Effects of Opiate Manipulations on Latent Inhibition in Rabbits: Sensitivity of the Medial Septal Region to Intracranial Treatments

The effects of opiate manipulations were examined on latent inhibition of classically conditioned heart rate in rabbits. Animals were exposed to an auditory stimulus that was later used as a conditioned stimulus (CS). Administration of the agonist levorphanol into the medial septal region either preceding or immediately following CS preexposure sessions attenuated latent inhibition of the conditioned response. This effect was observed in animals that showed no change in long-term retention of habituation. Administration of levorphanol into the central nucleus of the amygdala, a sensitive site for opiate manipulations on aversive conditioning tasks, was ineffective. Further analysis of the effect obtained within the septal region revealed pharmacological specificity. The effect of posttraining levorphanol was also time dependent and exhibited neuroanatomical specificity. Posttraining opiate antagonist administration into the medial septum produced a modest augmentation of latent inhibition. The medial septal region appears to provide a site where opiate manipulations alter some aspect of retention for events that occur in the absence of reinforcement.