hormone effects on entrained and free-running circadian activity rhythms in the developing diurnal rodent Octodon degus. The slowly maturing, long-lived rodent Octodon degus (degu) provides a unique opportunity to examine the development of the circadian system during adolescence. These studies characterize entrained and free-running activity rhythms in gonadally intact and prepubertally gonadectomized male and female degus across the first year of life to clarify the impact of sex and gonadal hormones on the circadian system during adolescence. Gonadally intact degus exhibited a delay in the phase angle of activity onset (⌿on) during puberty, which reversed as animals became reproductively competent. Gonadectomy before puberty prevented this phase delay. However, the effect of gonadal hormones during puberty on ⌿on does not result from changes in the period of the underlying circadian pacemaker. A sex difference in ⌿on and free-running period () emerged several months after puberty; these developmental changes are not likely to be related, since the sex difference in ⌿on emerged before the sex difference in . Changes in the levels of circulating hormones cannot explain the emergence of these sex differences, since there is a rather lengthy delay between the age at which degus reach sexual maturity and the age at which ⌿on and become sexually dimorphic. However, postnatal exposure to gonadal hormones is required for sexual differentiation of ⌿ on and , since these sex differences were absent in prepubertally gonadectomized degus. These data suggest that gonadal hormones modulate the circadian system during adolescent development and provide a new model for postpubertal sexual differentiation of a central nervous system structure. adolescent development; puberty; phase angle; tau; sexual differentiation ALTHOUGH DEVELOPMENTAL CHANGES within the circadian system have been well described, the existing research has focused primarily on changes during prenatal and early postnatal development or changes that occur with advanced age (23, 33). Adolescent development has received comparably little attention, despite the well-known phase delays in circadian rhythms exhibited by human adolescents (2, 3, 5, 10). There is little doubt that social factors play an important part in these developmental changes (e.g., increased academic responsibilities lead to later bedtimes); however, the phase delay in circadian rhythms is positively correlated with sexual maturity (5), which would suggest that gonadal hormones modulate the circadian system during adolescent development (6,29,32).It is possible that the phase delay in the circadian rhythms in human adolescents is the result of gonadal hormones acting to lengthen the endogenous period of the central circadian pacemaker (2). This hypothesis has not received much empirical support; although free-running period () appears to be longer in adolescent humans than in adults (2, 7), these age groups have not been directly compared in the same experiment. The rodent models commonly used...
The development of sexually dimorphic adult circadian period occurs after gonadal puberty is complete and requires the presence of gonadal steroids. The delay in development until after gonadal puberty is likely due to the delayed appearance of steroid receptors in the SCN. Phase is not sexually dimorphic and changes in the absence of steroid hormones.
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