Activation of AMP-Activated Protein Kinase Within the Ventromedial Hypothalamus Amplifies Counterregulatory Hormone Responses in Rats With Defective Counterregulation

Abstract

Defective counterregulatory responses (CRRs) to hypoglycemia are associated with a marked increase in the risk of severe hypoglycemia. The mechanisms leading to the development of defective CRRs remain largely unknown, although they are associated with antecedent hypoglycemia. Activation of AMP-activated protein kinase (AMPK) in the ventromedial hypothalamus (VMH) amplifies the counterregulatory increase in glucose production during acute hypoglycemia. To examine whether activation of AMPK in the VMH restores defective CRR, controlled hypoglycemia (ϳ2.8 mmol/l) was induced in a group of 24 Sprague-Dawley rats, all of which had undergone a 3-day model of recurrent hypoglycemia before the clamp study. Before the acute study, rats were microinjected to the VMH with either 5-aminoimidazole-4-carboxamide (AICAR; n ‫؍‬ 12), to activate AMPK, or saline (n ‫؍‬ 12). In a subset of rats, an infusion of H 3 -glucose was additionally started to calculate glucose turnover. Stimulation of AMPK within the VMH was found to amplify hormonal CRR and increase endogenous glucose production. In addition, analysis of tissue from both whole hypothalamus and VMH showed that recurrent hypoglycemia induces an increase in the gene expression of AMPK ␣ 1 and ␣ 2 . These findings suggest that the development of novel drugs designed to selectively activate AMPK in the VMH offer a future therapeutic potential for individuals with type 1 diabetes who have defective CRRs to hypoglycemia. Diabetes 55:1755-1760, 2006 S ingle or recurrent episodes of acute hypoglycemia in nondiabetic or type 1 diabetic (1-3) individuals are known to impair hormonal counterregulatory responses (CRRs) to a subsequent episode of hypoglycemia. Defective hormonal counterregulation to hypoglycemia is closely associated with both altered glucose thresholds for activation of the CRR and reduced symptomatic awareness of hypoglycemia, a combination of clinical syndromes that collectively have been termed hypoglycemia-associated autonomic failure (4). Defective hormonal CRR is in itself associated with a markedly increased risk of severe hypoglycemia (5).The mechanism through which recurrent hypoglycemia per se induces defective hormonal CRR remains largely unknown. Potential candidate mechanisms include alterations in key steps in the glucose-sensing pathway in the brain (6,7), increased glucose and/or alternate fuel uptake by the brain (8 -11), increases in the brain glycogen pool (12), and an effect of hypothalamopituitary axis activation (13,14). In addition, we have recently demonstrated that pharmacological activation of the serine/threonine kinase AMP-activated protein kinase (AMPK) in the ventromedial hypothalamus (VMH), a key central glucose-sensing region (15), amplifies the glucose CRR to acute hypoglycemia. In that study AMPK activation in the VMH during acute hypoglycemia resulted in a marked increase in endogenous glucose production (R a ), with a corresponding reduction in the requirement for exogenous glucose, in the absence of a change in the hormonal CRR (1...

Key Role for AMP-Activated Protein Kinase in the Ventromedial Hypothalamus in Regulating Counterregulatory Hormone Responses to Acute Hypoglycemia

“…We have previously shown that pharmacological activation of AMPK in the VMH, using AICAR, led to an amplification of the glucose CRR to acute hypoglycemia in normal Sprague-Dawley rats and in rats with defective counterregulation induced by prior recurrent hypoglycemia (14,15). Although these studies provided support for a role for AMPK in glucose sensing, they were limited by the potential for AICAR to activate other AMP-regulated kinases and/or to act through the adenosine receptor (36).…”

“…Hypothalamic AMPK has been implicated in the regulation of food intake (21)(22)(23)(24)(25) and is activated in response to fasting or central glucoprivation (22)(23)(24). Hypothalamic AMPK has also been implicated in the regulation of CRRs to acute hypoglycemia (14,15,26), and 5-aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside (AICAR), a pharmacological activator of AMPK, mimics the effect of low glucose to stimulate neuronal activity in VMH glucose-inhibited neurons (27). Although these studies provide good evidence of a role for AMPK in glucose sensing, they are limited because of the use of pharmacological interventions to manipulate hypothalamic AMPK or having examined for changes in AMPK activity in response to a systemic or intracerebroventricular manipulation.…”

“…Of the several brain glucose-sensing areas identified, the ventromedial hypothalamus (VMH) in particular has been shown to play a key role in the detection of hypoglycemia and generation of a counterregulatory response (CRR) (2)(3)(4)(12)(13)(14)(15)(16)(17)(18). The VMH contains neurons that are activated in response to an increase in circulating glucose (glucose-excited neurons) and others that are activated in response to a fall in circulating glucose (glucose-inhibited neurons) (19).…”

Hypothalamic ventromedial COUP-TFII protects against hypoglycemia-associated autonomic failure

“…5D). The HAAF observed in the SF1-COUP-TFII mice is associated with an overall defective AMPK activation (35).…”

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

“…As recent antecedent hypoglycemia reduces autonomic (epinephrine) and symptomatic (which normally prompts behavioral defenses such as eating) responses to subsequent hypoglycemia (6,7), a vicious cycle of recurrent hypoglycemia is created where hypoglycemia leads to further impairment of counterregulatory responses, which in turn begets more hypoglycemia and so forth (4,8). Although several mechanisms have been proposed for the development of HAAF, including increased brain glucose transport (9 -11), changes in brain fuel metabolism (12)(13)(14), altered hypothalamic activity (15,16), and changes in brain glucose sensing and signaling (17,18), the exact mechanism underlying this phenomenon is unclear.…”