The proposed behavioral experiments will explore the CNS pathways involved in the classical conditioning of heart rate responses in rabbits. Tone will be used as a conditioned stimulus (CS) and periorbital shock will serve as the unconditioned stimulus (US). In the rabbit, the conditioned heart rate response consists of heart rate slowing (bradycardia). The focus of the proposed project will be to describe the pathways mediating the CS and the conditioned response (CR), and to identify the structures in these pathways that are concerned with the discrimination between an associative CS (CS+) and a non-associative CS (CS-). Our strategy will involve: (a) using electrical stimulation techniques to elicit a cardiovascular response pattern from a given CNS site (e.g., primary bradycardia from sites along the CR pathway); (b) electrolytic lesions or administration of ibotenic acid along the pathway and assessment of the effects of the lesion or ibotenic acid administration on the differentially conditioned and/or unconditioned responses; (c) injection of horseradish peroxidase (HR), fluorescent tracers, or PHA-L into these CNS sites in order to trace anatomically anterograde and retrograde projections to and from the region; and (d) extracellular recording to determine if monosynaptic functional connections exist. Our research thus far has traced a bradycardia pathway originating in the central nucleus of the amygdala (ACE), which courses through the lateral hypothalamus, lateral zona incerta (LZI), and parabrachial nucleus of the pons, before terminating at the cells of origin of the vagus nerve in the medulla. Bilateral lesions of the LZI or ACE abolish the CR without influencing the UR, orienting response, or nictitating membrane conditioning, thereby suggesting that this pathway is the CR pathway. HRP injections into ACE produce cell body labeling in the medial geniculate nucleus (MGN), an auditory structure. Bilateral lesions of MGN abolish the ability to discriminate between a CS+ and CS- but do not affect the magnitude of the bradycardia CR, UR, or nictitating membrane differential conditioning. These data suggest that the CS pathway may be organized in a parallel fashion with rostral CS structures mediating discrimination, and more caudal CS structures mediating conditioned bradycardia responses. Knowledge of the pathways underlying learned cardiovascular adjustments could provide a biopsychological framework for studying possible learned aspects of cardiovascular pathophysiology and treatment.
