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

Evidence for Distinct Forms of Compulsivity in the SAPAP3 Mutant-Mouse Model for Obsessive-Compulsive Disorder

The specific mechanisms underlying compulsive behavior in obsessive-compulsive disorder (OCD) are unknown. It has been suggested that such compulsivity may have its origin in cognitive dysfunction such as impaired processing of feedback information, received after the completion of goal-directed actions. The signal attenuation (SA) task models such a processing deficit in animals by attenuating the association strength between food reward and audiovisual feedback (signal) presented after performance of an operant response. The compulsive-like responding resulting from SA is well characterized in rats, but was so far not established in mice, a species for which powerful genetic OCD models exist. Thus, first, we demonstrate that the SA task can be implemented in mice and show that attenuation of reward-associated response feedback produces similar behavior in C57BL/6 mice as previously reported in rats. Second, we tested the hypothesis that SAPAP3 knock-out mice (SAPAP3^-/-), prone to exhibit several OCD-like abnormalities including excessive grooming, show enhanced compulsive-like behavior in the SA task compared with their wild-type (WT) littermates. However, task-related compulsivity measures in SAPAP3^-/- and WT did not yield significant differences, neither following SA nor during “regular” extinction of operant behavior. Thus, compulsive-like instrumental behavior following feedback distortion was not potentiated in compulsively grooming mice, implicating specifically that (1) a general deficit in feedback processing is not related to excessive grooming in SAPAP3^-/- and (2) different manifestations of compulsivity may be driven by independent mechanisms.

ORBITOFRONTAL THICKNESS AS A MEASURE FOR TREATMENT RESPONSE PREDICTION IN OBSESSIVE-COMPULSIVE DISORDER

BACKGROUND

Early prediction of treatment response could reduce exposure to ineffective treatments and optimize the use of medical resources. Neuroimaging techniques have been used to identify biomarkers that are predictive of outcomes. The aims of this study were to investigate orbitofrontal cortex (OFC) thickness as a potential morphometric biomarker to discriminate outcomes in obsessive-compulsive disorder (OCD) and then to reexamine this biomarker in an independent cohort

METHODS

Using a logistic regression model based on the mean baseline thickness of subregions of the OFC, we estimated the probability of treatment response in 29 treatment-naïve OCD patients who participated in a clinical trial. That algorithm was then tested in an independent cohort of 12 patients with a confirmed diagnosis of refractory OCD RESULTS: Among the treatment-naïve OCD patients, measures of OFC thickness statistically significantly differentiated responders (n = 13) and nonresponders (n = 16), with an overall classification accuracy of ≈80%, a sensitivity of 77% (10/13), and a specificity of 81% (13/16). Of the refractory OCD patients in the second independent cohort, 67% were correctly classified as nonresponders. The most discriminative measures in the initial cohort of treatment-naïve patients were the thicknesses of the left and right medial OFC (P = .009 and P = .028, respectively)

CONCLUSIONS

We found OFC thickness to be a strong predictor of treatment response in treatment-naïve OCD patients. Although there are not yet any brain imaging biomarkers with clinical utility, our results highlight the potential of these measures as tools for predicting treatment outcomes in OCD.

Signal attenuation as a rat model of obsessive compulsive disorder

In the signal attenuation rat model of obsessive-compulsive disorder (OCD), lever-pressing for food is followed by the presentation of a compound stimulus which serves as a feedback cue. This feedback is later attenuated by repeated presentations of the stimulus without food (without the rat emitting the lever-press response). In the next stage, lever-pressing is assessed under extinction conditions (i.e., no food is delivered). At this stage rats display two types of lever-presses, those that are followed by an attempt to collect a reward, and those that are not. The latter are the measure of compulsive-like behavior in the model. A control procedure in which rats do not experience the attenuation of the feedback cue serves to distinguish between the effects of signal attenuation and of extinction. The signal attenuation model is a highly validated model of OCD and differentiates between compulsive-like behaviors and behaviors that are repetitive but not compulsive. In addition the measures collected during the procedure eliminate alternative explanations for differences between the groups being tested, and are quantitative, unbiased and unaffected by inter-experimenter variability. The major disadvantages of this model are the costly equipment, the fact that it requires some technical know-how and the fact that it is time-consuming compared to other models of OCD (11 days). The model may be used for detecting the anti- or pro-compulsive effects of pharmacological and non-pharmacological manipulations and for studying the neural substrate of compulsive behavior.

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.

Differential effects of serotonin-specific and excitotoxic lesions of OFC on conditioned reinforcer devaluation and extinction in rats

The orbitofrontal cortex (OFC) is critical for behavioral adaptation in response to changes in reward value. Here we investigated, in rats, the role of OFC and, specifically, serotonergic neurotransmission within OFC in a reinforcer devaluation task (which measures behavioral flexibility). This task used two visual cues, each predicting one of two foods, with the spatial position (left-right) of the cues above two levers pseudorandomized across trials. An instrumental action (lever press) was required for reinforcer delivery. After training, rats received either excitotoxic OFC lesions made by NMDA (N-methyl-d-aspartic acid), serotonin-specific OFC lesions made by 5,7-DHT (5,7-dihydroxytryptamine), or sham lesions. In sham-lesioned rats, devaluation of one food (by feeding to satiety) significantly decreased responding to the cue associated with that food, when both cues were presented simultaneously during extinction. Both types of OFC lesions disrupted the devaluation effect. In contrast, extinction learning was not affected by serotonin-specific lesions and was only mildly retarded in rats with excitotoxic lesions. Thus, serotonin within OFC is necessary for appropriately adjusting behavior toward cues that predict reward but not for reducing responses in the absence of reward. Our results are the first to demonstrate that serotonin in OFC is necessary for reinforcer devaluation, but not extinction.

Perseverative instrumental and Pavlovian responding to conditioned stimuli in serotonin transporter knockout rats

Environmental stimuli can influence behavior via the process of Pavlovian conditioning. Recent genetic research suggests that some individuals are more sensitive to environmental stimuli for behavioral guidance than others. One important mediator of this effect is serotonin transporter (5-HTT) genetic variance, which increases sensitivity to Pavlovian conditioned stimuli through changes in the build-up of corticolimbic circuits. As these stimuli can have reinforcing effects on instrumental responding, we here investigated their effects on instrumental behavior in 5-HTT knockout rats and their wild-type counterparts by means of the signal attenuation paradigm. In this paradigm animals acquired a Pavlovian association between a stimulus and food reward, and subsequently they had to lever press in order to gain access to this food reward-associated stimulus. Thereafter, half of the animals underwent extinction training during which extinction of the primary Pavlovian association was induced via non-reinforced stimulus presentations, whereas the other half did not receive this training. During a final test session all animals were tested for instrumental responding for the non-reinforced Pavlovian conditioned stimulus, as well as instrumental and Pavlovian responding to the stimulus after an initial lever-press. No genotype differences were observed during the training and extinction sessions. However, during the test session 5-HTT knockout rats that had not received prior extinction training displayed excessive instrumental responding. This was specifically observed during presentation of the stimulus (induced by the first lever press) and was accompanied by an increased number of feeder visits after termination of the stimulus presentation. An additionally performed c-Fos immunohistochemistry study revealed that the behaviors in these animals were associated with abnormal c-Fos immunoreactivity in the orbitofrontal cortex and basolateral amygdala, regions important for the acquisition and maintenance of Pavlovian conditioned stimuli. These findings complement earlier findings showing that 5-HTT knockout animals' behavior is heavily influenced by environmental stimuli and indicate that this extends to the instrumental domain.

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.

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.

Orbital frontal cortex in treatment-naïve pediatric obsessive-compulsive disorder

The orbital frontal cortex (OFC) has been implicated in obsessive-compulsive disorder (OCD). Participants comprised 28 treatment-naïve pediatric OCD patients and 21 controls, who were examined using magnetic resonance imaging. OCD patients had larger right but not left OFC white matter volume than controls. This is fresh evidence implicating white matter in OCD.

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.

The orbital cortex in rats topographically projects to central parts of the caudate-putamen complex

Disturbances of the orbitofrontal-striatal pathways in humans have been associated with several psychopathologies including obsessive-compulsive disorder and drug addiction. In nonhuman primates, different subareas of the orbitofrontal cortex project topographically to central and ventromedial parts of the striatum. Relatively little is known about the anatomical organization of the rat orbital cortex while there is a growing interest in this cortical area from a functional and behavioral point of view. The aim of the present neuroanatomical tracing study was to determine in rats the striatal target area of the projections of the orbital cortex as well as the topographical organization within these projections. To this end, anterograde tracers were injected in the different cytoarchitectonically distinct subareas of the orbital cortex. The results show that the individual orbital areas, i.e. medial orbital area, ventral orbital area, ventrolateral orbital area and lateral orbital area, project to central parts of the caudate-putamen, exhibiting a mediolateral and, to a lesser degree, rostrocaudal topographical arrangement. Orbital projections avoid the most dorsal, as well as rostral and caudal parts of the caudate-putamen. Terminal fields from cytoarchitectonically different areas show a considerable overlap. Superficial cortical layers project preferentially to the striatal matrix, deep layers to the patch compartment. The projections from the ventrolateral orbital area are strongest and occupy the most extensive striatal area. In addition to projections to the caudate-putamen, the ventrolateral, lateral and dorsolateral orbital areas have a scarce projection to the most lateral part of the nucleus accumbens shell in the ventral striatum. In contrast to nonhuman primates, the remainder of the rat nucleus accumbens is virtually free of orbital projections.

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.