‘Compulsive’ lever-pressing in rats is attenuated by the serotonin re-uptake inhibitors paroxetine and fluvoxamine but not by the tricyclic antidepressant desipramine or the anxiolytic diazepam

Opening new vistas on obsessive-compulsive disorder with the observing response task

Obsessive-compulsive disorder (OCD), a highly prevalent and debilitating disorder, is incompletely understood in terms of underpinning behavioural, psychological, and neural mechanisms. This is attributable to high symptomatic heterogeneity; cardinal features comprise obsessions and compulsions, including clinical subcategories. While obsessive and intrusive thoughts are arguably unique to humans, dysfunctional behaviours analogous to those seen in clinical OCD have been examined in nonhuman animals. Genetic, ethological, pharmacological, and neurobehavioural approaches all contribute to understanding the emergence and persistence of compulsive behaviour. One behaviour of particular interest is maladaptive checking, whereby human patients excessively perform checking rituals despite these serving no purpose. Dysfunctional and excessive checking is the most common symptom associated with OCD and can be readily operationalised in rodents. This review considers animal models of OCD, the neural circuitries associated with impairments in habit-based and goal-directed behaviour, and how these may link to the compulsions observed in OCD. We further review the Observing Response Task (ORT), an appetitive instrumental learning procedure that distinguishes between functional and dysfunctional checking, with translational application in humans and rodents. By shedding light on the psychological and neural bases of compulsive-like checking, the ORT has potential to offer translational insights into the underlying mechanisms of OCD, in addition to being a platform for testing psychological and neurochemical treatment approaches.

Bidirectional Behavioral Selection in Mice: A Novel Pre-clinical Approach to Examining Compulsivity

Obsessive-compulsive disorder (OCD) and related disorders (OCRD) is one of the most prevalent neuropsychiatric disorders with no definitive etiology. The pathophysiological attributes of OCD are driven by a multitude of factors that involve polygenic mechanisms, gender, neurochemistry, physiological status, environmental exposures and complex interactions among these factors. Such complex intertwining of contributing factors imparts clinical heterogeneity to the disorder making it challenging for therapeutic intervention. Mouse strains selected for excessive levels of nest- building behavior exhibit a spontaneous, stable and predictable compulsive-like behavioral phenotype. These compulsive-like mice exhibit heterogeneity in expression of compulsive-like and other adjunct behaviors that might serve as a valuable animal equivalent for examining the interactions of genetics, sex and environmental factors in influencing the pathophysiology of OCD. The current review summarizes the existing findings on the compulsive-like mice that bolster their face, construct and predictive validity for studying various dimensions of compulsive and associated behaviors often reported in clinical OCD and OCRD.

The Differential Effects of the Amount of Training on Sensitivity of Distinct Actions to Reward Devaluation

Shifting between goal-directed and habitual behaviors is essential for daily functioning. An inability to do so is associated with various clinical conditions, such as obsessive-compulsive disorder (OCD). Here we developed a new behavioral model in mice allowing us to produce and examine the development of different behaviors under goal-directed or habitual control. By using overtraining of instrumental associations between two levers and two rewards, and later devaluating one of the rewards, we differentiate and explore the motivational control of behaviors within the task which consequentially promotes what seems like excessive irrational behavior. Using our model, we found that the ability of instrumental behavior, to adapt to a change in the value of a known reward, is a function of practice. Once an instrumental action was practiced extensively it becomes habitual and, thus, under S-R control and could not be amended, not even when resulting in a noxious outcome. However, direct consummatory or Pavlovian actions, such as licking or checking, responds immediately to the change in value. This imbalance could render an instrumental behavior excessive and unresponsive to changes in outcome while the direct change in consumption implies that the change was in fact registered. This could suggest a system that, when out of balance, can create excessive behaviors, not adapting to an acknowledged change.

Neurocognitive Endophenotypes of OCD

In this chapter, I address the concept of endophenotypes for obsessive-compulsive disorder (OCD). Endophenotypes are objective and heritable quantitative traits hypothesized to be more biologically tractable than distal clinical phenotypes. This approach has been adopted to gain a better understanding of psychiatric conditions in general. It is theorized that endophenotypes will particularly assist in clarifying both the diagnostic status and aetiological origins of complex neuropsychiatric conditions such as OCD. At the cognitive level, separable constructs of relevance for OCD have been identified. The prevailing model for OCD assumes the development of abnormalities within fronto-striatal neural circuits leading to impairment of executive functions and their neuropsychological subcomponents. Here, I address whether this model can guide towards the identification of endophenotypes for this condition and discuss possible implications.

Chemical profiles, pharmacological properties, and in silico studies provide new insights on Cycas pectinata

The current study aimed to qualitatively and quantitatively determine the phytochemical components of Cycas pectinata methanol extract (MECP), along with its antioxidant, anti-inflammatory, thrombolytic, locomotor, anxiolytic, analgesic, and antidiarrheal activities. The in vitro antioxidant activity was evaluated by DPPH scavenging assay and the total phenol and total flavonoid contents, while the anti-inflammatory activity was evaluated by a protein denaturation assay. The in vivo locomotor effects were examined using the open field test and hole-cross test. The anxiolytic effect was examined using the elevated plus maze (EPM) test, hole-board test (HBT), and light-dark test (LDT), while the analgesic activity was investigated using the acetic acid-induced writhing test. The antidiarrheal effect was evaluated by castor oil-induced diarrhea and gastrointestinal motility. Ten bioactive compounds were selected on the basis of their biological activities and further investigated using in silico molecular docking simulation to correlate with the identified pharmacological properties. Additionally, the ADME properties of the compounds were evaluated according to their drug-likeness profile. MECP had a maximum total phenol content of 209.85 ± 3.40 gallic acid equivalents/g extract and a total flavonoid content of 105.17 ± 3.45 quercetin equivalents/g extract, with an IC50 value of 631.44 μg/mL. MECP (62.5-500 μg/mL) elicited 20.96-38.12% decreased protein denaturation compared to diclofenac sodium (65.40-83.50%), while a 35.72% (P < 0.001) clot lysis activity was observed for the 10 mg/mL concentration. MECP induced a dose-dependent reduction in locomotor activity, with a significant anxiolytic effect. In the analgesic test, MECP (200, 400 mg/kg) showed a 45.12% and 58.82% inhibition in analgesia, and the 400 mg/kg dose elicited a 27.5% inhibition in intestinal motility. These findings suggest that MECP might be effective in treating antioxidant, anti-inflammatory, and neuropharmacological defects, but this requires further study.

Dual-mode dopamine increases mediated by 5-HT1B and 5-HT2C receptors inhibition, inducing impulsive behavior in trained rats

Patients with eating disorders exhibit problems with appetitive impulse control. Interactions between dopamine and serotonin (5-HT) neuron in this setting are poorly characterized. Here we examined 5-HT receptor-mediated changes in extracellular dopamine during impulsive appetitive behavior in rats. Rats were trained to perform a cued lever-press (LP) task for a food reward such that they stopped experiencing associated dopamine increases. Trained rats were administered the mixed 5-HT1B/2C-receptor antagonist metergoline, the 5-HT2A/2C-receptor antagonist ketanserin, and p-chlorophenylalanine (PCPA). We measured dopamine changes in the ventral striatum using voltammetry and examined the number of premature LPs, reaction time (RT), and reward acquisition rate (RAR). Compared with controls, metergoline increased premature LPs and shortened RT significantly; ketanserin decreased premature LPs and lengthened RT significantly; and PCPA decreased premature LPs, lengthened RT, and decreased RAR significantly. Following metergoline administration, rats exhibited a fast phasic dopamine increase for 0.25-0.75 s after a correct LP, but only during LP for an incorrect LP. No dopamine increases were detected with ketanserin or PCPA, or in controls. After LP task completion, metergoline also caused dopamine to increase slowly and remain elevated; in contrast, ketanserin caused dopamine to increase slowly and decrease rapidly. No slow dopamine increase occurred with PCPA. Inhibition of 5-HT1B- and 5-HT2C-receptors apparently induced dual modes of extracellular dopamine increase: fast phasic, and slow long-lasting. These increases may be associated with the suppression of acquired prediction learning and retention of high motivation for reward, leading to impulsive excessive premature LPs.

Behavioral Biomarkers of Schizophrenia in High Drinker Rats: A Potential Endophenotype of Compulsive Neuropsychiatric Disorders

Psychogenic polydipsia, which is compulsive, non-regulatory fluid consumption, is present in 6%-20% of chronic psychiatric patients and frequently associated with the schizophrenia diagnosis. In the present study, we investigated the relation between schizophrenia-like symptoms and biomarkers with a compulsive drinking behavior phenotype in rats. Rats that were selected for low drinking vs high drinking behavior following schedule-induced polydipsia (SIP) were assessed in a latent inhibition (LI) paradigm using tone and electrical foot shock and in a spatial reversal learning task to evaluate behavioral inflexibility. We also analyzed the myelin basic protein in different brain areas of high drinker (HD) and low drinker (LD) rats. The HD rats, which were characterized by a compulsive drinking behavior on SIP, had a reduced level of LI effect and increased behavioral inflexibility in the spatial reversal learning task in comparison to the LD group. Moreover, HD rats showed less myelination in the center of the corpus callosum, striatum, and amygdala in comparison to LD rats. These findings strengthen the validity of HD rats that were selected by SIP as a possible phenotype of compulsive neuropsychiatric disorders, as evidenced by the existence of behaviors and biological markers that are related to schizophrenia and obsessive-compulsive disorder, including a reduced LI effect, behavioral inflexibility and reduced brain myelination. Future studies could contribute to the elucidation of the mechanisms underlying the compulsive phenotype of HD rats and its relation to vulnerability to schizophrenia.

Chronic pramipexole treatment induces compulsive behavior in rats with 6-OHDA lesions of the substantia nigra and ventral tegmental area

Dopamine replacement therapy (DRT) reduces motor symptoms in Parkinson's disease (PD), but also induces impulsive-compulsive behavior (ICB) in up to 25% of PD patients. These non-motor side effects of DRT generally follow a gradual transition from impulsive to compulsive-like-i.e. repetitive, compelled, and non-pleasurable-behavior. Here, we investigated the effect of chronic pramipexole (PPX) treatment on the onset of compulsive-like behavior, measured via the post-training signal attenuation (PTSA) procedure, in rats with dopaminergic lesions. Accordingly, we aimed to mimic chronic DRT in a PD context, and obtain data on the brain regions that potentially sustain this type of compulsive behavior pattern in rats. We observed that the lesion or treatment alone did not induce compulsive lever pressing in rats. However, rats with lesions of the substantia nigra and ventral tegmental area as well as with chronic PPX treatment developed strong compulsive lever-pressing behavior, as measured via PTSA. Furthermore, when chronic PPX treatment was discontinued before the PTSA test, the lesioned rats showed the same level of compulsive behavior as sham-operated rats. In fact, lesioned, treated, and compulsive-like rats showed significantly higher Fos expression in the orbitofrontal cortex and dorsal striatum. Thus, chronic PPX treatment in PD rats induced a strong compulsive-like behavior. Furthermore, Fos expression mapping suggests that the behavior was sustained via the activation of the orbitofrontal cortex and dorsal striatum.

Compulsivity in obsessive-compulsive disorder and addictions

Compulsive behaviors are driven by repetitive urges and typically involve the experience of limited voluntary control over these urges, a diminished ability to delay or inhibit these behaviors, and a tendency to perform repetitive acts in a habitual or stereotyped manner. Compulsivity is not only a central characteristic of obsessive-compulsive disorder (OCD) but is also crucial to addiction. Based on this analogy, OCD has been proposed to be part of the concept of behavioral addiction along with other non-drug-related disorders that share compulsivity, such as pathological gambling, skin-picking, trichotillomania and compulsive eating. In this review, we investigate the neurobiological overlap between compulsivity in substance-use disorders, OCD and behavioral addictions as a validation for the construct of compulsivity that could be adopted in the Research Domain Criteria (RDoC). The reviewed data suggest that compulsivity in OCD and addictions is related to impaired reward and punishment processing with attenuated dopamine release in the ventral striatum, negative reinforcement in limbic systems, cognitive and behavioral inflexibility with diminished serotonergic prefrontal control, and habitual responding with imbalances between ventral and dorsal frontostriatal recruitment. Frontostriatal abnormalities of compulsivity are promising targets for neuromodulation and other interventions for OCD and addictions. We conclude that compulsivity encompasses many of the RDoC constructs in a trans-diagnostic fashion with a common brain circuit dysfunction that can help identifying appropriate prevention and treatment targets.

A review of animal models of obsessive-compulsive disorder: a focus on developmental, immune, endocrine and behavioral models

INTRODUCTION

Obsessive-compulsive disorder (OCD) is a neuropsychiatric condition characterized by intrusive thoughts (obsessions) and/or repetitive behaviors (compulsions). Several models of OCD exist, many which employ behaviors such as over-grooming or hoarding as correlates for compulsive behaviors - often using a response to serotonergic agents as evidence for their validity. Recent discoveries in the genetics of OCD and the identification of aberrancies of glutamatergic, hormonal, and immune pathways in the OCD phenotype highlight a need to review existing of animal models of OCD. The focus of attention to these pathways may lead to possible new targets for drug discovery.

AREAS COVERED

In this review, the authors describe frameworks for animal models in OCD conceptualized as either biological (e.g., developmental, genetic, and endocrine pathways), or behavioral (e.g., repetitive grooming, and stereotypies). In addition, the authors give special attention to the emerging role of glutamate in OCD.

EXPERT OPINION

While many animal models for OCD demonstrate pathologic repetitive behavior phenotypes, which are relieved by serotoninergic agents, animal models based on reversal learning, perseverative responding, and neurodevelopmental mechanisms represent robust new paradigms. Glutamatergic influences in these new animal models suggest that drug discovery using neuroprotective approaches may represent a new stage for pharmacologic developments in OCD.

The aggression and behavioral abnormalities associated with monoamine oxidase A deficiency are rescued by acute inhibition of serotonin reuptake

The termination of serotonin (5-hydroxytryptamine, 5-HT) neurotransmission is regulated by its uptake by the 5-HT transporter (5-HTT), as well as its degradation by monoamine oxidase (MAO)-A. MAO-A deficiency results in a wide set of behavioral alterations, including perseverative behaviors and social deficits. These anomalies are likely related to 5-HTergic homeostatic imbalances; however, the role of 5-HTT in these abnormalities remains unclear. To ascertain the role of 5-HTT in the behavioral anomalies associated to MAO-A deficiency, we tested the behavioral effects of its blocker fluoxetine on perseverative, social and aggressive behaviors in transgenic animals with hypomorphic or null-allele MAO-A mutations. Acute treatment with the 5-HTT blocker fluoxetine (10 mg/kg, i.p.) reduced aggressive behavior in MAO-A knockout (KO) mice and social deficits in hypomorphic MAO-A(Neo) mice. Furthermore, this treatment also reduced perseverative responses (including marble burying and water mist-induced grooming) in both MAO-A mutant genotypes. Both MAO-A mutant lines displayed significant reductions in 5-HTT expression across the prefrontal cortex, amygdala and striatum, as quantified by immunohistochemical detection; however, the down-regulation of 5-HTT in MAO-A(Neo) mice was more pervasive and widespread than in their KO counterparts, possibly indicating a greater ability of the hypomorphic line to enact compensatory mechanisms with respect to 5-HT homeostasis. Collectively, these findings suggest that the behavioral deficits associated with low MAO-A activity may reflect developmental alterations of 5-HTT within 5-HTergic neurons. Furthermore, the translational implications of our results highlight 5-HT reuptake inhibition as an interesting approach for the control of aggressive outbursts in MAO-A deficient individuals.

Animal models of obsessive-compulsive spectrum disorders

Obsessive-compulsive disorder (OCD) and related conditions (trichotillomania, pathological skin-picking, pathological nail-biting) are common and disabling. Current treatment approaches fail to help a significant proportion of patients. Multiple tiers of evidence link these conditions with underlying dysregulation of particular cortico-subcortical circuitry and monoamine systems, which represent targets for treatment. Animal models designed to capture aspects of these conditions are critical for several reasons. First, they help in furthering our understanding of neuroanatomical and neurochemical underpinnings of the obsessive-compulsive (OC) spectrum. Second, they help to account for the brain mechanisms by which existing treatments (pharmacotherapy, psychotherapy, deep brain stimulation) exert their beneficial effects on patients. Third, they inform the search for novel treatments. This article provides a critique of key animal models for selected OC spectrum disorders, beginning with initial work relating to anxiety, but moving on to recent developments in domains of genetic, pharmacological, cognitive, and ethological models. We find that there is a burgeoning literature in these areas with important ramifications, which are considered, along with salient future lines of research.

The role of the cholinergic system in the signal attenuation rat model of obsessive-compulsive disorder

RATIONALE

In comparison to studies of the involvement of the serotonergic, dopaminergic, and glutamatergic systems in the pathophysiology of obsessive-compulsive disorder (OCD), research on the involvement of the cholinergic system in this disorder has remained sparse.

OBJECTIVES

The aim of this study was to test the role of the cholinergic system in compulsive behavior using the signal attenuation rat model of OCD. In this model, "compulsive" behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food.

METHODS

The acetylcholinesterase inhibitor physostigmine (0.05, 0.10, and 0.15 mg/kg), the nicotinic agonist nicotine (0.03, 0.06, 0.10, 0.30, 0.60, and 1.00 mg/kg), the nicotinic antagonist mecamylamine (1, 3, 5, and 8 mg/kg), the muscarinic agonist oxotremorine (0.0075, 0.0150, and 0.0300 mg/kg), and the muscarinic antagonist scopolamine (0.15, 0.50, 1.00, and 1.50 mg/kg) were acutely administered to rats just before assessing their lever-press responding following signal attenuation (experiments 1, 3, 5, 7, and 9, respectively). Because the effects of signal attenuation are assessed under extinction conditions, drug doses that were effective in the above experiments were also tested in an extinction session of lever-press responding that was not preceded by signal attenuation (experiments 2, 4, 6, 8, and 10).

RESULTS

Acute systemic administration of the cholinergic agents did not exert a selective anti- or pro-compulsive effect in the signal attenuation model.

CONCLUSIONS

Acetylcholine does not seem to play a role in the signal attenuation rat model of OCD.

Memantine add-on in moderate to severe obsessive-compulsive disorder: randomized double-blind placebo-controlled study

There is a growing body of evidence for the efficacy of memantine augmentation in patients with obsessive-compulsive disorder (OCD). However, to date, no double-blind study has addressed this issue. The objective of the present randomized double-blind placebo-controlled study was to evaluate efficacy and tolerability of memantine add-on treatment in patients with moderate to severe OCD. Forty-two patients with the diagnosis of OCD based on DSM-IV-TR who had a Yale-Brown Obsessive Compulsive Scale (Y-BOCS) score of ≥21 were randomly assigned to memantine (10 mg/day for the first week, and 20 mg/day for the rest of the trial) or placebo in addition to fluvoxamine for eight weeks. Patients were assessed using Y-BOCS every two weeks. Thirty-eight patients completed the study. Repeated measure ANOVA showed significant effect for time × treatment interaction in total scale [F (2.096, 75.470) = 5.280, P = 0.006] and obsession [F (2.340, 94.547) = 5.716, P = 0.002] and near significant effect for compulsion subscales [F (2.005, 79.179) = 2.841, P = 0.065]. By week eight, all patients in the memantine group and six (32%) patients in the placebo group [P value of Fisher's exact test <0.001] met the criteria for partial and complete response. At the end of the trial, 17 (89%) patients in the memantine group compared with six (32%) patients in the placebo group achieved remission (χ(2)(1) = 13.328, P < 0.001). Frequency of side-effects was not significantly different between the two groups. In summary, we showed that memantine add-on to fluvoxamine significantly improved short-term outcomes in patients with moderate to severe OCD.

The role of glutamate signaling in the pathogenesis and treatment of obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a common and often debilitating neuropsychiatric condition characterized by persistent intrusive thoughts (obsessions), repetitive ritualistic behaviors (compulsions) and excessive anxiety. While the neurobiology and etiology of OCD has not been fully elucidated, there is growing evidence that disrupted neurotransmission of glutamate within corticalstriatal-thalamocortical (CSTC) circuitry plays a role in OCD pathogenesis. This review summarizes the findings from neuroimaging, animal model, candidate gene and treatment studies in the context of glutamate signaling dysfunction in OCD. First, studies using magnetic resonance spectroscopy are reviewed demonstrating altered glutamate concentrations in the caudate and anterior cingulate cortex of patients with OCD. Second, knockout mouse models, particularly the DLGAP3 and Sltrk5 knockout mouse models, display remarkably similar phenotypes of compulsive grooming behavior associated with glutamate signaling dysfunction. Third, candidate gene studies have identified associations between variants in glutamate system genes and OCD, particularly for SLC1A1 which has been shown to be associated with OCD in five independent studies. This converging evidence for a role of glutamate in OCD has led to the development of novel treatment strategies involving glutamatergic compounds, particularly riluzole and memantine. We conclude the review by outlining a glutamate hypothesis for OCD, which we hope will inform further research into etiology and treatment for this severe neuropsychiatric condition.

Translational approaches to obsessive-compulsive disorder: from animal models to clinical treatment

Obsessive-compulsive disorder (OCD) is characterized by obsessions (intrusive thoughts) and compulsions (repetitive ritualistic behaviours) leading to functional impairment. Accumulating evidence links these conditions with underlying dysregulation of fronto-striatal circuitry and monoamine systems. These abnormalities represent key targets for existing and novel treatment interventions. However, the brain bases of these conditions and treatment mechanisms are still not fully elucidated. Animal models simulating the behavioural and clinical manifestations of the disorder show great potential for augmenting our understanding of the pathophysiology and treatment of OCD. This paper provides an overview of what is known about OCD from several perspectives. We begin by describing the clinical features of OCD and the criteria used to assess the validity of animal models of symptomatology; namely, face validity (phenomenological similarity between inducing conditions and specific symptoms of the human phenomenon), predictive validity (similarity in response to treatment) and construct validity (similarity in underlying physiological or psychological mechanisms). We then survey animal models of OC spectrum conditions within this framework, focusing on (i) ethological models; (ii) genetic and pharmacological models; and (iii) neurobehavioural models. We also discuss their advantages and shortcomings in relation to their capacity to identify potentially efficacious new compounds. It is of interest that there has been rather little evidence of 'false alarms' for therapeutic drug effects in OCD models which actually fail in the clinic. While it is more difficult to model obsessive cognition than compulsive behaviour in experimental animals, it is feasible to infer cognitive inflexibility in certain animal paradigms. Finally, key future neurobiological and treatment research areas are highlighted.

Increased expression of 5-HT₆ receptors in dorsolateral striatum decreases habitual lever pressing, but does not affect learning acquisition of simple operant tasks in rats

Serotonin-6 (5-HT(6)) receptors are densely expressed in the dorsolateral striatum (DLS), a brain region linked to habits. Medications acting on the serotonergic system, including 5-HT(6) receptors, can diminish habitual and repetitive behaviors associated with clinical syndromes such as obsessive-compulsive disorder, and may have implications for addiction as well. To examine the role of 5-HT(6) receptors in the acquisition and persistence of habitual behavior, we manipulated 5-HT(6) receptor expression in the DLS with herpes simplex virus vectors in combination with different behavioral procedures; control rats received a vector expressing enhanced green fluorescent protein. In one set of experiments, rats were tested under conditions that favor the acquisition of either discrete action-outcome responding or repetitive responding; increased 5-HT(6) receptor expression in DLS did not alter learning in either paradigm. In the next experiment, rats were over-trained on fixed- then variable-interval schedules, resulting in an escalation of lever pressing over sessions far in excess of that necessary to receive sucrose pellets. After training, rats received viral vector infusion into the DLS. Subsequently, half of each group underwent an omission contingency training session in which they received reinforcement for refraining from pressing the lever, while the other half served as yoked controls. A probe session under extinction conditions was performed the following day. Only rats that received both the 5-HT(6) vector and omission contingency training showed reduced lever pressing during the probe session. These results suggest that increasing 5-HT(6) receptor signaling in the DLS facilitates behavioral flexibility in the face of changing contingencies.

Towards mouse models of perseveration: a heritable component in extinction of operant behavior in fourteen standard and recombinant inbred mouse lines

Extinction of instrumental responses is an essential skill for adaptive behavior such as foraging. So far, only few studies have focused on extinction following appetitive conditioning in mice. We studied extinction of appetitive operant lever-press behavior in six standard inbred mouse strains (A/J, C3H/HeJ, C57BL/6J, DBA/2J, BALB/cByJ and NOD/Ltj) and eight recombinant inbred mouse lines. From the response rates at the end of operant and extinction training we computed an extinction index, with higher values indicating better capability to omit behavioral responding in absence of reward. This index varied highly across the mouse lines tested, and the variability was partially due to a significant heritable component of 12.6%. To further characterize the relationship between operant learning and extinction, we calculated the slope of the time course of extinction across sessions. While many strains showed a considerable capacity to omit responding when lever pressing was no longer rewarded, we found a few lines showing an abnormally high perseveration in lever press behavior, showing no decay in response scores over extinction sessions. No correlation was found between operant and extinction response scores, suggesting that appetitive operant learning and extinction learning are dissociable, a finding in line with previous studies indicating that these forms of learning are dependent on different brain areas. These data shed light on the heritable basis of extinction learning and may help develop animal models of addictive habits and other perseverative disorders, such as compulsive food seeking and eating.

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.

The selective serotonin reuptake inhibitor paroxetine does not alter consummatory concentration-dependent licking of prototypical taste stimuli by rats

Serotonin and the 5HT(1A) receptor are expressed in a subset of taste receptor cells, and the 5HT(3) receptor is expressed on afferent fibers innervating taste buds. Exogenous administration of the selective serotonin reuptake inhibitor, paroxetine, has been shown to increase taste sensitivity to stimuli described by humans as sweet and bitter. Serotonergic agonists also decrease food and fluid intake, and it is possible that modulations of serotonin may alter taste-based hedonic responsiveness; alternatively, or in combination, serotonin may interact with physiological state to impact ingestive behavior. In this study, the unconditioned licking of prototypical taste stimuli by rats in brief-access taste tests was assessed following paroxetine administration (0.3-10 mg/kg intraperitoneal). We also measured sucrose licking by rats in different deprivation states after paroxetine (5 mg/kg). In neither experiment did we find any evidence of an effect of paroxetine on licking relative to water to any of the taste stimuli in the brief-access test at doses that decreased food intake. However, in some conditions, paroxetine decreased trials initiated to tastants. Therefore, a systemic increase in serotonin via paroxetine administration can decrease appetitive behavior in brief-access tests but is insufficient to alter taste-guided consummatory behavior.

High but not low frequency stimulation of both the globus pallidus and the entopeduncular nucleus reduces 'compulsive' lever-pressing in rats

The anti-compulsive effects of high and low frequency stimulation (LFS, HFS) of the entopeduncular nucleus and globus pallidus (the rat's equivalent, respectively, of the primate's internal and external segments of the globus pallidus) were assessed in the signal attenuation rat model of obsessive-compulsive disorder (OCD). HFS, but not LFS, of the two nuclei exerted an anti-compulsive effect, suggesting that HFS of either segment of the globus pallidus may provide an additional therapeutic strategy for OCD.

The role of NMDA receptors in the signal attenuation rat model of obsessive-compulsive disorder

RATIONALE

In recent years, an increasing body of evidence points to the involvement of the glutamatergic system and specifically the glutamatergic ionotropic N-methyl-D-aspartate (NMDA) receptor in the pathophysiology of obsessive-compulsive disorder (OCD).

OBJECTIVES

To test the role of NMDA receptors in compulsive behavior using the signal attenuation rat model of OCD. In this model, 'compulsive' behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food.

METHODS

The NMDA antagonist, MK 801 (0.025-0.100 mg/kg) and the partial NMDA agonist, D-cycloserine (3-100 mg/kg) were administered to rats just before assessing their lever-press responding following signal attenuation (Experiments 1 and 2, respectively). Because the effects of signal attenuation are assessed under extinction conditions, drug doses that were effective in Experiments 1 and 2 were also tested in an extinction session of lever-press responding that was not preceded by signal attenuation (Experiment 3).

RESULTS

Systemic administration of D: -cycloserine (15 mg/kg) selectively decreased compulsive lever pressing, whereas systemic administration of MK 801 did not affect compulsive lever-pressing but dramatically increased resistance to extinction.

CONCLUSIONS

Activation of NMDA receptors may have an anti-compulsive effect in OCD patients.

The role of the striatum in compulsive behavior in intact and orbitofrontal-cortex-lesioned rats: possible involvement of the serotonergic system

In the signal attenuation rat model of obsessive-compulsive disorder (OCD), 'compulsive' behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food. We have recently found that lesions to the rat orbitofrontal cortex (OFC) led to an increase in compulsive lever-pressing that was prevented by systemic administration of the selective serotonin reuptake inhibitor paroxetine, and paralleled by an increase in the density of the striatal serotonin transporter. This study further explored the interaction between the OFC, the striatum, and the serotonergic system in the production of compulsive lever-pressing. Experiment 1 revealed that OFC lesions decrease the content of serotonin, dopamine, glutamate, and GABA in the striatum. Experiment 2 showed that intrastriatal administration of paroxetine blocked OFC lesion-induced increased compulsivity, but did not affect compulsive responding in intact rats. Experiments 3 and 4 found that pre-training striatal lesions had no effect on compulsive lever-pressing, whereas post-training striatal inactivation exerted an anticompulsive effect. These results strongly implicate the striatum in the expression of compulsive lever-pressing in both intact and OFC-lesioned rats. Furthermore, the results support the possibility that in a subpopulation of OCD patients a primary pathology of the OFC leads to a dysregulation of the striatal serotonergic system, which is manifested in compulsive behavior, and that antiobsessional/anticompulsive drugs exerts their effects, in these patients, by normalizing the dysfunctional striatal serotonergic system.

Cross-species models of OCD spectrum disorders

Several axis-I neuropsychiatric disorders are characterised by repetitive motor habits suggestive of underlying inhibitory dyscontrol, and may constitute members of a putative obsessive-compulsive (OC) spectrum. Notable examples include obsessive-compulsive disorder (OCD) and trichotillomania (repetitive hair-pulling). Multiple tiers of evidence link these conditions with underlying dysregulation of fronto-striatal circuitry and monoamine systems. These abnormalities represent key targets for existing and novel treatment interventions. Nonetheless, the brain bases of these conditions, and treatment mechanisms, remain poorly characterised. Animal models of repetitive habits and inhibitory control problems show great potential for augmenting our understanding of the pathophysiology and treatment of OC spectrum conditions. Here, we begin by describing clinical features of OC spectrum disorders, and criteria used to assess the validity of animal models of symptomatology. Namely, face validity (phenomenological similarity between inducing conditions and specific symptoms of the human phenomenon), predictive validity (similarity in response to treatment) and construct validity (similarity in underlying physiological or psychological mechanisms). We then survey animal models of OC spectrum conditions within this framework, focusing on (i) ethological models; (ii) genetic and pharmacological models; and (iii) behavioral models. Key future research directions are highlighted.

Ovarian hormones modulate 'compulsive' lever-pressing in female rats

Life events related to the female hormonal cycle may trigger the onset of obsessive-compulsive disorder (OCD) or exacerbate symptoms in women already suffering from it. These observations suggest a possible role for ovarian hormones in the course of this disorder. Yet, the mechanisms that may subserve the modulatory effect of ovarian hormones are currently unknown. The aim of the present study was therefore to test the role of ovarian hormones in the signal attenuation rat model of OCD. Experiment 1 compared the behavior of pre-pubertal and adult male and female rats in the model, and found no age and sex differences in compulsive responding. Experiment 2 found that compulsive responding fluctuates along the estrous cycle, being highest during late diestrous and lowest during estrous. Acute administration of estradiol to pre-pubertal female rats was found to attenuate compulsive behavior (Experiment 3), and withdrawal from chronic administration of estradiol was shown to increase this behavior (Experiment 4). These findings extend the use of the signal attenuation model of OCD to female rats, and by demonstrating that the model is sensitive to the levels of ovarian hormones, provide the basis for using the model to study the role of ovarian hormones in OCD. In addition, the present findings support the hypothesis that the increased risk of onset and exacerbation of OCD in women post-partum may be a result of the decrease in the level of estradiol, which was elevated during pregnancy.

High frequency stimulation and pharmacological inactivation of the subthalamic nucleus reduces 'compulsive' lever-pressing in rats

In recent years there have been several attempts to establish high frequency stimulation (HFS) as an additional treatment strategy for obsessive-compulsive disorder (OCD). Two studies reported that bilateral HFS of the subthalamic nucleus (STN) dramatically alleviated compulsions and improved obsessions in three patients with co-morbid Parkinson's disease and OCD. A recent study reported that HFS as well as pharmacological inactivation of the STN alleviate compulsive checking in the quinpirole rat model of OCD. As the quinpirole model is based on a dopaminergic manipulation, the aim of the present study was to test whether HFS and pharmacological inactivation of the STN exert an anti-compulsive effect also in the drug-naive brain, using the signal attenuation rat model of OCD. The main finding of the present study is that both HFS and pharmacological inactivation of the STN exerted an anti-compulsive effect, although the two manipulations differed in their effects on other behavioral measures. These findings support the possibility that HFS of the STN may provide an additional therapeutic strategy for 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.

Chronic blockade or constitutive deletion of the serotonin transporter reduces operant responding for food reward

The therapeutic effects of chronic selective serotonin reuptake inhibitors (SSRIs) are well documented, yet the elementary behavioral processes that are affected by such treatment have not been fully investigated. We report here the effects of chronic fluoxetine treatment and genetic deletion of the serotonin transporter (SERT) on food reinforced behavior in three paradigms: the progressive ratio operant task, the concurrent choice operant task, and the Pavlovian-to-Instrumental transfer task. We consistently find that chronic pharmacological blockade or genetic deletion of SERT result in similar behavioral consequences: reduced operant responding for natural reward. This is in line with previous studies reporting declines in operant responding for drugs and intracranial self-stimulation with fluoxetine treatment, suggesting that the effect of SERT blockade can be generalized to different reward types. Detailed analyses of behavioral parameters indicate that this reduction in operant responding affect both goal-directed and non-goal-directed behaviors without affecting the Pavlovian cue-triggered excessive operant responding. In addition, both pharmacological and genetic manipulations reduce locomotor activity in the open field novel environment. Our data contrast with the effect of dopamine in increasing operant responding for natural reward specifically in goal-directed behaviors and in increasing Pavlovian cue-triggered excessive operant responding. Serotonin and dopamine have been proposed to serve opposing functions in motivational processes. Our data suggest that their interactions do not result in simple opponency. The fact that pharmacological blockade and genetic deletion of SERT have similar behavioral consequences reinforces the utility of the SERT null mice for investigation of the mechanisms underlying chronic SSRIs treatment.

Deep brain stimulation for treatment-refractory obsessive-compulsive disorder: the search for a valid target

Obsessive-compulsive disorder (OCD) is a common psychiatric disease that is marked by recurring, anxiety-provoking thoughts (obsessions) accompanied by repetitive and time-consuming behaviors (compulsions). Among the controversies in the OCD literature is the issue of the origin of the disease and whether brain changes observed with modern imaging techniques are the causes or results of OCD behaviors and thoughts. These issues remain unresolved; however, significant strides have been made in understanding the illness from both phenomenological and pathophysiological perspectives. The current staple of OCD management remains pharmacological in nature and often occurs in conjunction with cognitive behavioral therapy. Refractory cases, however, are occasionally referred for neurosurgical consultation, and several procedures have been examined. Success in the treatment of Parkinson's disease, the reversibility of the therapy, and a relatively safe side-effect profile have allowed deep brain stimulation (DBS) to be examined as an alternative treatment for some psychiatric conditions. Here we assess the possibility of applying DBS to the treatment of OCD. Morphological, functional metabolic, and volumetric data point to several brain regions that are important to the etiology and maintenance of OCD. Converging evidence from the genetics and neurocircuitry literature suggests that several subcortical structures play prominent roles in the disease. The functional modification of these structures could potentially provide symptom relief. Here, we review the ablative and DBS procedures for refractory OCD, and provide a research-driven hypothesis that highlights the ventromedial head of the caudate nucleus, and structures up- and downstream from it, as potential DBS targets for treatment-resistant disease. We hope that a research-driven approach, premised on converging evidence and previous experience, will lead to a safe and effective DBS procedure that will benefit patients who remain disabled despite presently available therapies.

Lesions of dopaminergic neurons in the substantia nigra pars compacta and in the ventral tegmental area enhance depressive-like behavior in rats

Depression is the most common psychiatric complication in Parkinson's disease (PD). The pathophysiological events leading to PD-associated depression, however, remain largely unknown. The present study tested the differential implication of dopaminergic systems in depressive-like behavior in rats and its response to l-Dopa and the selective serotonin reuptake inhibitor citalopram. The learned helplessness model was used as a behavioral paradigm. Rats were lesioned in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) and assigned to subgroups with respect to the stereologically verified extent of the nigral and/or VTA degeneration. Both lesions increased depressive-like behavior in rats, which was reduced by both citalopram and l-Dopa treatment. We conclude that dopaminergic lesions of either the SNc or the VTA contribute to the manifestation of depressive-like behavior in rats. The effects of citalopram administration on depressive behavior induced by lesions of dopaminergic brain regions furthermore suggest an involvement of serotonergic pathways in dopaminergic cell loss-induced depression.

Strain differences in ‘compulsive’ lever-pressing

In the signal attenuation rat model of obsessive-compulsive disorder, 'compulsive' behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food. In recent years several studies have reported that Lewis rats, an inbred strain derived from the Sprague Dawley strain, exhibit addictive and/or compulsive tendencies. The aim of the present study was thus to test whether Lewis rats will also show increased compulsivity in the signal attenuation model. Because the model has been developed and validated using Wistar rats only, the present study compared the behavioral response to signal attenuation of Lewis, Sprague Dawley and Wistar rats, and assessed the effects of the anti-compulsive drug paroxetine on compulsive behavior in Lewis and Sprague Dawley rats. The results show that Lewis rats are more 'compulsive' than Sprague Dawley and Wistar rats in terms of both higher levels of compulsive lever-pressing and higher resistance to the anti-compulsive effect of paroxetine. The possibility that these strain differences are related to strain differences in the serotonergic and dopaminergic systems are discussed in light of current knowledge of the pathophysiology and pharmacotherapy of OCD.

The signal attenuation rat model of obsessive-compulsive disorder: a review

During the last 30 years, there have been many attempts to develop animal models of obsessive-compulsive disorder (OCD), in the hope that they may provide a route for furthering our understanding and treatment of this disorder. The present paper reviews a recently developed rat model of OCD, namely, signal attenuation. Results of pharmacological and lesion studies are presented and evaluated with respect to the pharmacology and pathophysiology of OCD. It is argued that signal attenuation is a rat model of OCD with construct (derived from similarity in the underlying mechanisms), predictive (derived from similarity in response to treatment), and face (derived from phenomenological similarity between "compulsive" behavior in the model and compulsions in OCD patients) validity.

Current animal models of obsessive compulsive disorder: a critical review

During the last 30 years there have been many attempts to develop animal models of obsessive compulsive disorder (OCD), in the hope that they may provide a route for furthering our understanding and treatment of this disorder. The present paper reviews current genetic, pharmacological and behavioral animal models of OCD, and evaluates their face validity (derived from phenomenological similarity between the behavior in the animal model and the specific symptoms of the human condition), predictive validity (derived from similarity in response to treatment) and construct validity (derived from similarity in the underlying mechanisms--physiological or psychological).

The effects of temporary inactivation of the orbital cortex in the signal attenuation rat model of obsessive compulsive disorder

Rats undergoing extinction of lever pressing after an external feedback for this behavior was attenuated by extinguishing its Pavlovian association with the reward (signal attenuation) exhibit compulsive lever pressing. The present study tested the effects of temporary inactivation of the orbital cortex in rats undergoing extinction of lever pressing that was or was not preceded by signal attenuation (post-training signal attenuation and regular extinction, respectively). Orbital inactivation led to a nonspecific decrease in lever pressing in rats undergoing post-training signal attenuation and to the emergence of compulsive-like behavior in rats undergoing regular extinction. These results suggest that orbital inactivation and extinguishing a Pavlovian stimulus-reinforcer contingency have a similar effect on lever pressing and are in line with previous findings implicating the orbital cortex in mediating the effects of previously acquired stimulus-reinforcer associations on operant behavior.

‘Compulsive’ lever pressing in rats is enhanced following lesions to the orbital cortex, but not to the basolateral nucleus of the amygdala or to the dorsal medial prefrontal cortex

In a new rat model of obsessive-compulsive disorder (OCD), 'compulsive' behaviour is induced by attenuating a signal indicating that a lever-press response was effective in producing food. We have recently found that compulsive lever pressing is increased following lesions to the rat orbital cortex, in accordance with several lines of evidence implicating the orbitofrontal cortex in the pathophysiology of OCD. In view of the functional similarities between the orbital cortex, the basolateral nucleus of the amygdala and the medial prefrontal cortex, the present study compared the effects of lesions to these three regions. The present study replicated the finding that lesions to the rat orbital cortex enhance compulsive lever pressing. In contrast, lesions to the dorsal medial prefrontal cortex and to the basolateral amygdala did not affect compulsive lever pressing. A comparison of these findings to current knowledge regarding similarities and differences in the functioning of the three regions sheds light on the mechanism by which signal attenuation induces compulsive lever pressing and on the role played by the orbital cortex in compulsive behaviour.

Role of the orbital cortex and of the serotonergic system in a rat model of obsessive compulsive disorder

The serotonergic system and the orbitofrontal cortex have been consistently implicated in the pathophysiology of obsessive compulsive disorder. Yet, the relations between these two systems and the ways they interact in producing obsessions and compulsions are poorly understood. The present study tested the hypothesis that pathology of the orbitofrontal cortex leads to a dysregulation of the serotonergic system which is manifested in compulsive behavior, using a new rat model of this disorder. In the model, 'compulsive' behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food. We found that lesion to the rat orbital cortex led to a selective increase in compulsive lever-pressing that was prevented by the serotonin re-uptake inhibitor, paroxetine, and was paralleled by an increase in the density of the striatal serotonin transporter, assessed using high affinity [3H]imipramine binding. These results suggest that the serotonergic system is involved in orbital lesion-induced compulsivity, and provide a possible account for the observed association between obsessions and compulsions and dysfunction of the orbitofrontal cortex and of the serotonergic system in obsessive compulsive disorder.