Ben Grayson

Animal models of cognitive dysfunction and negative symptoms of schizophrenia: Focus on NMDA receptor antagonism

Cognitive deficits in schizophrenia remain an unmet clinical need. Improved understanding of the neuro- and psychopathology of these deficits depends on the availability of carefully validated animal models which will assist the development of novel therapies. There is much evidence that at least some of the pathology and symptomatology (particularly cognitive and negative symptoms) of schizophrenia results from a dysfunction of the glutamatergic system which may be modelled in animals through the use of NMDA receptor antagonists. The current review examines the validity of this model in rodents. We review the ability of acute and sub-chronic treatment with three non-competitive NMDA antagonists; phencyclidine (PCP), ketamine and MK801 (dizocilpine) to produce cognitive deficits of relevance to schizophrenia in rodents and their subsequent reversal by first- and second-generation antipsychotic drugs. Effects of NMDA receptor antagonists on the performance of rodents in behavioural tests assessing the various domains of cognition and negative symptoms are examined: novel object recognition for visual memory, reversal learning and attentional set shifting for problem solving and reasoning, 5-Choice Serial Reaction Time for attention and speed of processing; in addition to effects on social behaviour and neuropathology. The evidence strongly supports the use of NMDA receptor antagonists to model cognitive deficit and negative symptoms of schizophrenia as well as certain pathological disturbances seen in the illness. This will facilitate the evaluation of much-needed novel pharmacological agents for improved therapy of cognitive deficits and negative symptoms in schizophrenia.

Atypical antipsychotics attenuate a sub-chronic PCP-induced cognitive deficit in the novel object recognition task in the rat.

The novel object recognition (NOR) task is a paradigm employed to detect both disruption and improvement of non-spatial memory in rats. PCP (phencyclidine) may be used to model aspects of schizophrenia symptomology in rats, in particular cognitive deficits. The aim of this study was to investigate the ability of typical and atypical antipsychotics to improve a sub-chronic PCP-induced impairment in cognition using the NOR task. Female hooded-Lister rats (195+/-12 g) received either vehicle (0.9% saline twice daily) or PCP (2 mg/kg, twice daily) for 7 days followed by 7-days drug free. Haloperidol (0.05 and 0.075 mg/kg), clozapine (1 and 5mg/kg), risperidone (0.05, 0.1 and 0.2 mg/kg) or vehicle (veh, saline) was administered i.p. 30 min prior to testing. Rats completed an acquisition trial followed by an inter-trial interval of 1 min, then a retention trial. Following sub-chronic vehicle treatment, rats spent significantly (p<0.05) more time exploring the novel compared to the familiar object, an effect that was abolished in the sub-chronic PCP treated animals. Clozapine (1.0 and 5.0 mg/kg) and risperidone (0.2 mg/kg) but not haloperidol significantly attenuated the PCP-induced impairment such that animals again spent significantly more time exploring the novel compared with familiar object (p<0.05). These results support our earlier work showing that acute PCP induces a robust object recognition deficit in female rats. Clozapine and risperidone but not haloperidol showed efficacy to reverse the deficit induced by sub-chronic PCP suggesting that this test may have some validity for assessing efficacy for improvement of cognitive deficit symptoms of schizophrenia.

Lu AE58054, a 5-HT6 antagonist, reverses cognitive impairment induced by subchronic phencyclidine in a novel object recognition test in rats.

The in-vitro potency and selectivity, in-vivo binding affinity and effect of the 5-HT(6)R antagonist Lu AE58054 ([2-(6-fluoro-1H-indol-3-yl)-ethyl]-[3-(2,2,3,3-tetrafluoropropoxy)-benzyl]-amine) on impaired cognition were evaluated. Lu AE58054 displayed high affinity to the human 5-HT(6) receptor (5-HT(6)R) with a Ki of 0.83 nm. In a 5-HT(6) GTPgammaS efficacy assay Lu AE58054 showed no agonist activity, but demonstrated potent inhibition of 5-HT-mediated activation. Besides medium affinity to adrenergic alpha(1A)- and alpha(1B)-adrenoreceptors, Lu AE58054 demonstrated >50-fold selectivity for more than 70 targets examined. Orally administered Lu AE58054 potently inhibited striatal in-vivo binding of the 5-HT(6) antagonist radioligand [(3)H]Lu AE60157 ([(3)H]8-(4-methylpiperazin-1-yl)-3-phenylsulfonylquinoline), with an ED(50) of 2.7 mg/kg. Steady-state modelling of an acute pharmacokinetic/5-HT(6)R occupancy time-course experiment indicated a plasma EC(50) value of 20 ng/ml. Administration of Lu AE58054 in a dose range (5-20 mg/kg p.o.) leading to above 65% striatal 5-HT(6)R binding occupancy in vivo, reversed cognitive impairment in a rat novel object recognition task induced after subchronic treatment for 7 d with phencyclidine (PCP 2 mg/kg b.i.d., i.p. for 7 d, followed by 7 d drug free). The results indicate that Lu AE58054 is a selective antagonist of 5-HT(6)Rs with good oral bioavailability and robust efficacy in a rat model of cognitive impairment in schizophrenia. Lu AE58054 may be useful for the pharmacotherapy of cognitive dysfunction in disease states such as schizophrenia and Alzheimers disease.

Isolation rearing impairs novel object recognition and attentional set shifting performance in female rats

It has been suggested that the isolation rearing paradigm models certain aspects of schizophrenia symptomatology. This study aimed to investigate whether isolation rearing impairs rats’ performance in two models of cognition: the novel object recognition (NOR) and attentional set-shifting tasks, tests of episodic memory and executive function, respectively. Two cohorts of female Hooded-Lister rats were used in these experiments. Animals were housed in social isolation or in groups of five from weaning, post-natal day 28. The first cohort was tested in the NOR test with inter-trial intervals (ITIs) of 1 min up to 6 h. The second cohort was trained and tested in the attentional set-shifting task. In the NOR test, isolates were only able to discriminate between the novel and familiar objects up to 1-h ITI, whereas socially reared animals remembered the familiar object up to a 4-h ITI. In the attentional set-shifting task, isolates were significantly and selectively impaired in the extra-dimensional shift phase of the task (P < 0.01). Rats reared in isolation show impaired episodic memory in the NOR task and reduced ability to shift attention between stimulus dimensions in the attentional set-shifting task. Because schizophrenic patients show similar deficits in performance in these cognitive domains, these data further support isolation rearing as a putative preclinical model of the cognitive deficits associated with schizophrenia.

PNU-120596, a positive allosteric modulator of α7 nicotinic acetylcholine receptors, reverses a sub-chronic phencyclidine-induced cognitive deficit in the attentional set-shifting task in female rats

The α7 nicotinic acetylcholine receptors (nAChRs) have been highlighted as a target for cognitive enhancement in schizophrenia. Adult female hooded Lister rats received sub-chronic phencyclidine (PCP) (2 mg/kg) or vehicle i.p. twice daily for 7 days, followed by 7 days’ washout. PCP-treated rats then received PNU-120596 (10 mg/kg; s.c.) or saline and were tested in the attentional set-shifting task. Sub-chronic PCP produced a significant cognitive deficit in the extra-dimensional shift (EDS) phase of the task (p < 0.001, compared with vehicle). PNU-120596 significantly improved performance of PCP-treated rats in the EDS phase of the attentional set-shifting task (p < 0.001). In conclusion, these data demonstrate that PNU-120596 improves cognitive dysfunction in our animal model of cognitive dysfunction in schizophrenia, most likely via modulation of α7 nACh receptors.

Assessment of disease-related cognitive impairments using the novel object recognition (NOR) task in rodents.

The novel object recognition test (NOR) test is a two trial cognitive paradigm that assesses recognition memory. Recognition memory is disturbed in a range of human disorders and NOR is widely used in rodents for investigating deficits in a variety of animal models of human conditions where cognition is impaired. It possesses several advantages over more complex tasks that involve lengthy training procedures and/or food or water deprivation. It is quick to administer, non-rewarded, provides data quickly, cost effective and most importantly, ethologically relevant as it relies on the animals natural preference for novelty. A PubMed search revealed over 900 publications in rats and mice using this task over the past 3 years with 34 reviews in the past 10 years, demonstrating its increasing popularity with neuroscientists. Although it is widely used in many disparate areas of research, no articles have systematically examined this to date, which is the subject of our review. We reveal that NOR may be used to study recognition memory deficits that occur in Alzheimers disease and schizophrenia, where research is extensive, in Parkinsons disease and Autism Spectrum Disorders (ASD) where we observed markedly reduced numbers of publications. In addition, we review the use of NOR to study cognitive deficits induced by traumatic brain injury and cancer chemotherapy, not disorders per se, but situations in which cognitive deficits dramatically reduce the quality of life for those affected, see Fig. 1 for a summary. Our review reveals that, in all these animal models, the NOR test is extremely useful for identification of the cognitive deficits observed, their neural basis, and for testing the efficacy of novel therapeutic agents. Our conclusion is that NOR is of considerable value for cognitive researchers of all disciplines and we anticipate that its use will continue to increase due to its versatility and several other advantages, as detailed in this review.

Activation of α7 nicotinic receptors improves phencyclidine-induced deficits in cognitive tasks in rats: implications for therapy of cognitive dysfunction in schizophrenia.

RATIONALE Nicotinic α7 acetylcholine receptors (nAChRs) have been highlighted as a target for cognitive enhancement in schizophrenia. AIM To investigate whether the deficits induced by sub-chronic phencyclidine (PCP) in reversal learning and novel object recognition could be attenuated by the selective α7 nAChR full agonist, PNU-282987. METHODS Adult female hooded-Lister rats received sub-chronic PCP (2mg/kg) or vehicle i.p. twice daily for 7days, followed by 7 days washout. In cohort 1, PCP-treated rats then received PNU-282987 (5, 10, 20mg/kg; s.c.) or vehicle and were tested in the reversal-learning task. In cohort 2, PCP-treated rats received PNU-282987 (10mg/kg; s.c.) or saline for 15days and were tested in the novel object recognition test on day 1 and on day 15, to test for tolerance. RESULTS Sub-chronic PCP produced significant deficits in both cognitive tasks (P<0.01-0.001). PNU-282987 attenuated the PCP-induced deficits in reversal learning at 10mg/kg (P<0.01) and 20mg/kg (P<0.001), and in novel object recognition at 10mg/kg on day 1 (P<0.01) and on day 15 (P<0.001). CONCLUSIONS These data show that PNU-282987 has efficacy to reverse PCP-induced deficits in two paradigms of relevance to schizophrenia. Results further suggest that 15-day once daily dosing of PNU-282987 (10mg/kg s.c.) does not cause tolerance in the rat. This study suggests that activation of α7 nAChRs, may represent a suitable strategy for improving cognitive deficits of relevance to schizophrenia.

Positive modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors reverses sub-chronic PCP-induced deficits in the novel object recognition task in rats.

Cognitive deficits are a major clinical unmet need in schizophrenia. The psychotomimetic drug phencyclidine (PCP) is widely applied in rodents to mimic symptoms of schizophrenia, including cognitive deficits. Previous studies have shown that sub-chronic PCP induces an enduring episodic memory deficit in female Lister Hooded rats in the novel object recognition (NOR) task. Here we show that positive modulation of AMPA receptor (AMPAR) mediated glutamate transmission alleviates cognitive deficits induced by sub-chronic PCP treatment. Female Lister hooded rats were treated sub-chronically with either vehicle (0.9% saline) or PCP (2mg/kg two doses per day for 7 days), followed by a 7 days washout period. 30 min prior to the acquisition trial of the NOR task animals were dosed with either vehicle, CX546 (10, 40 or 80 mg/kg) or CX516 (0.5, 2.5, 10, 40 or 80 mg/kg). Our results show that sub-chronic PCP treatment induced a significant decrease in the discrimination index (DI) and both ampakines CX546 and CX516 were able to reverse this disruption of object memory in rats in the novel object recognition task. These data suggest that positive AMPAR modulation may represent a mechanism for treatment of cognitive deficits in schizophrenia.

Effects of cariprazine, a novel antipsychotic, on cognitive deficit and negative symptoms in a rodent model of schizophrenia symptomatology.

Negative symptoms and cognitive impairment associated with schizophrenia are strongly associated with poor functional outcome and reduced quality of life and remain an unmet clinical need. Cariprazine is a dopamine D3/D2 receptor partial agonist with preferential binding to D3 receptors, recently approved by the FDA for the treatment of schizophrenia and manic or mixed episodes associated with bipolar I disorder. The aim of this study is to evaluate effects of cariprazine in an animal model of cognitive deficit and negative symptoms of schizophrenia. Following sub-chronic PCP administration (2mg/kg, IP for 7 days followed by 7 days drug-free), female Lister Hooded rats were administered cariprazine (0.05, 0.1, or 0.25mg/kg, PO) or risperidone (0.16 or 0.1mg/kg, IP) before testing in novel object recognition (NOR), reversal learning (RL), and social interaction (SI) paradigms. As we have consistently demonstrated, sub-chronic PCP significantly impaired behavior in these tests. Deficits were significantly improved by cariprazine, in a dose dependent manner in the operant RL test with efficacy at lower doses in the NOR and SI tests. Locomotor activity was reduced at the highest doses of 0.1mg/kg and 0.25mg/kg in NOR and SI. Risperidone also reversed the PCP-induced deficit in all tests. In conclusion, cariprazine was effective to overcome PCP-induced deficits in cognition and social behavior in a thoroughly validated rat model in tests representing specific symptom domains in schizophrenia patients. These findings support very recent results showing efficacy of cariprazine in the treatment of negative symptoms in schizophrenia patients.

Pay attention to impulsivity: modelling low attentive and high impulsive subtypes of adult ADHD in the 5-choice continuous performance task (5C-CPT) in female rats.

Varying levels of attention and impulsivity deficits are core features of the three subtypes of adult attention deficit-hyperactivity disorder (ADHD). To date, little is known about the neurobiological correlates of these subtypes. Development of a translational animal model is essential to improve our understanding and improve therapeutic strategies. The 5-choice continuous performance task (5C-CPT) in rats can be used to examine different forms of attention and impulsivity. Adult rats were trained to pre-set 5C-CPT criterion and subsequently separated into subgroups according to baseline levels of sustained attention, vigilance, premature responding and response disinhibition in the 5C-CPT. The behavioural subgroups were selected to represent the different subtypes of adult ADHD. Consequently, effects of the clinically used pharmacotherapies (methylphenidate and atomoxetine) were assessed in the different subgroups. Four subgroups were identified: low-attentive (LA), high-attentive (HA), high-impulsive (HI) and low-impulsive (LI). Methylphenidate and atomoxetine produced differential effects in the subgroups. Methylphenidate increased sustained attention and vigilance in LA animals, and reduced premature responding in HI animals. Atomoxetine also improved sustained attention and vigilance in LA animals, and reduced response disinhibition and premature responding in HI animals. This is the first study using adult rats to demonstrate the translational value of the 5C-CPT to select subgroups of rats, which may be used to model the subtypes observed in adult ADHD. Our findings suggest that this as an important paradigm to increase our understanding of the neurobiological underpinnings of adult ADHD-subtypes and their response to pharmacotherapy.