ATTENUATION OF THE MAGNITUDE OF SUCKLING-INDUCED PROLACTIN-RELEASE WITH ADVANCING LACTATION - MECHANISMS

To study why suckling-induced plasma prolactin levels decline in magnitude with advancing lactation, we examined prolactin release in lactating rats following suckling and pharmacologic manipulations during early, mid- and late lactation. on day 2 of lactation, litters were adjusted to 8 pups. on day 3, dams were implanted with an atrial catheter and experiments were conducted on lactation days 5, 11 and 17. To examine suckling-induced prolactin release, pups were removed at 0800 h, an extension was attached to the catheter at 1100 h, and pups returned to dams at 1200 h. Blood samples were obtained before, and at 10, 30, 60, 90 and 120 min after suckling started. Prolactin responses to sulpiride and thyrotropin releasing hormone (TRH) administration were studied in lactating rats separated from their litters for 4 hours. Blood samples were obtained before, and at 10, 30, 60 and 90 min after sulpiride (10 or 40 mu g/kg BW) and 5, 10, 20 and 30 min after TRH (1 or 4 mu g/kg BW) in rats pretreated with sulpiride. Prolactin release in response to suckling, administration of sulpiride or sulpiride and TRH diminished as lactation advanced. From these results, we conclude that refractoriness in anterior pituitary lactrotropes to prolactin-releasing stimuli is at least partially responsible for the decline in suckling-induced prolactin release with advancing lactation.