Effect of one-second light pulses on testicular function and locomotor activity in the golden hamster.

Abstract

The effects of 1-sec light pulses on testicular size and the circadian rhythm of locomotor activity in male golden hamsters were examined utilizing two different paradigms. First, adult male hamsters that had been maintained on 14L:1OD were transferred to either a 6L:18D or 6L:18D light cycle with a 1-sec light pulse interrupting the dark phase of 8 h after lights-off. Exposure to 6L:18D for 11 weeks induced complete testicular regression which was accompanied by low serum testosterone levels. In contrast, the paired testis weights and serum testosterone levels of animals receiving the 1-sec 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-sec night interruption exhibited two distinct patterns of entrainment: one in which the onset of activity occurred near the time of the 1-sec light pulse, and another in which activity was initiated daily near the offset of the 6-h light phase. In the second study, adult male hamsters that had been maintained on 14L:1OD were transferred to either constant darkness or to a 1-sec skeleton photoperiod consisting of two 1-sec light pulses separated by dark intervals of 14 h and 10 h (i.e., 1 sec L:14D:1 sec L:IOD). As expected, exposure to constant darkness for 11 weeks induced complete testicular regression. However, in 7 of 12 animals exposed to 2 sec of light per day, testicular size was maintained in the range of values normally observed in hamsters exposed to stimulator>’ long-day photoperiods. Following transfer to constant darkness, the circadian rhythm of locomotor activity was observed to free-run with a period greater than 24 h in all animals. In contrast, entrainment of the activity rhythm was observed in all animals exposed to the 1-sec skeleton photoperiod with the pattern of activity resembling that exhibited by animals maintained under a complete 14L:1OD photoperiod. These results demonstrate that as little as 2 sec 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.