LIMBIC THALAMIC, CINGULATE CORTICAL AND HIPPOCAMPAL NEURONAL CORRELATES OF DISCRIMINATIVE APPROACH LEARNING IN RABBITS

Previous research employing lesions and recording of neuronal activity has implicated cingulothalamic and hippocampal circuitry in the media tion of discriminative instrumental avoidance learning in rabbits. Thi s study was directed at the question of whether the cingulothalamic ci rcuitry is specialized for avoidance learning, or whether it is also i nvolved in appetitively motivated learning. Multi-unit neuronal record ings in the aforementioned areas were obtained as adult New-Zealand wh ite rabbits learned to approach and orally contact a drinking spout fo r water reward after a tone conditional stimulus (CS+), and to ignore the spout after a different, non-predictive tone conditional stimulus (CS-). As during avoidance learning, excitatory and discriminative tra ining-induced neuronal activity (TIA) developed during the course of a pproach learning. Discriminative TIA refers to development of greater neuronal firing response to the CS+ than to the CS-. Excitatory TIA re fers to increased neuronal discharge magnitude during training compare d to the activity elicited before training, when CS presentations were unpaired with foot-shock presentations. As during avoidance learning, TIA in anterior cingulate cortical and interconnected mediodorsal(MD) thalamic neuronal records preceded TIA in posterior cingulate cortica l and interconnected anterior ventral thalamic records. Delayed change s also occurred in area CAI of the hippocampus in parallel with change s in the posterior cingulate cortex and the anterior thalamic nuclei. In contrast to the avoidance-related activity, the changes in the thal amic areas preceded or occurred concurrently with changes in the relat ed cingulate cortical areas. This difference is hypothesized to be due to a reduced or absent contribution of amygdaloid efferents to the ap proach learning-related TIA. The overall magnitude of the elicited tra ining-induced neuronal responses was reduced, relative to the discharg es during avoidance conditioning. The discharge magnitude differences suggested a greater recruitment of limbic circuit functions during avo idance learning, possibly due to the aversiveness and high arousal ass ociated with the avoidance task. In general, the results indicate that the circuitry formed by interconnected cingulate cortical, limbic tha lamic and hippocampal neurons has fundamentally similar functions in b oth approach and avoidance learning.