Does gastric bypass surgery change body weight set point?

The relatively stable body weight during adulthood is attributed to a homeostatic regulatory mechanism residing in the brain which uses feedback from the body to control energy intake and expenditure. This mechanism guarantees that if perturbed up or down by design, body weight will return to pre-perturbation levels, defined as the defended level or set point. The fact that weight re-gain is common after dieting suggests that obese subjects defend a higher level of body weight. Thus, the set point for body weight is flexible and likely determined by the complex interaction of genetic, epigenetic and environmental factors. Unlike dieting, bariatric surgery does a much better job in producing sustained suppression of food intake and body weight, and an intensive search for the underlying mechanisms has started. Although one explanation for this lasting effect of particularly Roux-en-Y gastric bypass surgery (RYGB) is simple physical restriction due to the invasive surgery, a more exciting explanation is that the surgery physiologically reprograms the body weight defense mechanism. In this non-systematic review, we present behavioral evidence from our own and other studies that defended body weight is lowered after RYGB and sleeve gastrectomy. After these surgeries, rodents return to their preferred lower body weight if over- or underfed for a period of time, and the ability to drastically increase food intake during the anabolic phase strongly argues against the physical restriction hypothesis. However, the underlying mechanisms remain obscure. Although the mechanism involves central leptin and melanocortin signaling pathways, other peripheral signals such as gut hormones and their neural effector pathways likely contribute. Future research using both targeted and non-targeted 'omics' techniques in both humans and rodents as well as modern, genetically targeted, neuronal manipulation techniques in rodents will be necessary.

Appetite and body weight regulation after bariatric surgery

Bariatric surgery continues to be remarkably efficient in treating obesity and type 2 diabetes mellitus and a debate has started whether it should remain the last resort only or also be used for the prevention of metabolic diseases. Intense research efforts in humans and rodent models are underway to identify the critical mechanisms underlying the beneficial effects with a view towards non-surgical treatment options. This non-systematic review summarizes and interprets some of this literature, with an emphasis on changes in the controls of appetite. Contrary to earlier views, surgery-induced reduction of energy intake and subsequent weight loss appear to be the main drivers for rapid improvements of glycaemic control. The mechanisms responsible for suppression of appetite, particularly in the face of the large weight loss, are not well understood. Although a number of changes in food choice, taste functions, hedonic evaluation, motivation and self-control have been documented in both humans and rodents after surgery, their importance and relative contribution to diminished appetite has not yet been demonstrated. Furthermore, none of the major candidate mechanisms postulated in mediating surgery-induced changes from the gut and other organs to the brain, such as gut hormones and sensory neuronal pathways, have been confirmed yet. Future research efforts should focus on interventional rather than descriptive approaches in both humans and rodent models.

Roux-en-Y gastric bypass surgery increases number but not density of CCK-, GLP-1-, 5-HT-, and neurotensin-expressing enteroendocrine cells in rats

BACKGROUND

Roux-en-Y gastric bypass (RYGB) surgery is very effective in reducing excess body weight and improving glucose homeostasis in obese subjects. Changes in the pattern of gut hormone secretion are thought to play a major role, but the mechanisms leading to both changed hormone secretion and beneficial effects remain unclear. Specifically, it is not clear whether changes in the number of hormone-secreting enteroendocrine cells, or changes in the releasing stimuli, or both, are important.

METHODS

We estimated numbers of enteroendocrine cells after immunohistochemical staining in fixed tissue samples from rats at 10-11 months after RYGB.

KEY RESULTS

Numbers of glucagon-like peptide-1 (GLP-1) (L-cells, co-expressing peptide YY (PYY)), cholecystokinin (CCK), neurotensin, and 5-HT-immunoreactive cells were significantly increased in the Roux and common limbs, but not the biliopancreatic limb in RYGB rats compared with sham-operated, obese rats fed high-fat diet, and chow-fed controls. This increase was mostly accounted for by general hyperplasia of all intestinal wall layers of the nutrient-perfused Roux and common limbs, and less to increased density of expression. The number of ghrelin cells in the bypassed stomach was not different among the three groups.

CONCLUSIONS & INFERENCES

The findings suggest that the number of enteroendocrine cells increases passively as the gut adapts, and that the increased total number of L- and I-cells is likely to contribute to the higher circulating levels of GLP-1, PYY, and CCK, potentially leading to suppression of food intake and stimulation of insulin secretion. Whether changes in releasing stimuli also contribute to altered circulating levels will have to be determined in future studies.

Food reward functions as affected by obesity and bariatric surgery

Roux-en-Y gastric bypass surgery (RYGB) remains to be the most effective long-term treatment for obesity and its associated comorbidities, but the specific mechanisms involved remain elusive. Because RYGB patients appear to no longer be preoccupied with thoughts about food and are satisfied with much smaller meals and calorically dilute foods, brain reward mechanisms could be involved. Just as obesity can produce maladaptive alterations in reward functions, reversal of obesity by RYGB could normalize these changes or even further reset the food reward system through changes in gut hormone secretion, aversive conditioning and/or secondary effects of weight loss. Future studies with longitudinal assessments of reward behaviors and their underlying neural circuits before and after surgery will be necessary to uncover the specific mechanisms involved. Such new insights could be the base for future 'knifeless' pharmacological and behavioral approaches to obesity.

Could the mechanisms of bariatric surgery hold the key for novel therapies? report from a Pennington Scientific Symposium

Bariatric surgery is the most effective method for promoting dramatic and durable weight loss in morbidly obese subjects. Furthermore, type 2 diabetes is resolved in over 80% of patients. The mechanisms behind the amelioration in metabolic abnormalities are largely unknown but may be due to changes in energy metabolism, gut peptides and food preference. The goal of this meeting was to review the latest research to better understand the mechanisms behind the 'magic' of bariatric surgery. Replication of these effects in a non-surgical manner remains one of the ultimate challenges for the treatment of obesity and diabetes. Promising data on energy metabolism, gastrointestinal physiology, hedonic response and food intake were reviewed and discussed.

Roux-en-Y gastric bypass surgery changes food reward in rats

Context

Roux-en-Y gastric bypass surgery (RYGB) is currently the most effective treatment for morbid obesity and clinical studies suggest that RYGB patients change food preferences and the desire to eat.

Objective

To examine hedonic reactions to palatable foods and food choice behavior in an established rat model of Roux-en-Y gastric bypass surgery (RYGB).

Methods and Design

Male Sprague-Dawley rats and selected line obesity-prone rats that were rendered obese on a high-fat diet underwent RYGB or sham surgery and were tested for ‘liking’ and ‘wanting’ of palatable foods at different caloric densities 4 – 6 months after surgery.

Results

Compared with sham-operated (obese) and age-matched lean control rats, RYGB rats of both models exhibited more positive orofacial responses to low concentrations of sucrose but fewer to high concentrations. These changes in ‘liking’ by RYGB rats were translated into a shift of the concentration-response curve in the brief access test, with more vigorous licking of low concentrations of sucrose and corn oil, but less licking of the highest concentrations. The changes in hedonic evaluation also resulted in lower long-term preference/acceptance of high-fat diets compared with sham-operated (obese) rats. Furthermore, the reduced ‘wanting’ of a palatable reward in the incentive runway seen in sham-operated obese SD rats was fully restored after RYGB to the level found in lean control rats.

Conclusions

The results suggest that RYGB leads to a shift in hedonic evaluation, favoring low over high calorie foods and restores obesity-induced alterations in ‘liking’ and ‘wanting’. It remains to be determined whether these effects are simply due to weight loss or specific changes in gut-brain communication. Given the emerging evidence for modulation of cortico-limbic brain structures involved in reward mechanisms by gut hormones, RYGB-induced changes in the secretion of these hormones could potentially be mediating these effects.

Appetite control and energy balance regulation in the modern world: reward-driven brain overrides repletion signals

Powerful biological mechanisms evolved to defend adequate nutrient supply and optimal levels of body weight/adiposity. Low levels of leptin indicating food deprivation and depleted fat stores have been identified as the strongest signals to induce adaptive biological actions such as increased energy intake and reduced energy expenditure. In concert with other signals from the gut and metabolically active tissues, low leptin levels trigger powerful activation of multiple peripheral and brain systems to restore energy balance. It is not just neurons in the arcuate nucleus, but many other brain systems involved in finding potential food sources, smelling and tasting food, and learning to maximize rewarding effects of foods, that are affected by low leptin. Food restriction and fat depletion thus lead to a 'hungry' brain, preoccupied with food. By contrast, because of less (adaptive thrifty fuel efficiency) or lost (lack of predators) evolutionary pressure, the upper limits of body weight/adiposity are not as strongly defended by high levels of leptin and other signals. The modern environment is characterized by the increased availability of large amounts of energy-dense foods and increased presence of powerful food cues, together with minimal physical procurement costs and a sedentary lifestyle. Much of these environmental influences affect cortico-limbic brain areas concerned with learning and memory, reward, mood and emotion. Common obesity results when individual predisposition to deal with a restrictive environment, as engraved by genetics, epigenetics and/or early life experience, is confronted with an environment of plenty. Therefore, increased adiposity in prone individuals should be seen as a normal physiological response to a changed environment, not in the pathology of the regulatory system. The first line of defense should ideally lie in modifications to the environment and lifestyle. However, as such modifications will be slow and incomplete, it is equally important to gain better insight into how the brain deals with environmental stimuli and to develop behavioral strategies to better cope with them. Clearly, alternative therapeutic strategies such as drugs and bariatric surgery should also be considered to prevent or treat this debilitating disease. It will be crucial to understand the functional crosstalk between neural systems responding to metabolic and environmental stimuli, i.e. crosstalk between hypothalamic and cortico-limbic circuitry.

Vagal and hormonal gut-brain communication: from satiation to satisfaction

Studying communication between the gut and the brain is as relevant and exciting as it has been since Pavlov's discoveries a century ago. Although the efferent limb of this communication has witnessed significant advances, it is the afferent, or sensory, limb that has recently made for exciting news. It is now clear that signals from the gut are crucial for the control of appetite and the regulation of energy balance, glucose homeostasis, and more. Ghrelin, discovered just a few years ago, is the first gut hormone that increases appetite, and it may be involved in eating disorders. The stable analogue of glucagon-like peptide-1 has rapidly advanced to one of the most promising treatment options for type-2 diabetes. Changes in the signalling patterns of these and other gut hormones best explain the remarkable capacity of gastric bypass surgery to lower food intake and excess body weight. Given the enormous societal implications of the obesity epidemic, these are no small feats. Together with the older gut hormone cholecystokinin and abundant vagal mechanosensors, the gut continuously sends information to the brain regarding the quality and quantity of ingested nutrients, not only important for satiation and meal termination, but also for the appetitive phase of ingestive behaviour and the patterning of meals within given environmental constraints. By acting not only on brainstem and hypothalamus, this stream of sensory information from the gut to the brain is in a position to generate a feeling of satisfaction and happiness as observed after a satiating meal and exploited in vagal afferent stimulation for depression.

Melanin concentrating hormone innervation of caudal brainstem areas involved in gastrointestinal functions and energy balance

Neural signaling by melanin-concentrating hormone and its receptor (SLC-1) has been implicated in the control of energy balance, but due to the wide distribution of melanin-concentrating hormone-containing fibers throughout the neuraxis, its critical sites of action for a particular effect have not been identified. The present study aimed to anatomically and functionally characterize melanin-concentrating hormone innervation of the rat caudal brainstem, as this brain area plays an important role in the neural control of ingestive behavior and autonomic outflow. Using retrograde tracing we demonstrate that a significant proportion (5-15%) of primarily perifornical and far-lateral hypothalamic melanin-concentrating hormone neurons projects to the dorsal vagal complex. In the caudal brainstem, melanin-concentrating hormone-ir axon profiles are distributed densely in most areas including the nucleus of the solitary tract, dorsal motor nucleus of the vagus, and sympathetic premotor areas in the ventral medulla. Close anatomical appositions can be demonstrated between melanin-concentrating hormone-ir axon profiles and tyrosine hydroxylase, GABA, GLP-1, NOS-expressing, and nucleus of the solitary tract neurons activated by gastric nutrient infusion. In medulla slice preparations, bath application of melanin-concentrating hormone inhibited in a concentration-dependent manner the amplitude of excitatory postsynaptic currents evoked by solitary tract stimulation via a pre-synaptic mechanism. Fourth ventricular administration of melanin-concentrating hormone (10 microg) in freely moving rats decreased core body temperature but did not change locomotor activity and food and water intake. We conclude that the rich hypothalamo-medullary melanin-concentrating hormone projections in the rat are mainly inhibitory to nucleus of the solitary tract neurons, but are not involved in the control of food intake. Projections to ventral medullary sites may play a role in the inhibitory effect of melanin-concentrating hormone on energy expenditure.

Neuroanatomy of extrinsic afferents supplying the gastrointestinal tract

Here we discuss the neuroanatomy of extrinsic gastrointestinal (GI) afferent neurones, the relationship between structure and function and the role of afferents in disease. Three pathways connect the gut to the central nervous system: vagal afferents signal mainly from upper GI regions, pelvic afferents mainly from the colorectal region and splanchnic afferents from throughout. Vagal afferents mediate reflex regulation of gut function and behaviour, operating mainly at physiological levels. There are two major functional classes - tension receptors, responding to muscular contraction and distension, and mucosal receptors. The function of vagal endings correlates well with their anatomy: tracing studies show intramuscular arrays (IMAs) and intraganglionic laminar endings (IGLEs); IGLEs are now known to respond to tension. Functional mucosal receptors correlate with endings traced to the lamina propria. Pelvic afferents serve similar functions to vagal afferents, and additionally mediate both innocuous and noxious sensations. Splanchnic afferents comprise mucosal and stretch-sensitive afferents with low thresholds in addition to high-threshold serosal/mesenteric afferents suggesting diverse roles. IGLEs, probably of pelvic origin, have been identified recently in the rectum and respond similarly to gastric vagal IGLEs. Gastrointestinal afferents may be sensitized or inhibited by chemical mediators released from several cell types. Whether functional changes have anatomical correlates is not known, but it is likely that they underlie diseases involving visceral hypersensitivity.

Video-based spatio-temporal maps for analysis of gastric motility in vitro: effects of vagal stimulation in guinea-pigs

Our aim was to evaluate topographically specific gastric motility changes induced by graded vagal activation. A recently developed method of constructing spatio-temporal maps of motility from video movies was adapted to the in vitro perfused guinea-pig stomach with an intact vagal nerve supply. In the unstimulated preparation, spontaneous activity was low or absent. Bilateral vagal stimulation with frequencies as low as 0.2 Hz triggered weak anally, and in some cases orally, propagating antral contractions at rates of about 5-6 min-1. Upon stimulation with higher frequencies, antral contractions increased significantly in length (starting more proximally) and amplitude, and produced large pressure peaks of up to 25 hPa, with maximal effects at 2-4 Hz. In contrast, the speed of propagation and the interval between peristaltic waves did not change with vagal stimulation at any frequency. Vagal stimulation also produced a significant and frequency-dependent enlargement of the fundus with a maximal effect at 4 Hz. It is concluded that a very low tonic vagal activity is apparently necessary and sufficient to express basic antral motility, while more sustained vagal activity is necessary for high-amplitude gastric contractions and significant sustained fundic relaxation. The constant interval between propagating contractions supports the concept that vagal input impinges on intrinsic enteric neural circuits that have a modulatory role in the myogenic mechanism underlying slow-wave peristalsis, rather than directly on gastric musculature.

Food-related gastrointestinal signals activate caudal brainstem neurons expressing both NMDA and AMPA receptors

Vagal mechano- and chemosensors in the gastrointestinal tract and the portal-hepatic axis signaling the arrival of nutrients are major determinants of the satiation process. Although glutamate and its various receptor subtypes have been shown to transmit gustatory and cardiovascular sensory information at the level of the solitary nucleus (nucleus tractus solitarius; NTS), their involvement in the transmission of gastrointestinal satiety signals is not clear. Gastrointestinal sensors were stimulated by gastric balloon distension or by intraduodenal infusion of either linoleic acid or glucose in chronically catheterized, non-anesthetized rats, leading to activation of second order neurons in the NTS as detected by c-Fos immunohistochemistry. Subsequent (double)-immunohistochemistry for either NMDA or AMPA glutamate receptors was used to determine the percentage of activated neurons expressing a particular receptor subtype. Gastric distension and duodenal nutrient stimuli produced slightly, but significantly different patterns of c-Fos induction in the dorsal vagal complex. Expression of NMDA receptors, as detected by a NR2ab subunit-specific antibody, was abundant throughout the dorsal medulla. The percentage of neurons in the NTS activated by gastric distension (63.9+/-2.9%), linoleic acid (62.8+/-1.4%), and glucose (64.1+/-1.4%), expressing NMDA receptor was similar. Expression of AMPA receptors, as detected by a GLUR2/3 subunit-specific antibody, was equally abundant throughout the dorsal medulla. Again, the percentage of activated neurons expressing GLUR2/3 was similar for the gastric distension (59.8-65.6%) and duodenal linoleic acid (60.6-67.0%) stimuli, and for the various subnuclei of the NTS. Finally, GLUR1-specific immunoreactivity was much less abundant, with only a small percentage of distension-activated (4.4+/-0.4%) and linoleic acid-activated (5.1+/-0.4%) neurons expressing this receptor subunit. The results suggest a widespread, general involvement of both NMDA and AMPA receptors in primary afferent signal transmission at the level of the NTS, with no differential recruitment of the examined receptor subtypes by the different gastrointestinal sensory stimuli. This may indicate a high degree of convergence among sensory signals, or alternatively, the presence of other transmission systems such as peptides referring sensory specificity to second order neurons.

Immunohistochemical identification of cholecystokinin A receptors on interstitial cells of Cajal, smooth muscle, and enteric neurons in rat pylorus

One of the physiological functions of circulating cholecystokinin (CCK) is in the control of the pyloric sphincter and the subsequent delivery of nutrients to the small intestine. In order to identify the site(s) of action of CCK in the gastropyloric region, we performed immunohistochemistry using an antibody directed to the C-terminal region of the cholecystokinin A receptor (CCKAR). In the rat, cells that display strong CCKAR immunoreactivity and fit the morphological description of interstitial cells of Cajal (ICC) were found in the distal sphincter muscle and in the circular muscle of the proximal duodenum. Double labeling showed that these cells coexpressed vimentin, but that not all vimentin-positive cells expressed CCKAR. Confirmation that the CCKAR-expressing cells were ICC also came from kit double-labeling experiments in mice. In addition to ICC, circular smooth muscle cells at the tip of the comma-shaped sphincter muscle, but not elsewhere, also exhibited strong, membrane-bound CCKAR immunoreactivity. With higher antibody concentrations, the entire circular muscle displayed moderate CCKAR immunoreactivity, suggesting that circular smooth muscle cells express low levels of CCKAR. Select neurons in the myenteric ganglia near the sphincter muscle proper, the distal antrum, and proximal duodenum, as well as a few single neurons in the submucosa, also expressed strong CCKAR immunoreactivity. Finally, CCKAR-immunoreactive ICC and neurons were not specifically related to vagal afferent intramuscular and intraganglionic endings, and vagal afferents themselves did not exhibit any CCKAR immunoreactivity. These results suggest a role for ICC and enteric neurons in the mediation of CCK effects on pyloric sphincter pressure in addition to direct effects of the hormone on circular smooth muscle.

Vagal and spinal mechanosensors in the rat stomach and colon have multiple receptive fields

Mechano- and chemosensitive extrinsic primary afferents innervating the gastrointestinal tract convey important information regarding the state of ingested nutrients and specific motor patterns to the central nervous system via splanchnic and vagal nerves. Little is known about the organization of peripheral receptive sites of afferents and their correspondence to morphologically identified terminal structures. Mechano- and chemosensory characteristics and receptive fields of single vagal fibers innervating the stomach as well as lumbar splanchnic nerves innervating the distal colon were identified using an in vitro perifusion system. Twenty-three (17%) of one-hundred thirty-six vagal units identified were found to have multiple, punctate receptive fields, up to 35 mm apart, and were distributed throughout the stomach. Evidence was based on similarity of generated spike forms, occlusion, and latency determinations. Most responded with brief bursts of activity to mucosal stroking with von Frey hairs (10-200 mg) but not to stretch, and 32% responded to capsaicin (10(-5) M). They were classified as rapidly adapting mucosal receptors. Four (8%) of fifty-three single units recorded from the lumbar splanchnic nerve had more than one, punctate receptive field in the distal colon, up to 40 mm apart. They responded to blunt probing, particularly from the serosal side, and variously to chemical stimulation with 5-hydroxytryptamine and capsaicin. We conclude that a proportion of gastrointestinal mechanosensors has multiple receptive fields and suggest that they integrate mechanical and chemical information from an entire organ, constituting the generalists in visceral sensation.

Fourth ventricular injection of CART peptide inhibits short-term sucrose intake in rats

Expression of CART (cocaine-amphetamine-regulated transcript) in the rat hypothalamus is modulated by nutritional status, and injection of synthetic CART peptide into the forebrain ventricular system suppresses food intake, indicating a possible role in hypothalamic control of energy homeostasis. Its recent identification in cell bodies and central terminals of vagal afferent neurons additionally suggests a role in brainstem mechanisms of meal termination and satiety. We demonstrate here that CART[55-102] (0.2 nmol) suppresses short-term sucrose intake and overnight chow intake in non-food-deprived rats even more when delivered into the fourth ventricle as compared to the lateral ventricle. At the threshold dose (0.02-0.08 nmol) no readily noticeable motor impairments were observed. The results are consistent, but do not prove a site of action within the brainstem, possibly in mediating vagal satiety signals at the level of the NTS.

Vagal-enteric interface: vagal activation-induced expression of c-Fos and p-CREB in neurons of the upper gastrointestinal tract and pancreas

Many gastrointestinal and pancreatic functions are under strong modulatory control by the brain via the vagus nerve. To start identifying location and neurochemical phenotype of the enteric neurons receiving functional vagal efferent input, we activated vagal preganglionic neurons either by electrical or chemical stimulation and examined the expression of phosphorylated CREB (c-AMP response element binding protein) and the immediate early gene c-Fos. There was no spontaneous expression of both markers in the pancreas and considerable spontaneous expression of p-CREB but not Fos in the upper GI-tract. Unilateral electrical vagal stimulation-induced p-CREB was found in 40% of neurons in the head of the pancreas. Fos expression was found in 70-90% of neurons in the esophagus and stomach, in 20-30% of myenteric plexus neurons and 5-15% in submucosal neurons of the proximal duodenum. Double-labeling experiments showed that a majority of pancreatic neurons and about 25-35% of neurons in the stomach and duodenum contain NADPH-diaphorase and that many of these receive functional vagal input. Other neurons that can be vagally activated contain gastrin-releasing peptide or calretinin. Chemical stimulation of the dorsal surface of the caudal brainstem with the stable TRH analog RX77368 resulted in selective activation of vagal efferents with expression of Fos in a small number of gastric myenteric plexus neurons. The results demonstrate the suitability of this method to investigate magnitude and local distribution of vagal input to the enteric nervous system as well as specificity of its neurochemically coded pathways. They represent the first step in the identification of function-specific units of parasympathetic vagal outflow.

Vagal-enteric interface: vagal activation-induced expression of c-Fos and p-CREB in neurons of the upper gastrointestinal tract and pancreas

Many gastrointestinal and pancreatic functions are under strong modulatory control by the brain via the vagus nerve. To start identifying location and neurochemical phenotype of the enteric neurons receiving functional vagal efferent input, we activated vagal preganglionic neurons either by electrical or chemical stimulation and examined the expression of phosphorylated CREB (c-AMP response element binding protein) and the immediate early gene c-Fos. There was no spontaneous expression of both markers in the pancreas and considerable spontaneous expression of p-CREB but not Fos in the upper GI-tract. Unilateral electrical vagal stimulation-induced p-CREB was found in 40% of neurons in the head of the pancreas. Fos expression was found in 70-90% of neurons in the esophagus and stomach, in 20-30% of myenteric plexus neurons and 5-15% in submucosal neurons of the proximal duodenum. Double-labeling experiments showed that a majority of pancreatic neurons and about 25-35% of neurons in the stomach and duodenum contain NADPH-diaphorase and that many of these receive functional vagal input. Other neurons that can be vagally activated contain gastrin-releasing peptide or calretinin. Chemical stimulation of the dorsal surface of the caudal brainstem with the stable TRH analog RX77368 resulted in selective activation of vagal efferents with expression of Fos in a small number of gastric myenteric plexus neurons. The results demonstrate the suitability of this method to investigate magnitude and local distribution of vagal input to the enteric nervous system as well as specificity of its neurochemically coded pathways. They represent the first step in the identification of function-specific units of parasympathetic vagal outflow.

Functional and chemical anatomy of the afferent vagal system

The results of neural tracing studies suggest that vagal afferent fibers in cervical and thoracic branches innervate the esophagus, lower airways, heart, aorta, and possibly the thymus, and via abdominal branches the entire gastrointestinal tract, liver, portal vein, billiary system, pancreas, but not the spleen. In addition, vagal afferents innervate numerous thoracic and abdominal paraganglia associated with the vagus nerves. Specific terminal structures such as flower basket terminals, intraganglionic laminar endings and intramuscular arrays have been identified in the various organs and organ compartments, suggesting functional specializations. Electrophysiological recording studies have identified mechano- and chemo-receptors, as well as temperature- and osmo-sensors. In the rat and several other species, mostly polymodal units, while in the cat more specialized units have been reported. Few details of the peripheral transduction cascades and the transmitters for signal propagation in the CNS are known. Glutamate and its various receptors are likely to play an important role at the level of primary afferent signaling to the solitary nucleus. The vagal afferent system is thus in an excellent position to detect immune-related events in the periphery and generate appropriate autonomic, endocrine, and behavioral responses via central reflex pathways. There is also good evidence for a role of vagal afferents in nociception, as manifested by affective-emotional responses such as increased blood pressure and tachycardia, typically associated with the perception of pain, and mediated via central reflex pathways involving the amygdala and other parts of the limbic system. The massive central projections are likely to be responsible for the antiepileptic properties of afferent vagal stimulation in humans. Furthermore, these functions are in line with a general defensive character ascribed to the vagal afferent, paraventricular system in lower vertebrates.

Capsaicin-treated rats permanently overingest low- but not high-concentration sucrose solutions

The effect of capsaicin-induced chemical ablation of visceral afferents on 1-h liquid sucrose consumption was investigated in food-deprived rats. We first show that although 10% sucrose is permanently overconsumed by capsaicin-treated (CAPs) compared with vehicle-treated (VEHs) control rats, 40% sucrose is only overconsumed during the first but not subsequent 1-h exposures. Furthermore, one group of CAPs lost the overconsumption response at 20% when exposed to progressively increasing sucrose concentrations of 10-40%, and another group recovered the overconsumption response at 10% when exposed to a series of decreasing concentrations. Control rats ingested relatively constant volumes of sucrose over the range of 10, 15, and 20%, resulting in significantly different energy intakes. In contrast, CAPs generally showed a concentration-dependent decrease in volume intake, resulting in relatively constant energy intake. These results suggest that capsaicin-sensitive visceral afferents, likely from gastric distension and other preabsorptive sensors, provide major control over volume ingested. In the absence of these signals, rats initially overconsume, but rapidly learn to use other signals from capsaicin-resistant preabsorptive or postabsorptive sites, to control future intake. This redundant satiety system appears to be sensitive to the osmotic value or caloric content of the unfamiliar food, but only if this is above a threshold of about 15% sucrose.

Functional vagal input to gastric myenteric plexus as assessed by vagal stimulation-induced Fos expression

Immunohistochemical detection of c-Fos expression was used to identify gastric myenteric plexus neurons that receive excitatory input from vagal efferent neurons activated by electrical stimulation of the cervical vagi in anesthetized rats. Vagal stimulation-induced Fos expression increased with higher pulse frequency, so that with 16 Hz (rectangular pulses of 1 mA/0.5 ms for 30 min) approximately 30% and with 48 Hz 90% of all neurons near the lesser curvature were Fos positive. In sham-stimulated rats there was no Fos expression. The percentage of Fos-activated neurons was only slightly smaller (85% with 48 Hz) near the greater curvature. Prior atropine administration (1 mg/kg ip) had little effect on vagal stimulation-induced Fos expression, and in unilaterally stimulated rats there was no Fos expression on the contralateral (noninnervated) side of the stomach, ruling out mediation by gastric motility or secretory responses. However, polysynaptic recruitment of third- and higher-order neurons cannot be ruled out completely. These results support the idea that, at least in the stomach, functional excitatory innervation of myenteric plexus neurons by the efferent vagus is profuse and widespread, refuting the notion of only a few vagal "command neurons."

Conditioned taste aversion produced by inhibitors of fatty acid oxidation in rats

The aversive effects of mercaptoacetate (MA) and methyl palmoxirate (MP) were examined in the present experiment. We used a conditioned taste aversion (CTA) paradigm for Sprague-Dawley rats maintained on a low- or high-fat diet, and determined that MA and MP both produce profound aversions to a novel saccharin solution. Because it is known that the stimulation of food intake brought about by MA administration is blocked by destruction of vagal afferents. we repeated the CTA experiment in control and capsaicin-treated rats. Results show that although the capsaicin-treated rats did not increase food intake after MA administration, the CTA produced by MA remained. Therefore, the neural pathways for the aversive and orexigenic effects of MA are distinct. We conclude that MA and MP are aversive, and that the aversive signal generated by MA does not involve vagal afferents or other fibers damaged by capsaicin.

Functional vagal input to chemically identified neurons in pancreatic ganglia as revealed by Fos expression

The importance of neural elements in the control of both endocrine and exocrine pancreatic secretory functions and their coordination with gastrointestinal, hepatic, and general homeostatic functions is increasingly recognized. To better characterize the vagal efferent input to the pancreas, the capacity of electrical vagal stimulation to induce expression of c-Fos in neurochemically identified neurons of intrapancreatic ganglia was investigated. At optimal stimulation parameters, unilateral stimulation of either the left or right cervical vagus induced Fos expression in approximately 30% of neurons in the head and 10-20% of neurons in the body and tail of the pancreas. There was no Fos expression if no stimulation or stimulation with a distally cut vagus was applied. Large proportions of neurons contained nitric oxide synthase as assessed with NADPH diaphorase histochemistry (88%) and choline acetyltransferase. The proportion of nitrergic and nonnitrergic neurons receiving vagal input was not different. It is concluded that a significant proportion of pancreatic neurons receives excitatory synaptic input from vagal preganglionic axons and that many of these vagal postganglionic neurons can produce nitric oxide and acetylcholine.

Effect of brain stem NMDA-receptor blockade by MK-801 on behavioral and fos responses to vagal satiety signals

To test the possible role of N-methyl-D-aspartate (NMDA) glutamate receptors in the transmission of gastrointestinal satiety signals at the level of the nucleus of the solitary tract (NTS), we assessed the effect of fourth ventricular infusion of the noncompetitive NMDA receptor antagonist MK-801 on short-term sucrose intake and on gastric distension-induced Fos expression in the dorsal vagal complex of unanesthetized rats. MK-801, although not affecting initial rate of intake, significantly increased sucrose intake during the later phase of the meal (10-30 min, 8.9 +/- 1.0 vs. 2.9 +/- 0.8 ml, P < 0.01). In the medial subnucleus of the NTS, the area postrema, and the dorsal motor nucleus, MK-801 did not reduce gastric distension-induced Fos expression and itself did not significantly induce Fos expression. In the dorsomedial, commissural, and gelatinosus subnuclei, MK-801 in itself produced significant Fos expression and significantly reduced (-75%, P < 0.05) the ability of gastric distension to induce Fos expression, assuming an additive model with two separate populations of neurons activated by distension and the blocker. Although these results are consistent with NMDA receptor-mediated glutamatergic transmission of vagal satiety signals in general, they lend limited support for such a role in the transmission of specific gastric distension signals.

Vagal afferents from the uterus and cervix provide direct connections to the brainstem

Previous anatomical studies demonstrated vagal innervation to the ovary and distal colon and suggested the vagus nerve has uterine inputs. Recent behavioral and physiological evidence indicated that the vagus nerves conduct sensory information from the uterus to the brainstem. The present study was undertaken to identify vagal sensory connections to the uterus. Retrograde tracers, Fluorogold and pseudorabies virus were injected into the uterus and cervix. DiI, an anterograde tracer, was injected into the nodose ganglia. Neurectomies involving the pelvic, hypogastric, ovarian and abdominal vagus nerves were performed, and then uterine whole-mounts examined for sensory nerves containing calcitonin gene-related peptide. Nodose ganglia and caudal brainstem sections were examined for the presence of estrogen receptor-containing neurons in "vagal locales." Labeling of uterine-related neurons in the nodose ganglia (Fluorogold and pseudorabies virus) and in the brainstem nuclei (pseudorabies virus) was obtained. DiI-labeled nerve fibers occurred near uterine horn and uterine cervical blood vessels, in the myometrium, and in paracervical ganglia. Rats with vagal, pelvic, hypogastric and ovarian neurectomies exhibited a marked decrease in calcitonin gene-related peptide-immunoreactive nerves in the uterus relative to rats with pelvic, hypogastric, and ovarian neurectomies with intact vagus nerves. Neurons in the nodose ganglia and nucleus tractus solitarius were immunoreactive for estrogen receptors. These results demonstrated: (1) the vagus nerves serve as connections between the uterus and CNS, (2) the nodose ganglia contain uterine-related vagal afferent neuron cell bodies, and (3) neurons in vagal locales contain estrogen receptors.

Transient overconsumption of novel foods by deafferentated rats: effects of novel diet composition

We recently demonstrated that capsaicin-treated rats consume more of an unfamiliar high-fat diet than vehicle-treated controls, but only on initial exposure (Chavez et al, 1997). We hypothesized that negative feedback signals carried by capsaicin-sensitive visceral afferents are critical for the regulation of intake of novel foods, but redundant pathways take over during subsequent exposures. To examine the role of nutrient content of the novel diet, rats were systemically treated with capsaicin (n = 15) or vehicle (n = 10), and exposed to 1) a fat/olestra diet that was isocaloric with chow; 2) a readily accepted fat-free cake; and 3) pure corn oil. Each 3-h feeding trial was preceded by 24-h food deprivation. Treated rats did not overconsume familiar chow, but did consume 50% more than controls of both the fat/olestra diet and the corn oil on first exposure; this suggests that capsaicin eliminated visceral afferents that normally carry satiety signals. However, the effect with the fat/olestra mixture was due primarily to depressed intake by controls, unlike the pure fat diets; this apparent neophobic response was blunted in treated rats. Because treated rats failed to overconsume the fat-free cakes, the neural system damaged by capsaicin appears to be linked to energy or fat sensory mechanisms, and possibly to hedonic responsiveness.

Pulmonary intraepithelial vagal nodose afferent nerve terminals are confined to neuroepithelial bodies: an anterograde tracing and confocal microscopy study in adult rats

Our present understanding of the morphology of neuroepithelial bodies (NEBs) in mammalian lungs is comprehensive. Several hypotheses have been put forward regarding their function but none has been proven conclusively. Microscopic data on the innervation that appears to affect the reaction of NEBs to stimuli have given rise to conflicting interpretations. The aim of this study has been to check the validity of the hypothesis that pulmonary NEBs receive an extensive vagal sensory innervation. The fluorescent neuronal tracer DiI was injected into the vagal sensory nodose ganglion and NEBs were visualized in toto by using immunocytochemistry and confocal microscopy on 100-micrometer-thick frozen sections of the lungs of adult rats. The most striking finding was the extensive intraepithelial terminal arborizations of DiI-labelled vagal afferents in intrapulmonary airways, apparently always co-appearing with calcitonin gene-related peptide (CGRP)-immunoreactive NEBs. Not all NEBs received a traced nerve fibre. Intrapulmonary CGRP-containing nerve fibres, including those innervating NEBs, always appeared to belong to a nerve fibre population different from the DiI-traced fibres and hence did not arise from the nodose ganglion. Therefore, at least some of the pulmonary NEBs in adult rats are supplied with sensory nerve fibres that originate from the vagal nodose ganglion and form beaded ramifications between the NEB cells, thus providing support for the hypothesis of a receptor function for NEBs.

Vagal and spinal afferent innervation of the rat esophagus: a combined retrograde tracing and immunocytochemical study with special emphasis on calcium-binding proteins

Vagal afferent neurons contain a variety of neurochemical markers and neuroactive substances, most of which are present also in dorsal root ganglion cells. To test for the suitability of the calcium-binding protein calretinin as a specific marker for vagal afferent fibers in the periphery, immunocytochemistry for this protein was combined with retrograde tracing. Nerve fibers in the rat esophagus, as well as vagal and spinal sensory neurons innervating the esophagus, were investigated for co-localization of calretinin with calbindin, calcitonin gene-related peptide, and NADPH diaphorase. The results indicated that calretinin immunocytochemistry demonstrates neuronal structures known as vagal afferent from other studies, in particular intraganglionic laminar endings. A few enteric neurons whose distribution was unrelated to intraganglionic laminar endings also stained for calretinin. Strikingly, calretinin immunoreactivity was absent from spinal afferent neurons innervating the rat esophagus. In intraganglionic laminar endings and nodose ganglion cells calretinin was highly co-localized with calbindin but not with calcitonin gene-related peptide. On the other hand, calbindin was also found in spinal afferents to the esophagus where it was co-localized with calcitonin gene-related peptide. Vagal afferent neurons innervating the esophagus were never positive for NADPH diaphorase. Thus, calretinin appears to be a more specific marker for vagal afferent structures in the esophagus than calbindin, which is expressed by both vagal and spinal sensory neurons. Calretinin immunocytochemistry may be utilized as a valuable tool for investigations of subpopulations of vagal afferents in certain viscera.

Duodenal nutrient infusions differentially affect sham feeding and Fos expression in rat brain stem

Duodenal infusions of macronutrients inhibit sham and normal feeding. Neural substrates of this response were studied by infusing glucose, linoleic acid, an amino acid mixture, saline, or water into the duodenum of unanesthetized rats and then measuring sham feeding of 30% sucrose or Fos expression in the dorsal vagal complex. Linoleic acid and amino acids (both 1.5 kcal) and glucose (4.5 kcal) suppressed sham feeding relative to control infusions, and all three macronutrients triggered Fos expression in the nucleus of the solitary tract and area postrema. Although there were significant quantitative differences, the subnuclear distribution pattern of Fos-expressing neurons was not different for the three macronutrients and was largely localized to the medial, dorsomedial, and commissural subnuclei of the nucleus of the solitary tract and the area postrema. Linoleic acid suppressed intake and stimulated Fos expression similarly to glucose infusions of three times the caloric value. Amino acids strongly suppressed sham feeding but triggered relatively little Fos expression. These results indicate that the intake-suppressing potency of duodenal macronutrients is dependent on nutrient type, rather than simply caloric value, and that amino acids, although potent inducers of satiety, affect ingestion by processes different from those subserving lipids and carbohydrates. Furthermore, the similar patterns of neuronal activation after different duodenal infusions may indicate a large degree of convergence at the level of primary and second-order sensory neurons, whereas the distinctly different pattern obtained earlier with gastric distension indicates partially separate neural pathways for satiety signals generated by duodenal nutrients and gastric mechanoreceptors.

Vagal efferent and afferent innervation of the rat esophagus as demonstrated by anterograde DiI and DiA tracing: focus on myenteric ganglia

Anterograde tracing with the carbocyanine tracer DiI and the aminostyrol derivative DiA was used to selectively label fibers from the nucleus ambiguus, dorsal motor nucleus and nodose ganglion, respectively, terminating in the rat esophagus, and to compare them with the innervation of the gastric fundus in the same animals. Ambiguus neurons terminated on motor endplates distributed mainly to the ipsilateral half of the esophagus. There was no evidence of preganglionic innervation of myenteric ganglia from ambiguus neurons. Neurons of the dorsal motor nucleus supplied sparse fibers to only about 10% of enteric ganglia in the esophagus while they innervated up to 100% of myenteric ganglia in the stomach. Neurons of the nodose ganglion terminated profusely on more than 90% of myenteric ganglia of the esophagus and on about 50% of ganglia in the stomach. Afferent vagal fibers were also frequently found in smooth muscle layers starting at the esophago-gastric junction. In contrast, they were extremely rare in the striated muscle part of the esophagus. These morphological data suggest a minor influence of neurons of the dorsal motor nucleus and a prominent influence of vagal afferent terminals onto myenteric neurons in the rat esophagus.

Pulmonary neuroepithelial bodies are innervated by vagal afferent nerves: an investigation with in vivo anterograde DiI tracing and confocal microscopy

The pulmonary airway and alveolar epithelia contain distinctly innervated clusters of basally granulated cells: the neuroepithelial bodies. In the past, morphological criteria and the results of selective vagotomy have led to the interpretation that their innervation is sensory. Consequently, they are regarded as receptor organs. As a further test of this hypothesis, the present investigation set out to label vagal sensory nerve fibres to the lungs by anterograde neural tracing, and to establish the relationship between these fibres and the neuroepithelial bodies. A fluorescent neural tracer was injected unilaterally into the left or right nodose ganglion of adult rats. After suitable survival times, thick frozen sections of lung tissue were studied with laser scan confocal microscopy. Sensory nerve fibres were seen to run in the airway walls and occasionally penetrated the epithelium, where they formed complex terminals. The resulting intraepithelial sensory end organs showed a close morphological resemblance to the neuroepithelial bodies. Subsequently, electron microscopic investigation of such identified structures revealed the typical ultrastructural characteristics of neuroepithelial bodies: corpuscular cells containing dense cored secretory vesicles and contacted by mitochondria-rich nerve endings. We conclude that anterograde tracing of sensory nerves from the nodose ganglion confirms the receptor nature of the pulmonary neuroepithelial bodies, which may correspond to a subpopulation of the irritant and C-fibre receptors.

Vagal afferent nerve fibres contact mast cells in rat small intestinal mucosa

Mast cells degranulate when exposed to specific antigens (via surface bound IgE), resulting in the release of numerous pro-inflammatory mediators. Neuroregulatory substances also activate mast cells, and may effect differential mediator release, without degranulation, suggesting a role for nerves in modulating mast cell activity. We previously investigated the microanatomical relationships of intestinal mucosal mast cells (IMMC) with nerves and found extensive associations in the intestinal mucosae of rats and humans. The origins of nerves that contact IMMC have not been determined; however, recent morphological and functional studies suggest the possibility that the vagus nerve might be involved. In the current study we show that vagal afferent fibers (labeled by injecting DiI into the nodose ganglion) penetrate to the tips of jejunal villi; and that some of these nerves make intimate contact with IMMC. These data provide the microanatomical basis for direct neural communication between the central nervous system (CNS) and mast cells in the gastrointestinal mucosa.

Limited excitatory local effector function of gastric vagal afferent intraganglionic terminals in rats

Intraganglionic laminar endings (IGLEs) are complex terminal structures of vagal afferent origin, distributed throughout the myenteric plexus of the esophagus and gastrointestinal tract and without a known function. They may serve local effector function by means of peripheral axon reflexes, analogously to dorsal root afferents. To test this possibility, vagal afferent fibers were antidromically activated by suprathreshold electrical stimulation of the cervical vagus nerve in anesthetized rats that underwent prior supranodose vagotomy, and responses of myenteric neurons were monitored with Fos immunocytochemistry. Stimulation of vagal afferents produced Fos expression in a slightly, but significantly, higher proportion of myenteric plexus neurons of the gastric corpus (1.02 +/- 0.21%, P < 0.05) and esophagus (1.59 +/- 0.46%, P < 0.05) than in control animals with sham-stimulation (corpus, 0.12 +/- 0.05%; esophagus, 0.18 +/- 0.18%). Stimulation of vagal efferents produced widespread Fos induction in myenteric neurons. Given the many enteric neurons in close anatomic contact with IGLEs and the low proportion of Fos activated neurons after selective afferent stimulation, the results do not support a widespread excitatory local effector function of IGLEs. However, inhibitory effects and/or weak excitatory synaptic inputs that do not engage Fos expression cannot be ruled out.

Chemical lesion of visceral afferents causes transient overconsumption of unfamiliar high-fat diets in rats

Because it is commonly assumed that the major role of visceral afferents in food intake control is to terminate meals by carrying negative-feedback signals to the brain, we hypothesized that overconsumption should occur in rats with chemically lesioned visceral afferents if they were presented with an unfamiliar diet. Adult male Sprague-Dawley (SD) rats were treated with multiple doses of capsaicin or vehicle as a control. Five weeks later, a series of 3-h feeding tests after 24-h deprivation was carried out, first with chow and then with either a solid (vegetable shortening) or liquid (Ensure) unfamiliar high-fat diet. Both groups consumed similar amounts of their powdered chow maintenance diet, but capsaicin-treated rats consumed at least 50% more of either high-fat diet than vehicle controls (P < 0.01) at the beginning of the first trial. During second and third trials with the now-familiar high-fat diet, intake was no longer significantly different between the two groups, suggesting rapid engagement of redundant control mechanisms. These results support a role of capsaicin-sensitive visceral afferents in providing negative feedback for early meal termination during the ingestion of unfamiliar diets.

Distribution and structure of vagal afferent intraganglionic laminar endings (IGLEs) in the rat gastrointestinal tract

Intraganglionic laminar endings (IGLEs) are special terminal structures of vagal afferent fibers and have been demonstrated in the myenteric plexus of esophagus and stomach. In order to quantitatively map their presence and distribution over the entire gastrointestinal tract, including the small and large intestines, vagal afferents were anterogradely labeled in vivo by microinjections of the fluorescent carbocyanine dye DiI into the left or right nodose ganglion of adult male rats. In the most successfully labeled cases the highest density of IGLEs was found in the stomach, with about half to one-third of the myenteric ganglia receiving at least one IGLE. The proportion of myenteric ganglia innervated by IGLEs decreased in the small intestine; however, because of its large surface area this gut segment was estimated to contain the highest total number of IGLEs. Both the cecum and colon also contained significant numbers of IGLEs. In the stomach, this vagal afferent innervation by IGLEs was more or less lateralized, with less than 20% of labeled IGLEs found on the contralateral side with respect to the injection. The left/ventral vagus contributed a larger proportion of IGLEs to the proximal duodenum, while the right/dorsal vagus contributed a larger proportion of IGLEs to the distal duodenum and jejunum. Laser scanning confocal microscopy on select specimens revealed further structural details. The parent axon typically formed two or more branches that flanked the ganglia laterally, and in turn produced numerous highly arborizing laminar terminal branches that covered one or both flat sides of the ganglion in a dome-like fashion. The similar distribution patterns and structural details suggest a uniform function for the IGLEs throughout the gastrointestinal tract, but there is as yet no clear proof for any of the hypothesized roles as specialized mechanosensors or local effector terminals.

Capsaicin-resistant vagal afferent fibers in the rat gastrointestinal tract: anatomical identification and functional integrity

The presence and distribution of vagal fibers and terminals throughout esophagus and gastrointestinal tract that could be anterogradely labeled by nodose ganglion tracer injections was quantitatively assessed in capsaicin- and vehicle-pretreated adult rats, in order to identify the capsaicin-resistant population. Up to 90% of the intraganglionic laminar endings (IGLEs), in the myenteric plexus of the esophagus, and 70-90% in the stomach, as well as 57% of the intramuscular endings or arrays (IMAs) in the fundic stomach survived the capsaicin treatment, while in the upper small intestine only few and in the lower small intestine, the cecum and colon, virtually no IGLEs survived capsaicin treatment. Intramucosal terminals were not assessed. Furthermore, gastric balloon distension-induced c-Fos expression in the dorsal vagal complex was not significantly decreased in capsaicin-treated rats. It is concluded that among primary vagal afferents there is a capsaicin-resistant population that primarily innervates the esophagus and upper gastrointestinal tract, and a capsaicin-sensitive population that innervates mainly the lower tract. At least vagal gastric tension-sensitive afferents also seems to be functionally intact in that they may be capable of synaptically activating second-order neurons in the brainstem.

Gastric distension-induced c-fos expression in catecholaminergic neurons of rat dorsal vagal complex

Functionally specific vagal afferents were stimulated by gastric balloon distension in unanesthetized rats, followed by double c-fos/dopamine beta-hydroxylase (DBH) immunocytochemistry, to identify second-order neurons in the dorsal vagal complex. Continuous and repeated phasic distension with similar volumes produced similar numbers and patterns of c-fos expression, with most of the activated neurons in the medial and commissural nucleus of the solitary tract (NTS) and dorsal motor nucleus (DMNX). Larger distension activated significantly more neurons in all responsive areas but there was no differential effect. In most NTS subnuclei and the DMNX, a small (3-5%) proportion of gastric distension-activated neurons was DBH-immunoreactive (DBH-IR), and this proportion did not significantly change with type of distension. With continuous and repeated small distensions, 10-12% and, with the large distension, 22-30% of all DBH-IR neurons expressed c-fos. The results suggest a large degree of convergence between rapidly adapting mucosal receptors and slowly adapting tension receptors, but not between low- and high-threshold tension receptors, and a relatively minor role of catecholaminergic second-order neurons in the dissemination of distension signals in the brain.

Differential effects of baseline macronutrient preferences on macronutrient selection after galanin, NPY, and an overnight fast

Rats display individual patterns of fat and carbohydrate intakes when allowed to self-select among individual macronutrient diets. We investigated whether these individual preferences in macronutrient selection could be modified by an overnight fast or by two orexigenic peptides, galanin and neuropeptide Y (NPY), which may selectively stimulate fat and carbohydrate intake. Rats were grouped by preference based on the ratio of average baseline fat:carbohydrate intake. In counterbalanced tests conducted on separate days, saline, galanin, or NPY was infused into the paraventricular nucleus of the hypothalamus and 60-min food intake was measured. When the macronutrient intakes were expressed as percent of total caloric intake, galanin administered into the PVN did not increase fat consumption compared to saline injection in either preference group. NPY slightly enhanced the proportion of carbohydrate intake, but only in carbohydrate-preferring rats. When all three feeding stimuli were compared to baseline preferences, the only condition that significantly altered macronutrient selection was an overnight fast, which augmented fat intake. These data demonstrate that baseline preferences for fat or carbohydrate are not significantly modified by galanin or NPY but that an overnight fast increases fat preference.

Interaction between parasympathetic and sympathetic nerves in prevertebral ganglia: morphological evidence for vagal efferent innervation of ganglion cells in the rat

Vagal efferent preganglionic neurons were anterogradely labeled by injecting either DiI or DiA, fluorescent lipophilic carbocyanine dyes, into the dorsal motor nucleus of the vagus of the rat. All neurons of the peripheral nervous system (outside the blood-brain barrier) were then fluorescently counterstained in vivo by injecting Fluorogold (Fluorochrome, Inc., Englewood, CO) intraperitoneally. The upper abdominal prevertebral ganglia, including the numerous microganglia associated with the periarterial plexuses of the celiac and superior mesenteric arteries, were identified and dissected in formalin-fixed tissue under ultraviolet light and stereomicroscopic guidance. In 14 of 15 animals analyzed (93%), labeled vagal efferent fibers were found to penetrate into both the left and right celiac ganglia and the superior mesenteric ganglion, as well as into some of the associated microganglia. These projections formed varicose terminal-like structures, highly suggestive of synaptic contacts surrounding individual ganglion cells. In about half the animals, such vagal innervation was also seen in the left and right suprarenal ganglia. The specificity of vagal efferent labeling was confirmed by control experiments, which included injections in vagotomized animals and direct selective labeling of vagal afferents from the nodose ganglia. It is concluded that vagal efferent preganglionics innervate principal ganglion cells of prevertebral ganglia. These vagal contacts may either directly modulate the postganglionic outflow or else gate some or all of the potential modulatory inputs to these postganglionic neurons, thus allowing the vagal system to exert a more selective influence on sympathetic outflow. Finally, the use of laser scanning confocal microscopy and the in toto Fluorogold staining method for investigations of the peripheral nervous system are discussed.

Morphological analysis of vagal input to gastrin releasing peptide and vasoactive intestinal peptide containing neurons in the rat glandular stomach

Vagal preganglionic efferents to the rat stomach were labeled anterogradely by injecting the fluorescent carbocyanine dye DiA into the dorsal motor nucleus in vivo. Enteric neurons were labeled in toto by intraperitioneal administration of Fluorogold, and neurochemically characterized by simultaneous single- and double-label immunocytochemistry. Single peptide immunocytochemistry revealed that in all three major areas of the stomach, about one-third of all gastrin-releasing peptide immunoreactive (GRP-IR) neurons in the myenteric plexus, received vagal contacts. Because the proportion of GRP-IR neurons was 32% in the fundus, 23% in the corpus, and only 8% in the antrum, the absolute number of vagally contacted GRP-IR neurons per cm2 was also different. Double-label immunocytochemistry revealed colocalization of vasoactive intestinal peptide immunoreactivity (VIP-IR) in 45%, and of enkephalin immunoreactivity (ENK-IR) in about 30%, of the GRP-IR myenteric neurons. A subpopulation of myenteric neurons colocalized GRP-IR and VIP-IR and projects almost exclusively to the gastrin cell-rich basal mucosa of the antrum and the oxyntic mucosa of the corpus. Another subpopulation containing GRP-IR, but not VIP-IR, projects mainly to the myenteric plexus itself and the external muscle layers, particularly the longitudinal muscle. A third group of neurons containing VIP-IR but not GRP-IR projects heavily to the circular muscle layer, the muscularis mucosae, and to other myenteric neurons. Vagal input to these three subpopulations seems not to be selective, in that an equal proportion of about 20 to 30% of each group was vagally contacted. Vagal inputs to these neurochemically and topographically distinct enteric neurons provide the basis for the physiological vagal control of gastrin release, gastric acid secretion, and gastric motility.

Innervation of rat abdominal paraganglia by calretinin-like immunoreactive nerve fibers

The extended hepatic pedicle of the rat containing the vagal hepatic branch and associated paraganglia was dissected as whole mount, processed for calretinin immunocytochemistry, and analyzed by laser-scanning confocal microscopy. About half of the 8-12 paraganglia of the region were innervated to a various degree by calretinin-immunoreactive (CAL-IR) nerve fibers that formed basket-like varicose terminals around small clusters of glomus tissue. Neither the numerous capillaries, nor the occasional non-CAL-IR neuron accompanying some paraganglia, were recipients of CAL-IR terminals. In animals with prior left cervical vagotomy the number of CAL-IR fibers was decreased but not abolished. Since about 30% of neurons in the nodose ganglia, a few neurons within the vagal hepatic branch/hepatic hilus area, and numerous neurons in the gastric myenteric plexus were CAL-IR, these neurons represent other potential sources of CAL-IR innervation of the paraganglia.

Anatomical relationship between vagal afferent fibers and CCK-immunoreactive entero-endocrine cells in the rat small intestinal mucosa

There is evidence for a pathway involving small intestinal CCK-producing entero-endocrine cells and visceral afferent nerve fibers in signaling the effect of luminal nutrients on gastrointestinal and food intake regulation. In order to investigate the type of anatomical apposition that exists between CCK cells and vagal afferents, CCK immunocytochemistry was performed on tissue from rats whose vagal afferent fibers to the abdomen had previously been labeled in vivo by injecting the fluorescent carbocyanine dye DiI into the nodose ganglia. CCK immunoreactive (CCK-IR) cells were more abundant than vagal afferent fibers, but both were present throughout the small intestine as well as in crypts and villi. Few CCK-IR cells were in close (< 5 microns) anatomical contact with vagal afferent axons, and the latter did not produce suspicious terminal specializations near CCK-IR cells. Most labeled vagal afferent axons, which distributed strictly within the crypt and villous lamina propria, were at distances of tens to hundreds of microns to the nearest CCK-IR cell. These findings strongly support the idea that CCK released from entero-endocrine cells acts on vagal sensory fibers in a paracrine fashion, but do not rule out the presence of a few very close, neurocrine-like contacts or a humoral mode of action. Possible implications of such an arrangement on CCK-mediated satiety are discussed.

Anatomical demonstration of vagal input to nicotinamide acetamide dinucleotide phosphate diaphorase-positive (nitrergic) neurons in rat fundic stomach

Recent pharmacological evidence suggests that the nonadrenergic, noncholinergic (NANC) vagal inhibitory input responsible for receptive relaxation of the fundic stomach is mediated by nitric oxide-synthesizing enteric neurons. To demonstrate anatomically such direct vagal inputs to neurochemically identified enteric neurons, we utilized the nicotinamide acetamide dinucleotide phosphate (NADPH)-diaphorase histochemical reaction in conjunction with selective anterograde labeling of vagal efferents or afferents. Approximately 30% of all myenteric neurons of the fundic myenteric plexus stained positive for NADPH diaphorase, and the principal recipient of axonal projections from NADPH diaphorase-positive neurons was the circular muscle layer. In a group of animals showing the most complete labeling of vagal efferent preganglionics with the carbocyanine dye DiA, quantitative analysis of the half of the ventral fundic wall closer to the greater curvature revealed that 46.8% +/- 4.4% of all myenteric neurons received some degree of vagal contacts and that 30.5% +/- 6.6% of such vagally contacted neurons were also NADPH diaphorase positive. In another group of rats with the most successful selective labeling of vagal afferents through DiI injections into the left nodose ganglion, analysis of select ganglia throughout the ventral fundic wall revealed that, of a total of 454 neurons with vagal afferent contacts, 34.8% +/- 2.8% were NADPH diaphorase positive. These findings support the view that, in the fundic stomach, some vagal preganglionic efferents terminate on nitric oxide-synthesizing neurons that, in turn, project to and relax the external smooth muscle layers. Furthermore, vagal afferent endings also contact NADPH diaphorase-positive neurons, suggesting the possibility of local axon reflexes originating from smooth muscular in-series tension receptors and terminating on nitrergic neurons of the myenteric plexus.

Vagal sensors in the rat duodenal mucosa: distribution and structure as revealed by in vivo DiI-tracing

Results from functional studies point to the importance of chemoreceptive endings in the duodenum innervated by vagal afferents in the regulation of gastrointestinal functions such as gastric emptying and acid secretion, as well as in the process of satiation. In order to visualize the vagal sensory innervation of this gut segment, vagal afferents were selectively labeled in vivo by injecting the lipophilic carbocyanine dye DiI into either the left or the right nodose ganglion of young adult rats. Thick cryostat sections or whole-mounted peels of muscularis externa or submucosa of formalin-fixed tissue were analyzed with conventional and/or confocal microscopy. In the mucosa, many DiI-labeled vagal afferent fibers were found with terminal arborizations mainly between the crypts and the villous lamina propria. In both areas, vagal terminal branches came in close contact with the basal lamina, but did not appear to penetrate it so as to make direct contact with epithelial cells. Labeled vagal afferent fibers in the villous and cryptic lamina propria were found to be in intimate anatomical contact with fibrocyte-like cells that may belong to the class of interstitial cells of Cajal, and with small granular cells that might be granulocytes or histiocytes. Although our analysis was not quantitative, and considering that labeling was unilateral and not complete, it appears that the overall density of vagal afferent mucosal innervation was variable; many villi showed no evidence for innervation while other areas had quite dense networks of arborizing terminal fibers in several neighboring villi. Analysis of separate whole-mounted muscularis externa and submucosa peels revealed the presence of large bundles of labeled afferent fibers running within the myenteric plexus along the mesenteric attachment primarily in an aboral direction, with individual fibers turning towards the antimesenteric pole, and either penetrating into the submucosa or forming the characteristic intraganglionic laminar endings (IGLEs). Although the possibility of individual fibers issuing collaterals to myenteric IGLEs and at the same time to mucosal terminals was not demonstrated, it cannot be ruled out. These anatomical findings are discussed in the context of absorptive mechanisms for the different macronutrients and the implication of enteroendocrine cells such as CCK-containing cells that may function as intestinal "taste cells".

Vagal afferent innervation of rat abdominal paraganglia as revealed by anterograde DiI-tracing and confocal microscopy

Abdominal vagal afferent fibers were selectively labeled by injecting the fluorescent carbocyanine dye DiI into the left nodose ganglion of rats. Almost all paraganglia that were distributed along the five major abdominal vagal branches and their subbranches were found to be innervated by labeled vagal afferents. Laser scanning confocal microscopy with its single optical sectioning and three-dimensional reconstruction capabilities were used to analyze this innervation in more detail for paraganglia near the vagal hepatic branch and liver hilus. Furthermore, in double-labeling studies, it was demonstrated that a large percentage of the vagally innervated glomus cells were capable of catecholamine synthesis on the basis of their positive staining for tyrosine hydroxylase antibody. These findings support the concept of a chemoreceptive function for the abdominal paraganglia.

NADPH-diaphorase-positive nerve fibers associated with motor endplates in the rat esophagus: new evidence for co-innervation of striated muscle by enteric neurons

NADPH-diaphorase histochemistry was combined with demonstration of acetylcholinesterase and immunocytochemistry for calcitonin gene-related peptide to study esophageal innervation in the rat. Most of the myenteric neurons stained positively for NADPH-diaphorase, as did numerous varicose nerve fibers in the myenteric plexus, among striated muscle fibers, around arterial blood vessels, and in the muscularis mucosae. A majority of motor endplates (as demonstrated by acetylcholinesterase histochemistry or calcitonin gene-related peptide immunocytochemistry) were associated with fine varicose NADPH-diaphorase-positive nerve fibers. Analysis of brainstem nuclei, sensory vagal, spinal, and sympathetic ganglia in normal and neonatally capsaicin-treated rats, and comparison with anterogradely labeled vagal branchiomotor, preganglionic and sensory fibers led to the conclusion that NADPH-diaphorase-positive fibers on motor endplates originate in esophageal myenteric neurons. No association of NADPH-diaphorase-positive nerve fibers with motor endplates was found in other organs containing striated muscle. These results suggest extensive, presumably nitrergic, co-innervation of esophageal striated muscle fibers by enteric neurons. Thus, control of peristalsis in the esophagus of the rat may be more complex than hitherto assumed.

Vagal innervation of the rat pylorus: an anterograde tracing study using carbocyanine dyes and laser scanning confocal microscopy

In an attempt to identify the distribution and structure of vagal fibers and terminals in the gastroduodenal junction, vagal efferents were labeled in vivo by multiple injections of the fluorescent carbocyanine dye DiA into the dorsal motor nucleus (dmnX), and vagal afferents were anterogradely labeled by injections of DiI into the nodose ganglia of the same or separate rats. Thick frontal cryostat sections were analysed either with conventional or laser scanning confocal microscopy, using appropriate filter combinations and/or different wavelength laser excitation to distinguish the fluorescent tracers. Vagal efferent terminal-like structures were present in small ganglia within the circular sphincter muscle, which, in the absence of a well-developed, true myenteric plexus at this level, represent the myenteric ganglia. Furthermore, vagal efferent terminals were also present in submucosal ganglia, but were absent from mucosa, Brunner's glands and circular muscle fibers. Vagal afferent fibers and terminal-like structures were more abundant than efferents. The most prominent afferent terminals were profusely branching, large net-like aggregates of varicose fibers running within the connective tissue matrix predominantly parallel to the circular sphincter muscle bundles. Profusely arborizing, highly varicose endings were also present in large myenteric ganglia of the antrum and duodenum, in the modified intramuscular ganglia, and in submucosal ganglia. Additionally, afferent fibers and terminals were present throughout the mucosal lining of the gastroduodenal junction. The branching patterns of some vagal afferents suggested that individual axons produced multiple collaterals in different compartments. NADPH-diaphorase positive, possibly nitroxergic neurons were present in myenteric ganglia of the immediately adjacent antrum and duodenum, and fine varicose fibers entered the sphincter muscle from both sides, delineating the potential vagal inhibitory postganglionic innervation. These morphological results support the view of a rich and differentiated extrinsic neural control of this important gut region as suggested by functional studies.

Characterization of vagal innervation to the rat celiac, suprarenal and mesenteric ganglia

In order to shed light on the controversial issue of vagal innervation of the solar plexus ganglia, vagal efferent preganglionic fibers were anterogradely labeled by injecting the fluorescent carbocyanine dye Dil into the dorsal motor nucleus (dmnX). Additionally, Fluorogold was used to label the ganglia in toto, providing a counterstain and the possibility of UV light-guided dissection of the various ganglia. Using optical sectioning of whole mounted intact ganglia by means of laser scanning confocal microscopy, a considerable number of Dil-labeled vagal terminal-like structures were found in the major ganglia (celiac, superior mesenteric and suprarenal). Additionally, vagal efferent terminals were regularly found in microganglia associated with the periarterial plexuses of the celiac and superior mesenteric arteries, and in a few cases in small ganglia of the intermesenteric and renal plexuses. By using animals with prior selective vagal branch vagotomies, leaving only one (or a pair) of the three major abdominal divisions intact, it was concluded that the two celiac branches contribute the bulk of this vagal innervation, with the two gastric and the unpaired hepatic branch providing a small contribution mostly limited to the celiac ganglia. From control experiments, which involved Dil injections (1) into the dmnX in animals whose visceral afferents had been previously destroyed by capsaicin; (2) into the nodose ganglia, in order to anterogradely label vagal afferents; and (3) into the cervical vagus nerve as a control for uptake by fibers of passage, it was concluded that the identified terminal-like structures were vagal efferents and not inadvertently labeled afferents. We suggest that these vagal terminals have to be regarded either as ectopic parasympathetic junctions, or as part of a vagal mechanism for gating of sympathetic ganglionic transmission. Functionally, the parasympathetic innervation of the solar plexus may provide not only the classic vagal influence on gastrointestinal targets, but also vagal control of the adrenal glands and possibly other abdominal organs that have not been traditionally regarded as vagal targets.

An anterograde tracing study of the vagal innervation of rat liver, portal vein and biliary system

In order to investigate the distribution and structure of the vagal liver innervation, abdominal vagal afferents and efferents were selectively labeled by injecting WGA-HRP or Dil into the nodose ganglia, and DiA into the dorsal motor nucleus, respectively. Vagal afferent fibers produced characteristic terminal-like structures at three locations in the liver hilus: 1. Fine varicose endings preferentially surrounding, but not entering, the numerous peribiliary glands in the larger intra and extrahepatic bile ducts 2. Large, cup-shaped terminals in almost all paraganglia 3. Fine varicose endings in the portal vein adventitia. No fibers and terminals were found in the hepatic parenchyma. While about two thirds of the vagal afferent fibers that originate in the left nodose ganglion, and are contained in the hepatic branch, bypass the liver hilus area on their way to the gastroduodenal artery, a significant number (approx. 10% of the total) of vagal afferents that do innervate the area, originates from the right nodose ganglion, and projects to the periarterial plexus of the common hepatic artery and liver pedicle most likely through the dorsal celiac branch. Varicose vagal efferent fibers were present within the fascicles of the vagal hepatic branch and fine terminal-like structures in a small fraction of the paraganglia. No efferents were found to terminate in the hepatic parenchyma or on the few neurons embedded in nerves or paraganglia. In contrast to the paucity of vagal terminals in the hepatic parenchyma, an abundance of vagal efferent and afferent fibers and terminals with distinctive distribution patterns and structural characteristics was present in esophagus and gastrointestinal tract.(ABSTRACT TRUNCATED AT 250 WORDS)

Vagal afferent innervation of the rat fundic stomach: morphological characterization of the gastric tension receptor

Although the gastric tension receptor has been characterized behaviorally and electrophysiologically quite well, its location and structure remains elusive. Therefore, the vagal afferents to the rat fundus (forestomach or nonglandular stomach) were anterogradely labeled in vivo with injections of the carbocyanine dye Dil into the nodose ganglia, and the nerves and ganglia of the enteric nervous system were labeled in toto with intraperitoneal Fluorogold injection. Dissected layers and cryostat cross sections of the fundic wall were mounted in glycerin and analyzed by means of conventional and laser scanning confocal microscopy. Particularly in the longitudinal, and to a lesser extent in the circular, smooth muscle layers, Dil-labeled fibers and terminals were abundant. These processes, which originated from fibers coursing through the myenteric ganglia and connectives, entered either muscle coat and then ran parallel to the respective muscle fibers, often for several millimeters. They ran in close association with the Fluorogold-labeled network of interstitial cells of Cajal, upon which they appeared to form multiple spiny appositions or varicosities. In the myenteric plexus, two different types of afferent vagal structures were observed. Up to 300 highly arborizing endings forming dense accumulations of small puncta similar to the esophageal intraganglionic laminar endings (Rodrigo et al., '75 Acta Anat. 92:79-100) were found in the fundic wall ipsilateral to the injected nodose ganglion. They often covered small clusters of myenteric neurons or even single isolated ganglion cells (mean = 5.8 neurons) and tended to extend throughout the neuropil of the ganglia. In a second pattern, fine varicose fibers with less profuse arborizations innervated mainly the central regions of myenteric ganglia. Camera lucida analyses established that single vagal afferent fibers had separate collaterals in both a smooth muscle layer and the myenteric ganglia. Finally, Dil-labeled afferent vagal fibers were also found in the submucosa and mucosa. Control experiments in rats with supranodose vagotomy as well as rats with Dil injections directly in the distal cervical vagus ruled out the possibility of colabeling of afferent fibers of passage. In triple labeling experiments, in conjunction with Dil labeling of afferents and Fluorogold labeling of enteric neurons, the carbocyanine dye DiA was injected into the dorsal motor nucleus of the vagus to anterogradely label the efferent vagal fibers and terminals. The different distributions and morphological characteristics of the vagal afferents and efferents could be simultaneously compared. In some instances the same myenteric ganglion was apparently innervated by an afferent laminar ending and an efferent terminal.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology and distribution of efferent vagal innervation of rat pancreas as revealed with anterograde transport of Dil

Vagal efferent innervation of the pancreas was labeled by anterograde transport of Dil injected into the dorsal motor nucleus (dmnX). While over the entire organ only 19 +/- 3 (or 8 +/- 1%) of the 231 +/- 17 interlobular ganglia received Dil-labeled vagal fibers and terminals, the proximal duodenal lobe (or head) was significantly more densely innervated. Laser scanning confocal microscopy revealed further morphological details of the vagal terminals and their target ganglion cells. No vagal fibers or terminals were found in islets and acinar tissue.