d-Serine and a glycine transporter inhibitor improve MK-801-induced cognitive deficits in a novel object recognition test in rats

Abstract

Compounds enhancing N-methyl-d-aspartate (NMDA) glutamate receptor function have been reported to improve cognitive deficits. Since cognitive deficits are considered to be the core symptom of schizophrenia, enhancing NMDA receptor function represents a promising approach to treating schizophrenia. In the present study, we investigated whether d-serine or a glycine transporter inhibitor N-[3-(4′-fluorophenyl)-3-(4′-phenylphenoxy)propyl]sarcosine (NFPS), both of which enhance NMDA receptor function, could improve MK-801-induced cognitive deficits in rats, and compared their effects with those of the atypical antipsychotic clozapine and of the typical antipsychotic haloperidol. To assess cognitive function, we used a novel object recognition test in rats that measured spontaneous exploratory activity of a novel object when paired with a familiar object. We then evaluated the effects of the compounds on cognitive deficits induced by treatment with MK-801, the NMDA receptor antagonist. Pretreatment with clozapine (1, 5 mg/kg, i.p.) but not haloperidol (0.03, 0.1 mg/kg, i.p.) significantly improved MK-801-induced cognitive deficits. Pretreatment with d-serine at 800 mg/kg (i.p.) or NFPS (0.3, 1 mg/kg, i.p.) significantly improved MK-801-induced cognitive deficits under this test paradigm. These findings suggest that impaired preference for novel objects induced by MK-801 in the novel object recognition test could be a useful animal model for evaluating the efficacy of compounds targeting the cognitive deficits observed in schizophrenic patients. The results also suggest that enhancing NMDA receptor function is an effective way for treating the cognitive deficits associated with schizophrenia.

Introduction

In addition to the “dopamine hypothesis”, hypofunction of the glutamatergic system is now widely acknowledged to be implicated in the etiology of schizophrenia. This hypothesis stems from findings that N-methyl-d-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), MK-801 and ketamine induce a spectrum of positive and negative symptoms as well as cognitive deficits in human and rodents [17], [23], [32]. Thus, it has been hypothesized that schizophrenia may be associated with hypofunction of the NMDA receptor, and that enhancing NMDA receptor function may be beneficial in treating schizophrenia.

In addition to agonist-binding sites for glutamate, NMDA receptors contain co-agonist sites, called glycine modulatory site to which glycine and d-serine can bind. Activation of the channel by glutamate requires occupation of the glycine modulatory site [20]. It has been reported that exogenously administered glycine or d-serine can stimulate NMDA receptors without inducing the toxic effects observed following the administration of NMDA receptor agonists such as glutamate and NMDA [8]. Alternative way to stimulate the glycine modulatory site on a NMDA receptor involves inhibiting glycine transporter type 1 (GlyT1), which plays a role in maintaining glycine concentrations within synapses at sub-saturating levels [5]. Indeed, in animal models of schizophrenia, both d-serine and GlyT1 inhibitors have been reported to improve not only positive and negative symptoms but also cognitive deficits [18], [28]. It has been reported that adding both d-serine and sarcosine (a GlyT1 inhibitor) to antipsychotics may also help to alleviate positive and negative symptoms as well as cognitive deficits in schizophrenic patients [16], [24], [35], although some studies fail to support clinical efficacy [7], [13].

Cognitive function is markedly impaired in most patients with schizophrenia. Cognitive impairment in memory, executive functions and working memory is considered to be of major functional and symptomatic outcomes as well as quality of life of patients with schizophrenia [11], [14], [31], therefore, effective therapy for the cognitive deficits could be of great clinical importance. It is also well known that the typical antipsychotics seem to have a poor effect on cognitive deficits in patients with schizophrenia, and clozapine appears to have partial effect [29], [34]. In this regard, the beneficial effects of the drugs, which enhance NMDA receptor function, on cognitive deficits are intriguing.

A novel object recognition test for rodent is a non-reward paradigm based on spontaneous exploration of novel and familiar objects, a task reflecting non-spatial working memory [10]. It has been reported that schizophrenic patients demonstrate impaired performance in the recognition of visually presented objects [1], [6]. Therefore, the novel object recognition test has been increasingly used as one of a preclinical model to evaluate cognitive deficits observed in schizophrenia. In fact, systemic administration of NMDA receptor antagonists, which induce numerous cognitive dysfunctions in both human and rodents, has been reported to disrupt object recognition memory [4], [27], and PCP-induced cognitive deficits in the novel object recognition test in mice were restored by chronic treatment with clozapine, but not with haloperidol [15]. However, the effects of drugs that enhance NMDA receptor function under this test paradigm have yet to be examined.

In the present study, we evaluated the effects of the atypical antipsychotic clozapine and the typical antipsychotic haloperidol on cognitive deficits following MK-801 treatment, using a novel object recognition test in rats. We also evaluated the efficacy of d-serine and N-[3-(4′-fluorophenyl)-3-(4′-phenylphenoxy)propyl]sarcosine (NFPS), a selective and potent GlyT1 inhibitor, in treating MK-801-induced cognitive deficits.