Role of the pineal gland in ovine photoperiodism: regulation of seasonal breeding and negative feedback effects of estradiol upon luteinizing hormone secretion.

In this study we determined whether the pineal is required for the photoperiodic control of reproduction in ewes and, if so, whether its effect is mediated via negative feedback effects of estradiol on LH. Two groups of Suffolk ewes were pinealectomized and challenged with 90-day alternations between long (16 h of light, 8 h of darkness) and short (8 h of light, 16 h of darkness) photoperiods for 2 yr. One group of pinealectomized ewes had intact ovaries; the other was ovariectomized and implanted with estradiol to monitor feedback inhibition of LH. Reproductive effects of pinealectomy were correlated with elimination of the nighttime rise of serum melatonin. In pineal-intact control ewes, reproductive function was readily manipulated by photoperiod. Long days inhibited ovarian cyclicity and increased the negative feedback potency of estradiol regardless of time of year; short days had stimulatory effects on both variables. In most animals, pinealectomy abolished both the inductive effects of short days and the inhibitory effects of long days, and eliminated the nighttime rise in serum melatonin. Although seasonal changes in reproductive function persisted after pinealectomy, the timing of these changes coincided most closely with that of pineal-intact animals housed outdoors. With the passage of time after pinealectomy, however, these seasonal fluctuations became progressively damped. These results document pineal mediation of the photoperiodic control of seasonal breeding in ewes and indicate that the pineal exerts its effect, at least in part, by changing the capacity of estradiol to inhibit LH secretion. Furthermore, the pineal mediates the response to both stimulatory and inhibitory photoperiods. Finally, ewes that are unresponsive to photoperiod remain seasonal, either in response to other environmental cues or owing to the expression of an endogenous circannual rhythm of reproduction.