Evidence that exteroceptive stimuli other than suckling have no major effect on plasma prolactin in lactating rats.

The ability of exteroceptive stimuli from pups to increase plasma prolactin in lactating dams was investigated. Prolactin profiles were measured during 30 min of suckling followed by complete or partial separation from pups. Prolactin profiles were also measured subsequent to complete or partial isolation from pups in dams which had been with pups permanently before the experiment. In addition, plasma prolactin was measured in dams which after a night of isolation were partially united with pups. Finally, the effect of ether stress on prolactin profiles after interruption of suckling by pups was determined. Vigorous suckling after a period of isolation induced a sharp increase of plasma prolactin. Subsequent to pup removal, either partial or complete, prolactin profiles showed a widespread variation. Also dams which before experimentation were kept permanently with pups, showed a great variation in prolactin profiles subsequent to either complete or partial separation from pups. Plasma prolactin either decreased rapidly or gradually. In several dams a rapid or a gradual decline of plasma prolactin was interrupted by one or more episodes of prolactin release. Partial reunion with pups after a night of isolation, either in mid-lactation or in late lactation, did not induce a rise of plasma prolactin. It is concluded that exteroceptive stimuli from pups are not effective as prolactin releasing signal. Because ether stress did not induce a steep rise of plasma prolactin, we conclude that the episodic rises of plasma prolactin in dams, subsequent to partial or complete removal of pups, are due to spontaneous activity of the hypothalamo-pituitary system.