Failure to Demonstrate a Rôle for the Acute Release of Prolactin at Oestrus in the Ewe

Prolactin is readily secreted before and during oestrus in ewes (Kann, 1971; Cumming, Brown, Goding, Bryant & Greenwood, 1972) and Denamur, Martinet & Short (1973) indicated that this hormone has a luteotrophic effect. Although these findings suggest that prolactin contributes to normal reproduction in ewes, the physiological rôle of the prolactin surge at oestrus has as yet not been elucidated. In the rat and mouse, a surge of prolactin on the afternoon of pro-oestrus promotes luteolysis of corpora lutea (Malven & Sawyer, 1966; Grandison & Meites, 1972). Such an effect appears unlikely in sheep since luteal regression in this species commences several days before the onset of oestrus (Deane, Hay, Moor, Rowson & Short, 1966). Prolactin is also rapidly released into the circulation in response to the milking stimulus in lactating ewes (Fell, Beck, Brown, Catt, Cumming & Goding, 1972) and it has been suggested that the secretion of large