Ventromedial hypothalamic nitric oxide production is necessary for hypoglycemia detection and counterregulation.
OBJECTIVE
The response of ventromedial hypothalamic (VMH) glucose-inhibited neurons to decreased glucose is impaired under conditions where the counterregulatory response (CRR) to hypoglycemia is impaired (e.g., recurrent hypoglycemia). This suggests a role for glucose-inhibited neurons in the CRR. We recently showed that decreased glucose increases nitric oxide (NO) production in cultured VMH glucose-inhibited neurons. These in vitro data led us to hypothesize that NO release from VMH glucose-inhibited neurons is critical for the CRR.


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 ventromedial nucleus glucose-inhibited neurons was also assessed.


RESULTS
Hypoglycemia increased hypothalamic constitutive NO synthase (NOS) activity and neuronal NOS (nNOS) but not endothelial NOS (eNOS) phosphorylation in rats. Intracerebroventricular and VMH injection of the nonselective NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) slowed the recovery to euglycemia after hypoglycemia. VMH l-NMMA 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 than wild-type or eNOS knockout mice. Finally, VMH glucose-inhibited neurons were virtually absent in nNOS knockout mice.


CONCLUSIONS
We conclude that VMH NO production is necessary for glucose sensing in glucose-inhibited 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.
Fioramonti
Xavier
X
Department of Pharmacology and Physiology, New Jersey Medical School, Newark, New Jersey, USA.
Marsollier
Nicolas
N
Song
Zhentao
Z
Fakira
Kurt A
KA
Patel
Reema M
RM
Brown
Stacey
S
Duparc
Thibaut
T
Pica-Mendez
Arnaldo
A
Sanders
Nicole M
NM
Knauf
Claude
C
Valet
Philippe
P
McCrimmon
Rory J
RJ
Beuve
Annie
A
Magnan
Christophe
C
Routh
Vanessa H
VH
D000818:Animals; D002395:Catecholamines / Q000097:blood; D004837:Epinephrine / Q000378:metabolism; D005934:Glucagon / Q000097:blood; D015309:Glucose Clamp Technique; D006706:Homeostasis; D006946:Hyperinsulinism / Q000201:enzymology / Q000503:physiopathology; D007003:Hypoglycemia / Q000139:chemically induced / Q000175:diagnosis*; D007031:Hypothalamus / Q000201:enzymology*; D007328:Insulin / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009569:Nitric Oxide / Q000096:biosynthesis*; D019001:Nitric Oxide Synthase / Q000378:metabolism; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism; D052250:Nitric Oxide Synthase Type III / Q000172:deficiency / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014697:Ventromedial Hypothalamic Nucleus / Q000502:physiology*
D002395:Catecholamines; D007328:Insulin; D009569:Nitric Oxide; D005934:Glucagon; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; D052250:Nitric Oxide Synthase Type III; D004837:Epinephrine