Seasonal Changes in the Function of the Hypothalamic-Pituitary-Testicular Axis in the Syrian Hamster

Abstract

In the overwhelming majority of animal species, reproductive functions exhibit pronounced annual rhythmicity. Fertility is usually restricted to a particular and often a very brief period of the year, the breeding season. The Syrian (golden) hamster, Mesocricetus auratus, is a very popular model for the study of seasonal breeding, and a substantial part of what is known about the mechanisms of environmental control of mammalian reproduction is derived from experiments conducted in this species.

Golden hamsters have a relatively restricted geographic range and commercially available laboratory stocks are derived from a few animals collected over 50 years ago in northern Syria (1). In their natural habitat, hamsters breed in the spring and fall and hibernate during the winter. Under laboratory conditions, they remain reproductively active throughout the year, as long as they are exposed to long photoperiods, i.e., at least 12.5 hr of light per day. However, the transfer of adult animals to a short photoperiod (i.e., less than 12 hr/day) or complete darkness at any time of the year triggers a sequence of changes in their neuroendocrine and reproductive functions that appears to mimic the processes that normally occur during the fall, winter, and early spring. These changes include (i) repression of the testes, (ii) a period of gonadal atrophy and functional quiescence, and (iii) spontaneous recovery (“recrudescence”) of testicular size and activity. The amplitude of these changes is enormous, with reduction of testicular weight by at least 80%, comparable decreases in the size of other parts of the male reproductive system, complete cessation of spermatogenesis, substantial reduction in the blood levels of the male sex hormone, testosterone, and loss of libido (2, 3). These responses to a short photoperiod can be prevented by surgical removal of the pineal gland and mimicked by properly timed melatonin injections and thus are obviously mediated by photoperiod-related changes in the secretory function of the pineal (2).