Differential involvement of the lateral and medial divisions of the septal area on spatial learning processes as revealed by intracranial self-administration of morphine in mice.

The present study was conducted to investigate the effects of morphine self-administration into either the medial or lateral divisions of the septal region on spatial discrimination abilities in mice. To this end, BALB/c mice received a dose of 5 ng/50 nl of morphine sulfate, via a stainless steel injection cannula, inserted into either the medial septal area (MS) or the lateral septal nucleus (LS), when they are close to the end of one of the two choice arms of a Y-maze (acquisition period). In these conditions a discrimination between the reinforced arm and the neutral arm is acquired in MS as well as in LS mice. However both acquisition and subsequent extinction (vehicle only available) was more rapid in the LS group than in the MS group. Moreover, during the extinction period, numerous escape attempts from the Y-maze were observed for MS mice. When the dose of morphine was raised to 50 ng the pattern of acquisition was unchanged in the LS group. In contrast animals of the MS group learned to avoid the arm where the higher dose of morphine was delivered thus allowing us to conclude that in MS animals the drug effect became aversive at this higher dose. This possibility was directly investigated in a second experiment by closing the access to the neutral arm. Thus, for a 5-ng dose the rewarding effect of morphine was demonstrated in both MS and LS groups by the decrease of self-administration latencies which, furthermore, were notably lower than in the discrimination situation. Moreover, with the dose of 50 ng of morphine the latencies were identical for the two groups and at their lowest value thus indicating that morphine had similar appetitive effects in MS as well as in LS mice in this situation. Thus, the previously observed deficit of MS subjects, including escape attempts, disappeared when the dose of morphine was raised and the experimental context simplified. These results which demonstrate differential functional properties of these two septal divisions are discussed in terms of conflict resulting from the strongly appetitive effects of the morphine which induces, in parallel, deleterious consequences on cognitive processing in MS self-injected mice.