165-OR: Norepinephrine Acts on Glutamatergic Neurons in the Ventromedial Hypothalamus to Stimulate the Release of Counterregulatory Hormones

Release of glutamate in the ventromedial hypothalamus (VMH) is important for stimulating the counterregulatory hormone responses to hypoglycemia. It is currently not known whether VMH glutamatergic neurotransmission is influenced by adrenergic inputs. To address this question, we examined whether ß2-adrenergic receptors are expressed on VMH glutamatergic neurons using immunohistochemistry. Coronal brain sections collected from Sprague Dawley (SD) rats showed fairly extensive co-localization of Vglut2 (a marker of glutamatergic neurons) and ß2-adrenergic receptors. We then used local microinjection in combination with microdialysis to examine whether norepinephrine administration into the VMH stimulated the release of neurotransmitter glutamate and the counterregulatory hormones in the absence of hypoglycemia. About 7-8 rats were randomly allocated to one of two treatment groups: Controls, which received an infusion of artificial extracellular fluid, or the NE group, which received an infusion of 400 nM of norepinephrine. Our results indicate that NE administration into the VMH caused a marked increase in glutamate that was accompanied by an ∼3-fold increase in both plasma glucagon and epinephrine release (P<0.vs. Control) . Our data suggests that NE enhances the release of neurotransmitter glutamate in the VMH that in turn, may stimulate the release of counterregulatory hormones.