Testicular hormones modulate circadian rhythms of the diurnal rodent, Octodon degus

Sex differences have been identified in a variety of circadian rhythms, inc luding free-running rhythms, light-induced phase shifts, sleep patterns, ho rmonal fluctuations, and rates of reentrainment. In the precocial, diurnal rodent Octodon degus, sex differences have been found in length of free-run ning rhythm (tau), phase response curves, rates of reentrainment, and in th e use of social cues to facilitate reentrainment. Although gonadal hormones primarily organize circadian rhythms during early development, adult gonad al hormones have activational properties on various aspects of circadian rh ythms in a number of species examined. Gonadectomy of adult female O. degus did not influence tau, phase angle of entrainment, or activity patterns in previous experiments. The present experiment examined the role of gonadal hormones in adult male degus' circadian wheel-running rhythms. We predicted that male gonadal hormones would have an activational effect on some aspec ts of circadian rhythms, particularly those in which we see sex differences . Phase angles of entrainment, tau, length of the active period (alpha), ma ximum and mean activity levels, and activity amplitude were examined for in tact and castrated males housed in LD 12:12. Responses to light pulses whil e housed in constant darkness (DD) were also compared. Castration had no si gnificant effect on tau or light-induced phase shifts. However, castration significantly increased phase angle of entrainment and decreased activity l evels. The data indicate that adult gonadal steroids are not responsible fo r the sex differences in endogenous circadian mechanisms of O. degus (tau, PRC), although they influence activity level and phase angle of entrainment . This is most likely due to masking properties of testosterone, similar to the activity-increasing effects of estrogen during estrus in O. degus fema les, (C) 2000 Academic Press.