Increased GABAergic tone in the ventromedial hypothalamus contributes to suppression of counterregulatory responses after antecedent hypoglycemia.
OBJECTIVE
We have previously demonstrated that modulation of gamma-aminobutyric acid (GABA) inhibitory tone in the ventromedial hypothalamus (VMH), an important glucose-sensing region in the brain, modulates the magnitude of glucagon and sympathoadrenal responses to hypoglycemia. In the current study, we examined whether increased VMH GABAergic tone may contribute to suppression of counterregulatory responses after recurrent hypoglycemia.


METHODS
To test this hypothesis, we quantified expression of the GABA synthetic enzyme, glutamic acid decarboxylase (GAD), in the VMH of control and recurrently hypoglycemic rats. Subsequently, we used microdialysis and microinjection techniques to assess changes in VMH GABA levels and the effects of GABA(A) receptor blockade on counterregulatory responses to a standardized hypoglycemic stimulus.


RESULTS
Quantitative RT-PCR and immunoblots in recurrently hypoglycemic animals revealed that GAD(65) mRNA and protein were increased 33 and 580%, respectively. Basal VMH GABA concentrations were more than threefold higher in recurrently hypoglycemic animals. Furthermore, whereas VMH GABA levels decreased in both control and recurrently hypoglycemic animals with the onset of hypoglycemia, the fall was not significant in recurrently hypoglycemic rats. During hypoglycemia, recurrently hypoglycemic rats exhibited a 49-63% reduction in glucagon and epinephrine release. These changes were reversed by delivery of a GABA(A) receptor antagonist to the VMH.


CONCLUSIONS
Our data suggest that recurrent hypoglycemia increases GABAergic inhibitory tone in the VMH and that this, in turn, suppresses glucagon and sympathoadrenal responses to subsequent bouts of acute hypoglycemia. Thus, hypoglycemia-associated autonomic failure may be due in part to a relative excess of the inhibitory neurotransmitter, GABA, within the VMH.
Chan
Owen
O
Section of Endocrinology, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
Cheng
Haiying
H
Herzog
Raimund
R
Czyzyk
Daniel
D
Zhu
Wanling
W
Wang
Ajin
A
McCrimmon
Rory J
RJ
Seashore
Margretta R
MR
Sherwin
Robert S
RS
D000818:Animals; D017931:DNA Primers; D004195:Disease Models, Animal; D015971:Gene Expression Regulation, Enzymologic; D015309:Glucose Clamp Technique; D005968:Glutamate Decarboxylase / Q000235:genetics; D007003:Hypoglycemia / Q000503:physiopathology*; D008297:Male; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014697:Ventromedial Hypothalamic Nucleus / Q000503:physiopathology*; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
D017931:DNA Primers; D012333:RNA, Messenger; D005680:gamma-Aminobutyric Acid; D005968:Glutamate Decarboxylase