A negative feedback role for the ovaries of the mare on tonic LH secretion before the first ovulation of the breeding season.

: Eleven anoestrous mares were assigned randomly to Group A (intact, n = 6) or Group B (ovariectomized in January, n = 5). Jugular blood samples were collected during February to April. Ovarian activity was assessed by ultrasound and ovulation was confirmed by progesterone analysis. Intact mares ovulated between 2 and 28 April. Mean diameter of the largest follicle was less than 20 mm on 17 or 18 March (Period 3), but increased to 29 mm by 31 March or 1 April (Period 4). During Periods 1 and 2 (14 or 15 February and 3 or 4 March) mean luteinizing hormone (LH) concentrations were similar in both groups. In intact mares, circulating concentrations remained low during Periods 3 and 4. In contrast, in ovariectomized mares concentrations tended to increase during Period 3, and were significantly increased by Period 4, at which time concentrations were significantly (P less than 0.05) greater in ovariectomized as opposed to intact mares. Increased concentrations of LH in ovariectomized mares during Periods 3 and 4 were associated with well defined pulsatile profiles. In contrast, pulses in intact mares remained infrequent and low in amplitude during the same time periods. The absence of LH pulses in intact mares during Periods 3 and 4 may reflect a low pulse frequency or a decrease in amplitude such that pulses escape detection. These results support the hypothesis that in intact mares, low circulating concentrations of LH during the transition into the breeding season, in part, may reflect inhibition by a factor of ovarian origin. It remains to be determined whether this apparent inhibition of LH secretion undergoes a seasonal modification in effectiveness and plays a role in regulating the annual breeding cycle of the mare.