Neuronal activity in the mediodorsal and intralaminar nuclei of the dorsal thalamus during classical heart rate conditioning.

Multiple unit activity (MUA) was recorded from chronically implanted electrodes in either the mediodorsal (MD) or the intralaminar (IL) nuclei of the dorsal thalamus in separate groups of rabbits during (a) habituation of the cardiac orienting reflex, (b) Pavlovian heart rate (HR) conditioning, and (c) extinction of the HR conditioned response (CR). Other animals with similar recording electrodes received explicitly unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus (US). The cardiac orienting reflex and the HR CR consisted of bradycardia. However, tone-evoked tachycardia was obtained in animals that received CS/US unpaired presentations. MUA evoked by the CS consisted of a short latency (20-40 ms) increase under all conditions, which reached its maximum 200-300 ms after CS onset. This response habituated greatly during tone-alone pretraining, but was considerably greater in the paired than unpaired group during the later trials of conditioning in animals with MD, but not IL, placements. Instead, a longer latency increase (greater than 500 ms) in MUA occurred in the paired but not in the unpaired animals in the IL group. The MUA increases in both instances, including the early, short latency increase in the MD group, and the longer latency increase in the IL group, were trial-related, and declined to pretraining levels during extinction, indicating that these neuronal changes had an associative basis. These findings suggest that neuronal activity in both MD and IL is related to the early events involved in Pavlovian conditioning, but that the relative roles of these two closely related thalamic nuclei in associative learning must be somewhat different.