Disruption of social cognition in the sub-chronic PCP rat model of schizophrenia: Possible involvement of the endocannabinoid system

Inhibition of fatty acid amide hydrolase reverses aberrant prefrontal gamma oscillations in the sub-chronic PCP model for schizophrenia

Hypofunctioning of NMDA receptors, and the resulting shift in the balance between excitation and inhibition, is considered a key process in the pathophysiology of schizophrenia. One important manifestation of this phenomenon is changes in neural oscillations, those above 30 Hz (i.e., gamma-band oscillations), in particular. Although both preclinical and clinical studies observed increased gamma activity following acute administration of NMDA receptor antagonists, the relevance of this phenomenon has been recently questioned given the reduced gamma oscillations typically observed during sensory and cognitive tasks in schizophrenia. However, there is emerging, yet contradictory, evidence for increased spontaneous gamma-band activity (i.e., at rest or under baseline conditions). Here, we use the sub-chronic phencyclidine (PCP) rat model for schizophrenia, which has been argued to model the pathophysiology of schizophrenia more closely than acute NMDA antagonism, to investigate gamma oscillations (30-100 Hz) in the medial prefrontal cortex of anesthetized animals. While baseline gamma oscillations were not affected, oscillations induced by train stimulation of the posterior dorsal CA1 (pdCA1) field of the hippocampus were enhanced in PCP-treated animals (5 mg/kg, twice daily for 7 days, followed by a 7-day washout period). This effect was reversed by pharmacological enhancement of endocannabinoid levels via systemic administration of URB597 (0.3 mg/kg), an inhibitor of the catabolic enzyme of the endocannabinoid anandamide. Intriguingly, the pharmacological blockade of CB1 receptors by AM251 unmasked a reduced gamma oscillatory activity in PCP-treated animals. The findings are consistent with the observed effects of URB597 and AM251 on behavioral deficits reminiscent of the symptoms of schizophrenia and further validate the potential for cannabinoid-based drugs as a treatment for schizophrenia.

Changes in Sociability and Preference for Social Novelty in Female Rats in Prolonged Social Isolation

Chronic stress due to social isolation (SI) can lead to distress with negative consequences for both humans and animals. Numerous disorders caused by SI include disorders in the emotional-motivational domain and cognitive functions, as well as changes in social behavior. There are currently no data identifying the sequelae of SI when its duration is significantly increased. Although female rats have been shown to be highly sensitive to stress, research on them is lacking. The present study assessed sociability and preference for “social novelty” in a three-chamber social test in female Wistar rats in two series of experiments at different time points during prolonged SI, which began at adolescence and continued to ages 5.5 and 9.5 months. At two months of SI, rats showed an increased preference for a social object over a non-social object (increased sociability) simultaneously with the appearance of signs of a decrease in the preference for a new social object over an already familiar social object (signs of a decrease in the preference for social novelty). In a social interaction test, the rats also displayed increases in the durations of social contacts, including aggressive interactions; they showed a decrease in exploratory risk assessments (head dips from the open arms) in the elevated plus maze test and a decrease in exploratory activity. After SI lasting 8.5 months, the rats showed signs of social deficit and a marked decrease in the preference for social novelty. No signs of increased aggressiveness were found. Thus, the impact of SI on social behavior depended on its duration and, we believe, was accompanied by a change in coping strategies.

URB597 induces subtle changes to aggression in adult Lister Hooded rats

The endocannabinoid system has been implicated in both social and cognitive processing. The endocannabinoid metabolism inhibitor, URB597, dose-dependently improves non-social memory in adult Wistar and Sprague Dawley rats, whereas its effect on social interaction (SI) is affected by both rat strain and drug dose. Lister Hooded rats consistently respond differently to drug treatment in general compared with albino strains. This study sought to investigate the effects of different doses of URB597 on social and non-social memory in Lister Hooded rats, as well as analyzing the behavioral composition of the SI. Males were tested for novel object recognition (NOR), social preference (between an object and an unfamiliar rat), social novelty recognition (for a familiar vs. unfamiliar rat) and SI with an unfamiliar rat. URB597 (0.1 or 0.3 mg/kg) or vehicle was given 30 min before testing. During SI testing, total interaction time was assessed along with time spent on aggressive and explorative behaviors. Lister Hooded rats displayed expected non-social and social memory and social preference, which was not affected by URB597. During SI, URB597 did not affect total interaction time. However, the high dose increased aggression, compared to vehicle, and decreased anogenital sniffing, compared to the low dose of URB597. In summary, URB597 did not affect NOR, social preference or social recognition memory but did have subtle behavioral effects during SI in Lister hooded rats. Based on our findings we argue for the importance of considering strain as well as the detailed composition of behavior when investigating drug effects on social behavior.

Signal Transducer and Activator of Transcription 3 Activation in Hippocampal Neural Stem Cells and Cognitive Deficits in Mice Following Short-term Cuprizone Exposure

Recent studies have emphasized that adult hippocampal neurogenesis impairment may be associated with cognitive problems. Because cuprizone (CPZ), a copper-chelating reagent, was shown to decrease the production of new neurons, we aimed to further understand the involvement of adult hippocampal neurogenesis impairment in cognitive function by using a short-term (2-week) CPZ exposure paradigm. The CPZ-fed mice showed cognitive deficits, i.e., impaired sensorimotor gating and reduced social novelty preference, compared to normal-fed mice. Although a long-term (e.g., 5-week) CPZ exposure paradigm was found to cause demyelination, we encountered that the labeling for myelin in the hippocampus was unaffected by 2-week CPZ exposure. The densities of neuronal progenitor cells (NPCs) and newborn granule cells (NGCs) were lower in CPZ-fed mice than in normal-fed mice, while those of neural stem cells (NSCs) were comparable between groups. We then examined whether short-term CPZ exposure might induce activation of signal transducer and activator of transcription 3 (STAT3), which plays a major role in cytokine receptor signaling. The densities of phosphorylated STAT3-positive (pSTAT3⁺) NSCs were higher in CPZ-fed mice than in normal-fed mice, while those of pSTAT3⁺ NPCs/NGCs were very low in both groups. Interestingly, the densities of bromodeoxyuridine-positive (BrdU⁺) NSCs were higher in CPZ-fed mice than in normal-fed mice 2 weeks after BrdU injection, while those of BrdU⁺ NPCs/NGCs were lower in CPZ-fed mice than in normal-fed mice. These findings suggest that short-term CPZ exposure inhibits differentiation of NSCs into NPCs via activation of STAT3, which may in part underlie cognitive deficits.

Endocannabinoid-serotonin systems interaction in health and disease

Endocannabinoid (eCB) and serotonin (5-HT) neuromodulatory systems work both independently and together to finely orchestrate neuronal activity throughout the brain to strongly sculpt behavioral functions. Surprising parallelism between the behavioral effects of 5-HT and eCB activity has been widely reported, including the regulation of emotional states, stress homeostasis, cognitive functions, food intake and sleep. The distribution pattern of the 5-HT system and the eCB molecular elements in the brain display a strong overlap and several studies report a functional interplay and even a tight interdependence between eCB/5-HT signaling. In this review, we examine the available evidence of the interaction between the eCB and 5-HT systems. We first introduce the eCB system, then we describe the eCB/5-HT crosstalk at the neuronal and synaptic levels. Finally, we explore the potential eCB/5-HT interaction at the behavioral level with the implication for psychiatric and neurological disorders. The precise elucidation of how this neuromodulatory interaction dynamically regulates biological functions may lead to the development of more targeted therapeutic strategies for the treatment of depressive and anxiety disorders, psychosis and epilepsy.

From the Clinic to the Laboratory, and Back Again: Investigations on Cannabinoids and Endocannabinoid System Modulators for Treating Schizophrenia

The present mini-review focuses on animal models of schizophrenia that have explored the effects of cannabidiol (CBD; a non-psychoactive component of cannabis) or the pharmacological manipulation of the endocannabinoid system on behavioral and cognitive outcome measures. First, results of some relevant clinical studies in this area are summarized, and then pre-clinical work on animal models of schizophrenia based on NMDA receptor antagonism or neurodevelopmental manipulations are discussed. A brief overview is given of the theoretical framework on which these models are based, along with a concise summary of results that have been obtained. Clinical results using CBD for schizophrenia seem promising and its effects in animal models of schizophrenia support its potential as a useful pharmacotherapy. Animal models have been paramount for elucidating the actions of CBD and the function of the endocannabinoid system and for identifying novel pharmacological targets, such as cannabinoid receptors and anandamide. However, more attention needs to be placed on defining and applying independent variables and outcome measures that are comparable between pre-clinical and clinical studies. The objective of this review is, on the one hand, to emphasize the potential of such models to predict clinical response to experimental drugs, and on the other hand, to highlight areas in which research on such models could be improved.

Mesenchymal stem cells derived extracellular vesicles improve behavioral and biochemical deficits in a phencyclidine model of schizophrenia

Schizophrenia is a debilitating psychiatric disorder with a significant number of patients not adequately responding to treatment. Phencyclidine (PCP) is used as a validated model for schizophrenia, shown to reliably induce positive, negative and cognitive-like behaviors in rodents. It was previously shown in our lab that behavioral phenotypes of PCP-treated mice can be alleviated after intracranial transplantation of mesenchymal stem cells (MSC). Here, we assessed the feasibility of intranasal delivery of MSCs-derived-extracellular vesicles (EVs) to alleviate schizophrenia-like behaviors in a PCP model of schizophrenia. As MSCs-derived EVs were already shown to concentrate at the site of lesion in the brain, we determined that in PCP induced injury the EVs migrate to the prefrontal cortex (PFC) of treated mice, a most involved area of the brain in schizophrenia. We show that intranasal delivery of MSC-EVs improve social interaction and disruption in prepulse inhibition (PPI) seen in PCP-treated mice. In addition, immunohistochemical studies demonstrate that the EVs preserve the number of parvalbumin-positive GABAergic interneurons in the PFC of treated mice. Finally, MSCs-EVs reduced glutamate levels in the CSF of PCP-treated mice, which might explain the reduction of toxicity. In conclusion, we show that MSCs-EVs improve the core schizophrenia-like behavior and biochemical markers of schizophrenia and might be used as a novel treatment for this incurable disorder.

Differential effects of Δ9-tetrahydrocannabinol dosing on correlates of schizophrenia in the sub-chronic PCP rat model

Social withdrawal in the sub-chronic phencyclidine (PCP) rat model, a behavioral correlate of the negative symptoms of schizophrenia, results from deficits in brain endocannabinoid transmission. As cannabis intake has been shown to affect negatively the course and expression of psychosis, we tested whether the beneficial effects of endocannabinoid-mediated CB₁ activation on social withdrawal in PCP-treated rats (5 mg/kg, twice daily for 7 days)also occurred after administration of Δ9-tetrahydrocannabinol (THC; 0.1, 0.3, 1.0 mg/kg, i.p.). In addition, we assessed whether THC affected two correlates of positive symptoms: 1) motor activity induced by d-amphetamine (0.5 mg/kg, i.p.), and 2) dopamine neuron population activity in the ventral tegmental area (VTA). After the motor activity test, the brains from d-amphetamine-treated animals were collected and processed for measurements of endocannabinoids and activation of Akt/GSK3β, two molecular markers involved in the pathophysiology of schizophrenia. In control rats, THC dose-dependently produced social interaction deficits and aberrant VTA dopamine neuron population activity similar to those observed in PCP-treated animals. In PCP-treated rats, only the lowest dose of THC reversed PCP-induced deficits, as well as PCP-induced elevation of the endocannabinoid anandamide (AEA) in the nucleus accumbens. Last, THC activated the Akt/GSK3β pathway dose-dependently in both control and PCP-treated animals. Taken together, these data suggest that only low doses of THC have beneficial effects on behavioral, neurochemical and electrophysiological correlates of schizophrenia symptoms. This observation may shed some light on the controversial hypothesis of marijuana use as self-medication in schizophrenic patients.

The Benefits of Using New Tools for Behavioral Investigations in Animal Experimental Models

The animal experimental models of emotional disorders attempt to reproduce features of human psychiatric disorders in laboratory animals by correlating the physiological and behavioral changes associated with specific emotional states, the etiology of disorders, and responses on drug treatments. Animal experimental models that accurately recapitulate clinical pathology are indispensable for understanding molecular mechanisms and advancing preclinical studies. Behavioral tests on animal models have been developed over time in two directions: to enable the acquisition of as many valid behavior data as possible, and constructing experimental models and procedures that represent a parallel with certain conditions in humans. In this review we discuss more information for the new tools in behavioral investigations in animal experimental models. Here we described evoked beam-walking (EBW) test as a new test for estimation of anxiety levels. The reliability of that test was confirmed in our studies by using nandrolone decanoate (ND) and testosterone enanthate (TE) in supraphysiological doses. Also, we defined a new approach to estimation of exploratory activity by using these tests and an improvement of detectability in standard evaluation of depressive state levels. Taking into account that behavioral investigation in animal models still has to remain indispensable in conducting of preclinical studies, we assume that new tools that can be applied in this field may improve the quality of research.

Social modulation of drug use and drug addiction

This review aims to demonstrate how social science and behavioral neurosciences have highlighted the influence of social interactions on drug use in animal models. In neurosciences, the effect of global social context that are distal from drug use has been widely studied. For human and other social animals such as monkeys and rodents, positive social interactions are rewarding, can overcome drug reward and, in all, protect from drug use. In contrast, as other types of stress, negative social experiences facilitate the development and maintenance of drug abuse. However, interest recently emerged in the effect of so-called "proximal" social factors, that is, social interactions during drug-taking. These recent studies have characterized the role of the drug considered, the sharing of drug experience and the familiarity of the peer which interaction are made with. We also examine the few studies regarding the sensorial mediator of social behaviors and critically review the neural mediation of social factors on drug use. However, despite considerable characterization of the factors modulating distal influences, the mechanisms for proximal influences on drug use remain largely unknown. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Male rats treated with subchronic PCP show intact olfaction and enhanced interest for a social odour in the olfactory habituation/dishabituation test

The olfactory system participates in many sensory processes, and olfactory endophenotypes appear in a variety of neurological disorders such as Alzheimer's and Parkinson's disease, depression and schizophrenia. Social withdrawal is a core negative symptom of schizophrenia and animal models have proven to be invaluable for studying the neurobiological mechanisms and cognitive processes behind the formation of social relationships. The subchronic phencyclidine (PCP) rat model is a validated model for negative symptoms of schizophrenia, such as impaired sociability. However, the complete range of social behaviour and deficits in the model are still not fully understood. Intact rodent olfaction is essential for a wide range of social behaviour and disrupted olfactory function could have severe effects on social communication and recognition. In order to examine the olfactory ability of male rats treated with subchronic PCP, we conducted an olfactory habituation/dishabituation test including both non-social and social odours. The subchronic PCP-treated rats successfully recognized and discriminated among the odours, indicative of intact olfaction. Interestingly, the subchronic PCP-treated rats showed greater interest for a novel social odour compared to the saline-treated rats and the rationale remains to be elucidated. Our data indicate that subchronic PCP treatment does not disrupt olfactory function in male rats. By ruling out impaired olfaction as cause for the poor social interaction performance in subchronic PCP-treated rats, our data supports the use of NMDA receptor antagonists to model the negative symptoms of schizophrenia.

Oral haloperidol or olanzapine intake produces distinct and region-specific increase in cannabinoid receptor levels that is prevented by high fat diet

Clinical studies show higher levels of cannabinoid CB1 receptors (CB1R) in the brain of schizophrenic patients while preclinical studies report a significant functional interaction between dopamine D2 receptors and CB1Rs as well as an upregulation of CB1Rs after antipsychotic treatment. These findings prompted us to study the effects of chronic oral intake of a first and a second generation antipsychotic, haloperidol and olanzapine, on the levels and distribution of CB1Rs in the rat brain. Rats consumed either regular chow or high-fat food and drank water, haloperidol drinking solution (1.5mg/kg), or olanzapine drinking solution (10mg/kg) for four weeks. Motor and cognitive functions were tested at the end of treatment week 3 and upon drug discontinuation. Two days after drug discontinuation, rats were euthanized and brains were processed for in vitro receptor autoradiography. In chow-fed animals, haloperidol and olanzapine increased CB1R levels in the basal ganglia and the hippocampus, in a similar, but not identical pattern. In addition, olanzapine had unique effects in CB1R upregulation in higher order cognitive areas, in the secondary somatosensory cortex, in the visual and auditory cortices and the geniculate nuclei, as well as in the hypothalamus. High fat food consumption prevented antipsychotic-induced increase in CB1R levels in all regions examined, with one exception, the globus pallidus, in which they were higher in haloperidol-treated rats. The results point towards the hypothesis that increased CB1R levels could be a confounding effect of antipsychotic medication in schizophrenia that is circumveneted by high fat feeding.

Anxiety does not contribute to social withdrawal in the subchronic phencyclidine rat model of schizophrenia

Social withdrawal should not be considered a direct measure of the negative symptoms of schizophrenia as it may result not only from asociality (primary negative symptom) but also from other altered processes such as anxiety. To understand the contribution of these two factors to social deficit, we investigated whether the social withdrawal observed in the subchronic phencyclidine (PCP) rat model of schizophrenia could be attributed to increased anxiety. Compared to saline controls, PCP-treated rats (5 mg/kg, twice daily for 7 days, followed by a washout period) spent significantly less time in social interaction, but did not show anxiety-like behaviors in different relevant behavioral paradigms. In addition, their social deficit was not affected by a behavioral procedure known to reduce anxiety-like behavior (repeated exposure to the same partner) nor by systemic administration of the classical anxiolytic diazepam. In contrast, PCP-induced social withdrawal was reversed by the cannabinoid agonist CP55,940, a drug with known anxiogenic properties. Furthermore, when using the social approach task, PCP-treated animals performed similarly to control animals treated with diazepam, but not to those treated with the anxiogenic compound pentylenetetrazole. Taken together, our results indicate that PCP-induced social withdrawal cannot be attributed to increased anxiety. These data are discussed in the context of primary versus secondary negative symptoms and the deficit syndrome of schizophrenia.

A variant on promoter of the cannabinoid receptor 1 gene (CNR1) moderates the effect of valence on working memory

Cannabinoid receptor 1 gene (CNR1) variants have been related to affective information processing and, in particular, to stress release. Here, we aimed to examine whether the endocannabinoid system via CNR1 signaling modulates affective working memory, the memory system that transiently maintains and manipulates emotionally charged material. We focused on rs2180619 (A > G) polymorphism and examined genotype data collected from 231 healthy females. Analyses showed how a general positivity bias in working memory (i.e., better memory for positive words) emerged as task strings lengthened only in carriers of the major allele (AA/AG). Differently, GG carriers showed better memory for affective items in general (i.e., positive and negative words). These findings are some of the first to directly highlight the role of variant on promoter of the CNR1 gene in affective working memory and to evidence a differentiation among CNR1 genotypes in terms of larger difficulties in disengaging from negative stimuli in GG carriers.

Short- and long-term behavioral analysis of social interaction, ultrasonic vocalizations and social motivation in a chronic phencyclidine model

Phencyclidine (PCP) has been suggested to induce symptoms of schizophrenia. However, animal models using PCP administration have produced ambiguous results thus far. It seems that acute effects are similar to symptoms of schizophrenia, however, it is not clear if PCP can induce permanent behavioral changes that reflect schizophrenic-like symptoms. Therefore, we assessed the ability of chronic PCP administration (3mg/kg, 14 days) to induce short or long lasting behavioral changes in rats. Social behavior, including ultrasonic vocalizations and motivation for social contact were investigated at different time points, up to 29-36 days, after cessation of PCP treatment. During a social separation test, performed at 5 and 36 days, PCP treated rats spent less time near the divider that separates them from their familiar cage mate compared with saline (SAL) treated rats. Further, at short term, PCP was able to induce a decrease in social behavior. In contrast, at long-term, PCP treated animals spent more time in contact when exposed to an unfamiliar partner as compared to SAL treated rats. But, this difference was not observed when exposed to a familiar partner. We did not find any difference in ultrasonic vocalizations at all time points. The results of our study indicate that PCP is unable to induce overt long term deficits in social interaction behavior. Rather, it seems that PCP diminishes motivation for social contact. The long-term consequences of chronic PCP administration on social behavior in rodent models remain complex, and future studies addressing this are still needed.

Decreased CB receptor binding and cannabinoid signaling in three brain regions of a rat model of schizophrenia

Schizophrenia is a serious mental health disorder characterized by several behavioral and biochemicel abnormalities. In a previous study we have shown that mu-opioid (MOP) receptor signaling is impaired in specific brain regions of our three-hit animal model of schizophrenia. Since the cannabinoid system is significantly influenced in schizophrenic patients, in the present work we investigated cannabinoid (CB) receptor binding and G-protein activation in cortical, subcortical and cerebellar regions of control and 'schizophrenic' rats. Cannabinoid agonist (WIN-55,212-2 mesylate) mediated G-protein activation was consistently decreased in all areas tested, and the difference was extremely significant in membranes prepared from the cerebellum. Interestingly, the cerebellar activity of WIN-55,212-2 stimulated G-proteins was substantially higher than those of cerebral cortex and subcortical region in control animals, indicating a primordial role of the cannabinoid system in the cerebellum. At the level of radioligand binding, the affinities of the CB receptors were also markedly decreased in the model animals. Capacity of the [³H]WIN-55,212-2 binding was only higher in the cerebellum of 'schizophrenic' model rats. Taken together, in all three brain areas of model rats both cannabinoid receptor binding and cannabinoid agonist-mediated G-protein activation were regularly decreased. Our results revealed that besides the opioids, the endocannabinoid - cannabis receptor system also shows impairment in our rat model, increasing its face validity and translational utility.

Distinct neuronal activation patterns are associated with PCP-induced social withdrawal and its reversal by the endocannabinoid-enhancing drug URB597

The fatty acid amide hydrolase inhibitor, URB597, an endocannabinoid enhancing drug, reverses social withdrawal in the sub-chronic PCP rat model of schizophrenia, but reduces social interaction (SI) in controls. To identify the anatomical substrates associated with PCP-induced social withdrawal and the contrasting effects of URB597 on SI in PCP- versus saline-treated rats, we analyzed SI-induced c-Fos expression in 28 brain areas relevant to schizophrenia and/or social behavior following vehicle or URB597 administration. In saline-treated rats, SI was accompanied by changes in c-Fos expression in the infralimbic and orbitofrontal cortices, dorsomedial caudate putamen, ventrolateral nucleus of the septum, dorsolateral periaqueductal gray (dlPAG) and central amygdala. Except for the dlPAG, these changes were not observed in PCP-treated rats or in saline-treated rats receiving URB597. In the dorsomedial part of the bed nucleus of the stria terminalis (dmBNST), SI-induced c-Fos expression was observed only in PCP-treated rats. Interestingly, URB597 in PCP-treated rats restored a similar c-Fos expression pattern as observed in saline-treated rats: activation of the orbitofrontal cortex, inhibition of the central amygdala and suppression of activation of the dmBNST. These data suggest that orbitofrontal cortex, central amygdala and dmBNST play a critical role in the reversal of PCP-induced social withdrawal by URB597.