Insulin co-injection suppresses the thermogenic response to glutamate microinjection into the VMH in rats.

Selective stimulation of ventromedial hypothalamic nucleus (VMH) neurons by microinjection of the excitatory amino acid glutamate sharply increased interscapular brown adipose tissue (IBAT) and core temperatures in urethane-anaesthetized rats. This effect was blocked by co-injection of insulin (0.1-1 microgram) though not an inactive insulin analog, TNB3 insulin. Injection of insulin (1 microgram) into the contralateral VMH or systemic administration of insulin (1 microgram) had no effect on the thermogenic response to intra-VMH glutamate. These results complement those showing that intra-VMH insulin suppresses the basal firing rate of sympathetic nerves to IBAT and diminishes cold-induced nonshivering thermogenesis in BAT and add support to the view that insulin functions as an inhibitory signal on VMH neurons controlling thermogenesis in BAT.