Localization of inhibitory mechanisms to aggressiveness in the medial hypothalamus

Release of aggressive reactions was studied by placing a small lesion on the medial hypothalamus in the rat with previous massive destruction on the contralateral side of the hypothalamus. A set of tests for reactivity to provocations was applied to rats before the operation, after the unilateral lesion, and once more after the bilateral lesions. Bilateral lesions were accomplished in 53 rats. None of them increased food intake even though the ventromedial region was involved in many cases. Only 7 developed signs of aggressiveness. The unique histological feature of the animals showing aggressiveness was that the lesions were confined to the posterior hypothalamus usually sparing the ventromedial nucleus and were close to the base. It was suggested that the lesions might have interrupted the fibers projecting to the midbrain mechanisms for control of emotional reactivity.