The role of gastrointestinal vagal afferents in the control of food intake: current prospects

INGESTIVE BEHAVIOR AND OBESITY

The Role of Gastrointestinal Vagal Afferents in the Control of Food Intake: Current Prospects Gary J. Schwartz, PhD From the Edward W. Bourne Behavioral Research Laboratory, Weill Medical College of Cornell University, White Plains, New York, USA Meals are the functional units of food intake in humans and mammals, and physiologic approaches to understanding the controls of meal size have demonstrated that the presence of food in the upper gastrointestinal tract plays a critical role in determining meal size. The vagus nerve is the primary neuroanatomic substrate in the gut– brain axis, transmitting meal-related signals elicited by nutrient contact with the gastrointestinal tract to sites in the central nervous system that mediate ingestive behavior. This article describes progress in examining the role of the vagal gut– brain axis in the negative-feedback control of meal size from four perspectives: neuroanatomic, neurophysiologic, molecular, and behavioral. Vagal afferents are strategically localized to be sensitive to meal-related stimuli, and their central projections are organized viscerotopically in the caudal brainstem. Vagal afferents are sensitive to mechanical, chemical, and gut and peptide meal-related stimuli and can integrate multiple such modalities. Meal-elicited gastrointestinal stimuli activate distinct patterns of c-fos neural activation within caudal brainstem sites, where gut vagal afferents terminate. Results of selective chemical and surgical vagal deafferentation studies have refined our understanding of the sites and types of critical gastrointestinal feedback signals in the control of meal size. Recent behavioral, molecular, and neurophysiologic data have demonstrated brainstem sites where centrally acting neuropeptides may modulate the processing of gut vagal afferent meal-related signals to alter feeding. Investigations of the structure and function of splanchnic visceral afferents and enterics and characterization of the integrative capacities of the hindbrain and forebrain components of the gut– brain axis are critical next steps in this analysis. Nutrition 2000;16:866 – 873. ©Elsevier Science Inc. 2000 Key words: visceral afferents, vagotomy, gut– brain axis, meal size, nucleus of the solitary tract

INTRODUCTION Understanding the control of food intake relies critically on the understanding of the controls of individual meals, the functional units of ingestion common to humans and mammals. From a historical perspective, the biological analysis of the physiologic controls of meal size have in part sought to identify and characterize peripheral neural and humoral signals that may be elicited by the presence of food in the gut. The examination of peripheral targets stems from a wide range of observations suggesting that the presence of nutrients in the gastrointestinal tract is causally related to meal termination and meal size. One of the most compelling observations supporting this proposed relation is the striking increase in meal size seen during sham feeding, where ingested nutrients empty from the stomach without accumulating, precluding passage of nutrients into postgastric, small-intestinal sites critical for nutrient absorption. A putative negative-feedback role for meal-related stimuli in limiting meal size is also supported by studies demonstrating that upper gastrointestinal tract administration of nutrients and mechanical distention potently and dosedependently suppress food intake within a meal.1–3

This study was supported by NIH grant DK 47208. Correspondence to: Gary J. Schwartz, PhD, Edward W. Bourne Behavioral Research Laboratory, Weill Medical College of Cornell University, 21 Bloomingdale Road, White Plains, NY 10605, USA. E-mail: gjs2001@ med.cornell.edu Date accepted: July 19, 2000. Nutrition 16:866 – 873, 2000 ©Elsevier Science Inc., 2000. Printed in the United States. All rights reserved.

The past decade has witnessed significant advances in the understanding and appreciation of the peripheral physiologic mechanisms contributing to the control of meal size. Much of this progress has resulted from increased attention to the peripheral neural mediation of putative negative-feedback signals arising from contact of the upper gastrointestinal tract with ingested nutrients and from attempts to determine how these signals contribute to food intake. Together, the upper gastrointestinal tract, its intrinsic and extrinsic innervation, the extrinsic neural projection fields in the central nervous system, and their respective target neurons in the central nervous system comprise the neural gut– brain axis. This nomenclature emphasizes the direction of information flow along a neural pathway: afferent signals elicited by gut contact with ingested nutrients travel from sites in the upper gastrointestinal tract to sites in the central nervous system that mediate ingestive behavior. Although not considered here, gut– brain peptides may be released directly into the peripheral blood supply after nutrient ingestion, and these blood-borne factors may provide an alternative afferent signaling pathway to sites in the central nervous system that mediate ingestion by a hormonal rather than a direct neural route, the humoral gut– brain axis. Both the vagus nerve and nonvagal splanchnic mesenteric nerves supply preabsorptive upper gastrointestinal sites that handle ingested nutrients, and together they comprise the peripheral extrinsic neural component of the gut– brain axis. Within the gut wall, there also exist intrinsic, enteric neuronal elements, including the myenteric and submucosal plexuses, and these may mediate between gastrointestinal mucosal and muscular events and extrinsic neural signaling (see NEUROANATOMIC PERSPECTIVE). There is very little literature examining gut splanchnic afferent structure and function as it relates to ingestion; therefore, it seems 0899-9007/00/$20.00 PII S0899-9007(00)00464-0

Nutrition Volume 16, Number 10, 2000 premature to integrate these few findings into the current report. In contrast, vagal afferent axons comprise the vast majority of the entire abdominal vagus,4 and these appear to be the primary neuroanatomic substrate of the neural gut– brain axis. Consequently, this article discusses advances in our understanding of the role of gastrointestinal vagal afferents in the negative-feedback control of meal size. I examine a circumscribed view of the vagal gut– brain axis that includes (1) gastrointestinal tract sites innervated by afferent vagal fibers, (2) the branches and trunks of the subdiaphragmatic vagus nerve, (3) its primary cell bodies in the nodose ganglion, (4) the primary afferent terminal fields of gastrointestinal vagal afferent fibers in the caudal brainstem, and (5) the second-order sensory neurons in the brainstem nucleus of the solitary tract (NST). I choose this limited focus because it includes areas where the most progress has been made from neuroanatomic, neurophysiologic, molecular, and behavioral perspectives.

NEUROANATOMIC PERSPECTIVE Retrograde and anterograde tracing studies have suggested several important organizational principles of gut– brain-axis afferent projections in both the brainstem and the gut wall. Anterogradetransport studies after injections of tracer compounds directly into upper gastrointestinal sites (esophagus, stomach, cecum) have demonstrated a viscerotopic map of gut target-organ neuron pools in the nodose ganglion and in topographically discrete termination fields of the NTS.5 Anterograde tracing after injections into the individual subdiaphragmatic vagal branches (celiac, accessory celiac, hepatic, dorsal gastric, and ventral gastric) also show discrete, yet somewhat overlapping, NTS termination fields for each branch.6 Together these data support the notion that the vagal gut– brain axis maintains distinct pathways for information arising from the gastrointestinal tract into gut-organ– specific and vagal-branch–specific regions. This concept is fundamental to behavioral studies designed to evaluate the contribution of meal-related signals from individual gut organs or individual subdiaphragmatic vagal branches to the negativefeedback control of ingestion. The functional significance of the viscerotopic organization in the afferent gut– brain axis is bolstered by a parallel mapping in the efferent brain– gut axis. The dorsal motor vagal nucleus (DMN) is the primary location of gut vagal efferent motor neurons and is located just ventral to the NTS in the caudal brainstem. Retrograde-tracer studies of individual gut organs and individual subdiaphragmatic vagal branches have also demonstrated viscerotopic and branch-specific maps in longitudinal columns of the rat DMN.7 Powley et al. proposed that the columnar organization of gut vagal efferent motor neurons combined with the branchspecific gut vagal afferent terminations in the NTS form a sensorymotor lattice ideally suited to mediate discrete cephalic, gastric, hepatic, and intestinal reflexes accompanying the ingestion of food.8 In the periphery, elegant studies by Berthoud, Powley, and their colleagues have identified the morphologic structure and location of vagal afferent terminations in the gut wall of the rat. Nodose ganglion injections of the anterograde tracers DiI and wheat-germ agglutinin horseradish peroxidase label vagal afferent terminations throughout the upper gastrointestinal tract, including the esophagus, the fundus and antrum of the stomach, the pylorus, and the proximal duodenum.9 –13 In the stomach, DiI-labeled vagal afferent terminations permeate multiple regions of the gastric wall, including the circular and longitudinal muscle, the myenteric plexus, the submucosa, and mucosa. One subpopulation of vagal afferents enter either circular or longitudinal muscle walls, running in parallel with the muscle fibers. These intramuscular arrays (IMAs) appear well situated to detect contraction or tension in the surrounding muscle tissue.9,13 In addition, single gastric vagal afferent fibers with IMAs have

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collateral branches that appear to make synaptic contact with myenteric neurons, suggesting a direct pathway from enteric innervation to extrinsic afferent gut– brain pathways. Gastric mucosal vagal afferent endings have been identified, and their neurophysiologic response characteristics are consistent with a role in the transmission of meal-related negative-feedback signals (see NEUROPHYSIOLOGIC PERSPECTIVE). Pyloric and duodenal vagal afferents are distributed throughout the cross-sectional extent of the gut wall, including IMAs, and innervation near the mucosal crypts and the myenteric plexus, with much less dense labeling in the submucosa, submucosal plexus, and the lamina propria. There is a clear density gradient of vagal afferent terminations along the duodenum, with proximal sites near the duodenal bulb much more densely innervated than more distal regions.10,13 Some duodenal vagal afferents appear to terminate within a single epithelial-cell width of the duodenal luminal surface along the basal lamina of the epithelium.14,15 In addition, at the light microscopic level, some duodenal vagal afferents appear close to cholecystokinin (CCK)–immunoreactive cells in the duodenal villi.15 Exogenous CCK potently suppresses food intake by reducing meal size,16 and endogenous CCK is released by duodenal enteroendocrine cells by the luminal presence of nutrients.17 The satiety actions of exogenous and endogenous CCK have been demonstrated to be mediated by CCK’s interactions with the CCK-A–receptor subtype,18 and primary vagal sensory neurons in the nodose ganglion neurons contain mRNA for CCK-A receptors. Furthermore, all branches of the subdiaphragmatic vagus nerve have been shown to transport CCKbinding sites,19 CCK-binding sites are transported in vagal afferents,20 and there is CCK-A–receptor immunoreactivity in gastroduodenal vagal fibers.21 Thus, a subpopulation of duodenal vagal afferents with CCK-A receptors appear to be well placed to detect gut nutrient-elicited CCK release, most likely by a paracrine mode of action (see also NEUROPHYSIOLOGIC PERSPECTIVE). Taken together, the extensive vagal afferent terminations in the upper gut appear to be strategically and ideally located to convey mealelicited signals from the upper gastrointestinal tract to the NTS. The topography of rat vagal-branch–specific afferent terminations in the gut has not been as well documented as their terminations in the NTS.6 However, some important inferences may be drawn from studies of the topographic distribution of gut vagalbranch efferents.22 These data are instructive because they show that individual gut vagal-branch efferents do not innervate single gastrointestinal organs, as might be assumed from their gross anatomic appearance. These findings demonstrate that: (1) efferents within a single gut vagal branch supply more than one gastrointestinal target, and (2) vagal efferents from more than one gut vagal branch innervate the same general gastrointestinal region, and suggest that the same may be true of gut vagal-branch afferents. Specifically, (1) gastric-branch efferents supply gastric and duodenal sites, (2) hepatic-branch efferents supply the distal stomach and proximal duodenum (and presumably the adventitia of the hepatic portal vein of the liver),11 and (3) celiac branches supply duodenal, jejunal, cecal, and colonic sites. In the one recent finding investigating gut vagal-branch–specific afferent terminations, Phillips et al.22a showed that hepatic-branch vagal afferents supply the forestomach, antrum, pylorus, duodenum, and cecum, consistent with the general notion that individual gut vagal branches innervate multiple gastrointestinal-tract segments.

NEUROPHYSIOLOGIC PERSPECTIVE There has been significant progress in the identification and characterization of meal-related vagal afferent responses from the upper gastrointestinal tract that are consistent with a role for these signals in the negative-feedback control of ingestion. Vagal afferent fibers supplying the upper gastrointestinal tract have been shown to be sensitive to three classes of meal-related stimuli, each

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of which can reduce meal size: mechanical distention of the lumen or gut contraction, chemical properties of luminal contents, and gut peptides and neurotransmitters normally elicited by the presence of meals in the duodenum. In the rat stomach, gastric loads excite23 and dose dependently increase firing in vagal mechanosensitive fibers over a physiologic range,24 and Blackshaw and Grundy identified a population of vagal afferents responsive to mucosal stroking, acid and CCK in the ferret.25 Interestingly, load-sensitive vagal afferents in the rat stomach are dose dependently activated by increasing gastric volume independently of whether the gastric load consists of saline, carbohydrate, protein, or fat solutions.26 These data are consistent with behavioral studies demonstrating that gastric loads confined to the stomach 1) suppress meal size depending on gastric load volume and 2) suppress feeding independently of nutrient content,3,3a and support a primary role for gastric volume and not nutrient detection as an available negative-feedback signal arising from gastric contact with ingested foods. In the small intestine, studies in rats have demonstrated vagal afferent sensitivity to meal-related mechanical and chemical stimuli in the duodenum, jejunum, and ileum. Duodenal infusions of amino-acid and carbohydrate solutions elicit distinct patterns of duodenoantral motility and neurophysiologic activity in single vagal duodenal and gastric load-sensitive afferents.27 Duodenal infusions of peptone, a protein extract and plasma secretagog of CCK, produced greater duodenal and gastric antral contractions than did equiosmolar glucose solutions and greater vagal responses than would have been predicted based on the contractile response alone. These data demonstrate that duodenal nutrient-induced gut contractile responses are nutrient specific, that both duodenal and gastric vagal mechanosensitive afferents encode for these nutrient-specific elicited motility patterns, and suggest that nutrient elicited endogenous CCK can amplify the response to contraction in gut vagal afferents. Exogenous CCK dose dependently stimulates gastric and duodenal vagal mechanosensitive and mucosal afferents23,27–29 and amplifies the gastric vagal afferent neurophysiologic response to subsequent distending loads.24 Consistent with the observation that both gastric and hepatic branches supply the stomach and duodenum (vide supra), exogenous CCK has been shown to activate both gastric-branch30 and hepatic-branch31 vagal afferents. The vagal afferent response to exogenous CCK is 1) atropine insensitive, 2) unaffected by cervical vagotomy above the recording site, and 3) unchanged by pylorectomy, the major non-vagal CCK receptor site in the upper gastrointestinal tract. These data support the idea that gut vagal afferent responses to CCK are mediated by vagal CCK receptors and are not secondary to CCK-induced vagal efferent activation or pyloric contractility. A role for endogenous CCK acting at CCK-A–receptor types in the vagus in response to small-intestinal nutrients has been suggested by Eastwood et al.32 who showed that pretreatment with MK-329 blocks the mesenteric gut afferent excitation produced by jejunal exposure to casein, also a potent secretagog of CCK in the rat.33 In rats with elevated plasma CCK levels produced by bile pancreatic-juice diversion (BPJ), Li et al.33a demonstrated that primary sensory neurons in the rat nodose ganglia arising from subdiaphragmatic vagal fibers are responsive to BPJ. Vagal afferent neurophysiologic responses to BPJ and exogenous CCK can be blocked by pretreatment with the potent and specific CCK-Areceptor–specific antagonist MK-329 and are typically unaffected by the CCK-B-receptor–specific antagonist L 365-260.33a,34 Furthermore, vagal afferent responses to BPJ and exogenous CCK are also blocked by the CCK-A low-affinity antagonist CCK-JMV180. Together with the behavioral finding that gut afferent vagotomy blocks the satiety effects of CCK (see BEHAVIORAL PERSPECTIVE), these data are consistent with vagal CCK-A and, more specifically, CCK-A low affinity receptor mediation of CCK satiety in rats.18,34a Single vagal afferents from the ileum and jejunum are excited

Nutrition Volume 16, Number 10, 2000 by lipids, especially linoleic and oleic acid, but are much less sensitive to triacylglycerol corn oil and intralipid emulsions.35 Importantly for the putative small-intestinal vagal afferent mediation of the feeding-suppressive effects of intestinal lipids, application of the hydrophobic surfactant pluronic L-81 blocks the neurophysiological responses of celiac vagal afferents to ileal infusion of linoleic acid. L-81 blocks the export of absorbed fat from the enterocyte into the lymph as chylomicra and has been demonstrated to reverse the ability of intragastric corn oil or intraduodenal lipid-emulsion preloads to suppress food intake.36,37 Lipids are also potent CCK secretagogs, and recent work in humans has demonstrated that reductions in food intake and increased gastric-fullness sensations in response to gastric distention and duodenal lipid are attenuated or reversed by the CCK-A– receptor antagonist loxiglumide.38,39 In the context of the rat vagal neurophysiologic sensitivity to CCK at CCK-A receptors, these data suggest a role for gut vagal afferents in the fullness responses and reductions in food intake elicited by lipids. Integrative Capacity of the Afferent Gut Vagus During ingestion of a meal, food accumulates in the stomach and begins to empty into the duodenum, where it elicits the secretion of gut-brain peptides. Thus, both gastric distention and gut peptide release are likely to be present simultaneously within a meal. Behavioral data in rats, monkeys, and humans have demonstrated that combinations of gastric distending loads and subthreshold doses of exogenous CCK or CCK analogs will potently suppress feeding.40 – 42 More recent data in humans has shown that CCK in combination with an orally consumed nutrient preload increased fullness and decreased hunger ratings and that this was completely reversed by the administration of the specific CCK-A–receptor antagonist loxiglumide.43 A sufficient gut vagal afferent substrate for this behavioral synergy is suggested by the finding that combinations of gastric loads and exogenous CCK excite gastric vagal mechanoreceptors to a greater degree than either stimulus alone.24 Neurophysiologic data have demonstrated that this synergistic interaction is mediated by the CCK-A receptor in rats, and gastric vagal afferent responses to CCK and gastric loads may be pharmacologically dissociated. CCK-A–receptor antagonism with MK-329 blocks the ability of CCK but not of gastric loads to stimulate gastric vagal load-sensitive fibers.32 Together these data 1) suggest that single gut vagal afferents have distinct transduction mechanisms for different classes of meal-related negative feedback signals and 2) demonstrate that the vagus has the capacity to integrate these signals to form a coherent message representing the simultaneous presence of two meal-related stimuli, each of which can limit meal size. Much less is known about second-order gut sensory neurons in the caudal brainstem, where gut vagal afferents terminate. Zhang et al.44 demonstrated discrete populations of gastric and duodenal load-sensitive neurons in the rat NTS. Neurons sensitive to duodenal but not to gastric distention are located in the subpostremal and commisural NTS subnuclei, and gastric-sensitive neurons are located in the gelatinous subnucleus. Neurons that responded to both stimuli were predominantly localized in the medial subnucleus.44 Raybould et al.45 showed that gastric-load–responsive medial NTS neurons also are activated by local gut vascular administration of CCK, but the extent and nature of the integrative properties of gut-sensitive NTS neurons remains to be explored.

MOLECULAR PERSPECTIVE— C-FOS STUDIES Molecular biological and immunocytochemical techniques have begun to be exploited to show the pattern of neural activation in the caudal brainstem region of the vagal gut– brain axis after gastrointestinal administration of meal-related stimuli. These findings rely on immunocytochemical localization of c-fos protein, an

Nutrition Volume 16, Number 10, 2000 oncoprotein that can be detected in neurons 20 to 90 min after depolarization, and whose expression dissipates within 4 to 72 h. Rats that consume a normal meal to satiety express significant c-fos–like immunoreactivity in the area postrema (AP) and in the medial nucleus of the NTS, where gastrointestinal vagal afferents terminate.46,47 The contribution of pregastric alimentary-tract sites in this pattern of activation was evaluated by examining c-fos expression in response to sham feeding of liquid diet with an open gastric fistula, allowing consumed nutrients to drain from the stomach without accumulating. Sham feeding significantly attenuates meal-induced c-fos labeling in medial and commisural NTS subnuclei, where gut afferents terminate, but not in more rostral NTS regions that receive input from oropharyngeal afferents.47,48 Just as three classes of meal-related stimuli—mechanical, nutrient chemical, and gut peptide—are capable of activating gut vagal afferent fibers supplying the upper gastrointestinal tract, representatives of these stimulus groups also produce c-fos–like immunoreactivity in the caudal brainstem termination regions of gut vagal afferents. Physiologic and repeated balloon distention at rates and volumes that mimic those that occur during meal ingestion produce c-fos–like immunoreactivity in the medial and commisural nuclei of the NTS.48,49 Duodenal infusions of individual macronutrient solutions into the small intestine that suppress sham feeding also produce significant c-fos–like immunoreactivity in the AP and in the medial, dorsomedial, and commisural subnuclei of the NTS.50 –52 There is some suggestion that the rostrocaudal location of duodenal nutrient-induced brainstem c-fos immunoreactivity in the NTS depends on the macronutrient content. Glucose and linoleic acid induce more c-fos activation in the medial NTS caudal to the AP than amino acids; at the AP and in the NTS near and rostral to the AP, amino acids are more effective.52 The ability of duodenal infusions of Intralipid to produce NTS c-fos–like immunoreactivity is significantly attenuated by perivagal capsaicin administration,51 thus supporting a role for capsaicin-sensitive gut vagal afferents in mediating the central nervous system representation of duodenal nutrient stimuli. In terms of brainstem activation by gut-brain peptides, peripheral CCK administration over a range of doses that dose dependently suppress food intake also dose dependently increase c-fos– like immunoreactivity in the neuronal NTS regions, where gut vagal afferents terminate.53,54,54a A role for vagal afferents in CCK-induced c-fos expression is supported by the finding that perivagal capsaicin treatment blocks this response.53 Taken together, the results of these studies 1) confirm and supplement our understanding of the neurophysiologic response properties of gut vagal afferents, 2) reflect the ability of meal-related stimuli to activate brainstem vagal afferent termination sites along the gut– brain axis, and 3) support a vagal afferent role for some aspects of the brainstem representation of a meal.

BEHAVIORAL PERSPECTIVE Interruption of the neural gut– brain axis by surgical transection or chemical damage continues to provide the primary experimental approach to understanding the role of gut vagal afferents in the negative-feedback control of ingestion. Total subdiaphragmatic vagotomy 1) blocks the ability of gastric loads confined to the stomach to suppress meal size,3 2) attenuates or completely blocks the suppression of sham feeding induced by intestinal nutrient infusions, 3) blocks the ability of cafeteria diets to induce sleep,55 a critical component of the satiety response to meals,56 and 4) reverses the satiety effects of doses of CCK during both real and sham feeding.57–59 These results lend support to a negativefeedback role for gut vagal afferent signals in the control of meal size and, in conjunction with neuroanatomic data, spurred attempts to identify which vagal branches were carrying critical afferent meal-related information from discrete compartments of the upper gastrointestinal tract.

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Although from an efferent-labeling perspective individual vagal branches have been shown to supply multiple targets in the upper gastrointestinal tract,22 it remains unclear which branches specifically contain vagal afferents that are critical to the feedingsuppressive effects of gastrointestinal meal-related stimuli. In a comprehensive set of studies, Phillips and Powley examined which subdiaphragmatic vagal branches contribute to the ability of gastric contents to reduce meal size.3 Rats were equipped with pyloric cuffs that, when closed, confined consumed nutrients and gastric saline preloads to the stomach. Rats with closed pyloric cuffs receiving gastric preloads demonstrated volume-dependent suppression of meal size, with larger volumes producing greater reductions in intake. In addition, gastric loads suppressed feeding independently of their nutrient content,3a suggesting that gastric volume and not nutrient content is critical in the gastric negativefeedback control of ingestion. This conclusion is supported by the response properties of mechanosensitive gastric vagal afferents (see NEUROPHYSIOLOGIC PERSPECTIVE). Rats with selective gut vagal branch vagotomies demonstrated differential gastric load-induced suppression of feeding, consistent with the density of gastric innervation provided by each branch. Hepatic and gastric branches provide the most dense innervation of the stomach, and surgical transection of both of these sets of branches significantly attenuates the ability of gastric loads to suppress meal size.3 In studies designed to evaluate the gut vagal branch neural mediation of the feeding-suppressive effects of CCK, Smith et al. found that surgical transection of both gastric vagal branches, but not any of the other subdiaphragmatic branches, completely blocked the feeding-suppressive effects of CCK.57 Atropine methyl nitrate administration, which blocks peripheral vagal efferent cholinergic neurotransmission, failed to block CCK’s feedingsuppressive actions, suggesting that vagal afferents rather than efferents were critical in CCK-induced reductions in food intake and meal size. These findings are consistent with the neurophysiologic sensitivity to and CCK-receptor presence in gut vagal afferents. However, these vagotomies also inevitably block gut vagal efferent traffic, which plays a significant role in the modulation of gastric motility, secretion, and emptying. Thus, total or branch-selective vagotomies fail to specifically address the role of vagal afferent negative-feedback signals in the control of ingestion independently of the adverse gastrointestinal consequences of concomitant vagal efferent damage. Neuroanatomic-tracing studies had demonstrated that gut vagal afferents terminate in an organized, viscerotopic pattern, strongly suggesting discrete contributions from gut vagal afferents supplying distinct gut regions. To specifically address the role of gut vagal afferents in the negative-feedback control of ingestion, Norgren and Smith59 developed and employed a selective gut vagal deafferentation technique to surgically interrupt all gut vagal afferent traffic, while leaving half of the gut vagal innervation intact.58 This approach took advantage of the fact that, in the rat, the vagus nerve trunk bifurcates into distinct sensory and motor rootlets as it meets the caudal brainstem. Using a ventral approach to the base of the skull and brainstem, they were able to expose vagal afferent and efferent rootlets for discrete transections. (A similar surgical outcome with a dorsal rather than a ventral skull approach to the brainstem was developed by Walls et al.62) This subdiaphragmatic vagal deafferentation (SDA) procedure and the interpretations that it permitted represented a critical turning point in the conceptualization of the role of the gut– brain axis in the control of meal size because it was the first to experimentally focus on the relatively selective, yet complete, interruption of gut vagal afferent traffic. Using this technique, a critical refinement and confirmation of Smith et al.’s earlier report came in 1985, when the selective gut vagal deafferentation was found to block the ability of CCK to suppress food intake.59 Subsequently, Walls et al.61 found that subdiaphragmatic vagal deafferentation blocked the suppression of meal size produced by intestinal infusions of glu-

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cose, L-phenylalanine, oleic acid, casein hydrolysate, and liquidnutrient diets. Most recently, my colleagues and I reported that SDA, using a ventral surgical approach, failed to attenuate the feedingsuppressive effects of a dose range of gastric glucose, peptone, and fat infusions in rats during a scheduled 30-min access to glucose solution after a 6-h daytime food deprivation. Although SDA did not block the feeding-suppressive actions in this case, it did chronically increase spontaneous meal size, as measured by total licking of the nutritionally complete Ensure liquid diet, during the first 6 h of the dark cycle.62 Our study and that by Walls et al.61 differ in terms of the route of intestinal delivery (duodenal versus gastric infusions of nutrients) and in that the effects of infused nutrient were measured during spontaneous61 versus scheduled62 meals. Overall, however, these findings after selective transection of gut vagal afferents support a critical role for gut vagal signals in the negative-feedback control of meal size.

FUNCTIONAL AND ANATOMIC CONSEQUENCES OF CHEMICAL DAMAGE TO GUT VAGAL AFFERENTS USING THE NEUROTOXIN CAPSAICIN A growing literature examining gut vagal afferent function has evolved around the neurotoxic effects of capsaicin, the active vanilloid ingredient found in red peppers. Systemic capsaicin administration has been demonstrated to damage a subpopulation of small-diameter, poorly myelinated and unmyelinated sensory neurons and produces neuronal degeneration in gut vagal afferent termination regions and neurons within the NTS,63 similar to that produced by nodose ganglionectomy. Systemic capsaicin treatment also produces a long-lasting loss of vagal afferent terminations in the rat small-intestinal myenteric plexus.64 Overnight food-deprived rats with systemic capsaicin treatment overconsume a 10% sucrose solution relative to untreated controls, suggesting that a capsaicin-sensitive neural pathway mediates the gastrointestinal negative-feedback control of ingestion. However, this overconsumption may not be specific to nutrient ingestion, in that capsaicin treated rats also overconsumed 1) water after hyperosmotic and hypovolemic simuli and 2) saline after mineralocorticoid treatment.65 Systemic capsaicin treatment has also been shown to cause transient overconsumption of unfamiliar but not of familiar high-fat diets in rats66 and attenuates but does not completely block the feeding-suppressive effects of duodenal carbohydrate, fatty-acid, and amino-acid infusions and CCK.67,68 In terms of gut-brain peptide signals, neonatal systemic capsaicin treatment attenuates vagal axonal transport and brainstem CCK binding and attenuates but does not eliminate the feedingsuppressive effects of CCK.69 Systemic neonatal capsaicin treatment also completely blocks the feeding-suppressive effects of systemic bombesin,70 a member of a family of feeding-suppressive gut-brain peptides including neuromedin B and gastrin-releasing peptide, which potently stimulates gut vagal afferents.71 Because systemic capsaicin treatment affects somatic and visceral capsaicin-sensitive fibers, discrete vagal or gut capsaicin application has been used to selectively evaluate the negativefeedback contributions of capsaicin-sensitive gut afferents. Direct application of capsaicin to the subdiaphragmatic vagus nerve trunks damages a subpopulation of unmyelinated and poorly myelinated fibers, with concomitant degeneration and reductions of terminal tracer labeling in the subdiaphragmatic vagal projection fields of the NST.72 From a functional perspective, peripheral vagal capsaicin application in adult rats significantly attenuates CCK-induced feeding suppression.73 In the duodenum, capsaicin treatment or anesthetization of the luminal surface has been administered to evaluate the role of small-intestinal innervation in the negative-feedback control of ingestion. Greenberg et al.2 found that the addition of the local

Nutrition Volume 16, Number 10, 2000 anesthetic tetracaine to an intralipid fat emulsion significantly reversed but did not completely block the suppression of sham feeding produced by duodenal lipid infusions. Tamura and Ritter74 found that intestinal capsaicin administration only transiently attenuates the suppression of sham feeding produced by duodenal oleate infusions. These findings demonstrate that capsaicin-induced neural damage significantly attenuates the suppression of food intake produced by a number of gastrointestinal meal-related stimuli and suggest a significant role for capsaicin-sensitive visceral afferents in the negative-feedback control of meal size. Systemic capsaicin treatment remains a quick way of addressing the likelihood that gut visceral afferents—vagal, splanchnic, or both—play a role in the negative-feedback control of ingestion and provides an important starting point for the further identification and quantification of the extent to which each set of afferents contributes to this control. However, the feeding-suppressive effects of meal-related gastrointestinal stimuli should be systematically assessed after selective vagal and splanchnic capsaicin application. From an interpretative standpoint, it is important to note that the results of these studies must be limited to evaluating the contribution of the subpopulation of capsaicin-sensitive afferents, because there are clearly insensitive vagal afferents that survive capsaicin treatment.75 There is a proximal-to-distal gradient in vagal afferent capsaicin sensitivity along the upper gastrointestinal tract. In anterograde-tracing studies, Berthoud et al.75 demonstrated that systemic capsaicin treatment (1) slightly reduces the extent of DiI-labeled vagal afferents in the esophagus and stomach, (2) significantly attenuates labeling in the duodenum, and (3) virtually eliminates labeling of vagal afferents at lower gastrointestinal sites. The functional integrity of gastric non– capsaicinsensitive afferents in the negative-feedback control of ingestion is supported by the parallel finding that systemic capsaicin treatment fails to attenuate the number of c-fos–like–immunoreactive cells in the NTS induced by gastric distention.75 Capsaicin excitotoxically depolarizes primary sensory dorsal root ganglion neurons through its interactions with the vanilloid-1 (VR-1) receptor, a non-selective Ca2⫹ channel protein. In addition, capsaicin has a direct excitatory effect on rat gastrointestinal vagal afferents and can desensitize some muscular and mucosal afferent responses to subsequent mechanical stimulation.76 Rat vagal afferents were originally reported not to express VR-1 receptors,77 but the same research group has recently shown that vagal afferents and brainstem NTS and AP sites do express dense VR-1–like immunoreactivity,78 although the discrepancy between these findings remains to be reconciled. Given the identification of VR-1– like immunoreactivity in vagal afferents, it may be quite revealing to assess ingestive behavior and the feeding-suppressive efficacy of meal-related gastrointestinal stimuli (e.g., distention, gastrointestinal nutrient exposure, gut-brain peptides) in the recently developed mice lacking the VR-1 receptor.79 It has been suggested that one or multiple subtypes of VR-1 receptor exist in vagal afferents80 and that these differ from VR-1 receptors found in primary sensory dorsal root ganglion neurons. The molecular identification and characterization of vagal afferent vanilloid receptors would facilitate the design and evaluation of specific ligands in the analysis of vagal mealrelated negative-feedback signals.

RECENT DEVELOPMENTS: FOREBRAIN MODULATION OF GUT VAGAL AFFERENT SIGNALS IN CAUDAL BRAINSTEM—DIRECT AND INDIRECT CONTROLS OF MEAL SIZE This article has focused on vagal afferent signals arising from upper gastrointestinal contact with ingested nutrients. Such signals comprise direct controls of meal size, classified by Smith80a as

Nutrition Volume 16, Number 10, 2000 influences on meal size that result from direct contact with nutrients. Furthermore, by definition, all controls of meal size that are not direct can be classified as indirect controls. These include a wide range of environmental, physiologic, learned, metabolic, rhythmic, and cognitive influences. According to Smith, all indirect controls act by modulating the potency of the direct controls, and reciprocal neural connections between the forebrain and brainstem are necessary for this modulation (Smith, this issue). Over the past decade, there has been a dramatic increase in the number of potential indirect physiologic controls of meal size in the identification and characterization of many potent orexigenic and anorexigenic neuropeptides, including agouti-related protein, melanocortins, orexins, cocaine amphetamine-related transcript, and leptin. Most of these neuropeptides have signaling elements within forebrain hypothalamic nuclei. Although central nervous system administration of these peptides has profound effects on food intake and body weight, the ways in which they contribute to the control of meal size are not well understood. Perhaps the most is known about the anorexic effects of leptin, a circulating adiposity signal that is transported across the blood– brain barrier and can act at specific receptors in the hypothalamic arcuate nucleus to mediate energy balance.81 Leptin reduces food intake by specifically reducing meal size, with no change in meal frequency.82 According to the framework of direct and indirect controls, leptin is an indirect control; therefore, its ability to suppress meal size is achieved by modulating the direct controls of meal size. Recent work has begun to focus on the caudal brainstem in the search for the sites and ways in which the indirect controls modulate the direct controls of meal size. This focus stems from three critical features of the gut– brain axis: 1) gut vagal afferent signals that arise from gastrointestinal nutrient contact and are capable of reducing meal size terminate in the caudal brainstem NTS, 2) forebrain neuroanatomic components of the gut– brain axis, especially the hypothalamic paraventricular nucleus, have reciprocal afferent and efferent connections with the NTS,83 and 3) work in decerebrate rats has shown that the caudal brainstem contains sufficient neural circuitry to mediate oromotor ingestive responses and respond to direct controls of meal size84 (Smith, this issue). An experimental approach has been developed to examine behavioral evidence for and the neural representation of indirect controls modulating direct controls of ingestion. This approach relies on the application of meal-related direct control stimuli in combination with central nervous system administration of hypothalamically active neuropeptide factors demonstrated to alter food intake. To specifically address whether centrally administered leptin would alter the potency of peripherally administered CCK within the context of an individual meal, Emond et al.85 examined the effects of exogenous CCK administration in combination with third cerebroventricular administration of leptin. After a 6-h daytime food-deprivation period, CCK alone suppressed 30-min glucose intake, leptin alone did not, and the combination of CCK and leptin produced significantly greater suppression than either CCK or leptin. The greater the dose of central leptin, the greater the reduction in intake when administered in combination with CCK. Neuronal activation as measured by c-fos–like immunoreactivity was also examined in both brainstem AP and NTS and in the forebrain hypothalamic paraventricular nucleus. CCK alone stimulated NTS c-fos activation, leptin alone stimulated paraventricular nuclear c-fos activation, and the behaviorally effective combination of CCK and leptin showed dramatic increases in c-fos–like immunoreactivity in the NTS, AP, and paraventricular nucleus. In complementary single-cell neurophysiologic studies, we reported that gastric load-sensitive neurons in the NTS had increased responses to gastric loads after the third ventricular leptin administration.86 Furthermore, Zhang et al.87 recently reported that electrical stimulation of the paraventricular nucleus decreases the baseline activity of gastric and duodenal distention-sensitive neu-

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rons in the rat NTS and in a few cases altered their responses to distention.* These interactions begin to provide behavioral and neural evidence of indirect controls modulating the direct controls of meal size at the level of the caudal brainstem. In addition, the cumulative effects of such interactions may have important consequences for daily food intake and body-weight regulation. Matson and colleagues89,90 demonstrated potent and long-lasting synergistic effects of CCK and leptin in reducing daily food intake and body weight to a greater degree than the effect of either peptide alone. Further experiments designed 1) to reveal the structural and functional circuitry of the more rostral levels of the gut– brain axis, in particular reciprocal projections between the NTS and hypothalamic forebrain regions, and 2) to identify and characterize the nature of the interactions between the NTS and the hypothalamus will become critical in establishing ways for indirect controls to modulate direct controls.

CONCLUDING REMARKS This review examines significant new neuroanatomic, neurophysiologic, molecular, and behavioral data demonstrating an important role for gut vagal afferent signals in the negative-feedback control of meal size. The strategic and wide-ranging vagal afferent innervation of the upper gut, the vagal afferent neurophysiologic sensitivity to mechanical, chemical, and gut-peptide meal-related stimuli, the significant patterns of gut– brain axis neuronal activation produced by meals and by meal-related stimulation of the upper gastrointestinal tract, and the behavioral consequences of novel selective gut afferent vagotomies provide converging lines of evidence in support of this role. There is now a sufficient phenomenologic database from these individual perspectives to begin to examine the processing and integration of vagal gut and brain meal-related signals in the context of the indirect controls of meal size. Three current areas of relative ignorance concerning the structure and function of the gut– brain axis demand experimental attention. First, there is a striking lack of information concerning the role of nonvagal, splanchnic extrinsic gut afferent innervation in the control of food intake. Second, given the apparent anatomic connectivity between enteric neurons and extrinsic gut afferents,9 the interplay of intrinsic and extrinsic afferent gut innervation seems a ripe target for further investigation. Third, the examination of interactions between the forebrain and hindbrain and, more specifically, forebrain modulation of gut-brain afferent signals represent a critical new research direction geared toward understanding the physiologic controls of meal size. Successful experimental approaches to each of these three areas are currently available and should continue to elucidate the contributions of gut vagal afferent signals to the control of food intake.

REFERENCES 1. Yox DP, Stokesberry H, Ritter RC. Vagotomy attenuates suppression of sham feeding induced by intestinal nutrients. Am J Physiol 1991;260:R503 2. Greenberg D, Smith GP, Gibbs J. Intraduodenal infusions of fats elicit satiety in sham-feeding rats. Am J Physiol 1990;259:R110 3. Phillips RJ, Powley TL. Gastric volume detection after selective vagotomies in rats. Am J Physiol 1998;274:R1626 3a.Phillips RJ, Powley TL. Gastric volume rather than nutrient content inhibits food intake. Am J Physiol 1996;271:R766

*The principle of forebrain modulation of NTS visceral afferent signaling extends to nongut viscera as well. Duan et al.88 showed that electrical stimulation of the paraventricular nucleus modulates the NTS processing of baroreflex afferent input, thereby inhibiting the expression of pressor responses.

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4. Prechtl JC, Powley TL. The fiber composition of the abdominal vagus of the rat. Anat Embryol 1990;181:101 5. Altschuler SM, Bao X, Bieger D, et al. Viscerotopic representation of the upper alimentary tract in the rat: sensory ganglia and nuclei of the solitary and spinal trigeminal tracts. J Comp Neurol 1989;283:248 6. Norgren R, Smith GP. Central distribution of subdiaphragmatic vagal branches in the rat. J Comp Neurol 1988;273:207 7. Fox EA, Powley TL. Longitudinal columnar organization within the dorsal motor nucleus represents separate branches of the abdominal vagus. Brain Res 1985; 341:269 8. Powley TL, Berthoud H-R, Fox EA, Laughton W. The dorsal vagal complex forms a sensory-motor lattice: the circuitry of gastrointestinal reflexes. In: Ritter S, Ritter RC, Barnes CD, eds. Neuroanatomy and physiology of abdominal vagal afferents. Ann Arbor: CRC Press, 1992:57 9. Berthoud H-R, Powley TL. Vagal afferent innervation of the rat fundic stomach: morphological characterization of the gastric tension receptor. J Comp Neurol 1992;319:261 10. Berthoud HR, Jedrzejewska A, Powley TL. Simultaneous labeling of vagal innervation of the gut and afferent projections from the visceral forebrain with dil injected into the dorsal vagal complex in the rat. J Comp Neurol 1990;301:65 11. Berthoud HR, Kressel M, Neuhuber WL. An anterograde tracing study of the vagal innervation of rat liver, portal vein, and biliary system. Anat Embryol 1992;186:431 12. Kressel M, Berthoud H-R, Neuhuber WL. Vagal innervation of the rat pylorus; an anterograde study using carbocyanine dyes and laser scanning confocal microscopy. Cell Tissue Res 1994;275:109 13. Wang FB, Powley TL. Topographic inventories of vagal afferents in gastrointestinal muscle. J Comp Neurol 2000;421:302 14. Berthoud H-R, Kressel M, Raybould HE, Neuhuber WL. Vagal sensors in the rat duodenal mucosa: distribution and structure as revealed by in vivo Di-I tracing. Anat Embryol 1995;191:203 15. Berthoud HR, Patterson LM. Anatomical relationship between vagal afferent fibers and CCK-immunoreactive entero-endocrine cells in the rat small intestinal mucosa. Acta Anat 1996;156:123 16. Gibbs J, Young RC, Smith GP. cholecystokinin decreases food intake in rats. J Comp Physiol Psychol 1973;84:488 17. Liddle RA. Regulation of cholecystokinin synthesis and secretion in rat intestine. J Nutr (suppl) 1994;124:1308S 18. Moran TH, Ameglio PJ, Schwartz GJ, et al. Blockade of type A, not type B CCK receptors attenuates satiety actions of exogenous and endogenous CCK. Am J Physiol 1992;262:R46 19. Moran TH, Smith GP, Hosetetler AM, et al. Transport of cholcesytokinin (CCK) binding sites in subdiaphragmatic vagal branches. Brain Res 1987;415:149 20. Moran TH, Norgren R, Crosby RJ, et al. Central and peripheral vagal transport of choelcystokinin occurs in afferent fibers. Brain Res 1990;526:95 21. Sternini C, Wong H, Pham T, et al., Expression of cholecystokinin A receptors in neurons innervating the rat stomach and intestine. Gastroenterology 1999;117: 1136 22. Berthoud H-R, Carlson NR, Powley TL. Topography of efferent vagal innervation of the rat gastrointestinal tract. Am J Physiol 1991;260:R200 22a.Phillips RJ, Baronowsky EA, Powley TL. Afferent innervation of the gastrointestinal tract smooth muscle by the hepatic branch of the vagus. J Comp Neurol 1997;384:248 23. Davison JS, Clarke GD. Mechanical properties and sensitivity to CCK of vagal gastric slowly adapting mechanoreceptors. Am J Physiol 1988;255:G55 24. Schwartz GJ, McHugh PR, Moran TH. Integration of vagal afferent responses to gastric loads and cholecystokinin in rats. Am J Physiol 1991;261:R64 25. Blackshaw LA, Grundy D. Effects of cholecystokinin (CCK-8) on two classes of gastroduodenal vagal afferent fibre. J Auton Nerv Syst 1990;31:191 26. Mathis C, Moran TH, Schwartz GJ. Load-sensitive rat gastric vagal afferents encode volume but not gastric nutrients. Am J Physiol 1998;274:R280 27. Schwartz GJ, Moran TH. Duodenal nutrient exposure elicits nutrient-specific gut motility and vagal afferent signals in rat. Am J Physiol 1998;274:R1236 28. Schwartz GJ, Tougas G, Moran TH. Integration of vagal afferent responses to duodenal loads and exogenous CCK. Peptides 1995;16:707 29. Richards W, Hillsley K, Eastwood C, et al. Sensitivity of vagal mucosal afferents to cholecystokinin and its role in afferent signal transduction in the rat. J Physiol (Lond) 1996;497:473 30. Kurosawa M, Bucinskaite V, Taniguchi T, et al. Response of the gastric vagal afferent activity to cholecystokinin in rats lacking type A cholecystokinin receptors. J Auton Nerv Syst 1999;75:51 31. Cox JE, Randich A. CCK-8 activates hepatic vagal C-fiber afferents. Brain Res 1997;776:189 32. Eastwood C, Maubach K, Kirkup AJ, et al. The role of endogenous cholecystokinin in the sensory transduction of luminal nutrient signals in the rat jejunum. Neurosci Lett 1998;254:145

Nutrition Volume 16, Number 10, 2000 33. Lewis LD, Williams JA. Regulation of cholecystokinin secretion by food, hormones, and neural pathways in the rat. Am J Physiol 1990;258:G512 33a.Li Y, Zhu J, Owyang C. Electrical physiological evidence for high and low affinity vagal CCK-A receptors. Am J Physiol 1999;277:G469 34. Schwartz GJ, McHugh PR, Moran TH. Pharmacological dissociation of responses to CCK and gastric loads in rat mechanosensitive vagal afferents. Am J Physiol 1994;267:R303 34a.Weatherford SC, Laughton WB, Salabarria J, et al. CCK satiety is differentially mediated by high- and low-affinity CCK receptors in mice and rats. Am J Physiol 1993;264:R244 35. Randich A, Tyler WJ, Cox JE, et al. Responses of celiac and cervical vagal afferents to infusions of lipids in the jejunum or ileum of the rat. Am J Physiol 2000;278:R34 36. Sakata Y, Fujimoto K, Ogata S-I, et al. Postabsorptive factors are important for satiation in rats after a liquid meal. Am J Physiol 1996;271:G438 37. Meyer JH, Hlinka M, Khatibi A, et al. Role of small intestine in caloric compensations to oil premeals in rats. Am J Physiol 1998;275:R1320 38. Feinle C, D’Amato M, Read NW. Cholecystokinin-A receptors modulate gastric sensory and motor responses to gastric distension and duodenal lipid. Gastroenterology 2000;110:1379 39. Matzinger D, Gutzwiller J-P, Drewe J, et al. Inhibition of food intake in response to intestinal lipid is mediated by cholecystokinin in humans. Am J Physiol 1999;277:R1718 40. Moran TH, McHugh PR. Cholecystokinin suppresses food intake by inhibiting gastric emptying. Am J Physiol 1982;242:R491 41. Schwartz GJ, Netterville LA, McHugh PR, et al. Gastric loads potentiate inhibition of food intake produced by a cholecystokinin analogue. Am J Physiol 1991;261:R1141 42. Muurahainen NE, Kissileff HR, Lachaussee J, et al. Effect of a soup preload on reduction of food intake by cholecystokinin in humans. Am J Physiol 1991;260: R672 43. Gutzwiller J-P, Drewe J, Ketterer S, et al. Interactions between CCK and a preload on reduction of food intake is mediated by CCK-A receptors in humans. Am J Physiol 2000;279:R189 44. Zhang X, Fogel R, Renehan WE. Relationships between the morphology and function of gastric- and intestine-sensitive neurons in the nucleus of the solitary tract. J Comp Neurol 1995;363:37 45. Raybould HE, Gayton RJ, Dockray GJ. CNS effects of circulating CCK8: involvement of brainstem neurones responding to gastric distension. Brain Res 1985;342:187 46. Fraser KA, Davison JS. Meal-induced c-fos expression in brainstem is not dependent on cholecystokinin release. Am J Physiol 1993;265:R235 47. Rinaman L, Baker EA, Hoffman GE, Stricker EM, Verbalis JG. Medullary c-Fos activation in rats after ingestion of a satieting meal. Am J Physiol 1988;275:R262 48. Fraser KA, Raizada E, Davison JS. Oral–pharyngeal– esophageal and gastric cues contribute to meal-induced c-fos expression. Am J Physiol 1995;268:R223 49. Willing AE, Berthoud H-R. Gastric distension-induced c-fos expression in catecholaminergic neurons of rat dorsal vagal complex. Am J Physiol 1997;272:R59 50. Zittel TT, DeGioggio R, Sternini C, Raybould HE. Fos protein expression in the nucleus of the solitary tract in response to intestinal nutrients in awake rats. Brain Res 1994;663:266 51. Mo¨nnikes H, Lauer G, Bauer C, et al. Pathways of Fos expression in locus ceruleus, dorsal vagal complex, and PVN in response to intestinal lipid. Am J Physiol 1997;273:R2059 52. Phifer CB, Berthoud H-R. Duodenal nutrient infusions differentially affect sham feeding and Fos expression in rat brain stem. Am J Physiol 1998;274:R1275 53. Fraser KA, Davison JS. Cholecystokinin-induced c-fos expression in the rat brain stem is influenced by vagal nerve integrity. Exp Physiol 1992;77:225 54. Rinaman L, Verbalis JG, Stricker EM, Hoffman GE. Distribution and neurochemical phenotypes of caudal medullary neurons activated to express cFos following peripheral administration of cholecystokinin. J Comp Neurol 1993; 338:475 54a.Zittel TT, Glatzle J, Kreis ME, et al. C-fos protein expression in the nucleus of the solitary tract correlates with cholecystokinin dose injected and food intake in rats. Brain Res 1999;846:1 55. Hansen MK, Kapas L, Fang J, et al. Cafeteria diet-induced sleep is blocked by subdiaphragmatic vagotomy in rats. Am J Physiol 1998;274:R168 56. Antin J, Gibbs J, Holt J, Young RC, Smith GP. Cholecystokinin elicits the complete behavioral sequence of satiety in rats. J Comp Physiol Psychol 1975; 89:784 57. Smith GP, Jerome C, Cushin BJ, et al. Abdominal vagotomy blocks the satiety effect of cholecystokinin in the rat. Science 1981;213:1036 58. Norgren R, Smith GP. A method for selective section of vagal afferent or efferent axons in the rat. Am J Physiol 1994;267:R1136 59. Smith GP, Jerome C, Norgren R. Afferent axons in abdominal vagus mediate satiety effect of cholecystokinin in rats. Am J Physiol 1985;249:R638

Nutrition Volume 16, Number 10, 2000 60. Walls EK, Wang FB, Holst M-C, et al., Selective vagal rhizotomies: a new dorsal surgical approach used for intestinal deafferentations. Am J Physiol 1995;269: R1279 61. Walls EK, Phillips RJ, Wang FB, et al. Suppression of meal size by intestinal nutrients is eliminated by celiac vagal deafferentation. Am J Physiol 1995;269: R1410 62. Schwartz GJ, Salorio CF, Skoglund C, et al. Gut vagal afferent lesions increase meal size but do not block gastric preload-induced feeding suppression. Am J Physiol 1999;276:R1623 63. Ritter S, Dinh TT. Capsaicin-induced neuronal degeneration; silver impregnation of cell bodies, axons, and terminals in the central nervous system of the adult rat. J Comp Neurol 1988;271:79 64. Jagger A, Grahn J, Ritter RC. Reduced vagal sensory innervation of the small intestinal myenteric plexus following capsaicin treatment of adult rats. Neurosci Lett 1997;236:103 65. Curtis KS, Stricker EM. Enhanced fluid intake by rats after capsaicin treatment. Am J Physiol 1997;272:R704 66. Chavez M, Kelly L, York DA, et al. Chemical lesion of visceral afferents causes transient overconsumption of unfamiliar high-fat diets in rats. Am J Physiol 1997;272:R1657 67. Yox DP, Ritter RC. Capsaicin attenuates suppression of sham feeding induced by intestinal nutrients. Am J Physiol 1988;255:R569 68. Lucas F, Sclafani A. Capsaicin attenuates feeding suppression but not reinforcement by intestinal nutrients. Am J Physiol 1996;270:R1059 69. Mercer JG, Farningham DA, Lawrence CB. Effect of neonatal capsaicin treatment on cholecystokinin-(CCK8) satiety and axonal transport of CCK binding sites in the rat vagus nerve. Brain Res 1992;569:311 70. Michaud D, Anisman H, Merali Z. Capsaicin-sensitive fibers are required for the anorexic action of systemic but not central bombesin. Am J Physiol 1999;276: R1617 71. Schwartz GJ, Moran TH, White WO, Ladenheim EE. Relationships between gastric motility and gastric vagal afferent responses to CCK and GRP in rats differ. Am J Physiol 1997;272:R1726 72. Bernier SS, South EH, Ritter RC. Capsaicin-induced damage to subdiaphragmatic vagal fibers and central afferent terminals from the stomach. Soc Neurosci Abstr 1987; v.13, p. 529 73. South EH, Ritter RC. Capsaicin application to central or peripheral vagal fibers attenuates CCK satiety. Peptides 1988;9:601 74. Tamura CS, Ritter RC. Intestinal capsaicin transiently attenuates suppression of sham feeding by oleate. Am J Physiol 1994;267:R561

Meal-Elicited Gut Vagal Afferent Signals Limit Meal Size

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75. Berthoud HR, Patterson LM, Willing AE, et al. Capsaicin-resistant vagal afferent fibers in the rat gastrointestinal tract: anatomical identification and functional integrity. Brain Res 1997;746:195 76. Blackshaw LA, Page AJ, Partosoedarso ER. Acute effects of capsaicin on gastrointestinal vagal afferents. Neuroscience 2000;96:407 77. Caterina MJ, Schumacher MA, Tominaga M. The capsaicin receptor: a hatactivated ion channel in the pain pathway. Nature 1997;389:816 78. Tominaga M, Cateriana MJ, Malmbergm AB, et al., the cloned capsaicin receptor integrates multiple pain-producing stimuli. Neuron 1998;21:531 79. Caterina MJ, Leffler A, Malmberg AB, et al. Impaired nociception and pain sensation in mice lacking the capsaicin receptor. Science 2000;288:306 80. Holzer P. Neural injury, repair, and adaptation in the GI tract II. The elusive action of capsaicin on the vagus nerve. Am J Physiol 1998;275:G8 80a.Smith GP. The direct and indirect controls of meal size. Neurosci Biobehav Rev 1996;20:41 81. Schwartz MW, Seeley RJ, Campfield LA, et al. Identification of targets of leptin action in the rat hypothalamus. J Clin Invest 1996;98:1101 82. Eckel LA, Langhans W, Kahler A, et al. Chronic administration of OB protein decreases food intake by selectively reducing meal size in female rats. Am J Physiol 1998;275:R186 83. Toth Z, Gallatz K, Fodor M, et al. Decussations of the descending paraventricular pathways to the brainstem and spinal cord autonomic centers. J Comp Neurol 1999;414:255 84. Grill HJ, Kaplan JM. Sham feeding in intact and chronic decerebrate rats. Am J Physiol 1992;262:R1070 85. Emond M, Schwartz GJ, Ladenheim EE, Moran TH. Leptin modulates behavioral and neural responsivity to CCK. Am J Physiol 1999;276:R1545 86. Schwartz GJ, Moran TH. NPY and leptin produce opposing modulatory actions on meal-related negative feedback signals in the rat nucleus of the solitary tract. Soc Neurosci Abstr 1998; v. 24, p. 13 87. Zhang X, Fogel R, Renehan WE. Stimulation of the paraventricular nucleus modulates the activity of gut-sensitive neurons in the vagal complex. Am J Physiol 1999;277:G79 88. Duan YF, Kopin IJ, Goldstein DS. Stimulation of the paraventricular nucleus modulates firing of neurons in the nucleus of the solitary tract. Am J Physiol 1999;277:R403 89. Matson CA, Reid DF, Cannon TA, Ritter RC. Cholecystokinin and leptin act synergistically to reduce body weight. Am J Physiol 2000;278:R882 90. Matson CA, Ritter RC. Long-term CCK-leptin synergy suggests a role for CCK in the regulation of body weight. Am J Physiol 1999;276:R1038