Brain insulin action regulates hypothalamic glucose sensing and the counterregulatory response to hypoglycemia.
OBJECTIVE
An impaired ability to sense and appropriately respond to insulin-induced hypoglycemia is a common and serious complication faced by insulin-treated diabetic patients. This study tests the hypothesis that insulin acts directly in the brain to regulate critical glucose-sensing neurons in the hypothalamus to mediate the counterregulatory response to hypoglycemia.


METHODS
To delineate insulin actions in the brain, neuron-specific insulin receptor knockout (NIRKO) mice and littermate controls were subjected to graded hypoglycemic (100, 70, 50, and 30 mg/dl) hyperinsulinemic (20 mU/kg/min) clamps and nonhypoglycemic stressors (e.g., restraint, heat). Subsequently, counterregulatory responses, hypothalamic neuronal activation (with transcriptional marker c-fos), and regional brain glucose uptake (via (14)C-2deoxyglucose autoradiography) were measured. Additionally, electrophysiological activity of individual glucose-inhibited neurons and hypothalamic glucose sensing protein expression (GLUTs, glucokinase) were measured.


RESULTS
NIRKO mice revealed a glycemia-dependent impairment in the sympathoadrenal response to hypoglycemia and demonstrated markedly reduced (3-fold) hypothalamic c-fos activation in response to hypoglycemia but not other stressors. Glucose-inhibited neurons in the ventromedial hypothalamus of NIRKO mice displayed significantly blunted glucose responsiveness (membrane potential and input resistance responses were blunted 66 and 80%, respectively). Further, hypothalamic expression of the insulin-responsive GLUT 4, but not glucokinase, was reduced by 30% in NIRKO mice while regional brain glucose uptake remained unaltered.


CONCLUSIONS
Chronically, insulin acts in the brain to regulate the counterregulatory response to hypoglycemia by directly altering glucose sensing in hypothalamic neurons and shifting the glycemic levels necessary to elicit a normal sympathoadrenal response.
Diggs-Andrews
Kelly A
KA
Division of Endocrinology, Metabolism and Lipid Research, Department of Internal Medicine, Washington University School of Medicine, Saint Louis, Missouri, USA.
Zhang
Xuezhao
X
Song
Zhentao
Z
Daphna-Iken
Dorit
D
Routh
Vanessa H
VH
Fisher
Simon J
SJ
D000818:Animals; D001786:Blood Glucose / Q000378:metabolism*; D001921:Brain / Q000502:physiology*; D003433:Crosses, Genetic; D005786:Gene Expression Regulation; D005941:Glucokinase / Q000235:genetics; D006720:Homozygote; D006358:Hot Temperature; D007003:Hypoglycemia / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D007031:Hypothalamus / Q000502:physiology*; D007328:Insulin / Q000502:physiology*; D019426:Integrases / Q000235:genetics; D007381:Intermediate Filament Proteins / Q000235:genetics; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009419:Nerve Tissue Proteins / Q000235:genetics; D064231:Nestin; D009474:Neurons / Q000502:physiology; D011401:Promoter Regions, Genetic; D016760:Proto-Oncogene Proteins c-fos / Q000235:genetics / Q000502:physiology; D051381:Rats; D011972:Receptor, Insulin / Q000172:deficiency* / Q000235:genetics; D012149:Restraint, Physical; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013315:Stress, Psychological / Q000097:blood
D001786:Blood Glucose; D007328:Insulin; D007381:Intermediate Filament Proteins; D009419:Nerve Tissue Proteins; C577485:Nes protein, mouse; C577486:Nes protein, rat; D064231:Nestin; D016760:Proto-Oncogene Proteins c-fos; D005941:Glucokinase; D011972:Receptor, Insulin; C045073:Cre recombinase; D019426:Integrases