Glucose is the primary energy source for cellular metabolism. Thus, the maintenance of serum glucose levels is critical for survival. Glucoprivation triggers defensive physiological and behavioral mechanisms that are aimed at mobilizing stored carbohydrate during periods of fasting and physiological emergency. A critical site for the defense against glucopenia is the caudal medulla. This region contains at least two potential detection/ effector control sites; the nucleus of the solitary tract (NST) and the ventrolateral medulla (VLM). Both regions are important to physiological and behavioral responses to hypoglycemia. These defensive counter-regulatory responses (CRR) include increases in serum glucagon and corticosteroids, increased food intake, an increase in adrenergic tone, and a dramatic acceleration in gastric motility. Recent controversial data suggest that detection of a low glucose state by brainstem astrocytes could be critical to the initiation of CRR. For example, transgenic mice whose GLUT2 transporter (critical component of most glucodetection mechanisms) is knocked out do not demonstrate CRR. However, CRR defects are rescued by the selective re-expression of GLUT2 in astrocytes, but not neurons. Our recent calcium (Ca++) imaging studies demonstrated that astrocytes in NST increase cytoplasmic Ca++ in response to cyto- glucopenia. Our in vivo, neurophysiological studies showed that glucoprivic challenges alter the sensitivity of medullary vago-vagal reflex neurons, resulting in an increase in gastric motility. This modulatory effect on NST neurons appears to be dependent on normal, functioning astrocytes. Studies in intact animals verified that both dorsal medullary and systemic glucoprivation significantly increases gastric motility. Significantly, astrocyte inactivation blocked this gastric component of CRR. These results explain century-old observations connecting hypoglycemia with increased gastric motility and accelerated digestion in CRR. We will now address questions relevant to broader aspects of astrocyte involvement in glucopenia defense. We hypothesize that intact hindbrain astrocyte signaling is essential to counter-regulatory control over not only gastric motility, but other aspects of CRR such as the initiation of glucoprivic feeding and the critical and rapid hormonal changes that provide a physiological defense against hypoglycemia. The pathways and mechanisms connecting glucodetection and triggering defensive counter-regulatory responses are not well understood. The role of the astrocyte in the CRR trigger mechanism is not understood at all. However, a discovery that astrocytes serve as the critical initiators of CRR will significantly advance understanding gluco- regulatory mechanisms and provide the basis for work on astrocyte involvement in pathological dysfunction of CRR, especially hypoglycemia-associated autonomic failure (HAAF). The mechanism and relevance of astrocyte glucodetection and control over CRR circuits will be examined by live cell in vitro Ca++ imaging and electrophysiology as well as physiological and behavioral studies conducted in awake, unstressed rats.
The proposed studies are focused on understanding the role of astrocytes to detect hypoglycemia, and, to initiate mechanisms providing a behavioral and physiological defense against dangerously low extracellular glucose concentrations. We hypothesize that astrocytes function as critical sensory elements that communicate directly with neurons involved in triggering glucoprivic feeding and the release of an array of hormones acting to counteract the effects of hypoglycemia. The results of this work will be directly applicable to the development of therapies useful in combating the long term effects of treatment-induced hypoglycemia in individuals with diabetes.
| Rogers, Richard C; Ritter, Sue; Hermann, Gerlinda E (2016) Hindbrain cytoglucopenia-induced increases in systemic blood glucose levels by 2-deoxyglucose depend on intact astrocytes and adenosine release. Am J Physiol Regul Integr Comp Physiol 310:R1102-8 |
