Role of the Pineal Gland in Photoperiod-Induced Changes in Hypothalamic-Pituitary Sensitivity to Testosterone Feedback in Castrated Male Hamsters*

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Abstract
An exposure to short days induces testicular atrophy in intact, but not pinealectomized, golden hamsters. Since recent evidence suggests that an alteration in the sensitivity of the hypothalamic-hypophyseal system to steroid feedback may be one way in which the photoperiod acts to alter neuroendocrine- gonadal activity in seasonally breeding animals, the present investigation was designed to determine if the pineal gland is involved in this photic-induced change to steroid feedback. Castrated adult male hamsters that had been sham-pinealectomized (Sham-Px) or pinealectomized (Px) were maintained on either stimulatory long days [14 h of light, 10 h of darkness (LD 14:10)] or nonstimulatory short days (LD 6:18) for 8 weeks. The animals were then implanted with various sized testosteronefilled capsules and serum was collected 10 and 21 days later for hormone analysis. Capsules which contained 2 or 4 mm powdered testosterone maintained serum testosterone levels between about 0.8-1.5 ng/ml, and induced at least a 10-fold reduction in serum levels of immunoreactive LH and FSH in Sham- Px animals maintained on LD 6:18. These size capsules had no significant effect on serum gonadotropin levels in Sham-Px animals maintained on LD 14:10. Higher doses of testosterone (8 and 20 mm long testosterone-filled capsules) suppressed serum gonadotropin levels in all Sham-Px animals. Pinealectomy did not significantly alter the inhibition of pituitary gonadotropin release by testosterone in animals on LD 14:10. In contrast, the increased responsiveness of the gonadotropin control center to steroid feedback observed in Sham-Px animals on LD 6:18 was prevented or suppressed by pinealectomy. These results indicate that the short day-induced increase in sensitivity of the hamster hypothalamic-hypophyseal axis to testosterone feedback is mediated, at least in part, by the pineal gland.