Multiple-unit activity of the prefrontal cortex and mediodorsal thalamic nucleus during acquisition of discriminative avoidance behavior in rabbits.

Multiple-unit activity of the prefrontal cortex (PFC) and the mediodorsal (MD) thalamic nucleus was recorded during discriminative conditioning of a locomotory avoidance response in rabbits. A major objective was to compare the results with those previously obtained from the posterior limbic (cingulate) cortex, and the anteroventral (AV) thalamic nucleus. The results indicated the development, with training, of discriminative neuronal activity, i.e., greater neuronal discharges to presentation of the positive conditional stimulus (CS+, a tone paired with a footshock unconditional stimulus), relative to the negative conditional stimulus (CS-, a tone not paired with the footshock). The rostral-sulcal subfield of the PFC developed the discriminative activity during the first session of conditioning, prior to the acquisition of discriminative behavioral responding. This effect persisted throughout training to the asymptote of behavioral acquisition. The medial subdivision of the MD nucleus, with which the rostral-sulcal PFC is interconnected, did not develop discriminative effects. The caudal (supragenual) subfield of the PFC also developed discriminative activity in the first session of training but the effect declined progressively in the remaining acquisition sessions. The lateral subdivision of the MD nucleus, with which the caudal PFC is interconnected, manifested robust discriminative activity in the late acquisition sessions, concomitant with acquisition of significant behavioral discrimination. Thus, as in the cingulate cortical AV nuclear system, a corticothalamic sequential progression of discriminative activity occurred in the PFC-MD nuclear system during behavioral acquisition. However, the effect was confined to one subsystem: the caudal subfield of the PFC and the lateral subdivision of the MD nucleus. Also, discriminative activity developed more rapidly in the PFC than in the cingulate cortex, and it developed more rapidly in the MD nucleus than in the AV nucleus. Implications are considered concerning the contributions to learning and memory processes, of the PFC-MD thalamic system.