CHANGES IN LOCOMOTOR-ACTIVITY ASSOCIATED WITH THE PHOTOPERIODIC RESPONSE OF THE TESTES IN MALE GOLDEN-HAMSTERS

Male golden hamsters were exposed to either 14 h of light/24 h (LD [light:dark] 14:10) or LD 6:18 for 210 days, to assess the effects of the light-dark cycle and changes in the reproductive system upon locomotor activity. Wheel-running activity was recorded continuously, and testicular size and serum testosterone concentrations were measured periodically throughout the study. The short photoperiod induced a decrease in testicular width, serum testosterone levels and the number of wheel revolutions per 24-h period, when compared to the levels of these variables exhibited by LD 14:10 hamsters. Prolonged exposure to LD 6:18 resulted in a spontaneous increase in these 3 parameters to levels indistinguishable from those observed in LD 14:10 animals. Neither testicular width, serum testosterone concentration nor quantity of activity significantly changed in LD 14:10 hamsters throughout the study. LD 6:18 exposure also induced an expansion of the daily activity time; this increased duration of the daily active phase persisted, even after the spontaneous increase in testicular width, serum testosterone concentration and number of wheel revolutions/day. The duration of the daily active phase was unchanged in LD 14:10 animals through the course of the experiment. The onset of activity in all hamsters exposed to LD 14:10 occurred between 0-1 h after lights-off and did not appreciably vary for individual animals during the 210 days of LD 14:10. The time required for stable reentrainment following a shift from LD 14:10 to LD 6:18 varied from 30-120 days among individual hamsters and the phase relationship of activity onset to lights-off after 200 days of LD 6:18 ranged between -1 and -6 h. The time course of reentrainment to short days did not appear to be affected by changing testosterone levels. Coincident with LD 6:18-induced testicular regression was an increased lability of the time of activity onset each day. A return to stability in the day-to-day time of activity onset was coincident with spontaneous testicular recrudescence in LD 6:18 hamsters. Hamsters maintained on LD 14:10 exhibited a stable time of daily activity onset throughout the investigation. The number of wheel revolutions/day and the lability of daily activity onset are evidently correlated with light-induced changes in the hamster reproductive system. The duration of the daily active phase and the phase relationship between activity onset and lights-off are relatively independent of changes in the reproductive system and are a function of the entraining light-dark cycle.