Testosterone and photoperiod interact to regulate locomotor activity in male hamsters

Testicular size, plasma testosterone levels, copulatory behavior, and daily locomotor activity are reduced in male hamsters after 10 weeks of exposure to short days. The role of testosterone in the short day-induced decline in locomotor activity was investigated, determining whether or not photoperiod could alter the effect of testosterone on activity. Castrated adult hamsters were allowed to acclimate to running wheels (wired to digital counters) and then were kept on either long (L:D 14:10) or short (L:D 6:18) days for 60 days. On Day 60, half of the animals on each light cycle were implanted with 12-mm-long testosterone-filled Silastic capsules; half received empty capsules. Digital counting of wheel-running activity continued for another 140 days. Blood samples taken on Day 200 confirmed L:D 14:10 and L:D 6:18 testosterone-treated hamsters had equivalent plasma testosterone levels. After an initial decline in activity, L:D 14:10 animals exhibited a progressive rise in mean running activity (from ~2000 to ~5000 wheel revolutions per day) through 100 days after the initiation of testosterone treatment. In contrast, activity levels in testosterone-treated L:D 6:18 animals remained uniform (~2000 wheel revolutions per day) during this time, indicating exposure to short days rendered the hamsters less sensitive to the stimulatory effect of testosterone on activity. Of further interest was a marked increase in activity after 160–200 short days in animals treated with either testosterone-filled or empty capsules. It appears the total amount of daily locomotor activity in the hamster is modulated by circulating testosterone levels in a manner which is dependent upon the environmental photoperiod.