Conditioned suppression of medial forebrain bundle and septal intracranial self-stimulation in the rat: evidence for a fear-relief mechanism of the septum.

A conditioned emotional response (CER) paradigm was presented to two groups of rats during intracranial self-stimulation (ICSS). One group bar pressed for medial forebrain bundle (MFB) stimulation reward; the other group bar pressed for septal stimulation reward. The MFB ICSS was found to be suppressed by the CER procedure, but this procedure failed to suppress septal ICSS. This difference between the two sites was found only when both MFB and septal ICSS current intensities were available at their optimal levels. When ICSS current intensities were lowered to either threshold or medium level, both groups exhibited the CER suppression effect. The animals were also tested for a possible analgesic effect produced by the ICSS. The MFB stimulation was found to produce some degree of analgesia, but septal stimulation failed to produce any analgesic effect. Thus, the possibility that the attenuation of the CER suppression effect in the septal group was due to analgesia was excluded. The difference in MFB and septal ICSS behavior during the presentation of the aversive stimulus suggested a possible qualitative distinction between the reward functions of these two sites, and a possible fear-reduction property of the septal area.