Core temperature in the female rat: effect of pinealectomy or altered lighting

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Abstract
Radiotelemetry of core temperature in unrestrained, mature female rats revealed the existence of a 24 h rhythm that was bimodal. The principal peak occurred during the night under control conditions of 14 h light and 10 h darkness, and a less pronounced, secondary peak occurred 3-4 h after the onset of the light phase. Shifts in the phase of the photoperiod or alteration of the proportion of light per day revealed that the temperature rhythm was entrained by light, but that the 2 component peaks were governed by different aspects of the lighting regimen. Exposure of rats to continuous darkness, continuous light, or to a 20 h photoperiod revealed that the primary rhythm was endogenous, entrained by circadian photoperiods only, whereas the secondary rhythm was exogenous, requiring a circadian light/dark rhythm. A relationship between mean core temperature and the proportion of light per day was severely altered after pinealectomy, but no effect on rhythmicity was noted. The pineal gland regulates a light/dark-dependent set point about which oscillations in core temperature may be generated.

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