Astrocytes, glucose detection, and counter-regulation

项目总结 葡萄糖是细胞新陈代谢的主要能量来源。因此，维持血糖水平 水平是生存的关键。葡萄糖代谢触发防御生理和行为机制， 目的是在禁食和生理紧急情况下调动储存的碳水化合物。 防御血糖减少症的一个关键部位是尾侧延髓。此区域至少包含两个 潜在检测/效应器控制部位；孤束核和延髓腹外侧部 (VLm)。这两个区域对低血糖的生理和行为反应都很重要。这些 防御性反调节反应(CRR)包括血清胰高血糖素和皮质类固醇的增加， 食物摄入量增加，肾上腺素能张力增加，胃动力显著加快。 最近有争议的数据表明，脑干星形胶质细胞检测到低血糖状态可能是 对启动CRR至关重要。例如，GLUT2转运蛋白(GLUT2转运蛋白的关键成分)的转基因小鼠 大多数糖检测机制)被敲除，不显示CRR。然而，CRR缺陷是 通过选择性地在星形胶质细胞中重新表达GLUT2，而不是在神经元中。我们最近的钙(Ca++) 影像研究表明，NST中的星形胶质细胞对细胞内钙离子的反应是胞浆内钙离子增加 葡萄糖缺乏症。我们的活体神经生理学研究表明，挑战葡聚糖改变了对 延髓迷走神经-迷走神经反射神经元，导致胃动力增加。这种对NST的调制作用 神经元似乎依赖于正常的、功能正常的星形胶质细胞。对完整动物的研究证实，两者都 延髓背侧和全身性糖化可显著增强胃动力。值得注意的是，星形细胞 失活阻断了CRR的这一胃成分。这些结果解释了百年前的观察结果 CRR患者出现低血糖，胃动力增强，消化加速。我们现在将回答问题 与星形胶质细胞参与血糖减少防御的更广泛的方面有关。 我们假设完整的后脑星形胶质细胞信号对于NOT的反调控控制是必不可少的 只有胃动力，但CRR的其他方面，如开始葡萄糖喂养和关键和 快速的荷尔蒙变化，提供对低血糖的生理防御。这些小路和 糖检测和触发防御性反调节反应之间的联系机制不是很好 明白了。星形胶质细胞在CRR触发机制中的作用完全不清楚。然而，a 星形胶质细胞是CRR的关键启动者，这一发现将极大地促进对糖皮质激素受体的理解 并为星形胶质细胞参与病理功能障碍的研究奠定了基础 CRR，尤其是低血糖相关性自主神经衰竭(HAAF)。其机制和关联性 星形胶质细胞的血糖检测和CRR回路的控制将通过体外活细胞钙成像和 电生理学以及在清醒的非应激大鼠中进行的生理和行为研究。

Project summary 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.