Prolactin levels and duration of postpartum anoestrus in lactating ewes.

Abstract

In both lactating women and lactating ewes suckling is accompanied by large discharges of oxytocin and prolactin. The prolactin secretion decreases as lactation progresses being almost nonexistent by the time of return of cyclical ovarian activity. It has been suggested that prolactin has an antigonadal action. This study investigates ovarian and pituitary activities in lactating ewes after denervation of the mammary gland to inhibit the afferent arc of the suckling reflex. All nervous connections between the mammary gland and the body were severed in 5 Prealpes du Sud ewes during late pregnancy. The ewes were allowed to suckle their lambs after normal parturition as were 8 unoperated control ewes which delivered at the same time. Blood samples were drawn twice daily for radioimmunoassay of prolactin and luteinizing hormone (LH). Estrous behavior was checked after blood sampling using a vasectomized ram. All ewes were subjected to laparoscopy between days 12-15 25-31 and 40-50 postpartum for examination of ovarian status. The normal prepartum surge of prolactin secretion and basal levels during lactation were not affected by denervation but the suckling stimulus was not followed by the normal reflex discharge of prolactin. Control animals exhibited neither ovulation nor an LH peak before day 25 postpartum. Silent ovulations preceded by an LH surge occurred in 5 of 8 animals between days 25-50 postpartum; only 3 came into estrous during this period. In contrast 3 of 5 operated animals had ovulated without showing estrous before day 14 postpartum and all 5 animals had ovulated and shown estrous before day 50. The findings indicate that it is the surges of prolactin secretion induced by suckling which suppress the resumption of cyclical ovarian activity.