NMDA receptor antagonists do not block the development of sensitization of catalepsy, but make its expression state-dependent.

Dopamine (DA) receptor blockade induces catalepsy in rats which increases in strength upon retesting. This increase in catalepsy represents a form of sensitization which has been shown to be completely context dependent. Sensitization of catalepsy therefore represents a good model for studying the neurobiological mechanisms underlying the interaction between the cellular effect of a drug (DA-receptor blockade) and the context. This study investigated whether glutamatergic mechanisms are involved in the development of sensitization. Rats were treated with either haloperidol or haloperidol plus an N-methyl-D-aspartate (NMDA) receptor antagonist. Haloperidol consistently induced catalepsy which developed sensitization upon retesting. Co-administration of D-CPPene (5 mg/kg and 10 mg/kg, i.p.), eliprodil (30 mg/kg, i.p.) or Ro 25-6981 (15 mg/kg, i.p.) did not have any effect on sensitization, although all three drugs exerted some anticataleptic effects. When sensitization developed under haloperidol plus NMDA receptor antagonist, the sensitized response was expressed only in the presence of the NMDA receptor antagonist. This strongly suggests that the NMDA receptor antagonists represent contextual stimuli to which catalepsy has been conditioned, and this implies that the expression of sensitization has been rendered state-dependent.