The Regulation of Glucose-Excited Neurons in the Hypothalamic Arcuate Nucleus by Glucose and Feeding-Relevant Peptides R. Wang, 1 X. Liu, 1 S.T. Hentges, 2 A.A. Dunn-Meynell, 3,4 B.E. Levin, 1,3,4 W. Wang, 1 and V.H. Routh 1,3 Glucosensing neurons in the hypothalamic arcuate nu- cleus (ARC) were studied using electrophysiological and immunocytochemical techniques in neonatal male Sprague-Dawley rats. We identified glucose-excited and -inhibited neurons, which increase and decrease, re- spectively, their action potential frequency (APF) as extracellular glucose levels increase throughout the physiological range. Glucose-inhibited neurons were found predominantly in the medial ARC, whereas glu- cose-excited neurons were found in the lateral ARC. ARC glucose-excited neurons in brain slices dose- dependently increased their APF and decreased their ATP-sensitive K  channel (K ATP channel) currents as extracellular glucose levels increased from 0.1 to 10 mmol/l. However, glucose sensitivity was greatest as extracellular glucose decreased to <2.5 mmol/l. The glucokinase inhibitor alloxan increases K ATP single- channel currents in glucose-excited neurons in a man- ner similar to low glucose. Leptin did not alter the activity of ARC glucose-excited neurons. Although insu- lin did not affect ARC glucose-excited neurons in the presence of 2.5 mmol/l (steady-state) glucose, they were stimulated by insulin in the presence of 0.1 mmol/l glucose. Neuropeptide Y (NPY) inhibited and -melano- cyte–stimulating hormone stimulated ARC glucose-ex- cited neurons. ARC glucose-excited neurons did not show pro-opiomelanocortin immunoreactivity. These data suggest that ARC glucose-excited neurons may serve an integrative role in the regulation of energy balance. Diabetes 53:1959 –1965, 2004 T he hypothalamic arcuate nucleus (ARC) is in a pivotal position for involvement in the central control of glucose homeostasis. The ARC houses neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons, which have opposing effects on the regulation of food intake and energy balance. NPY neu- rons project to the hypothalamic paraventricular nucleus (PVN). This pathway favors anabolic processes, including increased food intake and decreased energy expenditure (1–3). In contrast, the ARC POMC neurons mediate cata- bolic processes (4). ARC NPY and POMC neurons receive input from central and peripheral metabolic signals in- volved in the regulation of food intake and energy balance (e.g., monoamines, insulin, and leptin) (1,2,5). Further- more, they project to the sympathetic cell bodies in the spinal cord (6,7). The ARC also possesses glucosensing neurons (8,9). Thus, the ARC is a critical center for the integration and regulation of systems involved in the central control of energy homeostasis. To date, there are few electrophysiological studies char- acterizing ARC glucosensing neurons (8,9). Moreover, these studies of ARC glucosensing neurons have been performed using nonphysiological levels of extracellular glucose. That is, ARC glucosensing neurons have been characterized by decreasing extracellular glucose from 10 or 20 to 0 mmol/l. An extracellular glucose level of 0 mmol/l is incompatible with life, and brain glucose levels may never exceed 5 mmol/l. Studies in anesthetized rats using a glucose oxidase electrode have shown that glucose levels in the ventromedial hypothalamus (VMH; which contains the ARC and the ventromedial hypothalamic nucleus [VMN]) vary from 0.2 to 4.5 mmol/l as plasma glucose levels increase from 2 to 18 mmol/l, with steady- state levels being 2.5 mmol/l (10). A recent report using microdialysis (zero-net flux method) (11) in awake ani- mals showed that VMH glucose levels decreased from 1.5 mmol/l in fed rats to 0.73 mmol/l after an overnight fast (12). Therefore, it is necessary to reevaluate and redefine the electrophysiological characteristics of purported ARC glucosensing neurons under more physiological condi- tions to determine their relevance to physiological glucose sensing. We have recently shown that VMN glucosensing neurons are sensitive to physiological levels of extracellu- lar glucose (13). In this study we found two types of VMN neurons that respond directly to physiological changes in extracellular glucose. Glucose-excited neurons increase From the 1 Department of Pharmacology and Physiology, New Jersey Medical School (UMDNJ), Newark, New Jersey; the 2 Oregon Health and Science University, Vollum Institute, Portland, Oregon; the 3 Department of Neuro- sciences, New Jersey Medical School (UMDNJ), Newark, New Jersey; and the 4 Neurology Service, Veterans Administration Medical Center, East Orange, New Jersey. Address correspondence and reprint requests to Vanessa H. Routh, PhD, Department of Pharmacology and Physiology, New Jersey Medical School (UMDNJ), P.O. Box 1709, Newark, NJ 07101-1709. E-mail: routhvh@ umdnj.edu. Received for publication 6 February 2004 and accepted in revised form 23 April 2004. W.W. is currently affiliated with Robert S. Dow Neurobiology Laboratories, Legacy Research, Portland, Oregon. ACSF, artificial cerebrospinal fluid; APF, action potential frequency; ARC, arcuate nucleus; K ATP channel, ATP-sensitive K + channel; -MSH, -melano- cyte–stimulating hormone; NPY, neuropeptide Y; POMC, proopiomelanocor- tin; PVN, paraventricular nucleus; VMH, ventromedial hypothalamus; VMN, ventromedial hypothalamic nucleus. © 2004 by the American Diabetes Association. DIABETES, VOL. 53, AUGUST 2004 1959