Role of Neuronal Glucosensing in the Regulation of Energy Barry E. Levin,1,2 Ling Kang,2 Nicole M. Sanders,3 and Ambrose A. Dunn-Meynell1,2

Glucosensing is a property of specialized neurons in the studies of damage to the pointed to the brain that regulate their membrane potential and firing brain as the primary regulator of energy homeostasis. rate as a function of ambient glucose levels. These neurons Lesions of the ventromedial hypothalamus (VMH) produce have several similarities to ␤- and ␣-cells in the pancreas, increased food intake (hyperphagia), (1), and which are also responsive to ambient glucose levels. Many defective autonomic function in organs involved in the use glucokinase as a rate-limiting step in the production of ATP and its effects on membrane potential and ion channel regulation of energy expenditure (2,3). On the other hand, function to glucose. Glucosensing neurons are orga- electrical stimulation of the VMH leads to generalized nized in an interconnected distributed network throughout sympathoadrenal activation (4) with increased activity in the brain that also receives afferent neural input from thermogenic tissues (5). Lesions of the lateral hypotha- glucosensors in the liver, carotid body, and small intes- lamic area (LHA) reduce food intake and increase sympa- tines. In addition to glucose, glucosensing neurons can use thetic activity and eventually establish a new lower other metabolic substrates, hormones, and peptides to defended body weight (3,5,6). Whereas such early studies regulate their firing rate. Consequently, the output of pointed to the hypothalamus as the central controller of these “metabolic sensing” neurons represents their in- tegrated response to all of these simultaneous inputs. energy homeostasis, later studies suggested that energy The efferents of these neurons regulate feeding, neu- homeostasis is controlled by a distributed network of roendocrine and autonomic function, and thereby energy specialized neurons that use glucose, as well as a variety of expenditure and storage. Thus, glucosensing neurons play metabolic substrates and hormones, to regulate their a critical role in the regulation of energy homeostasis. membrane potential and firing rate (7–14) (Fig. 1). Defects in the ability to sense glucose and regulatory These “glucosensing” neurons are localized in a variety hormones like and may underlie the predis- of brain sites that are involved in the regulation of energy position of some individuals to develop diet-induced obe- sity. Diabetes 55 (Suppl. 2):S122–S130, 2006 homeostasis. These central neurons are part of a larger network of glucosensors that are located in peripheral organs. Such peripheral glucosensors are located in the hepatic portal vein (15), carotid body (16), and the gut he term “energy homeostasis” is a modification (17). Their vagal and sympathetic neural afferents termi- of the second law of thermodynamics whereby nate predominantly in the nucleus tractus solitarius (NTS) the amount of energy taken in as food equals the in the medulla (Fig. 1). The neurons in the NTS represent Tamount expended as heat (thermogenesis). a critical nodal point where hardwired inputs from meta- When intake exceeds expenditure, the excess is stored bolic, hormone, and peptide signals from the periphery primarily as and , and these stores are used to converge and are integrated. Because many NTS neurons supply fuel when food is in short supply. This process is are also glucosensing neurons, this allows them to sum- regulated over different time frames and by a variety of mate the direct effects of glucose, other metabolic sub- physiological and metabolic systems; dysregulation of strates, and hormones such as leptin and insulin at the either intake or expenditure can lead to obesity. Early level of their membrane potential with those arriving via neural afferents from peripheral glucosensors (18). NTS From the 1Neurology Service, Department of Veterans Affairs New Jersey neurons project widely to other brainstem and forebrain Health Care System, East Orange, New Jersey; the 2Department of Neurology nuclei such as the rostral and caudal ventrolateral medulla and Neurosciences, New Jersey Medical School, University of Medicine and and raphe pallidus and obscurus (RPa/Ob); the hypotha- Dentistry, Newark, New Jersey; and the 3VA Puget Sound Health Care System, Metabolism and Endocrinology Division and Department of Psychiatry and lamic paraventricular nucleus (PVN), arcuate nucleus Behavioral Sciences, University of Washington, Seattle, Washington. (ARC), ventromedial nucleus (VMN), and dorsomedial Address correspondence and reprint requests to Barry E. Levin, Neurology nucleus; and LHA, the substantia nigra, and ventral teg- Service (127C), VA Medical Center, 385 Tremont Ave., East Orange, NJ 07018. E-mail: [email protected]. mental and central nucleus of the amygdala, most of which Received for publication 22 March 2006 and accepted in revised form 15 contain glucosensing neurons and are also involved in May 2006. autonomic function and energy homeostasis (Figs. 1 and This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educa- 2) (19–22). tional grant from Servier. Because of historical precedents, a majority of early 5TG, 5-thioglucose; ARC, arcuate nucleus; CRR, counterregulatory re- studies focused on hypothalamic neurons as regulators of sponse; GE, glucose excited; GI, glucose inhibited; GK, glucokinase; KATP channel, ATP-sensitive Kϩ channel; LHA, lateral hypothalamic area; NPY, energy homeostasis. This has led to the realization that Y; NTS, nucleus tractus solitarius; POMC, proopiomelanocortin; manipulations of the VMH most often affected function in PVN, paraventricular nucleus; VMH, ventromedial hypothalamus; VMN, ven- both the VMN and ARC (Fig. 2). In fact, it is the ARC that tromedial nucleus. DOI: 10.2337/db06-S016 may be the more important of these two nuclei, since it © 2006 by the American Diabetes Association. contains two sets of neurons whose primary function The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance appears to be the regulation of energy homeostasis. Medial with 18 U.S.C. Section 1734 solely to indicate this fact. ARC neurons that express (NPY) are

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FIG. 1. Location and afferent inputs to central glucosensing neurons. Vagal, glossopharyngeal, and sympathetic afferents from metabolic sensors (denoted by four-pointed stars) in the portal vein, carotid body, stomach, and small intestines converge on NTS metabolic sensing neurons (denoted by five-pointed stars) in the medulla and are integrated along with metabolic and hormonal signals from the periphery, which are transported across the blood-brain barrier or cross through the fenestrated barrier in the area postrema adjacent to the NTS. In addition to the NTS, noradrenergic, adrenergic, neuropeptide Y (NPY), glucagon-like peptide 1 (GLP-1), and ␣-melanocyte–stimulating hormone (␣-MSH) metabolic sensing neurons in the caudal ventrolateral medulla (CVLM) and rostral ventrolateral medulla (RVLM) also integrate these incoming signals and project to various hypothalamic nuclei (arcuate [ARC], ventromedial [VMN], dorsomedial [DMN], paraventricular [PVN]) and the LHA. Metabolic sensing serotonin neurons in the raphe pallidus and obscurus (RPa/Ob) project to sympathetic preganglionic neurons in the intermediolateral cell column of the spinal cord. Metabolites and hormones also interact with metabolic sensing neurons in the hypothalamus. Neurons in the ARC project to additional metabolic sensing neurons in the PVN and LHA, and the LHA also projects to the PVN, which, along with the LHA, gives rise to descending outputs to autonomic outs in the medulla and spinal cord. Adrenal medullary cells also are responsive to low glucose levels as one possible mechanism for their release of epinephrine to mobilize hepatic glucose stores during hypoglycemia (L.K., B.E.L., unpublished data). Dotted lines denote polysynaptic pathways. classified as anabolic because release of this peptide onto both leptin and insulin inhibit NPY and stimulate POMC target neurons in the PVN and LHA potently stimulates gene transcription. Insulin and leptin also have acute food intake and inhibits energy expenditure by decreasing effects on NPY and POMC neuronal activity (19). sympathetic activity in thermogenic organs. Laterally Two other groups of metabolic sensing neurons in the placed ARC neurons produce proopiomelanocortin LHA produce the anabolic peptides orexin (hypocretin) (POMC), which is a pro-hormone for ␣-melanocyte–stim- and melanocyte concentrating hormones. As opposed to ulating hormone. This catabolic peptide interacts with ARC NPY and POMC neurons, these LHA neurons are melanocortin 3 and 4 receptors (MC3/4-R) to inhibit intake involved in a much wider spectrum of metabolic, physio- and stimulate sympathetic activity and thermogenesis by logical, and behavioral processes (24,25) (Fig. 2). Orexin acting on some of the same PVN and LHA neurons that are neurons are inhibited (glucose-inhibited [GI] neurons) and NPY targets. Some ARC POMC neurons also project to the melanocyte concentrating hormone neurons are excited intermedio-lateral cell column of the spinal cord, which (glucose-excited [GE] neurons) by glucose (26), and both positions them to modulate the activity of sympathetic receive inputs from ARC NPY and POMC neurons (27). preganglionic neurons (23). ARC NPY neurons also pro- They also receive inputs from thalamic areas involved in duce and co-release agouti-related peptide, a unique pep- nociception, limbic, and striatal areas involved in motiva- tide that acts as a functional antagonist of MC3/4-R. Thus, tion and reward and hypothalamic and brainstem net- activation of ARC NPY neurons releases a strong anabolic works that coordinate foraging, gustatory, feeding, and peptide and an equally potent inhibitor of catabolic path- defensive behaviors. In turn, melanocyte concentrating ways. Importantly, ARC NPY and POMC neurons are hormones and orexin LHA neurons project to brain areas prototypic metabolic sensing neurons; NPY neurons are involved in nonhomeostatic reward systems such as the inhibited and POMC neurons are excited by glucose, while nucleus accumbens, ventral tegmental area, prefrontal

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FIG. 2. Efferents of glucosensing neurons involved in energy homeostasis. Metabolic sensing neurons (stars) are localized throughout the hypothalamus and brainstem as well as other areas, including the substantia nigra and ventral tegmental area (VTA). ARC NPY and POMC (␣-melanocyte–stimulating hormone [␣-MSH]) neurons are prototypic metabolic sensing neurons that project to the PVN and to orexin and melanin concentrating (MCH) metabolic sensing neurons in the LHA (heavy dotted lines). ARC POMC neurons, which also express cocaine- and amphetamine-related transcript, also project to the intermediolateral (IML) preganglionic sympathetic neurons. PVN efferents project to the median eminence and pituitary to modulate neuroendocrine outputs and to sympathetic and parasympathetic output (dorsal vagal complex [DVC]) neurons in the medulla for control of visceral functions involved in energy homeostasis. Unlike the relatively restricted output of ARC NPY and POMC neurons, those of the orexin and MCH LHA neurons project widely to brain areas involved in a large variety of behavioral and physiological functions, some of which are involved in the regulation of various aspects of energy homeostasis. The dotted box around the VMN and ARC denotes the area generally referred to as the VMH (ventromedial hypothalamus) in many stimulation and lesion studies of energy homeostasis. cortex, amygdale, and bed nucleus of the stria terminalis ing neurons: GE neurons increase, while GI neurons (which collectively are commonly referred to as the ex- decrease, their activity as ambient glucose levels rise. tended amygdala); neocortical and hippocampal areas They reverse this pattern as glucose levels fall. GE neu- involved in cognition and memory; striatal areas involved rons function similarly in many ways to pancreatic ␤-cells, in motor activity; hindbrain areas involved in conscious- whereas GI neurons have some similarities to ␣-cells ness and arousal (locus coeruleus, dorsal raphe, central (14,19,35–39). As with pancreatic ␤- and ␣-cells, we know gray); and thalamic areas involved in stress responsivity a great deal about the way in which GE neurons sense (ventro-posterior thalamus). They also project to auto- glucose, but much less about the way in which GI neurons nomic efferent areas in the central nucleus of the amyg- do this. As with most neurons, the majority of GE and GI dala, PVN, NTS, and dorsal vagal complex (28) (Fig. 2). In neurons use the high-capacity high-affinity glucose trans- addition to neurons in the ARC and LHA, other glucosens- porter 3 to import glucose (37). While hexokinase I ing neurons include VMN ␥-aminobutyric acid neurons, accounts for the majority of glucose phosphorylation in substantia nigra and ventral tegmental area dopamine neurons, many glucosensing neurons also express the neurons, and others in the NTS, rostral and caudal ventro- pancreatic form glucokinase (GK) (hexokinase IV), the lateral medulla, and dorsal vagal complex cholinergic rate-limiting step in ␤-cell glucosensing (36,38,40–42). The neurons (19,29–32). There are likely to be other collec- Km for GK activity in the brain, as in the pancreas, is tions of such neurons scattered in other brain sites that ϳ9–11 mmol/l (40,43). Because brain glucose levels are remain to be discovered. only ϳ20% of those seen in blood under most conditions, this means that GK functions at the lower end of its HOW DO NEURONS SENSE GLUCOSE? response curve in glucosensing neurons. Also, because Whereas most neurons require glucose to fuel their meta- hexokinase I is the primary means of phosphorylating bolic activity, glucosensing neurons also use glucose in a glucose in glucosensing neurons, we have postulated concentration-dependent manner as a signaling molecule that GK may be compartmentalized beneath the plasma to regulate their membrane potential and neural activity membrane together with mitochondria, close to the ion (8,14,33,34). There are two broad categories of glucosens- channels that would be affected by the ATP formed by

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GK-regulated oxidation of glucose (19). Approximately 65% of GE and 45% of GI neurons express GK mRNA (37). In those neurons, pharmacological inhibition of GK activ- ity decreases activity in GE and increases activity in GI neurons at glucose concentrations comparable to those seen in the brain (2.5 mmol/l) under euglycemic conditions in the periphery (36,37). On the other hand, pharmacolog- ical activation of GK inhibits activity of GI and enhances activity of GE neurons held at glucose concentrations comparable to those seen in the brain (0.5 mmol/l) under hypoglycemic conditions (42). These data strongly suggest that GK is a critical regulator of neuronal activity in many GE and GI neurons. In GE neurons, glucose metabolism increases the ratio of ATP to ADP. This causes ATP to bind to the ATP- ϩ sensitive K (KATP) channel composed of a Kir6.2 pore- forming unit for potassium and a sulfonylurea receptor (44). Binding of either ATP or sulfonylureas inactivates FIG. 3. Levels of blood and brain extracellular (EC) glucose at which (closes) the channel and depolarizes the cell membrane. various components of the counterregulatory response, self-reported , and defects in cognitive function develop during a hyperinsu- Depolarization is followed by influx of extracellular cal- linemic-hypoglycemic clamp in humans. Data are extrapolated from cium through a voltage-dependent calcium channel (36,37) studies in which simultaneous blood and brain EC glucose levels were and is often associated with increased action potential measured in rats (62,63) and from studies in humans (64,65). frequency. Glucose-induced closure of the KATP channel at nerve terminals on some GE neurons can also release GLUCOSENSING NEURONS AND THE REGULATION OF independently of action potentials prop- ENERGY HOMEOSTASIS agated from the cell body (45). Whereas the KATP channel Glucosensing and the control of feeding. Mayer (53) appears to be the final common pathway involved in GE proposed that “. . . the passage of potassium ions into glucosensing, much less is known about GI neuronal glucoreceptor cells along with the glucose phosphate glucosensing. A ClϪ channel (35), the Naϩ-Kϩ ATP pump represents the point at which effective glucose levels are (46), and an ATP-responsive Kϩ channel (47) have all been translated into an electric or neural mechanism. . . ” for the proposed as possible final common pathways, but the control of feeding. He suggested that these cells were actual effector of GI glucosensing is unknown. In addition located in the hypothalamus even though it would be to this uncertainty, it is also unclear how the GE and GI another 11 years before such glucosensing neurons were neurons that do not express GK regulate their ability to first identified (7,8) and 35 years before the KATP channel use glucose as a signaling molecule. would be recognized as the mechanisms by which GE Glucosensing neurons do not function in isolation. They neurons sense glucose (34). Despite Mayer’s “glucostatic are surrounded by and provided with metabolic support by hypothesis” for the control of food intake, we still do not glia. Astrocytes readily take up and store transported know whether normal excursions of blood glucose that glucose as glycogen, which is hydrolyzed to release lactate occur during the diurnal cycle play an important role in the into the extracellular space. Extracellular lactate is taken regulation of normal feeding behavior. Unquestionably, up by neurons and converted to pyruvate, which is then severe glucoprivation can stimulate feeding in rodents (54) and feelings of hunger in humans (55), and high concen- oxidized in mitochondria to provide ATP (37,48). By this trations of glucose placed into the brain can terminate or mechanism, lactate can reverse the inhibition of GE activ- reduce feeding (56–59). There is also no question that ity, which occurs at low ambient glucose levels (49). glucosensing neurons can respond to very small changes Glucosensing neurons are also responsive to fatty acids in ambient glucose concentrations (0.1–0.3 mmol/l) that (9,50), ketone bodies (51), leptin (12,19), and insulin are comparable to the very small decrements in blood (13,14). As with lactate, the effects of these other metab- glucose levels (0.5–2 mmol/l) that precede some meals in olites and hormones on glucosensing neurons depend on both humans and rodents (14,35,42,60–63). However, ambient glucose levels. For example, both leptin and when blood glucose levels are lowered in a stepwise insulin inhibit neuronal activity in VMH GE neurons held fashion, humans report hunger only at levels slightly at very high (10 mmol/l) glucose levels (12,13), but they higher than those associated with impaired cognitive either stimulate or have no effect on activity in these function (64,65) (Fig. 3). In fact, no one has ever demon- neurons held at glucose levels seen in the brain during strated that meal initiation or termination can be manipu- euglycemic or hypoglycemic conditions (14,19). In addi- lated by altering brain glucose levels within the limits tion, glucosensing neurons are, as expected, subject to found during normal ingestive cycles. modulatory transmitter and peptide inputs from surround- Nevertheless, there does appear to be an intrinsic sys- ing neurons (14,52). Thus, the response of glucosensing tem within the brain (and hepatic portal system [66]) that neurons to changes in glucose levels highly depends on the responds to severe cellular glucopenia by stimulating neural, metabolic, and hormonal milieu in which they find feeding. Generalized inhibition of brain glucose metabo- themselves. Because of these multiple influences on their lism by injection of 2-deoxyglucose into the lateral cere- activity and because they are critical to the control of bral ventricle stimulates feeding (67). Although Mayer energy homeostasis, the term “metabolic sensing” is prob- postulated that glucosensing neurons in the hypothalamus ably more apt than “glucosensing” to describe their func- were important for feeding, direct unilateral injection of tion (19). the potent 2-deoxyglucose analog, 5-thioglucose (5TG),

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FIG. 4. Sites of hindbrain glucokinase (GK) expression and responses to 5TG- vs. alloxan-induced feeding and hyperglycemia. A: In situ hybridization autoradiograms demonstrating GK mRNA expression in the nucleus tractus solitarius (NTS), raphe obscurus (ROb), and raphe pallidus (RPa) and in the caudal medulla and the site at which alloxan and 5TG were injected (vertical dotted line). Top autoradiogram is rostral and bottom is caudal. B: Food intake over 3 h after 200-nl injections of 24 ␮g 5TG, pharmacological dose of alloxan (1 ␮g in saline, pH 3.0), and per group) before and 7 days after injection of a toxic dose of alloxan 7 ؍ saline (pH 3.0) into the caudal dorsomedial medulla in satiated rats (n (40 ␮g) into the same site. Each injection was separated by at least 4 days. Coordinates were based on those of Ritter et al. (30). Both alloxan and 5TG stimulated food intake comparably, but only the response to alloxan was prevented by pretreatment with a toxic dose of alloxan to destroy GK-expressing neurons. *P < 0.05 when saline group was compared with the others. C: Blood glucose levels over 120 min after the caudal dorsomedial medulla injection of the same amount of 5TG (pre- and post-toxic alloxan dose [40 ␮g]), alloxan, or saline injected into a separate group of seven rats. The 5TG but not alloxan injections provoked hyperinsulinemia both before and after putative destruction of GK-expressing neurons. D: Bilateral 1-␮l injections of 4 ␮g alloxan into the ventromedial hypothalamus (VMH) had no effect on 3-h food intake (compared with .(per group 13 ؍ saline; n into the VMH does not stimulate feeding (30). On the other 5TG nor alloxan injections into the VMH stimulate feeding, hand, avid feeding results from small localized injections feeding provoked by 5TG injections into the hindbrain of 5TG into areas of the caudal ventrolateral medulla and serotonin neurons is completely blocked when VMH GK dorsomedial medulla. The former site corresponds to the mRNA is upregulated after third ventricular injections of A1/C1 norepinephrine/epinephrine neurons (30), and de- toxic doses of alloxan (70). Such data demonstrate a struction of the rostrally projecting catecholamine neu- complex interplay between hindbrain and hypothalamic rons from this site inhibits feeding in response to systemic sites involved in the regulation of glucoprivic feeding. injections of 2-deoxyglucose (68). The latter glucosensing Whereas “emergency” feeding can be stimulated by pro- site overlaps with serotonin neurons in the raphe pallidus ducing focal glucoprivation in select hindbrain glucosens- and obscurus that also express GK (Fig. 4A) (30,69). ing neurons, their projections to rostral hypothalamic sites Inhibition of GK activity in neurons in that area with focal are clearly required for full expression of this response. injection of pharmacological doses of alloxan stimulates It is important to point out that glucose does not feeding comparably to injecting 5TG (Fig. 4B). This effect necessarily act alone on central glucosensing neurons to is completely inhibited by pretreatment with toxic doses alter feeding. First, infusions of glucose into the portal of alloxan, which should selectively destroy GK-express- vein can reduce food intake (71). Second, the complex ing neurons. However, such putative destruction of GK interaction between central glucosensing neurons and neurons does not block 5TG-induced food intake, suggest- other metabolic substrates and hormones is illustrated by ing that different sets of neurons mediate these feeding studies demonstrating the role of insulin and the VMH effects. While alloxan stimulates feeding when injected KATP channel activity in feeding. First, chronic (2-day) into the caudal dorsomedial medulla, similar bilateral carotid infusions of glucose alone have little effect on injections of alloxan into the VMH have no effect on feeding, independent of the caloric load infused, whereas feeding (Fig. 4D). Despite the fact that neither unilateral addition of insulin to the infusate decreases intake out of

S126 DIABETES, VOL. 55, SUPPLEMENT 2, DECEMBER 2006 B.E. LEVIN AND ASSOCIATES proportion to the infused (72). Additionally, cen- systemic glucoprivation is greatly attenuated 4 days after tral infusions of oleic acid reduce food intake, and this third ventricular administration of toxic doses of alloxan, effect is antagonized by pharmacologic inhibition of KATP which increase VMH GK mRNA expression (70). Finally, channel activity in the VMH (73). VMH GK expression is elevated in obesity-prone rats, Glucosensing and the control of energy expenditure which also demonstrate a reduced adrenomedullary re- and glucose homeostasis. It has been known for several sponse to hypoglycemia (36,94). years that infusions of glucose can both increase general But the VMH is not the only site from which the sympathetic activity (as evidenced by increased plasma counterregulatory response can be stimulated. Injection of norepinephrine levels) (74,75) and produce an increase in 5TG into the caudal dorsomedial medulla evokes a brisk thermogenesis, which is partly due to such sympathetic hyperglycemic response, as do injections into the nearby activation (76). While hyperinsulinemia has been evoked caudal ventrolateral medulla (30,70) (Fig. 4C). The hyper- as a stimulant of sympathetic activity, it is clear that glycemic response to systemic 2-deoxyglucose can be glucose can evoke this activity when given alone (in blocked by lesions of the rostral C1 epinephrine–contain- insulin-deficient animals) (75) and directly into the fore- ing neurons that project to the sympathetic preganglionic brain via the carotid artery without altering plasma insulin neurons in the spinal cord (68). However, unlike the levels (77,78). Such forebrain infusions activate neurons in stimulatory effect that alloxan injections into the caudal several hypothalamic areas known to contain glucosens- dorsomedial medulla have on feeding, such injections do ing neurons such as those in the PVN that project directly not produce hyperglycemia (Fig. 4C). to autonomic outflow areas of the medulla and spinal cord As with feeding, peripheral glucosensing sites are also (79,80). Intracarotid glucose infusions also increase effer- important contributors to the effect of glucose on energy ent vagal nerve activity in the pancreas (81). Although homeostasis and the counterregulatory response to hypo- such activation should increase insulin secretion, absence glycemia. Raising portal vein glucose levels leads to a of change in plasma insulin levels after such infusions is decrease in vagal afferent discharges impinging upon NTS probably because of the concomitant sympathetic activa- neurons, which are themselves inhibited by direct appli- tion that inhibits insulin secretion (78,82). The thermo- cation of glucose. Sympathetic efferents to the adrenal, genic effects of glucose are also probably mediated by liver, splanchnic bed, and pancreas are activated, whereas hypothalamic glucosensing neurons, since intracarotid pancreatic vagal afferents are inhibited after such infu- and direct injections of glucose into the VMH and PVN sions. Because all of these reflex efferent outputs are produce increased activity in the sympathetic efferents to blocked by hepatic vagotomy, it appears that signals from brown in the rat (83,84). high levels of portal glucose are transmitted to the brain- Again, the interaction of hormones with glucosensing stem through hepatic vagal afferents (18,95). On the other neurons in the control of energy homeostasis is suggested hand, counterregulatory response to moderate systemic by studies showing that third cerebral ventricular insulin hypoglycemia is attenuated (but not completely blocked) infusions reduce hepatic glucose production, and this by clamping the liver at euglycemic levels, and this effect effect is inhibited by central administration of KATP chan- is disrupted by interruption of sympathetic (but not vagal) nel inactivators (85) or mimicked by third ventricular afferents from the hepatic portal circulation (96,97). Such infusion of a KATP channel activator (86). However, these data suggest that the hepatic portal vein, like the brain, results alone do not definitively implicate glucosensing may have different glucose sensors that respond to either neurons, since pharmacological manipulation of the KATP high or low glucose levels. Other glucosensors are also channel can alter neuronal activity even in GI and nonglu- present in carotid body glomus cells that are activated by cosensing neurons, neither of which normally use the KATP low glucose levels and contribute to the release of gluca- channel to sense glucose (36,37). gon from the pancreas in response to hypoglycemia As with food intake, cellular glucopenia can stimulate a (98,99). These signals reach the brainstem via glossopha- counterregulatory response associated with increased ryngeal afferents whose cell bodies reside in the inferior plasma norepinephrine, epinephrine, glucagon, and corti- petrosal ganglion (Fig. 1). Finally, the small intestinal costeroid levels that helps mobilize blood glucose levels myenteric plexes contain glucosensing neurons that ex- by promoting hepatic glycogenolysis and fatty acids by press the KATP channel, which is the same channel used by promoting lipolysis (65,87). The brain appears to be an pancreatic ␤-cells and GE neurons in the brain (17,33). important mediator of this effect, since clamping the brain Glucosensing and the predisposition to obesity. In at euglycemic levels greatly attenuates the counterregula- humans, functional magnetic resonance imaging demon- tory response to systemic hypoglycemia (88). Unlike glu- strates inhibition of hypothalamic indexes of neuronal coprivic feeding, the VMH appears to play a major role in activation after ingestion of an oral glucose load. Interest- the counterregulatory response to glucoprivation. Bilat- ingly, this response is both delayed and attenuated in eral injections of 2-deoxyglucose into the VMH mimic the obese individuals (100). Unfortunately, human studies effects of systemic hypoglycemia (89), whereas VMH infu- involving brain function are seriously hampered by the low sions of glucose greatly attenuate the counterregulatory resolution and relative nonspecificity of most imaging response to systemic hypoglycemia (90). Also, inhibition techniques. For this reason, rodent models of obesity are of KATP channel activity in the hypothalamus attenuates useful surrogates for the studies of human obesity, partic- (91), whereas enhancing channel activity selectively in the ularly those involving the brain. We have used a polygenic VMH increases, the counterregulatory response to hypo- model of rodent obesity that has many similarities to glycemia (92). Even a single bout of hypoglycemia leads to human obesity and has the advantage of being able to blunting of subsequent counterregulatory response to hy- study such animals before the onset of obesity to identify poglycemia (93). GK may be an important modulator of potential predisposing factors. One of the most striking this reduced counterregulatory response, since its expres- features of this model is the fact that obesity-prone rats sion in the VMH is elevated commensurate with this have a number of defects in their ability to sense and blunting (36). Similarly, the counterregulatory response to respond to glucose. These include defective glucose-in-

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