Reversal of phencyclidine-induced prepulse inhibition deficits by clozapine in monkeys

Rationale. Prepulse inhibition (PPI) of the acoustic startle reflex is a measure of sensorimotor gating, which occurs across species and is deficient in severe neuropsychiatric disorders such as schizophrenia. In monkeys, as in rodents, phencyclidine (PCP) induces schizophrenia-like deficits in PPI. In rodents, in general, typical antipsychotics (e.g. haloperidol) reverse PPI deficits induced by dopamine (DA) agonists (e.g. apomorphine), but not those induced by N-methyl-d-aspartate (NMDA) receptor antagonists [e.g. phencyclidine (PCP)], whereas atypical antipsychotics (e.g. clozapine) reverse PPI deficits induced by DA agonists and NMDA antagonists. However, some discrepancies exist with some compounds and strains of rodents. Objectives. This study investigated whether a typical (haloperidol, 0.035 mg/kg) and an atypical (clozapine, 2.5 mg/kg) antipsychotic could be distinguished in their ability to reverse PCP-induced deficits in PPI in eight monkeys (Cebus apella). Methods. First, haloperidol dose was determined by its ability to attenuate apomorphine-induced deficits in PPI. Then, haloperidol and clozapine were tested in eight monkeys with PCP-induced deficits of PPI. Experimental parameters were similar to standard human PPI procedures, with 115 dB white noise startle pulses, either alone or preceded by 120 ms with a prepulse 16 dB above the 70 dB background noise. Results. Clozapine reversed PCP-induced PPI deficits. In contrast, haloperidol did not significantly attenuate PCP-induced PPI deficits even at doses that significantly attenuated apomorphine effects. Conclusions. In this primate model, clozapine was distinguishable from haloperidol by its ability to attenuate PCP-induced deficits in PPI. The results provide further evidence that PPI in nonhuman primates may provide an important animal model for the development of novel anti-schizophrenia medications.