Daily rhythms in glucose metabolism: suprachiasmatic nucleus output to peripheral tissue.

The body has developed several control mechanisms to maintain plasma glucose concentrations within strict boundaries. Within those physiological boundaries, a clear daily rhythm in plasma glucose concentrations is present; this rhythm depends on the biological clock, which is located in the hypothalamic suprachiasmatic nucleus (SCN), and is independent of the daily rhythm in food intake. Interestingly, there is also a daily rhythm in glucose uptake, which also depends on the SCN and follows the same pattern as the daily rhythm in plasma glucose concentrations; both rise before the onset of activity. Thus, the SCN prepares the individual for the upcoming activity period in two different ways: by increasing plasma glucose concentrations and by facilitating tissue glucose uptake. In addition to this anticipation of glucose metabolism to expected glucose demands, the SCN also influences, depending on the time of the day, the responses of pancreas and liver to abrupt glucose changes (such as a glucose rise after a meal or hypoglycaemia). This review presents the view that the SCN uses different routes to (i) maintain daily glucose balance and (ii) set the level of the endocrine response to abrupt blood glucose changes.