Chemical stimulation of vagal afferent neurons and sympathetic vasomotor tone

Vagal afferents innervate a diverse range of structures of the thoracic and abdominal viscera. While a proportion of these afferents function as mechanoreceptors and respond to changes in intramural tension within the structures that they innervate, many also sense a broad range of chemical substances ranging from peptides, sugars and lipids present in the intraluminal contents of the gastrointestinal tract, as well as tissue prostanoids, cytokines and monoamines in the cardiopulmonary circulation. This review examines the effects of chemical stimulation of vagal afferents on circulatory and sympathetic vasomotor function. Notably, the von Bezold–Jarisch reflex is a cardiorespiratory reflex produced by chemical activation of cardiopulmonary vagal afferents. Classical stimulants of the von Bezold–Jarisch reflex include the Veratrum alkaloids and 5-HT₃ receptor agonists. Atrial natriuretic peptides are agents which also produce a von Bezold–Jarisch reflex-like response or a sensitisation of this reflex via an action on vagal afferents. Cholecystokinin (CCK) activates abdominal visceral vagal afferents, which apart from a clear role in mediation of satiety, also produces selective sympathetic vasomotor inhibition probably by inhibition of sub-groups of presympathetic vasomotor neurons of the rostral ventrolateral medulla. These actions of CCK may constitute a novel gastrointestinal-cardiovascular reflex. The afferent vagus transmits a diverse array of signals to the central nervous system, influencing sympathetic vasomotor and cardiomotor function, gastrointestinal function, neuroimmune function and endocrine function.     

Abdominal vagal afferent pathways and their distributions of intraganglionic laminar endings in the rat duodenum

We examined abdominal vagal afferents (n = 33) and the distributions of their intraganglionic laminar endings (IGLEs) in the duodenum. Rats (male, Sprague-Dawley) received a partial subdiaphragmatic vagotomy that spared a single branch. Wheat germ agglutinin-horseradish peroxidase (0.5-1.0 μl) was injected into the nodose ganglion ipsilateral to the vagotomized side. We observed that the hepatic branch does not project to the stomach, that the accessory celiac and celiac branches course along the celiac artery and innervate the intestines, and that the left nodose afferents innervate predominantly the duodenum. The hepatic branch innervates the duodenum via the "hepatoduodenal" subbranch and has the densest IGLE distribution in both the dorsoventral and the rostrocaudal extensions of the first 4-cm segment. Both gastric branches have two subbranches that innervate the duodenum; the "lesser curvature" subbranches follow the lesser curvature artery and may join the "hepatoduodenal" subbranch, whereas the "pyloric" subbranches run through the antrum and pylorus to reach the proximal duodenum. Moreover, the subbranches of ventral and dorsal gastric branches innervate more in the ventral and dorsal parts of the duodenum, respectively, and have more IGLEs in the rostral region than in the caudal. A posteriori comparisons indicate that, in the first-centimeter segment, the ventral gastric branch has significantly more IGLEs, whereas, in the third- and fourth-centimeter segments, the hepatic branch has more IGLEs. The finding that three different vagal branches innervate the duodenum with different densities of afferent endings might indicate a viscerotopic receptive field that coordinates digestive functions in feeding.     

Evaluation of vagal afferent modulation of the digastric reflex in cats

In the present study, we have examined the relative ability of cervical, thoracic, cardiac and diaphragmatic vagal stimulation to modulate the digastic reflex produced by tooth-pulp stimulation in anesthetized cats. The right maxillary tooth pulp was stimulated and the digastric reflex was recorded from the right digastric muscle. Cervical vagal stimulation produced a biphasic effect on the digastric reflex. The reflex was facilitated at conditioning test intervals less than 20 ms and inhibited at conditioning test intervals between 100 ms and 500 ms. Cardiac and thoracic vagal stimulation did not significantly facilitate the digastric reflex but inhibited the reflex at conditioning test intervals between 50 ms and 500 ms with maximum inhibition observed at 200 ms. In contrast, diaphragmatic vagal stimulation produced a weaker inhibition of the digastric reflex. The relative ability of different vagal segments to inhibit the digastric reflex was:thoracic=cardiac=cervical> diaphragmatic. The inhibitory effects were not related to cardiovascular responses to vagal afferent stimulation. These findings suggest cardiopulmonary vagal afferents represent an important source of vagal afferents which modulate the digastric reflex in the cat.     

The development of the emetic reflex in the house musk shrew, Suncus murinus

The emetic (retching and vomiting) reflex is an important component of the body's defence system against accidentally ingested toxins and emesis is also a common symptom of disease and a side-effect of a number of pharmacological therapies. The development of the reflex has been the subject of few systematic studies. The aim of this study was to characterise the development of the emetic reflex in Suncus murinus (the house musk shrew) using emetic stimuli acting via three different afferent pathways: motion via the vestibular system, pyrogallol via abdominal vagal afferents and resiniferatoxin (a capsaicin analog) via the brainstem. The emetic reflex was not present to any stimulus prior to postnatal day 10 but the onset of the response to motion lagged behind that to the other stimuli in not being present until postnatal day 15. Body weight was not a determinant of the presence of the reflex. It is proposed that the delayed presence of the emetic reflex in Suncus makes it an ideal species in which to investigate factors regulating its development.     

Ventricular reflex interactions during transmural myocardial ischemia

The role of left ventricular receptors with sympathetic afferent fibers in the reflex response to myocardial ischemia is controversial, particularly in the canine model. Previous experiments have shown that reflex excitatory responses mediated by left ventricular sympathetic afferents can be detected in sinoaortic denervated and vagotomized dogs during transmural myocardial ischemia. The purpose of these experiments was to determine if reflex excitatory responses occur in dogs with intact left ventricular vagal afferents. Experiments were performed in 27 chloralose-anesthetized dogs with sinoaortic denervation. Responses of efferent renal sympathetic nerve activity, arterial, and left atrial pressures to transmural and non-transmural inferoposterior myocardial ischemia were measured before and after interruption of left ventricular sympathetic afferents by stellectomy. The adequacy of sympathetic deafferentation was assessed by measurement of renal nerve responses to epicardial bradykinin. Prior to stellectomy, excitatory responses were observed in 10 animals and inhibitory responses in 9 animals. The remaining animals had no responses and were excluded from analysis. In the excitatory group, reflex increases in renal nerve activity during both transmural and non-transmural inferoposterior ischemia were abolished by stellectomy and not replaced by inhibitory responses. In the inhibitory group, non-transmural inferoposterior ischemia elicited greater reflex decreases in renal nerve activity when left ventricular sympathetic afferents were intact. After stellectomy, transmural ischemia elicited greater reflex inhibition of renal nerve activity. Renal nerve responses to epicardial bradykinin were abolished by stellectomy. These results indicate that reflex excitatory responses mediated by left ventricular receptors with sympathetic afferent fibers can be elicited in dogs with intact vagal afferents. These excitatory responses are most apparent during transmural myocardial ischemia. In dogs with inhibitory responses to coronary occlusion, activation of sympathetic afferents during transmural ischemia appears to attenuate reflex inhibitory responses mediated by left ventricular vagal afferents.     

TNFalpha: a trigger of autonomic dysfunction.

During disease, infection, or trauma, the cytokine tumor necrosis factor alpha (TNF alpha) causes fever, fatigue, malaise, allodynia, anorexia, gastric stasis associated with nausea, and emesis via interactions with the central nervous system. Our studies have focused on how TNF alpha produces a profound gastric stasis by acting on vago-vagal reflex circuits in the brainstem. Sensory elements of this circuit (i.e., nucleus of the solitary tract [NST] and area postrema) are activated by TNF alpha. In response, the efferent elements (i.e., dorsal motor neurons of the vagus) cause gastroinhibition via their action on the gastric enteric plexus. We find that TNF alpha presynaptically modulates the release of glutamate from primary vagal afferents to the NST and can amplify vagal afferent responsiveness by sensitizing presynaptic intracellular calcium-release mechanisms. The constitutive presence of TNF alpha receptors on these afferents and their ability to amplify afferent signals may explain how TNF alpha can completely disrupt autonomic control of the gut.     

Afferent innervation of the lower oesophageal sphincter of the cat. An HRP study

Labeling of afferent neurons by the retrograde axonal transport of horseradish peroxidase (HRP) was performed on anaesthetized cats in order to examine the afferent innervation of the lower oesophageal sphincter (LOS), involving both the vagal and the sympathetic nerves. The labeled cells, whose fibres follow the sympathetic pathways were found in dorsal root ganglia from T1 to L2. Nerve section experiments indicated that the main pathways involved were the splanchnic nerves, as expected from classical data. Additional pathways passing through the sympathetic cardiac branch emerging from the stellate ganglion and the thoracic sympathetic branches were also evidenced. This work corroborated the electrophysiological data showing the richness of the LOS sensory vagal innervation. Nevertheless, in this case the difficulties related to the HRP technique are particularly enhanced since the abdominal sensory vagal fibres can be affected by HRP injections.     

Area specific reflexes from normal and supernumerary hindlimbs of Xenopus laevis.

Two area specific reflexes elicited by natural stimulation of different regions of the hindlimbs of Xenopus laevis have been identified. Light or intense mechanical stimulation of the foot evokes reflex activity in the ipsilateral knee flexor nerve; moderate pressure applied to the calf evokes reflex activity predominantly in the ipsilateral knee extensor nerve. The reflex responses have been recorded electrophysiologically to overcome the limitations of behavioral observations for determining the presence of activity in particular muscles. Normal area specific reflexes are elicited in the normal ipsilateral hindlimb by stimulation of grafted supernumerary hindlimbs innervated either by hindlimb (lumbar) or by non-limb (thoracic) spinal cord segments. The area specific reflexes can be elicited only if the limb is grafted to a host younger than stage 54-55 of Nieuwkoop and Faber ('56), the stage at which reflex movements are first observed behaviorally. Abnormal reflex responses are evoked by stimulation of supernumerary limbs innervated by either thoracic or lumbar segments when the limb buds are grafted to older larvae. Supernumerary forelimbs grafted at early stages and innervated by either thoracic or lumbar spinal cord segments generally fail to elicit area specific reflex responses in the normal hindlimb. Single-unit recordings of afferent fibers supplying the normal and supernumerary hindlimbs show that each limb receives a separate nerve supply. No evidence for branched afferent fibers has been found. The implications of these results for theories of neuronal specification are discussed, particularly the hypothesis that peripheral tissues are able to specify the central actions of afferent fibers that innervate them.     

Organ- and function-specific anatomical organization of vagal fibers supports fascicular vagus nerve stimulation

Vagal fibers travel inside fascicles and form branches to innervate organs and regulate organ functions. Existing vagus nerve stimulation (VNS) therapies activate vagal fibers non-selectively, often resulting in reduced efficacy and side effects from non-targeted organs. The transverse and longitudinal arrangement of fibers inside the vagal trunk with respect to the functions they mediate and organs they innervate is unknown, however it is crucial for selective VNS. Using micro-computed tomography imaging, we tracked fascicular trajectories and found that, in swine, sensory and motor fascicles are spatially separated cephalad, close to the nodose ganglion, and merge caudad, towards the lower cervical and upper thoracic region; larynx-, heart- and lung-specific fascicles are separated caudad and progressively merge cephalad. Using quantified immunohistochemistry at single fiber level, we identified and characterized all vagal fibers and found that fibers of different morphological types are differentially distributed in fascicles: myelinated afferents and efferents occupy separate fascicles, myelinated and unmyelinated efferents also occupy separate fascicles, and small unmyelinated afferents are widely distributed within most fascicles. We developed a multi-contact cuff electrode to accommodate the fascicular structure of the vagal trunk and used it to deliver fascicle-selective cervical VNS in anesthetized and awake swine. Compound action potentials from distinct fiber types, and physiological responses from different organs, including laryngeal muscle, cough, breathing, and heart rate responses are elicited in a radially asymmetric manner, with consistent angular separations that agree with the documented fascicular organization. These results indicate that fibers in the trunk of the vagus nerve are anatomically organized according to functions they mediate and organs they innervate and can be asymmetrically activated by fascicular cervical VNS.     

The complex role of spindle afferent input, as evidenced by the study of posture control in normal subjects and patients

This report reviews recent findings from our laboratory on the connectivity of the group II spindle afferent input and the role played by these afferents in the control of quiet and perturbed human stance. At variance with group Ia fibres, which subserve the monosynaptic stretch reflex, the group II fibres, after having entered the spinal cord, make synamptic contacts with a short chain of interneurons which impinge on homonymous motoneurons. Analysis of the short- and medium-latency responses evoked in foot and leg muscles by perturbations of upright stance under different experimental conditions has revealed a role of group II fibres in the production of the medium-latency response. The conduction velocity of group II spindle afferent fibres and their central delay have also been estimated. Furthermore, data from normal subjects and from neuropathic and hemiparetic patients are in favour of a prevailing role of the input from group II fibres in the afferent control of quiet and perturbed stance. Since Ia fibres innervate receptors more sensitive to the veolocity of muscle stretch, and II fibres innervate receptors more sensitive to the absolute value of muscle length, it is hypothesised that the major role of the latter in the reflex control of stance reflects the slow velocity and amplitude of sway during quiet upright posture. Indirect evidence supports the conclusion that, also in humans, monoaminergic descending pathways from brainstem nuclei modulate the excitability of the circuits mediating the group II input.     

The complex role of spindle afferent input,as evidenced by the study of posture control in normal subjects and patients

This report reviews recent findings from our laboratory on the connectivity of the group II spindle afferent input and the role played by these afferents in the control of quiet and perturbed human stance. At variance with group Ia fibres, which subserve the monosynaptic stretch reflex, the group II fibres, after having entered the spinal cord, make synamptic contacts with a short chain of interneurons which impinge on homonymous motoneurons. Analysis of the short- and medium-latency responses evoked in foot and leg muscles by perturbations of upright stance under different experimental conditions has revealed a role of group II fibres in the production of the medium-latency response. The conduction velocity of group II spindle afferent fibres and their central delay have also been estimated. Furthermore, data from normal subjects and from neuropathic and hemiparetic patients are in favour of a prevailing role of the input from group II fibres in the afferent control of quiet and perturbed stance. Since Ia fibres innervate receptors more sensitive to the veolocity of muscle stretch, and II fibres innervate receptors more sensitive to the absolute value of muscle length, it is hypothesised that the major role of the latter in the reflex control of stance reflects the slow velocity and amplitude of sway during quiet upright posture. Indirect evidence supports the conclusion that, also in humans, monoaminergic descending pathways from brainstem nuclei modulate the excitability of the circuits mediating the group II input.     

Central vagal sensory and motor connections: human embryonic and fetal development

The embryonic and fetal development of the nuclear components and pathways of vagal sensorimotor circuits in the human has been studied using Nissl staining and carbocyanine dye tracing techniques. Eight fetal brains ranging from 8 to 28 weeks of development had DiI (1,1'-dioctadecyl-3,3,3',3' tetramethylindocarbocyanine perchlorate) inserted into either the thoracic vagus nerve at the level of the sternal angle (two specimens of 8 and 9 weeks of gestation) or into vagal rootlets at the surface of the medulla (at all other ages), while a further five were used for study of cytoarchitectural development. The first central labeling resulting from peripheral application of DiI to the thoracic vagus nerve was seen at 8 weeks. By 9 weeks, labeled bipolar cells at the ventricular surface around the sulcus limitans (sl) were seen after DiI application to the thoracic vagus nerve. Subnuclear organization as revealed by both Nissl staining and carbocyanine dye tracing was found to be advanced at a relatively early fetal age, with afferent segregation in the medial Sol apparent at 13 weeks and subnuclear organization of efferent magnocellular divisions of dorsal motor nucleus of vagus nerve noticeable at the same stage. The results of the present study also confirm that vagal afferents are distributed to the dorsomedial subnuclei of the human nucleus of the solitary tract, with particular concentrations of afferent axons in the gelatinosus subnucleus. These vagal afferents appeared to have a restricted zone of termination from quite early in development (13 weeks) suggesting that there is no initial exuberance in the termination field of vagal afferents in the developing human nucleus of the solitary tract. On the other hand, the first suggestion of afferents invading 10N from the medial Sol was not seen until 20 weeks and was not well developed until 24 weeks, suggesting that direct monosynaptic connections between the sensory and effector components of the vagal sensorimotor complex do not develop until this age.     

Anatomical, Physiological, and Theoretical Basis for the Antiepileptic Effect of Vagus Nerve Stimulation

Summary: The vagus is a mixed nerve carrying somatic and visceral afferents and efferents. The majority of vagal nerve fibers are visceral afferents and have a wide distribution throughout the central nervous system (CNS) either monosynaptically or via the nucleus of the solitary tract. Besides activation of well-defined reflexes, vagal stimulation produces evoked potentials recorded from the cerebral cortex, the hippocampus, the thalamus, and the cerebellum. Activation of vagal afferents can depress monosynaptic reflexes, decrease the activity of spinothalamic neurons, and increase pain threshold. Depending on the stimulation parameters, vagal afferent stimulation in experimental animals can produce electroencephalo-graphic (EEG) synchronization or desynchronization and has been shown to affect sleep states. The desynchronization of the EEG appears to depend on activation of afferent fibers that have conduction velocities of ≤ 15 m/s. Vagal afferent stimulation can also influence the activity of interictal cortical spikes produced by topical strychnine application, and either attenuate or stop seizures produced by pentylenetetrazol, 3-mercaptoproprionic acid, maximal electroshock, and topical alumina gel. The mechanisms for the antiepileptic effects of vagal stimulation are not fully understood but probably relate to effects on the reticular activating system. The vagus provides an easily accessible, peripheral route to modulate CNS function.     

Hepato-vagal pathway associated with nicotine's anorectic effect in the rat.

Nicotine reduces appetite and body weight. Because the hepato-portal area senses different types of nutrients that transmit signals via vagal afferent nerves to the hypothalamus to modify food intake and feeding pattern, we investigated the effect of nicotine on a hepato-vagal-hypothalamic pathway. Low doses of nicotine (< 10 ng) injected into portal vein (i.p.v.) decreased, while high doses of nicotine increased (> or = 10 ng) electrophysiological activity of hepatic vagal afferents. Stimulatory effect of high dose of nicotine on vagal hepatic afferents was blocked by a prior i.p.v. injection of curare (30 microg) or hexamethonium (1 mg). Furthermore, activities of gastric vagal and adrenal sympathetic efferents were suppressed by low-dose, but stimulated by high-dose i.p.v. nicotine. These reflex effects did not occur in hepatic vagotomized rats. Results of experiments demonstrate that in addition to nicotine's anorectic effect being mediated via a direct central action, nicotine also acts peripherally via hepatic vagal afferents from sensors of nicotine in the hepato-portal region.     

Anatomical,Physiological, and Theoretical Basis for the Antiepileptic Effect of Vagus Nerve Stimulation

Summary: The vagus is a mixed nerve carrying somatic and visceral afferents and efferents. The majority of vagal nerve fibers are visceral afferents and have a wide distribution throughout the central nervous system (CNS) either monosynaptically or via the nucleus of the solitary tract. Besides activation of well-defined reflexes, vagal stimulation produces evoked potentials recorded from the cerebral cortex, the hippocampus, the thalamus, and the cerebellum. Activation of vagal afferents can depress monosynaptic reflexes, decrease the activity of spinothalamic neurons, and increase pain threshold. Depending on the stimulation parameters, vagal afferent stimulation in experimental animals can produce electroencephalo-graphic (EEG) synchronization or desynchronization and has been shown to affect sleep states. The desynchronization of the EEG appears to depend on activation of afferent fibers that have conduction velocities of ≤ 15 m/s. Vagal afferent stimulation can also influence the activity of interictal cortical spikes produced by topical strychnine application, and either attenuate or stop seizures produced by pentylenetetrazol, 3-mercaptoproprionic acid, maximal electroshock, and topical alumina gel. The mechanisms for the antiepileptic effects of vagal stimulation are not fully understood but probably relate to effects on the reticular activating system. The vagus provides an easily accessible, peripheral route to modulate CNS function.     

Signalling the state of the digestive tract

The gastrointestinal tract has a rich sensory innervation. Extrinsic afferents in vagal, splanchnic and pelvic nerves project to the CNS where gut reflex function is coordinated and integrated with behavioural responses (e.g. regulation of food intake) and mediate sensations. The afferent information conveyed by vagal and spinal mechanosensitive afferents can be very different. Vagal afferents have low thresholds of activation and reach maximal responses within physiological levels of distension. In contrast, spinal afferents, although many have corresponding thresholds for activation, are able to respond beyond the physiological range and encode both physiological and noxious levels of stimulation. However, mechanosensitivity is not fixed but can be influenced by a wide range of chemical mediators released as a consequence of ischemia, injury and inflammation. Indeed, previously mechanical insensitive afferents can develop mechanosensitivity during inflammation and a variety of chemical mediators are implicated in this sensitisation process. Chemosensitivity is also a property of vagal mucosal afferents that detect the chemical milieu for chemicals absorbed across the epithelium or released from enteroendocrine cells that are strategically positioned to "taste" luminal contents. Thus, there exists a complex interplay between immunomodulators, neurotransmitters and neuroendocrine factors that underlie gastrointestinal sensing mechanisms and enable orchestration of appropriate host responses.     

Chemical coding and central projections of gastric vagal afferent neurons

Abstract  Vagal afferents that innervate gastric muscle or mucosa transmit distinct sensory information from their endings to the nucleus of the tractus solitarius (NTS). While these afferent subtypes are functionally distinct, no neurochemical correlate has been described and it is unknown whether they terminate in different central locations. This study aimed to identify gastric vagal afferent subtypes in the nodose ganglion (NG) of ferrets, their terminal areas in NTS and neurochemistry for isolectin-B4 (IB4) and calcitonin gene-related peptide (CGRP). Vagal afferents were traced from gastric muscle or mucosa and IB4 and CGRP labelling assessed in NG and NTS. 7 ± 1% and 6 ± 1% of NG neurons were traced from gastric muscle or mucosa respectively; these were more likely to label for CGRP or for both CGRP and IB4 than other NG neurons ( P  <   0.01). Muscular afferents were also less likely than others to label with IB4 ( P  <   0.001). Less than 1% of NG neurons were traced from both muscle and mucosa. Central terminals of both afferent subtypes occurred in the subnucleus gelatinosus of the NTS, but did not overlap completely. This region also labelled for CGRP and IB4. We conclude that while vagal afferents from gastric muscle and mucosa differ little in their chemical coding for CGRP and IB4, they can be traced selectively from their peripheral endings to NG and to overlapping and distinct regions of NTS. Thus, there is an anatomical substrate for convergent NTS integration for both types of afferent input.     

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.     

Genetic tracing of Nav1.8-expressing vagal afferents in the mouse

Nav1.8 is a tetrodotoxin-resistant sodium channel present in large subsets of peripheral sensory neurons, including both spinal and vagal afferents. In spinal afferents, Nav1.8 plays a key role in signaling different types of pain. Little is known, however, about the exact identity and role of Nav1.8-expressing vagal neurons. Here we generated mice with restricted expression of tdTomato fluorescent protein in all Nav1.8-expressing afferent neurons. As a result, intense fluorescence was visible in the cell bodies, central relays, and sensory endings of these neurons, revealing the full extent of their innervation sites in thoracic and abdominal viscera. For instance, vagal and spinal Nav1.8-expressing endings were seen clearly within the gastrointestinal mucosa and myenteric plexus, respectively. In the gastrointestinal muscle wall, labeled endings included a small subset of vagal tension receptors but not any stretch receptors. We also examined the detailed innervation of key metabolic tissues such as liver and pancreas and evaluated the anatomical relationship of Nav1.8-expressing vagal afferents with select enteroendocrine cells (i.e., ghrelin, glucagon, GLP-1). Specifically, our data revealed the presence of Nav1.8-expressing vagal afferents in several metabolic tissues and varying degrees of proximity between Nav1.8-expressing mucosal afferents and enteroendocrine cells, including apparent neuroendocrine apposition. In summary, this study demonstrates the power and versatility of the Cre-LoxP technology to trace identified visceral afferents, and our data suggest a previously unrecognized role for Nav1.8-expressing vagal neurons in gastrointestinal functions.