Role of Hypothalamic Adenosine 5′-Monophosphate-Activated Protein Kinase in the Impaired Counterregulatory Response Induced by Repetitive Neuroglucopenia

Abstract

Antecedent hypoglycemia blunts counterregulatory responses that normally restore glycemia, a phenomenon known as hypoglycemia-associated autonomic failure (HAAF). The mechanisms leading to impaired counterregulatory responses are largely unknown. Hypothalamic AMP-activated protein kinase (AMPK) acts as a glucose sensor. To determine whether failure to activate AMPK could be involved in the etiology of HAAF, we developed a model of HAAF using repetitive intracerebroventricular (icv) injection of 2-deoxy-D-glucose (2DG) resulting in transient neuroglucopenia in normal rats. Ten minutes after a single icv injection of 2DG, both alpha1- and alpha2-AMPK activities were increased 30-50% in arcuate and ventromedial/dorsomedial hypothalamus but not in other hypothalamic regions, hindbrain, or cortex. Increased AMPK activity persisted in arcuate hypothalamus at 60 min after 2DG injection when serum glucagon and corticosterone levels were increased 2.5- to 3.4-fold. When 2DG was injected icv daily for 4 d, hypothalamic alpha1- and alpha2-AMPK responses were markedly blunted in arcuate hypothalamus, and alpha1-AMPK was also blunted in mediobasal hypothalamus 10 min after 2DG on d 4. Both AMPK isoforms were activated normally in arcuate hypothalamus at 60 min. Counterregulatory hormone responses were impaired by recurrent neuroglucopenia and were partially restored by icv injection of 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside, an AMPK activator, before 2DG. Glycogen content increased 2-fold in hypothalamus after recurrent neuroglucopenia, suggesting that glycogen supercompensation could be involved in down-regulating the AMPK glucose-sensing pathway in HAAF. Thus, activation of hypothalamic AMPK may be important for the full counterregulatory hormone response to neuroglucopenia. Furthermore, impaired or delayed AMPK activation in specific hypothalamic regions may play a critical role in the etiology of HAAF.

Glucose Regulates Hypothalamic Long-chain Fatty Acid Metabolism via AMP-activated Kinase (AMPK) in Neurons and Astrocytes

“…The absence of glucose effect on cortical P-AMPK is in agreement with previous reports showing that high glucose, hypoglycemia, or neuroglucopenia does not modulate AMPK activity in cortical areas (12,39,40). Interestingly, the lack of response to glucose was not related to a lower capacity to metabolize glucose as the -fold increase in glucose utilization and oxidation rates in response to glucose were similar in hypothalamic versus cortical astrocytes and in MBH versus cortical slices.…”

“…Interestingly, the lack of response to glucose was not related to a lower capacity to metabolize glucose as the -fold increase in glucose utilization and oxidation rates in response to glucose were similar in hypothalamic versus cortical astrocytes and in MBH versus cortical slices. This suggests a dissociation between glucose metabolism and AMPK activity that may involve differences in ATP generation and/or in the expression of the ␣1-AMPK isoform in cortical versus hypothalamic regions that is less sensitive to glucose compared with the ␣2 isoform (12,39,41). Although it is difficult to speculate on mRNA levels, there is also a possibility that the increased expression of fatty acid synthase and MCD in cortical astrocytes translates into lower levels of malonyl-CoA generated in response to glucose in cortical versus hypothalamic astrocytes.…”

“…Four 300-m-thick sections containing the hypothalamus were cut from each rat using a Vibratome while being continuously immersed in ice-cold cutting solution. The MBH, which includes the arcuate nucleus plus the ventro-and dorsomedian hypothalamus, and the mediolateral area of the motor and somatosensory cortex were dissected on each section as previously described (12,32).…”

“…Intracellular metabolism of glucose inhibits the activity of a key energy sensing enzyme, AMP-activated kinase (AMPK). Several studies have established the importance of AMPK in the mediobasal hypothalamus (MBH) in glucose sensing (9,10), action (11), and the counterregulatory response to hypoglycemia (12,13). Glucose inhibition of hypothalamic AMPK leads to the activation of acetyl-CoA carboxylase (ACC), thereby leading to the generation of malonyl-CoA from glucose-derived acetyl-CoA (14,15).…”

Brain Glycogen Content and Metabolism in Subjects with Type 1 Diabetes and Hypoglycemia Unawareness

“…In addition, studies of the hypoglycemia response of glycogen in patients with type 1 diabetes, both during and after induced hypoglycemia similar to our prior studies in healthy volunteers (Ö z et al, 2009), may provide additional insight and more clinically relevant physiological conditions. Also note that the current results were obtained in a localized brain region in the occipital lobe and we cannot exclude the possibility that glycogen supercompensation in the hypothalamus, similar to what was seen with recurrent neuroglucopenia in rats (Alquier et al, 2007), may contribute to the development of hypoglycemia-associated autonomic failure. Unfortunately, the current in vivo methodology lacks the sensitivity to acquire glycogen data from a very small and deep brain region such as the hypothalamus.…”