Lactational amenorrhea in monkeys: effects of suckling on prolactin secretion.

To determine the acute and chronic effects of suckling on maternal PRL secretion in monkeys, five mother-infant pairs were studied longitudinally on days 40, 80, 120, and 10 after weaning (day 160). Mothers were chronically cannulated and, during blood collections, wore protective nylon vests with mobile tethers. Studies were undertaken during the day and night with the mother and infant undisturbed, during the daytime, before and after the removal of the infant, and during the day and night before and after the reunion of mother and infant. Maternal PRL levels were significantly (P less than 0.05) higher at night than during the day in undisturbed mother-infant pairs. This nocturnal elevation was probably induced by a more intensive interaction of the mother and infant at night than during the day. Basal PRL concentrations in samples collected during these undisturbed settings significantly (P less than 0.05) declined as the postpartum interval continued. The removal of the infant did not perturb maternal PRL patterns. Typically, after reunion of mother and infant, maternal PRL levels were increased significantly (P less than 0.05), reaching maximal levels approximately 2 h after reunion. If PRL secretion, induced by the suckling stimulus, is instrumental in sustaining puerperal infertility, then the increased secretion of PRL that occurs at night during the protracted interval of intense mother-infant interaction may be of particular significance in inhibition of the hypothalamic-pituitary-ovarian axis.