Time-dependent effect of insulin in suprachiasmatic nucleus on blood glucose

Studies were made on the effects of insulin (30 or 300 microU) injected into the suprachiasmatic nucleus (SCN) of the hypothalamus on the plasma glucose and insulin concentrations in pentobarbital-anesthetized rats, which had been kept under 12:12 light-dark conditions. Insulin injection caused a decrease in the plasma glucose concentration in the light period but an increase in the dark period, these changes being significant within 2 min after injection. Insulin injection also caused a decrease in plasma insulin concentration in the light period but a tendency for its increase in the dark period. Intraperitoneal injection of insulin at the same dose did not affect the plasma glucose or insulin concentration in either the light or dark period. Insulin injection into the SCN decreased glycogen phosphorylase a activity but increased glycogen synthase I activity in the liver in the light period, but the insulin injection did not affect the activities of these enzymes in the dark period. Lesions including bilateral SCN completely eliminated the responses of the plasma glucose and insulin concentrations to insulin injection both in the light and dark periods. These findings suggest that insulin injection into the SCN causes time-dependent changes in plasma glucose through changes in activities of glycogen metabolizing enzymes in the liver in the light period but through other mechanisms in the dark period.