Sensory vagal innervation of the rat esophagus and cardia: a light and electron microscopic anterograde tracing study

An in vitro study of the properties of vagal afferent fibres innervating the ferret oesophagus and stomach

1. A novel preparation of the oesophagus with attached vagus nerve from the ferret maintained in vitro was used to study the properties of single vagal afferent nerve fibres with identified receptive fields. 2. Recordings were made from three types of gastro-oesophageal vagal afferent fibres that were classified on the basis of their sensitivity to mechanical stimulation. There were those responding to mucosal stroking (mucosal receptors), to circular tension (tension receptors) and those responding to mucosal stroking and circular tension, which we have termed tension/mucosal (TM) receptors. 3. The conduction velocities for mucosal, TM and tension receptor fibres were 6.38 +/- 1.22 m s-1 (n = 22), 6.20 +/- 1.49 m s-1 (n = 13) and 5.33 +/- 0.86 m s-1 (n = 22), respectively. 4. Receptive fields of afferents showed random topographical distribution by fibre type and conduction velocity. They were found mainly distal but also occasionally proximal to the point of vagal dissection. 5. Twenty-eight per cent of mucosal, 63% of TM and 43% of tension receptors responded to one or more drugs or chemical stimuli applied to the receptive field. 6. In conclusion, this experimental preparation provides evidence for the existence of three types of oesophageal vagal afferent fibre, namely mucosal, tension and the newly identified tension/mucosal receptors.

Gastric volume detection after selective vagotomies in rats

Rats receiving intragastric infusions of 2.5, 5.0, 7.5, or 10.0 ml of normal saline while their pylori are reversibly occluded suppress meal size to the smallest infusion and display a dose-dependent reduction across volumes [Phillips, R. J., and T. L. Powley. Am. J. Physiol. 271 (Regulatory Integrative Comp. Physiol. 40): R766-R779, 1996]. To evaluate the contributions of the vagus to this detection of gastric volume, groups prepared with different selective vagotomies and equipped with pyloric cuffs and gastric catheters were tested. Liquid diet consumption during a 30-min feeding bout was measured after infusions of 5.0 and 10.0 ml of normal saline on cuff-open and cuff-closed trials. Consistent with earlier observations, sham animals with cuffs closed exhibited volume-dependent suppression of food intake to the infusions, and completely vagotomized animals did not inhibit feeding in response to the loads. In cuff-closed trials, the suppression function slopes of the selective vagotomy groups were intermediate to those of the shams and the completely vagotomized animals. Furthermore, for the different groups, the extent of suppression after vagotomy was proportional to the density of the afferent innervation respective branches supplied to the stomach. Specifically, the group with the gastric branches spared (nonsignificantly attenuated in comparison to shams) and the group with only the hepatic branch spared (significantly attenuated with respect to shams) both still exhibited significant dose-dependent suppression slopes (compared with completes), whereas the group with only celiac branches spared was not significantly different from completely vagotomized animals. In sum, the vagus nerve mediates the detection of the gastric volumes tested, and the different branches of the vagus make distinctive contributions to this afferent feedback.

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.

Vagovagal reflex motility patterns of the rat esophagus

Esophageal reflex motility and its neural correlates were investigated in 94 urethan-anesthetized adult male albino rats. When distended by means of a stationary balloon, the cervical and thoracic esophageal portion responded with a single pressure wave (type I response), whereas the diaphragmatic (intercrural) segment exhibited rhythmic contractions (type II response). Balloon deflation resulted in an off response aboral to the balloon. Bilateral cervical vagotomy or systemic D-tubocurarine abolished all types of reflex responses. Both type I and type II responses were associated with multiunit discharges in the central subnucleus of the solitary tract complex (NTSC) and the compact formation of the nucleus ambiguus (AMBC). Type I discharges, consisting of single bursts, and type II discharges, consisting of rhythmic 0.6-Hz bursts, preceded intraesophageal pressure waves in a fixed phase relationship, persisted after contralateral vagotomy, and were eliminated by ipsilateral vagotomy. During neuromuscular paralysis, peak intraburst discharge rates were reduced in both the NTSC and AMBC, with a concomitant decrease in rhythmicity. It is concluded that bolusevoked peristalsis of the rat esophagus is 1) segmentally organized; 2) effected by a bilateral uncrossed reflex arc consisting of vagal viscerosensory, NTSC premotor, and AMBC motoneurons innervating the striated muscle tunic and 3) strongly facilitated by reafferent feedback.

Tyramide amplification allows anterograde tracing by horseradish peroxidase-conjugated lectins in conjunction with simultaneous immunohistochemistry

Current protocols for a combined approach of anterograde tracing with carbocyanine dyes or horseradish peroxidase (HRP) conjugates and immunohistochemistry represent a compromise between sensitive detection of the tracer and the immunohistochemical procedure. Therefore, it was investigated whether the use of tyramide amplification allows sensitive anterograde tracing with wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) in conjunction with simultaneous immunohistochemistry. Vagal afferents were anterogradely labeled by injection of WGA-HRP into the nodose ganglion of rats. By use of tyramide-biotin amplification, a dense fiber plexus of vagal afferents was visualized centrally in the nucleus of the solitary tract and in retrogradely labeled neurons in the dorsal vagal nucleus. In the esophagus and duodenum, large- and small-caliber vagal fibers and terminals could be demonstrated comparably to conventional tracing technique using carbocyanine dyes or WGA-HRP and TMB histochemistry. Combination with immunohistochemistry could easily be done, requiring only one more incubation step, and did not result in loss of sensitivity of the tracing. With this method and confocal microscopy, the presence of Ca binding proteins in vagal afferent terminals could be demonstrated. Tyramide amplification allows sensitive anterograde tracing with low background staining in conjunction with immunohistochemistry of a-axonal markers.

Vagal efferent fibre responses to gastric and oesophageal mechanical and chemical stimuli in the ferret

Gastric and oesophageal afferent inputs to vagal efferent fibres were investigated in Urethane anaesthetized ferrets. Mechanical, chemical, and pharmacological stimuli were tested and efferent activity recorded from single cervical vagal fibres. Fibres showed either no basal discharge or low frequency, irregular patterns of resting discharge; only those which showed > 50% excitation or inhibition of basal activity with both gastric distension and oesophageal balloon distension were studied further. These responses were rapid and maintained only for the duration of the stimuli. 18/32 efferent fibres tested also showed changes in discharge in response to acid infused slowly into the distal oesophagus. These responses were larger after repeated acid infusions. Subsequent intra-oesophageal capsaicin elicited a similar response in 7/8 fibres. These responses were reproducible with repeated capsaicin infusions in 2/4 fibres and desensitized in 2/4 fibres. 2 capsaicin-responsive fibres were unresponsive to oesophageal acidification. 4/12 fibres tested responded to close intraarterial injections of capsaicin and 9/12 to close intraarterial bradykinin. These responses were brief and of short latency. Vagal efferent responses to mechanical and chemical stimuli above were unchanged after the NK-1 receptor antagonist CP96,345 (4 mg/kg i.v.). Subsequently, bilateral vagotomy caudal to the recording site abolished the basal activity in 4/7 fibres. In the 3 fibres where spontaneous activity remained, none of these responded to oesophageal distension or intra-oesophageal acid (2/2 fibres tested) after vagotomy, whereas 2/2 fibres tested still responded to gastric distension. The response of 1 fibre to intraarterial bradykinin and capsaicin was unchanged by vagotomy. We conclude that vagal efferent neurones respond to gastro-oesophageal mechanical inputs and also receive convergent input from oesophageal acid-sensitive and gastrointestinal bradykinin- and capsaicin-sensitive afferents. These afferent inputs are not mediated via NK-1 receptors. There also exists a nonvagal afferent input onto vagal efferent neurones which is probably spinal and likewise non NK-1 receptor mediated.

Lower oesophageal sphincter responses to noxious oesophageal chemical stimuli in the ferret: involvement of tachykinin receptors

Repeated oesophageal acidification is a definitive feature of gastro-oesophageal reflux disease, which in turn is caused by relaxation of the lower oesophageal sphincter (LOS). This study in anaesthetised ferrets investigates the reflex pathways involved in effects of oesophageal acidification on motor function of the LOS, with particular focus on the role of tachykinins. LOS pressure was monitored with a perfused micromanometric sleeve assembly. Oesophageal acidification reduced LOS pressure by 48 +/- 5% until washout with saline. This reduction became larger with repeated tests, and was unaffected in amplitude by acute bilateral vagotomy, although the response became slower in onset. Intra-oesophageal capsaicin (0.5% solution) caused a 68 +/- 17% decrease in LOS pressure which remained unchanged with repeated tests. The NK-1 receptor antagonist CP96,345 (1-5 mg/kg intravenous (i.v.) blocked the post-vagotomy LOS responses to both intra-luminal acid and capsaicin. Close intra-arterial (i.a.) injections of capsaicin (1-100 micrograms) gut induced LOS relaxation which was neither vagally nor NK-1 receptor-mediated. Substance P or the selective NK-1 receptor agonist [Sar9, Met(O2)11] substance P (25-500 ng close i.a.) caused a biphasic LOS response, consisting of initial brief contraction followed by prolonged, dose-dependent relaxation. Tetrodotoxin (10 micrograms/kg close i.a.) changed the biphasic response to substance P to excitation only. The neurokinin-1 (NK-1) receptor antagonist CP96,345 (0.3-10 mg/kg i.v.) dose-dependently reduced the inhibitory response to substance P. The excitatory phase of the response to substance P was larger and prolonged after guanethidine (5 mg/kg, i.v.), or propranolol (1 mg/kg, i.v.). L-NAME (100 mg/kg i.v.) reduced the inhibitory phase. The selective NK-2 receptor agonist [beta-Ala8] neurokinin A(4-10) caused LOS excitation only. These data indicate that intra-oesophageal acid causes substance P release from extrinsic afferent nerve endings which activates local inhibitory pathways to the LOS via NK-1 receptors.

Ultrastructural relationships of spinal primary afferent fibres with neuronal and non-neuronal cells in the myenteric plexus of the cat oesophago-gastric junction

Spinal primary afferent fibres innervating the myenteric area in the oesophago-gastric junction of the cat were selectively labelled by anterogradely transported cholera toxin B subunit-horseradish peroxidase conjugate injected into thoracic dorsal root ganglia. The ultrastructure of these labelled primary afferent fibres was studied in order to determine whether they display close relationships with specific cell types in the myenteric plexus. Horseradish peroxidase was revealed with tetramethylbenzidine stabilized with ammonium heptamolybdate or with the tetramethylbenzidine/tungstate reaction in order to visualize the cytoplasmic organelles and the axolemma, respectively. The labelled primary afferent fibres were unmyelinated. Two kinds of profiles of labelled fibres containing vesicles and mitochondrial accumulations were found: (i) fibres running in myenteric connectives in isolated nerve bundles, and (ii) fibres within the myenteric ganglia. The first kind had small areas of axolemma with no glial cell covering, whereas the second kind had little or no glial cell covering (termed naked primary afferent fibres). In addition, labelled fibres containing few vesicles and mitochondria and running in nerve bundles surrounded by perineurium were numerous. Within the myenteric ganglia, naked primary afferent fibres contacted myenteric neurons. The contacts were mainly axosomatic. No synaptic specializations were distinguished. In the interganglionic area, some labelled fibres terminated close to blood vessels. The intraganglionic naked primary afferent fibres are suggested to be mechanoreceptors. Their exposed axolemma might allow both mechanotransduction and release of neurotransmitters which could act on myenteric neurons. Because they are protected by their glial cell sheath and by bundles of collagen fibrils, interganglionic primary afferent fibres are likely to be less exposed to deformation.

Development of nicotinic receptor clusters and innervation accompanying the change in muscle phenotype in the mouse esophagus

During development, the external muscle of the mouse esophagus undergoes a transdifferentiation from smooth to striated muscle (Patapoutian et al. [1995] Science 270:1818-1821). We now report on the development of the innervation accompanying the change in phenotype of the external muscle of the mouse esophagus. The phenotype of the muscle was monitored by using light and electron microscopy. Nicotinic acetylcholine receptors were localised by using a fluorescence conjugate of alpha-bungarotoxin, and neural elements were localised by using antisera to synaptophysin (a synaptic vesicle protein that was used to label all nerve terminals), the vesicular acetylcholine transporter (VAChT), calcitonin gene-related peptide (CGRP), nitric oxide synthase (NOS), and vasoactive intestinal peptide (VIP). CGRP and VAChT were co-localised in the terminals of vagal motoneurons that innervate the external muscle, and NOS and VIP were co-localised in intrinsic (enteric) neurons, which provide some terminals that are associated with motor endplates. Cells exhibiting striations were first observed in the outer layers of the most rostral regions of the esophagus of embryonic day 15 (E15) mice. Clusters of nicotinic acetylcholine receptors were also first observed at the rostral end of the esophagus of E15 mice, and developed in a rostrocaudal progression that coincided with the appearance of striations within the muscle cells. Synaptophysin-, VAChT- and NOS-immunoreactive nerve terminals were present within the external muscle prior to the formation of receptor clusters, and their appearance did not follow any apparent rostrocaudal sequence. Surprisingly, not all of the receptor clusters at E15 had synaptophysin- and VAChT-immunoreactive nerve terminals closely associated with them. However, from E18 on, almost all of the clusters had synaptophysin-immunoreactive nerve terminals in close association. At late embryonic and early postnatal stages, there was a rostrocaudal gradient in the proportion of receptor clusters having VAChT-immunoreactive nerve terminals associated with them. Nerve terminals associated with nicotinic receptor clusters did not show detectable CGRP-immunoreactivity until one to two weeks after the appearance of synaptophysin- and VAChT-immunoreactivity. The NOS-immunoreactive neurons did not show detectable VIP-immunoreactivity until three days after NOS could be detected. These results show that the appearance of clusters of nicotinic receptors in the external muscle of the esophagus coincides with the expression of a striated muscle phenotype, but not with the presence of ingrowing nerve terminals. However, many of the receptor clusters that were observed first were apparently uninnervated.

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.

Location of sensory nerve cells that provide calbindin-containing laminar nerve endings in myenteric ganglia of the rat esophagus

To determine the origin of the calbindin-containing laminar nerve endings in the myenteric ganglia of the rat esophagus, retrograde tracing experiments combined with immunohistochemistry using an antibody for calbindin were carried out. After Fast blue was injected into the cervical portion of the esophagus, labeled neurons were found bilaterally in the nodose ganglion and dorsal root ganglia of C1 to T3. 80% of the total neurons in the nodose ganglion and 20% of those in the dorsal root ganglia showed calbindin immunoreactivity. Moreover, 79% of Fast-blue-labeled neurons found in the nodose ganglion and 18% of those in the dorsal root ganglia were immunoreactive for calbindin. These results suggest that the calbindin antibody we used is useful as a marker for identifying esophageal vagal afferents derived from the nodose ganglion. The calbindin-immunoreactive nerve fibers forming the laminar endings in the myenteric ganglia of the rat cervical esophagus are mainly derived from sensory neurons in the nodose ganglion and partly derived from those in the cervical and upper thoracic dorsal root ganglia. Calbindin-containing laminar nerve endings may be related to mechanoreceptors in the esophagus.

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.

Galanin-immunoreactive nerve terminals innervating the striated muscle fibers of the rat esophagus

Galanin (GAL) immunohistochemistry combined with acetylcholinesterase (AChE) histochemistry was applied to demonstrate the innervation of the rat esophageal muscle coats. GAL immunoreactivity was found in a number of nerve cell bodies in the myenteric ganglia and in numerous varicose and non-varicose nerve fibers in the myenteric plexus and around blood vessels. Many GAL-positive varicose fibers ran in the internodal strands and along the striated muscle fibers. They often ramified and terminated on the muscle fibers to form arborizing structures, which were most abundant in the thoracic portion of the esophagus. Such GAL-positive terminals were localized in most (87.7%) of AChE-reactive motor endplates on the esophageal striated muscles. Left supranodose vagotomy caused a significant decrease of the GAL-arborizing terminals on the striated muscles of the esophagus. This suggests that they are terminals of efferent fibers in the vagus nerve.

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".

Three-dimensional reconstructions of autonomic projections to the gastrointestinal tract

Three-dimensional reconstruction protocols in confocal microscopy are typically considered in terms of rendering separate stacks of optical sections. Single stacks, however, include volumes that are often too small to permit descriptions of entire neurons, complete axonal arbors, or complex neural networks. Furthermore, traditional tissue preparation protocols generally yield specimens too limited to permit reconstructions of complex neural systems. For 3-D analyses of extensive networks such as the autonomic nervous system projections within the viscera, it is critical to incorporate appropriate tissue techniques, including suitable tracer protocols, into the reconstruction strategy. This report summarizes complementary technologies, including whole mount procedures, tracer techniques for identifying single fibers in situ, and methods of examining stacks of optical images, which make it practical to describe the complete terminal field of an individual axon in the gastrointestinal tract. Such methods establish that vagal motor axons travel long distances within their target organs, collateralize frequently, and ramify extensively. Vagal afferents have extensive, complex, and, in some cases, polytopic arbors within target tissues.

Immunohistochemical demonstration of calbindin-containing nerve endings in the rat esophagus

Immunoreactivity for calbindin was found in nerve endings with irregular laminar shapes in the rat esophagus. In the myenteric ganglia, laminar endings of a range of sizes formed a complex network and appeared to lie at the surface of the ganglion. The myenteric ganglia that contained nerve endings were most abundant in the upper portion of the esophagus, their number decreasing orally to anally. Calbindin-immunoreactive nerve cell bodies were scattered throughout the esophagus. Laminar terminals were found in the connective tissue of the lamina propria immediately beneath the epithelium and in the muscularis mucosae. Occasional nerve branches formed a network of aborizing endings that surrounded part of the submucosal arterioles. Immunoreactive nerve endings in the mucosa and submucosa were present only in the upper part of the cervical esophagus. Unilateral vagotomy caused a remarkable decrease in the number of the myenteric ganglia containing the calbindin-immunoreactive laminar endings after 15 days or survival; in some of ganglia, the laminar structures disappeared and nerve endings showing weak immunoreactivity had an indistinct appearance, so that the outline of the ganglia became obscure. In operated rats at 24 days, the number of innervated ganglia was about half that in normal rats. However, there was no change in the morphology and the occurrence of the immunoreactive laminar structures in the mucosa and submucosa after denervation. The results show that many of the laminar endings that are immunoreactive for calbindin in the myenteric ganglia are derived from the vagus nerve. Thus, the calbindin-immunoreactive nerve endings with laminar expansions that are found in the rat esophageal wall could be sensory receptors.

Morphological relationships of choleragenoid horseradish peroxidase-labeled spinal primary afferents with myenteric ganglia and mucosal associated lymphoid tissue in the cat esophagogastric junction

The goal of the present study was to gain insight into the environmental factors influencing the activity of primary spinal afferent fibers in the different layers of the esophagogastric junction of the cat and, thus, to analyze the relationships of these afferents with various cellular components. Spinal primary afferent fibers were selectively labeled by anterogradely transported choleragenoid horseradish peroxidase conjugate (B-HRP). B-HRP was injected into the thoracic dorsal root ganglion at the T8-T13 levels. 6-Hydroxydopamine-induced sympathectomy was performed prior to B-HRP injection in order to prevent otherwise unavoidable labeling of sympathetic fibers in the gut wall. Numerous labeled fibers ran between, around, and within the myenteric ganglia. Others crossed the muscle layers directly and entered the mucosa, where some ran near granulocytes and around or through solitary lymphoid follicles. Labeled fibers were observed in the squamous esophageal epithelium but not in the fundic glandular epithelium. The fibers in the myenteric area are probably connected to the muscular tension receptors that have been detected by electrophysiologic techniques. This assumption is based on the observation that only a few fibers appear to terminate in muscle layers and on the fact that the myenteric area is very narrow and subject to powerful forces. Fibers in the myenteric ganglia could be involved in local efferent functions. Fibers in the mucosa could act as nociceptors and might be involved in local immunological responses.

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.

Rectospinal neurons: cell bodies, pathways, immunocytochemistry and ultrastructure

A novel class of enteric neurons projecting directly from the rectal wall to the spinal cord, "rectospinal neurons", was investigated in rats by combined retrograde neuronal tracing, immunocytochemistry and electron microscopy. Rectospinal neurons were almost confined to myenteric ganglia of the distal rectum below the pelvic diaphragm and were labeled preferentially by injections into spinal cord segments L6/S1. Injections into more rostral spinal cord segments resulted in hardly any labeled enteric neurons. Dorsal and ventral rhizotomy experiments indicated an almost exclusive projection of rectospinal neurons through dorsal roots L6/S1 to the respective spinal cord segments. Among various peptides immunostained, vasoactive intestinal polypeptide and calcitonin gene-related peptide were selectively found in rectospinal neurons, which were also shown to contain calbindin, neurofilament protein- and peripherin-immunoreactivity. Vasoactive intestinal polypeptide- and calbindin-immunostaining were frequently co-localized in the same perikarya, while calcitonin gene-related peptide-immunoreactive rectospinal neurons probably represented a separate population. Neonatal capsaicin treatment did not significantly reduce the number of rectospinal neurons. Electron microscopy revealed synaptic contacts on the surface of rectospinal neurons. Taken together, these results establish rectospinal neurons as an anatomically and neurochemically distinct class of enteric neurons. Synaptic contacts on rectospinal neurons suggest that these neurons may function as a direct link from the enteric to the central nervous system, thus indicating that connections between these two networks are reciprocal.

The brainstem esophagomotor network pattern generator: a rodent model

The evidence reviewed in this essay supports the following working model of the central function generator for esophageal peristalsis in the rat: solitarial subnucleus centralis (NTSc) neurons operate in a dual capacity as esophagomotor reflex interneurons and as command neurons programming respective outputs from nucleus ambiguus compact formation (AMBc) motoneurons during secondary and primary peristalsis. In both conditions, there is a critical requirement for cholinergic input which enables NTSc neurons to generate the timed sequence of AMBc motoneuronal activity. In primary peristalsis, the cholinergic coupling mechanism is activated centrally, probably via projections from deglutitive premotor neurons to the parvicellular reticular formation and thence to the NTS. In reflex (or secondary) peristalsis, the cholinergic input could in part be generated by cholinergic vagal viscerosensory fibers innervating the esophagus. Postulated connections between NTS deglutitive neurons and the parvicellular cholinergic neurons of the intermediate reticular formation have yet to be demonstrated. Premotor input from NTSc to AMBc is generated by somatostatinergic and excitatory aminoacidergic neurons. Coactivation of both inputs by cholinergic afferents is necessary to generate esophagomotor output from AMBc neurons. The model under study is derived from investigations into central mechanisms governing striated muscle peristaltic activity. Whether the basic operational principles revealed thus far apply to peristaltic pattern generation in species with a smooth muscle esophagus, requires further investigation.

Tyrosine-hydroxylase-containing vagal afferent neurons in the rat nodose ganglion are independent from neuropeptide-Y-containing populations and project to esophagus and stomach

Immunoreactivity to the rate limiting enzyme of catecholamine synthesis, tyrosine hydroxylase, has been described in the inferior sensory (= nodose) ganglion of the vagal nerve in the rat. The aim of the present study was to characterize further this neuronal population. The neurons do not represent displaced autonomic efferent neurons, since they do not receive synaptic input, as indicated by the absence of synaptophysin-immunoreactive terminals. In addition to the immunoreactivity to tyrosine hydroxylase, a tyrosine hydroxylase cRNA probe hybridizes with nodose ganglion neurons as demonstrated by in situ hybridization and Northern blotting. Many but not all of the tyrosine hydroxylase-immunoreactive neurons are also immunoreactive to the dopamine synthesizing enzyme, aromatic-L-amino-acid-decarboxylase, but lack the noradrenaline-synthesizing enzyme, dopamine-beta-hydroxylase, thus favoring synthesis of dopamine. Neuropeptide Y, which is often colocalized with catecholamines, is also present in a subset of nodose ganglion neurons, as indicated by immunohistochemistry, in situ hybridization and Northern blotting. However, double-labeling immunofluorescence has revealed that these two antigens are localized in different cell populations. Retrograde neuronal tracing utilizing fluorescent dyes (Fast blue, Fluoro-gold) combined with tyrosine hydroxylase immunohistochemistry has demonstrated that the esophagus and stomach are peripheral targets of tyrosine-hydroxylase-containing vagal viscero-afferent neurons.

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)

WGA-HRP and choleragenoid-HRP as anterogradely transported tracers in vagal visceral afferents and binding of WGA and choleragenoid to nodose ganglion neurons in rodents

The axonal and terminal labelling pattern in the brain stem resulting from the injection of horseradish peroxidase (HRP) conjugate of wheat germ agglutinin (WGA) or choleragenoid into the nodose ganglion of guinea pigs was examined. In addition, the binding profiles of WGA and choleragenoid in the nodose ganglion of guinea pig and rat were examined. The results show that WGA-HRP and choleragenoid-HRP (B-HRP) produce almost identical distribution of axonal and terminal labelling, the difference being some contralateral fibre labelling present only with B-HRP. However, WGA-HRP shows the strongest labelling at short survival times, whereas B-HRP requires longer postoperative survival times to reach maximum labelling intensity. All nodose ganglion neurons appear to bind WGA as well as choleragenoid although to a varying degree. The results of this and previous studies support the view that visceral sensory ganglion cells and the large light subpopulation of somatic dorsal root ganglion cells both bind choleragenoid, whereas the small dark somatic cells show affinity for WGA but rarely for choleragenoid.

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)

Evidence for dorsal root projection to somatostatin-immunoreactive structures in laminae I–II of the spinal dorsal horn

In order to determine if somatostatin (SOM)-immunoreactive (I) cell bodies and processes in lamina (L) II of the rat spinal cord receive dorsal root input, the latter were anterogradely labeled by wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). SOM-I structures were demonstrated by immunohistochemistry. Cell bodies labeled transscellularly or transsynaptically by WGA-HRP and immunohistochemically for SOM were present in L II. In addition, a L I cell was double labeled. These results suggest that some dorsal root axons innervate SOM-I neurons in L I-II of the spinal cord. In addition to confirming immunohistochemical observations in published reports, we have revealed SOM-I central terminals in the type II glomerulus. Further, a SOM-I CI-terminal, presumed to be of primary afferent origin, contacted a SOM-I dendrite in L II. Since SOM has been implicated in nociceptive function in the dorsal horn, it is possible that some of the SOM-I structures identified are involved in nociceptive processing.

Effects of tachykinins on myenteric neurones of the guinea-pig gastric corpus: involvement of NK-3 receptors

Responses of gastric myenteric neurones evoked by the mammalian tachykinins substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) were investigated using conventional intracellular recording methods. Application of the tachykinins caused a long lasting depolarization of the membrane potential which was associated with increased spike discharge and augmented excitability of the cells. The responses slowly desensitized. Additionally, cross desensitization occurred between SP, NKA and NKB. Both the NK-1 receptor agonist [Sar9,MetO2(11)]SP and the NK-2 receptor agonist [beta-Ala8]NKA(4-10) had no effect on the electrical properties of the neurones. Only the NK-3 receptor agonist [MePhe7]NKB mimicked the excitatory response observed during SP, NKA and NKB applications. [MePhe7]NKB-induced desensitization abolished the response to SP, NKA and NKB. However, long lasting applications of [Sar9,MetO2(11)]SP or [beta-Ala8]NKA(4-10) had no effect on the SP, NKA or NKB responses. The excitatory effect of SP, NKA and NKB remained unchanged during application of the tachykinin analogues [D-Arg1,D-Trp7,9,Leu11]SP and [Tyr5,D-Trp6,8,9,Arg10]NKA(4-10). The results indicate that SP, NKA and NKB act as excitatory neuromodulators within the enteric nervous system of the stomach. The effects of SP, NKA and NKB appeared to be mediated by activation of NK-3 receptors.

Distribution of vagal afferent fibers of the guinea pig heart labeled by anterograde transport of conjugated horseradish peroxidase

To determine the distribution of vagal afferent fibers in the heart, wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) or choleragenoid conjugated horseradish peroxidase (B-HRP) was injected into nodose ganglia of guinea pigs. Anterogradely labeled fibers and beaded, terminal-like arborizations were observed in the ascending aorta and aortic arch, the pulmonary trunk and arteries, posterior atrial walls, atrioventricular valves, and ventricles. Control experiments with injection of B-HRP into the cervical vagus nerve indicated that labeled fibers observed in the heart originated from sensory neurons in the nodose ganglia. Neither the density nor distribution of labeling differed between WGA-HRP and B-HRP. Injection of tracer into the left or right nodose ganglion shows that these regions of the heart are bilaterally innervated, although labeling in the left or right posterior atrium was denser after injection into the ipsilateral ganglion. Comparison with a previous study on the distribution of sympathetic afferent fibers in the guinea pig heart suggests that the two afferent systems maintain a complementary, but not mutually exclusive, distribution within the ventricles. Whereas afferents with their source in the spinal ganglia are mainly distributed with coronary arteries on the anterior-superior surface of the ventricles, afferent fibers with their source in the nodose ganglia are concentrated within peri-arterial regions of the posterior-inferior ventricular epicardium and the posterior septal ventricular endocardium. These differences in distributions of afferent systems could play a role in post-infarction autonomic dysfunction and in the symptoms that accompany angina pectoris.

Role of peptidergic sensory neurons in gastric mucosal blood flow and protection

The present findings have revealed a new aspect of how mechanisms of gastric mucosal resistance to injury are called into effect and are coordinated by the nervous system. Capsaicin-sensitive sensory neurons in the stomach play a physiological role in monitoring acid influx into the superficial mucosa. Once activated, they strengthen gastric mucosal defense against deep injury, with a key process in this respect being an increase in blood flow through the gastric mucosa. This concept opens up completely new perspectives in the physiology and pathophysiology of the gastric mucosa if we consider that the long-term integrity of the gastric mucosa may be under the subtle control of acid-sensitive sensory neurons and that, vice versa, improper functioning of these neutral control mechanisms may predispose to gastric ulcer disease. The present observations also indicate that some of the peptides contained in gastric sensory nerve endings might fulfill a transmitter or mediator role in controlling gastric mucosal blood flow and integrity. Whereas substance P and neurokinin A are unlikely to play a role in the regulation of gastric mucosal blood flow, there is severalfold evidence that CGRP is very important in this respect. This peptide, which in the rat gastric mucosa originates exclusively from spinal sensory neurons, is released upon stimulation of sensory nerve endings and is extremely potent in facilitating gastric mucosal blood flow and in protecting the mucosa from injurious factors. Selective ablation of spinal sensory neurons containing CGRP weakens the resistance of the gastric mucosa against acid injury, which is most likely due to inhibition of protective vasodilator reflexes. We now aim at providing direct pharmacological evidence that antagonism of endogenously released CGRP results in similar pathophysiological consequences as ablation of capsaicin-sensitive sensory neurons.

Modulation of gastric contractions in response to tachykinins and bethanechol by extrinsic nerves

1. Extrinsic reflexes elicited by changes in gastric wall tension play an important role in regulating gastric tone. The present study investigated whether such reflexes modulate gastric contractions induced by close arterially administered neurokinin A (NKA), substance P (SP), SP-methylester and bethancehol in anaesthetized rats. 2. Reflex pathways were acutely interrupted by either subdiaphragmatic vagotomy or prevertebral ganglionectomy. C-fibre afferent nerve activity was abolished by pretreating rats with capsaicin 10 to 16 days before the experiments. 3. The order of potency in inducing gastric contractions was NKA greater than SP greater than bethanechol. SP-methylester was markedly less effective than SP and its effects did not fit sigmoid dose-response curves (DRCs). The maximal responses to NKA, SP, and bethanechol were similar, whilst the DRC for SP was significantly flatter than those for NKA or bethanechol. Pretreatment of the rats with the peptidase inhibitors phosphoramidon or captopril did not increase the contractile response to SP. 4. Prevertebral ganglionectomy had no significant effect on the DRCs for SP and NKA, whereas vagotomy shifted the DRCs for all three test substances to the left. 5. Capsaicin pretreatment did not change the DRC for NKA in rats with intact vagus but shifted that for bethanechol to the left. The leftward of the DRC for NKA caused by vagotomy was prevented in capsaicin-pretreated rats whereas the vagotomy-induced shift of the DRC for bethanechol remained unaltered. The shift of the DRC for SP seen in response to vagotomy was only slightly reduced by capsaicin pretreatment. 6. These data may be interpreted as demonstrating two neuronal mechanisms for modulating drug-induced gastric contractions. First, the contractions themselves activate a vago-vagal negative feedback involving capsaicin-sensitive afferents. Second, NKA, and to a lesser degree SP, seem to induce a nonvagal non-splanchnic mechanism which via capsaicin-sensitive afferent neurones reinforces tachykinininduced gastric contractions.

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.

Central neural control of esophageal motility: a review

We review recent studies on the central neural control of esophageal motility, emphasizing the anatomy and chemical coding of esophageal pathways in the spinal cord and medulla. Sympathetic innervation of the proximal esophagus is derived primarily from cervical and upper thoracic paravertebral ganglia, whereas that of the lower esophageal sphincter and proximal stomach is derived from the celiac ganglion. In addition to noradrenaline, many sympathetic fibers in the esophagus contain neuropeptide Y (NPY), and both noradrenaline and NPY appear to decrease blood flow and motility. Preganglionic neurons innervating the cervical and upper thoracic ganglia are located at lower cervical and upper thoracic spinal levels. The preganglionic innervation of the celiac ganglion arises from lower thoracic spinal levels. Both acetylcholine (ACh) and enkephalin (ENK) have been localized in sympathetic preganglionic neurons, and it has been suggested that ENK acts to pre-synaptically inhibit ganglionic transmission. Spinal afferents from the esophagus are few, but have been described in lower cervical and thoracic dorsal root ganglia. A significant percentage contain calcitonin gene-related peptide (CGRP) and substance P (SP). The central distribution of spinal afferents, as well as their subsequent processing within the spinal cord, have not been addressed. Medullary afferents arise from the nodose ganglion and terminate peripherally both in myenteric ganglia, where they have been postulated to act as tension receptors, and, to a lesser extent, in more superficial layers. Centrally, these afferents appear to end in a discrete part of the nucleus of the solitary tract (NTS) termed the central subnucleus. The transmitter specificity of the majority of these afferents remains unknown. The central subnucleus, in turn, sends a dense and topographically discrete projection to esophageal motor neurons in the rostral portion of the nucleus ambiguous (NA). Both somatostatin-(SS) and ENK-related peptides have been localized in this pathway. Finally, motor neurons from the rostral NA innervate striated portions of the esophagus. In addition to ACh, these esophageal motor neurons contain CGRP, galanin (GAL), N-acetylaspartylglutamate (NAAG), and brain natriuretic peptide (BNP). The physiological effect of these peptides on esophageal motility remains unclear. Medullary control of smooth muscle portions of the esophagus have not been thoroughly investigated.

Vagal afferent fibers almost exclusively innervate islets in the rat pancreas as demonstrated by anterograde tracing

Anterograde tracing from the nodose ganglion with wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was utilized to investigate vagal afferent innervation of the rat pancreas. Labelled afferent fibers were consistently detected in islets in all animals. Only about 10% of islets were labelled even in best cases. In only 5 out of 16 rats, acini and excretory ducts received afferent innervation. Injections into the right nodose ganglion resulted in labelling preferentially within the splenic lobe, whereas injections into the left ganglion labelled fibers predominantly in the duodenal lobe. These results point to a non-random distribution of vagal afferent fibers within the pancreas, and suggest a role for these afferents in the regulation of endocrine pancreatic function.

Presence of cholinergic neurons in the vagal afferent system: biochemical and immunohistochemical approaches

The presence of cholinergic fibers in the afferent vagal system of various species was shown using biochemical and immunohistochemical methods. Biochemical activity of choline acetyl transferase, the synthesizing enzyme for acetylcholine, was detected in the nodose ganglion of cat, rabbit, dog and sheep. Immunohistochemistry, using a monoclonal antibody raised against choline acetyl transferase, revealed labelled cell bodies in the nodose ganglion of the rabbit. Acetylcholine endogenous content, measured in nodose ganglia devoid of efferent fibers, was twice as high in the right ganglion as compared to the left. Enzyme transport and choline acetyl transferase activity analysis were each determined on separate peripheral vagus nerves. These results are discussed in terms of functional properties of the vagal afferent neurons, including the modulation of vagal afferent messages at the level of the nodose ganglion and the eventual control of peripheral intrinsic neurons by sensory vagal terminals.

Vagal paraganglia of the rat

Paraganglia are associated with every branch of the rat vagus nerve except the pharyngeal branch. Some of the paraganglia closely resemble the glomus caroticum, whereas others appear like small, intensely fluorescent (SIF) cells of autonomic ganglia. The paraganglionic cells of SIF cell-like bodies (SLB) store catecholamines (the most abundant is probably noradrenaline) and in some cases neurotensin. The innervation pattern of SLB is variable and their physiological role remains unclear. Paraganglionic cells of glomus-like bodies (GLB) predominantly store dopamine and probably also to a lesser extent noradrenaline. These putative chemoreceptor organs receive sensory innervation from nodose ganglion neurons as revealed by degeneration experiments and by anterograde neuronal tracing. Substance P- and calcitonin gene-related peptide-immunoreactive fibres seen in the region of vascular entry into the GLB may account for some of these sensory fibres, but the peptide/classical transmitter stored in sensory terminals synapsing on paraganglionic cells is unknown. Ultrastructural immunocytochemistry revealed vasoactive intestinal polypeptide (VIP)-immunoreactive fibres lying in the interstitial space between paraganglionic cells and large capillaries. These fibres may originate from VIP-immunoreactive neurons, being frequently attached to GLB. The major difference between GLB and the glomus caroticum concerns their blood supply and related innervation: Arteries and arterioles do not penetrate into GLB and, accordingly, noradrenaline- and neuropeptide Y-containing nerve fibres are lacking within GLB. This peculiar arrangement of paraganglionic parenchyma and arterial blood supply may be one of the reasons for the different physiological properties of vagal and carotid arterial chemoreceptors.

Viscerotopic representation of the upper alimentary tract in the rat: sensory ganglia and nuclei of the solitary and spinal trigeminal tracts

The aim of this study was to map the viscerotopic representation of the upper alimentary tract in the sensory ganglia of the IXth and Xth cranial nerves and in the subnuclei of the solitary and spinal trigeminal tracts. Therefore, in 172 rats 0.5-65 microliters of horseradish peroxidase (HRP), wheat germ agglutinin-HRP, or cholera toxin-HRP were injected into the trunks and major branches of the IXth and Xth cranial nerves as well as into the musculature and mucosa of different levels of the upper alimentary and respiratory tracts. The results demonstrate that the sensory ganglia of the IXth and Xth nerves form a fused ganglionic mass with continuous bridges of cells connecting the proximal and distal portions of the ganglionic complex. Ganglionic perikarya were labeled in crude, overlapping topographical patterns after injections of tracers into nerves and different parts of the upper alimentary tract. After injections into the soft palate, pharynx, esophagus, and stomach, anterograde labeling was differentially distributed in distinct subnuclei in the nucleus of the tractus solitarius (NTS). Palatal and pharyngeal injections resulted primarily in labeling of the interstitial and intermediate subnuclei of the NTS and in the paratrigeminal islands (PTI) and spinal trigeminal complex. Esophageal and stomach wall injections resulted in labeling primarily of the subnucleus centralis and subnucleus gelatinosus, respectively. The distribution of upper alimentary tract vagal-glossopharyngeal afferents in the medulla oblongata has two primary groups of components, i.e., a viscerotopic distribution in the NTS involved in ingestive and respiratory reflexes and a distribution coextensive with fluoride-resistant acid-phosphatase-positive regions of the PTI and spinal trigeminal nucleus presumably involved in visceral reflexes mediated by nociceptive or chemosensitive C fibers.

Simultaneous immunohistochemical demonstration of intra-axonally transported markers and neuropeptides in the peripheral nervous system of the guinea pig

Projections and peptide neurotransmitter/neuromodulator content of autonomic and visceral afferent neurons of the guinea pig were studied after application of the subunit B of cholera toxin (CTB) with or without horseradish peroxidase (HRP) as retrograde and anterograde tracers and subsequent immunohistochemical processing for double staining using antibodies raised to CTB, HRP and various neuropeptides. The results demonstrate that substance P (SP)- and calcitonin gene-related peptide (CGRP)-containing dorsal root ganglion cells project to the pylorus as well as to the celiac superior mesenteric and stellate ganglia as demonstrated with both retrograde and anterograde transport methodology. Binding studies revealed that a small number of the CTB-binding dorsal root ganglion cells contains immunoreactivity to SP and CGRP. The majority of the CTB-binding cells is SP- and CGRP-negative and terminate in the deeper parts of the dorsal horn. After injection of CTB conjugated to HRP (B-HRP) into the nodose ganglion, both motor and sensory elements were labeled in the medulla oblongata. Some of the CTB labeled vagal sensory nerve fibers in the nucleus tractus solitarii (NTS) were also found to contain immunoreactivity to SP or CGRP. The tracer was also transported through the peripheral branch of the nodose ganglion cells and labeled terminals in the esophagus.