Serotonergic dissection of obsessive compulsive symptoms: a challenge study with m-chlorophenylpiperazine and sumatriptan

Serotonergic agonism and pharmacologically-induced adolescent stress cause operant-based learning deficits in mice

BACKGROUND

The interplay between environmental stress and genetic factors is thought to play an important role in the pathogenesis and maintenance of obsessive-compulsive disorder (OCD). However, the relative contribution of these causative antecedents in the manifestation of cognitive inflexibility-a phenotype often seen in obsessive-compulsive (OC)- spectrum disorders-is not fully understood.

METHOD

In this study, we treated mice with 50 mg/L corticosterone (CORT, a glucocorticoid stress hormone) in their drinking water during adolescence. In adulthood, we assessed anxiety-like behaviour and locomotor activity; along with operant-based discrimination and reversal learning. RU-24969, a selective serotonin receptor 5-HT1A/1B receptor agonist, was used as an acute pharmacological model of OC-like behaviour. RU-24969 (5 mg/kg) was administered prior to each reversal learning testing session.

RESULTS

We found that acute treatment with 5 mg/kg RU-24969 induced stereotyped hyperlocomotion in vehicle- and CORT-treated mice. Furthermore, pre-treatment with CORT in adolescence produced subtle anxiety-like behaviour in adult mice, and also resulted in an impairment to late-stage discrimination learning and alterations to reversal learning. Finally, acute treatment with 5 mg/kg RU-24969 caused an impairment to early-stage reversal learning.

CONCLUSION

Whilst we revealed dissociable detrimental effects of adolescent CORT treatment and acute 5-HT1A/1B receptor agonism on discrimination and reversal learning, respectively, we did not find evidence of additive deleterious effects of these two treatments. We therefore suggest that while disrupted serotonergic signalling is likely to be involved in the cognitive phenotype of OC-spectrum disorders, distinct neuropathological pathways may be at play in mediating the role of stress as an antecedent in OCD and related illnesses.

Psilocybin as a lead candidate molecule in preclinical therapeutic studies of psychiatric disorders: A systematic review

Psilocybin is the main psychoactive compound found in hallucinogenic/magic mushrooms and can bind to both serotonergic and tropomyosin receptor kinase b (TrkB) receptors. Psilocybin has begun to show efficacy for a range of neuropsychiatric conditions, including treatment-resistant depression and anxiety disorders; however, neurobiological mechanisms are still being elucidated. Clinical research has found that psilocybin can alter functional connectivity patterns in human brains, which is often associated with therapeutic outcomes. However, preclinical research affords the opportunity to assess the potential cellular mechanisms by which psilocybin may exert its therapeutic effects. Preclinical rodent models can also facilitate a more tightly controlled experimental context and minimise placebo effects. Furthermore, where there is a rationale, preclinical researchers can investigate psilocybin administration in neuropsychiatric conditions that have not yet been researched clinically. As a result, we have systematically reviewed the knowledge base, identifying 82 preclinical studies which were screened based on specific criteria. This resulted in the exclusion of 44 articles, with 34 articles being included in the main review and another 2 articles included as Supporting Information materials. We found that psilocybin shows promise as a lead candidate molecule for treating a variety of neuropsychiatric conditions, albeit showing the most efficacy for depression. We discuss the experimental findings, and identify possible mechanisms whereby psilocybin could invoke therapeutic changes. Furthermore, we critically evaluate the between-study heterogeneity and possible future research avenues. Our review suggests that preclinical rodent models can provide valid and translatable tools for researching novel psilocybin-induced molecular and cellular mechanisms, and therapeutic outcomes.

A closer look to neural pathways and psychopharmacology of obsessive compulsive disorder

The intricate neural pathways involved in obsessive-compulsive disorder (OCD) affect areas of our brain that control executive functioning, organization, and planning. OCD is a chronic condition that can be debilitating, afflicting millions of people worldwide. The lifetime prevalence of OCD in the US is 2.3%. OCD is predominantly characterized by obsessions consisting of intrusive and unwanted thoughts, often with impulses that are strongly associated with anxiety. Compulsions with OCD encompass repetitive behaviors or mental acts to satisfy their afflicted obsessions or impulses. While these factors can be unique to each individual, it has been widely established that the etiology of OCD is complex as it relates to neuronal pathways, psychopharmacology, and brain chemistry involved and warrants further exploration.

Lopez S, Amodeo DA. Impact of specific serotonin receptor modulation on restricted repetitive behaviors

Restricted, repetitive behaviors (RRBs) are commonly divided into two behavioral categories, lower-order and higher-order RRBs. Individuals displaying lower-order motoric RRBs may express repetitive hand flapping behaviors, body rocking back and forth movements, and continuous body spinning. Higher-order RRBs most commonly cover the behavior inflexibility and cognitive rigidity commonly found in disorders such as autism spectrum disorder and obsessive-compulsive disorder. Various neuropsychiatric disorders are plagued by RRBs yet no FDA-approved treatments have been identified. In rodents, lower-order RRBs are commonly measured through various tasks, such as repetitive self-grooming, marble burying, and stereotypic motor behaviors. This review focuses on the effects that modulation of specific serotonin receptors have on lower-order RRBs. Although there is research examining how changes in 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT6, and 5-HT7 receptor modulation, more research has focused on the 5-HT1A, 5-HT2A, and 5-HT2C receptors. The accumulating data suggest that increasing 5-HT1A activation decreases RRBs while blocking 5-HT1A activation has no effect on RRBs. While there are mixed findings regarding the impact of 5-HT2A modulation on RRBs, the general trend shows mixed effects of 5-HT2A receptor activation RRB expression, whereas blockade generally decreases RRBs. 5-HT2C receptor activation can modulate RRBs in either direction depending on the 5-HT2C drug used, blocking 5-HT2C activation only seems to show therapeutic properties when 5-HT2C activation is already elevated. The other 5-HT receptors have been explored far less but show promise as potential targets for regulating RRBs. Although it is less clear due to the involvement of 5-HT1D, 5-HT1A activation increases RRBs, and blocking 5-HT1A tends to decrease RRBs. 5-HT2B activation could reduce RRBs, while inhibiting 5-HT2B does not impact RRBs. Increasing 5-HT3 has not been shown to affect RRBs. Yet, increases in RRBs have been observed in Htr3a KO mice. 5-HT6 receptor activation can increase RRBs, while blocking 5-HT6 activity tends to decrease RRBs. Lastly, neither increasing or blocking 5-HT7 activity can reduce RRBs. In sum, there is no uniform pattern in whether all specific 5-HT receptors affect RRBs in either direction, instead, there is evidence suggesting that different 5-HT receptors can modulate RRBs in different directions. Further researching the less explored receptors and aiming to understand why these receptors can differently modulate RRBs, may play a key role in developing therapeutics that treat RRBs.

Meta-Chlorophenylpiperazine-Induced Behavioral Changes in Obsessive-Compulsive Disorder Research: A Systematic Review of Rodent Studies

Meta-chlorophenylpiperazine (mCPP) was one of the first compounds used in clinical and preclinical studies that demonstrated the role of serotonin in Obsessive-Compulsive Disorder (OCD). This systematic review aimed to (a) identify publications that report in rodents the effects of mCPP relevant to OCD, (b) explore the methodological characteristics of these studies, and (c) summarize the profile of mCPP effects. A comprehensive literature search was performed using PubMed, Scopus, and Web of Science. Search terms were a combination of obsessive-compulsive disorder or OCD and meta-chlorophenylpiperazine or mCPP. Twenty-nine articles were included in the review. The years of publication ranged from 1993 to 2021. Most studies used adult male Wistar or Sprague-Dawley rats. The most frequent dose of mCPP was 1.0 mg/kg administered acutely, intraperitoneally. In general, available preclinical evidence suggests increased defensive and compulsive behaviors associated with a decreased locomotor activity. But other results besides these and the absence of significant mCPP effects were also observed. Among the factors that may contribute to the variability of mCPP effects, differences in methods are highlighted, such as characteristics of the species/strains studied, mCPP doses and treatment regimens used. The heterogeneity of the OCD-like behaviors evaluated and the interaction of mCPP with different receptors may also be critical variables for discrepancies in the findings with mCPP. The information described in this review may contribute to a better understanding of how mCPP-induced behavioral changes in rodents have been used to study OCD, highlighting the main challenges for future investigations in this field.

The prefrontal cortex and OCD

Obsessive Compulsive Disorder (OCD) is a highly prevalent and severe neuropsychiatric disorder, with an incidence of 1.5-3% worldwide. However, despite the clear public health burden of OCD and relatively well-defined symptom criteria, effective treatments are still limited, spotlighting the need for investigation of the neural substrates of the disorder. Human neuroimaging studies have consistently highlighted abnormal activity patterns in prefrontal cortex (PFC) regions and connected circuits in OCD during both symptom provocation and performance of neurocognitive tasks. Because of recent technical advances, these findings can now be leveraged to develop novel targeted interventions. Here we will highlight current theories regarding the role of the prefrontal cortex in the generation of OCD symptoms, discuss ways in which this knowledge can be used to improve treatments for this often disabling illness, and lay out challenges in the field for future study.

Distinct behavioral traits and associated brain regions in mouse models for obsessive-compulsive disorder

BACKGROUND

Obsessive-compulsive disorder (OCD) is a mental disease with heterogeneous behavioral phenotypes, including repetitive behaviors, anxiety, and impairments in cognitive functions. The brain regions related to the behavioral heterogeneity, however, are unknown.

METHODS

We systematically examined the behavioral phenotypes of three OCD mouse models induced by pharmacological reagents [RU24969, 8-hydroxy-DPAT hydrobromide (8-OH-DPAT), and 1-(3-chlorophenyl) piperazine hydrochloride-99% (MCPP)], and compared the activated brain regions in each model, respectively.

RESULTS

We found that the mouse models presented distinct OCD-like behavioral traits. RU24969-treated mice exhibited repetitive circling, anxiety, and impairments in recognition memory. 8-OH-DPAT-treated mice exhibited excessive spray-induced grooming as well as impairments in recognition memory. MCPP-treated mice showed only excessive self-grooming. To determine the brain regions related to these distinct behavioral traits, we examined c-fos expression to indicate the neuronal activation in the brain. Our results showed that RU24969-treated mice exhibited increased c-fos expression in the orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), prelimbic cortex (PrL), infralimbic cortex (IL), nucleus accumbens (NAc), hypothalamus, bed nucleus of the stria terminalis, lateral division, intermediate part (BSTLD), and interstitial nucleus of the posterior limb of the anterior commissure, lateral part (IPACL), whereas in 8-OH-DPAT-treated mice showed increased c-fos expression in the ACC, PrL, IL, OFC, NAc shell, and hypothalamus. By contrast, MCPP did not induce higher c-fos expression in the cortex than control groups.

CONCLUSION

Our results indicate that different OCD mouse models exhibited distinct behavioral traits, which may be mediated by the activation of different brain regions.

Pharmacotherapeutic Strategies and New Targets in OCD

Effective pharmacological and psychotherapeutic treatments are well established for obsessive-compulsive disorder (OCD). Serotonin reuptake inhibitors (SRIs) are first-line treatment and are of benefit to about half of patients. Augmentation of SRI treatment with low-dose neuroleptics is an evidence-based second-line strategy. Specialty psychotherapy is also used as both first-line and second-line treatment and can benefit many. However, a substantial number of patients do not respond to these treatments. New alternatives are urgently needed. This review summarizes evidence for these established pharmacotherapeutic strategies, and for others that have been investigated in refractory disease but are not supported by the same level of evidence. We focus on three neurotransmitter systems in the brain: serotonin, dopamine, and glutamate. We summarize evidence from genetic, neuroimaging, animal model, and other lines of investigation that probe these three systems in patients with OCD. We also review recent work on predictors of response to current treatments. While many studies suggest abnormalities that may provide insight into the pathophysiology of the disorder, most studies have been small, and non-replication of reported findings has been common. Nevertheless, the gradual accrual of evidence for neurotransmitter dysregulation may in time lead the way to new pharmacological strategies.

The Psychopharmacology of Obsessive-Compulsive Disorder: A Preclinical Roadmap

This review evaluates current knowledge about obsessive-compulsive disorder (OCD), with the goal of providing a roadmap for future directions in research on the psychopharmacology of the disorder. It first addresses issues in the description and diagnosis of OCD, including the structure, measurement, and appropriate description of the disorder and issues of differential diagnosis. Current pharmacotherapies for OCD are then reviewed, including monotherapy with serotonin reuptake inhibitors and augmentation with antipsychotic medication and with psychologic treatment. Neuromodulatory therapies for OCD are also described, including psychosurgery, deep brain stimulation, and noninvasive brain stimulation. Psychotherapies for OCD are then reviewed, focusing on behavior therapy, including exposure and response prevention and cognitive therapy, and the efficacy of these interventions is discussed, touching on issues such as the timing of sessions, the adjunctive role of pharmacotherapy, and the underlying mechanisms. Next, current research on the neurobiology of OCD is examined, including work probing the role of various neurotransmitters and other endogenous processes and etiology as clues to the neurobiological fault that may underlie OCD. A new perspective on preclinical research is advanced, using the Research Domain Criteria to propose an adaptationist viewpoint that regards OCD as the dysfunction of a normal motivational system. A systems-design approach introduces the security motivation system (SMS) theory of OCD as a framework for research. Finally, a new perspective on psychopharmacological research for OCD is advanced, exploring three approaches: boosting infrastructure facilities of the brain, facilitating psychotherapeutic relearning, and targeting specific pathways of the SMS network to fix deficient SMS shut-down processes. SIGNIFICANCE STATEMENT: A significant proportion of patients with obsessive-compulsive disorder (OCD) do not achieve remission with current treatments, indicating the need for innovations in psychopharmacology for the disorder. OCD may be conceptualized as the dysfunction of a normal, special motivation system that evolved to manage the prospect of potential danger. This perspective, together with a wide-ranging review of the literature, suggests novel directions for psychopharmacological research, including boosting support systems of the brain, facilitating relearning that occurs in psychotherapy, and targeting specific pathways in the brain that provide deficient stopping processes in OCD.

Ketamine induces immediate and delayed alterations of OCD-like behavior

RATIONALE

Obsessive-compulsive disorder (OCD) is a psychiatric disorder characterized by intrusive obsessive thoughts and/or compulsive behaviors. Currently, serotonin reuptake inhibitors (SRIs) provide the only pharmacological monotherapy for OCD, but response rates are insufficient. Ketamine, a noncompetitive NMDA receptor antagonist, was reported to have rapid, sustained therapeutic effects in OCD patients. However, the mechanisms remain unknown.

OBJECTIVES

Here, we aimed to provide a platform for investigating mechanisms underlying anti-OCD effects of ketamine treatment by assessing whether ketamine pretreatment could alleviate 5-HT1B receptor (5-HT1BR)-induced OCD-like behavior in mice.

METHODS

We assessed whether acute ketamine (0, 3, 10, 30 mg/kg), administered at two pretreatment time points (30 min, 24 h), would modulate 5-HT1BR-induced OCD-like behavior in mice. Behavioral measures were perseverative hyperlocomotion in the open field and deficits in prepulse inhibition (PPI) induced by acute pharmacological 5-HT1BR challenge.

RESULTS

Three milligrams per kilogram of ketamine reduced 5-HT1BR-induced perseverative hyperlocomotion, but not PPI deficits, 24 h postinjection. In contrast, higher doses of ketamine were either ineffective (10 mg/kg) or exacerbated (30 mg/kg) 5-HT1BR-induced perseverative hyperlocomotion 30 min postinjection. At 24 h postinjection, 30 mg/kg ketamine reduced perseverative hyperlocomotion across all groups.

CONCLUSIONS

Our results suggest that the 5-HT1BR-induced model of OCD-like behavior is sensitive to a low dose of ketamine, a potential fast-acting anti-OCD treatment, and may provide a tool for studying mechanisms underlying the rapid therapeutic effects of ketamine in OCD patients.

Dissecting the roles of β-arrestin2 and GSK-3 signaling in 5-HT1BR-mediated perseverative behavior and prepulse inhibition deficits in mice

Serotonin-1B receptors (5-HT1BRs) modulate perseverative behaviors and prepulse inhibition (PPI) in humans and mice. These inhibitory G-protein-coupled receptors signal through a canonical G-protein-coupled pathway that is modulated by GSK-3β, and a noncanonical pathway mediated by the adaptor protein β-arrestin2 (Arrb2). Given the development of biased ligands that differentially affect canonical versus noncanonical signaling, we examined which signaling pathway mediates 5-HT1BR agonist-induced locomotor perseveration and PPI deficits, behavioral phenotypes observed in both obsessive-compulsive disorder (OCD) and autism spectrum disorder (ASD). To assess the role of canonical 5-HT1BR signaling, mice received acute pretreatment with a GSK-3 inhibitor (SB216763 or AR-A014418) and acute treatment with the 5-HT1A/1B receptor agonist RU24969 prior to assessing perseverative locomotor behavior in the open field, and PPI. To determine the role of noncanonical 5-HT1BR signaling, Arrb2 wild-type (WT), heterozygous (HT), and knockout (KO) mice received acute RU24969 treatment prior to behavioral testing. GSK-3 inhibition increased locomotor perseveration overall, and also failed to influence the RU24969-induced perseverative locomotor pattern in the open field. Yet, GSK-3 inhibition modestly reduced RU24969-induced PPI deficits. On the other hand, Arrb2 HT and KO mice showed reduced locomotion and no changes in perseveration overall, in addition to modest reductions in RU24969-induced locomotion and PPI deficits. In conclusion, our data do not support use of either GSK-3 inhibitors or β-arrestin2 inhibition in treatment of perseverative behaviors.

The 5-HT1B receptor - a potential target for antidepressant treatment

Major depressive disorder (MDD) is the leading cause of disability worldwide. The serotonin hypothesis may be the model of MDD pathophysiology with the most support. The majority of antidepressants enhance synaptic serotonin levels quickly, while it usually takes weeks to discern MDD treatment effect. It has been hypothesized that the time lag between serotonin increase and reduction of MDD symptoms is due to downregulation of inhibitory receptors such as the serotonin 1B receptor (5-HT1BR). The research on 5-HT1BR has previously been hampered by a lack of selective ligands for the receptor. The last extensive review of 5-HT1BR in the pathophysiology of depression was published 2009, and based mainly on findings from animal studies. Since then, selective radioligands for in vivo quantification of brain 5-HT1BR binding with positron emission tomography has been developed, providing new knowledge on the role of 5-HT1BR in MDD and its treatment. The main focus of this review is the role of 5-HT1BR in relation to MDD and its treatment, although studies of 5-HT1BR in obsessive-compulsive disorder, alcohol dependence, and cocaine dependence are also reviewed. The evidence outlined range from animal models of disease, effects of 5-HT1B receptor agonists and antagonists, case-control studies of 5-HT1B receptor binding postmortem and in vivo, with positron emission tomography, to clinical studies of 5-HT1B receptor effects of established treatments for MDD. Low 5-HT1BR binding in limbic regions has been found in MDD patients. When 5-HT1BR ligands are administered to animals, 5-HT1BR agonists most consistently display antidepressant-like properties, though it is not yet clear how 5-HT1BR is best approached for optimal MDD treatment.

How can preclinical mouse models be used to gain insight into prefrontal cortex dysfunction in obsessive-compulsive disorder?

Obsessive-compulsive disorder is a debilitating psychiatric disorder that is characterised by perseverative thoughts and behaviours. Cognitive and affective disturbances play a central role in this illness, and it is therefore not surprising that clinical neuroimaging studies have demonstrated widespread alterations in prefrontal cortex functioning in patients. Preclinical mouse experimental systems provide the opportunity to gain mechanistic insight into the neurobiological changes underlying prefrontal cortex dysfunction through new technologies that allow measurement and manipulation of activity in discrete neural populations in awake, behaving mice. However, recent preclinical research has focused on striatal dysfunction, and has therefore provided relatively little insight regarding the role of the prefrontal cortex in obsessive-compulsive disorder-relevant behaviours. Here, we will discuss a number of translational prefrontal cortex-dependent paradigms, including obsessive-compulsive disorder-relevant tasks that produce compulsive responding, and how they can be leveraged in this context. Drawing on recent examples that have led to mechanistic insight about specific genes, cell types and circuits that mediate prefrontal cortex contributions to distinct aspects of cognition, we will provide a framework for applying similar strategies to identify neural mechanisms underlying obsessive-compulsive disorder-relevant behavioural domains. We propose that research using clinically relevant paradigms will accelerate translation of findings from preclinical mouse models, thus supporting the development of novel therapeutics targeted to specific pathophysiological mechanisms.

Fear extinction in an obsessive-compulsive disorder animal model: Influence of sex and estrous cycle

Obsessive-compulsive disorder (OCD) is a neuropsychiatric condition that affects men and women equally, but with a sexually dimorphic pattern of development. Reproductive cycle events can influence symptom severity of OCD in females, indicating that ovarian hormones or their interaction with distinct neurotransmitter systems may play a role in OCD pathophysiology. Clinical studies and animal models have confirmed the importance of the serotonergic (5-HT) system in the neurobiology and treatment of OCD. Accordingly, the non-selective 5-HT2c agonist, meta-chlorophenylpiperazine (mCPP), exacerbates symptoms in untreated OCD patients. In rodents, it evokes repetitive behaviors that engage brain areas that are homologous with those found to be dysfunctional in OCD patients. These regions, including the medial prefrontal and anterior cingulate cortices, are also involved in fear inhibition, which is impaired in OCD. Here, we treated rats with mCPP (0.5 and 3.0 mg/kg) to evaluate its influence on self-grooming behavior and assess potential fear extinction retention deficits, taking into account sex differences and females' estrous cycle. We found that mCPP exacerbated grooming in male and female rats, irrespective of the estrous cycle phase. Fear extinction retention, however, was impaired only in females. Moreover, females undergoing fear extinction training during the metestrus/diestrus phases of the estrous cycle were more sensitive to the impairments induced by mCPP. Our results indicate that mCPP can induce OCD-like symptoms, exacerbating self-grooming and impairing fear extinction. It suggests that changes in 5-HT signaling through 5-HT2c receptors may have an important role in the OCD pathophysiology and that the influence of gonadal hormones in OCD should be further investigated.

Rodent models of obsessive compulsive disorder: Evaluating validity to interpret emerging neurobiology

Obsessive Compulsive Disorder (OCD) is a common neuropsychiatric disorder with unknown molecular underpinnings. Identification of genetic and non-genetic risk factors has largely been elusive, primarily because of a lack of power. In contrast, neuroimaging has consistently implicated the cortico-striatal-thalamo-cortical circuits in OCD. Pharmacological treatment studies also show specificity, with consistent response of OCD symptoms to chronic treatment with serotonin reuptake inhibitors; although most patients are left with residual impairment. In theory, animal models could provide a bridge from the neuroimaging and pharmacology data to an understanding of pathophysiology at the cellular and molecular level. Several mouse models have been proposed using genetic, immunological, pharmacological, and optogenetic tools. These experimental model systems allow testing of hypotheses about the origins of compulsive behavior. Several models have generated behavior that appears compulsive-like, particularly excessive grooming, and some have demonstrated response to chronic serotonin reuptake inhibitors, establishing both face validity and predictive validity. Construct validity is more difficult to establish in the context of a limited understanding of OCD risk factors. Our current models may help us to dissect the circuits and molecular pathways that can elicit OCD-relevant behavior in rodents. We can hope that this growing understanding, coupled with developing technology, will prepare us when robust OCD risk factors are better understood.

OCD is associated with an altered association between sensorimotor gating and cortical and subcortical 5-HT1b receptor binding

Obsessive-compulsive disorder (OCD) is characterized by impaired sensorimotor gating, as measured using prepulse inhibition (PPI). This effect may be related to abnormalities in the serotonin (5-HT) system. 5-HT1B agonists can impair PPI, produce OCD-like behaviors in animals, and exacerbate OCD symptoms in humans. We measured 5-HT1B receptor availability using (11)C-P943 positron emission tomography (PET) in unmedicated, non-depressed OCD patients (n=12) and matched healthy controls (HC; n=12). Usable PPI data were obtained from 20 of these subjects (10 from each group). There were no significant main effects of OCD diagnosis on 5-HT1B receptor availability ((11)C-P943 BPND); however, the relationship between PPI and (11)C-P943 BPND differed dramatically and significantly between groups. 5-HT1B receptor availability in the basal ganglia and thalamus correlated positively with PPI in controls; these correlations were lost or even reversed in the OCD group. In cortical regions there were no significant correlations with PPI in controls, but widespread positive correlations in OCD patients. Positive correlations between 5-HT1B receptor availability and PPI were consistent across diagnostic groups only in two structures, the orbitofrontal cortex and the amygdala. Differential associations of 5-HT1B receptor availability with PPI in patients suggest functionally important alterations in the serotonergic regulation of cortical/subcortical balance in OCD.

Clinically effective OCD treatment prevents 5-HT1B receptor-induced repetitive behavior and striatal activation

RATIONALE

Serotonin-1B receptor (5-HT1BR) agonist treatment induces obsessive-compulsive disorder (OCD)-like behaviors including locomotor stereotypy, prepulse inhibition deficits, and delayed alternation disruptions, which are selectively prevented by clinically effective OCD treatment. However, the role of 5-HT1BRs in modulating other repetitive behaviors or OCD-like patterns of brain activation remains unclear.

OBJECTIVES

We assessed the effects of 5-HT1BR agonism on digging, grooming, and open field behaviors in mice. We also quantified effects on neuronal activation in brain regions overactivated in OCD. Finally, we assessed whether effects of the 5-HT1BR challenge could be blocked by clinically effective, but not ineffective, drug treatments.

METHODS

Mice were tested in open field, dig, and splash tests after acute treatment with saline, 1, 3, 5, or 10 mg/kg RU24969 (5-HT1B/1A agonist). Behavioral effects of RU24969 were also tested following co-treatment with vehicle, 1 mg/kg WAY100635 (5-HT1A antagonist) and 5 or 10 mg/kg GR127935 (5HT1B/D antagonist). Separate mice were behaviorally assessed following chronic pretreatment with vehicle with 10 mg/kg fluoxetine or 20 mg/kg desipramine and acute treatment with saline or 10 mg/kg RU24969. Brains were analyzed for Fos expression in the orbitofrontal cortex, the dorsal striatum, and the cerebellum.

RESULTS

RU24969 induced robust locomotor stereotypy and decreased rearing, digging, and grooming. Effects were blocked by GR127935 but not by WAY100635. RU24969 also increased Fos expression in the dorsal striatum. Chronic fluoxetine, but not desipramine, alleviated 5-HT1BR-induced effects.

CONCLUSIONS

We report novel 5-HT1BR-induced behaviors and striatal activation that were alleviated only by clinically effective pharmacological OCD treatment. Studying the mechanisms underlying these effects could provide insight into OCD pathophysiology.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Activation of serotonin 5-HT2A receptors inhibits high compulsive drinking on schedule-induced polydipsia

RATIONALE

Schedule-induced polydipsia (SIP) is an established model for studying compulsive behaviour in rats. Serotoninergic drugs effectively reduce compulsive drinking on SIP, and high compulsive drinker rats selected by SIP have shown differences in serotoninergic brain activity. However, the specific serotoninergic receptors that modulate compulsive SIP remain unclear.

OBJECTIVE

We investigated the functional role of serotonin 5-hydroxytryptamine 2A or C (5-HT2A/C) receptors in compulsive SIP behaviour.

METHODS

Rats were selected for low (LD) versus high drinking (HD) behaviour on SIP. The effects of the systemic administration of the selective serotonin reuptake inhibitor citalopram, selective norepinephrine reuptake inhibitor atomoxetine, serotonin 5-HT2A/C receptor agonist DOI hydrochloride ((±)-2,5-dimethoxy-4-iodoamphetamine), serotonin 5-HT2C receptor antagonist SB242084, serotonin 5-HT2A receptor antagonist ketanserin and M100907 were assessed on SIP. Subsequently, the effects of DOI were tested after the pre-administration of SB242084, ketanserin and M100907 on SIP.

RESULTS

Citalopram and DOI reduced compulsive drinking in HD compared with LD rats on SIP. In contrast, SB242084 increased compulsive drinking in HD compared with LD rats on SIP. Atomoxetine, ketanserin and M100907 had no effect on SIP. The reduction in water intake produced by DOI was blocked by ketanserin and M100907, but not by SB242084 administration, in HD rats.

CONCLUSIONS

These findings highlight the contribution of serotoninergic 5-HT2A/C receptors compared with noradrenergic mechanisms on SIP and reveal the "therapeutic" activation of serotonin 5-HT2A in the inhibition of the compulsive drinking behaviour in HD rats. Thus, it may represent a potentially new marker of vulnerability and provides additional insight for potential treatments on compulsive behaviours in neuropsychiatric populations.

Separate mechanisms for development and performance of compulsive checking in the quinpirole sensitization rat model of obsessive-compulsive disorder (OCD)

RATIONALE

Acute administration of serotonergic agonist, meta-chlorophenylpiperazine (mCPP), attenuates performance of compulsive checking in an animal model of obsessive-compulsive disorder (OCD). It is not known whether mCPP has a similar effect on development of compulsive checking.

OBJECTIVES

The objective of the study was to examine whether similar mechanisms mediate the development versus the performance of compulsive checking in the rat model.

METHODS

Four groups of male rats (N = 14/group) were tested: two experimental groups co-injected with D2/D3 dopamine agonist quinpirole (0.25 mg/kg) and mCPP (0.625 mg/kg or 1.25 mg/kg), and two control groups, one co-injected with quinpirole and saline, the other receiving injections of saline. The time course of development of compulsive checking across injections 1 to 10 in quinpirole-treated rats was compared to rats co-injected with quinpirole and mCPP.

RESULTS

Results showed that during the course of chronic treatment, mCPP (1.25 mg/kg) significantly attenuated performance of checking behavior. However, when these rats were retested for expression of compulsive checking (that is, with an injection of quinpirole only), their profile of compulsive checking was no different from the control rats treated throughout with quinpirole only.

CONCLUSIONS

Findings show that mCPP inhibits performance of compulsive checking but does not block quinpirole from inducing the neural substrate underlying this compulsive behavior. Hence, a separate mechanism underlies the induction of compulsive checking and the performance of it. It is suggested that development of the OCD endophenotype reflects neuroplastic changes produced by repeated dopamine D2/D3 receptor stimulation, while stimulation of serotonergic receptors mediates a negative feedback signal that shuts down the motor performance of checking.

Compulsivity in anorexia nervosa: a transdiagnostic concept

The compulsive nature of weight loss behaviors central to anorexia nervosa (AN), such as relentless self-starvation and over-exercise, has led to the suggestion of parallels between AN and other compulsive disorders such as obsessive-compulsive disorder (OCD) and addictions. There is a huge unmet need for effective treatments in AN, which has high rates of morbidity and the highest mortality rate of any psychiatric disorder, yet a grave paucity of effective treatments. Viewing compulsivity as a transdiagnostic concept, seen in various manifestations across disorders, may help delineate the mechanisms responsible for the persistence of AN, and aid treatment development. We explore models of compulsivity that suggest dysfunction in cortico-striatal circuitry underpins compulsive behavior, and consider evidence of aberrancies in this circuitry across disorders. Excessive habit formation is considered as a mechanism by which initially rewarding weight loss behavior in AN may become compulsive over time, and the complex balance between positive and negative reinforcement in this process is considered. The physiological effects of starvation in promoting compulsivity, positive reinforcement, and habit formation are also discussed. Further research in AN may benefit from a focus on processes potentially underlying the development of compulsivity, such as aberrant reward processing and habit formation. We discuss the implications of a transdiagnostic perspective on compulsivity, and how it may contribute to the development of novel treatments for AN.

The 5-HT1D/1B receptor agonist sumatriptan enhances fear of simulated speaking and reduces plasma levels of prolactin

This study measured the effects of the preferential 5-HT1D/1B receptor agonist sumatriptan in healthy volunteers who performed the Simulated Public Speaking Test (SPST), which recruits the neural network involved in panic disorder and social anxiety disorder. In a double-blind, randomised experiment, 36 males received placebo (12), 50 mg (12) or 100 mg (12) of sumatriptan 2 h before the SPST. Subjective, physiological and hormonal measures were taken before, during and after the test. The dose of 100 mg of sumatriptan increased speech-induced fear more than either a 50mg dose of the drug or placebo. The largest dose of sumatriptan also enhanced vigilance more than placebo, without any change in blood pressure, heart rate or electrical skin conductance. Sumatriptan decreased plasma levels of prolactin. A significant but moderate increase in plasma cortisol after SPST occurred, independent of treatment. Because sumatriptan decreases 5-HT release into the extracellular space, the potentiation of SPST-induced fear caused by the drug supports the hypothesis that 5-HT attenuates this emotional state. As acute administration of antidepressants has also been shown to enhance speaking fear and increase plasma prolactin, in contrast to sumatriptan, the 5-HT regulation of stress-hormone release is likely to be different from that of emotion.

Effects of chronic fluoxetine treatment on serotonin 1B receptor-induced deficits in delayed alternation

RATIONALE

Obsessive-compulsive disorder (OCD) patients show overactivation of the orbitofrontal cortex and deficits in cognitive tasks that require proper orbitofrontal functioning including delayed alternation tests of spatial working memory. We recently showed that OCD-like behavior is induced in mice by activating orbitofrontal serotonin 1B receptors (5-HT1Bs). However, the role of 5-HT1Bs in delayed alternation remains unclear.

OBJECTIVES

We examined the effects of 5-HT1B receptor activation on delayed alternation task (DAT) performance. We also assessed the ability of an effective OCD treatment, fluoxetine, to prevent 5-HT1B-induced deficits in DAT performance.

METHODS

Mice were tested on the DAT after acute treatment with saline, 3 or 6 mg/kg RU24969 (5-HT1B/1A agonist), 0.3 or 3 mg/kg 8-OH-DPAT (5-HT1A agonist), or co-injection with 3 mg/kg RU24969 and 5 mg/kg GR127935 (5-HT1B/1D antagonist). Separate mice were pretreated chronically (28 days) with 10 mg/kg fluoxetine and then tested on the DAT after acute treatment with 3 mg/kg RU24969, 0.3 mg/kg 8-OH-DPAT, or saline.

RESULTS

Both doses of RU24969 decreased accuracy and increased latency on the DAT, and GR127935 blocked RU24969-induced effects on accuracy. The 0.3 mg/kg 8-OH-DPAT did not affect the DAT performance, whereas 3 mg/kg increased omissions on the DAT. Finally, RU24969-induced DAT deficits were absent in fluoxetine-pretreated mice.

CONCLUSIONS

We show that 5-HT1B receptor activation disrupts DAT performance in mice, and chronic fluoxetine pretreatment blocks these 5-HT1B-induced deficits. Our findings suggest that 5-HT1B receptors play an important role in modulating orbitofrontal-dependent delayed alternation. Moreover, 5-HT1B-induced DAT deficits may provide a mouse model for DAT deficits in OCD.

Effects of the serotonergic agonist mCPP on male rats in the quinpirole sensitization model of obsessive-compulsive disorder (OCD)

RATIONALE

The serotonergic agonist, meta-chlorophenylpiperazine (mCPP), produces inconsistent effects on obsessive-compulsive disorder (OCD) symptoms, perhaps because clinical studies have not utilized a homogenous OCD subgroup of patients.

OBJECTIVES

This study aimed to evaluate mCPP effects on functional components of compulsive checking, using the quinpirole sensitization rat model of OCD.

METHODS

In study 1, the effects of mCPP were evaluated in quinpirole rats with compulsive checking. Two experimental groups were co-injected with quinpirole (0.125 mg/kg) and mCPP (0.625 or 1.25 mg/kg), while one control group was co-injected with quinpirole (0.125 mg/kg) and saline and the other control group received co-injections of saline. In study 2, mCPP (0, 0.3125, 0.625, and 1.25 mg/kg) was administered repeatedly to naïve rats and induction of compulsive checking evaluated.

RESULTS

mCPP significantly attenuated quinpirole-induced compulsive checking behavior by reducing vigor of checking (indexed by frequency of checking and length of check) and increasing rest after a bout of checking (indexed by time to the next checking bout), but it did not affect focus on the task of checking (indexed by recurrence time of checking and number of stops before returning to check). In naïve rats, mCPP did not induce compulsive behavior, but the highest dose reduced vigor of checking performance compared to saline controls.

CONCLUSIONS

mCPP did not exacerbate or induce compulsive checking behavior. Instead, it ameliorated compulsive checking by reducing vigor of checking and increasing post-checking satiety, without affecting focus on checking. Ameliorative effects of mCPP may involve 5HT2A/2C receptors in substantia nigra pars reticulata that inhibit expression of motor vigor.

Neurotrophic factors in obsessive-compulsive disorder

In this cross-sectional study, we assessed the levels of neurotrophins (NF) of patients with obsessive-compulsive disorder (OCD) in different stages of treatment and their relationship with OCD clinical features. Forty patients with OCD and 40 healthy controls had Brain-Derived Neurotrophic Factor (BDNF), Nerve Growth Factor (NGF), and Glial Cell-Derived Neurotrophic Factor (GNDF) plasma levels measured by enzyme-linked immunosorbent assay (ELISA). Patients with OCD were further examined with the Obsessive-Compulsive Inventory-Revised, the Beck Depression Inventory, the Beck Anxiety Inventory, and the Sheehan Disability Scale (SDS). Patients with OCD exhibited significantly lower levels of BDNF and significantly increased levels of NGF as compared to healthy controls. In OCD, statistically significant negative correlations between BDNF levels and number of working days lost per week were found. Additional analyses revealed a statistically significant positive correlation between both NGF and GDNF and severity of washing symptoms. Plasma levels of NF were not affected by age, age at OCD onset, gender, major depressive disorder, the relative dose of serotonin-reuptake inhibitors being prescribed, or the use of antipsychotics. Our findings suggest that patients with OCD may exhibit a particular NF profile, with functional impairment correlating with BDNF levels and severity of washing symptoms correlating with NGF and GDNF levels.

Compulsive features in behavioural addictions: the case of pathological gambling

AIMS

To describe, in the context of DSM-V, how a focus on addiction and compulsion is emerging in the consideration of pathological gambling (PG).

METHODS

A systematic literature review of evidence for the proposed re-classification of PG as an addiction.

RESULTS

Findings include: (i) phenomenological models of addiction highlighting a motivational shift from impulsivity to compulsivity associated with a protracted withdrawal syndrome and blurring of the ego-syntonic/ego-dystonic dichotomy; (ii) common neurotransmitter (dopamine, serotonin) contributions to PG and substance use disorders (SUDs); (iii) neuroimaging support for shared neurocircuitries between 'behavioural' and substance addictions and differences between obsessive-compulsive disorder (OCD), impulse control disorders (ICDs) and SUDs; (iv) genetic findings more closely related to endophenotypic constructs such as compulsivity and impulsivity than to psychiatric disorders; (v) psychological measures such as harm avoidance identifying a closer association between SUDs and PG than with OCD; (vi) community and pharmacotherapeutic trials data supporting a closer association between SUDs and PG than with OCD. Adapted behavioural therapies, such as exposure therapy, appear applicable to OCD, PG or SUDs, suggesting some commonalities across disorders.

CONCLUSIONS

PG shares more similarities with SUDs than with OCD. Similar to the investigation of impulsivity, studies of compulsivity hold promising insights concerning the course, differential diagnosis and treatment of PG, SUDs, and OCD.

Essential role for orbitofrontal serotonin 1B receptors in obsessive-compulsive disorder-like behavior and serotonin reuptake inhibitor response in mice

BACKGROUND

Perseveration and sensorimotor gating deficits are core features of obsessive-compulsive disorder (OCD). Serotonin 1B receptor (5-HT1BR) agonists exacerbate OCD symptoms in patients and induce perseveration and sensorimotor gating deficits in mice. Serotonin reuptake inhibitors (SRIs), but not noradrenaline reuptake inhibitors (NRIs), reduce OCD symptoms following 4 to 8 weeks of treatment. Using mice, we compared the effects of chronic SRI versus NRI treatment on 5-HT1BR-induced OCD-like behavior and 5-HT1BR sensitivity in orbitofrontal-subcortical OCD circuits. Furthermore, we localized the 5-HT1BR population that mediates OCD-like behavior.

METHODS

Mice chronically received the SRI clomipramine or the NRI desipramine and were examined for 5-HT1BR-induced OCD-like behavior or 5-HT1BR binding and G-protein coupling in caudate putamen, nucleus accumbens, and orbitofrontal cortex. Separate mice were tested for OCD- or depression-like behavior following 4, 14, 21, 28, or 56 days of SRI treatment. Finally, OCD-like behavior was assessed following intra-orbitofrontal 5-HT1BR agonist infusion or intra-orbitofrontal 5-HT1BR antagonist infusion coupled with systemic 5-HT1BR agonist treatment.

RESULTS

Effective, but not ineffective, OCD treatments reduced OCD-like behavior in mice with a time course that parallels the delayed therapeutic onset in OCD patients and downregulated 5-HT1BR expression in the orbitofrontal cortex. Intra-orbitofrontal 5-HT1BR agonist infusion induced OCD-like behavior, and intra-orbitofrontal 5-HT1BR antagonist infusion blocked OCD-like effects of systemic 5-HT1BR agonist treatment.

CONCLUSIONS

These results indicate that orbitofrontal 5-HT1BRs are necessary and sufficient to induce OCD-like behavior in mice and that SRI pharmacotherapy reduces OCD-like behavior by desensitizing orbitofrontal 5-HT1BRs. Our findings suggest an essential role for orbitofrontal 5-HT1BRs in OCD pathophysiology and treatment.

Glutamate abnormalities in obsessive compulsive disorder: neurobiology, pathophysiology, and treatment

Obsessive compulsive disorder is prevalent, disabling, incompletely understood, and often resistant to current therapies. Established treatments consist of specialized cognitive-behavioral psychotherapy and pharmacotherapy with medications targeting serotonergic and dopaminergic neurotransmission. However, remission is rare, and more than a quarter of OCD sufferers receive little or no benefit from these approaches, even when they are optimally delivered. New insights into the disorder, and new treatment strategies, are urgently needed. Recent evidence suggests that the ubiquitous excitatory neurotransmitter glutamate is dysregulated in OCD, and that this dysregulation may contribute to the pathophysiology of the disorder. Here we review the current state of this evidence, including neuroimaging studies, genetics, neurochemical investigations, and insights from animal models. Finally, we review recent findings from small clinical trials of glutamate-modulating medications in treatment-refractory OCD. The precise role of glutamate dysregulation in OCD remains unclear, and we lack blinded, well-controlled studies demonstrating therapeutic benefit from glutamate-modulating agents. Nevertheless, the evidence supporting some important perturbation of glutamate in the disorder is increasingly strong. This new perspective on the pathophysiology of OCD, which complements the older focus on monoaminergic neurotransmission, constitutes an important focus of current research and a promising area for the ongoing development of new therapeutics.

Animal models of obsessive-compulsive disorder: exploring pharmacology and neural substrates

During the last 30 years there have been many attempts to develop animal models of obsessive-compulsive disorder (OCD). Most models have not been studied further following the original publication, and in the past few years, most papers present studies employing a few established animal models, exploring the neural basis of compulsive behavior and developing new treatment strategies. Here we summarize findings from the five most studied animal models of OCD: 8-OHDPAT (8-hydroxy-2-(di-n-propylamino)-tetralin hydrobromide) induced decreased alternation, quinpirole-induced compulsive checking, marble burying, signal attenuation and spontaneous stereotypy in deer mice. We evaluate each model's face validity, derived from similarity between the behavior in the model and the specific symptoms of the human condition, predictive validity, derived from similarity in response to treatment (pharmacological or other), and construct validity, derived from similarity in the mechanism (physiological or psychological) that induces behavioral symptoms and in the neural systems involved. We present ideas regarding future clinical research based on each model's findings, and on this basis, also emphasize possible new approaches for the treatment of OCD.

Brain regional α-[11C]methyl-L-tryptophan trapping in medication-free patients with obsessive-compulsive disorder

CONTEXT

The hypothesis of a serotonin (5-hydroxytryptamine [5-HT]) dysfunction in obsessive-compulsive disorder (OCD) stems largely from the clinical efficacy of 5-HT reuptake inhibitors. Serotonergic abnormalities in the unmedicated symptomatic state, however, remain to be fully characterized.

OBJECTIVE

To investigate brain regional 5-HT synthesis, as indexed by positron emission tomography and the α-[(11)C]methyl-L-tryptophan trapping constant (K*), in treatment-free adults meeting criteria for OCD.

DESIGN

Between-group comparison.

SETTING

Department of Psychiatry and Montreal Neurological Institute, McGill University, and Department of Psychology, McGill University Health Centre, Quebec, Canada.

PARTICIPANTS

Twenty-one medication-free patients with OCD (15 men with a mean [SD] age of 33.2 [9.3] years and 6 women with a mean [SD] age of 35.8 [7.1] years) and 21 healthy controls matched for age and sex (15 men with a mean [SD] age of 32.9 [10.1] years and 6 women with a mean [SD] age of 36.5.5 [8.6] years). Main Outcome Measure The α-[(11)C]methyl-L-tryptophan brain trapping constant K*, which was analyzed with Statistical Parametric Mapping (SPM8) and with proportional normalization (extent threshold of 100 voxels with a peak threshold of P ≤ .005).

RESULTS

Compared with healthy controls, the patients with OCD exhibited significantly greater α-[(11)C]methyl-L-tryptophan trapping in the right hippocampus and left temporal gyrus (Brodmann area 20). In the larger subsample of all men, these same differences were also evident, as well as higher K* values in the caudate nucleus. Individual differences in symptom severity correlated positively with K* values sampled from the caudate and temporal lobe of the patients with OCD, respectively. There were no regions where the patients exhibited abnormally low K* values. Volumetric analyses found no morphometric alterations that would account for the group differences.

CONCLUSION

The results support previous reports of greater striatal and temporal lobe activity in patients with OCD than in healthy controls and suggest that these disturbances include a serotonergic component. Previously reported glucose metabolic disturbances in OCD involving the orbitofrontal and cingulate cortices, in comparison, might reflect postsynaptic changes in the serotonergic system.

The relationship between impulse-control disorders and obsessive-compulsive disorder: a current understanding and future research directions

Impulse-control disorders (ICDs) constitute a heterogeneous group of conditions linked diagnostically by difficulties in resisting "the impulse, drive, or temptation to perform an act that is harmful to the person or to others." Specific ICDs share clinical, phenomenological and biological features with obsessive-compulsive disorder (OCD) that have suggested that these disorders might be categorized together. However, other data suggest significant differences between OCD and ICDs. In this article, clinical, phenomenological and biological features of the formal ICDs are reviewed and compared and contrasted with those of OCD. Available data indicate substantial differences between ICDs and OCD that suggest independent categorizations. Existing research gaps are identified and avenues for future research suggested.

Assessing the validity of current mouse genetic models of obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD is a major cause of disability; however, the genetic factors and pathophysiological mechanisms underlying this complex, heterogeneous disorder remain largely unknown. During the past decade, a number of putative mouse genetic models of OCD have been developed for the purpose of studying the neural mechanisms underlying this disorder and developing novel treatments. This review presents and evaluates these experimental preparations to date. Models using knockout or transgenic approaches, as well as those examining variation in genetically diverse populations, are evaluated and discussed.

Chronic reductions in serotonin transporter function prevent 5-HT1B-induced behavioral effects in mice

BACKGROUND

Obsessive-compulsive disorder (OCD) is characterized by intrusive thoughts, images, or impulses and/or repetitive stereotypical behavior. Obsessive-compulsive disorder patients exhibit reduced prepulse inhibition (PPI) and symptom exacerbation after challenge with 5-HT1B receptor agonists. Recently, gain-of-function alleles of the serotonin transporter (5-HTT) have been associated with OCD. We tested the hypothesis that reducing 5-HTT function chronically, either genetically or via serotonin reuptake inhibitor (SRI) treatment, attenuates PPI deficits and perseverative hyperlocomotion induced by 5-HT1B agonists in mice.

METHODS

Mice received subchronic or chronic pretreatment with the SRI fluoxetine and acute treatment with RU24969 (5-HT1A/1B agonist) or 8-OH-DPAT (5-HT1A agonist) and were assessed for PPI, locomotor activity, and spatial patterns of locomotion. The same measures were evaluated in 5-HTT wild-type (WT), heterozygous (HT), and knockout (KO) mice after RU24969 treatment. The effects of WAY100635 (5-HTA antagonist) or GR127935 (5-HT1B/D antagonist) pretreatment on RU24969-induced effects were evaluated. Finally, 5-HT1B binding and functional coupling were assessed in 5-HTT-WT, -HT, and -KO mice, and normal fluoxetine-treated mice.

RESULTS

Chronic, but not subchronic, fluoxetine treatment prevented RU24969-induced PPI deficits and perseverative hyperlocomotion. These RU24969-induced effects were mediated via 5-HT1B and not 5-HT1A receptors. 5-HTT-KO mice showed no effects of RU24969, and 5-HTT-HT mice exhibited intermediate phenotypes. 5-HT1B binding and functional coupling were reduced in the globus pallidus and substantia nigra of 5-HTT-KO mice.

CONCLUSIONS

Our results demonstrate that chronic, but not subchronic, fluoxetine treatment and 5-HTT knockout robustly attenuate 5-HT1B agonist-induced PPI deficits and perseverative hyperlocomotion. These results may have implications for the etiology and treatment of OCD.

Short onset of action of a serotonin reuptake inhibitor when used to reduce premenstrual irritability

Several studies suggest that serotonin reuptake inhibitors (SRIs) exert a more rapid effect when used for the treatment of symptoms such as anger and irritability then when used for depression, obsessive-compulsive disorder, or anxiety. In line with this, premenstrual irritability can be effectively dampened by intermittent administration of an SRI, from ovulation to menstruation, indicating an onset of action of 10 days or less. How fast this effect appears, in terms of hours or days, is of considerable theoretical interest, but has previously not been studied in detail. To explore this issue, 22 women with marked premenstrual irritability, who previously had responded to paroxetine, were given this compound during two menstrual cycles and placebo during one cycle in a double-blind, cross-over fashion. The women were asked to start medication in the midst of the luteal phase when irritability had been intense for 2 days. The paroxetine cycles differed significantly from the placebo cycle as early as 14 h after drug intake with respect to the number of subjects experiencing sustained reduction in irritability. When the different cycles were compared with respect to irritability-rating scores for each time of assessment, the difference was significant at day 3. The side effect nausea had an even more rapid onset (4 h), but usually disappeared within 4 days. To summarize, this controlled trial shows that an SRI reduces premenstrual irritability already within a few days after the onset of treatment.

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.

The role of 5-HT2A and 5-HT2C receptors in the signal attenuation rat model of obsessive-compulsive disorder

Serotonin 5-HT2A and 5-HT2C receptors have been implicated in the pathophysiology of obsessive-compulsive disorder (OCD) and in the mechanism mediating the anti-compulsive effects of serotonin reuptake inhibitors. Yet it is currently unclear whether activation or blockade of these receptors would have an anti-compulsive effect. The present study tested the effects of 5-HT2A and 5-HT2C activation and blockade in the signal attenuation rat model of OCD. In this model, 'compulsive' behaviour is induced by attenuating a signal indicating that a lever-press response was effective in producing food. Experiments1-4 revealed that systemic administration of the 5-HT2C antagonist RS 102221 (2 mg/kg) selectively decreases compulsive lever-pressing, whereas systemic administration of the 5-HT2A antagonist MDL11,939(0.2-5 mg/kg) or of the 5-HT2A/2C agonist DOI (0.05-5 mg/kg) did not have a selective effect on this behaviour. Experiments 5 and 6 found that systemic co-administration of DOI (0.5 mg/kg) withMDL11,939 (1 mg/kg) or with RS 102221 (2 mg/kg) had a non-selective effect on lever-press responding,with the former manipulation increasing and the latter manipulation decreasing lever-pressing. Finally,experiment 7 demonstrated that administration of RS 102221 directly into the orbitofrontal cortex also exerts an anti-compulsive effect. The results of these experiments suggest that blockade of 5-HT2Creceptors may have an anti-compulsive effect in OCD patients, and that this effect may be mediated by5-HT2C receptors within the orbitofrontal cortex.

Stereotypic behaviour in the deer mouse: pharmacological validation and relevance for obsessive compulsive disorder

Stereotypy is an important manifestation of obsessive compulsive disorder (OCD). OCD involves disturbed serotonin and dopamine pathways, and demonstrates a selective response to serotonin reuptake inhibitors (SRI), with limited to no response to noradrenaline reuptake inhibitors (NRI). Deer mice (Peromyscus maniculatus bairdii) engage in various spontaneous stereotypic behaviours, including somersaulting, jumping and pattern running, and has to date not been explored for possible relevance for OCD. We studied the population diversity of spontaneous stereotypy in these animals, followed by assessing behavioural response to chronic high and low dose SRI (viz. fluoxetine) and NRI (viz. desipramine) treatment (both 10 mg/kg; 20 mg/kg x 21 days). We also studied behavioural responses to the 5-HT(2A/C) agonist, meta-chlorophenylpiperazine (mCPP) and the D2 agonist, quinpirole (2 mg/kg and 5 mg/kg respectively x 4 days). Deer mice showed a distinct separation into high and low stereotypic behaviour populations, with high and low dose fluoxetine, but not desipramine, significantly reducing stereotypic behaviour in both populations. A significant attenuation of stereotypy was also observed in both groups following quinpirole or mCPP challenge. In its response to drug treatment, spontaneous stereotypic behaviour in deer mice demonstrates predictive validity for OCD. States of spontaneous stereotypy are attenuated by 5-HT(2A/C) and dopamine D2 receptor agonists.

Acute hormonal changes after IV citalopram and treatment response in OCD

RATIONALE

Serotonergic pharmacological challenges have failed to produce consensual results in patients with obsessive-compulsive disorder (OCD), suggesting a heterogeneous 5-hydroxytryptamine (5-HT) activity in this disorder.

OBJECTIVES

The aim of this study was to compare the neuroendocrine response to a serotonergic challenge in OCD patient responders (RP) and nonresponders (NR) to serotonin reuptake inhibitors treatment and healthy volunteers.

MATERIALS AND METHODS

Thirty OCD treatment NR, 30 RP, and 30 controls (CN) matched for sex and age were included. Each subject received 20 mg of intravenous citalopram. Prolactin, cortisol, and growth hormone plasma concentration were measured at times-20, 0, 20, 40, 60, 80, 100, 120, 140, and 160 min after the onset of citalopram infusion.

RESULTS

Citalopram did not induce anxiety or OCD symptoms in patients. Citalopram was associated with stronger prolactin response in the CN group (maximal percentage variation [max%Delta] = 65.76 +/- 105.1) than in NR (max%Delta = 17.41 +/- 31.06) and RP groups (max%Delta = 15.87 +/- 31.71; p = 0.032; Friedman chi (2) = 6.87; df = 2). On the other hand, cortisol response did not differ between CN and RP groups and was blunted in the NR group (NR max%Delta = 20.98 +/- 58.14 vs RP max%Delta = 47.69 +/- 66.94; CN max%Delta = 63.58 +/- 88.4; p = 0.015; Friedman chi (2) = 8.60; df = 2).

CONCLUSIONS

Compared to CN, both treatment RP and NR patients showed blunted prolactin response to citalopram, but only NR patients showed an attenuated cortisol response, suggesting a more disrupted central serotonergic transmission in this group.

The genetic studies of obsessive-compulsive disorder and its future directions

Obsessive-compulsive disorder (OCD) is characterized by recurrent and persistent thoughts (obsessions), and repetitive behaviors or mental acts (compulsions). In Korea, an epidemiological study reported that the lifetime prevalence of OCD in the population was greater than two percent. The exact cause of OCD is still unknown. Evidence from familial, twin and segregation studies supports the role of a genetic component in the etiology of OCD. In addition, there is growing evidence that OCD has a specific neurochemical and neuroanatomical basis. According to this evidence, researchers have selected various candidate genes which have been implicated in the neurophysiology of OCD, and differences of allelic variants in OCD patients and controls have been analyzed. In this review we will introduce the results of previous genetic studies of OCD which have been performed in other populations, including twin studies, family studies, segregation analyses, linkage analyses, and association studies. In addition to these studies, we will present the results of our genetic studies of OCD performed in Korea.

The current status of association studies in obsessive-compulsive disorder

During the last 2 decades, a large number of association studies have been dedicated to disentangling the genetic components that may be involved in the etiology of OCD. The preliminary and frequently in consistent nature of the data represented in the majority of OCD psychiatric genetic-association studies may seem discouraging. Failure to replicate, and thus to confirm, previously identified susceptibility loci could result from a number of reasons, including the potential for population admixture, the clinical heterogeneity of OCD, small sample sizes (and subsequent lack of power),publication bias, epistasis, or failure to account for multiple testing. Various methods of accounting for these confounders do exist and should be implemented in any genetic-association study that is to be regarded as robust and replicable. Discrepancy between results, however, might be ascribed to the underlying genetic differences between the populations in the respective studies (ie, the investigated variant may be in linkage disequilibrium with the causal variant in one population but not in another). Such discrepancies are difficult to reconcile in single-locus association studies; haplotype analyses(in which a number of variants, usually single-nucleotide polymorphisms occurring on the same gene, are analyzed as a unit) may be able to resolve these uncertainties. Investigating epistatic interactions between variants in other genes that might be involved in the same physiologic pathways would be an alternative means of deciphering the reason for discrepant genetic association results.A valid means of increasing the power (by reducing background noise)would be to stratify the patient sample according to clinically defined sub-types, such as obsession and compulsion subtypes, age at onset of the disorder, and severity of the disorder. Although many of the OCD genetics studies have incorporated investigations of these subtypes [65,66,68,77,84-86,89,107,118,132,133,145,148,149], the number of subjects decreases after stratification, thereby limiting the power of the studies. It may therefore be useful to employ other quantitative approaches in the design of the investigation: the possibility should be considered that OCD symptoms can be broken down into multiple dimensions that are continuous with the normal population [150]. This division would represent an important route to disentangling the complex inheritance of OCD. The results obtained from genetic investigations should be incorporated with clinical and epidemiologic parameters to elucidate correctly the cause of OCD. Future studies should also be extended to incorporate the screening of more polymorphisms, because high-resolution mapping within specific chromosomes will improve knowledge regarding the impact of genetic diversity within the genes or linked chromosomal regions in OCD. The advantages ofa gene-based over a single-nucleotide polymorphism based approach are becoming ever more apparent [151]. Therefore, a more complete assessment of candidate genes, possibly using haplotype blocks that span larger regions,is proposed. In addition, increasing the amount of information on human genome sequences and polymorphisms will make it possible to characterize the amount of sequence variation expressed in the brain and to delineate the potential effects that these variations may have on the development of OCD. Knowledge of new functional variants will emerge as researchers gain an understanding of the potential for genetic variants in the coding and regulatory regions to impact gene expression.