Intrahypothalamic injection of insulin decreases firing rate of sympathetic nerves.

Injection of picomolar quantities of insulin into the ventromedial hypothalamus of rats significantly reduced the firing rate of sympathetic nerves that supply interscapular brown adipose tissue. The minimal firing rate was reached in 2 min, and the effect was gone within 4 min. The effect of insulin was dose-related and did not occur when comparable volumes of physiological saline were injected into the ventromedial hypothalamus. Destruction of neurons in the ventromedial hypothalamus by injection of kainic acid abolished the inhibitory effects of insulin. These data suggest that insulin may play a role in modulating the sympathetic firing rate to thermogenically important tissues.