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.

Acute hyperinsulinemia and its reversal by vagotomy after lesions of the ventromedial hypothalamus in anesthetized rats

The acute effect of bilateral electrolytic ventromedial hypothalamic lesions (20-25-m Coulomb stainless steel electrodes) on plasma levels of insulin and glucose was studied in anesthetized rats to determine early effects that would occur before hyperphagia and obesity. In rats fed ad libitum, lesions in the ventromedial hypothalamus (VMH) but not in the cortex produced a marked increase in circulating insulin levels (starting at 20 min postlesion) and a small increase in glycemia which, however, was not significant and could therefore not be the cause of increased insulin secretion. Hyperinsulinemia after VMH lesions was more pronounced when glucose was infused iv at a rate of 7-8 mg/kg . min. Bilateral subdiaphragmatic vagotomy, performed 50 min after VMH lesions, immediately and completely reversed the observed hyperinsulinemia. With the exception of a tendency of lesions producing the highest degree of hyperinsulinemia to be slightly larger than the lesions not producing any hyperinsulinemia, no statement about the critical involvement of a specific hypothalamic locus can be made. It is concluded that electrolytic VMH destruction causes immediate hypersecretion of the pancreatic B cell, an effect that requires the integrity of the vagus nerves. Further localization of the central circuitry responsible for this mechanism, however, will require more specific methods than electrolytic lesions.

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)

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.

Cephalic phase, reflex insulin secretion neuroanatomical and physiological characterization

Using chronically catheterized, freely moving male Wistar rats, we have shown that the sweet taste of a saccharin solution reliably triggers a rapid cephalic phase insulin response (CPIR), in the absence of any significant change of glycemia. To establish the neural mediation of this reflex response we used rats that were cured from streptozotocin diabetes by intrahepatic islet-transplantation as a denervated B-cell preparation. The complete lack of any saccharin-induced CPIR in these rats suggests that it is indeed mediated by the peripheral autonomic nervous system, and that the insulin-stimulating gastrointestinal hormones are not involved in this response. It was further found that this reflex insulin secretion is not easily extinguishable and thus might have an unconditioned component. To investigate the central neural pathways involved in this reflex response we used both electrophysiological methods in anesthetized and semi-micro CNS manipulations in freely moving rats. On the basis of our preliminary results, and several reports, using the decerebrate rat preparation for measuring behavioral or saliva secretory oral taste reactivity, it appears that CPIR might be organized at the brain stem/midbrain level, receiving strong modulatory influences from the diencephalon. But much further work has to be done to establish the central nervous circuitry. Finally, in two experiments, aiming at the question of how important and physiologically relevant the CPIR might be, we found that, on one hand, its lack can result in pathological oral glucose tolerance and on the other hand its exaggeration might contribute to the behavioral reaction to highly palatable sweet food and the resulting development of dietary obesity.

Topography of efferent vagal innervation of the rat gastrointestinal tract

The gastrointestinal territories innervated by the gastric, celiac, and hepatic abdominal vagi were identified in rats with selective branch vagotomies by means of 1) anterograde tracing with the carbocyanine dye DiI injected into the dorsal motor nucleus and 2) measurement of cervical vagal stimulation-induced motility responses throughout the gut axis. Presence of DiI-labeled vagal terminals in the myenteric plexus and evoked motility responses were well correlated across the sampled gastrointestinal (GI) sites. In animals with only the two gastric branches intact, the entire stomach and the most proximal duodenum showed significant motility responses and were densely innervated, having DiI-labeled vagal terminals in almost every ganglion. The hepatic branch was found to primarily innervate the duodenum, with minor projections to the distal antral stomach and the intestines. The two celiac branches were found to almost exclusively innervate the jejunum, ileum, cecum and entire colon, and, together with the other vagal branches, the duodenum. Therefore, while there is some degree of specific innervation by the abdominal vagal branches of the oral-to-anal gut axis, which could be called "viscerotopic," the considerably overlapping innervation of the duodenum does not satisfy a viscerotopy criterion and needs further functional analysis.

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

The vagal innervation of the different layers of the rat gastrointestinal wall was identified with the fluorescent carbocyanine dye Dil, injected into the dorsal motor nucleus of the vagus (dmnX). Multiple, bilateral injections were used to label all dmnX preganglionic motoneurons, and as a consequence, most of the vagal primary afferents that terminate in the adjacent nucleus of the solitary tract (nts) were also retrogradely and transganglionically labeled. With Fluorogold used to label the enteric nervous system completely and specifically, the Dil-labeled vagal profiles could be visualized and quantified in their anatomical relation to the neurons of the myenteric and submucous ganglia. In the myenteric plexus, vagal fibers and terminals were found throughout the gastrointestinal tract as far caudal as the descending colon, but there was a general decreasing proximodistal gradient in the density of vagal innervation. All parts of the gastric myenteric plexus (fundus, corpus, antrum), as well as the proximal duodenum, were extremely densely innervated, with vagal fibers and terminals in virtually every ganglion and connective. Further caudally, both the percentage of innervated myenteric ganglia and the average density of label within the ganglia rapidly decreased, with the exception of the cecum and proximal colon, where up to 65% of the ganglia were innervated. In the gastric and duodenal submucosa very few and in the mucosa no vagal fibers and terminals were found. With both normal epifluorescence and laser scanning confocal microscopy, highly varicose or beaded terminal structures of various size and geometry could be identified. The Dil injections, which impregnated the dmnX as well as the adjacent nts, resulted in retrograde and anterograde labeling of all the previously reported forebrain connections with the dorsal vagal complex. We conclude that the myenteric plexus is the primary target of vagal innervation throughout the gastrointestinal tract, and that its innervation is more complete than previously assumed. In contrast, vagal afferent (and efferent) innervation of mucosa and submucosa seems conspicuously sparse or absent. Furthermore, the use of more focal injections of Dil offers the prospect to simultaneously identify specific subsets of vagal preganglionics and their central nervous inputs.

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 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".

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.

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.

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 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.

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.

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.

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.

Diet and cephalic phase insulin responses

Cephalic phase digestive responses may be particularly critical in determining our various reactions to different diets, since these responses are the first physiological adjustments to food. The potential importance of the cephalic responses is also underscored by the fact that many of the most important food attributes for humans--color, appearance, flavor, aroma, and texture--can influence the individual's gastrointestinal physiology solely by affecting these early metabolic responses. The present survey examines in some detail the data available for one of the responses, the cephalic phase insulin response. Specific shortcomings of the existing analyses are discussed. In addition, given the possible significance of these reflexes, several suggestions for improvements of experimental protocols are considered, and a summary of major experimental questions is provided.

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.

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.

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.

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.

Increases in plasma insulin levels in response to electrical stimulation of the dorsal motor nucleus of the vagus nerve

In order to investigate the physiological counterpart of the anatomical finding showing that the dorsal motor nucleus of the vagus nerve (DMX) is a source of efferent vagal fibers innervating the pancreas, unilateral electrical stimulation using monopolar electrodes (50 microA, 30 Hz, 0.2 msec) at a glycemia of 150 mg/100 ml was performed in normal anesthetized rats. DMX stimulation resulted in rapid (within 1 min) rise in plasma insulin levels (greater than or equal to 200%). Stimulation of the nucleus of tractus solitarius, anatomically connected to DMX, also produced a 50% increase in insulinemia. The effect of DMX stimulation was almost completely abolished by atropine pretreatment or acute bilateral subdiaphragmatic vagotomy. The effect of DMX stimulation was not potentiated by the alpha-adrenergic blocker (infusion of phentolamine) indicating that no inhibitory fiber was recruited during DMX stimulation. It is concluded that DMX is connected to the endocrine pancreas exclusively via vagal fibers and has a role in neurally mediated insulin release.

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.

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.

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.