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Ventral Hippocampal Afferents to Nucleus Accumbens Encode Both Latent Vulnerability and Stress-Induced Susceptibility

Open AccessPublished:May 25, 2020DOI:https://doi.org/10.1016/j.biopsych.2020.05.021

      Abstract

      Background

      Stress is a major risk factor for depression, but not everyone responds to stress in the same way. Identifying why certain individuals are more susceptible is essential for targeted treatment and prevention. In rodents, nucleus accumbens (NAc) afferents from the ventral hippocampus (vHIP) are implicated in stress-induced susceptibility, but little is known about how this pathway might encode future vulnerability or specific behavioral phenotypes.

      Methods

      We used fiber photometry to record in vivo activity in vHIP-NAc afferents during tests of depressive- and anxiety-like behavior in male and female mice, both before and after a sex-specific chronic variable stress protocol, to probe relationships between prestress neural activity and behavior and potential predictors of poststress behavioral adaptation. Furthermore, we examined chronic variable stress–induced alterations in vHIP-NAc activity in vivo and used ex vivo slice electrophysiology to identify the mechanism of this change.

      Results

      We identified behavioral specificity of the vHIP-NAc pathway to anxiety-like and social interaction behavior. We also showed that this activity is broadly predictive of stress-induced susceptibility in both sexes, while prestress behavior is predictive only of anxiety-like behavior. We observed a stress-induced increase in in vivo vHIP-NAc activity coincident with an increase in spontaneous excitatory postsynaptic current frequency.

      Conclusions

      We implicate vHIP-NAc in social interaction and anxiety-like behavior and identify markers of vulnerability in this neural signal, with elevated prestress vHIP-NAc activity predicting increased susceptibility across behavioral domains. Our findings indicate that individual differences in neural activity and behavior play a role in predetermining susceptibility to later stress, providing insight into mechanisms of vulnerability.
      Depression is the leading cause of disability worldwide (
      • Whiteford H.A.
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      • Rehm J.
      • Baxter A.J.
      • Ferrari A.J.
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      Global burden of disease attributable to mental and substance use disorders: Findings from the Global Burden of Disease Study 2010.
      ). Current therapies treat observable symptoms, rather than known mechanisms, and remain ineffective for many. Depression is a chronic, recurrent disorder, with 50% of individuals experiencing repeated episodes, and each episode further increases risk of recurrence (
      • Burcusa S.L.
      • Iacono W.G.
      Risk for recurrence in depression.
      ). Strategies to prevent emergence of the disorder before symptoms occur would have enormous clinical and societal impact (
      • Akil H.
      • Gordon J.
      • Hen R.
      • Javitch J.
      • Mayberg H.
      • McEwen B.
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      Treatment resistant depression: A multi-scale, systems biology approach.
      ). Stress is a major risk factor, yet only a minority of people who encounter stress will develop depression. Identifying at-risk individuals and the neural mechanisms underlying differential vulnerability is essential to developing targeted interventions for treatment and prevention.
      Chronically stressed rodents exhibit individual differences in stress susceptibility, such that some develop depressive- and anxiety-like behavior while others remain resilient (
      • Krishnan V.
      • Han M.H.
      • Graham D.L.
      • Berton O.
      • Renthal W.
      • Russo S.J.
      • et al.
      Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions.
      ). Differential adaptation can reveal mechanisms of vulnerability to stress-induced disorders such as depression (
      • Muir J.
      • Lorsch Z.S.
      • Ramakrishnan C.
      • Deisseroth K.
      • Nestler E.J.
      • Calipari E.S.
      • et al.
      In vivo fiber photometry reveals signature of future stress susceptibility in nucleus accumbens.
      ,
      • Francis T.C.
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      Molecular basis of dendritic atrophy and activity in stress susceptibility.
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      Hippocampal neurogenesis confers stress resilience by inhibiting the ventral dentate gyrus.
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      Multidimensional predictors of susceptibility and resilience to social defeat stress.
      ). However, most rodent research has exclusively considered males, despite the increased depression prevalence in women (
      • Girgus J.S.
      • Yang K.
      Gender and depression.
      ). Chronic variable stress (CVS) (
      • LaPlant Q.
      • Chakravarty S.
      • Vialou V.
      • Mukherjee S.
      • Koo J.W.
      • Kalahasti G.
      • et al.
      Role of nuclear factor kappaB in ovarian hormone-mediated stress hypersensitivity in female mice.
      ,
      • Hodes G.E.
      • Pfau M.L.
      • Purushothaman I.
      • Ahn H.F.
      • Golden S.A.
      • Christoffel D.J.
      • et al.
      Sex differences in nucleus accumbens transcriptome profiles associated with susceptibility versus resilience to subchronic variable stress.
      ), a model for depression in which mice experience repeated inescapable stressors, is easily applied to males and females and can reveal interesting sex differences in stress susceptibility, with female mice developing depressive-like behavior following shorter stress durations. With sufficient stress, both sexes exhibit behavioral susceptibility (
      • Hodes G.E.
      • Pfau M.L.
      • Purushothaman I.
      • Ahn H.F.
      • Golden S.A.
      • Christoffel D.J.
      • et al.
      Sex differences in nucleus accumbens transcriptome profiles associated with susceptibility versus resilience to subchronic variable stress.
      ,
      • Labonte B.
      • Engmann O.
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      • Menard C.
      • Wang J.
      • Tan C.
      • et al.
      Sex-specific transcriptional signatures in human depression.
      ,
      • Lorsch Z.S.
      • Loh Y.E.
      • Purushothaman I.
      • Walker D.M.
      • Parise E.M.
      • Salery M.
      • et al.
      Estrogen receptor alpha drives pro-resilient transcription in mouse models of depression.
      ), rendering this is a useful model for studying stress-induced neural adaptations accompanying comparable behavioral susceptibility in both sexes.
      The ventral hippocampus (vHIP) is sensitive to early-life experience (
      • Nguyen H.B.
      • Bagot R.C.
      • Diorio J.
      • Wong T.P.
      • Meaney M.J.
      Maternal care differentially affects neuronal excitability and synaptic plasticity in the dorsal and ventral hippocampus.
      ) and regulates anxiety-like behavior in male rodents (
      • Jimenez J.C.
      • Su K.
      • Goldberg A.R.
      • Luna V.M.
      • Biane J.S.
      • Ordek G.
      • et al.
      Anxiety cells in a hippocampal-hypothalamic circuit.
      ,
      • Adhikari A.
      • Topiwala M.A.
      • Gordon J.A.
      Synchronized activity between the ventral hippocampus and the medial prefrontal cortex during anxiety.
      ,
      • Padilla-Coreano N.
      • Bolkan S.S.
      • Pierce G.M.
      • Blackman D.R.
      • Hardin W.D.
      • Garcia-Garcia A.L.
      • et al.
      Direct ventral hippocampal-prefrontal input is required for anxiety-related neural activity and behavior.
      ), suggesting a potential role in encoding latent vulnerability to stress. Through projections to the nucleus accumbens (NAc), the vHIP regulates susceptibility to chronic social defeat stress in male mice (
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ). Immediate early gene expression in vHIP-NAc projecting neurons and altered glutamate release probability suggest increased vHIP-NAc activity in susceptible mice. Optogenetically increasing vHIP-NAc activity increased stress-induced susceptibility, decreasing social interaction and increasing passive coping, whereas attenuating neural activity increased social interaction, fostering resilience. Stress may also increase neuronal excitability in vHIP-NAc projecting neurons in female mice, inducing reduced sucrose preference, which is commonly interpreted as an indicator of anhedonia (
      • Williams E.S.
      • Manning C.E.
      • Eagle A.L.
      • Swift-Gallant A.
      • Duque-Wilckens N.
      • Chinnusamy S.
      • et al.
      Androgen-dependent excitability of mouse ventral hippocampal afferents to nucleus accumbens underlies sex-specific susceptibility to stress.
      ). While ex vivo electrophysiology and optogenetic experiments broadly implicate this pathway in stress-induced susceptibility, how in vivo physiological vHIP-NAc activity regulates specific behavioral phenotypes is unknown. Moreover, the prior focus on consequences of chronic stress cannot differentiate whether the vHIP-NAc pathway is simply a locus of stress-induced alterations or whether preexisting differences in vHIP-NAc activity drive differential vulnerability. This is an important distinction, because if the latter is true, the vHIP-NAc may shape the initial development of susceptibility to chronic stress, providing a potential therapeutic target for early intervention.
      Despite sex differences in prevalence, depression affects both men and women, and it is essential to understand mechanisms of depressive-like behavior in both sexes. To this end, we employed sex-specific stress protocols to probe the role of the vHIP-NAc under conditions in which behavioral susceptibility is observed in each sex. We interrogated vHIP-NAc neural activity in both male and female mice during depressive- and anxiety-like behavior tests both before and after chronic stress to investigate the behavioral significance of this neural signal and whether variation in this signal predicts future susceptibility or is modified by stress or both. We hypothesized not only that CVS would alter vHIP-NAc neural activity but that prestress differences in basal activity may identify susceptible individuals.

      Methods and Materials

      See Supplemental Methods for extended details.

      Chronic Variable Stress

      CVS was performed as previously described (
      • LaPlant Q.
      • Chakravarty S.
      • Vialou V.
      • Mukherjee S.
      • Koo J.W.
      • Kalahasti G.
      • et al.
      Role of nuclear factor kappaB in ovarian hormone-mediated stress hypersensitivity in female mice.
      ), with 1 of 3 stressors administered for 1 hour daily (100 foot-shocks, tail suspension, restraint). Male and female mice were subjected to sex-specific CVS protocols of differing lengths: 21-day CVS for males and 4-day CVS for females.

      Behavioral Assessments

      Open field, social preference, and tail suspension tests assessed depressive- and anxiety-like behavior before and again after CVS.

      In Vivo Fiber Photometry

      To measure calcium-associated fluorescence changes, we recorded vHIP-NAc projecting cells during behavior before and after stress (
      • Muir J.
      • Lorsch Z.S.
      • Ramakrishnan C.
      • Deisseroth K.
      • Nestler E.J.
      • Calipari E.S.
      • et al.
      In vivo fiber photometry reveals signature of future stress susceptibility in nucleus accumbens.
      ). Data were extracted and normalized to ΔF/F using custom written MATLAB scripts (The MathWorks, Inc., Natick, MA).

      Statistics

      Statistical analyses were performed using GraphPad Prism 7 (GraphPad Software, San Diego, CA).

      Results

      vHIP-NAc Activity Is Associated With Baseline Anxiety-like Behavior and Social Interaction But Not Passive Coping

      To examine in vivo behaviorally relevant modulation of vHIP-NAc neural activity, we injected retrograding AAV-GCaMP7f into the NAc and recorded calcium ion–associated fluorescence via an implanted optic fiber above the vHIP during tests of anxiety- and depressive-like behaviors in stress-naïve male and female mice and interrogated how the photometry signal varied with behavior (Figure 1). The open field test is a standard test for anxiety-like behavior wherein reduced center time is commonly interpreted as indicating increased anxiety-like behavior. Using linear regression, we found that peak frequency but not peak amplitude of vHIP-NAc events negatively correlated with center time in females (Figure 2A) and males (Figure 2B), implicating increased vHIP-NAc activity in increased anxiety-like behavior. Passive coping is a behavioral strategy employed during prolonged challenge that may help manage energy expenditure (
      • Koolhaas J.M.
      • Korte S.M.
      • De Boer S.F.
      • Van Der Vegt B.J.
      • Van Reenen C.G.
      • Hopster H.
      • et al.
      Coping styles in animals: Current status in behavior and stress-physiology.
      ,
      • Andalman A.S.
      • Burns V.M.
      • Lovett-Barron M.
      • Broxton M.
      • Poole B.
      • Yang S.J.
      • et al.
      Neuronal dynamics regulating brain and behavioral state transitions.
      ). This behavior can become maladaptive and has been associated with depression and other mood disorders (
      • Willner P.
      Chronic mild stress (CMS) revisited: Consistency and behavioural-neurobiological concordance in the effects of CMS.
      ). Passive coping, assessed from time immobile in a tail suspension test, did not correlate with peak frequency in either sex (Figure 2C, D), suggesting that the vHIP-NAc is not a primary mediator of this behavior.
      Figure thumbnail gr1
      Figure 1(A) Schematic of virus injection and fiber implantation sites depicting site of retrogradely infected cells and fiber placement. (B) Immunohistochemistry images show retrograde GCaMP7f expression. Scale bar = 300 μm (leftmost image), and scale bar = 75 μm (rightmost image). (C) Experimental timeline. D, day; NAc, nucleus accumbens; OFT, open field test; SP, social preference; TST, tail suspension test; vHIP, ventral hippocampus.
      Figure thumbnail gr2
      Figure 2Baseline relationships between ventral hippocampus–nucleus accumbens activity and behavior. Increased peak frequency is predictive of decreased time in center in the open field test before stress in (A) females (F1,10 = 5.36, n = 12, r = .59, p = .04) and (B) males (F1,10 = 7.11, n = 12, r = .65, p = .02). (C, D) Peak frequency is not related to time immobile in the tail suspension test in either sex: (C) females (F1,9 = 0.003, n = 11, r = .019, p = .93); (D) males (F1,11 = 0.1095, n = 13, r = .099, p = .74). (E) Increased peak frequency is predictive of an increased number of nose-to-grid events during a social interaction test in females (F1,11 = 5.45, n = 13, r = .58, p = .04), while (F) increased mean z score is predictive of an increased number of nose-to-grid events in (F) males (F1,11 = 6.33, n = 13, r = .61, p = .03). NTG, nose to grid; PF, peak frequency.
      Mice generally seek out social interaction (
      • Hung L.W.
      • Neuner S.
      • Polepalli J.S.
      • Beier K.T.
      • Wright M.
      • Walsh J.J.
      • et al.
      Gating of social reward by oxytocin in the ventral tegmental area.
      ); however, chronically stressed mice display social avoidance, reflecting dysregulation in reward circuitry (
      • Krishnan V.
      • Han M.H.
      • Graham D.L.
      • Berton O.
      • Renthal W.
      • Russo S.J.
      • et al.
      Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions.
      ).The vHIP-NAc mediates reward (
      • Britt J.P.
      • Benaliouad F.
      • McDevitt R.A.
      • Stuber G.D.
      • Wise R.A.
      • Bonci A.
      Synaptic and behavioral profile of multiple glutamatergic inputs to the nucleus accumbens.
      ), but paradoxically its increased activity is also implicated in stress-induced social avoidance in defeated mice (
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ). The interpretation of this test as indicating reduced social reward has been challenged, as the social target is commonly an aggressive mouse conspecific to social aggressors (
      • Ayash S.
      • Schmitt U.
      • Muller M.B.
      Chronic social defeat-induced social avoidance as a proxy of stress resilience in mice involves conditioned learning.
      ). To examine whether vHIP-NAc regulates social interaction in stress-naïve animals, we probed activity during proximal social interaction, defined as nose-to-grid (NTG) bouts, in a social preference (3-chamber sociability) test wherein a mouse freely explores a 3-chamber arena that contains both an empty grid enclosure and a same-sex conspecific. In females, increased peak frequency of vHIP-NAc events within individual NTG bouts associated with increased NTG interaction bout number (Figure 2E). In males, a related metric, higher mean z score during NTG bouts, associated with increased bout number (Figure 2F). There was no relationship between social interaction behavior and mean z score in females (Figure S1A) or peak frequency in males (Figure S1B). Our data implicate increased vHIP-NAc activity in preference for same-sex social interaction in stress-naïve animals of both sexes.

      Predictors of Stress-Induced Increases in Depressive- and Anxiety-like Behavior

      Stress-induced changes in vHIP-NAc synaptic activity (
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ) and excitability (
      • Williams E.S.
      • Manning C.E.
      • Eagle A.L.
      • Swift-Gallant A.
      • Duque-Wilckens N.
      • Chinnusamy S.
      • et al.
      Androgen-dependent excitability of mouse ventral hippocampal afferents to nucleus accumbens underlies sex-specific susceptibility to stress.
      ) regulate differential susceptibility. A key clinically relevant challenge is identifying individuals that will be vulnerable to stress-induced pathology, before stress. We hypothesized that individual differences in the vHIP-NAc may precede stress, representing a latent vulnerability that is revealed by future stress. To test this hypothesis, following baseline behavioral testing, we exposed the same mice to sex-specified intermediate CVS to induce individual variability in stress-induced anxiety- and depressive-like behavior in both sexes. While previous work has identified sex differences in stress susceptibility using identical stress protocols in both sexes (
      • Hodes G.E.
      • Pfau M.L.
      • Purushothaman I.
      • Ahn H.F.
      • Golden S.A.
      • Christoffel D.J.
      • et al.
      Sex differences in nucleus accumbens transcriptome profiles associated with susceptibility versus resilience to subchronic variable stress.
      ,
      • Williams E.S.
      • Manning C.E.
      • Eagle A.L.
      • Swift-Gallant A.
      • Duque-Wilckens N.
      • Chinnusamy S.
      • et al.
      Androgen-dependent excitability of mouse ventral hippocampal afferents to nucleus accumbens underlies sex-specific susceptibility to stress.
      ), depression affects both men and women, and thus we asked whether vHIP-NAc activity modulates stress-induced behavioral adaptation in both sexes under the relevant conditions under which behavioral susceptibility is observed in each sex. In females, 6-day CVS induces robust susceptibility and 3-day CVS results in a subthreshold effect (
      • Hodes G.E.
      • Pfau M.L.
      • Purushothaman I.
      • Ahn H.F.
      • Golden S.A.
      • Christoffel D.J.
      • et al.
      Sex differences in nucleus accumbens transcriptome profiles associated with susceptibility versus resilience to subchronic variable stress.
      ), whereas, consistent with reported sex differences in stress susceptibility and lower incidence of mood and anxiety disorders in men, 28-day CVS is necessary to elicit robust depressive- and anxiety-like behavior in males (
      • Hodes G.E.
      Sex, stress, and epigenetics: Regulation of behavior in animal models of mood disorders.
      ). Thus, we exposed females and males to 4-day and 21-day CVS, respectively, and then we recorded vHIP-NAc activity during behavior tests.
      Intermediate length CVS robustly increased depressive- and anxiety-like behavior in both males and females, with considerable within-group variability. On average, CVS decreased open field center time, indicating increased anxiety-like behavior (females: Figure 3A; males: Figure 3B). Both male and female mice showed increased immobility during tail suspension, suggesting that passive coping increased (females: Figure 3C; males: Figure 3D). Having previously identified a role for the vHIP-NAc in defeat-induced social avoidance (
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ), we examined whether CVS, which does not involve social aggression, also affects social behavior. Although total social preference (time in social/time in nonsocial area) was unaffected (Figure S2), proximal interaction (NTG) with the social target after CVS was reduced in both sexes (females: Figure 3E; males: Figure 3F), indicating that even the nonsocial stress of CVS can induce social avoidance.
      Figure thumbnail gr3
      Figure 3Chronic variable stress modulates depressive and anxiety-like behavior. (A, C, E) Female mice underwent 4 days of stress, and (B, D, F) male mice underwent 21 days of stress. Decreased time in center on the open field test was observed in both (A) females (n = 12, p = .01) and (B) males (n = 13, p = .03); increased immobile time in the tail suspension test was observed in both (C) females (n = 12, p = .04) and (D) males (n = 13, p < .01); and decreased time interacting with a social target in a social preference test was observed in both (E) females (n = 12, p = .02) and (F) males (n = 13, p = .04) following stress. NTG, nose to grid. ∗p < .05.
      Having established stress-induced susceptibility across anxiety- and depressive-like behaviors in both sexes, with considerable individual variability, we then examined whether the degree of susceptibility could be predicted from either prestress behavior or neural activity in the vHIP-NAc projecting neurons. In an open field test, both prestress behavior and neural activity predicted poststress anxiety-like behavior. In females, center time poststress was robustly predicted by prestress peak frequency (Figure 4B) and by prestress center time (Figure 4E). Similarly, in males, stress-induced anxiety-like behavior was marginally, although not statistically significantly, predicted by prestress peak frequency (Figure 4C) and prestress center time (Figure 4F). Thus, stress-induced anxiety-like behavior can be predicted by prestress individual differences in either vHIP-NAc activity or anxiety-like behavior. In contrast, prestress vHIP-NAc activity did not predict poststress passive coping in either sex (Figure 4H, I), further suggesting that vHIP-NAc is not a critical mediator of this behavior. Poststress passive coping could be predicted from prestress behavior in females (Figure 4K) but not males (Figure 4L). Consistent with previous findings (
      • Muir J.
      • Lorsch Z.S.
      • Ramakrishnan C.
      • Deisseroth K.
      • Nestler E.J.
      • Calipari E.S.
      • et al.
      In vivo fiber photometry reveals signature of future stress susceptibility in nucleus accumbens.
      ), poststress social interaction (cumulative duration of social target NTG bouts) was not predicted by prestress social interaction in either sex (Figure 4R, Q). Neural activity had greater predictive value. Poststress proximal interaction frequency (number of NTG events) was marginally predicted by prestress vHIP-NAc peak frequency during social NTG bouts in females (Figure 4N) and by prestress social NTG bout mean z score in males (Figure 4O). The number of social interaction bouts was not predicted by mean z score in females (Figure S1C) or peak frequency in males (Figure S1D). This finding suggests that vHIP-NAc may be one, although likely not the primary, mediator of stress-induced social interaction deficits.
      Figure thumbnail gr4
      Figure 4Predictors of anxiety and depressive-like behavior. (A) In the open field test, prestress peak frequency predicts poststress time in center in both (B) females (F1,10 = 8.44, n = 12, r = .69, p = .02) and (C) males (F1,8 = 4.70, n = 10, r = .61, p = .06). (D) Prestress time in center also predicts poststress time in center in (E) females (F1,10 = 10.08, n = 12, r = 0.71, p < .01) and weakly in (F) males (F1,11 = 4.42, n = 13, r = .54, p = .06). (G) In the tail suspension test, prestress peak frequency did not predict time immobile in either (H) females (F1,9 = 0.01, n = 11, r = .04, p = .90) or (I) males (F1,8 = 1.64, n = 10, r = 0.41 p = 0.23). (J) Prestress time immobile predicts poststress time immobile in (K) females (F1,10 = 5.77, n = 12, r = .60, p = .04) but not (L) males (F1,10 = 0.0032, n = 12, r < .02, p = .97). (M) In the social preference test, the number of nose-to-grid interaction events following stress is predicted by peak frequency during these bouts in (N) females (F1,9 = 5.09, n = 11, r = .60, p = .05) and by mean z score during these events in (O) males (F1,10 = 4.41, n = 12, r = .57, p = .06). (P) Prestress time interacting does not predict time interacting after stress in (Q) females (F1,9 = 0.63, n = 11, r = .25, p = .448) or (R) males (F1,11 = 0.02, n = 13, r = .04, p = .90). NTG, nose to grid; PF, peak frequency.

      Stress Modulates In Vivo vHIP-NAc Neural Activity Across Behavioral Domains

      After identifying that prestress vHIP-NAc activity predicts specific domains of stress-induced behavioral adaptation, we next asked whether in vivo vHIP-NAc activity is also altered by chronic stress. Previous ex vivo analyses found stress-induced alterations in glutamate release probability in males after chronic social defeat stress (
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ) and increased excitability in females after CVS (
      • Williams E.S.
      • Manning C.E.
      • Eagle A.L.
      • Swift-Gallant A.
      • Duque-Wilckens N.
      • Chinnusamy S.
      • et al.
      Androgen-dependent excitability of mouse ventral hippocampal afferents to nucleus accumbens underlies sex-specific susceptibility to stress.
      ), but how the integrated impact of synaptic and intrinsic changes regulate behaviorally relevant in vivo neural signaling is not known. In females, CVS increased vHIP-NAc peak amplitude (Figure 5E) and marginally increased peak frequency (Figure 5C) while mice explored an open field (Figure 5A). In males, CVS increased peak frequency (Figure 5D) but not amplitude (Figure 5F). To probe the evolution of increased neural activity with emerging behavioral susceptibility, we examined an intermediate time point in males. Fifteen-day CVS did not affect anxiety-like behavior (Figure S3A); however, peak frequency was already significantly increased (Figure S3B) and did not increase further after 21-day CVS, indicating that changes in vHIP-NAc activity precede emergence of stress-induced behavioral adaptations.
      Figure thumbnail gr5
      Figure 5Chronic variable stress increases ventral hippocampus–nucleus accumbens activity. (A, B) During the open field test, in females, chronic variable stress increases (C) peak frequency (n = 11, p = .05) and (E) peak amplitude (n = 10, p = .03), while in males, (D) peak frequency (n = 11, p = .02) is increased but (F) amplitude (n = 10, p = .12) is not. (G, H) During the tail suspension test, chronic variable stress increases peak frequency in (J) males (n = 12, p = .01) but not (I) females (n = 10, p = .30), and it increases peak amplitude in both (K) females (n = 10, p < .01) and (L) males (n = 11, p = .02). (M, N) In the social preference test, during nose-to-grid interaction bouts, chronic variable stress increases (O) peak frequency (n = 10, p < .01) and (Q) mean z score (n = 9, p = .05) in females but neither (P) peak frequency (n = 11, p = .58) nor mean z score (R) (n = 9, p = .25) in males. Panels (B, H, N) show representative traces for their respective conditions. CVS, chronic variable stress; NTG, nose to grid; OFT, open field test; PA, peak amplitude; PF, peak frequency; SoP, social preference; TST, tail suspension test. ∗∗p < .005, ∗p < .05, #p < .1.
      CVS also increased peak amplitude during tail suspension (Figure 5G) in both sexes (Figure 5K, L), but peak frequency was significantly increased only in males (Figure 5J). In contrast to broadly consistent CVS-induced increases in vHIP-NAc activity during open field and tail suspension tests in both sexes, neural activity during social interaction revealed sex differences. Following CVS, both mean z score (Figure 5Q) and peak frequency (Figure 5O) of fluorescence increased during proximal social interaction (NTG bouts) in females (Figure 5M). This finding is consistent with a general stress-induced increase in vHIP-NAc activity across behavioral contexts. However, in males, CVS did not affect either measure (Figure 5P,R), suggesting that there is greater task-specificity in the modulation of vHIP-NAc neural activity in males.

      CVS Increases Presynaptic Input to vHIP-NAc Projection Neurons

      Fiber photometry identified key relationships between in vivo vHIP-NAc neural activity and specific anxiety- and depressive-like behaviors and further revealed stress-induced increases in neural activity in multiple behaviorally relevant contexts. While fiber photometry is a powerful technique for probing in vivo neural activity during behavior, capturing gross changes in neural activity, as the photometry signal represents integrated activity of a population of neurons, complementary approaches are needed to resolve precise mechanisms of altered neural activity. To investigate mechanisms of stress-induced increase in vHIP-NAc activity, we used ex vivo patch-clamp electrophysiology. We injected mice with retrograding AAVrg-hSyn1-GCaMP6s-P2A-nis-dTomato (Figure 6A, B) to visualize and target recordings to the same population of neurons studied in vivo (i.e., vHIP neurons projecting to the NAc) and exposed mice to 21-day (males) or 4-day (females) CVS. We then prepared acute brain slices (Figure 6C). We examined spontaneous excitatory postsynaptic currents (sEPSCs) to assay pre- and postsynaptic alterations. CVS increased sEPSC frequency relative to that of stress-naïve control mice in both sexes (Figure 6D, E, H, I), but amplitude was unaffected (Figure 6F, G), suggesting that CVS has a presynaptic effect. We analyzed the AMPA:NMDA receptor ratio and confirmed that CVS also did not alter this metric of postsynaptic function in either sex (Figure 6J–M). Thus, CVS increases neural activity in the vHIP-NAc pathway via a presynaptic alteration that increases synaptic input to vHIP-NAc projecting neurons similarly in both males and females.
      Figure thumbnail gr6
      Figure 6Chronic variable stress increases spontaneous excitatory postsynaptic current frequency in both male and female mice. (A) Experimental timeline. (B) Schematic showing the location of viral injection, retrograde expression, and patch-clamp recordings. (C) Immunohistochemistry image shows GCaMP6s-dTomato expression (scale bar = 100 μm in both images). Increases in spontaneous excitatory postsynaptic current frequency were found in both (D) females (ncontrol = 9, nstress = 9, p = .03) and (E) males (ncontrol = 7, nstress = 9, p = .03. No increase in amplitude was found in either (F) females (ncontrol = 9, nstress = 9, p = .52) or (G) males (ncontrol = 7, nstress = 9, p = .71). The AMPA:NMDA receptor ratio was also unchanged for both (J) females (ncontrol = 10, nstress = 7, p = .52) and (K) males (ncontrol = 6, nstress = 7, p = .71). Panels (H, I, L, M) show representative traces. ∗p < .05. CVS, chronic variable stress; NAc, nucleus accumbens; sEPSC, spontaneous excitatory postsynaptic current.

      Discussion

      Identifying the neural mechanisms that shape individual differences in stress adaptation in both sexes is an essential foundation for developing novel strategies to redirect the course of stress-related psychiatric disorders in vulnerable individuals. Here, we interrogated neural activity in the vHIP-NAc pathway, finding that baseline vHIP-NAc activity correlates with individual differences in both anxiety-like and social interaction behavior but not passive coping. Critically, our findings demonstrate that baseline neural activity differences are predictive of stress-induced alterations in these behavioral domains. Furthermore, we identified a presynaptic mechanism by which stress increases neural activity in this pathway to mediate stress-induced susceptibility in both sexes.
      By probing in vivo neural activity across a range of behaviors, we identified behavioral specificity in vHIP-NAc neural activity, finding that properties of the neural signal relate to anxiety-like behavior and social interaction in both sexes, but they do not relate to passive coping. Depression is a heterogeneous disorder marked by a wide range of symptoms. Our findings support the idea that depression reflects dysfunction in distributed circuits that underlie specific behaviors or symptoms rather than a unitary pathology (
      • Drysdale A.T.
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      • Downar J.
      • Dunlop K.
      • Mansouri F.
      • Meng Y.
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      Resting-state connectivity biomarkers define neurophysiological subtypes of depression.
      ). The vHIP has long been implicated in anxiety-like behavior, with lesions reducing stress reactivity and avoidance of anxiogenic environments (
      • Jimenez J.C.
      • Su K.
      • Goldberg A.R.
      • Luna V.M.
      • Biane J.S.
      • Ordek G.
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      Anxiety cells in a hippocampal-hypothalamic circuit.
      ,
      • Adhikari A.
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      Synchronized activity between the ventral hippocampus and the medial prefrontal cortex during anxiety.
      ,
      • Padilla-Coreano N.
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      • Blackman D.R.
      • Hardin W.D.
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      Direct ventral hippocampal-prefrontal input is required for anxiety-related neural activity and behavior.
      ,
      • Kjelstrup K.G.
      • Tuvnes F.A.
      • Steffenach H.-A.
      • Murison R.
      • Moser E.I.
      • Moser M.-B.
      Reduced fear expression after lesions of the ventral hippocampus.
      ). Recent in vivo imaging and optogenetic manipulations identified a role specifically for vHIP projections to the hypothalamus in regulating avoidance of anxiogenic contexts in male mice (
      • Jimenez J.C.
      • Su K.
      • Goldberg A.R.
      • Luna V.M.
      • Biane J.S.
      • Ordek G.
      • et al.
      Anxiety cells in a hippocampal-hypothalamic circuit.
      ). Here, our findings show that vHIP-NAc projections may encode elements of anxiety-like behavior in both sexes. Intriguingly, our data also implicate vHIP-NAc activity in social interaction in stress-naïve animals. Although activity in the vHIP-NAc pathway correlates with both social interaction and anxiety-like behavior, these behaviors do not correlate with each other (males: F1,12 = 0.18, n = 14, r = −.12, p = .68; females: F1,11 = 0.22, n = 13, r = −.14, p = .65), indicating that these relationships may represent distinct factors of stress vulnerability. Mice will optogenetically self-stimulate the vHIP-NAc (
      • Britt J.P.
      • Benaliouad F.
      • McDevitt R.A.
      • Stuber G.D.
      • Wise R.A.
      • Bonci A.
      Synaptic and behavioral profile of multiple glutamatergic inputs to the nucleus accumbens.
      ), suggesting that this pathway may encode some element of reward or reinforcement. Consistent with this finding, social interaction, widely considered rewarding (
      • Hung L.W.
      • Neuner S.
      • Polepalli J.S.
      • Beier K.T.
      • Wright M.
      • Walsh J.J.
      • et al.
      Gating of social reward by oxytocin in the ventral tegmental area.
      ), associates with increased activity in this pathway in male and female mice. The association of vHIP-NAc neural activity with both increased social interaction and increased anxiogenic behavior is at first surprising. We suggest that the vHIP-NAc neural signal encodes a general property of salience rather than specifically reward or anxiety. This suggestion might also account for social preference data whereby vHIP-NAc activity correlates with increased interaction before stress yet also predicts decreased interaction time following stress. In support of this idea, an intra-vHIP pharmacological manipulation that increases neuronal activity in the NAc shell facilitates both subthreshold contextual fear conditioning and conditioned place preference, suggesting that communication between the vHIP and the NAc may exert a valence-independent regulation of emotional salience (
      • Loureiro M.
      • Kramar C.
      • Renard J.
      • Rosen L.G.
      • Laviolette S.R.
      Cannabinoid transmission in the hippocampus activates nucleus accumbens neurons and modulates reward and aversion-related emotional salience.
      ). In this light, previous findings that the vHIP-NAc pathway supports reward may be reinterpreted as evidence that this pathway encodes salience. Indeed, mice will robustly work for salient stimuli with no intrinsic motivational value (
      • Olsen C.M.
      • Winder D.G.
      Operant sensation seeking engages similar neural substrates to operant drug seeking in C57 mice.
      ). Thus, rather than encoding specific behaviors or information about valence, the vHIP may communicate information about context or stimulus salience to the NAc to modulate behavioral output. Electrophysiological studies demonstrate that the vHIP can gate other glutamatergic projections to the NAc, inducing a bi-stable state in medium spiny neurons that may facilitate the processing of other inputs (
      • O'Donnell P.
      • Grace A.A.
      Synaptic interactions among excitatory afferents to nucleus accumbens neurons: Hippocampal gating of prefrontal cortical input.
      ,
      • MacAskill A.F.
      • Little J.P.
      • Cassel J.M.
      • Carter A.G.
      Subcellular connectivity underlies pathway-specific signaling in the nucleus accumbens.
      ,
      • Goto Y.
      • O'Donnell P.
      Network synchrony in the nucleus accumbens in vivo.
      ), providing a circuit mechanism for the vHIP to exert modulatory control of motivated behavior. Thus, we would not necessarily expect the relationship between prestress vHIP-NAc activity and prestress social interaction behavior to be the same as the predictive relationship or the relationship with anxiety-like behavior. However, it is clear that vHIP-NAc neural activity under basal conditions is telling of an animal’s vulnerability.
      A fundamental unanswered question is what drives the initial divergence such that apparently similar individuals respond differently to the same stress. That is, do differences that exist prior to stress render some individuals vulnerable to stress-induced pathology? Identifying these specific mechanistic risk factors creates the potential for precise interventions targeted to vulnerable individuals to treat the underlying dysfunction before additional pathological consequences accrue. This treatment goal is especially relevant in depression, given that increasing chronicity is accompanied by ever-increasing treatment resistance (
      • Burcusa S.L.
      • Iacono W.G.
      Risk for recurrence in depression.
      ). In this context, there has been a recent push to identify factors that predispose subjects to susceptibility (
      • Muir J.
      • Lorsch Z.S.
      • Ramakrishnan C.
      • Deisseroth K.
      • Nestler E.J.
      • Calipari E.S.
      • et al.
      In vivo fiber photometry reveals signature of future stress susceptibility in nucleus accumbens.
      ,
      • Anacker C.
      • Luna V.M.
      • Stevens G.S.
      • Millette A.
      • Shores R.
      • Jimenez J.C.
      • et al.
      Hippocampal neurogenesis confers stress resilience by inhibiting the ventral dentate gyrus.
      ,
      • Nasca C.
      • Menard C.
      • Hodes G.
      • Bigio B.
      • Pena C.
      • Lorsch Z.
      • et al.
      Multidimensional predictors of susceptibility and resilience to social defeat stress.
      ,
      • Hultman R.
      • Ulrich K.
      • Sachs B.D.
      • Blount C.
      • Carlson D.E.
      • Ndubuizu N.
      • et al.
      Brain-wide electrical spatiotemporal dynamics encode depression vulnerability.
      ). Here, we find that anxiety-like behavior prior to stress predicts poststress anxiety-like behavior, which is consistent with the findings of previous work (
      • Nasca C.
      • Menard C.
      • Hodes G.
      • Bigio B.
      • Pena C.
      • Lorsch Z.
      • et al.
      Multidimensional predictors of susceptibility and resilience to social defeat stress.
      ,
      • Castro J.E.
      • Diessler S.
      • Varea E.
      • Marquez C.
      • Larsen M.H.
      • Cordero M.I.
      • et al.
      Personality traits in rats predict vulnerability and resilience to developing stress-induced depression-like behaviors, HPA axis hyper-reactivity and brain changes in pERK1/2 activity.
      ). However, neither poststress passive coping nor social interaction deficits were predicted by prestress behavior. Importantly, we show that vHIP-NAc neural activity predicts both anxiety-like and social interaction behavior, suggesting that differences in the neural processing of anxiogenic and rewarding stimuli predict future susceptibility and may influence individual stress adaptation. Our findings suggest that certain neural and behavioral abnormalities observed following stress may actually define preexisting vulnerability states, in other words, those that precede stress.
      Our findings add to accumulating evidence that links anxiety and vulnerability to stress-induced depressive-like behavior (
      • Nasca C.
      • Menard C.
      • Hodes G.
      • Bigio B.
      • Pena C.
      • Lorsch Z.
      • et al.
      Multidimensional predictors of susceptibility and resilience to social defeat stress.
      ,
      • Castro J.E.
      • Diessler S.
      • Varea E.
      • Marquez C.
      • Larsen M.H.
      • Cordero M.I.
      • et al.
      Personality traits in rats predict vulnerability and resilience to developing stress-induced depression-like behaviors, HPA axis hyper-reactivity and brain changes in pERK1/2 activity.
      ,
      • Larrieu T.
      • Cherix A.
      • Duque A.
      • Rodrigues J.
      • Lei H.
      • Gruetter R.
      • et al.
      Hierarchical status predicts behavioral vulnerability and nucleus accumbens metabolic profile following chronic social defeat stress.
      ,
      • Weger M.
      • Sandi C.
      High anxiety trait: A vulnerable phenotype for stress-induced depression.
      ). Rodents with elevated anxiety-like behavior at baseline are more vulnerable to chronic stress (
      • Nasca C.
      • Bigio B.
      • Zelli D.
      • Nicoletti F.
      • McEwen B.S.
      Mind the gap: Glucocorticoids modulate hippocampal glutamate tone underlying individual differences in stress susceptibility.
      ). Such trait anxiety also links to a variety of factors associated with stress vulnerability, including enhanced hypothalamic-pituitary-adrenal axis activation by acute stress (
      • Jakovcevski M.
      • Schachner M.
      • Morellini F.
      Individual variability in the stress response of C57BL/6J male mice correlates with trait anxiety.
      ,
      • Jakovcevski M.
      • Schachner M.
      • Morellini F.
      Susceptibility to the long-term anxiogenic effects of an acute stressor is mediated by the activation of the glucocorticoid receptors.
      ), social rank (
      • Larrieu T.
      • Cherix A.
      • Duque A.
      • Rodrigues J.
      • Lei H.
      • Gruetter R.
      • et al.
      Hierarchical status predicts behavioral vulnerability and nucleus accumbens metabolic profile following chronic social defeat stress.
      ), and metabolic changes (
      • Larrieu T.
      • Cherix A.
      • Duque A.
      • Rodrigues J.
      • Lei H.
      • Gruetter R.
      • et al.
      Hierarchical status predicts behavioral vulnerability and nucleus accumbens metabolic profile following chronic social defeat stress.
      ,
      • Cherix A.
      • Larrieu T.
      • Grosse J.
      • Rodrigues J.
      • McEwen B.
      • Nasca C.
      • et al.
      Metabolic signature in nucleus accumbens for anti-depressant-like effects of acetyl-L-carnitine.
      ). In humans, anxiety is the most frequently co-occurring symptom with depression, contributing to increased severity and recurrence. Trait anxiety also contributes to neuroticism (vulnerability to negative emotions), a trait with significant heritability that mediates the largest genetic risk for depression (
      • Power R.A.
      • Pluess M.
      Heritability estimates of the Big Five personality traits based on common genetic variants.
      ), associating with variation in many genes (
      • Minelli A.
      • Bonvicini C.
      • Scassellati C.
      • Sartori R.
      • Gennarelli M.
      The influence of psychiatric screening in healthy populations selection: A new study and metaanalysis of functional 5-HTTLPR and rs25531 polymorphisms and anxiety-related personality traits.
      ,
      • Hettema J.M.
      • An S.S.
      • Neale M.C.
      • Bukszar J.
      • van den Oord E.J.
      • Kendler K.S.
      • et al.
      Association between glutamic acid decarboxylase genes and anxiety disorders, major depression, and neuroticism.
      ), including some associated with anxiety in rodents (
      • Jakovcevski M.
      • Schachner M.
      • Morellini F.
      Individual variability in the stress response of C57BL/6J male mice correlates with trait anxiety.
      ,
      • Jakovcevski M.
      • Schachner M.
      • Morellini F.
      Susceptibility to the long-term anxiogenic effects of an acute stressor is mediated by the activation of the glucocorticoid receptors.
      ,
      • Marques F.Z.
      • Eikelis N.
      • Bayles R.G.
      • Lambert E.A.
      • Straznicky N.E.
      • Hering D.
      • et al.
      A polymorphism in the norepinephrine transporter gene is associated with affective and cardiovascular disease through a microRNA mechanism.
      ,
      • Kim S.E.
      • Kim H.N.
      • Yun Y.J.
      • Heo S.G.
      • Cho J.
      • Kwon M.J.
      • et al.
      Meta-analysis of genome-wide SNP- and pathway-based associations for facets of neuroticism.
      ). Standard tests of anxiety-like behavior probe an animal’s willingness to engage potential threat and may reveal preexisting abnormalities in circuit function and hormonal signaling that in turn mediate individual differences in exploration, cognitive processing and, ultimately, adaptation to stress and aversive experiences to mediate risk for depressive-like behavior.
      The distal causes of preexisting differences in behavior and neural activity associated with differential stress adaptation remain to be fully explored. Individual differences in early-life experience may play a role. Maternal care exerts a robust and enduring influence on the hippocampus such that the relative amount of licking and grooming in the first week of life associates with individual differences in hippocampal synaptic plasticity and excitability, morphology, glucocorticoid and mineralocorticoid expression levels, and hippocampus-dependent fear learning in adulthood (
      • Bagot R.C.
      • van Hasselt F.N.
      • Champagne D.L.
      • Meaney M.J.
      • Krugers H.J.
      • Joels M.
      Maternal care determines rapid effects of stress mediators on synaptic plasticity in adult rat hippocampal dentate gyrus.
      ,
      • Bagot R.C.
      • Tse Y.C.
      • Nguyen H.B.
      • Wong A.S.
      • Meaney M.J.
      • Wong T.P.
      Maternal care influences hippocampal N-methyl-D-aspartate receptor function and dynamic regulation by corticosterone in adulthood.
      ,
      • Bagot R.C.
      • Zhang T.Y.
      • Wen X.
      • Nguyen T.T.
      • Nguyen H.B.
      • Diorio J.
      • et al.
      Variations in postnatal maternal care and the epigenetic regulation of metabotropic glutamate receptor 1 expression and hippocampal function in the rat.
      ). Several of these factors associate with future susceptibility to chronic stress (
      • Nasca C.
      • Menard C.
      • Hodes G.
      • Bigio B.
      • Pena C.
      • Lorsch Z.
      • et al.
      Multidimensional predictors of susceptibility and resilience to social defeat stress.
      ). Furthermore, rodents characterized as dominant versus subordinate exhibit increased anxiety-like behavior at baseline and increased susceptibility after stress (
      • Larrieu T.
      • Cherix A.
      • Duque A.
      • Rodrigues J.
      • Lei H.
      • Gruetter R.
      • et al.
      Hierarchical status predicts behavioral vulnerability and nucleus accumbens metabolic profile following chronic social defeat stress.
      ). Levels of certain metabolites in the NAc are lower in subordinate animals at baseline but are increased by stress, specifically in these lower-rank animals. Acetyl-L-carnitine, an endogenous compound that supports energy metabolism and has promising antidepressant effects, partially abolishes vulnerability in dominant animals (
      • Cherix A.
      • Larrieu T.
      • Grosse J.
      • Rodrigues J.
      • McEwen B.
      • Nasca C.
      • et al.
      Metabolic signature in nucleus accumbens for anti-depressant-like effects of acetyl-L-carnitine.
      ). These and other environmental factors may contribute to differential vulnerability. Alternatively, individual differences may arise through the compounding of stochastic differences during development. Ultimately, whatever the source, uncovering mechanisms of preexisting vulnerability is a critical step in developing strategies for early intervention in at-risk populations.
      Beyond identifying preexisting differences in the vHIP-NAc pathway, we show that subsequent exposure to stress that is sufficient to increase anxiety- and depressive-like behavior (4-day CVS for females, 21-day CVS for males) robustly increases vHIP-NAc activity in both sexes. Stress increased vHIP-NAc activity in vivo across a range of behaviors. Interestingly, while increased vHIP-NAc activity was observed in both males and females in the anxiogenic context of an open field and during tail suspension, in a test of social behavior, increased neural activity was observed only in female mice and only while they were actively interacting with a social target, suggesting that behavioral context regulates stress-induced alterations in neural activity within this pathway. To probe the mechanism of this in vivo stress-induced increased neural activity, we examined pre- and postsynaptic function in vHIP-NAc projecting neurons and found that stress increased sEPSC frequency but not amplitude and did not alter the AMPA:NMDA ratio. This finding points to a presynaptic mechanism of increased input to these projection neurons and suggests that it will be important to probe upstream to further resolve the circuit-level origin of stress-induced alterations in vHIP-NAc signaling. Previous work has found a role for the vHIP-NAc pathway in stress-induced susceptibility in males (
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ) and females (
      • Williams E.S.
      • Manning C.E.
      • Eagle A.L.
      • Swift-Gallant A.
      • Duque-Wilckens N.
      • Chinnusamy S.
      • et al.
      Androgen-dependent excitability of mouse ventral hippocampal afferents to nucleus accumbens underlies sex-specific susceptibility to stress.
      ). Increased vHIP-NAc neuron excitability in females may account for increased female susceptibility to stress-induced sucrose preference deficits. Here we demonstrate that while females are initially more vulnerable to developing stress-induced depressive- and anxiety-like behavior, once evidenced, behavioral susceptibility is accompanied by similar increases in neural activity in this pathway in both sexes. These findings extend the mounting body of work implicating enhanced activity in the vHIP, and specifically the vHIP-NAc pathway, in stress susceptibility by identifying both preexisting differences and stress-induced alterations in stress susceptibility in both sexes (
      • Anacker C.
      • Luna V.M.
      • Stevens G.S.
      • Millette A.
      • Shores R.
      • Jimenez J.C.
      • et al.
      Hippocampal neurogenesis confers stress resilience by inhibiting the ventral dentate gyrus.
      ,
      • Nasca C.
      • Menard C.
      • Hodes G.
      • Bigio B.
      • Pena C.
      • Lorsch Z.
      • et al.
      Multidimensional predictors of susceptibility and resilience to social defeat stress.
      ,
      • Bagot R.C.
      • Parise E.M.
      • Pena C.J.
      • Zhang H.X.
      • Maze I.
      • Chaudhury D.
      • et al.
      Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.
      ,
      • Williams E.S.
      • Manning C.E.
      • Eagle A.L.
      • Swift-Gallant A.
      • Duque-Wilckens N.
      • Chinnusamy S.
      • et al.
      Androgen-dependent excitability of mouse ventral hippocampal afferents to nucleus accumbens underlies sex-specific susceptibility to stress.
      ,
      • Hultman R.
      • Ulrich K.
      • Sachs B.D.
      • Blount C.
      • Carlson D.E.
      • Ndubuizu N.
      • et al.
      Brain-wide electrical spatiotemporal dynamics encode depression vulnerability.
      ). Potentially, interventions designed to target this mechanism could be effective in treating and even protecting against stress-induced pathology in both sexes.
      We used a longitudinal design to probe predictive relationships. A question that may arise is whether changes in behavior and neural activity are stress induced or result from habituation or repeated behavioral testing. The fact that ex vivo electrophysiology in mice not exposed to behavioral testing revealed increases in sEPSC frequency in stressed compared with unstressed control animals is evidence against such a possibility. Increased sEPSC frequency measured ex vivo is consistent with in vivo activity changes revealed by fiber photometry, suggesting that stress increased neural vHIP-NAc activity. Behavioral and neural adaptations after stress were similar in both sexes, despite significant differences in experimental design (4 days vs. 21 days between pre- and posttest), offering further evidence against a nonspecific phenomenon resulting from repeated testing, in which we would expect to see variation due to time elapsed since testing. Our experimental design employed differing lengths of stress in males and females to induce comparable behavioral susceptibility, given known sex differences in effects of stress. While acute versus chronic stress differentially affects neural circuit activity in male rats (
      • Moore H.
      • Rose H.J.
      • Grace A.A.
      Chronic cold stress reduces the spontaneous activity of ventral tegmental dopamine neurons.
      ), this phenomenon is unlikely to account for sex-specific increases in in vivo activity during social interaction observed during comparable behavior. Furthermore, despite behavior-specific differences in neural changes, ex vivo electrophysiology revealed similar stress-induced neural adaptations in both sexes. Future research could explicitly examine how systematically varying stress chronicity affects neural circuit function in both males and females.
      Employing a longitudinal pathway-specific in vivo recording approach combined with targeted ex vivo interrogation of synaptic function, we shed new light on the role of the vHIP-NAc pathway in specific stress-related behaviors and how this signal is modified by stress in males and females. Furthermore, we established not only that this pathway is active in these behavioral contexts but that the degree of activity is predictive of individual differences in adaptation to future stress. Effective treatments for depression remain limited. Inherent heterogeneities of the disorder, with different individuals exhibiting different symptoms and varying responses to treatment, coupled with the emergence of treatment resistance across recurrent episodes, represent major obstacles. Identifying specific mechanisms underlying these behavioral abnormalities at early time points, even preceding emergence of the full disorder, will open the door not only to targeted treatments but also to new strategies for prevention.

      Acknowledgments and Disclosures

      This work was supported by funding from the Ludmer Centre for Neuroinformatics and Mental Health, a Canadian Institutes of Health Research Project grant (Grant No. 201709PJT-391173-BSA-CFAA-178116 [to RCB]), a John R. Evans Leaders Fund grant from the Canada Foundation for Innovation (Grant No. 35849 [to RCB]), a Canada Research Chair award (Grant No. PT83583 [to RCB]), and a Canadian Institutes of Health Research graduate scholarship (Grant No. 201810GSD-4221 05-DRA-CFAA-297096 [to JM]).
      Presented at the Canadian Association for Neuroscience annual meeting, May 22 to 25, 2019, Toronto, Ontario, Canada.
      The authors report no biomedical financial interests or potential conflicts of interest.

      Supplementary Material

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      Linked Article

      • New Insights Into the Role of Ventral Hippocampus–Nucleus Accumbens Activity Encoding Stress Vulnerability
        Biological PsychiatryVol. 88Issue 11
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          The mechanistic knowledge to determine an individual’s future vulnerability to stress is an incredibly valuable clinical tool as well as a significant advance toward developing preventative therapeutics. Advances in this field have focused on genetics, biomarkers isolated from blood, and behavioral measurements before stress, all of which can be used to make predictions of stress vulnerability (1). Conversely, advancements of a mechanistic circuit level understanding of stress susceptibility involve overcoming the technical hurdles of prospective studies.
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