Midline thalamic lesions enhance conditioned bradycardia and the cardiac orienting reflex in rabbits

Abstract

Learned heart rate changes were assessed during two sessions of differential Pavlovian (classical) conditioning in three groups of New Zealand albino rabbits. One group received ibotenic acid lesions of the midline nuclei of the thalamus, a second received vehicle injections in the midline nuclei, and a third group served as unoperated control animals. During an initial session, the cardiac component of the orienting reflex was assessed in all three groups. The rabbits with lesions of midline thalamic nuclei demonstrated enhanced bradycardiac conditioned responses, relative to either vehicle-control or unoperated animals. The heart rate component of the orienting response to nonreinforced tones, which also consisted of stimulus-evoked bradycardia, was likewise larger in lesioned animals. These results suggest that the midline thalamic nuclei, like the interconnected mediodorsal thalamic nucleus, may be involved in the regulation of sympathetically mediated autonomic adjustments. © 1989, Psychonomic Society, Inc.. All rights reserved.