The temporary inactivation of the red nucleus affects performance of both conditioned and unconditioned nictitating membrane responses in the rabbit

Abstract

These experiments are part of a series of studies examining the role of the red nucleus in the performance of the conditioned and unconditioned nictitating membrane reflexes in the rabbit. Specifically, the experiments test the hypothesis that the temporary inactivation of the red nucleus selectively affects the performance of the conditioned reflex. The experiments were designed to assess the effects of lidocaine and control saline microinjections on conditioned as well as unconditioned responses in both paired and unpaired trials. Rabbits were chronically implanted with cannulae through which small injecting tubes were passed stereotaxically to the red nucleus. The animals were conditioned using a delay paradigm in which a 1 kHz tone and an air puff applied to the cornea were used as the unconditioned and conditioned stimulus, respectively. Once conditioned, the effects of either lidocaine or saline injection were evaluated while alternating paired trials with unpaired trials in which only the air puff was applied. The principal finding of this study was that the amplitudes of both the conditioned and unconditioned responses were reduced following lidocaine injection into the red nucleus. The effect on the unconditioned response amplitude could not be ascribed to any interaction between the conditioned and unconditioned responses, since it also was present in the unpaired trials. The reduction in amplitude of the conditioned and unconditioned responses was shown to be correlated with changes in other characteristics of the same responses. The data suggest that the red nucleus contributes to the performance of both the conditioned and unconditioned nictitating membrane reflexes and consequently is not likely to be involved only in pathways responsible for mediating and/or storing the engram for the conditioned reflex.