Evaluation of NMDA receptor models of schizophrenia: divergences in the behavioral effects of sub-chronic PCP and MK-801.

The hypothesis of hypo-functionality of NMDA receptors in schizophrenia originates from the observation that administration of the NMDA antagonist phencyclidine (PCP) induces psychotic states that closely resemble schizophrenic symptoms and that persist after drug discontinuation. A large number of animal studies have used PCP and the NMDA antagonist dizocilpine (MK-801) almost interchangeably to model schizophrenia. However, PCP interacts with pharmacological targets other than NMDA receptors that are not affected by MK-801. In addition, although acute administration of either compound produces similar effects in animals, there is little information whether withdrawal from chronic MK-801 causes behavioral deficits that mimic schizophrenia symptoms as in the case of PCP. To clarify this issue, we compared the following behaviors in rats subjected to withdrawal from sub-chronic administration (2 x 7 days) of either PCP (5 mg/kg, i.p.) or MK-801 (0.5 mg/kg, i.p.): (1) working memory in a variable-delayed alternation task in a T-maze, (2) social interaction, and (3) motor activity in response to a (a) novel environment, (b) mild stressor, and (c) d-amphetamine challenge. Withdrawal from sub-chronic PCP caused a delay-dependent impairment of working memory, reduced social interaction and enhanced d-amphetamine-induced motor activity. These results were not replicated in animals sub-chronically treated with MK-801, which displayed only a slight decrease in social interaction. These data suggest that pharmacological antagonism at NMDA receptors is not sufficient to explain the full spectrum of PCP psychotomimetic properties.