Evidence for pineal induced seasonal changes in reproductive physiology of male hamsters kept under natural environmental conditions

Pinealectomized and intact male hamsters were kept under natural daylight and temperature conditions at 29° 33' N latitude and 30° 29' W longitude beginning either Nov. 22, 1971, or Sept. 20, 1972. Testes and accessory sex organs of hamsters killed in late October, late November and mid January were grossly and microscopically atrophic. In March, near the vernal equinox, reproductive organs of intact hamsters showed considerable regeneration but were not normal in size or in histological appearance. By mid May, however, reproductive organs exhibited complete recovery. Pinealectomized hamsters never experienced gonadal atrophy during the winter, i.e., surgical removal of the pineal gland prevented the normal winter period of sexual dormancy. Reproductive organs of pinealectomized animals were always similar to those of hamsters kept in the laboratory with 14 hours of light per day. Plasma LH levels and pituitary prolactin levels were also diminished significantly during the winter months in intact hamsters while in the pinealectomized controls they were comparable to summer values at all seasons tested. Hamsters exposed to near total darkness during the summer months (after having experienced gonadal atrophy the previous winter) never experienced a second reproductive regression possibly indicating a summer refractoriness to the pineal.