AMPHETAMINE-INDUCED DISRUPTION AND HALOPERIDOL-INDUCED POTENTIATION OF LATENT INHIBITION DEPEND ON THE NATURE OF THE STIMULUS

If a stimulus (e.g. light) is repeatedly preexposed without consequenc es, it subsequently develops a weaker association with a reinforcer (e .g. foot shock) than does a non-preexposed stimulus. This retarded con ditioning to the preexposed as compared to the non-preexposed stimulus , is latent inhibition (LI). It is well documented that LI is disrupte d by low doses of amphetamine and potentiated by neuroleptic drugs, an d there is evidence that the action of these agents on LI can be modif ied by changes in the parameters of preexposure or conditioning. The p resent experiments tested whether the effects of DA agents on LI are i nfluenced by the nature of the stimulus. In two experiments, LI was as sessed using an off-baseline conditioned emotional response (CER) proc edure in rats licking for water, consisting of three stages: preexposu re, in which the stimulus (a light) to be conditioned, was repeatedly presented without being followed by reinforcement; conditioning, in wh ich the preexposed stimulus was paired with reinforcement (a foot-shoc k); and test, in which LI was indexed by animals' degree of suppressio n of licking during stimulus presentation. In both experiments, differ ent groups of animals were preexposed and conditioned with four differ ent preexposed visual stimuli: three steady side-lights, three flashin g side-lights, one flashing side-light, and a flashing houselight. Exp eriment 1 used 40 stimulus preexposures and tested the effects of I mg /kg D-amphetamine, whereas experiment 2 used 10 preexposures and teste d the effects of 0.1 mg/kg haloperidol. The results showed that of the four stimuli used, both drugs were effective with only one and the sa me stimulus, namely, flashing houselight. This demonstrates that the d isruptive effect of amphetamine and the potentiating effect of haloper idol on LI, are modifiable by manipulating the nature of the preexpose d stimulus. (C) 1997 Elsevier Science B.V.