Influence of photoperiodism on testicular function and sebaceous glands in Syrian hamster

The photoperiod (i.e., the daylight fraction of the 24-h day and its seasonal changes) influences the annual cycle of many mammalian species. Especially the Syrian hamster (Mesocricetus auratus), which is an appropriate animal model to investigate the sebaceous gland activity, shows a strong photoperiodism controlling the sexual development as well as the function of androgen-controlled organs such as sebaceous glands. Short photoperiods with accompanying long dark periods lead to a sexual regression while long photoperiods stimulate the recrudescence. In light-physiologic studies Syrian hamsters were exposed to different light schedules. The daily light exposure was increased from 8 to 12, 13, 14, and 16 h. Sebaceous gland areas, weight of testes and accessory glands, tubular areas, and plasma levels of testosterone were determined. Syrian hamsters are sexually stimulated at a daily light exposure of 14 h. Below this light threshold the sexual regression begins. At a light schedule of 8 h the testes shrink, plasma testosterone levels and sebaceous gland areas show a significant reduction ("photoperiodic castration"). Therefore, in experiments of androgen-controlled organs of the Syrian hamster a minimum daily light period of 14 or 16 h is necessary for a sufficient testicular function and therefore for an effective stimulation of the sebaceous gland activity. Control animals of the same age and the same light schedule should be required to avoid pitfalls of photoperiodic effects.