P.3.d.021 Physostigmine reverses MK-801-induced but not amphetamine-induced effects on latent inhibition: focus on cholinergic treatments in schizophrenia

Abstract

Latent inhibition (LI) refers to retarded conditioning to a stimulus that has been repeatedly presented without reinforcement. LI is impaired in schizophrenic patients and in rats treated with amphetamine. Neuroleptic drugs produce two effects in this test paradigm: antagonism of amphetamine-induced disruption of LI, and enhancement of LI when administered on their own. The present experiments tested the effects of the atypical neuroleptic, clozapine, on LI. The experiments used a conditioned emotional response procedure in rats licking for water, consisting of three stages: preexposure, in which the to-be-conditioned stimulus (tone) was repeatedly presented without reinforcement; conditioning, in which the preexposed stimulus was paired with reinforcement (foot shock); and test, in which LI was indexed by animals' degree of suppression of licking during tone presentation. In experiments 1 and 2, the effects of 5.0 and 10.0 mg/kg clozapine on LI were assessed following 20 or 10 tone preexposures, respectively. Experiments 3 and 4 used 40 preexposures and investigated antagonism of amphetamine-induced disruption of LI by 5.0 and 10.0 mg/kg clozapine, respectively. The results demonstrated that clozapine possesses a neuroleptic profile in the LI model, namely, it facilitates the development of LI and antagonizes amphetamine-induced disruption of LI.