title: Increased GABAergic output in the ventromedial hypothalamus contributes to impaired hypoglycemic counterregulation in diabetic rats.
abstract: OBJECTIVE
  Impaired glucose counterregulation during hypoglycemia is well documented in patients with type 1 diabetes; however, the molecular mechanisms underlying this defect remain uncertain. We reported that the inhibitory neurotransmitter γ-aminobutyric acid (GABA), in a crucial glucose-sensing region within the brain, the ventromedial hypothalamus (VMH), plays an important role in modulating the magnitude of the glucagon and epinephrine responses to hypoglycemia and investigated whether VMH GABAergic tone is altered in diabetes and therefore might contribute to defective counterregulatory responses.
  
  
  METHODS
  We used immunoblots to measure GAD(65) protein (a rate-limiting enzyme in GABA synthesis) and microdialysis to measure extracellular GABA levels in the VMH of two diabetic rat models, the diabetic BB rat and the streptozotocin (STZ)-induced diabetic rat, and compared them with nondiabetic controls.
  
  
  RESULTS
  Both diabetic rat models exhibited an ~50% increase in GAD(65) protein as well as a twofold increase in VMH GABA levels compared with controls under baseline conditions. Moreover, during hypoglycemia, VMH GABA levels did not change in the diabetic animals, whereas they significantly declined in nondiabetic animals. As expected, glucagon responses were absent and epinephrine responses were attenuated in diabetic rats compared with their nondiabetic control counterparts. The defective counterregulatory response in STZ-diabetic animals was restored to normal with either local blockade of GABA(A) receptors or knockdown of GAD(65) in the VMH.
  
  
  CONCLUSIONS
  These data suggest that increased VMH GABAergic inhibition is an important contributor to the absent glucagon response to hypoglycemia and the development of counterregulatory failure in type 1 diabetes.
authors.affiliation: Section of Endocrinology, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, USA. owen.chan@yale.edu
authors.forename: Owen
authors.initials: O
authors.lastname: Chan
authors.forename: Sachin
authors.initials: S
authors.lastname: Paranjape
authors.forename: Daniel
authors.initials: D
authors.lastname: Czyzyk
authors.forename: Adam
authors.initials: A
authors.lastname: Horblitt
authors.forename: Wanling
authors.initials: W
authors.lastname: Zhu
authors.forename: Yuyan
authors.initials: Y
authors.lastname: Ding
authors.forename: Xiaoning
authors.initials: X
authors.lastname: Fan
authors.forename: Margretta
authors.initials: M
authors.lastname: Seashore
authors.forename: Robert
authors.initials: R
authors.lastname: Sherwin
mesh_terms: D000818:Animals; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism*; D015309:Glucose Clamp Technique; D005968:Glutamate Decarboxylase / Q000235:genetics / Q000378:metabolism; D007003:Hypoglycemia / Q000378:metabolism*; D015151:Immunoblotting; D008297:Male; D017551:Microdialysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D014697:Ventromedial Hypothalamic Nucleus / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D005680:gamma-Aminobutyric Acid; D005968:Glutamate Decarboxylase; C401141:glutamate decarboxylase 2