Circadian and Photoperiodic Effects of Brief Light Pulses in Male Djungarian Hamsters1

The effects of brief light pulses (1–60 min in duration) on the circadian rhythm of locomotor activity and/or the neuroendocrine-gonadal axis was investigated in male Djungarian hamsters. Exposure of hamsters free-running in constant darkness to a single 1-h pulse of light induced phase-dependent phase shifts in the rhythm of locomotor activity. The general shape of the “phase-response curve” was similar to that observed in other animals; phase-delays and phase-advances were induced by light pulses delivered in the early and late subjective night, respectively, while light pulses during the subjective day induced little or no phase-shift in the activity rhythm. Animals exposed for 7 days to 1-min of light during the night in animals otherwise exposed to 6L:18D resulted in increased levels of serum FSH and testicular weight. Daily exposure to two 1-h or two 10-min pulses of light (but not two 1-min pulses) for 10 days resulted in stable entrainment of the activity rhythm as well as testicular weight gains and serum FSH increases. When two 10-min pulses of light were presented 8 and 16 h apart, some animals showed a short-day entrainment pattern (i.e., locomotor activity confined to the long period of darkness) while other animals showed a long-day entrainment pattern (i.e., locomotor activity confined to the short period of darkness). Importantly, the stimulatory effects of light on neuroendocrine-gonadal activity were clearly dependent on the phase-relationship between the light pulses and the circadian rhythm of locomotor activity. While exposure to two 10-mm pulses of light per day induced testicular growth in all the hamsters in which the activity rhythm showed a long-day entrainment pattern, exposure to the same light cycle did- not result in testicular growth in any of the animals in which the activity rhythm showed a short-day entrainment pattern. Taken together, these results demonstrate that short pulses of light can induce a rapid increase in neuroendocrine-gonadal function in the Djungarian hamster and that the effects of light on reproductive function depend upon the phase-relationship between the light and the circadian system.