Reduced midbrain-pons serotonin transporter binding in patients with obsessive-compulsive disorder

Serotonin depletion impairs both Pavlovian and instrumental reversal learning in healthy humans

Serotonin is involved in updating responses to changing environmental circumstances. Optimising behaviour to maximise reward and minimise punishment may require shifting strategies upon encountering new situations. Likewise, autonomic responses to threats are critical for survival yet must be modified as danger shifts from one source to another. Whilst numerous psychiatric disorders are characterised by behavioural and autonomic inflexibility, few studies have examined the contribution of serotonin in humans. We modelled both processes, respectively, in two independent experiments (N = 97). Experiment 1 assessed instrumental (stimulus-response-outcome) reversal learning whereby individuals learned through trial and error which action was most optimal for obtaining reward or avoiding punishment initially, and the contingencies subsequently reversed serially. Experiment 2 examined Pavlovian (stimulus-outcome) reversal learning assessed by the skin conductance response: one innately threatening stimulus predicted receipt of an uncomfortable electric shock and another did not; these contingencies swapped in a reversal phase. Upon depleting the serotonin precursor tryptophan-in a double-blind randomised placebo-controlled design-healthy volunteers showed impairments in updating both actions and autonomic responses to reflect changing contingencies. Reversal deficits in each domain, furthermore, were correlated with the extent of tryptophan depletion. Initial Pavlovian conditioning, moreover, which involved innately threatening stimuli, was potentiated by depletion. These results translate findings in experimental animals to humans and have implications for the neurochemical basis of cognitive inflexibility.

Functional connectivity of the raphe nucleus as a predictor of the response to selective serotonin reuptake inhibitors in obsessive-compulsive disorder

Selective serotonin reuptake inhibitors (SSRIs) are first-line pharmacological agents for treating obsessive-compulsive disorder (OCD). However, because nearly half of patients show insufficient SSRI responses, serotonergic dysfunction in heterogeneous OCD patients should be investigated for precision medicine. We aimed to determine whether functional connectivity (FC) of the raphe nucleus (RN), the major source of most serotonergic neurons, was altered in OCD patients and could predict the SSRI response. A total of 102 medication-free OCD patients and 101 matched healthy control (HC) subjects participated in resting-state functional magnetic resonance imaging. Among them, 54 OCD patients were treated with SSRIs for 16 weeks, resulting in 26 responders and 28 nonresponders. Seed-based whole brain FC with the RN as a seed region was compared between the OCD and HC groups, as well as between SSRI responders and nonresponders. FC cluster values showing significant group differences were used to investigate factors correlated with symptomatic severity before treatment and predictive of SSRI response. Compared to HCs, OCD patients exhibited significantly larger FC between the RN and temporal cortices including the middle temporal gyrus (MTG), paracingulate gyrus, amygdala, hippocampus, putamen, thalamus, and brain stem. Greater RN-left MTG FC was positively correlated with OC symptom severity at baseline. In addition, larger FC of the RN-left MTG was also found in SSRI nonresponders compared to responders, which was a significant predictor of SSRI response after 16 weeks. The FC of RN may reflect the neurobiological underpinning of OCD and could aid future precision medicine as a differential brain-based biomarker.

Serotonin transporter binding is increased in Tourette syndrome with Obsessive Compulsive Disorder

While the importance of the serotonergic system in obsessive compulsive disorder (OCD) is well established, its role in Tourette syndrome (TS) is uncertain. Particularly in TS patients with comorbid OCD (TS + OCD), decreased serotonin transporter (SERT) binding has been suggested. Here, we investigated for the first time SERT binding in TS patients with and without OCD (TS - OCD) compared to both healthy controls (HC) and OCD patients as well as the influence of escitalopram using the potent SERT imaging ligand [¹²³I]2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([¹²³I]ADAM) and single-photon emission tomography (SPECT). We included 33 adult subjects (10 HC, 10 TS - OCD, 8 TS + OCD and 5 OCD). In patients with OCD and TS + OCD [¹²³I]ADAM SPECT was repeated after 12-16 weeks treatment with escitalopram. SERT binding was normal in patients with OCD and TS - OCD, but significantly increased (p < 0.05) in those with TS + OCD, particularly in caudate and midbrain compared to both HC and TS - OCD. Treatment with escitalopram resulted in a significant overall reduction in SERT binding (range, 19 to 79%, p values between 0.0409 and <0.0001) without any correlation with clinical improvement. Our results provide further evidence that alterations in the serotonergic system in TS are related to comorbid OCD and do not represent the primary cause of the disease.

Oral N-acetylcysteine in the treatment of obsessive-compulsive disorder: A systematic review of the clinical evidence

Obsessive-compulsive disorder (OCD) is characterized by the presence of obsessions and/or compulsions. It is a leading cause of morbidity worldwide, as it can interfere with all aspects of life. Despite the adequate treatment trials, half of patients preserve residual or impairing symptoms and selective serotonin reuptake inhibitors (SSRIs) are not free from adverse side effects. This work aims to systematically review the current evidence available concerning the efficacy of N-acetylcysteine (NAC) in the treatment of OCD. Five randomized placebo-controlled trials (RCTs), 3 case reports and 2 case series were included. The studies developed so far are somehow contradictory. However, our pooled result from the 4 observational studies (n = 13) showed a mean reduction in Y-BOCS score after NAC treatment of -11 points (p = .01). Pooled mean difference from 4 of the 5 RCTs included was 3.35, with a95% confidence interval of -0.21-6.91 and a p-value barely below statistical significance (p = .07). This result trends to favour the use of NAC over placebo in OCD patients. NAC has an optimal tolerability profile, even in higher doses, and the most frequently reported adverse events were gastrointestinal. Despite the degree of evidence being D, in our opinion the potential of NAC is underestimated. Considering its exceptional tolerability profile, the use as an add-on agent should be contemplated, on an ad hoc basis.

Peripheral SLC6A4 Gene Expression in Obsessive-Compulsive Disorder in the Han Chinese Population

BACKGROUND

Serotonergic system dysfunction has been implicated in obsessive-compulsive disorder (OCD). This study examined peripheral SLC6A4 gene expression in OCD patients and healthy controls to explore the relationship between SLC6A4 and OCD.

METHODS

Participants included 50 first episode OCD patients and 60 age and gender-matched healthy controls. Relative SLC6A4 gene expression were examined by real-time quantitative reverse transcription polymerase chain reaction (RT-PCR) in peripheral leukocytes of all the subjects. The Yale-Brown Obsessive Compulsive Scale (Y-BOCS) was used to assess the severity and subtype of OCD.

RESULTS

SLC6A4 gene expression, normalized by glyceraldehyde-3-phosphate dehydrogenase (GAPDH), were not significantly different between the OCD patients and healthy controls(z=-0.79, p=0.428). Male OCD patients showed a tendency of low gene expression of SLC6A4 in peripheral blood (z=-1.66, p=0.096). We did not find a significant correlation between SLC6A4 expression and the severity and subtype of OCD.

CONCLUSION

There is no correlation between SLC6A4 expression levels and the severity and subtype of OCD, but male OCD patients showed a tendency of low gene expression of SLC6A4 in peripheral blood. These results suggest that gene expression of SLC6A4 in peripheral blood may not be a useful biomarker of OCD in the Han Chinese population.

A review of the role of serotonin system genes in obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a debilitating neuropsychiatric disorder that causes the patient to experience intrusive thoughts and/or to carry out repetitive, ritualized behaviors that are time consuming and impairing. OCD is familial and heritable. The genetic factors responsible for pathogenesis, however, remain largely unknown despite the numerous candidate gene studies conducted. Based on efficacy of serotonin reuptake inhibitors (SRIs) in treating OCD, serotonin system genes have been a dominant focus in OCD candidate gene studies. We review the most commonly studied candidate serotonin system gene variants (specifically in SLC6A4, HTR2A, HTR1B, and HTR2C) and their association with OCD. Although findings to date are mixed, serotonin transporter polymorphism 5-HTTLPR and HTR2A polymorphism rs6311 (or rs6313) are most consistently associated with OCD. Mixed findings may be the result of genetic complexity and phenotypic heterogeneity that future studies should account for. Homogenous patient subgroups reflecting OCD symptom dimensions, OCD subtypes, and sex should be used for gene discovery.

Core, social and moral disgust are bounded: A review on behavioral and neural bases of repugnance in clinical disorders

Disgust is a multifaceted experience that might affect several aspects of life. Here, we reviewed research on neurological and psychiatric disorders that are characterized by abnormal disgust processing to test the hypothesis of a shared neurocognitive architecture in the representation of three disgust domains: i) personal experience of 'core disgust'; ii) social disgust, i.e., sensitivity to others' expressions of disgust; iii) moral disgust, i.e., sensitivity to ethical violations. Our review provides some support to the shared neurocognitive hypothesis and suggests that the insula might be the "hub" structure linking the three domains of disgust sensitivity, while other brain regions may subserve specific facets of the multidimensional experience. Our review also suggests a role of serotonin core and moral disgust, supporting "neo-sentimentalist" theories of morality, which posit a causal role of affect in moral judgment.

Motivation and value influences in the relative balance of goal-directed and habitual behaviours in obsessive-compulsive disorder

Our decisions are based on parallel and competing systems of goal-directed and habitual learning, systems which can be impaired in pathological behaviours. Here we focus on the influence of motivation and compare reward and loss outcomes in subjects with obsessive-compulsive disorder (OCD) on model-based goal-directed and model-free habitual behaviours using the two-step task. We further investigate the relationship with acquisition learning using a one-step probabilistic learning task. Forty-eight OCD subjects and 96 healthy volunteers were tested on a reward and 30 OCD subjects and 53 healthy volunteers on the loss version of the two-step task. Thirty-six OCD subjects and 72 healthy volunteers were also tested on a one-step reversal task. OCD subjects compared with healthy volunteers were less goal oriented (model-based) and more habitual (model-free) to reward outcomes with a shift towards greater model-based and lower habitual choices to loss outcomes. OCD subjects also had enhanced acquisition learning to loss outcomes on the one-step task, which correlated with goal-directed learning in the two-step task. OCD subjects had greater stay behaviours or perseveration in the one-step task irrespective of outcome. Compulsion severity was correlated with habitual learning in the reward condition. Obsession severity was correlated with greater switching after loss outcomes. In healthy volunteers, we further show that greater reward magnitudes are associated with a shift towards greater goal-directed learning further emphasizing the role of outcome salience. Our results highlight an important influence of motivation on learning processes in OCD and suggest that distinct clinical strategies based on valence may be warranted.

Abnormal resting-state activities and functional connectivities of the anterior and the posterior cortexes in medication-naïve patients with obsessive-compulsive disorder

Background

Obsessive-compulsive disorder (OCD) is a mental illness characterized by the loss of control. Because the cingulate cortex is believed to be important in executive functions, such as inhibition, we used functional magnetic resonance imaging (fMRI) techniques to examine whether and how activity and functional connectivity (FC) of the cingulate cortex were altered in drug-naïve OCD patients.

Methods

Twenty-three medication-naïve OCD patients and 23 well-matched healthy controls received fMRI scans in a resting state. Functional connectivities of the anterior cingulate (ACC) and the posterior cingulate (PCC) to the whole brain were analyzed using correlation analyses based on regions of interest (ROI) identified by the fractional amplitude of low-frequency fluctuation (fALFF). Independent Component Analysis (ICA) was used to identify the resting-state sub-networks.

Results

fALFF analysis found that regional activity was increased in the ACC and decreased in the PCC in OCD patients when compared to controls. FC of the ACC and the PCC also showed different patterns. The ACC and the PCC were found to belong to different resting-state sub-networks in ICA analysis and showed abnormal FC, as well as contrasting correlations with the severity of OCD symptoms.

Conclusions

Activity of the ACC and the PCC were increased and decreased, respectively, in the medication-naïve OCD patients compared to controls. Different patterns in FC were also found between the ACC and the PCC with respect to these two groups. These findings implied that the cardinal feature of OCD, the loss of control, may be attributed to abnormal activities and FC of the ACC and the PCC.

Sex determines which section of the SLC6A4 gene is linked to obsessive-compulsive symptoms in normal Chinese college students

Previous case-control and family-based association studies have implicated the SLC6A4 gene in obsessive-compulsive disorder (OCD). Little research, however, has examined this gene's role in obsessive-compulsive symptoms (OCS) in community samples. The present study genotyped seven tag SNPs and two common functional tandem repeat polymorphisms (5-HTTLPR and STin2), which together cover the whole SLC6A4 gene, and investigated their associations with OCS in normal Chinese college students (N = 572). The results revealed a significant gender main effect and gender-specific genetic effects of the SLC6A4 gene on OCS. Males scored significantly higher on total OCS and its three dimensions than did females (ps < .01). The 5-HTTLPR in the promoter region showed a female-specific genetic effect, with the l/l and l/s genotypes linked to higher OCS scores than the s/s genotype (ps < .05). In contrast, a conserved haplotype polymorphism (rs1042173| rs4325622| rs3794808| rs140701| rs4583306| rs2020942) covering from intron 3 to the 3' UTR of the SLC6A4 gene showed male-specific genetic effects, with the CGAAGG/CGAAGG genotype associated with lower OCS scores than the other genotypes (ps < .05). These effects remained significant after controlling for OCS-related factors including participants' depressive and anxiety symptoms as well as stressful life events, and correction for multiple tests. These results are discussed in terms of their implications for our understanding of the sex-specific role of the different sections of the SLC6A4 gene in OCD.

Serotonin 2A receptor, serotonin transporter and dopamine transporter alterations in dogs with compulsive behaviour as a promising model for human obsessive-compulsive disorder

Neuro-imaging studies have shown altered, yet often inconsistent, serotonergic and dopaminergic neurotransmission in patients with obsessive-compulsive disorder (OCD). We investigated both serotonergic and dopaminergic neurotransmission in 9 drug-naïve dogs with compulsive behaviour, as a potential model for human OCD. Single photon emission computed tomography was used with (123)I-R91150 and (123)I-FP-CIT, in combination with (99m)Tc-ECD brain perfusion co-registration, to measure the serotonin (5-HT) 2A receptor, dopamine transporter (DAT) and serotonin transporter (SERT) availability. Fifteen normally behaving dogs were used as reference group. Significantly lower 5-HT2A receptor radioligand availability in frontal and temporal cortices (bilateral) was observed. Further, in 78% of the compulsive dogs abnormal DAT ratios in left and right striatum were demonstrated. Interestingly, both increased and decreased DAT ratios were observed. Finally, significantly lower subcortical perfusion and (hypo)thalamic SERT availability were observed in the compulsive dogs. This study provides evidence for imbalanced serotonergic and dopaminergic pathways in the pathophysiology of compulsions in dogs. The similarities with the altered neurotransmission in human OCD provide construct validity for this non-induced, natural canine model, suggesting its usefulness for future investigations of the pathophysiology of human OCD as well as the effectiveness of psychopharmacological interventions.

The serotonin transporter availability in untreated early-onset and late-onset patients with obsessive-compulsive disorder

The pathogenetic role of central serotonin transporters (SERT) in obsessive-compulsive disorder (OCD) has been investigated in vivo by positron emission tomography (PET) or single-photon emission computed tomography (SPECT) studies with inconsistent results. This might reflect methodological differences but possibly also the pathophysiological heterogeneity of the disorder, i.e. the age at onset of OCD. The aim of our study was to compare SERT availability in patients with OCD to healthy controls (HC) taking into account the onset type, other factors and covariates (e.g. SERT genotype, age, depression level, gender). We studied 19 drug-naive OCD patients (36±13 yr, eight females) with early onset (EO-OCD, n=6) or with late onset (LO-OCD, n=13), and 21 HC (38±8 yr, nine females) with PET and the SERT-selective radiotracer [11C]DASB. Statistical models indicated that a variety of covariates and their interaction influenced SERT availability measured by distribution volume ratios (DVR). These models revealed significant effects of onset type on DVR with lower values in LO-OCD (starting at age 18 yr) compared to EO-OCD and HC in limbic (e.g. the amygdala), paralimbic brain areas (the anterior cingulate cortex), the nucleus accumbens and striatal regions, as well as borderline significance in the thalamus and the hypothalamus. The putamen, nucleus accumbens and hypothalamus were found with significant interaction between two SERT gene polymorphisms (SERT-LPR and VNTR). These findings suggest that late but not early onset of OCD is associated with abnormally low SERT availability. In part, functional polymorphisms of the SERT gene might determine the differences.

Nutraceuticals in the treatment of obsessive compulsive disorder (OCD): a review of mechanistic and clinical evidence

Obsessive-compulsive disorder (OCD) is a debilitating mental illness which has a significant impact on quality of life. First-line SSRI treatments for OCD typically are of limited benefit to only 40-60% of patients, and are associated with a range of adverse side effects. Current preclinical research investigating nutraceuticals (natural products) for OCD, reveals encouraging novel activity in modulating key pathways suggested to be involved in the pathogenesis of OCD (glutamatergic and serotonergic pathway dysregulation). Emerging clinical evidence also appears to tentatively support certain nutrients and plant-based interventions with known active constituents which modulate these pathways: N-acetlycysteine, myo-inositol, glycine, and milk thistle (Silybum marianum). The serotonin precursor tryptophan is unlikely to be of use in treating OCD while 5-HTP may possibly be a more effective precursor strategy. However, there is currently no clinical evidence to test the efficacy of either of these substances. Currently the balance of clinical evidence does not support the use of St. John's wort (Hypericum perforatum) in OCD. While clinical research in this area is in its infancy, further research into nutraceuticals is warranted in light of the promising preclinical data regarding their mechanisms of action and their favourable side effect profiles in comparison to current SSRI treatments. It is recommended that future clinical trials of nutraceutical treatments for OCD utilize randomized placebo-controlled study designs and considerably larger sample sizes in order to properly test for efficacy.

Association of SLC6A4 variants with obsessive-compulsive disorder in a large multicenter US family study

Genetic association studies of SLC6A4 (SERT) and obsessive-compulsive disorder (OCD) have been equivocal. We genotyped 1241 individuals in 278 pedigrees from the OCD Collaborative Genetics Study for 13 single-nucleotide polymorphisms, for the linked polymorphic region (LPR) indel with molecular haplotypes at rs25531, for VNTR polymorphisms in introns 2 and 7 and for a 381-bp deletion 3' to the LPR. We analyzed using the Family-Based Association Test (FBAT) under additive, dominant, recessive and genotypic models, using both OCD and sex-stratified OCD as phenotypes. Two-point FBAT analysis detected association between Int2 (P = 0.0089) and Int7 (P = 0.0187) (genotypic model). Sex-stratified two-point analysis showed strong association in females with Int2 (P<0.0002), significant after correction for linkage disequilibrium, and multiple marker and model testing (P(Adj) = 0.0069). The SLC6A4 gene is composed of two haplotype blocks (our data and the HapMap); FBAT whole-marker analysis conducted using this structure was not significant. Several noteworthy nonsignificant results have emerged. Unlike Hu et al., we found no evidence for overtransmission of the LPR L(A) allele (genotype relative risk = 1.11, 95% confidence interval: 0.77-1.60); however, rare individual haplotypes containing L(A) with P<0.05 were observed. Similarly, three individuals (two with OCD/OCPD) carried the rare I425V SLC6A4 variant, but none of them passed it on to their six OCD-affected offspring, suggesting that it is unlikely to be solely responsible for the 'OCD plus syndrome', as reported by Ozaki et al. In conclusion, we found evidence of genetic association at the SLC6A4 locus with OCD. A noteworthy lack of association at the LPR, LPR-rs25531 and rare 425V variants suggests that hypotheses about OCD risk need revision to accommodate these new findings, including a possible gender effect.

Cortical GABA, striatal dopamine and midbrain serotonin as the key players in compulsive and anxiety disorders--results from in vivo imaging studies

Various factors are discussed in the pathophysiology of anxiety disorders, including dysfunctions of the (DA)ergic, serotonin (5-HT)ergic and GABAergic system. We assessed the contribution of the individual synaptic constituents by subjecting all available in vivo imaging studies on patients with anxiety disorders to a retrospective analysis. On a total of 504 patients with obsessive-compulsive disorder (OCD), generalized anxiety disorder (GAD), panic disorder (PD), phobia, or posttraumatic stress-disorder (PTSD) and 593 controls, investigations of VMAT2, DAT, SERT, D1, D2, 5-HTIA, 5-HT2A, GABA(A), and NK1 receptor binding in neostriatum, ventral striatum, thalamus, neocortex, limbic system, cingulate, midbrain/ pons or cerebellum were performed using either PET or SPECT. Separate analyses of the individual disorders showed significant decreases of striatal D2 receptors in OCD (-18%), mesencephalic SERT in OCD (-13%), frontocortical GABAA receptors in PD (-13%) and temporocortical GABAA receptors in GAD (-16%). Pooling of all disorders yielded a significant reduction of mesencephalic SERT (-13%), mesencephalic (-27%) as well as cingulate 5-HT1A receptors (-18%), striatal D2 receptors (-21%) and frontal (-14%), temporal (-14%), occipital (-13%) and cingulate GABAA receptors (-15%). The results show that DA, 5-HT, and GABA play a major role in all subtypes of anxiety disorders. In particular, the findings imply that the regulation state of DA as modulated by GABA and 5-HT may be crucial for the development of anxiety- and compulsion-related disorders. As GABA and 5-HT inhibit DAergic neurotransmission, the reductions of GABAA, 5-HT1A and SERT can be assumed to result in an enhanced activity of the mesolimbic DAergic system. This notion is also reflected by the decrease of striatal D2 receptor binding, which is indicative of an increased availability of synaptic DA.

Diencephalic serotonin transporter availability predicts both transporter occupancy and treatment response to sertraline in obsessive-compulsive checkers

BACKGROUND

To our knowledge, no studies have investigated the predictive value of central serotonin transporter (SERT) availability for treatment response to serotonin reuptake inhibitors (SSRIs) in patients with obsessive-compulsive disorder (OCD). This study used brain imaging to examine the relationship between pretreatment SERT availability and transporter occupancy as well as treatment response by sertraline in patients displaying prominent behavioral checking compulsions (OC checkers).

METHODS

Single photon emission computed tomography (SPECT) was used to measure thalamic-hypothalamic SERT availability with [(123)I]-2beta-carbomethoxy-3beta-(4-iodophenyl)-tropane in 28 nondepressed OC checkers at baseline and after 14 weeks of treatment with sertraline (175 mg daily). SERT availability was correlated with OC severity and treatment response as assessed with the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS). Associations between individual transporter occupancies and clinical parameters were investigated.

RESULTS

1) Correlation analyses between thalamic-hypothalamic SERT availability and OC severity showed significant negative associations at baseline and after treatment with sertraline. 2) Pretreatment SERT availability correlated significantly with both transporter occupancy and treatment response; in addition, a positive association was found between transporter occupancy and treatment response directly. 3) Using multivariate statistical models, the data demonstrated that higher pretreatment SERT availability significantly predicted higher occupancy rates as well as better treatment response 14 weeks later.

CONCLUSIONS

Higher pretreatment thalamic-hypothalamic SERT availability may predict both higher occupancy rates and better treatment response to sertraline. The data suggest a strong connection between transporter occupancy and treatment response.

Role of serotonin in obsessive–compulsive disorder

Obsessive–compulsive disorder (OCD) is a psychiatric disorder consisting of obsessions and compulsions. Over the past two decades, it has been suggested that OCD might be related to the functioning of brain serotonin systems, mainly because of the antiobsessional efficacy of selective serotonin-reuptake inhibitors (SSRIs). Although the efficacy of SSRIs suggests a role of the serotonergic system in OCD, the exact function of serotonin is still unclear. Is the serotonergic system implicated in the pathophysiology of OCD, or is it implicated in the treatment effect in OCD? Do SSRIs compensate for a fundamental abnormality of the serotonergic system, or do SSRIs modulate an intact serotonergic system to compensate for another neurotransmitter mechanism? This review summarizes evidence supporting a role for the serotonin transporter and serotonin receptor subtypes in the pathophysiology and treatment of OCD.

In vivo PET study of 5HT(2A) serotonin and D(2) dopamine dysfunction in drug-naive obsessive-compulsive disorder

There are several lines of evidence, the majority indirect, suggesting that changes in serotonergic or dopaminergic neurotransmission may contribute to the pathogenesis of obsessive-compulsive disorder (OCD). We evaluated the co-occurrence of serotonergic and dopaminergic dysfunctions in OCD subjects, all drug-naive, with no co-morbidity and homogeneous for symptoms. Each subject underwent two positron emission tomography (PET) scans to measure in vivo both serotonin (5-HT(2A)) and dopamine (D(2)) receptor distribution. For this, we used [11C]MDL and [11C]Raclopride, highly selective antagonists of 5-HT(2A) and D(2) receptors, respectively. The comparison with a control group was carried out using both voxel-wise (SPM2) and regions of interest (ROI) approaches. There was a significant reduction of 5-HT(2A) receptor availability in frontal polar, dorsolateral, and medial frontal cortex, as well as in parietal and temporal associative cortex of OCD patients. We also found a significant correlation between 5-HT(2A) receptor availability in orbitofrontal and dorsolateral frontal cortex and clinical severity, suggesting a specific role for serotonin in determining the OCD symptoms. There was also a significant reduction of [11C]Raclopride uptake in the whole striatum, particularly in the ventral portion, possibly reflecting endogenous dopaminergic hyperactivity. The co-existence of serotonergic and dopaminergic dysfunction in the same homogeneous group of drug-naive OCD patients provides in vivo evidence for the complex molecular mechanisms of OCD, and represents the basis for further studies on the effect of therapeutic agents with specific modulatory effects on these neurotransmission systems.