evidence of lower synaptic vesicle density in schizophrenia.
        Mol Psychiatry. 2021; 26: 7690-7698
        • Glantz L.A.
        • Lewis D.A.
        Decreased dendritic spine density on prefrontal cortical pyramidal neurons in schizophrenia.
        Arch Gen Psychiatry. 2000; 57: 65-73
        • Berdenis van Berlekom A.
        • Muflihah C.H.
        • Snijders G.J.L.J.
        • MacGillavry H.D.
        • Middeldorp J.
        • Hol E.M.
        • et al.
        Synapse pathology in schizophrenia: A meta-analysis of postsynaptic elements in postmortem brain studies.
        Schizophr Bull. 2020; 46: 374-386
        • Goldman-Rakic P.
        Cellular basis of working memory.
        Neuron. 1995; 14: 477-485
        • Glausier J.R.
        • Lewis D.A.
        Dendritic spine pathology in schizophrenia.
        Neuroscience. 2013; 251: 90-107
        • Kolluri N.
        • Sun Z.
        • Sampson A.R.
        • Lewis D.A.
        Lamina-specific reductions in dendritic spine density in the prefrontal cortex of subjects with schizophrenia.
        Am J Psychiatry. 2005; 162: 1200-1202
        • Roberts R.C.
        • Barksdale K.A.
        • Roche J.K.
        • Lahti A.C.
        Decreased synaptic and mitochondrial density in the postmortem anterior cingulate cortex in schizophrenia.
        Schizophr Res. 2015; 168: 543-553
        • Garey L.J.
        • Ong W.Y.
        • Patel T.S.
        • Kanani M.
        • Davis A.
        • Mortimer A.M.
        • et al.
        Reduced dendritic spine density on cerebral cortical pyramidal neurons in schizophrenia.
        J Neurol Neurosurg Psychiatry. 1998; 65: 446-453
        • Moyer C.E.
        • Delevich K.M.
        • Fish K.N.
        • Asafu-Adjei J.K.
        • Sampson A.R.
        • Dorph-Petersen K.A.
        • et al.
        Intracortical excitatory and thalamocortical boutons are intact in primary auditory cortex in schizophrenia.
        Schizophr Res. 2013; 149: 127-134
        • McKinney B.C.
        • MacDonald M.L.
        • Newman J.T.
        • Shelton M.A.
        • DeGiosio R.A.
        • Kelly R.M.
        • et al.
        Density of small dendritic spines and microtubule-associated-protein-2 immunoreactivity in the primary auditory cortex of subjects with schizophrenia.
        Neuropsychopharmacology. 2019; 44: 1055-1061
        • Jacobsen C.
        Studies of cerebral functions in primates.
        Comparative Psychology Monographs. 1936; 13: 1-60
        • Fuster J.
        The Prefrontal Cortex.
        Raven Press, New York1989
        • Robbins T.W.
        Dissociating executive functions of the prefrontal cortex.
        Phil Trans R Soc Lond B Biol Sci. 1996; 351: 1463-1471
        • Szczepanski S.M.
        • Knight R.T.
        Insights into human behavior from lesions to the prefrontal cortex.
        Neuron. 2014; 83: 1002-1018
        • Goldman-Rakic P.S.
        • Selemon L.D.
        • Schwartz M.L.
        Dual pathways connecting the dorsolateral prefrontal cortex with the hippocampal formation and parahippocampal cortex in the rhesus monkey.
        Neuroscience. 1984; 12: 719-743
        • Giguere M.
        • Goldman-Rakic P.S.
        Mediodorsal nucleus: Areal, laminar, and tangential distribution of afferents and efferents in the frontal lobe of rhesus monkeys.
        J Comp Neurol. 1988; 277: 195-213
        • Selemon L.D.
        • Goldman-Rakic P.S.
        Common cortical and subcortical targets of the dorsolateral prefrontal and posterior parietal cortices in the rhesus monkey: Evidence for a distributed neural network subserving spatially guided behavior.
        J Neurosci. 1988; 8: 4049-4068
        • Goldman-Rakic P.S.
        Circuitry of the primate prefrontal cortex and the regulation of behavior by representational memory.
        in: Plum F. Handbook of Physiology, The Nervous System, Higher Functions of the Brain. American Physiological Society, Bethesda1987: 373-417
        • Fuster J.
        • Alexander G.
        Neuron activity related to short-term memory.
        Science. 1971; 173: 652-654
        • Funahashi S.
        • Bruce C.J.
        • Goldman-Rakic P.S.
        Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex.
        J Neurophysiol. 1989; 61: 331-349
        • Li D.
        • Constantinidis C.
        • Murray J.D.
        Trial-to-trial variability of spiking delay activity in prefrontal cortex constrains burst-coding models of working memory.
        J Neurosci. 2021; 41: 8928-8945
        • González-Burgos G.
        • Barrionuevo G.
        • Lewis D.A.
        Horizontal synaptic connections in monkey prefrontal cortex: An in vitro electrophysiological study.
        Cereb Cortex. 2000; 10: 82-92
        • Wang M.
        • Yang Y.
        • Wang C.J.
        • Gamo N.J.
        • Jin L.E.
        • Mazer J.A.
        • et al.
        NMDA receptors subserve working memory persistent neuronal firing in dorsolateral prefrontal cortex.
        Neuron. 2013; 77: 736-749
        • Wang X.J.
        Synaptic basis of cortical persistent activity: The importance of NMDA receptors to working memory.
        J Neurosci. 1999; 19: 9587-9603
        • Yang Y.
        • Paspalas C.D.
        • Jin L.E.
        • Picciotto M.R.
        • Arnsten A.F.T.
        • Wang M.
        Nicotinic α7 receptors enhance NMDA cognitive circuits in dorsolateral prefrontal cortex.
        Proc Nat Acad Sci U S A. 2013; 110: 12078-12083
        • Galvin V.C.
        • Yang S.T.
        • Paspalas C.D.
        • Yang Y.
        • Jin L.E.
        • Datta D.
        • et al.
        Muscarinic M1 receptors modulate working memory performance and activity via KCNQ potassium channels in primate prefrontal cortex.
        Neuron. 2020; 106: 649-661
        • Vierra N.C.
        • Kirmiz M.
        • van der List D.
        • Santana L.F.
        • Trimmer J.S.
        Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons.
        Elife. 2019; 8e49953
        • Paspalas C.D.
        • Wang M.
        • Arnsten A.F.T.
        Constellation of HCN channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex—potential substrate for working memory deficits in schizophrenia.
        Cereb Cortex. 2013; 23: 1643-1654
        • Arnsten A.F.T.
        • Datta D.
        • Wang M.
        The genie in the bottle—magnified calcium signaling in dorsolateral prefrontal cortex.
        Mol Psychiatry. 2021; 26: 3684-3700
        • Datta D.
        • Enwright J.F.
        • Arion D.
        • Paspalas C.D.
        • Morozov Y.M.
        • Lewis D.A.
        • et al.
        Mapping phosphodiesterase 4D (PDE4D) in macaque dorsolateral prefrontal cortex: Postsynaptic compartmentalization in higher-order layer III pyramidal cell circuits.
        Front Neuroanat. 2020; 14: 578483
        • Wang M.
        • Ramos B.
        • Paspalas C.
        • Shu Y.
        • Simen A.
        • Duque A.
        • et al.
        Alpha2A-adrenoceptor stimulation strengthens working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex.
        Cell. 2007; 129: 397-410
        • Yang S.T.
        • Wang M.
        • Paspalas C.P.
        • Crimins J.L.
        • Altman M.T.
        • Mazer J.A.
        • et al.
        Core differences in synaptic signaling between primary visual and dorsolateral prefrontal cortex.
        Cereb Cortex. 2018; 28: 1458-1471
        • El-Hassar L.
        • Datta D.
        • Chatterjee M.
        • Arnsten A.F.T.
        • Kaczmarek L.K.
        Interaction between HCN and Slack channels regulates mPFC pyramidal cell excitability and working memory function.
        Neurosci Abstracts. 2019; 462: 05
        • Arnsten A.F.T.
        • Wang M.
        • Paspalas C.D.
        Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses.
        Neuron. 2012; 76: 223-239
        • Datta D.
        • Leslie S.N.
        • Wang M.
        • Yang S.
        • Morozov Y.
        • Mentone S.
        • et al.
        Age-related calcium dysregulation linked with tau pathology and impaired cognition in non-human primates.
        Alzheimers Dement. 2021; 17: 920-932
        • Vijayraghavan S.
        • Wang M.
        • Birnbaum S.G.
        • Bruce C.J.
        • Williams G.V.
        • Arnsten A.F.T.
        Inverted-U dopamine D1 receptor actions on prefrontal neurons engaged in working memory.
        Nat Neurosci. 2007; 10: 376-384
        • Gamo N.J.
        • Lur G.
        • Higley M.J.
        • Wang M.
        • Paspalas C.D.
        • Vijayraghavan S.
        • et al.
        Stress impairs prefrontal cortical function via D1 dopamine receptor interactions with HCN channels.
        Biol Psychiatry. 2015; 78: 860-870
        • González-Burgos G.
        • Miyamae T.
        • Krimer Y.
        • Gulchina Y.
        • Pafundo D.E.
        • Krimer O.
        • et al.
        Distinct properties of layer 3 pyramidal neurons from prefrontal and parietal areas of the monkey neocortex.
        J Neurosci. 2019; 39: 7277-7290
        • Mustafa T.
        Pituitary adenylate cyclase-activating polypeptide (PACAP): A master regulator in central and peripheral stress responses.
        Adv Pharmacol. 2013; 68: 445-457
        • Birnbaum S.B.
        • Yuan P.
        • Wang M.
        • Vijayraghavan S.
        • Bloom A.
        • Davis D.
        • et al.
        Protein kinase C overactivity impairs prefrontal cortical regulation of working memory.
        Science. 2004; 306: 882-884
        • Datta D.
        • Yang S.T.
        • Galvin V.C.
        • Solder J.
        • Luo F.
        • Morozov Y.M.
        • et al.
        Noradrenergic α1-adrenoceptor actions in the primate dorsolateral prefrontal cortex.
        J Neurosci. 2019; 39: 2722-2734
        • Liang M.
        • Eason M.G.
        • Jewell-Motz E.A.
        • Williams M.A.
        • Theiss C.T.
        • Dorn 2nd, G.W.
        • et al.
        Phosphorylation and functional desensitization of the alpha2A-adrenergic receptor by protein kinase C.
        Mol Pharmacol. 1998; 54: 44-49
        • Zhu Q.
        • Qi L.J.
        • Shi A.
        • Abou-Samra A.
        • Deth R.C.
        Protein kinase C regulates alpha(2A/D)-adrenoceptor constitutive activity.
        Pharmacology. 2004; 71: 80-90
        • Macek T.A.
        • Schaffhauser H.
        • Conn P.J.
        Protein kinase C and A3 adenosine receptor activation inhibit presynaptic metabotropic glutamate receptor (mGluR) function and uncouple mGluRs from GTP-binding proteins.
        J Neurosci. 1998; 18: 6138-6146
        • Arnsten A.F.T.
        Stress signaling pathways that impair prefrontal cortex structure and function.
        Nat Rev Neurosci. 2009; 10: 410-422
        • Arnsten A.F.
        Stress weakens prefrontal networks: Molecular insults to higher cognition.
        Nat Neurosci. 2015; 18: 1376-1385
        • Baldessarini R.J.
        • Huston-Lyons D.
        • Campbell A.
        • Marsh E.
        • Cohen B.M.
        Do central antiadrenergic actions contribute to the atypical properties of clozapine?.
        Br J Psychiatry. 1992; Suppl: 12-16
        • Barsegyan A.
        • Mackenzie S.M.
        • Kurose B.D.
        • McGaugh J.L.
        • Roozendaal B.
        Glucocorticoids in the prefrontal cortex enhance memory consolidation and impair working memory by a common neural mechanism.
        Proc Natl Acad Sci U S A. 2010; 107: 16655-16660
        • van Stegeren A.H.
        • Roozendaal B.
        • Kindt M.
        • Wolf O.T.
        • Joëls M.
        Interacting noradrenergic and corticosteroid systems shift human brain activation patterns during encoding.
        Neurobiol Learn Mem. 2010; 93: 56-65
        • Grundemann D.
        • Schechinger B.
        • Rappold G.A.
        • Schomig E.
        Molecular identification of the cortisone-sensitive extraneuronal catecholamine transporter.
        Nat Neurosci. 1998; 1: 349-351
        • Liston C.
        • Miller M.M.
        • Goldwater D.S.
        • Radley J.J.
        • Rocher A.B.
        • Hof P.R.
        • et al.
        Stress-induced alterations in prefrontal cortical dendritic morphology predict selective impairments in perceptual attentional set-shifting.
        J Neurosci. 2006; 26: 7870-7874
        • Radley J.J.
        • Rocher A.B.
        • Miller M.
        • Janssen W.G.
        • Liston C.
        • Hof P.R.
        • et al.
        Repeated stress induces dendritic spine loss in the rat medial prefrontal cortex.
        Cereb Cortex. 2006; 16: 313-320
        • Ota K.T.
        • Liu R.J.
        • Voleti B.
        • Maldonado-Aviles J.G.
        • Duric V.
        • Iwata M.
        • et al.
        REDD1 is essential for stress-induced synaptic loss and depressive behavior.
        Nat Med. 2014; 20: 531-535
        • Woo E.
        • Sansing L.H.
        • Arnsten A.F.T.
        • Datta D.
        Chronic stress weakens connectivity in the prefrontal cortex: architectural and molecular changes.
        Chronic Stress (Thousand Oaks). 2021; 5 (24705470211029254)
        • Hains A.B.
        • Vu M.A.
        • Maciejewski P.K.
        • van Dyck C.H.
        • Gottron M.
        • Arnsten A.F.
        Inhibition of protein kinase C signaling protects prefrontal cortex dendritic spines and cognition from the effects of chronic stress.
        Proc Natl Acad Sci U S A. 2009; 106: 17957-17962
        • Hains A.B.
        • Yabe Y.
        • Arnsten A.F.T.
        Chronic stimulation of alpha-2A-adrenoceptors with guanfacine protects rodent prefrontal cortex dendritic spines and cognition from the effects of chronic stress.
        Neurobiol Stress. 2015; 2: 1-9
        • Rosenberg D.R.
        • Lewis D.A.
        Changes in the dopaminergic innervation of monkey prefrontal cortex during late postnatal development: A tyosine hydroxylase immunohistochemical study.
        Biol Psychiatry. 1994; 36: 272-277
        • Abi-Dargham A.
        • Xu X.
        • Thompson J.L.
        • Gil R.
        • Kegeles L.S.
        • Urban N.B.
        • et al.
        Increased prefrontal cortical D1 receptors in drug naive patients with schizophrenia: A PET study with [11C]NNC112.
        J Psychopharmacol. 2012; 26: 794-805
        • Schizophrenia Working Group of the Psychiatric Genomics Consortium
        Biological insights from 108 schizophrenia-associated genetic loci.
        Nature. 2014; 511: 421-427
        • Sekar A.
        • Bialas A.R.
        • de Rivera H.
        • Davis A.
        • Hammond T.R.
        • Kamitaki N.
        • et al.
        Schizophrenia risk from complex variation of complement component 4.
        Nature. 2016; 530: 177-183
        • Zhou Y.
        • Dong F.
        • Lanz T.A.
        • Reinhart V.
        • Li M.
        • Liu L.
        • et al.
        Interactome analysis reveals ZNF804A, a schizophrenia risk gene, as a novel component of protein translational machinery critical for embryonic neurodevelopment.
        Mol Psychiatry. 2018; 23: 952-962
        • Kirov G.
        • Rujescu D.
        • Ingason A.
        • Collier D.A.
        • O’Donovan M.C.
        • Owen M.J.
        Neurexin 1 (NRXN1) deletions in schizophrenia.
        Schizophr Bull. 2009; 35: 851-854
        • Datta D.
        • Arion D.
        • Corradi J.P.
        • Lewis D.A.
        Altered expression of CDC42 signaling pathway components in cortical layer 3 pyramidal cells in schizophrenia.
        Biol Psychiatry. 2015; 78: 775-785
        • Datta D.
        • Arion D.
        • Roman K.M.
        • Volk D.W.
        • Lewis D.A.
        Altered expression of ARP2/3 complex signaling pathway genes in prefrontal layer 3 pyramidal cells in schizophrenia.
        Am J Psychiatry. 2017; 174: 163-171
        • Hjelm B.E.
        • Rollins B.
        • Mamdani F.
        • Lauterborn J.C.
        • Kirov G.
        • Lynch G.
        • et al.
        Evidence of mitochondrial dysfunction within the complex genetic etiology of schizophrenia.
        Mol Neuropsychiatry. 2015; 1: 201-219
        • Arion D.
        • Corradi J.P.
        • Tang S.
        • Datta D.
        • Boothe F.
        • He A.
        • et al.
        Distinctive transcriptome alterations of prefrontal pyramidal neurons in schizophrenia and schizoaffective disorder.
        Mol Psychiatry. 2015; 20: 1397-1405
        • Schulmann A.
        • Ryu E.
        • Goncalves V.
        • Rollins B.
        • Christiansen M.
        • Frye M.A.
        • et al.
        Novel complex interactions between mitochondrial and nuclear DNA in schizophrenia and bipolar disorder.
        Mol Neuropsychiatry. 2019; 5: 13-27
        • Elston G.N.
        Specialization of the neocortical pyramidal cell during primate evolution.
        in: Kaas J.H. Striedter G.F. Bullock T.H. Preuss T.M. Rubenstein J. Krubutzer L.A. Evolution of Nervous Systems. Academic Press, Oxford2006: 191-242
        • Elston G.N.
        • Benavides-Piccione R.
        • Elston A.
        • Zietsch B.
        • Defelipe J.
        • Manger P.
        • et al.
        Specializations of the granular prefrontal cortex of primates: Implications for cognitive processing.
        Anat Rec A Discov Mol Cell Evol Biol. 2006; 288: 26-35
        • Elston G.N.
        • Benavides-Piccione R.
        • Elston A.
        • Manger P.R.
        • Defelipe J.
        Pyramidal cells in prefrontal cortex of primates: marked differences in neuronal structure among species.
        Front Neuroanat. 2011; 5: 2
        • Lencz T.
        • Malhotra A.
        Targeting the schizophrenia genome: A fast track strategy from GWAS to clinic.
        Mol Psychiatry. 2015; 20: 820-826
        • Stephan A.H.
        • Barres B.A.
        • Stevens B.
        The complement system: An unexpected role in synaptic pruning during development and disease.
        Annu Rev Neurosci. 2012; 35: 369-389
        • Myers S.J.
        • Yuan H.
        • Kang J.Q.
        • Tan F.C.K.
        • Traynelis S.F.
        • Low C.M.
        Distinct roles of GRIN2A and GRIN2B variants in neurological conditions.
        F1000Res. 2019; 8 (F1000 Faculty Rev-1940)
        • Di Prisco S.
        • Merega E.
        • Bonfiglio T.
        • Olivero G.
        • Cervetto C.
        • Grilli M.
        • et al.
        Presynaptic, release-regulating mGlu2-preferring and mGlu3-preferring autoreceptors in CNS: Pharmacological profiles and functional roles in demyelinating disease.
        Br J Pharmacol. 2016; 173: 1465-1477
        • Jin L.E.
        • Wang M.
        • Galvin V.C.
        • Lightbourne T.C.
        • Conn P.J.
        • Arnsten A.F.T.
        • et al.
        mGluR2 vs. mGluR3 in primate prefrontal cortex: Postsynaptic mGluR3 strengthen cognitive networks.
        Cerebral Cortex. 2018; 28: 974-987
        • Saini S.M.
        • Mancuso S.G.
        • Mostaid M.S.
        • Liu C.
        • Pantelis C.
        • Everall I.P.
        • et al.
        Meta-analysis supports GWAS-implicated link between GRM3 and schizophrenia risk.
        Transl Psychiatry. 2017; 7e1196
        • Ghose S.
        • Gleason K.A.
        • Potts B.W.
        • Lewis-Amezcua K.
        • Tamminga C.A.
        Differential expression of metabotropic glutamate receptors 2 and 3 in schizophrenia: A mechanism for antipsychotic drug action?.
        Am J Psychiatry. 2009; 166: 812-820
        • Arnsten A.F.T.
        • Wang M.
        The evolutionary expansion of mGluR3-NAAG-GCPII signaling: Relevance to human intelligence and cognitive disorders.
        Am J Psychiatry. 2020; 177: 1103-1106
        • Egan M.F.
        • Straub R.E.
        • Goldberg T.E.
        • Yakub I.
        • Callicott J.H.
        • Hariri A.R.
        • et al.
        Variation in GRM3 affects cognition, prefrontal glutamate, and risk for schizophrenia.
        Proc Natl Acad Sci U S A. 2004; 101: 12604-12609
        • Zink C.
        • Barker P.
        • Sawa A.
        • Weinberger D.
        • Wang A.
        • Quillian H.
        • et al.
        Missense mutation in FOLH1 is associated with decreased NAAG levels and impaired working memory circuitry and cognition.
        Am J Psychiatry. 2020; 177: 1129-1139
        • Millar J.K.
        • Wilson-Annan J.C.
        • Anderson S.L.
        • Christie S.
        • Taylor M.S.
        • Semple C.A.
        • et al.
        Disruption of two novel genes by a translocation co-segregating with schizophrenia.
        Hum Mol Genet. 2000; 9: 1415-1423
        • Millar J.K.
        • Pickard B.S.
        • Mackie S.
        • James R.S.
        • Christie S.
        • Buchanan S.R.
        • et al.
        DISC1 and PDE4B are interacting genetic factors in schizophrenia that regulate cAMP signaling.
        Science. 2005; 310: 1187-1191
        • Arnsten A.F.
        • Jin L.E.
        Guanfacine for the treatment of cognitive disorders: A century of discoveries at Yale.
        Yale J Biol Med. 2012; 85: 45-58
        • Gamo N.J.
        • Duque A.
        • Paspalas C.D.
        • Kata A.
        • Fine R.
        • Boven L.
        • et al.
        Role of disrupted in schizophrenia 1 (DISC1) in stress-induced prefrontal cognitive dysfunction.
        Transl Psychiatry. 2013; 3: e328
        • Vacic V.
        • McCarthy S.
        • Malhotra D.
        • Murray F.
        • Chou H.H.
        • Peoples A.
        • et al.
        Duplications of the neuropeptide receptor gene VIPR2 confer significant risk for schizophrenia.
        Nature. 2011; 47: 499-503
        • Datta D.
        • Mentone S.A.
        • Morozov Y.
        • Arnsten A.
        Subcellular localization of schizophrenia risk genes encoding Cav1. 2 (CACNA1C) and VIPR2 in rhesus macaque dorsolateral prefrontal cortex.
        Biol Psychiatry. 2021; 89: S308
        • Andrade A.
        • Brennecke A.
        • Mallat S.
        • Brown J.
        • Gomez-Rivadeneira J.
        • Czepiel N.
        • et al.
        Genetic associations between voltage-gated calcium channels and psychiatric disorders.
        Int J Mol Sci. 2019; 20: 3537
        • Yoshimizu T.
        • Pan J.Q.
        • Mungenast A.E.
        • Madison J.M.
        • Su S.
        • Ketterman J.
        • et al.
        Functional implications of a psychiatric risk variant within CACNA1C in induced human neurons.
        Mol Psychiatry. 2015; 20: 162-169
        • Bhat S.
        • Dao D.T.
        • Terrillion C.E.
        • Arad M.
        • Smith R.J.
        • Soldatov N.M.
        • et al.
        CACNA1C (Cav1.2) in the pathophysiology of psychiatric disease.
        Prog Neurobiol. 2012; 99: 1-14
        • Ripke S.
        • O’Dushlaine C.
        • Chambert K.
        • Moran J.L.
        • Kähler A.K.
        • et al.
        Genome-wide association analysis identifies 13 new risk loci for schizophrenia.
        Nat Genet. 2013; 45: 1150-1159
        • Gordovez F.J.A.
        • McMahon F.J.
        The genetics of bipolar disorder.
        Mol Psychiatry. 2020; 25: 544-559
        • Zink C.F.
        • Giegerich M.
        • Prettyman G.E.
        • Carta K.E.
        • van Ginkel M.
        • O’Rourke M.P.
        • et al.
        Nimodipine improves cortical efficiency during working memory in healthy subjects.
        Transl Psychiatry. 2020; 10: 372
        • Thimm M.
        • Kircher T.
        • Kellermann T.
        • Markov V.
        • Krach S.
        • Jansen A.
        • et al.
        Effects of a CACNA1C genotype on attention networks in healthy individuals.
        Psychol Med. 2011; 41: 1551-1561
        • Cosgrove D.
        • Mothersill O.
        • Kendall K.
        • Konte B.
        • Harold D.
        • Giegling I.
        • et al.
        Cognitive characterization of schizophrenia risk variants involved in synaptic transmission: Evidence of CACNA1C’s role in working memory.
        Neuropsychopharmacology. 2017; 42: 2612-2622
        • Catterall W.A.
        Regulation of cardiac calcium channels in the fight-or-flight response.
        Curr Mol Pharmacol. 2015; 8: 12-21
        • Pardiñas A.F.
        • Holmans P.A.
        • Pocklington A.J.
        • Escott-Price V.
        • Ripke S.
        • Carrera N.
        • et al.
        Common schizophrenia alleles are enriched in mutation-intolerant genes and in regions under strong background selection.
        Nat Genet. 2018; 50: 381-389
        • Greenwood T.A.
        • Lazzeroni L.C.
        • Maihofer A.X.
        • Swerdlow N.R.
        • Calkins M.E.
        • Freedman R.
        • et al.
        Genome-wide association of endophenotypes for schizophrenia from the Consortium on the Genetics of Schizophrenia (COGS) study.
        JAMA Psychiatry. 2019; 76: 1274-1284
        • Refisch A.
        • Chung H.Y.
        • Komatsuzaki S.
        • Schumann A.
        • Mühleisen T.W.
        • Nöthen M.M.
        • et al.
        A common variation in HCN1 is associated with heart rate variability in schizophrenia.
        Schizophr Res. 2020; 229: 73-79
        • Prasad K.M.
        • Chowdari K.V.
        • D’Aiuto L.A.
        • Iyengar S.
        • Stanley J.A.
        • Nimgaonkar V.L.
        Neuropil contraction in relation to complement C4 gene copy numbers in independent cohorts of adolescent-onset and young adult-onset schizophrenia patients—a pilot study.
        Transl Psychiatry. 2018; 8: 134
        • Breier A.
        • Wolkowitz O.
        • Pickar D.
        Stress and schizophrenia: Advances in neuropsychiatry and psychopharmacology.
        in: Tamminga C. Schult S. Schizophrenia Research. Raven Press, New York1991
        • Parrott J.M.
        • O’Connor J.C.
        Kynurenine 3-monooxygenase: An influential mediator of neuropathology.
        Front Psychiatry. 2015; 6: 116
        • Gál E.M.
        • Sherman A.D.
        L-kynurenine: Its synthesis and possible regulatory function in brain.
        Neurochem Res. 1980; 5: 223-239
        • Pullan L.M.
        • Cler J.A.
        Schild plot analysis of glycine and kynurenic acid at the N-methyl-D-aspartate excitatory amino acid receptor.
        Brain Res. 1989; 497: 59-63
        • Albuquerque E.X.
        • Schwarcz R.
        Kynurenic acid as an antagonist of α7 nicotinic acetylcholine receptors in the brain: Facts and challenges.
        Biochem Pharmacol. 2013; 85: 1027-1032
        • Plitman E.
        • Iwata Y.
        • Caravaggio F.
        • Nakajima S.
        • Chung J.K.
        • Gerretsen P.
        • et al.
        Kynurenic acid in schizophrenia: A systematic review and meta-analysis.
        Schizophr Bull. 2017; 43: 764-777
        • Sathyasaikumar K.V.
        • Stachowski E.K.
        • Wonodi I.
        • Roberts R.C.
        • Rassoulpour A.
        • McMahon R.P.
        • et al.
        Impaired kynurenine pathway metabolism in the prefrontal cortex of individuals with schizophrenia.
        Schizophr Bull. 2011; 37: 1147-1156
        • Kindler J.
        • Lim C.K.
        • Weickert C.S.
        • Boerrigter D.
        • Galletly C.
        • Liu D.
        • et al.
        Dysregulation of kynurenine metabolism is related to proinflammatory cytokines, attention, and prefrontal cortex volume in schizophrenia.
        Mol Psychiatry. 2020; 25: 2860-2872
        • Schwarcz R.
        • Pellicciari R.
        Manipulation of brain kynurenines: Glial targets, neuronal effects, and clinical opportunities.
        J Pharmacol Exp Ther. 2002; 303: 1-10
        • Blanco-Ayala T.
        • Sathyasaikumar K.V.
        • Uys J.D.
        • Pérez-de-la-Cruz V.
        • Pidugu L.S.
        • Schwarcz R.
        N-acetylcysteine inhibits kynurenine aminotransferase II.
        Neuroscience. 2020; 444: 160-169
        • Vornov J.J.
        • Hollinger K.R.
        • Jackson P.F.
        • Wozniak K.M.
        • Farah M.H.
        • Majer P.
        • et al.
        Still NAAG’ing after all these years: The continuing pursuit of GCPII inhibitors.
        Adv Pharmacol. 2016; 76: 215-255
        • Datta D.
        • Leslie S.N.
        • Woo E.
        • Amancharla N.
        • Elmansy A.
        • Lepe M.
        • et al.
        Glutamate carboxypeptidase II in aging rat prefrontal cortex impairs working memory performance.
        Front Aging Neurosci. 2021; 13: 760270
        • Zhang Z.
        • Bassam B.
        • Thomas A.G.
        • Williams M.
        • Liu J.
        • Nance E.
        • et al.
        Maternal inflammation leads to impaired glutamate homeostasis and up-regulation of glutamate carboxypeptidase II in activated microglia in the fetal/newborn rabbit brain.
        Neurobiol Dis. 2016; 94: 116-128
        • Duraisamy S.
        • Bajpai M.
        • Bughani U.
        • Dastidar S.G.
        • Ray A.
        • Chopra P.
        MK2: A novel molecular target for anti-inflammatory therapy.
        Expert Opin Ther Targets. 2008; 12: 921-936
        • MacKenzie K.F.
        • Wallace D.A.
        • Hill E.V.
        • Anthony D.F.
        • Henderson D.J.
        • Houslay D.M.
        • et al.
        Phosphorylation of cAMP-specific PDE4A5 (phosphodiesterase-4A5) by MK2 (MAPKAPK2) attenuates its activation through protein kinase A phosphorylation.
        Biochem J. 2011; 435: 755-769
        • Houslay K.F.
        • Christian F.
        • MacLeod R.
        • Adams D.R.
        • Houslay M.D.
        • Baillie G.S.
        Identification of a multifunctional docking site on the catalytic unit of phosphodiesterase-4 (PDE4) that is utilised by multiple interaction partners.
        Biochem J. 2017; 474: 597-609
        • Li J.T.
        • Xie X.M.
        • Yu J.Y.
        • Sun Y.X.
        • Liao X.M.
        • Wang X.X.
        • et al.
        Suppressed calbindin levels in hippocampal excitatory neurons mediate stress-induced memory loss.
        Cell Rep. 2017; 21: 891-900