Unit activity of the septo-hippocampal system in classical conditioning with rewarding brain stimulation

Neuronal activity in dentate, hippocampus, septum and medial forebrain bundle (MFB) was recorded with chronically implanted electrodes in rats. Responses to tone were measured before and after differential conditioning reinforced by rewarding brain stimulation. Differential unit response changes were found in hippocampus and MFB. The hippocampal response was enhanced in the early phase to the CS+ and gradually subsided. A similar pattern was obtained in the dentate, with a less remarkable differentiation. In contrast, MFB showed sustained enhancement to the CS+ during the whole period, but not to the CS--. It was proposed that the CS-UCS connection was formed in the MFB after conditioning, and that this connection had an effect that was the activation of this system by the CS+ itself.