Decrease in plasma glucose concentration after microinjection of insulin into VMN

The influence of insulin on hypothalamic regulation of blood sugar homeostatis was studied in anesthetized rats. Insulin was injected directly into the ventromedial hypothalamic nucleus (VMN), the lateral hypothalamic area (LHA), the parietal cortex, or the third cerebral ventricle, and changes in hepatic venous plasma glucose concentrations were studied. After injection of 100 microU insulin into the parietal cortex or the third ventricle, hepatic venous plasma glucose concentration did not differ from that of the control rats, which received saline injection into the same CNS regions. Saline injection into the LHA raised the hepatic venous plasma glucose concentration in control rats, where injection of 100 microU insulin into the LHA resulted in a modest but significant decrease of glycemia in the 2-, 5-, and 10-min postinjection samples. As little as 10 microU insulin injected into the VMN eliminated the hyperglycemic response seen in control rats after saline injection into this site. The divergence between insulin-treated rats and their saline-treated controls was further amplified, and an actual fall of plasma glucose was observed in rats given injections of 50 or 100 microU insulin into the VMN. Increasing quantities of insulin (from 10 to 100 microU) injected into the VMN resulted in graded decreases of hepatic venous plasma glucose concentrations, suggestive of a dose-response curve. These observations support the hypothesis that the VMN and the LHA are parts of an insulin-sensitive CNS glucoregulator system that exerts influences on the systemic blood glucose levels by causing rapid alterations in hepatic glucose metabolism.