We have revealed that circadian body temperature (Tb) rhythm is significantly influenced by fasting/fasting-related hormones. The effect of circadian mechanism and fasting/fasting-related hormones on thermoregulation was examined. Fasting decreases Tb during the light phase in rodents. For the regulation, the suprachiasmatic nucleus (SCN) and clock genes, such as Cry and Clock, are necessary. In addition, ghrelin and several hypothalamic nuclei, that is, the medial preoptic area, paraventricular nucleus (PVN), and arcuate nucleus (ARC), play a key role in the Tb rhythm. During the light phase, fasting and ghrelin affect the hypothalamic areas. The activity of the SCN increases and that of the ARC decreases. The SCN sends inhibitory signals to the PVN, which may result in a lower heat production in the interscapular brown adipose tissue (iBAT) and Tb. By contrast, during the dark phase, the activity of the SCN decreases and that of the ARC increases. The inhibitory signal from the SCN is less, and the PVN is activated. Heat production of the iBAT increases and Tb is maintained. There are functional and anatomical connections between the circadian and thermoregulation systems. The circadian system modulates thermoregulatory response to hypothermia and/or cold depending on time and feeding condition.
Part of the book: Homeostasis