Group rearing abolishes hyperdefensiveness induced in weanling rats by lateral septal or medial accumbens lesions but not by medial hypothalamic lesions.

Lesions of the medial hypothalamus, medial accumbens, or septum were made in 21- to 25-day-old male hooded rats. Half of the animals in each group were subsequently reared in groups and the other half in isolation. When tested for defensiveness toward the experimenter at 31, 34, and 37 days postoperatively, rats with medial hypothalamic lesions were most hyperdefensive toward the experimenter if reared in isolation but were significantly more defensive than sham-lesioned animals even when reared in groups. Rats with septal lesions were significantly more defensive than sham-lesioned animals only when reared in isolation while rats with medial accumbens lesions were not different from controls whether reared individually or in groups. These results suggest that the medial hypothalamus may have a special importance in determining temperament since the hyperdefensiveness that results from interference with its functioning is resistant to experiential remediation.