A suprachiasmatic nucleus generated rhythm in basal glucose concentrations.

The daily rhythm in feeding activity in mammals, as driven by the biological clock, largely determines the daily fluctuations in basal concentrations of glucose and insulin. To investigate a possible direct impact of the suprachiasmatic nucleus (SCN) on these parameters, we subjected intact rats and SCN-lesioned rats to a fasting regimen of 36 h, or to a scheduled feeding regimen of six identical meals equally distributed over the light:dark-cycle. Plasma profiles of glucose and insulin in rats during the final 24 h of the 36 h of fasting, and in rats subjected to the scheduled feeding regimen were compared to profiles in rats fed ad libitum. In rats fed ad libitum, in fasted rats and in rats subjected to a scheduled feeding regimen basal glucose concentrations showed a pronounced 24-h rhythm that was not found in rats that had been SCN-lesioned. Basal insulin levels showed a 24-h rhythm in 50% of the rats fed ad libitum and in 50% of the rats subjected to a scheduled feeding regimen; neither rhythms were present in SCN-lesioned rats. However, none of the fasted rats showed a 24-h rhythm in basal insulin concentrations. These data provide clear evidence that the SCN directly controls basal glucose concentrations independent of its influence on feeding activity. At the same time, we found no consistent evidence for a strong impact of the SCN on basal insulin concentrations.