Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) were studied using visually guided slicepatch recording techniques in brain slices from 14-to 21-day-old male Sprague-Dawley rats. Whole-cell current-clamp recordings were made as extracellular glucose levels were increased (from 2.5 to 5 or 10 mmol/l) or decreased (from 2.5 to 0.1 mmol/l). Using these physiological conditions to define glucosensing neurons, two subtypes of VMN glucosensing neurons were directly responsive to alterations in extracellular glucose levels. Another three subtypes were not directly glucose-sensing themselves, but rather were presynaptically modulated by changes in extracellular glucose. Of the VMN neurons, 14% were directly inhibited by decreases in extracellular glucose (glucose-excited [GE]), and 3% were directly excited by decreases in extracellular glucose (glucose-inhibited [GI]). An additional 14% were presynaptically excited by decreased glucose (PED neurons). The other two subtypes of glucosensing neurons were either presynaptically inhibited (PIR; 11%) or excited (PER; 8%) when extracellular glucose was raised to >2.5 mmol/l. GE neurons sensed decreased glucose via an ATP-sensitive K ؉ (K ATP ) channel. The inhibitory effect of increased glucose on PIR neurons appears to be mediated by a presynaptic ␥-aminobutyric acid-ergic glucosensing neuron that probably originates outside the VMN. Finally, all types of glucosensing neurons were both fewer in number and showed abnormal responses to glucose in a rodent model of diet-induced obesity and type 2 diabetes.

Insulin and leptin receptors are present in hypothalamic regions that control energy homeostasis, and these hormones reduce food intake and body weight in lean, but not obese, Zucker rats. Here we demonstrate that insulin, like leptin, hyperpolarizes lean rat hypothalamic glucose-responsive (GR) neurons by opening KATP channels. These findings suggest hypothalamic K ATP channel function is crucial to physiological regulation of food intake and body weight.

Specialized neurons utilize glucose as a signaling molecule to alter their firing rate. Glucose-excited (GE) neurons increase and glucose-inhibited (GI) neurons reduce activity as ambient glucose levels rise. Glucoseinduced changes in the ATP-to-ADP ratio in GE neurons modulate the activity of the ATP-sensitive K ؉ channel, which determines the rate of cell firing. The GI glucosensing mechanism is unknown. We postulated that glucokinase (GK), a high-Michaelis constant (K m ) hexokinase expressed in brain areas containing populations of GE and GI neurons, is the controlling step in glucosensing. Double-label in situ hybridization demonstrated neuron-specific GK mRNA expression in locus ceruleus norepinephrine and in hypothalamic neuropeptide Y, pro-opiomelanocortin, and ␥-aminobutyric acid neurons, but it did not demonstrate this expression in orexin neurons. GK mRNA was also found in the area postrema/nucleus tractus solitarius region by RT-PCR. Intracarotid glucose infusions stimulated c-fos expression in the same areas that expressed GK. At 2.5 mmol/l glucose, fura-2 Ca 2؉ imaging of dissociated ventromedial hypothalamic nucleus neurons demonstrated GE neurons whose intracellular Ca 2؉ oscillations were inhibited and GI neurons whose Ca 2؉ oscillations were stimulated by four selective GK inhibitors. Finally, GK expression was increased in rats with impaired central glucosensing (posthypoglycemia and diet-induced obesity) but was unaffected by a 48-h fast. These data suggest a critical role for GK as a regulator of glucosensing in both GE and GI neurons in the brain. S pecialized neurons utilize glucose as a signaling molecule rather than as an energy substrate. Glucose-excited (GE) neurons increase, whereas glucose-inhibited (GI) neurons decrease, their firing rate as ambient glucose levels rise (1-3). It is unclear how GI neurons sense glucose, but GE neurons function much like the pancreatic ␤-cell, which utilizes the ATPsensitive K ϩ (K ATP ) channel to sense glucose and regulate insulin secretion (2-7). Although many neurons contain K ATP channels, few exhibit glucosensing properties (8,9). This makes control of glycolysis a major candidate as a regulator of ATP production and K ATP channel activity (6). The pancreatic form of glucokinase (GK; hexokinase IV, ATP:D-glucose 6-phosphotransferase) is selectively expressed in brain areas where glucosensing neurons reside (5,10 -12). GK has the physiological properties that would make it an ideal regulator of glucosensing in neurons (6,13). Previous studies examined a role for GK in hypothalamic glucosensing neurons in brain slice preparations, where presynaptic inputs could not be excluded, and under conditions outside the physiological range of brain glucose (4,5). The current studies provide new data on the neurotransmitter and peptide phenotype of neurons expressing GK, the regulatory role of GK in isolated hypothalamic GE and GI neuron activity, and the in vivo conditions associated with altered brain GK expression. RESEARCH DESIGN AND METHODSAnimals. All ...

To evaluate potential mechanisms for neuronal glucosensing, fura-2 Ca 2؉ imaging and single-cell RT-PCR were carried out in dissociated ventromedial hypothalamic nucleus (VMN) neurons. Glucose-excited (GE) neurons increased and glucose-inhibited (GI) neurons decreased intracellular Ca 2؉ ([Ca 2؉ ] i ) oscillations as glucose increased from 0.5 to 2.5 mmol/l. The Kir6.2 subunit mRNA of the ATP-sensitive K ؉ channel was expressed in 42% of GE and GI neurons, but only 15% of nonglucosensing (NG) neurons. Glucokinase (GK), the putative glucosensing gatekeeper, was expressed in 64% of GE, 43% of GI, but only 8% of NG neurons and the GK inhibitor alloxan altered [Ca 2؉ ] i oscillations in ϳ75% of GK-expressing GE and GI neurons. Insulin receptor and GLUT4 mRNAs were coexpressed in 75% of GE, 60% of GI, and 40% of NG neurons, although there were no statistically significant intergroup differences. Hexokinase-I, GLUT3, and lactate dehydrogenase-A and -B were ubiquitous, whereas GLUT2, monocarboxylate transporters-1 and -2, and leptin receptor and GAD mRNAs were expressed less frequently and without apparent relationship to glucosensing capacity. Thus, although GK may mediate glucosensing in up to 60% of VMN neurons, other regulatory mechanisms are likely to control glucosensing in the remaining ones. Diabetes 53:549 -559, 2004

Glucosensing neurons are specialized cells that use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Glucokinase (GK) appears to be the primary regulator of most neuronal glucosensing, but other regulators almost certainly exist. Glucose-excited neurons increase their activity when glucose levels rise, and most use GK and an ATP-sensitive K ؉ channel as the ultimate effector of glucose-induced signaling. Glucose-inhibited (GI) neurons increase their activity at low glucose levels. Although many use GK, it is unclear what the final pathway of GI neuronal glucosensing is. Glucosensing neurons are located in brain sites and respond to and integrate a variety of hormonal, metabolic, transmitter, and peptide signals involved in the regulation of energy homeostasis and other biological functions. Although it is still uncertain whether daily fluctuations in blood glucose play a specific regulatory role in these physiological functions, it is clear that large decreases in glucose availability stimulate food intake and counterregulatory responses that restore glucose levels to sustain cerebral function. Finally, glucosensing is altered in obesity and after recurrent bouts of hypoglycemia, and this altered sensing may contribute to the adverse outcomes of these conditions. Thus, although much is known, much remains to be learned about the physiological function of brain glucosensing neurons.