SINGLE-UNIT ACTIVITY IN THE DORSOMEDIAL PREFRONTAL CORTEX DURING THE EXPRESSION OF DISCRIMINATIVE BRADYCARDIA IN RABBITS

Much recent evidence suggests that the medial prefrontal cortex (PFCm) participates in the development and/or expression of learned, primary bradycardiac adjustments in rabbits. Accordingly, the present experiments were undertaken to determine whether single-unit activity in the precentral agranular and anterior cingulate regions of the PFCm is systematically affected by differential aversive Pavlovian conditioning and, if so, whether such activity is correlated with concomitant heart rate (HR) variables. In these studies, extracellular recordings were made during a training procedure that involved two distinctive, tone conditioned stimuli (CSs); one of these (CS+) was often paired with eye-shock (unconditioned stimulus, US), whereas the other (CS-) was never so paired. Of the 100 spontaneously-active cells whose activity was so evaluated, 74 were found to be tone-responsive and could be classified as belonging to one of 5 subpopulations on the basis of their specific patterns of evoked changes in discharge; additional data suggested that these cells were also generally responsive to the eye-shock US. Regarding the effects of training, the vast majority of cells in these subpopulations showed greater evoked activity changes on CS+, as opposed to CS-, trials, irrespective of the sign (i.e. increase and/or decrease) of their responses. This finding was not confounded by differences in baseline activity preceding CS+ vs CS- trials, which was generally characterized as being both irregular and unrelated to concomitant HR variables; nor could it be attributed to unconditioned response biases. Thus, the discriminative activity patterns of these cells appeared to reflect the differential Pavlovian contingencies in effect. Moreover, approximately half of these cells exhibited tone-evoked activity changes that were reliably correlated with concomitant HR changes on a trial-by-trial basis. Consequently, the present data indicate that training-induced changes in the CS-evoked activity of PFCm cells are significantly related to aversively conditioned bradycardia in rabbits.