Photoperiodism and Seasonality in Hamsters: Role of the Pineal Gland

Abstract

During the past two decades, a large body of evidence has been accumulated to indicate a central role for the pineal gland in the regulation of photoperiodic responses in mammals (Goldman and Darrow 1983). Most of this evidence has come from studies of the effects of pinealectomy on the reproductive system in photoperiodic mammals. The Syrian hamster has been the most intensively studied of these species, and frequently has been presented as a “model” for the role of the pineal in regulating reproductive activity. This species is a long-day breeder and exhibits gonadal regression following several weeks of exposure to short days—i.e., daylengths of less than 12.5-h illumination/24-h cycle (Elliott 1976). Removal of the pineal completely prevents this response to short-day exposure; pinealectomized Syrian hamsters remain reproductively active under all photoperiodic conditions (Reiter 1969, Reiter 1974). The photoperiod dependency of the response to pinealectomy in this species has been emphasized. In long days, pinealectomized and intact Syrian hamsters show identical levels of reproductive activity; the effect of pinealectomy becomes apparent only during exposure to short days (Reiter 1974). The closely related Turkish hamster is also a long-day breeder and requires daylengths of nearly 16 h to stimulate reproductive activity (Hong et al. 1986). Pinealectomy also has a photoperiod-dependent effect on reproductive activity in this species, but the nature of the response to pinealectomy is almost the reverse of that seen in Syrian hamsters. In Turkish hamsters, pinealectomy actually triggers testicular regression in long-day-housed animals; short-day-exposed Turkish hamsters exhibit gonadal regression regardless of whether or not they have been pinealectomized (Carter et al. 1982).