Elsevier

Physiology & Behavior

Volume 58, Issue 3, September 1995, Pages 573-585
Physiology & Behavior

Article
Estrus- and steroid-induced changes in circadian rhythms in a diurnal rodent, Octodon degus

https://doi.org/10.1016/0031-9384(95)00096-2Get rights and content

Abstract

Diurnal Octodon degus exhibited marked alterations in activity and temperature in conjunction with the 3 wk estrous cycle when housed in LD12:12 light cycle. On the day of estrus, mean daily activity increases 109%, mean core temperature rises .4°C, activity onset is advanced 2 h, and amplitudes of both rhythms decline compared with the 3 days prior to estrus. On the day following estrus, activity onset was delayed 4.9 h, and mean activity and core temperature fell below that of the preestrus period. Ovariectomy significantly reduced mean temperature (.98°C) but did not significantly alter mean activity, and eliminated cyclic effects of estrus. Estrogen replacement led to a nonsignificant elevation in mean activity and core temperature with no change in the phase angle of entrainment. Progesterone replacement significantly reduced mean core temperature and mean activity, while only the phase angle difference between temperature minimum and activity onset was significantly altered. Intact degus maintained in constant darkness displayed only transient fluctuations in activity onset and temperature minimum during and after estrus. Estrogen or progesterone treatment of ovariectomized, free-running degus altered mean temperature and activity levels, but did not influence tau. Changes in phase angle of entrainment during estrus are not the result of hormone effects on the circadian clock but likely reflect increased or decreased levels of activity.

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      Bilu and Kronfeld-Schor (2013), however, did find that melatonin administration increased anxiety-like behaviors in captive degus during the daytime. Compared to males, adult female degus have a free-running circadian rhythm that is ∼30 min longer (Labyak and Lee, 1995). This sex difference is likely due to organizational effects of sex steroid hormones during the pre-pubertal stage, as degus gonadectomized before puberty show no sex difference in free-running circadian rhythm (Hummer et al., 2007).

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