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Dopaminergic Function in Cannabis Users and Its Relationship to Cannabis-Induced Psychotic Symptoms

  • Michael A.P. Bloomfield
    Affiliations
    Psychiatric Imaging Group, Medical Research Council Clinical Sciences Centre, Institute of Clinical Sciences, Hammersmith Hospital, Imperial College London

    Department of Psychosis Studies, Institute of Psychiatry, King’s College London (King’s Health Partners), London, United Kingdom
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  • Celia J.A. Morgan
    Affiliations
    Clinical Psychopharmacology Unit, Division of Psychology and Language Sciences, University College London
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  • Alice Egerton
    Affiliations
    Psychiatric Imaging Group, Medical Research Council Clinical Sciences Centre, Institute of Clinical Sciences, Hammersmith Hospital, Imperial College London

    Department of Psychosis Studies, Institute of Psychiatry, King’s College London (King’s Health Partners), London, United Kingdom
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  • Shitij Kapur
    Affiliations
    Department of Psychosis Studies, Institute of Psychiatry, King’s College London (King’s Health Partners), London, United Kingdom
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  • H. Valerie Curran
    Affiliations
    Clinical Psychopharmacology Unit, Division of Psychology and Language Sciences, University College London
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  • Oliver D. Howes
    Correspondence
    Address correspondence to Oliver D. Howes, D.M., Ph.D., M.A., B.M., B.Ch., M.R.C.Psych., Psychiatric Imaging Group, Francis Fraser Laboratories, MRC Clinical Sciences Centre, Hammersmith Hospital, Du Cane Road, London W12 0NN United Kingdom
    Affiliations
    Psychiatric Imaging Group, Medical Research Council Clinical Sciences Centre, Institute of Clinical Sciences, Hammersmith Hospital, Imperial College London

    Department of Psychosis Studies, Institute of Psychiatry, King’s College London (King’s Health Partners), London, United Kingdom
    Search for articles by this author

      Background

      Cannabis is the most widely used illicit drug globally, and users are at increased risk of mental illnesses including psychotic disorders such as schizophrenia. Substance dependence and schizophrenia are both associated with dopaminergic dysfunction. It has been proposed, although never directly tested, that the link between cannabis use and schizophrenia is mediated by altered dopaminergic function.

      Methods

      We compared dopamine synthesis capacity in 19 regular cannabis users who experienced psychotic-like symptoms when they consumed cannabis with 19 nonuser sex- and age-matched control subjects. Dopamine synthesis capacity (indexed as the influx rate constant Kicer) was measured with positron emission tomography and 3,4-dihydroxy-6-[18F]-fluoro-l-phenylalanine ([18F]-DOPA).

      Results

      Cannabis users had reduced dopamine synthesis capacity in the striatum (effect size: .85; t36 = 2.54, p = .016) and its associative (effect size: .85; t36 = 2.54, p = .015) and limbic subdivisions (effect size: .74; t36 = 2.23, p = .032) compared with control subjects. The group difference in dopamine synthesis capacity in cannabis users compared with control subjects was driven by those users meeting cannabis abuse or dependence criteria. Dopamine synthesis capacity was negatively associated with higher levels of cannabis use (r = −.77, p < .001) and positively associated with age of onset of cannabis use (r = .51, p = .027) but was not associated with cannabis-induced psychotic-like symptoms (r = .32, p = .19).

      Conclusions

      These findings indicate that chronic cannabis use is associated with reduced dopamine synthesis capacity and question the hypothesis that cannabis increases the risk of psychotic disorders by inducing the same dopaminergic alterations seen in schizophrenia.

      Key Words

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      ).
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      Cannabinoids enhance subsecond dopamine release in the nucleus accumbens of awake rats.
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      Dopaminergic sensitisation to THC occurs in animals (
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      Chronic (-)-delta9-tetrahydrocannabinol treatment induces sensitization to the psychomotor effects of amphetamine in rats.
      ), suggesting that dopaminergic effects are greater with regular cannabis exposures. Studies in recently abstinent and ex-cannabis users have not found abnormal striatal dopamine release (
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      Dopamine release in chronic cannabis users: A [(11)C]raclopride positron emission tomography study.
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      We therefore sought to study presynaptic dopaminergic function in active cannabis users who experienced cannabis-induced psychotic-like symptoms because these individuals are most at risk of psychosis (
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      Methods and Materials

      The study was approved by the National Research Ethics Service and the Administration of Radioactive Substances Advisory Committee. The study was conducted in accordance with the Declaration of Helsinki. All subjects provided informed written consent to participate.

      Study Population

      Inclusion criteria for all subjects were as follows: minimum age 18 years, good physical health with no history of major medical condition, and capacity to give written informed consent. Exclusion criteria for all subjects were current or past psychiatric illness (except cannabis use disorders in the cannabis user group and nicotine use disorder in all subjects) using the Structured Clinical Interview for DSM-IV (
      • First M.B.
      • Spitzer R.L.
      • Gibbon M.
      • Williams J.B.W.
      Structured Clinical Interview for DSM-IV Axis I Disorders, Clinician Version (SCID-CV).
      ), history of serious mental illness (including psychosis) in a first-degree relative determined via the Family Interview for Genetic Studies (
      NIMH Genetics Initiative
      Family Interview for Genetic Studies(FIGS).
      ), evidence of an At Risk Mental State for psychosis (
      • Phillips L.J.
      • Yung A.R.
      • McGorry P.D.
      Identification of young people at risk of psychosis: Validation of Personal Assessment and Crisis Evaluation Clinic intake criteria.
      ), DSM-IV-TR (
      American Psychiatric Association
      Diagnostic and Statistical Manual of Mental Disorders, 4th ed., text revision (DSM-IVTR).
      ) substance dependency or abuse (other than cannabis in the cannabis user group and tobacco for all subjects), and contraindications to positron emission tomography (PET; including pregnancy and breast-feeding). None of the subjects were taking psychotropic medication at the time of study participation.
      Detailed drug histories were obtained from all subjects using the Cannabis Experience Questionnaire (
      • Barkus E.J.
      • Stirling J.
      • Hopkins R.S.
      • Lewis S.
      Cannabis-induced psychosis-like experiences are associated with high schizotypy.
      ), structured interview and timeline follow-back. Lifetime cannabis use was estimated as the total number of “spliffs” (cannabis cigarettes; “joints”) consumed. The time taken to smoke an “eighth” of cannabis (one-eighth ounce; approximately 3.5 g, representing the standard unit of sale in Britain) was chosen as the primary index of cannabis use because this provides a measure of the amount of current drug consumption (shorter time indicating greater consumption). This is likely to be more accurate than subjective recall of the number of spliffs consumed because of variability in cannabis dose between spliffs and inconsistencies in self-reported cannabis use (
      • Akinci I.H.
      • Tarter R.E.
      • Kirisci L.
      Concordance between verbal report and urine screen of recent marijuana use in adolescents.
      ).

      Cannabis User Group

      We recruited cases from an ongoing cohort study in which more than 400 cannabis users were tested when intoxicated with cannabis and when not intoxicated (
      • Morgan C.J.A.
      • Gardener C.
      • Schafer G.
      • Swan S.
      • Demarchi C.
      • Freeman T.P.
      • et al.
      Sub-chronic impact of cannabinoids in street cannabis on cognition, psychotic-like symptoms and psychological well-being.
      ). Subjects met the following criteria: current, at least weekly use of cannabis and the induction of psychotic-like symptoms in response to smoking cannabis, which was defined as a positive change on the psychotic items score of the Psychotomimetic States Inventory (PSI) (
      • Mason O.J.
      • Morgan C.J.
      • Stefanovic A.
      • Curran H.V.
      The Psychotomimetic States Inventory (PSI): Measuring psychotic-type experiences from ketamine and cannabis.
      ) measured 5 minutes after smoking their usual amount of cannabis (i.e., when acutely intoxicated) compared with when not intoxicated with the drug. Cannabis users consumed their own cannabis, and testing occurred in the presence of a researcher in the environment where users habitually consumed cannabis in their usual drug-taking context (e.g., at home) because drug effects are typically larger in naturalistic as opposed to laboratory environments (
      • Barkus E.J.
      • Stirling J.
      • Hopkins R.S.
      • Lewis S.
      Cannabis-induced psychosis-like experiences are associated with high schizotypy.
      ). Cannabis-induced psychotic-like symptoms abated within 2 hours of consumption, and no subject met the DSM-IV TR criteria for a diagnosis of a psychotic disorder. The psychotic items from the PSI covered “Delusional Thinking,” “Perceptual Distortions,” “Cognitive Disorganization” (thought disorder), and “Paranoia.” Each item is rated on a 4-point scale from “not at all” (score = 0) to “strongly” (score = 3). Examples of items include “People can put thoughts into your mind” and “You can sense an evil presence around you, even though you cannot see it.” A sample of the cannabis that each participant smoked was taken on the day of testing and analyzed for levels of THC (Forensic Science Service, Birmingham, United Kingdom).

      Control Group

      Nonuser control subjects were recruited from the same geographic area by public advertisement. Controls were required to have no lifetime history of cannabis dependence or abuse (DSM-IV), no more than 10 total uses of cannabis in their lifetime, no report of the induction of psychotic symptoms by cannabis, and no history of cannabis use in the preceding 3 months. Community surveys indicate that more than 30% of young adults in England report trying cannabis in their lifetime (
      ). We therefore permitted control subjects to have had a minimal exposure to cannabis to ensure the control group was representative of the same general population from which we recruited the cannabis users.

      PET Data Acquisition

      All subjects underwent a 3,4-dihydroxy-6-[18F]-fluoro-/-phenylalanine ([18F]-DOPA) scan on an ECAT HR+ 962 PET scanner (CTI/Siemens, Knocksville, Tennessee) in three-dimensional mode, with an axial field of view of 15.5 cm, performed as previously reported (
      • Lieberman J.A.
      • Kane J.M.
      • Alvir J.
      Provocative tests with psychostimulant drugs in schizophrenia.
      ). Subjects were asked to fast and abstain from cannabis for 12 hours and to refrain from smoking tobacco for 2 hours before imaging. On the day of the PET scan, urine drug screen (Monitect HC12, Branan Medical Corporation, Irvine, California) confirmed no recent drug use (other than cannabis in the user group), and a negative urinary pregnancy test was required in all female subjects. A research clinician assessed psychotic symptoms using the Positive and Negative Syndrome Scale at the time of scanning. No subjects had psychotic symptoms at the time of scanning (mean [SD] Positive and Negative Syndrome Scale positive score cannabis users = 7.3 [.5]; control subjects = 7.2 [.4]). Subjects received carbidopa 150 mg and entacapone 400 mg orally 1 hour before imaging (
      • Sawle G.V.
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      • Morrish P.K.
      • Lammertsma A.A.
      • Snow B.J.
      • Luthra S.
      • et al.
      The effect of entacapone on brain [18F]-6-L-fluorodopa metabolism: Implications for levodopa therapy of Parkinson’s disease.
      ) to reduce the formation of radiolabeled [18F]-DOPA metabolites (
      • Cumming P.
      • Léger G.C.
      • Kuwabara H.
      • Gjedde A.
      Pharmacokinetics of plasma 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]Fdopa) in humans.
      ,
      • Guttman M.
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      • Reches A.
      • Evans A.
      • Kuwabara H.
      • Cedarbaum J.M.
      • Gjedde A.
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      ). Head position was marked and monitored via laser crosshairs and a camera and minimized using a head-strap. A 10-minute transmission scan was performed before radiotracer injection for attenuation and scatter correction. Approximately 180 MBq of [18F]-DOPA was administered by bolus intravenous injection 30 seconds after the start of PET imaging. We acquired emission data in list mode for 95 minutes, rebinned into 26 timeframes (30-second background frame, four 60-second frames, three 120-second frames, three 180-second frames, and fifteen 300-second frames).

      Volume of Interest Analysis

      To correct for head movement, nonattenuation-corrected dynamic images were denoised using a level 2, order 64 Battle-Lemarie wavelet filter (
      • Turkheimer F.E.
      • Brett M.
      • Visvikis D.
      • Cunningham V.J.
      Multiresolution analysis of emission tomography images in the wavelet domain.
      ), and individual frames were realigned to a single frame acquired 10 minutes after the [18F]-DOPA injection using a mutual information algorithm (
      • Studholme C.
      • Hill D.L.
      • Hawkes D.J.
      Automated 3-D registration of MR and CT images of the head.
      ). Transformation parameters were then applied to the corresponding attenuation-corrected frames, and the realigned frames were combined to create a movement-corrected dynamic image (from 6 to 95 minutes following [18F]-DOPA administration) for analysis.
      After movement correction, we defined standardized volumes of interest (VOIs) bilaterally in the whole striatum, the limbic (ventral), associative (precommisural dorsal caudate, precommisural dorsal putamen, and postcommisural caudate), and sensorimotor (postcommisural putamen) striatal functional subdivisions and the cerebellar reference region in Montreal Neurologic Institute space (
      • Martinez D.
      • Slifstein M.
      • Broft A.
      • Mawlawi O.
      • Hwang D.R.
      • Huang Y.
      • et al.
      Imaging human mesolimbic dopamine transmission with positron emission tomography. Part II: amphetamine-induced dopamine release in the functional subdivisions of the striatum.
      ,
      • Egerton A.
      • Demjaha A.
      • McGuire P.
      • Mehta M.A.
      • Howes O.D.
      The test–retest reliability of 18F-DOPA PET in assessing striatal and extrastriatal presynaptic dopaminergic function.
      ). An [18F]-DOPA template was normalized with the VOI map to each individual PET summation (add) image using statistical parametric mapping software (SPM5, http://fil.ion.ucl.ac.uk/spm), allowing VOIs to be placed automatically on individual [18F]-DOPA PET images without observer bias. We calculated [18F]-DOPA uptake, relative to the cerebellum [Kicer (min–1)], for each VOI using the Patlak graphic analysis adapted for a reference tissue input function (
      • Hartvig P.
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      • Tedroff J.
      • Reibring L.
      • Agren H.
      • Bjurling P.
      • et al.
      Positron emission tomography of L-dopa utilization in the brains of rhesus monkeys and human volunteers.
      ,
      • Hartvig P.
      • Torstenson R.
      • Tedroff J.
      • Watanabe Y.
      • Fasth K.J.
      • Bjurling P.
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      Amphetamine effects on dopamine release and synthesis rate studied in the rhesus monkey brain by positron emission tomography.
      ,
      • Hoshi H.
      • Kuwabara H.
      • Léger G.
      • Cumming P.
      • Guttman M.
      • Gjedde A.
      6-[18F]fluoro-L-dopa metabolism in living human brain: A comparison of six analytical methods.
      ,
      • Patlak C.S.
      • Blasberg R.G.
      Graphical evaluation of blood-to-brain transfer constants from multiple-time uptake data. Generalizations.
      ). We have previously demonstrated good test–retest reliability for striatal Kicer determined this way (
      • Egerton A.
      • Demjaha A.
      • McGuire P.
      • Mehta M.A.
      • Howes O.D.
      The test–retest reliability of 18F-DOPA PET in assessing striatal and extrastriatal presynaptic dopaminergic function.
      ).

      Voxelwise Analysis

      We complemented the VOI analysis with an independent voxelwise analysis using a wavelet-based Patlak method (
      • Turkheimer F.E.
      • Aston J.A.
      • Asselin M.C.
      • Hinz R.
      Multi-resolution Bayesian regression in PET dynamic studies using wavelets.
      ) as previously described (
      • Howes O.D.
      • Montgomery A.J.
      • Asselin M.C.
      • Murray R.M.
      • Valli I.
      • Tabraham P.
      • et al.
      Elevated striatal dopamine function linked to prodromal signs of schizophrenia.
      ). The parametric image for each participant was then normalized into standard space using the participants PET summation image and the [18F]-DOPA template (
      • Howes O.D.
      • Montgomery A.J.
      • Asselin M.C.
      • Murray R.M.
      • Valli I.
      • Tabraham P.
      • et al.
      Elevated striatal dopamine function linked to prodromal signs of schizophrenia.
      ). Statistical parametric mapping was conducted using SPM5 and a striatal mask defined according to previously described criteria (
      • Egerton A.
      • Demjaha A.
      • McGuire P.
      • Mehta M.A.
      • Howes O.D.
      The test–retest reliability of 18F-DOPA PET in assessing striatal and extrastriatal presynaptic dopaminergic function.
      ) to compare groups. Results are presented corrected for multiple comparisons using random field theory as applied in SPM5 (p < .05, corrected at the family-wise error rate).

      Statistical Analysis

      We assessed normality of distributions using the one-sample Kolmogorov-Smirnov test. Between-group comparisons were made with two-tailed independent t tests for normally distributed data and Mann-Whitney U tests for nonnormally distributed data. Relationships among Kicer, levels of cannabis use, and cannabis-induced psychotic-like symptom severity were tested using Pearson’s product–moment correlation coefficient. Potential confounding effects of other substance use were explored using a single analysis of covariance (ANCOVA) with subject group as the fixed factor; Kicer as the dependent variable and levels of use of each substance other than cannabis as separate covariates, and using Pearson’s product–moment correlation coefficient to determine if there was a relationship between Kicer and levels of tobacco smoking. Statistical significance was defined as p < .05 (two-tailed). Our primary outcome measure was Kicer in the whole striatum. Exploratory analyses were conducted in the striatal subdivisions (presented uncorrected for multiple comparisons).

      Results

      Subject Characteristics and Scan Parameters

      Twenty cannabis users were recruited to the study. Owing to tomograph malfunction during one scan, complete data were available on nineteen users. All cannabis users consumed the drug as a spliff. The mean (SD) age of first cannabis use was 15.5 (1.6) years, and the mean (SD) duration of at least weekly use was 4.7 (3.1) years. The median (interquartile range) time taken to smoke an eighth and lifetime exposure to cannabis was 4.0 (13.5) days and 2340 (6240) spliffs, respectively. Within the user group, the median (interquartile range) time between the scan and the last cannabis exposure and self-reported cannabis-induced psychotic-like symptoms was 14.0 (23.8) hours. Ten users met DSM-IV criteria for cannabis dependence (n = 5) or abuse (n = 5). Mean (SD) time to smoke an eighth was 2.3 (2.2) days in users who met dependency/abuse criteria and 6.9 (4.7) days in users who did not meet criteria. Mean (SD) age of first cannabis consumption was 14.8 (1.6) years in users who met dependency/abuse criteria, and 16.2 (1.3) years in users who did not meet criteria. Nineteen control subjects were matched to the user group for age (±5 years) and sex. Subjects’ characteristics are reported in Table 1. Urine drug screen was positive for THC and negative for all other substances (amphetamine, opiates, cocaine, methamphetamine, benzodiazepines) in every cannabis user and negative for all drugs (including cannabis) in every control subject. There was a significant group difference in current cannabis consumption, as expected, and also in tobacco and ecstasy use (Table 1).
      Table 1Sample Characteristics and Scan Parameters
      Controls (n = 19)Cannabis Users (n = 19)p
      Independent-samples t tests for variables with normal data distributions; Mann-Whitney U tests for variables with nonnormal data distributions; χ2 tests for dichotomous variables.
      Sample Characteristic
       Age (years), mean (SD)22.3 (2.8)20.8 (1.7).07
       Sex (n)2 female, 17 male2 female, 17 male1.00
       Handedness (n)2 left, 17 right4 left, 15 right.37
       Ethnicity (n)4 AB, 3 BB, 1 ME, 11 WB4 AB, 15 WB.16
      Groups were compared on a dichotomized ethnicity variable (white British vs. ethnic minority).
      Current Drug Use
      Drug use reported in 3 months before scan. Drug user defined as any drug use in the 3 months before scan.
      ,
      There was no reported lysergic acid diethylamide, benzodiazepine, opiate, or methamphetamine use in the 3 months before scanning.
       Cannabis users (n)0 users, 19 nonusers19 users, 0 nonusers1.00
       Cannabis use (grams of cannabis/month), median (IQR).0 (.0)26.3 (90.0).00
       THC content of cannabis (%), mean (SD)8.7 (3.8)
       Time since last cannabis exposure (hours), median (IQR)14.0 (23.8)
       Time taken to smoke an “eighth” of cannabis (days), median (IQR)4.0 (13.5)
       Age of onset of regular cannabis use (years), mean (SD)15.5 (1.6)
       Tobacco cigarette smokers (n)8 users, 11 nonusers15 users, 4 nonusers.02
       Tobacco use in whole sample (cigarettes/day), median (IQR).0 (.0)4.0 (7.0).01
       Tobacco use in smokers (cigarettes/day), median (IQR) (tobacco users)1.0 (9.0)7.0 (8.0)
       Alcohol use in past 3 months (n)19 users, 0 nonusers19 users, 0 nonusers1.00
       Alcohol
      1 UK alcohol unit = 10 mL (~7.88 g) alcohol.
      use (United Kingdom alcohol units/week), median (IQR)
      9.0 (12.0)12.0 (21.0).34
       MDMA use in past 3 months (n)5 users, 14 nonusers11 users, 8 nonusers.05
       MDMA use in whole sample (grams of MDMA/month), median (IQR).0 (.0).3 (1.0).02
       MDMA use in MDMA users (grams of MDMA/month), median (IQR).3 (.8)1.0 (1.7)
       Cocaine use in past 3 months (n)3 users, 16 nonusers3 users, 16 nonusers1.00
       Cocaine use in whole sample (grams of cocaine/month), median (IQR).0 (.0).0 (.0).60
       Cocaine use in cocaine users (grams of cocaine/month), median (IQR)<.1 (<.1)<.1 (1.0)
       Amphetamine use in past 3 months (n)1 user, 18 nonusers4 users, 15 nonusers.15
       Amphetamine use in whole sample (grams of amphetamine/month), median (IQR).0 (.0).0 (.0).27
       Amphetamine use in amphetamine users (grams of amphetamine/month), median (IQR)<.1.5 (.3)
       Ketamine use in past 3 months (n)1 user, 18 nonusers6 users, 13 nonusers.04
       Ketamine use in whole sample (grams of ketamine/month), median (IQR).0 (.0)<.1 (.5).10
       Ketamine use in ketamine users (grams of ketamine/month), median (IQR)<.11.5 (2.9)
       Psilocybin use in past 3 months (n)1 user, 18 nonusers1 user, 18 nonusers1.00
       Psilocybin use in whole sample (grams of “magic mushrooms”/month), median (IQR).0 (.0).0 (.0).80
       Psilocybin use in psilocybin users (grams of “magic mushrooms”/month)<.12.0
      Scan Parameter
       Injected dose (MBq), mean (SD)180.6 (7.2)184.4 (5.2).11
       Specific activity (MBq/µmol), mean (SD)31.1 (17.3)30.5 (14.0).92
       Whole striatal volume (mm3), mean (SD)17,587.82 (1729.50)17,942.90 (1286.73).48
       Associative striatal volume (mm3), mean (SD)10,801.19 (1134.46)10,772.76 (1161.24).94
       Limbic striatal volume (mm3), mean (SD)2080.30 (234.77)2276.51 (977.85).40
       Sensorimotor striatal volume (mm3), mean (SD)4706 (106.60)4668.98 (443.16).80
      AB, Asian British; BB, black British; IQR, interquartile range; MDMA, 3,4-methylenedioxy-N-methylamphetamine (“Ecstasy”); ME, mixed ethnicity; WB, white British.
      a Independent-samples t tests for variables with normal data distributions; Mann-Whitney U tests for variables with nonnormal data distributions; χ2 tests for dichotomous variables.
      b Groups were compared on a dichotomized ethnicity variable (white British vs. ethnic minority).
      c Drug use reported in 3 months before scan. Drug user defined as any drug use in the 3 months before scan.
      d There was no reported lysergic acid diethylamide, benzodiazepine, opiate, or methamphetamine use in the 3 months before scanning.
      e 1 UK alcohol unit = 10 mL (~7.88 g) alcohol.
      There was no significant group difference in the amount of radioactivity or specific activity injected (Table 1). There was no significant difference in whole striatal or subdivision volumes between the groups. There was no relationship between age and Kicer in the striatum or its subdivisions in the whole sample or in either group (data available on request).

      Striatal Dopaminergic Function

      Kicer was significantly reduced in cannabis users relative to controls in the whole striatum (Figure 1). Secondary analysis in each striatal subdivision showed that this reduction reached significance in the limbic and associative subdivisions (Table 2). The finding of reduced Kicer in cannabis users remained significant after covarying for other drugs used, with the amount of use of each of the drugs listed in Table 1 included as separate covariates in the ANCOVA, in the whole striatum (F1,37 = 4.65, p = .040) and its associative (F1,37 = 5.00, p = .034) and limbic (F1,37 = 7.358, p = .011) subdivisions.
      Figure thumbnail gr1
      Figure 1Striatal dopamine synthesis capacity in regular cannabis users (n = 19) and nonuser control subjects (n = 19). Dopamine synthesis capacity was significantly reduced in cannabis users compared with nonusers (t36 = 2.54, p = .016). Error bars indicate standard deviations.
      Table 2[18F]-DOPA Kicer (min–1) by Group
      Control Subjects (n = 19)Cannabis Users (n = 19)Group Comparisons
      Independent-samples t tests.
      Effect Size
      VOIMeanSDMeanSDtdfp(Cohen’s d)
      STR.0134.0009.0127.00072.5436.016.85
      AST.0127.0009.0121.00075.5436.015.85
      LST.0138.0009.0132.00082.2336.032.74
      SMST.0146.0014.0139.00081.8536.070.62
      AST, associative striatum; Kicer, influx rate constant; LST, limbic striatum; SMST, sensorimotor striatum; STR, whole striatum; VOI, volume of interest.
      a Independent-samples t tests.
      Voxel-based analysis confirmed reduced Kicer in the cannabis user group relative to nonuser control subjects with peak statistical significance in the right putamen (Montreal Neurological Institute coordinates: 28, 6, −8; p = .048 [corrected for familywise error]; Figure 2). There were no voxels where there was a significant elevation in Kicer in cannabis users relative to control subjects.
      Figure thumbnail gr2
      Figure 2Reduced striatal dopamine synthesis capacity in regular cannabis users relative to nonuser controls. The image shows a statistical parametric map of significant reductions (p < .05) in dopamine synthesis capacity, relative to healthy comparison subjects (n = 19), in regular cannabis users who experienced transient psychotic-like symptoms (n = 19). The most significant reduction was in the right putamen (Montreal Neurological Institute coordinates: 28, 6, −8; p = .048, corrected at the family-wise error rate). The color bar indicates the t statistic for each voxel.

      The Relationship Between Striatal Dopamine Synthesis Capacity and Cannabis Use

      Within the cannabis user group, greater levels of current cannabis use (less time to smoke an eighth of cannabis) were associated with lower Kicer in the whole striatum (r = −.77, p < .001; Figure 3A). Secondary analysis in each striatal subdivision showed that this pattern reached significance in the associative (r = –.68, p = .001) and sensorimotor (r = –.84, p < .001) subdivisions but not the limbic subdivision (r = –.26, p = .290). In addition, there was a significant correlation between age of onset of cannabis use and Kicer in the whole striatum (r = .51, p = .027; Figure 3B) and in its associative subdivision (r = .56, p = .013), which remained significant after controlling for current age (r = .49, p = .04 [whole striatum]; r = .54, p = .02 [associative]), with no significant correlation in the sensorimotor (r = .34, p = .158) or limbic (r = .36, p = .126) subdivisions. There was no significant correlation between age of first cannabis use and current cannabis use (r = .16, p = .52).
      Figure thumbnail gr3
      Figure 3(A) The correlation between level of cannabis use (time to smoke an “eighth” [~3.5 g] of cannabis; days) and striatal dopamine synthesis capacity, indexed as Kicer (min–1), in cannabis users (r = –.77, p < .001). (B) The correlation between age of onset of cannabis use and Kicer in the whole striatum (r = .51, p = .027), which remained significant when controlling for current age (r = .49, p = .04).
      Across the whole sample and within the control group, there was no significant difference between Kicer in tobacco smokers and nontobacco smokers in any of the regions examined (all ps >.1). Within the whole sample and within each group, there was no relationship between Kicer and daily cigarette use among tobacco cigarette smokers in the whole striatum (r = .26, p = .91 [whole sample], r = .10, p = .81 [control subjects]; r = .18, p = .52 [cannabis users]) and its functional subdivisions (data available on request). Within the whole sample and within each group, there were no significant relationships (all ps > .1) between whole striatal Kicer and other substances used (listed in Table 1).
      To examine whether cannabis dependency/abuse was associated with reduced Kicer, we divided the cannabis user group into subjects that met DSM-IV diagnostic criteria for cannabis dependency or abuse (n = 10) and those who did not meet criteria (n = 9). One-way analysis of variance found a significant effect of group on whole striatal Kicer (F2,37 = 4.02, p = .027, Figure 4). Post hoc t tests showed significant differences between the cannabis dependency/abuse and nondependency/nonabuse cannabis user subgroups (t17 = 2.80, p = .012) and between the cannabis dependency/abuse subgroup and control subjects (t27 = 2.67, p = .013), but not between the nondependency/nonabuse subgroup and the control group (p = .60). When examining the striatal subdivisions, significant differences in Kicer between the cannabis dependency/abuse and nondependency/nonabuse cannabis user subgroups were observed in the associative subdivision only (t17 = 2.89, p = .010).
      Figure thumbnail gr4
      Figure 4Striatal dopamine synthesis dopamine synthesis capacity in subjects who met DSM-IV criteria for a diagnosis of cannabis dependence or abuse (n = 10), regular cannabis users who did not meet diagnostic criteria (n = 9), and nonuser control subjects (n = 19). There were significant differences between cannabis dependence/abuse versus cannabis users who did not meet criteria (t17 = 2.80, p = .012) and cannabis dependence/abuse versus control group (t27 = 2.67, p = .013). There was no significant difference between controls versus cannabis users who did not meet dependence/abuse criteria (t26 = .54, p = .60). Error bars indicate standard deviations.

      The Relationship Between Striatal Dopamine Synthesis Capacity and Cannabis-Induced Psychotic Symptoms

      Within the cannabis user group, the mean (SD) increase in PSI psychotic symptom subscale score after consuming cannabis was 9.9 (5.1). There was no significant correlation between striatal Kicer and increase in transient psychotic-like symptoms following cannabis use (r = .32, p = .19; Figure 5).
      Figure thumbnail gr5
      Figure 5The relationship between the striatal influx rate constant Kicer and transient induction of cannabis-induced psychotic-like symptoms in the cannabis users. There was no significant relationship between the two variables (r = .32, p = .19). PSI, Psychotomimetic States Inventory.

      Discussion

      Our main finding is that striatal dopamine synthesis capacity is lower in current cannabis users than matched nonuser control subjects. In users, lower dopamine synthesis capacity was associated with greater current cannabis use, which explained 59% of variance in striatal dopamine synthesis capacity, and earlier age of onset of use, but not with cannabis-induced psychotic-like symptoms.
      Importantly, we also found that the lower levels of dopamine synthesis capacity in cannabis users compared with nonusers were driven by users who met diagnostic criteria for abuse and dependence. These findings are inconsistent with our hypothesis that elevated dopamine synthesis capacity underlies the link between cannabis and risk of psychosis.
      Our results extend previous findings in current (
      • Albrecht D.S.
      • Skosnik P.D.
      • Vollmer J.M.
      • Brumbaugh M.S.
      • Perry K.M.
      • Mock B.H.
      • et al.
      Striatal D(2)/D(3) receptor availability is inversely correlated with cannabis consumption in chronic marijuana users.
      ) and recently abstinent cannabis users (
      • Urban N.B.
      • Slifstein M.
      • Thompson J.L.
      • Xu X.
      • Girgis R.R.
      • Raheja S.
      • et al.
      Dopamine release in chronic cannabis users: A [(11)C]raclopride positron emission tomography study.
      ), which found reduced dopamine receptor density was associated with higher current cannabis use and lower dopamine release in the associative striatum was associated with earlier age of onset of cannabis. Although these studies (
      • Urban N.B.
      • Slifstein M.
      • Thompson J.L.
      • Xu X.
      • Girgis R.R.
      • Raheja S.
      • et al.
      Dopamine release in chronic cannabis users: A [(11)C]raclopride positron emission tomography study.
      ,
      • Sevy S.
      • Smith G.S.
      • Ma Y.
      • Dhawan V.
      • Chaly T.
      • Kingsley P.B.
      • et al.
      Cerebral glucose metabolism and D2/D3 receptor availability in young adults with cannabis dependence measured with positron emission tomography.
      ,
      • Albrecht D.S.
      • Skosnik P.D.
      • Vollmer J.M.
      • Brumbaugh M.S.
      • Perry K.M.
      • Mock B.H.
      • et al.
      Striatal D(2)/D(3) receptor availability is inversely correlated with cannabis consumption in chronic marijuana users.
      ) and a further study in ex-users (
      • Stokes P.R.
      • Egerton A.
      • Watson B.
      • Reid A.
      • Lappin J.
      • Howes O.D.
      • et al.
      History of cannabis use is not associated with alterations in striatal dopamine D2/D3 receptor availability.
      ) have reported estimates of the number of lifetime uses of cannabis and our sample is comparable to these, measures of the amount or type of cannabis consumed have not been reported, such that direct comparisons of cannabis use across the studies cannot be made. Our findings of reduced dopamine synthesis capacity in dependent subjects may reflect a “blunted” dopamine system, as observed with other drugs of addiction (
      • Volkow N.D.
      • Wang G.J.
      • Fowler J.S.
      • Tomasi D.
      • Telang F.
      Addiction: Beyond dopamine reward circuitry.
      ). Taken with findings from these and other studies (
      • Heinz A.
      • Siessmeier T.
      • Wrase J.
      • Buchholz H.G.
      • Gründer G.
      • Kumakura Y.
      • et al.
      Correlation of alcohol craving with striatal dopamine synthesis capacity and D2/3 receptor availability: A combined [18F]DOPA and [18F]DMFP PET study in detoxified alcoholic patients.
      ,
      • Wu J.C.
      • Bell K.
      • Najafi A.
      • Widmark C.
      • Keator D.
      • Tang C.
      • et al.
      Decreasing striatal 6-FDOPA uptake with increasing duration of cocaine withdrawal.
      ,
      • Martinez D.
      • Carpenter K.M.
      • Liu F.
      • Slifstein M.
      • Broft A.
      • Friedman A.C.
      • et al.
      Imaging dopamine transmission in cocaine dependence: Link between neurochemistry and response to treatment.
      ,
      • Wang G.J.
      • Smith L.
      • Volkow N.D.
      • Telang F.
      • Logan J.
      • Tomasi D.
      • et al.
      Decreased dopamine activity predicts relapse in methamphetamine abusers.
      ), there is mounting evidence that dopaminergic dysfunction provides a biomarker of addiction severity.
      Although the case–control design of this study is not able to detect a causative relationship between cannabis use and dopamine dysfunction, our findings suggestive of dose effects warrant further research into potential causative mechanisms. Animal studies indicate increased dopaminergic function in response to acute THC treatment. However, there is evidence of a biphasic dose-dependent dopamine response to THC (
      • Bloom A.S.
      Effect of delta9-tetrahydrocannabinol on the synthesis of dopamine and norepinephrine in mouse brain synaptosomes.
      ), suggesting higher cannabis exposures may reduce dopamine synthesis capacity, in line with our findings. Furthermore, with the exception of perinatal studies (
      • Walters D.E.
      • Carr L.A.
      Perinatal exposure to cannabinoids alters neurochemical development in rat brain.
      ), animal data on dopaminergic effects of long-term and high dose cannabis exposures are sparse, and the longest duration of THC administration has been 21 days (
      • Gorriti M.A.
      • Rodríguez de Fonseca F.
      • Navarro M.
      • Palomo T.
      Chronic (-)-delta9-tetrahydrocannabinol treatment induces sensitization to the psychomotor effects of amphetamine in rats.
      ,
      • Wu X.
      • French E.D.
      Effects of chronic delta9-tetrahydrocannabinol on rat midbrain dopamine neurons: An electrophysiological assessment.
      ,
      • Ginovart N.
      • Tournier B.B.
      • Moulin-Sallanon M.
      • Steimer T.
      • Ibanez V.
      • Millet P.
      Chronic Δ9-tetrahydrocannabinol exposure induces a sensitization of dopamine D2/3 receptors in the mesoaccumbens and nigrostriatal systems.
      ,
      • Behan A.T.
      • Hryniewiecka M.
      • O’Tuathaigh C.M.
      • Kinsella A.
      • Cannon M.
      • Karayiorgou M.
      • et al.
      Chronic adolescent exposure to delta-9-tetrahydrocannabinol in COMT mutant mice: Impact on indices of dopaminergic, endocannabinoid and GABAergic pathways.
      ). Of these, one study (
      • Ginovart N.
      • Tournier B.B.
      • Moulin-Sallanon M.
      • Steimer T.
      • Ibanez V.
      • Millet P.
      Chronic Δ9-tetrahydrocannabinol exposure induces a sensitization of dopamine D2/3 receptors in the mesoaccumbens and nigrostriatal systems.
      ) in Sprague-Dawley rats reported that long-term treatment with THC was associated with reduced striatal tyrosine hydroxylase gene expression and concurrent supersensitivity of D2/3 receptors, and a separate study (
      • Behan A.T.
      • Hryniewiecka M.
      • O’Tuathaigh C.M.
      • Kinsella A.
      • Cannon M.
      • Karayiorgou M.
      • et al.
      Chronic adolescent exposure to delta-9-tetrahydrocannabinol in COMT mutant mice: Impact on indices of dopaminergic, endocannabinoid and GABAergic pathways.
      ) in catechol-O-methyltransferase mutant mice found chronic treatment with THC in adolescence was associated with reduced dopaminergic cell size in the ventral tegmental area.
      One explanation for our findings is that chronic cannabis use is associated with dopaminergic down-regulation. This might underlie amotivation and reduced reward sensitivity in chronic cannabis users (
      • van Hell H.H.
      • Vink M.
      • Ossewaarde L.
      • Jager G.
      • Kahn R.S.
      • Ramsey N.F.
      Chronic effects of cannabis use on the human reward system: an fMRI study.
      ). Alternatively, preclinical evidence suggests that low dopamine neurotransmission may predispose an individual to substance use (
      • Nader M.A.
      • Morgan D.
      • Gage H.D.
      • Nader S.H.
      • Calhoun T.L.
      • Buchheimer N.
      • et al.
      PET imaging of dopamine D2 receptors during chronic cocaine self-administration in monkeys.
      ). However, this is inconsistent with findings that recently abstinent and former cannabis users show neither altered dopamine receptor availability (
      • Sevy S.
      • Smith G.S.
      • Ma Y.
      • Dhawan V.
      • Chaly T.
      • Kingsley P.B.
      • et al.
      Cerebral glucose metabolism and D2/D3 receptor availability in young adults with cannabis dependence measured with positron emission tomography.
      ,
      • Stokes P.R.
      • Egerton A.
      • Watson B.
      • Reid A.
      • Lappin J.
      • Howes O.D.
      • et al.
      History of cannabis use is not associated with alterations in striatal dopamine D2/D3 receptor availability.
      ,
      • Albrecht D.S.
      • Skosnik P.D.
      • Vollmer J.M.
      • Brumbaugh M.S.
      • Perry K.M.
      • Mock B.H.
      • et al.
      Striatal D(2)/D(3) receptor availability is inversely correlated with cannabis consumption in chronic marijuana users.
      ) nor altered dopamine release (
      • Urban N.B.
      • Slifstein M.
      • Thompson J.L.
      • Xu X.
      • Girgis R.R.
      • Raheja S.
      • et al.
      Dopamine release in chronic cannabis users: A [(11)C]raclopride positron emission tomography study.
      ), suggesting that altered dopaminergic function during chronic cannabis use is normalized by abstinence, as is observed with amphetamine in vervet monkeys (
      • Melega W.P.
      • Raleigh M.J.
      • Stout D.B.
      • Huang C.S.
      • Phelps M.E.
      Ethological and 6-[18F]fluro-L-DOPA-PET profiles of long-term vulnerability to chronic amphetamine.
      ).
      In this study, we investigated dopaminergic function in cannabis users who experience a transient increase in psychotic-like experiences when acutely intoxicated with cannabis. The lack of relationship between the induction of psychotic-like experiences and dopaminergic function suggests that our findings would generalize to cannabis users in general, but this requires confirmation in future studies.
      Our findings suggest that elevated striatal dopamine synthesis capacity is unlikely to be the mechanism underlying the link between cannabis and psychosis. Our study focused on the striatum because dopaminergic changes there have been reliably linked to psychosis (
      • Howes O.D.
      • Kambeitz J.
      • Kim E.
      • Stahl D.
      • Slifstein M.
      • Abi-Dargham A.
      • et al.
      The Nature of dopamine dysfunction in schizophrenia and what this means for treatment: Meta-analysis of imaging studies.
      ) but we cannot exclude the possibility that dopaminergic changes in extrastriatal regions underlie cannabis-induced psychotic symptoms. A previous study (
      • Glenthoj B.Y.
      • Mackeprang T.
      • Svarer C.
      • Rasmussen H.
      • Pinborg L.H.
      • Friberg L.
      • et al.
      Frontal dopamine D(2/3) receptor binding in drug-naive first-episode schizophrenic patients correlates with positive psychotic symptoms and gender.
      ) using single photon emission computed tomography reported a significant increase in temporal cortex D2/3 receptor availability in antipsychotic-naive first-episode patients with psychosis who tested positive for cannabis compared with those who did not. Alternatively, the mechanism may be mediated via nondopaminergic systems, such as direct effects on cannabinoid receptors (
      • Hirvonen J.
      • Goodwin R.S.
      • Li C.T.
      • Terry G.E.
      • Zoghbi S.S.
      • Morse C.
      • et al.
      Reversible and regionally selective downregulation of brain cannabinoid CB(1) receptors in chronic daily cannabis smokers.
      ).
      Nevertheless, findings that striatal dopamine release in patients with comorbid schizophrenia and substance dependence is blunted but still associated with amphetamine-induced psychotic symptoms (
      • Thompson J.L.
      • Urban N.
      • Slifstein M.
      • Xu X.
      • Kegeles L.S.
      • Girgis R.R.
      • et al.
      Striatal dopamine release in schizophrenia comorbid with substance dependence [published online ahead of print August 7].
      ) supports the possibility that other aspects of striatal dopaminergic function are altered by cannabis or that cannabis use interacts with other risk factors for schizophrenia to induce hyperdopaminergia. In support of this, early work in Wistar rats (
      • Littleton J.M.
      • Maclean K.I.
      • Brownlee G.
      Proceedings: Alterations in dopamine uptake in rat corpus striatum induced by combinations of stress and delta8-tetrahydrocannabinol (delta8-THC).
      ) found THC decreased striatal dopamine uptake compared with vehicle, but increases in striatal dopamine uptake were observed when THC-treated rats were housed under “stressful” versus “normal” conditions. Earlier age of onset of cannabis use increases psychosis risk and may interfere with normal brain development (
      • Ashtari M.
      • Cervellione K.
      • Cottone J.
      • Ardekani B.A.
      • Sew S.
      • Kumra S.
      Diffusion abnormalities in adolescents and young adults with a history of heavy cannabis use.
      ). Another possibility is thus that cannabis use during key developmental periods alters the regulation of dopaminergic function to make it more susceptible to subsequent stressors that could underlie an increased risk of psychosis. Additional prospective studies on the effects of chronic cannabis exposure are therefore warranted.

      Study Limitations

      One potential limitation of this study is that subjects consumed their own cannabis rather than a standard preparation. However, we tested individuals while intoxicated, measured levels of THC in samples of the cannabis our subjects were using, and confirmed it contained high levels of THC in all subjects (mean THC content = 8.7%). There was no fixed interval between cannabis exposure and PET, meaning that heavier cannabis users may have had a shorter interval between exposure and scan. It therefore remains possible that differences in the time since last cannabis use contribute to the differences between the dependent/abuser and nondependent groups, rather than dependency or abuse per se. In addition, lack of association between cannabis-induced psychotic symptoms may be due to variable interval between cannabis exposure and PET. However, in terms of acute effects of cannabis, only one of three molecular imaging studies of the acute effects of THC in healthy volunteers have found evidence of dopamine release (
      • Bossong M.G.
      • van Berckel B.N.
      • Boellaard R.
      • Zuurman L.
      • Schuit R.C.
      • Windhorst A.D.
      • et al.
      Delta 9-tetrahydrocannabinol induces dopamine release in the human striatum.
      ,
      • Stokes P.R.
      • Mehta M.A.
      • Curran H.V.
      • Breen G.
      • Grasby P.M.
      Can recreational doses of THC produce significant dopamine release in the human striatum?.
      ,
      • Barkus E.
      • Morrison P.D.
      • Vuletic D.
      • Dickson J.C.
      • Ell P.J.
      • Pilowsky L.S.
      • et al.
      Does intravenous Δ9-tetrahydrocannabinol increase dopamine release? A SPET study.
      ), suggesting that acute effects of THC on dopaminergic function may not be large or consistent in humans. Given that THC and its metabolites have an elimination half-life of about 7 days (
      • Maykut M.O.
      Health consequences of acute and chronic marihuana use.
      ) and all our cannabis users were regular, long-term users who had consumed cannabis within the past 7 days (median time since last consumption = 14 hours), our subjects were unlikely to be acutely withdrawing.
      Our measures of substance use rely on self-report, and we were not able to independently verify substance use histories beyond ongoing cannabis use in the user group and no recent use of other drugs in all participants. As would be expected, higher rates of other substance use were reported in cannabis users, although, with the exception of tobacco, the use of other substances was low in both groups. Our findings remained significant after covarying for all other drug use, suggesting that use of other substances does not underlie our findings, although it should be noted that ANCOVA may be less able to adjust for factors when groups differ significantly in covariates (
      • Miller G.A.
      • Chapman J.P.
      Misunderstanding analysis of covariance.
      ) and should be considered exploratory. We therefore cannot exclude the possibility that group differences in other drug use contributed to the results observed.
      Although cannabis users in our sample reported higher levels of ecstasy use than control subjects, ecstasy has been associated with increased dopamine synthesis capacity (
      • Tai Y.F.
      • Hoshi R.
      • Brignell C.M.
      • Cohen L.
      • Brooks D.J.
      • Curran H.V.
      • et al.
      Persistent nigrostriatal dopaminergic abnormalities in ex-users of MDMA (“Ecstasy”): An 18F-dopa PET study.
      ), so this is unlikely to explain our findings. More of the cannabis users smoked cigarettes than control subjects. The effects of cigarette smoking on presynaptic dopamine function are unclear; tobacco use has been associated with reduced amphetamine-induced dopamine release (
      • Busto U.E.
      • Redden L.
      • Mayberg H.
      • Kapur S.
      • Houle S.
      • Zawertailo L.A.
      Dopaminergic activity in depressed smokers: A positron emission tomography study.
      ) but increased dopamine synthesis capacity (
      • Salokangas R.K.
      • Vilkman H.
      • Ilonen T.
      • Taiminen T.
      • Bergman J.
      • Haaparanta M.
      • et al.
      High levels of dopamine activity in the basal ganglia of cigarette smokers.
      ). In addition, tobacco smoking may influence [18F]-DOPA kinetics via cerebral blood flow effects (
      • Domino E.F.
      • Ni L.
      • Xu Y.
      • Koeppe R.A.
      • Guthrie S.
      • Zubieta J.K.
      Regional cerebral blood flow and plasma nicotine after smoking tobacco cigarettes.
      ), which, if regionally selective, could affect our outcome measure. However, we did not find a relationship between levels of cigarette consumption and dopamine synthesis capacity, suggesting this did not influence our results, although additional research is needed to determine the effect of tobacco smoking on dopaminergic function.

      Conclusion

      Our results show that regular long-term cannabis use is associated with a dose-dependent reduction in dopamine synthesis capacity in the corpus striatum, particularly in those meeting diagnostic criteria for cannabis abuse or dependence. However, we found no relationship between dopaminergic function and cannabis-induced psychotic-like symptoms. These findings question the prevailing assumption that cannabis increases the risk of schizophrenia by inducing the same dopaminergic alterations seen in schizophrenia.
      This study was funded by a Medical Research Council (United Kingdom) grant to Dr. Howes (Grant no. MC-A656-5QD30 ), a National Institute of Health Research Biomedical Research Council grant to King’s College London , and a Medical Research Council (United Kingdom) grant to Professor Curran and Dr Morgan.
      We thank our subjects; the radiographers and staff of GE Imanet for their assistance with the positron emission tomography scans; Dr. Gianpaolo Tomasi for assistance with scan analysis software; Mr. Anthony Lewis for assistance with illustrations; and Professor Federico Turkheimer for statistical and methodologic advice.
      These data were presented orally at the British Association of Psychopharmacology and via posters at the European College of Neuropsychopharmacology, the Royal College of Psychiatrists, the Schizophrenia International Research Society, and the 9th International Symposium on Functional Neuroreceptor Mapping of the Living Brain.
      The authors reported no biomedical financial interests or potential conflicts of interest.

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