Electrical self-stimulation in the medial and lateral septum as compared to the lateral hypothalamus: differential intervention of reward and learning processes?

The characteristics of the electrical self-stimulation behavior elicited from the lateral hypothalamus (LH) and from both medial (MS) and lateral (LS) parts of the septal nucleus have been compared in male mice of the BALB/c strain. Using two different experimental situations (the lever-press box and a spatial discrimination test in a Y-maze) the self-stimulation rate-current intensity relations and the performances during both acquisition and reversal of a spatial discrimination were tested successively. In the lever-press box, it was observed that highest self-stimulation rates were obtained from LH placements while both MS and LS rates were lower. However, MS animals showed higher self-stimulation rates and lower self-stimulation thresholds than LS animals. Acquisition of the spatial discrimination in the Y-maze was achieved by all 3 groups with similar time courses. However, when tested for the reversal of the discrimination, the LS implanted mice were much more perturbed than the two other groups and exhibited marked perseveration. The incidence of convulsive episodes was more frequent in LS mice than in either MS or LH implanted animals during both phases of the discrimination task. These differences in the self-stimulation behavior elicited from medial and lateral parts of the septal complex are discussed in relation to the operation of positive reinforcement mechanisms and to internal inhibition processes operating during acquisition and reversal of the spatial discrimination.