Effect of psychosocial stimuli and limbic lesions on prolactin at rest and following stress.

Many of the factors altering corticosterone in the rat have similar effects on prolactin. Resting prolactin, like corticosterone, is higher in animals housed singly than in those multiply housed. Unlike corticosterone, gentling did not lower prolactin. An exquisite sensitivity to environmental stimuli which results in rapid elevations in prolactin levels has been indentified in the rat which is considerably greater than that for corticosterone. Like corticosterone, the pattern of prolactin stress responses varies according to time of day. This differential sensitivity may well account for the diurnal variation in prolactin reported by some investigators. The limbic system has an important influence on prolactin. Lesions of the septal region do not modify resting levels but alter the pattern of prolactin stress responses in ways which are only partly similar to the effects of such lesions on corticosterone. In rhesus monkeys, individual variation in resting prolactin levels was observed, similar to that seen in the human. Levels are stable over periods of up to 50 days and are characteristic of the individual animal. In response to the stress of electric shock or shaping to a bar press response, prolactin elevation occurs only in animals with high resting levels, suggesting that a common mechanism is responsible for controlling both resting levels and magnitude of stress responses.