Effects of scopolamine and methylscopolamine on classical conditioning of the rabbit nictitating membrane response.

Classical conditioning of the rabbit nictitating membrane response was accomplished by presenting tone- and light-conditioned stimuli for 800 msec before delivery of a 100-msec shock as the unconditioned stimulus. Scopolamine significantly retarded the rate of acquisition and final asymptotic performance of conditioned responses to the tone- and light-conditioned stimuli. Methylscopolamine was approximately 20 times less potent than scopolamine in retarding the rate of acquisition, and had no effect on the final asymptotic performance of conditioned responses. The retardation in acquisition of conditioned responses produced by scopolamine could still be detected 5 days after cessation of drug injections, indicating that the effects of scopolamine were on acquisition and not performance. In contrast, scopolamine and methylscopolamine had no affect on the development of long-term habituation produced by the unpaired presentations of tone, light and shock stimuli. Control experiments indicated that the acquisition of conditioned responses by animals injected with saline, scopolamine or methylscopolamine was not contaminated by the presence of changes in base-line responding, sensitization or pseudoconditioning. In addition, scopolamine and methylscopolamine did not affect the unconditioned nictitating membrane reflex. In previously trained animals, scopolamine produced a significant, approximately 25-db elevation in the intensity threshold of a tone-conditioned stimulus for elicitation of conditioned responses. It was concluded that scopolamine blocks the excitatory properties of tone stimuli and this accounts for its ability to retard the rate of acquisition of conditioned responses.