Seasonal and photoperiod-induced changes in serum prolactin and pituitary responsiveness to thyrotropin-releasing hormone in the mare.

Experiments were conducted in the horse mare to study the effects of photoperiod and season on serum prolactin and pituitary responsiveness to thyrotropin-releasing hormone (TRH). Increasing the photoperiod to 16 hr light:8 hr dark beginning in December (Experiment 1) and September (Experiment 2) increased serum prolactin, but the rate of increase was greater when the photoperiod treatment was initiated in September. In addition, TRH-induced prolactin secretion was found to be affected by season, in that pituitary secretion (net increase in prolactin and total prolactin secreted) was significantly greater in June compared with that of January (Experiment 3). These data suggest that in the mare, photoperiod plays an important role in controlling circulating levels of prolactin, but that in addition to photoperiod, other seasonally related factors, such as temperature, are involved in modulating the seasonal rhythm of serum prolactin.