Blockade of GABAA Receptors in the Ventromedial Hypothalamus Further Stimulates Glucagon and Sympathoadrenal but Not the Hypothalamo-Pituitary-Adrenal Response to Hypoglycemia

Abstract

Hypoglycemia provokes a multifaceted counterregulatory response involving the sympathoadrenal system, stimulation of glucagon secretion, and the hypothalamo-pituitaryadrenal axis that is commonly impaired in diabetes. We examined whether modulation of inhibitory input from ␥-aminobutyric acid (GABA) in the ventromedial hypothalamus (VMH), a major glucose-sensing region within the brain, plays a role in affecting counterregulatory responses to hypoglycemia. Normal Sprague-Dawley rats had carotid artery and jugular vein catheters chronically implanted, as well as bilateral steel microinjection guide cannulas inserted down to the level of the VMH. Seven to 10 days following surgery, the rats were microinjected with artificial extracellular fluid, the GABA A receptor agonist muscimol (1 nmol/side), or the GABA A receptor antagonist bicuculline methiodide (12.5 pmol/side) before being subjected to a hyperinsulinemic-hypoglycemic (2.5 mmol/l) glucose clamp for 90 min. Following VMH administration of bicuculline methiodide, glucose infusion rates were significantly suppressed, whereas muscimol raised glucose infusion rates significantly compared with controls. Glucagon and epinephrine responses were elevated with the antagonist and suppressed with the agonist compared with controls. Corticosterone responses, however, were unaffected by either administration of the agonist or antagonist into the VMH. These data demonstrate that modulation of the GABAergic system in the VMH alters both glucagon and sympathoadrenal, but not corticosterone, responses to hypoglycemia. Our findings are consistent with the hypothesis that GABAergic inhibitory tone within the VMH can modulate glucose counterregulatory responses. Diabetes

Increased GABAergic Tone in the Ventromedial Hypothalamus Contributes to Suppression of Counterregulatory Reponses After Antecedent Hypoglycemia

“…It is noteworthy that the increased VMH GABA levels do not influence baseline counterregulatory hormone concentrations. This observation is consistent with previously published data from our laboratory in which the GABA A receptor agonist, muscimol, was administered before a hyperinsulinemic-euglycemic clamp (32). In that study, activation of GABA A receptors did not influence counterregulatory hormone release, suggesting that under conditions of euglycemia, counterregulatory responses may already be under tonic inhibition.…”

“…To examine this hypothesis further, we microinjected the GABA A receptor antagonist bicuculline methiodide into the VMH of recurrently hypoglycemic rats. In these studies, acute blockade of GABAergic inhibition in the VMH of rats exposed to antecedent hypoglycemia diminished glucose infusion rates required to maintain the standardized hypoglycemic stimulus to control levels and reversed the defects in the secretion of the primary counterregulatory hormones glucagon and epinephrine to an extent similar to that seen in our control animals described previously (32). Although this observation is consistent with our hypothesis that increases in VMH GABAergic tone after antecedent hypoglycemia may contribute to impaired glucose counterregulation, we cannot be certain that these two phenomena are mechanistically linked.…”

“…Recently, we reported that when GABAergic tone is specifi- cally modulated in the VMH, a key glucose-sensing region within the brain, using GABA A receptor agonists and antagonists, stimulation of glucose counterregulatory mechanisms during hypoglycemia were markedly affected (32). These findings raised the possibility that increased GABAergic tone in the VMH may be an important factor contributing to defective glucose counterregulation in HAAF.…”

“…Surgery. On day 0, the animals underwent vascular surgery in which catheters were placed into the left carotid artery and right jugular vein, as described previously (32). In addition, bilateral stainless steel guide cannulas were stereotaxically positioned in close proximity to the VMH according to the coordinates of Paxinos and Watson (33) (from bregma: Ϫ2.6 mm anteriorposterior, Ϯ3.8 mm medial-lateral, and Ϫ8.3 mm dorsal-ventral at an angle of 20°for microinjection and Ϫ8.9 mm dorsal-ventral at an angle of 16°for microdialysis).…”

“…Furthermore, it has been reported that GABA levels in VMH microdialysate samples show a biphasic rise in GABA levels during glucoprivation that may be attributed to a corresponding rise in local norepinephrine concentrations (28). On the other hand, we recently reported that modulation of GABAergic tone in the VMH of normal rats specifically affected the magnitude of glucagon and epinephrine responses to an acute episode of hypoglycemia (32). Increasing GABAergic tone suppressed, while decreasing GABAergic tone amplified, these counterregulatory responses.…”