Association of genetic ancestry with striatal dopamine D2/D3 receptor availability

Socioenvironmental Factors are Associated With Dopamine Transporter Availability in Healthy Individuals but not in Parkinson's Disease

OBJECTIVE

Social factors can influence the brain's dopaminergic function. This study investigated the relationship between socioenvironmental factors and dopamine transporter (DaT) availability in healthy individuals (n = 74) and those with Parkinson's disease (PD) (n = 240).

METHODS

All single photon emission computed tomography (SPECT) DaT data and clinical data used in this study were obtained from the Parkinson's Progression Markers Initiative (PPMI) dataset. Socioenvironmental data was obtained from Social Explorer analyses of the American Community Survey (2014-2018) using the residential ZIP codes of the subjects available in the PPMI dataset.

RESULTS

Participants resided in 302 ZIP code tabulation areas across 38 U.S. states. In healthy individuals, DaT signals were significant and negatively correlated in the caudate with median household income (r = -0.27, P = 0.02) and educational level of the living area (r = -0.23, P = 0.04), but not significant in the putamen (r = -0.21, P = 0.08; r = -0.11, P = 0.37 respectively). Also, there was a significant positive correlation between DaT signals in caudate and poverty rates (r = 0.29, P = 0.01), but not in the putamen (r = 0.16, P = 0.19) in healthy subjects. No significant associations were observed in the PD group for any variables.

CONCLUSION

The study findings suggest that socioenvironmental factors, such as median household income, education level, and poverty rate, are significantly associated with DaT availability in the caudate of healthy individuals but not in those with PD. This indicates that PD might disrupt the connection between the social environment and dopaminergic function. These results underscore the importance of considering socioenvironmental variables when studying dopaminergic function in the human brain.

Chronic social stressors and striatal dopamine functioning in humans: A systematic review of SPECT and PET studies

The dopamine hypothesis of schizophrenia posits that elevated striatal dopamine functioning underlies the development of psychotic symptoms. Chronic exposure to social stressors increases psychosis risk, possibly by upregulating striatal dopamine functioning. Here we systematically review single photon emission computed tomography (SPECT) and positron emission tomography (PET) studies that examined the relationship between chronic social stress exposure and in vivo striatal dopamine functioning in humans. We searched the scientific databases PubMed and PsycINFO from inception to August 2023. The quality of the included studies was evaluated with the ten-item Observational Study Quality Evaluation (PROSPERO: CRD42022308883). Twenty-eight studies were included, which measured different aspects of striatal dopamine functioning including dopamine synthesis capacity (DSC), vesicular monoamine transporter type 2 binding, dopamine release following a pharmacological or behavioral challenge, D2/3 receptor binding, and dopamine transporter binding. We observed preliminary evidence of an association between childhood trauma and increased striatal DSC and dopamine release. However, exposure to low socioeconomic status, stressful life events, or other social stressors was not consistently associated with altered striatal dopamine functioning. The quality of available studies was generally low. In conclusion, there is insufficient evidence that chronic social stressors upregulate striatal dopamine functioning in humans. We propose avenues for future research, in particular to improve the measurement of chronic social stressors and the methodological quality of study designs.

Smoking-Related Increases in Alcohol Outcomes and Preliminary Evidence for the Protective Effect of a Functional Nicotine Receptor Gene (CHRNA5) Variant on Alcohol Consumption in Individuals Without Alcohol Use Disorder

BACKGROUND

Alcohol and nicotine interact with the nicotinic acetylcholine receptor system to alter reward-related responses, thereby contributing to the co-use and misuse of these drugs. A missense polymorphism rs16969968 (G>A) in the CHRNA5 gene has shown a strong association with nicotine-related phenotypes. However, less is known about the impact of this variant on alcohol-related phenotypes.

METHODS

We assessed the main and interactive effect of smoking and rs16969968 polymorphism on alcohol consumption using the Alcohol Use Disorders Identification Test (AUDIT), Timeline Follow Back (TLFB), and Lifetime Drinking History (LDH) in 980 healthy adults without alcohol use disorder. We further examined the effect of the rs16969968 polymorphism on acute alcohol consumption using a free-access i.v. alcohol self-administration (IV-ASA) human laboratory paradigm in a subset of 153 nonsmoking participants. Subjective alcohol responses, alcohol sensitivity, and expectancy measures were compared between genotype groups (GG; AA/AG).

RESULTS

We observed a significant association of smoking with AUDIT, TLFB, and LDH measures across genotype groups, with smokers showing higher scores compared with nonsmokers. Additionally, we found an association between genotype and TLFB-total drinks in the IV-ASA subset, with the GG group showing higher scores than AA/AG group. Relatedly, the alcohol negative expectancy score was significantly lower in the GG group than the AA/AG group.

CONCLUSIONS

Our findings underscore the association of smoking with alcohol measures. We found preliminary evidence for the protective effect of the functional CHRNA5 polymorphism on alcohol consumption and its association with increased negative alcohol expectancies, which highlights the substantial heterogeneity in alcohol responses.

Ethnic differences in response to atypical antipsychotics in patients with schizophrenia: individual patient data meta-analysis of randomised placebo-controlled registration trials submitted to the Dutch Medicines Evaluation Board

BACKGROUND

Little is known about the effect of ethnicity on the response to antipsychotic medication in patients with schizophrenia.

AIMS

To determine whether ethnicity moderates the response to antipsychotic medication in patients with schizophrenia, and whether this moderation is independent of confounders.

METHOD

We analysed 18 short-term, placebo-controlled registration trials of atypical antipsychotic medications in patients with schizophrenia (N = 3880). A two-step, random-effects, individual patient data meta-analysis was applied to establish the moderating effect of ethnicity (White versus Black) on symptom improvement according to the Brief Psychiatric Rating Scale (BPRS) and on response, defined as >30% BPRS reduction. These analyses were corrected for baseline severity, baseline negative symptoms, age and gender. A conventional meta-analysis was performed to determine the effect size of antipsychotic treatment for each ethnic group separately.

RESULTS

In the complete data-set, 61% of patients were White, 25.6% of patients were Black and 13.4% of patients were of other ethnicities. Ethnicity did not moderate the efficacy of antipsychotic treatment: pooled β-coefficient for the interaction between treatment and ethnic group was -0.582 (95% CI -2.567 to 1.412) for mean BPRS change, with an odds ratio of 0.875 (95% CI 0.510-1.499) for response. These results were not modified by confounders.

CONCLUSIONS

Atypical antipsychotic medication is equally effective in both Black and White patients with schizophrenia. In registration trials, White and Black patients were overrepresented relative to other ethnic groups, limiting the generalisability of our findings.

Low status, humiliation, dopamine and risk of schizophrenia

The social defeat hypothesis of schizophrenia, which proposes that the chronic experience of outsider status or subordinate position leads to increased striatal dopamine activity and thereby to increased risk, has been criticized. The aims of this paper are to improve the definition of defeat and to integrate the social defeat hypothesis with the neurodevelopmental hypothesis. Marmot advanced the idea that low status is pathogenic in that it is associated with a lack of social participation and a lack of autonomy. Given the similarity with outsider status and subordinate position, we re-define social defeat as low status. From this new perspective it is also likely that pre-schizophrenic impairments (of neurodevelopmental origin or not) are pathogenic in that they contribute to low status. The effect of low status may be enhanced by repeated exposure to humiliation, but few studies have measured this variable. Since most individuals exposed to low status do not develop schizophrenia, we propose that this risk factor increases the risk of disorder in the presence of a poor homeostatic control of dopamine neurons in midbrain and dorsal striatum. This is consistent with studies of healthy subjects which report a negative association between low socio-economic status and dopamine D2/D3 receptor availability in the dorsal striatum. In this new version of the social defeat hypothesis we propose that the combination of low status, repeated humiliation and poor homeostatic control of dopamine neurons in midbrain and dorsal striatum leads to increased striatal dopamine activity and thereby to an increased risk of schizophrenia.

A polygenic score indexing a DRD2-related co-expression network is associated with striatal dopamine function

The D2 dopamine receptor (D2R) is the primary site of the therapeutic action of antipsychotics and is involved in essential brain functions relevant to schizophrenia, such as attention, memory, motivation, and emotion processing. Moreover, the gene coding for D2R (DRD2) has been associated with schizophrenia at a genome-wide level. Recent studies have shown that a polygenic co-expression index (PCI) predicting the brain-specific expression of a network of genes co-expressed with DRD2 was associated with response to antipsychotics, brain function during working memory in patients with schizophrenia, and with the modulation of prefrontal cortex activity after pharmacological stimulation of D2 receptors. We aimed to investigate the relationship between the DRD2 gene network and in vivo striatal dopaminergic function, which is a phenotype robustly associated with psychosis and schizophrenia. To this aim, a sample of 92 healthy subjects underwent 18F-DOPA PET and was genotyped for genetic variations indexing the co-expression of the DRD2-related genetic network in order to calculate the PCI for each subject. The PCI was significantly associated with whole striatal dopamine synthesis capacity (p = 0.038). Exploratory analyses on the striatal subdivisions revealed a numerically larger effect size of the PCI on dopamine function for the associative striatum, although this was not significantly different than effects in other sub-divisions. These results are in line with a possible relationship between the DRD2-related co-expression network and schizophrenia and extend it by identifying a potential mechanism involving the regulation of dopamine synthesis. Future studies are needed to clarify the molecular mechanisms implicated in this relationship.

Addressing Structural Racism in Psychiatry With Steps to Improve Psychophysiologic Research

IMPORTANCE

The American Medical Association has acknowledged the public health threat posed by racism in medicine. While clinicians in psychiatry have echoed the sentiment, the research community has largely been silent. Current understanding of the biological domains that underlie psychiatric disorders was historically established by studying White populations, often leaving widely used treatments ineffective for Asian, Black, Hispanic, Indigenous, and other racial and ethnic minority individuals. This article addresses how undersampling of racial and ethnic minority individuals has led to overgeneralized physiological findings, the implications for development of psychiatric treatments, and steps to improve service to racially diverse communities.

OBSERVATIONS

Three primary observations regarding differences associated with race and ethnicity have been addressed in the existing psychiatric research: misdiagnosis, medication nonadherence, and treatment efficacy and expression of adverse effects. While cultural factors have been discussed as potential factors associated with these differences, a lack of understanding of physiologic systems may be foundational to each of these issues. Recent evidence points to race differences in psychophysiological measures, likely attributed to factors including the lived experience of racism as opposed to inherent biological differences. This mounting evidence supports a reassessment of existing work to examine potential divergent patterns within racial and ethnic groups. The following strategies may improve understanding of the influence of racism on physiology, allowing clinicians to better address psychiatric symptoms and improve existing treatment approaches. Thus, psychiatric researchers need to (1) understand the historic and current terminology for race and ethnicity and use appropriate terms and categories as defined by sociologists, population health experts, and databases while respecting individuals' right to self-identify, (2) refine research questions, and (3) reexamine research data to determine whether patterns observed in largely White populations can extend to other groups. To appropriately implement these steps, researchers must accept the discomfort that accompanies growth, invite scientists from diverse backgrounds to participate, and use resources to increase diversity in recruitment of study participants. This will require a commitment from funding agencies to provide adequate support to recruit and investigate large, diverse samples.

CONCLUSIONS AND RELEVANCE

To create more suitable medical treatments and improve the quality of care received by those with psychiatric conditions, further discussion is needed surrounding the physiologic toll that racism has had on multiple generations of racial and ethnic minority groups and how that may alter responsivity to biobehavioral interventions. To better inform psychiatric research, the resources provided must be expanded, basic physiologic studies should be replicated with more diverse samples and adequate analyses, and psychiatry scientists must reconsider approaches to clinical research.

The increasing challenge of the possible impact of ethnicity on psychopharmacology

Ethnic differences may significantly influence the outcome of psychopharmacological treatment, in terms of prescription, adherence, clinical response, emergence of side effects, as well as pharmacokinetics and pharmacodynamics. The purpose of this review was to explore the available literature in order to provide general suggestions to help clinicians in choosing the best therapeutic option for patients, taking into account ethnicity. Although findings are sometimes controversial, the overall published studies suggest that ethnicities other than Caucasians tend to show a lower response to antidepressants and a reduced compliance. Africans tend to be more prescribed with antipsychotics, probably due to cultural stereotypes, except with clozapine, probably for their chronic benign neutropenia. Asians usually require less antipsychotic dosages than Caucasians. The differential response and side effect profile of antidepressants and antipsychotics have been related to individual intrinsic factors, to genetic make-up, but also to cultural and contextual variables. Interestingly, albeit limited data suggest ethnic-related genetic heterogeneity at the level of the serotonin transporters, the cytochromes and some neuroreceptors. Taken together, no conclusive findings are available about the role and impact of ethnicity in psychopharmacology. One of the main problems is that the majority of the studies in psychopharmacology have been conducted on Caucasians, so that there is an urgent need to have data in other populations. Furthermore, in the era of precision medicine, the role of ethnicity may be also supported by genetic analysis.

Neurological correlates of brain reward circuitry linked to opioid use disorder (OUD): Do homo sapiens acquire or have a reward deficiency syndrome?

The extant literature confirms that an array of polymorphic genes related to- neurotransmitters and second messengers govern the net release of dopamine in the Nucleus Accumbens (NAc) in the mesolimbic region of the brain. They are linked predominantly to motivation, anti-stress, incentive salience (wanting), and wellbeing. Notably, in 2000 the Nobel Prize was awarded to Carlsson, Greengard, and Kandel for their work on the molecular and cellular function of dopaminergic activity at neurons. This historical psychopharmacological work involved neurotransmission of serotonin, endorphins, glutamate, and dopamine, and the seminal work of Blum, Gold, Volkow, Nestler, and others related to neurotransmitter function and related behaviors. Currently, Americans are facing their second and worst opioid epidemic, prescribed opioids, and easy access drive this epidemic of overdoses, and opioid use disorders (OUDs). Presently the clinical consensus is to treat OUD, as if it were an opioid deficiency syndrome, with long-term to life-long opioid substitution therapy. Opioid agonist administration is seen as necessary to replace missing opioids, treat OUD, and prevent overdoses, like insulin is used to treat diabetes. Treatment of OUD and addiction, in general, is similar to the endocrinopathy conceptualization in that it views opioid agonist MATs as an essential core to therapy. Is this approach logical? Other than as harm reduction, is using opioids to treat OUD therapeutic or harmful in the long term? This historical Trieste provides a molecular framework to understand the current underpinnings of endorphinergic/dopaminergic mechanisms related to opioid deficiency syndrome and generalized reward processing depletion. WC 249.

Cocaine-induced changes in behaviour and DNA methylation in rats are influenced by inter-individual differences in spontaneous exploratory activity

BACKGROUND

Individual differences in behavioural traits influence susceptibility to addictive disorders. Drug addiction involves changes in gene expression, proposed to occur via DNA methylation (DNAm).

AIMS

To investigate DNAm changes in reward-related brain structures (nucleus accumbens (NAc), lateral habenula (LHb)) in response to cocaine exposure in rats differing in spontaneous exploratory activity.

METHODS

Rats were observed in the exploration box and categorised as high- (HE) or low explorers (LE). Rats were administered vehicle or cocaine (12 mg/kg, i.p.) for 7 days, followed by a 14-day withdrawal period and cocaine challenge (7 mg/kg); horizontal locomotor activity was recorded. Brain tissue was dissected after 24 h; we analysed messenger RNA (mRNA) and activity levels of epigenetic DNA modifiers (DNMTs and TETs) as well as mRNA and promoter methylation levels at selected genes previously linked to addictive behaviours.

RESULTS

The cocaine challenge dose stimulated locomotor activity in both LE- and HE rats only when administered after a repeated cocaine schedule, suggesting development of behavioural sensitisation. Quantitative polymerase chain reaction analyses demonstrated higher basal expression of Dnmt3a, Tet2 and Tet3 in the LHb of HE- vs. LE rats, and we observed differential effects of cocaine exposure on the expression and activity of epigenetic DNA modifiers in the NAc and LHb of HE- and LE rats. Furthermore, cocaine exposure differentially altered promoter methylation levels of A_2AR, Ppp1cc, and Taar7b in the NAc and LHb of HE- and LE rats.

CONCLUSIONS

DNAm might play a role in the HE- and LE phenotypes as well as mediate behavioural effects of LE- and HE rats in response to drugs of abuse.

Epigenetic aging is accelerated in alcohol use disorder and regulated by genetic variation in APOL2

To investigate the potential role of alcohol use disorder (AUD) in aging processes, we employed Levine's epigenetic clock (DNAm PhenoAge) to estimate DNA methylation age in 331 individuals with AUD and 201 healthy controls (HC). We evaluated the effects of heavy, chronic alcohol consumption on epigenetic age acceleration (EAA) using clinical biomarkers, including liver function test enzymes (LFTs) and clinical measures. To characterize potential underlying genetic variation contributing to EAA in AUD, we performed genome-wide association studies (GWAS) on EAA, including pathway analyses. We followed up on relevant top findings with in silico expression quantitative trait loci (eQTL) analyses for biological function using the BRAINEAC database. There was a 2.22-year age acceleration in AUD compared to controls after adjusting for gender and blood cell composition (p = 1.85 × 10^-5). This association remained significant after adjusting for race, body mass index, and smoking status (1.38 years, p = 0.02). Secondary analyses showed more pronounced EAA in individuals with more severe AUD-associated phenotypes, including elevated gamma-glutamyl transferase (GGT) and alanine aminotransferase (ALT), and higher number of heavy drinking days (all ps < 0.05). The genome-wide meta-analysis of EAA in AUD revealed a significant single nucleotide polymorphism (SNP), rs916264 (p = 5.43 × 10^-8), in apolipoprotein L2 (APOL2) at the genome-wide level. The minor allele A of rs916264 was associated with EAA and with increased mRNA expression in hippocampus (p = 0.0015). Our data demonstrate EAA in AUD and suggest that disease severity further accelerates epigenetic aging. EAA was associated with genetic variation in APOL2, suggesting potential novel biological mechanisms for age acceleration in AUD.

The effects of psychosocial stress on dopaminergic function and the acute stress response

Chronic psychosocial adversity induces vulnerability to mental illnesses. Animal studies demonstrate that this may be mediated by dopaminergic dysfunction. We therefore investigated whether long-term exposure to psychosocial adversity was associated with dopamine dysfunction and its relationship to psychological and physiological responses to acute stress. Using 3,4-dihydroxy-6-[¹⁸F]-fluoro-l-phenylalanine ([¹⁸F]-DOPA) positron emission tomography (PET), we compared dopamine synthesis capacity in n = 17 human participants with high cumulative exposure to psychosocial adversity with n = 17 age- and sex-matched participants with low cumulative exposure. The PET scan took place 2 hr after the induction of acute psychosocial stress using the Montréal Imaging Stress Task to induce acute psychosocial stress. We found that dopamine synthesis correlated with subjective threat and physiological response to acute psychosocial stress in the low exposure group. Long-term exposure to psychosocial adversity was associated with dampened striatal dopaminergic function (p=0.03, d = 0.80) and that psychosocial adversity blunted physiological yet potentiated subjective responses to acute psychosocial stress. Future studies should investigate the roles of these changes in vulnerability to mental illnesses.

The effect of a genetic variant at the schizophrenia associated AS3MT/BORCS7 locus on striatal dopamine function: A PET imaging study

One of the most statistically significant loci to result from large-scale GWAS of schizophrenia is 10q24.32. However, it is still unclear how this locus is involved in the pathoaetiology of schizophrenia. The hypothesis that presynaptic dopamine dysfunction underlies schizophrenia is one of the leading theories of the pathophysiology of the disorder. Supporting this, molecular imaging studies show evidence for elevated dopamine synthesis and release capacity. Thus, altered dopamine function could be a potential mechanism by which this genetic variant acts to increase the risk of schizophrenia. We therefore tested the hypothesis that the 10q24.32 region confers genetic risk for schizophrenia through an effect on striatal dopamine function. To this aim we investigated the in vivo relationship between a GWAS schizophrenia-associated SNP within this locus and dopamine synthesis capacity measured using [18F]-DOPA PET in healthy controls. 92 healthy volunteers underwent [18F]-DOPA PET scans to measure striatal dopamine synthesis capacity (indexed as Kicer) and were genotyped for the SNP rs7085104. We found a significant association between rs7085104 genotype and striatal Kicer. Our findings indicate that the mechanism mediating the 10q24.32 risk locus for schizophrenia could involve altered dopaminergic function. Future studies are needed to clarify the neurobiological pathway implicated in this association.

Disrupted-in-schizophrenia 1 functional polymorphisms and D2 /D3 receptor availability: A [11 C]-(+)-PHNO imaging study

The disrupted-in-schizophrenia 1 (DISC1) protein has been implicated in a range of biological mechanisms underlying chronic mental disorders such as schizophrenia. Schizophrenia is associated with abnormal striatal dopamine signalling, and all antipsychotic drugs block striatal dopamine 2/3 receptors (D_2/3 Rs). Importantly, the DISC1 protein directly interacts and forms a protein complex with the dopamine D₂ receptor (D₂ R) that inhibits agonist-induced D₂ R internalisation. Moreover, animal studies have found large striatal increases in the proportion of D₂ R receptors in a high affinity state (D₂ ^high R) in DISC1 rodent models. Here, we investigated the relationship between the three most common polymorphisms altering the amino-acid sequence of the DISC1 protein (Ser704Cys (rs821616), Leu607Phe (rs6675281) and Arg264Gln (rs3738401)) and striatal D_2/3 R availability in 41 healthy human volunteers, using [¹¹ C]-(+)-PHNO positron emission tomography. We found no association between DISC1 polymorphisms and D_2/3 R availability in the striatum and D₂ R availability in the caudate and putamen. Therefore, despite a direct interaction between DISC1 and the D₂ R, none of its main functional polymorphisms impact striatal D_2/3 R binding potential, suggesting DISC1 variants act through other mechanisms.

Lack of Association Between Serotonin Transporter Gene (SLC6A4) Promoter Methylation and Amygdala Response During Negative Emotion Processing in Individuals With Alcohol Dependence

AIMS

Differences in DNA methylation of the serotonin transporter gene (SLC6A4) have been shown to alter SLC6A4 expression and predict brain functions in healthy individuals. This study investigated the association between SLC6A4 promoter methylation and threat-related amygdala activation in individuals with alcohol dependence (AD).

METHODS

Methylation of the SLC6A4 promoter region was assessed using peripheral blood DNA from 45 individuals with AD and 45 healthy controls (HCs). All participants completed an emotional face matching task in a 3-T magnetic resonance imaging (MRI) scanner.

RESULTS

Results did not reveal any association between SLC6A4 promoter methylation variation and threat-related amygdala activation in HCs or individuals with AD. Furthermore, methylation in the promoter region of SLC6A4 did not significantly differ between the groups.

CONCLUSIONS

Our results do not replicate a previous finding that increased methylation in the promoter region of SLC6A4 is associated with threat-related amygdala activation in healthy individuals and further show that there is no such association in individuals with AD. Given that the number of imaging epigenetics studies on SLC6A4 is very limited to date, these inconsistent results indicate that future research is needed to clarify its association with amygdala reactivity in both healthy and clinical populations.

Endocannabinoid signaling in psychiatric disorders: a review of positron emission tomography studies

Endocannabinoid signaling is implicated in an array of psychopathologies ranging from anxiety to psychosis and addiction. In recent years, radiotracers targeting the endocannabinoid system have been used in positron emission tomography (PET) studies to determine whether individuals with psychiatric disorders display altered endocannabinoid signaling. We comprehensively reviewed PET studies examining differences in endocannabinoid signaling between individuals with psychiatric illness and healthy controls. Published studies evaluated individuals with five psychiatric disorders: cannabis use disorder, alcohol use disorder, schizophrenia, post-traumatic stress disorder, and eating disorders. Most studies employed radiotracers targeting cannabinoid receptor 1 (CB1). Cannabis users consistently demonstrated decreased CB1 binding compared to controls, with normalization following short periods of abstinence. Findings in those with alcohol use disorder and schizophrenia were less consistent, with some studies demonstrating increased CB1 binding and others demonstrating decreased CB1 binding. Evidence of aberrant CB1 binding was also found in individuals with anorexia nervosa and post-traumatic stress disorder, but limited data have been published to date. Thus, existing evidence suggests that alterations in endocannabinoid signaling are present in a range of psychiatric disorders. Although recent efforts have largely focused on evaluating CB1 binding, the synthesis of new radiotracers targeting enzymes involved in endocannabinoid degradation, such as fatty acid amide hydrolase, will allow for other facets of endocannabinoid signaling to be evaluated in future studies.

Effects of sedative drug use on the dopamine system: a systematic review and meta-analysis of in vivo neuroimaging studies

Use of alcohol, cannabis and opioids is highly prevalent and is associated with global disease burden and high economic costs. The exact pathophysiology of abuse or addiction associated with these sedative substances is not completely understood, but previous research implicates the important role of the striatal dopamine system in the addiction process. Multiple studies investigated changes in the striatal dopamine systems of users of sedative substances, but currently these results are very heterogeneous. Therefore, we conducted a meta-analysis of in vivo neuroimaging studies investigating dopaminergic alterations in the striatum of users of alcohol, opioids or cannabis. Analyses for each substance were conducted separately for the availability of D2/D3 dopamine receptors, dopamine transporters and dopamine synthesis capacity. In total, 723 substance users and 752 healthy controls were included. The results indicated a significant lower striatal D2/D3 receptor availability in alcohol users compared to controls (g = 0.46) but no difference in dopamine transporter availability or dopamine synthesis capacity. Our analysis indicated that changes of dopamine receptors and transporters are moderated by the duration of abstinence. Comparing opioid users with controls revealed a significant lower D2/D3 receptor availability (g = 1.17) and a significantly lower transporter availability (g = 1.55) in opioid users. For cannabis users, there was no significant difference in receptor availability compared to controls and too few studies provided information on dopamine transporter availability or synthesis capacity. Our analysis provides strong evidence for a central role of the striatal dopamine system in use of alcohol or opioids. Further studies are needed to clarify the impact of the dopamine system in cannabis users.

Expectation effects on brain dopamine responses to methylphenidate in cocaine use disorder

The response to drugs of abuse is affected by expectation, which is modulated in part by dopamine (DA), which encodes for a reward prediction error. Here we assessed the effect of expectation on methylphenidate (MP)-induced striatal DA changes in 23 participants with an active cocaine use disorder (CUD) and 23 healthy controls (HC) using [¹¹C]raclopride and PET both after placebo (PL) and after MP (0.5 mg/kg, i.v.). Brain dopamine D2 and D3 receptor availability (D2R: non-displaceable binding potential (BP_ND)) was measured under four conditions in randomized order: (1) expecting PL/receiving PL, (2) expecting PL/receiving MP, (3) expecting MP/receiving PL, and (4) expecting MP/receiving MP. Expecting MP increased pulse rate compared to expecting PL. Receiving MP decreased D2R in striatum compared to PL, indicating MP-induced striatal DA release, and this effect was significantly blunted in CUD versus HC consistent with prior findings of decreased striatal dopamine responses both in active and detoxified CUD. There was a group × challenge × expectation effect in caudate and midbrain, with expectation of MP increasing MP-induced DA release in HC but not in CUD, and expectation of PL showing a trend to increase MP-induced DA release in CUD but not in HC. These results are consistent with the role of DA in reward prediction error in the human brain: decreasing DA signaling when rewards are less than expected (blunted DA increases to MP in CUD) and increasing them when greater than expected (for PL in CUD reflecting conditioned responses to injection). Our findings also document disruption of the expectation of drug effects in dopamine signaling in participants with CUD compared to non-addicted individuals.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

The relationship between cortical glutamate and striatal dopamine in first-episode psychosis: a cross-sectional multimodal PET and magnetic resonance spectroscopy imaging study

BACKGROUND

The pathophysiology of psychosis is incompletely understood. Disruption in cortical glutamatergic signalling causing aberrant striatal dopamine synthesis capacity is a proposed model for psychosis, but has not been tested in vivo. We therefore aimed to test the relationship between cortical glutamate concentrations and striatal dopamine synthesis capacity, and psychotic symptoms.

METHODS

In this cross-sectional multimodal imaging study, 28 individuals with first-episode psychosis and 28 healthy controls underwent 18F-DOPA PET (measuring striatal dopamine synthesis capacity), and proton magnetic resonance spectroscopy (measuring anterior cingulate cortex glutamate concentrations). Participants were recruited from first-episode psychosis services in London, UK and were required to be in the first episode of a psychotic illness, with no previous illness or treatment episodes. Exclusion criteria for all participants were: history of substantial head trauma, dependence on illicit substances, medical comorbidity (other than minor illnesses), and contraindications to scanning (such as pregnancy). Symptoms were measured using the Positive and Negative Syndrome Scale. The primary endpoint was the relationship between anterior cingulate cortex glutamate concentrations and striatal dopamine synthesis capacity in individuals with their first episode of psychosis as shown by imaging, examined by linear regression. Linear regression was used to examine relationships between measures.

FINDINGS

Glutamate concentrations showed a significant inverse relationship with striatal dopamine synthesis capacity in patients with psychosis (R2=0·16, p=0·03, β -1·71 × 10-4, SE 0·76 × 10-4). This relationship remained significant after the addition of age, gender, ethnicity, and medication status to the model (p=0·015). In healthy controls, there was no significant relationship between dopamine and glutamate measures (R2=0·04, p=0·39). Positive and Negative Syndrome Scale positive psychotic symptoms were positively associated with striatal dopamine synthesis capacity (R2=0·14, p=0·046, β 2546, SE 1217) and showed an inverse relationship with anterior cingulate glutamate concentrations (R2=0·16, p=0·03, β -1·71 × 10-4, SE 7·63 × 10-5). No relationships were seen with negative symptoms (positive symptoms, mean [SD] -18·4 (6·6) negative symptoms, mean [SD] -15·4 [6·1]).

INTERPRETATION

These observations are consistent with the hypothesis that cortical glutamate dysfunction is related to subcortical dopamine synthesis capacity and psychosis. Although the precise mechanistic relationship between cortical glutamate and dopamine in vivo remains unclear, our findings support further studies to test the effect of modulating cortical glutamate in the treatment of psychosis.

FUNDING

Medical Research Council, Wellcome Trust, Biomedical Research Council, South London and Maudsley NHS Foundation Trust, JMAS Sim Fellowship, Royal College of Physicians (Edinburgh) (SJ).