Effect of One-Second Light Pulses on Testicular Function and Locomotor Activity in the Golden Hamster 1

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Abstract
The effects of 1-s light pulses on testicular size and the circadian rhythm of locomotor activity in male golden hamsters were examined utilizing 2 different paradigms. Adult male hamsters that had been maintained on 14L:10D [L:D, light:dark] were transferred to either a 6L:18D or 6L:18D light cycle with a 1-s light pulse interrupting the dark phase of 8 h after lights-off. Exposure to 6L:18D for 11 wk induced complete testicular regression which was accompanied by low serum testosterone levels. The paired testis weights and serum testosterone levels of animals receiving the 1-s night interruption were maintained at values comparable to those found in hamsters maintained on photostimulatory long days. The daily onset of activity occurred 4-6 h after lights-off in the animals exposed to 6L:18D. Animals receiving the 1-s night interruption exhibited 2 distinct patterns of entrainment: one in which the onset of activity occurred near the time of the 1-s light pulse, and another in which activity was initiated daily near the offset of the 6-h light phase. In the 2nd study, adult male hamsters that had been maintained on 14L:10D were transferred to either constant darkness or to a 1-s skeleton photoperiod consisting of two 1-s light pulses separated by dark intervals of 14 h and 10 h (i.e., 1-s L:14D:1-s L:10D). As expected, exposure to constant darkness for 11 wk induced complete testicular regression. In 7 of 12 animals exposed to 2-s of light per day, testicular size was maintained in the range of values normally observed in hamsters exposed to stimulatory long-day photoperiods. Following transfer to constant darkness, the circadian rhythm of locomotor activity was observed to free-run with a period > 24 h in all animals. In contrast, entrainment of the activity rhythm was observed in all animals exposed to the 1-s skeleton photoperiod with the pattern of activity resembling that exhibited by animals maintained under a complete 14L:10D photoperiod. As little as 2-s of light per day can influence the neuroendocrine events underlying the photoperiodic maintenance of gonadal function, and can be effective stimuli for the entrainment of the circadian rhythm of locomotor activity.