Glucosensing neurons in the hypothalamic arcuate nucleus (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, respectively, 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 glucose-excited neurons were found in the lateral ARC. ARC glucose-excited neurons in brain slices dosedependently 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 singlechannel currents in glucose-excited neurons in a manner similar to low glucose. Leptin did not alter the activity of ARC glucose-excited neurons. Although insulin 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 ␣-melanocyte-stimulating hormone stimulated ARC glucose-excited 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 neurons 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 catabolic processes (4). ARC NPY and POMC neurons receive input from central and peripheral metabolic signals involved in the regulation of food intake and energy balance (e.g., monoamines, insulin, and leptin) (1,2,5). Furthermore, 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 characterizing 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 gluc...