Circadian rhythm of liver glycogen metabolism in rats: effects of hypothalamic lesions

The effects of electrolytic lesions of the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMH) of the hypothalamus on the circadian rhythm of liver glycogen metabolism were studied in rats fed during the dark period (2000–0800 h). Bilateral lesions of the SCN did not affect appreciably the circadian rhythmicity in three parameters of liver glycogen metabolism, i.e., glycogen content and glycogen synthetase I and phosphorylase a activities. In contrast, bilateral lesions of the VMH resulted in almost complete loss of circadian rhythmicity in phosphorylase a activity by lowering the high activity normally observed in late evening. VMH lesions also reduced the amplitudes of the rhythms of synthetase I activity and glycogen content, but did not abolish their rhythmicities. These results suggest that the integrity of the VMH, rather than the SCN, is important for regulation or expression of the food-triggered circadian rhythm of liver phosphorylase a activity. The implication of the lateral hypothalamic nucleus in regulation of the circadian rhythm of liver synthetase I is also discussed.