Comparison of Luteinizing Hormone and Prolactin Levels in Cycling and Anestrous Ewes

Concentrations of luteinizing hormone (LH) and prolactin (Prl) in the serum of anestrous and cycling ewes were compared. The concentration of LH in serum and the frequency of episodic releases of LH were lower in anestrous than in cycling ewes. Concentrations of LH in serum rose as rapidly after ovariectomy of anestrous ewes as after ovariectomy of cycling ewes. Prl levels were higher during anestrus than at any stage of the estrous cycle; however, high Prl levels were not necessarily associated either with low LH levels or the absence of episodic release of LH. These data suggest: (1) that anestrus is not due to a reduction in the intrinsic ability of the hypothalamo-hypophyseal system to secrete LH, and (2) that high Prl levels may be a symptom rather than a cause of seasonal anestrus.