Electrophysiological characteristics of amygdaloid central nucleus neurons during Pavlovian fear conditioning in the rabbit.

Recent evidence suggests that the amygdaloid central nucleus (ACE) may contribute importantly to cardiovascular adjustments in response to the presentation of conditioned emotional stimuli, possibly via direct ACE projections to cardiovascular regulatory nuclei in the medulla. The present experiment was conducted to obtain additional data relevant to this suggestion. Extracellular single-unit recordings were obtained from 85 histologically-verified ACE neurons during Pavlovian differentially conditioned heart-rate responding in rabbits. Conditioning involved pairing one tone (CS+), but not a second tone (CS-), with paraorbital shock. Those ACE neurons which project to the lower brainstem were identified by their antidromic responses to stimulation of a mesencephalic region through which descending ACE projections course. Under these conditions it was possible to classify ACE neurons as conforming to one of 6 general categories based on their spontaneous activity and conditioned response characteristics. In addition, it was determined that: (1) the electrophysiological characteristics of many ACE neurons were differentially altered in response to presentations of the CS+ versus the CS-; (2) the responses of many ACE neurons to presentations of the CS+ were correlated with the magnitudes of concomitant conditioned alterations in heart rate; and (3) the activity of antidromically-identified ACE neurons which project to the lower brainstem was decreased in response to presentations of each CS. These data provide additional support for the notion that the ACE contributes to cardiovascular regulation during the presentation of emotionally-arousing stimuli.