Photic threshold for stimulation of testicular growth and pituitary FSH release in male Djungarian hamsters.

While the effects of photoperiod on neuroendocrine-gonadal activity have been extensively studied in a number of species, surprisingly little information concerning the quantitative aspects of light regulating reproductive activity is available. In the present experiment, Djungarian hamsters were exposed to two 10 min pulses of light per day and the light irradiance was systematically varied to determine the threshold for photostimulation by white light. After 10 days testes weight and serum follicle-stimulating hormone (FSH) levels were determined. The data indicate that the irradiance threshold necessary for induction of significant increases of both serum FSH levels and testes weight lies between 0.1 and 0.34 microW/cm2 for 10 min pulses of light. These results demonstrate a strong correlation between the effects of light on serum FSH levels and testes weight and provide the first quantitative assessment of the irradiance threshold for light involved in photoperiodic stimulation of the hypothalamic-pituitary gonadal axis of a mammalian species.