Ultradian and seasonal rhythms in plasma gonadotropins, prolactin, cortisol, and testosterone in pinealectomized rams

Plasma concentrations of PRL, LH, cortisol, FSH, and testosterone were determined in pinealectomized (Px) and control rams at four times during a year. Basal levels of PRL decreased between spring and winter in controls from 44 +/- 2 ng/ml (mean +/- SE) to 16 +/- 3 ng/ml, while in Px rams, a different biphasic pattern was observed (56 +/- 9 ng/ml in the spring; 12 +/- 3 ng/l in the summer, 45 +/- 8 ng/ml in the autumn, and 25 +/- 5 ng/ml in the winter). There were no other significant seasonal differences or treatment effects in PRL episodes frequency or peak height, nor did pinealectomy alter the ultradian rhythm. The control rams had a significant seasonal difference in LH peak frequency, being higher in summer than at any other time. The Px rams had no seasonal changes in this or any other parameter of LH secretion. There were no significant differences between Px and control rams in any aspect of cortisol secretion and no seasonal trends. No seasonal or treatment effects were observed in plasma testosterone levels. Plasma FSH tended to be highest in controls in autumn, but this peak was not observed in Px rams. These results indicate that the pineal gland of sheep is involved in some aspects of seasonal breeding, possibly synchronizing hormone secretion with environmental changes. (Endocrinology 108: 639, 1981)