Amygdaloid central nucleus in rabbits: Role in differential bradycardiac conditioning and examination of subcortical afferent projections

In Experiment 1, rabbits with bilateral ibotenic acid (IA) lesions of either the amygdaloid central nucleus (ACE) or control sites were subjected to 7 days of differential Pavlovian bradycardiac conditioning. IA lesions in ACE prevented the acquisition of the differential heart rate (HR) conditioned response (CR), yet had no effect on the HR orienting response (OR), unconditioned response (UR), baseline, or corneo-retinal potential (CRP) conditioned responses. The major finding of this study is that intrinsic neurons of ACE, not fibers of passage, participate in the expression of the bradycardiac CR. In Experiment 2, animals received IA injections bilaterally in lateral zona incerta (LZI) or control sites and were subjected to 7 days of differential Pavlovian bradycardiac conditioning. IA lesions had no effect on the acquisition of the HR CR, OR, UR, baseline, or CRP CR. In light of previous research which demonstrated that electrolytic lesions of LZI abolished the HR CR (Jarrell et al., 1986), the present data suggests that fibers of passage through LZI, and not intrinsic neurons, participate in the expression of the conditioned bradycardiac response. Experiment 3 used the Fluoro-gold method of retrograde transport to determine the subcortical nuclei which project to ACE. Retrograde labeling was demonstrated in various thalamic and hypothalamic nuclei as well as parabrachial regions