The subchronic phencyclidine rat model: relevance for the assessment of novel therapeutics for cognitive impairment associated with schizophrenia

Effects of chronic stress on cognitive function - From neurobiology to intervention

Exposure to chronic stress contributes considerably to the development of cognitive impairments in psychiatric disorders such as depression, generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and addictive behavior. Unfortunately, unlike mood-related symptoms, cognitive impairments are not effectively treated by available therapies, a situation in part resulting from a still incomplete knowledge of the neurobiological substrates that underly cognitive domains and the difficulty in generating interventions that are both efficacious and safe. In this review, we will present an overview of the cognitive domains affected by stress with a specific focus on cognitive flexibility, behavioral inhibition, and working memory. We will then consider the effects of stress on neuronal correlates of cognitive function and the factors which may modulate the interaction of stress and cognition. Finally, we will discuss intervention strategies for treatment of stress-related disorders and gaps in knowledge with emerging new treatments under development. Understanding how cognitive impairment occurs during exposure to chronic stress is crucial to make progress towards the development of new and effective therapeutic approaches.

NU-1223, a simplified analog of alstonine, with 5-HT2cR agonist-like activity, rescues memory deficit and positive and negative symptoms in subchronic phencyclidine mouse model of schizophrenia

The serotonin (5-HT)2 C receptor(R) is a widely distributed G-protein-coupled receptor, expressed abundantly in the central nervous system. Alstonine is a natural product that has significant properties of atypical antipsychotic drugs (AAPDs), in part attributed to 5-HT2 CR agonism. Based on alstonine, we developed NU-1223, a simplified β carboline analog of alstonine, which shows efficacies comparable to alstonine and to other 5-HT2 CR agonists, Ro-60-0175 and lorcaserin. The 5-HT2 CR antagonism of some APDs, including olanzapine, contributes to weight gain, a major side effect which limits its tolerability, while the 5-HT2 CR agonists and/or modulators, may minimize weight gain. We used the well-established rodent subchronic phencyclidine (PCP) model to test the efficacy of NU-1223 on episodic memory, using novel object recognition (NOR) task, positive (locomotor activity), and negative symptoms (social interaction) of schizophrenia (SCH). We found that NU-1223 produced both transient and prolonged rescue of the subchronic PCP-induced deficits in NOR and SI. Further, NU-1223, but not Ro-60-0175, blocked PCP and amphetamine (AMPH)-induced increase in LMA in subchronic PCP mice. These transient efficacies in LMA were blocked by the 5-HT2 CR antagonist, SB242084. Sub-chronic NU-1223 treatment rescued NOR and SI deficits in subchronic PCP mice for at least 39 days after 3 days injection. Chronic treatment with NU-1223, ip, twice a day for 21 days, did not increase average body weight vs olanzapine. These findings clearly indicate NU-1223 as a class of small molecules with a possible 5-HT2 CR-agonist-like mechanism of action, attributing to its efficacy. Additional in-depth receptor mechanistic studies are warranted, as this small molecule, both transiently and chronically rescued PCP-induced deficits. Furthermore, NU-1223 did not induce weight gain post long-term administrations vs AAPDs such as olanzapine, making NU-1223 a putative therapeutic compound for SCH.

Connecting Neurobiological Features with Interregional Dysconnectivity in Social-Cognitive Impairments of Schizophrenia

Schizophrenia (SZ) is a devastating psychiatric disorder affecting about 1% of the world's population. Social-cognitive impairments in SZ prevent positive social interactions and lead to progressive social withdrawal. The neurobiological underpinnings of social-cognitive symptoms remain poorly understood, which hinders the development of novel treatments. At the whole-brain level, an abnormal activation of social brain regions and interregional dysconnectivity within social-cognitive brain networks have been identified as major contributors to these symptoms. At the cellular and subcellular levels, an interplay between oxidative stress, neuroinflammation and N-methyl-D-aspartate receptor hypofunction is thought to underly SZ pathology. However, it is not clear how these molecular processes are linked with interregional dysconnectivity in the genesis of social-cognitive symptoms. Here, we aim to bridge the gap between macroscale (connectivity analyses) and microscale (molecular and cellular mechanistic) knowledge by proposing impaired myelination and the disinhibition of local microcircuits as possible causative biological pathways leading to dysconnectivity and abnormal activity of the social brain. Furthermore, we recommend electroencephalography as a promising translational technique that can foster pre-clinical drug development and discuss attractive drug targets for the treatment of social-cognitive symptoms in SZ.

Introduction of a pharmacological neurovascular uncoupling model in rats based on results of mice

Our aim was to establish a pharmacologically induced neurovascular uncoupling (NVU) method in rats as a model of human cognitive decline. Pharmacologically induced NVU with subsequent neurological and cognitive defects was described in mice, but not in rats so far. We used 32 male Hannover Wistar rats. NVU was induced by intraperitoneal administration of a pharmacological "cocktail" consisting of N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide (MSPPOH, a specific inhibitor of epoxyeicosatrienoic acid-producing epoxidases, 5 mg kg-1), L-NG-nitroarginine methyl ester (L-NAME, a nitric oxide synthase inhibitor, 10 mg kg-1) and indomethacin (a nonselective inhibitor of cyclooxygenases, 1 mg kg-1) and injected twice daily for 8 consecutive days. Cognitive performance was tested in the Morris water-maze and fear-conditioning assays. We also monitored blood pressure. In a terminal operation a laser Doppler probe was used to detect changes in blood-flow (CBF) in the barrel cortex while the contralateral whisker pad was stimulated. Brain and small intestine tissue samples were collected post mortem and examined for prostaglandin E2 (PGE2) level. Animals treated with the "cocktail" showed no impairment in their performance in any of the cognitive tasks. They had higher blood pressure and showed cca. 50% decrease in CBF. Intestinal bleeding and ulcers were found in some animals with significantly decreased levels of PGE2 in the brain and small intestine. Although we could evoke NVU by the applied mixture of pharmacons, it also induced adverse side effects such as hypertension and intestinal malformations while the treatment did not cause cognitive impairment. Thus, further refinements are still required for the development of an applicable model.

Repeated administration of rapastinel produces exceptionally prolonged rescue of memory deficits in phencyclidine-treated mice

Rapastinel, a positive N-methyl-D-aspartate receptor (NMDAR) modulator with rapid-acting antidepressant properties, rescues memory deficits in rodents. We have previously reported that a single intravenous dose of rapastinel, significantly, but only transiently, prevented and rescued deficits in the novel object recognition (NOR) test, a measure of episodic memory, produced by acute or subchronic administration of the NMDAR antagonists, phencyclidine (PCP) and ketamine. Here, we tested the ability of single and multiple subcutaneous doses per day of rapastinel to restore NOR and operant reversal learning (ORL) deficits in subchronic PCP-treated mice. Rapastinel, 1 or 3 mg/kg, administered subcutaneously, 30 min before NOR or ORL testing, respectively, transiently rescued both deficits in subchronic PCP mice. This effect of rapastinel on NOR and ORL was mammalian target of rapamycin (mTOR)-dependent. Most importantly, 1 mg/kg rapastinel given twice daily for 3 or 5 days, but not 1 day, restored NOR for at least 9 and 10 weeks, respectively, which is an indication of neuroplastic effects on learning and memory. Both rapastinel (3 mg/kg) and ketamine (30 mg/kg), moderately increased the efflux of dopamine, norepinephrine, and serotonin in medial prefrontal cortex; however, only ketamine increased cortical glutamate efflux. This observation was likely the basis for the contrasting effects of the two drugs on cognition.

Wisket rat model of schizophrenia: Impaired motivation and, altered brain structure, but no anhedonia

It is well-known that the poor cognition in schizophrenia is strongly linked to negative symptoms, including motivational deficit, which due to, at least partially, anhedonia. The goal of this study was to explore whether the schizophrenia-like Wisket animals with impaired motivation (obtained in the reward-based hole-board test), also show decreased hedonic behavior (investigated with the sucrose preference test). While neurochemical alterations of different neurotransmitter systems have been detected in the Wisket rats, no research has been performed on structural changes. Therefore, our additional aim was to reveal potential neuroanatomical and structural alterations in different brain regions in these rats. The rats showed decreased general motor activity (locomotion, rearing and exploration) and impaired task performance in the hole-board test compared to the controls, whereas no significant difference was observed in the sucrose preference test between the groups. The Wisket rats exhibited a significant decrease in the frontal cortical thickness and the hippocampal area, and moderate increases in the lateral ventricles and cell disarray in the CA3 subfield of hippocampus. To our knowledge, this is the first study to investigate the hedonic behavior and neuroanatomical alterations in a multi-hit animal model of schizophrenia. The results obtained in the sucrose preference test suggest that anhedonic behavior might not be involved in the impaired motivation obtained in the hole-board test. The neuropathological changes agree with findings obtained in patients with schizophrenia, which refine the high face validity of the Wisket model.

Contrasting Typical and Atypical Antipsychotic Drugs

The beliefs that antipsychotic drugs (APDs) are 1) effective only to treat delusions and hallucinations (positive symptoms), 2) that typical and atypical APDs differ only in ability to cause extrapyramidal side effects, and 3) that their efficacy as antipsychotics is due solely to their dopamine D₂ receptor blockade are outmoded concepts that prevent clinicians from achieving optimal clinical results when prescribing an APD. Atypical APDs are often more effective than typical APDs in treating negative symptoms, cognitive impairment, and mood symptoms as well as reducing the risk for suicide and decreasing aggression. This applies not only to those diagnosed with schizophrenia or schizoaffective disorder but also to bipolar disorder, major depression, and other psychiatric diagnoses. The greater advantage of an atypical APD is not evident in all patients for every atypical APD due, in part, to individual differences in genetic and epigenetic endowment and differences in the pharmacology of the atypical APDs, their mode of action being far more complex than that of the typical APDs. A common misconception is that among the atypical APDs, only clozapine is effective for reducing psychosis in treatment-resistant schizophrenia. Aripiprazole, lurasidone, olanzapine, and risperidone also can be more effective than typical APDs for treatment-resistant schizophrenia; clozapine is uniquely indicated for reducing the risk for suicide. The ability of the atypical APDs to improve cognition and negative symptoms in some patients together with lower propensity to cause tardive dyskinesia (an underappreciated advantage) leads to better overall outcomes. These advantages of the atypical APDs in efficacy and safety are due, in part, to initiation of synaptic plasticity via direct and indirect effects of the atypical APDs on a variety of proteins, especially G proteins, and release of neurotrophins (e.g., brain-derived neurotrophic factor). The typical APDs beneficial effects on psychosis are mainly the result of D₂ receptor blockade, which can be associated with serious side effects and lack of tolerability.

Mismatch negativity as EEG biomarker supporting CNS drug development: a transnosographic and translational study

The lack of translation from basic research into new medicines is a major challenge in CNS drug development. The need to use novel approaches relying on (i) patient clustering based on neurobiology irrespective to symptomatology and (ii) quantitative biomarkers focusing on evolutionarily preserved neurobiological systems allowing back-translation from clinical to nonclinical research has been highlighted. Here we sought to evaluate the mismatch negativity (MMN) response in schizophrenic (SZ) patients, Alzheimer's disease (AD) patients, and age-matched healthy controls. To evaluate back-translation of the MMN response, we developed EEG-based procedures allowing the measurement of MMN-like responses in a rat model of schizophrenia and a mouse model of AD. Our results indicate a significant MMN attenuation in SZ but not in AD patients. Consistently with the clinical findings, we observed a significant attenuation of deviance detection (~104.7%) in rats subchronically exposed to phencyclidine, while no change was observed in APP/PS1 transgenic mice when compared to wild type. This study provides new insight into the cross-disease evaluation of the MMN response. Our findings suggest further investigations to support the identification of neurobehavioral subtypes that may help patients clustering for precision medicine intervention. Furthermore, we provide evidence that MMN could be used as a quantitative/objective efficacy biomarker during both preclinical and clinical stages of SZ drug development.

Phosphodiesterase type 1 inhibition alters medial prefrontal cortical activity during goal-driven behaviour and partially reverses neurophysiological deficits in the rat phencyclidine model of schizophrenia

Positive modulation of cAMP signalling by phosphodiesterase (PDE) inhibitors has recently been explored as a potential target for the reversal of cognitive and behavioural deficits implicating the corticoaccumbal circuit. Previous studies show that PDE type 1 isoform B (PDE1B) inhibition may improve memory function in rodent models; however, the contribution of PDE1B inhibition to impulsivity, attentional and motivational functions as well as its neurophysiological effects have not been investigated. To address this, we recorded single unit activity in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in Lister Hooded rats treated with the PDE1B inhibitor Lu AF64386 and tested in the 5-choice serial reaction time task (5-CSRTT). We also asked whether PDE1B inhibition modulates neurophysiological deficits produced by subchronic phencyclidine (PCP) treatment, a rat pharmacological model of schizophrenia. Lu AF64386 significantly affected behavioural parameters consistent with a reduction in goal-directed behaviour, however without affecting accuracy. Additionally, it reduced mPFC neuronal activity. Pre-treatment with PCP did not affect behavioural parameters, however it significantly disrupted overall neuronal firing while increasing phasic responses to reward-predicting cues and disrupting mPFC-NAc cross-talk. The latter two effects were reversed by Lu AF64386. These findings suggest PDE1B inhibition may be beneficial in disorders implicating a dysfunction of the mPFC-NAc network.

Phencyclidine-induced cognitive impairments in repeated touchscreen visual reversal learning tests in rats

Reversal learning, a component of executive functioning, is commonly impaired among schizophrenia patients and is lacking effective treatment. N-methyl-ᴅ-aspartate (NMDA) receptor antagonists, such as phencyclidine (PCP), impair reversal learning of rodents. Touchscreen-based pairwise visual discrimination and reversal test is a translational tool to assess reversal learning in rodents. However, to fully exploit this task in testing of novel compounds, it is necessary to perform several reversal learning experiments with trained animals. Firstly, we assessed whether PCP-induced deficits in visual reversal learning in rats would be detectable with a short (5 sessions) reversal learning phase, and whether the short reversal phases could be repeated with novel stimulus pairs. Secondly, we assessed whether the PCP-induced deficits in reversal learning could be seen upon repeated PCP challenges with the same animals. Finally, we tested the effect of a novel compound, a selective α_2C adrenoceptor antagonist, ORM-13070, to reverse PCP-induced cognitive deficits in this model. A 4-day PCP treatment at a dose of 1.5 mg/kg/day impaired early reversal learning in male Lister Hooded rats without inducing non-specific behavioral effects. We repeated the reversal learning experiment four times using different stimulus pairs with the same animals, and the PCP-induced impairment was evident in every single experiment. The α_2C adrenoceptor antagonist ameliorated the PCP-induced cognitive deficits. Our results suggest that repeated PCP challenges in the touchscreen set-up induce schizophrenia-like cognitive deficits in visual reversal learning, improve throughput of the test and provide a protocol for testing novel drugs.

α7 nicotinic receptor agonist and positive allosteric modulators differently improved schizophrenia-like cognitive deficits in male rats

The majority of schizophrenia patients have cognitive deficits as a separate symptom cluster independent of positive or negative symptoms. Current medicines, unfortunately, cannot provide clear benefits for cognitive symptoms in patients. Recent findings showed decreased α7 nicotinic acetylcholine receptor (nAChR) expressions in subjects with schizophrenia. α7 nAChR full/partial agonists and positive allosteric modulators (PAMs) may be valuable drug candidates to treat cognitive deficits of disease. This study comparatively investigated the effect of α7 nAChR agonist (A-582941), type I PAM (CCMI), type II PAM (PNU-120596), and the antipsychotic drug (clozapine) on behavioral, molecular, and immunohistochemical parameters in a subchronic MK-801 model of schizophrenia in male rats. Novel object recognition (NOR) and Morris water maze (MWM) tests were performed to evaluate recognition and spatial memories, respectively. Gene and protein expressions of parvalbumin, glutamic acid decarboxylase-67 (GAD67), and α7 nAChR were examined in the rats' hippocampal tissue. The subchronic MK-801 administration produced cognitive deficits in the NOR and MWM tests. It also decreased the protein and gene expressions of parvalbumin, GAD67, and α7 nAChR in the hippocampus. Clozapine, A-582941, and PNU-120596 but not CCMI increased the parvalbumin and α7 nAChR expressions and provided benefits in recognition memory. Interestingly, clozapine and CCMI restored the MK-801 induced deficits on GAD1 expression and spatial memory while A-582941 and PNU-120596 were ineffective. These results indicated that α7 nAChR agonist, type I and type II PAMs may provide benefits in different types of cognitive deficits rather than a complete treatment in schizophrenia.

Aerobic exercise improves memory and prevents cognitive deficits of relevance to schizophrenia in an animal model

INTRODUCTION AND OBJECTIVES

Cognitive impairment associated with schizophrenia (CIAS) greatly reduces patients' functionality, and remains an unmet clinical need. The sub-chronic phencyclidine (scPCP) rat model is commonly employed in studying CIAS. We have previously shown that voluntary exercise reverses impairments in novel object recognition (NOR) induced by scPCP. However, there has not been a longitudinal study investigating the potential protective effects of exercise in a model of CIAS. This study aimed to investigate the pro-cognitive and protective effects of exercise on CIAS using the translational NOR and attentional set-shifting tasks (ASST).

METHODS

Female Lister Hooded rats were either exercised (wheel running for one hour per day, five days per week, for six weeks; n=20) or not (n=20) and then tested in a natural-forgetting NOR test. Rats in each group were then administered either PCP (2 mg/kg intraperitoneally (i.p.)) or saline solution (1 mL/kg i.p.) for seven days, followed by seven days washout. Three NOR tests were conducted immediately and two and nine weeks after washout, and a natural-forgetting NOR test was carried out again eight weeks post washout. Rats were trained and tested in ASST from week 6 to week 10 post washout.

RESULTS

Non-exercised rats displayed a deficit in both of the natural-forgetting NOR tests, whereas exercised rats did not. The scPCP exercise group did not show the expected deficit in NOR at any time point, and had a significantly ameliorated deficit in the ASST compared to the scPCP control group.

CONCLUSION

Voluntary exercise has long-lasting pro-cognitive and protective effects in two cognitive domains. Exercise improves cognition and could provide protection against CIAS.

Glutamatergic and Serotonergic Modulation of Rat Medial and Lateral Orbitofrontal Cortex in Visual Serial Reversal Learning

Adapting behavior to a dynamic environment requires both steadiness when the environment is stable and behavioral flexibility in response to changes. Much evidence suggests that cognitive flexibility, which can be operationalized in reversal learning tasks, is mediated by cortico-striatal circuitries, with the orbitofrontal cortex (OFC) playing a prominent role. The OFC is a functionally heterogeneous region, and we have previously reported differential roles of lateral (lOFC) and medial (mOFC) regions in a touchscreen serial visual reversal learning task for rats using pharmacological inactivation. Here, we investigated the effects of pharmacological overactivation of these regions using a glutamate transporter 1 (GLT-1) inhibitor, dihydrokainate (DHK), which increases extracellular glutamate by blocking its reuptake. We also tested the impact of antagonism of the serotonin 2A receptor (5-HT_2AR), which modulates glutamate action, in the mOFC and lOFC on the same task. Overactivation induced by DHK produced dissociable effects in the mOFC and lOFC, with more prominent effects in the mOFC, specifically improving performance in the early, perseveration phase. Intra-lOFC DHK increased the number of omitted responses without affecting errors. In contrast, blocking the 5-HT_2AR in the lOFC impaired reversal learning overall, while mOFC 5-HT_2AR blockade had no effect. These results further support dissociable roles of the rodent mOFC and lOFC in deterministic visual reversal learning and indicate that modulating glutamate transmission through blocking the GLT-1 and the 5-HT_2AR have different roles in these two structures.

This study further supports dissociable roles of specific orbitofrontal subregions, as well as glutamatergic and serotonergic transmission in these subregions, in cognitive flexibility. This knowledge will add to the understanding of specific neural mechanisms underlying inflexible behaviour across psychiatric disorders.

Global brain volume reductions in a sub-chronic phencyclidine animal model for schizophrenia and their relationship to recognition memory

BACKGROUND

Cognitive deficits and structural brain changes co-occur in patients with schizophrenia. Improving our understanding of the relationship between these is important to develop improved therapeutic strategies. Back-translation of these findings into rodent models for schizophrenia offers a potential means to achieve this goal.

AIMS

The purpose of this study was to determine the extent of structural brain changes and how these relate to cognitive behaviour in a sub-chronic phencyclidine rat model.

METHODS

Performance in the novel object recognition task was examined in female Lister Hooded rats at one and six weeks after sub-chronic phencyclidine (2 mg/kg intra-peritoneal, n=15) and saline controls (1 ml/kg intra-peritoneal, n=15). Locomotor activity following acute phencyclidine challenge was also measured. Brain volume changes were assessed in the same animals using ex vivo structural magnetic resonance imaging and computational neuroanatomical analysis at six weeks.

RESULTS

Female sub-chronic phencyclidine-treated Lister Hooded rats spent significantly less time exploring novel objects (p<0.05) at both time-points and had significantly greater locomotor activity response to an acute phencyclidine challenge (p<0.01) at 3-4 weeks of washout. At six weeks, sub-chronic phencyclidine-treated Lister Hooded rats displayed significant global brain volume reductions (p<0.05; q<0.05), without apparent regional specificity. Relative volumes of the perirhinal cortex however were positively correlated with novel object exploration time only in sub-chronic phencyclidine rats at this time-point.

CONCLUSION

A sustained sub-chronic phencyclidine-induced cognitive deficit in novel object recognition is accompanied by global brain volume reductions in female Lister Hooded rats. The relative volumes of the perirhinal cortex however are positively correlated with novel object exploration, indicating some functional relevance.

NMDA receptor dependence of reversal learning and the flexible use of cognitively demanding search strategies in mice

Cognitive flexibility helps organisms to respond adaptively to environmental changes. Deficits in this executive function have been associated with a variety of brain disorders, and it has been shown to rely on various concomitant neurobiological mechanisms. However, the involvement of the glutamatergic system in general, and NMDA receptors in particular, has been debated. Therefore, we injected C57BL/6 mice repeatedly with low-doses of the non-competitive NMDA receptor antagonist MK-801 (dizocilpine, 0.1 mg/kg, i.p.). Reversal learning and the use of specific cognitive strategies were assessed in a non-spatial discrimination touchscreen task and the Morris water maze (MWM) spatial learning task. In addition, mice were subjected to a non-mnemonic test battery. Although initial acquisition learning was not affected by MK-801 administration, it did induce deficits in reversal learning, both in the non-spatial and spatial task. Defects in non-spatial reversal learning appeared to be caused by perseverative errors. Also, MK-801 administration induced perseverative behaviours as well as inefficient spatial strategy use during MWM reversal learning. These effects could not be reduced to changes in exploratory (anxiety-related) behaviours, nor to motor deficits. This was consistent with results in the non-mnemonic test battery, during which MK-801 evoked hyperlocomotion and subtle motor defects, but failed to alter general motor activity and exploratory behaviours. In conclusion, NMDA receptors appear to be involved in the flexible cognitive processes that underlie reversal learning in spatial as well as non-spatial tasks. Our results also indicate that reversal learning as well as the use of cognitively demanding strategies are more sensitive to NMDA receptor blockage than some other functions that have been suggested to be NMDA receptor dependent.

The Dopamine Allosteric Agent, PAOPA, Demonstrates Therapeutic Potential in the Phencyclidine NMDA Pre-clinical Rat Model of Schizophrenia

PAOPA, a potent analog of prolyl-leucyl-glycinamide, has shown therapeutic potential at the preclinical stage for dopaminergic related illnesses, including animal models of schizophrenia, Parkinson’s disease and haloperidol-induced extrapyramidal movement disorders. PAOPA’s unique allosteric mechanism and dopamine D2 receptor specificity provide a unique composition of properties for the development of potential therapeutics for neuropsychiatric illnesses. We sought to investigate PAOPA’s therapeutic prospects across the spectrum of schizophrenia-like symptoms represented in the established phencyclidine-induced rat model of schizophrenia, (5 mg/kg PCP twice daily for 7 days, followed by 7 days of drug withdrawal). PAOPA was assessed for its effect on brain metabolic activity and across a battery of behavioral tests including, hyperlocomotion, social withdrawal, sensorimotor gating, and novel object recognition. PAOPA showed therapeutic efficacy in behavioral paradigms representing the negative (social withdrawal) and cognitive-like (novel object recognition) symptoms of schizophrenia. Interestingly, some behavioral indices associated with the positive symptoms of schizophrenia that were ameliorated in PAOPA’s prior examination in the amphetamine-sensitized model of schizophrenia were not ameliorated in the PCP model; suggesting that the deficits induced by amphetamine and PCP—while phenotypically similar—are mechanistically different and that PAOPA’s effects are restricted to certain mechanisms and systems. These studies provide insight on the potential use of PAOPA for the safe and effective treatment of schizophrenia.

An Overview of Animal Models Related to Schizophrenia

Schizophrenia is a heterogeneous psychiatric disorder that is poorly treated with current therapies. In this brief review, we provide an update regarding the use of animal models to study schizophrenia in an attempt to understand its aetiology and develop novel therapeutic strategies. Tremendous progress has been made developing and validating rodent models that replicate the aetiologies, brain pathologies, and behavioural abnormalities associated with schizophrenia in humans. Here, models are grouped into 3 categories-developmental, drug induced, and genetic-to reflect the heterogeneous risk factors associated with schizophrenia. Each of these models is associated with varied but overlapping pathophysiology, endophenotypes, behavioural abnormalities, and cognitive impairments. Studying schizophrenia using multiple models will permit an understanding of the core features of the disease, thereby facilitating preclinical research aimed at the development and validation of better pharmacotherapies to alter the progression of schizophrenia or alleviate its debilitating symptoms.

Implication of nigral dopaminergic lesion and repeated L-dopa exposure in neuropsychiatric symptoms of Parkinson's disease

This study aims to investigate the contribution of nigral dopaminergic (DA) cell loss, repeated exposure to DA medication and the combination of both to the development of neuropsychiatric symptoms observed in Parkinson's disease (PD). A bilateral 6-OHDA lesion of the substantia nigra pars compacta (SNc) was performed in rats. A set of animals was repeatedly administered with L-dopa (20 mg/kg/day) and benserazide (5 mg/kg/day) over 10 days starting from day 11 post-lesion. Behavioural testing was performed in week 3 post-lesion: novel object recognition (NOR), elevated plus maze (EPM) social interaction (SI) tests, and amphetamine-induced hyperlocomotion (AIH). Immunohistochemical analysis revealed a significant partial lesion (48%) in 6-OHDA versus sham rats. This lesion was not associated with motor impairment. However, lesioned rats displayed a significant deficit in the NOR, which was reversed by acute treatment with l-dopa/benserazide (12.5 mg/kg and 15 mg/kg respectively). Lesioned rats also displayed a deficit in the EPM which was not reversed by acute treatment with l-dopa. No difference was observed in the SI test or in the AIH assay. In all assays, no effect of chronic l-dopa exposure was observed. This study provides new insights into the neuropathophysiology associated with neuropsychiatric symptoms of PD. Our data strongly emphasises a not previously clearly identified critical role in cognition for the SNc. The results suggest that DA pathways were less directly involved in lesion-induced anxiety-like behaviour. We did not report any effect of chronic l-dopa exposure in the context of partial nigral cell loss.

TPA-023 attenuates subchronic phencyclidine-induced declarative and reversal learning deficits via GABAA receptor agonist mechanism: possible therapeutic target for cognitive deficit in schizophrenia

GABAergic drugs are of interest for the treatment of anxiety, depression, bipolar disorder, pain, cognitive impairment associated with schizophrenia (CIAS), and other neuropsychiatric disorders. Some evidence suggests that TPA-023, (7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b] pyridazine), a GABA_A α2,3 subtype-selective GABA_A partial agonist and α_1/5 antagonist, and the neurosteroid, pregnenolone sulfate, a GABA_A antagonist, may improve CIAS in pilot clinical trials. The goal of this study was to investigate the effect of TPA-023 in mice after acute or subchronic (sc) treatment with the N-methyl-D-aspartate receptor (NMDAR) antagonist, phencyclidine (PCP), on novel object recognition (NOR), reversal learning (RL), and locomotor activity (LMA) in rodents. Acute TPA-023 significantly reversed scPCP-induced NOR and RL deficits. Co-administration of sub-effective dose (SED) TPA-023 with SEDs of the atypical antipsychotic drug, lurasidone, significantly potentiated the effect of TPA-023 in reversing the scPCP-induced NOR deficit. Further, scTPA-023 co-administration significantly prevented scPCP-induced NOR deficit for 5 weeks. Also, administration of TPA-023 for 7 days following scPCP reversed the NOR deficit for 1 week. However, TPA-023 did not blunt acute PCP-induced hyperactivity, suggesting lack of efficacy as a treatment for psychosis. Systemic TPA-023 significantly blocked lurasidone-induced increases in cortical acetylcholine, dopamine, and glutamate without affecting increases in norepinephrine and with minimal effect on basal release of these neurotransmitters. TPA-023 significantly inhibited PCP-induced cortical and striatal dopamine, serotonin, norepinephrine, and glutamate efflux. These results suggest that TPA-023 and other GABA_A agonists may be of benefit to treat CIAS.

Prefrontal cortex executive processes affected by stress in health and disease

Prefrontal cortical executive functions comprise a number of cognitive capabilities necessary for goal directed behavior and adaptation to a changing environment. Executive dysfunction that leads to maladaptive behavior and is a symptom of psychiatric pathology can be instigated or exacerbated by stress. In this review we survey research addressing the impact of stress on executive function, with specific focus on working memory, attention, response inhibition, and cognitive flexibility. We then consider the neurochemical pathways underlying these cognitive capabilities and, where known, how stress alters them. Finally, we review work exploring potential pharmacological and non-pharmacological approaches that can ameliorate deficits in executive function. Both preclinical and clinical literature indicates that chronic stress negatively affects executive function. Although some of the circuitry and neurochemical processes underlying executive function have been characterized, a great deal is still unknown regarding how stress affects these processes. Additional work focusing on this question is needed in order to make progress on developing interventions that ameliorate executive dysfunction.

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.

Neurosteroid pregnenolone sulfate, alone, and as augmentation of lurasidone or tandospirone, rescues phencyclidine-induced deficits in cognitive function and social interaction

BACKGROUND

Pregnenolone sulfate (PregS), an endogenous neurosteroid, which negatively and positively modulates gamma amino butyric acid subunit A (GABA_A) and N-methyl D-aspartate (NMDA) receptors (R) respectively, among other potential neuroplastic changes on synaptic processes, has shown some beneficial effects on treating cognitive impairment associated with schizophrenia (CIAS) and negative symptoms. Lurasidone (Lur), an atypical antipsychotic drug (AAPD), and tandospirone (Tan), a 5-HT_1A R partial agonist, have also been reported to improve cognitive or negative symptoms, or both, in some schizophrenia patients.

METHODS

We tested whether PregS, by itself, and in combination with Lur or Tan could rescue persistent deficits produced by subchronic treatment with the NMDAR antagonist, phencyclidine (PCP)-in episodic memory, executive functioning, and social behavior, using novel object recognition (NOR), operant reversal learning (ORL), and social interaction (SI) tasks, in male C57BL/6 J mice.

RESULTS

PregS (10, but not 3 mg/kg) significantly rescued subchronic PCP-induced NOR and SI deficits. Co-administration of sub-effective doses (SEDs) of PregS (3 mg/kg) + Lur (0.1 mg/kg) or Tan (0.03 mg/kg) rescued scPCP-induced NOR and SI deficits. Further, PregS (30, but not 10 mg/kg) rescued PCP-induced ORL deficit, as did the combination of SED PregS (10 mg/kg) +SED Lur (1 mg/kg) or Tan (1 mg/kg).

CONCLUSION

PregS was effective alone and as adjunctive treatment for treating two types of cognitive impairments and negative symptoms in this schizophrenia model. Further study of the mechanisms by which PregS alone and in combination with AAPDs and 5-HT_1A R partial agonists, rescues the deficits in cognition and SI in this preclinical model is indicated.

Estradiol and luteinizing hormone regulate recognition memory following subchronic phencyclidine: Evidence for hippocampal GABA action

The cognitive symptoms of schizophrenia are poorly understood and difficult to treat. Estrogens may mitigate these symptoms via unknown mechanisms. To examine these mechanisms, we tested whether increasing estradiol (E) or decreasing luteinizing hormone (LH) could mitigate short-term episodic memory loss in a phencyclidine (PCP) model of schizophrenia. We then assessed whether changes in cortical or hippocampal GABA may underlie these effects. Female rats were ovariectomized and injected subchronically with PCP. To modulate E and LH, animals received estradiol capsules or Antide injections. Short-term episodic memory was assessed using the novel object recognition task (NORT). Brain expression of GAD67 was analyzed via western blot, and parvalbumin-containing cells were counted using immunohistochemistry. Some rats received hippocampal infusions of a GABA_A agonist, GABA_A antagonist, or GAD inhibitor before behavioral testing. We found that PCP reduced hippocampal GAD67 and abolished recognition memory. Antide restored hippocampal GAD67 and rescued recognition memory in PCP-treated animals. Estradiol prevented PCP's amnesic effect in NORT but failed to restore hippocampal GAD67. PCP did not cause significant differences in number of parvalbumin-expressing cells or cortical expression of GAD67. Hippocampal infusions of a GABA_A agonist restored recognition memory in PCP-treated rats. Blocking hippocampal GAD or GABA_A receptors in ovx animals reproduced recognition memory loss similar to PCP and inhibited estradiol's protection of recognition memory in PCP-treated animals. In summary, decreasing LH or increasing E can lessen short-term episodic memory loss, as measured by novel object recognition, in a PCP model of schizophrenia. Alterations in hippocampal GABA may contribute to both PCP's effects on recognition memory and the hormones' ability to prevent or reverse them.

Dopamine dysregulation in the prefrontal cortex relates to cognitive deficits in the sub-chronic PCP-model for schizophrenia: A preliminary investigation

RATIONALE

Dopamine dysregulation in the prefrontal cortex (PFC) plays an important role in cognitive dysfunction in schizophrenia. Sub-chronic phencyclidine (scPCP) treatment produces cognitive impairments in rodents and is a thoroughly validated animal model for cognitive deficits in schizophrenia. The aim of our study was to investigate the role of PFC dopamine in scPCP-induced deficits in a cognitive task of relevance to the disorder, novel object recognition (NOR).

METHODS

Twelve adult female Lister Hooded rats received scPCP (2 mg/kg) or vehicle via the intraperitoneal route twice daily for 7 days, followed by 7 days washout. In vivo microdialysis was carried out prior to, during and following the NOR task.

RESULTS

Vehicle rats successfully discriminated between novel and familiar objects and this was accompanied by a significant increase in dopamine in the PFC during the retention trial ( p < 0.01). scPCP produced a significant deficit in NOR ( p < 0.05 vs. control) and no PFC dopamine increase was observed.

CONCLUSIONS

These data demonstrate an increase in dopamine during the retention trial in vehicle rats that was not observed in scPCP-treated rats accompanied by cognitive disruption in the scPCP group. This novel finding suggests a mechanism by which cognitive deficits are produced in this animal model and support its use for investigating disorders in which PFC dopamine is central to the pathophysiology.

RP5063, an atypical antipsychotic drug with a unique pharmacologic profile, improves declarative memory and psychosis in mouse models of schizophrenia

Various types of atypical antipsychotic drugs (AAPDs) modestly improve the cognitive impairment associated with schizophrenia (CIAS). RP5063 is an AAPD with a diverse and unique pharmacology, including partial agonism at dopamine (DA) D₂, D₃, D₄, serotonin (5-HT)_1A, and 5-HT_2A receptors (Rs), full agonism at α₄β₂ nicotinic acetylcholine (ACh)R (nAChR), and antagonism at 5-HT_2B, 5-HT₆, and 5-HT₇Rs. Most atypical APDs are 5-HT_2A inverse agonists. The efficacy of RP5063 in mouse models of psychosis and episodic memory were studied. RP5063 blocked acute phencyclidine (PCP)-as well as amphetamine-induced hyperactivity, indicating antipsychotic activity. Acute administration of RP5063 significantly reversed subchronic (sc)PCP-induced impairment in novel object recognition (NOR), a measure of episodic memory, but not reversal learning, a measure of executive function. Co-administration of a sub-effective dose (SED) of RP5063 with SEDs of a 5-HT₇R antagonist, a 5-HT_1BR antagonist, a 5-HT_2AR inverse agonist, or an α₄β₂ nAChR agonist, restored the ability of RP5063 to ameliorate the NOR deficit in scPCP mice. Pre-treatment with a 5-HT_1AR, a D₄R, antagonist, but not an α₄β₂ nAChR antagonist, blocked the ameliorating effect of RP5063. Further, co-administration of scRP5063 prior to each dose of PCP prevented the effect of PCP to produce a deficit in NOR for one week. RP5063, given to scPCP-treated mice for one week restored NOR for one week only. Acute administration of RP5063 significantly increased cortical DA efflux, which may be critical to some of its cognitive enhancing properties. These results indicate that RP5063, by itself, or as an adjunctive treatment has a multifaceted basis for improving some cognitive deficits associated with schizophrenia.

Spatial and temporal boundaries of NMDA receptor hypofunction leading to schizophrenia

The N-methyl-d-aspartate receptor hypofunction is one of the most prevalent models of schizophrenia. For example, healthy subjects treated with uncompetitive N-methyl-d-aspartate receptor antagonists elicit positive, negative, and cognitive-like symptoms of schizophrenia. Patients with anti-N-methyl-d-aspartate receptor encephalitis, which is likely caused by autoantibody-mediated down-regulation of cell surface N-methyl-d-aspartate receptors, often experience psychiatric symptoms similar to schizophrenia initially. However, where and when N-methyl-d-aspartate receptor hypofunction occurs in the brain of schizophrenic patients is poorly understood. Here we review the findings from N-methyl-d-aspartate receptor antagonist and autoantibody models, postmortem studies on N-methyl-d-aspartate receptor subunits, as well as the global and cell-type-specific knockout mouse models of subunit GluN1. We compare various conditional GluN1 knockout mouse strains, focusing on the onset of N-methyl-d-aspartate receptor deletion and on the cortical cell-types. Based on these results, we hypothesize that N-methyl-d-aspartate receptor hypofunction initially occurs in cortical GABAergic neurons during early postnatal development. The resulting GABA neuron maturation deficit may cause reduction of intrinsic excitability and GABA release, leading to disinhibition of pyramidal neurons. The cortical disinhibition in turn could elicit glutamate spillover and subsequent homeostatic down regulation of N-methyl-d-aspartate receptor function in pyramidal neurons in prodromal stage. These two temporally-distinct N-methyl-d-aspartate receptor hypofunctions may be complimentary, as neither alone may not be able to fully explain the entire schizophrenia pathophysiology. Potential underlying mechanisms for N-methyl-d-aspartate receptor hypofunction in cortical GABA neurons are also discussed, based on studies of naturally-occurring N-methyl-d-aspartate receptor antagonists, neuregulin/ErbB4 signaling pathway, and theoretical analysis of excitatory/inhibitory balance.

The neural basis of reversal learning: An updated perspective

Reversal learning paradigms are among the most widely used tests of cognitive flexibility and have been used as assays, across species, for altered cognitive processes in a host of neuropsychiatric conditions. Based on recent studies in humans, non-human primates, and rodents, the notion that reversal learning tasks primarily measure response inhibition, has been revised. In this review, we describe how cognitive flexibility is measured by reversal learning and discuss new definitions of the construct validity of the task that are serving as a heuristic to guide future research in this field. We also provide an update on the available evidence implicating certain cortical and subcortical brain regions in the mediation of reversal learning, and an overview of the principal neurotransmitter systems involved.

Applications of the Neonatal Quinpirole Model to Psychosis and Convergence upon the Dopamine D2 Receptor

This mini review focuses on the importance of the dopamine D₂-like receptor family and its importance in psychosis. Past findings from this laboratory along with collaborators have been that neonatal quinpirole (a dopamine D₂-like receptor agonist) results in increases in dopamine D₂ receptor sensitivity that persists throughout the animal's lifetime. Findings from this model have been shown to have particular application and validity to schizophrenia, but may have broader implications toward other psychoses, which is reviewed in the present manuscript. In the present review, we also highlight other models of psychoses that have been centered on the subchronic administration of quinpirole to rats in order to model certain psychoses, which has uncovered some interesting and valid behavioral findings. This review highlights the importance of the combination of behavioral findings and neurobiological mechanisms focusing on neural plasticity in discovering underlying pathologies in these disorders that may lead to treatment discoveries, as well as the value of animal models across all psychoses.