CIRCADIAN RHYTHMICITY IN THE WHEEL-RUNNING ACTIVITY OF RODENTS: FACTORS AFFECTING DEVELOPMENT OF THE PACEMAKER

The effects of environmental factors on the development of the circadian pacemaker that underlies wheel-running activity/rest rhythmicity were investigated in mice (Mus musculus) and hamsters (Mesocricetus auratus). In the first of these studies, the influence of the period of environmental rhythmicity was examined by raising mice from the time they were conceived to adulthood on non-24-hour light/dark cycles (periods of 20 and 28 hours). The mothers of these mice were entrained to the cycles so that the mice also developed under non-24-hour maternal rhythmicity. Comparison of groups thus raided, showed only a temporary difference in freerunning period, similar to a difference seen in mice exposed to these cycles as adults only. The groups were not different in the ratio of activity time to rest time or in their abilities to subsequently entrain to light/dark cycles with periods of 20 or 28 hours. It is concluded that environmental periodicity has no influence on the development of circadian pacemaker properties.; In the same study, the temporary effects of entrainment on free-running period were found to last longer in older animals. Age differences in period and in activity/rest ratios were also observed, suggesting a general age difference in circadian rhythmicity.; The second study investigated the entrainment of the circadian rhythm in wheel-running activity of young hamsters, specifically entrainment by the mother and the possibility of entrainment by a light/dark cycle via extraretinal photoreceptors. No evidence for extraretinal photoreception was observed. Evidence for maternal entrainment was seen as synchronization among the individuals of a litter and as phaseshifts of pups following those of mothers.; It was found in the third study that blinding hamster pups within 24-hours of birth, and thereby depriving an area of the brain important to the generation of circadian rhythms (the suprachiasmatic nuclei) of retinal afferents, does not affect the period of the pacemaker when measured after three weeks of age. Level and pattern of activity was also unaffected, although a small difference in the ratio of activity time to rest time was observed.; A final series of experiments led to the discovery and characterization of sex differences in the circadian pacemaker underlying wheel-running activity of hamsters. Intact as well as castrated males and females differ in their ability to entrain to a light/dark cycle with a period of 24.75 hours. This difference is accounted for by small sex differences in freerunning period and in the magnitude of light-induced phase-shifts of the rhythm. Intact females were also found to begin their activity earlier than males when entrained to L:D, 14:10. This difference is consistent with the described pacemaker differences, but may also be due to a difference in the temporal relationship between overt activity and the pacemaker.; These sex differences suggest that the mechanisms of sexual differentiation affect the circadian pacemaker. Effects of castration on entrainment were found to be different in males and females. In females, these effects are influenced by the age of castration.