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Abstract
The daily changes in plasma glucose, FFA, insulin and glucagon concentrations in rats under 12 hr-12 hr light-dark conditions, and the role of the suprachiasmatic nucleus (SCN) of the hypothalamus in these changes were examined. In sham-operated rats, the four parameters showed significant daily rhythms. However, after bilateral lesions of the SCN, daily rhythms could not be detected in these parameters under the present experimental conditions. Furthermore, after the SCN lesions the plasma glucose concentration remained at the minimum level of that in sham-operated rats, while the plasma insulin and glucagon concentrations reduced to approximately the mean level and about half the minimum level of sham-operated rats, respectively, and the FFA concentration lowered to somewhat below the minimum level. Gradual increase in the plasma insulin concentration at the end of the light period was observed in intact rats even after starvation for 24 hr. These findings suggest that the SCN is essential for generation of the daily changes in the plasma glucose, FFA, insulin and glucagon concentrations and also that it plays critical roles in regulation of the secretion of pancreatic hormones. The gradual increase in the plasma insulin level observed at the end of the light period is discussed in connection with initiation of spontaneous feeding behaviour.