Ventromedial Hypothalamic Nitric Oxide Production Is Necessary for Hypoglycemia Detection and Counterregulation

Abstract Objective- The response of ventromedial hypothalamic (VMH) glucose-inhibited (GI) neurons to decreased glucose is impaired under conditions where the counter-regulatory response (CRR) to hypoglycemia is impaired (e.g., recurrent hypoglycemia). This suggests a role for GI neurons in the CRR. We recently showed that decreased glucose increases nitric oxide (NO) production in cultured VMH GI neurons. These in vitro data lead us to hypothesize that NO release from VMH GI neurons is critical for the CRR. Research Design And Methods- The CRR was evaluated in rats and mice in response to acute insulin-induced hypoglycemia and hypoglycemic clamps after modulation of brain NO signaling. The glucose sensitivity of VMN GI neurons was also assessed. Results- Hypoglycemia increased hypothalamic constitutive NO synthase (NOS) activity and nNOS but not eNOS phosphorylation in rats. Intracerebroventricular (ICV) and VMH injection of the non-selective NOS inhibitor N G -Nitro-L-arginine (LNMMA) slowed the recovery to euglycemia following hypoglycemia. VMH LNMMA injection also increased the glucose infusion rate (GIR) and decreased epinephrine secretion during hyperinsulinemic/hypoglycemic clamp in rats. The GIR required to maintain the hypoglycemic plateau was higher in nNOS knockout (KO) than wildtype (WT) or eNOS KO mice. Finally, VMH GI neurons were virtually absent in nNOS KO mice. Conclusion- We conclude that VMH NO production is necessary for glucose sensing in GI neurons and full generation of the CRR to hypoglycemia. These data suggest that potentiating NO signaling may improve the defective CRR resulting from recurrent hypoglycemia in patients using intensive insulin therapy.