Neuronal correlates of classically conditioned bradycardia in the rabbit: studies of the medial prefrontal cortex.

Multiple-unit activity (MUA) in the agranular medial prefrontal cortex (PFC) was recorded in conscious rabbits during a classical conditioning procedure involving repeated pairings of a 4-s tone conditioned stimulus (CS) with eye shock. Prior to behavioral training, significant tone-evoked increases in prefrontal MUA were observed; the magnitude of this evoked cortical discharge declined with repeated presentations of the tone alone. However, the first few paired presentations of tone with eye shock served to rapidly reestablish the evoked activity, and subsequent pairings resulted in a significant enhancement of CS-evoked discharge, relative to pretraining response levels. These associative training-induced changes in prefrontal MUA appeared to parallel the development of bradycardiac conditioned responses (CRs), and, in fact, significant correlations between the neuronal and behavioral responses were observed. In contrast to these associative effects, non-associative training procedures involving either unpaired presentations of tone and eye shock or repeated presentations of the tone alone resulted in progressive attenuation of tone-evoked MUA, relative to pretraining levels. We had previously reported that bilateral destruction of the medial PFC seriously compromises the development of discriminative heart rate CRs in the rabbit. In light of this finding, our present results lend further support to the suggestion that associative training-induced changes in CS-evoked neuronal activity in this cortical region contribute, at least in part, to the development of learned cardiovascular adjustments in this animal.