Identification of different types of spinal afferent nerve endings that encode noxious and innocuous stimuli in the large intestine using a novel anterograde tracing technique.

In mammals, sensory stimuli in visceral organs, including those that underlie pain perception, are detected by spinal afferent neurons, whose cell bodies lie in dorsal root ganglia (DRG). One of the major challenges in visceral organs has been how to identify the different types of nerve endings of spinal afferents that transduce sensory stimuli into action potentials. The reason why spinal afferent nerve endings have been so challenging to identify is because no techniques have been available, until now, that can selectively label only spinal afferents, in high resolution. We have utilized an anterograde tracing technique, recently developed in our laboratory, which facilitates selective labeling of only spinal afferent axons and their nerve endings in visceral organs. Mice were anesthetized, lumbosacral DRGs surgically exposed, then injected with dextran-amine. Seven days post-surgery, the large intestine was removed. The characteristics of thirteen types of spinal afferent nerve endings were identified in detail. The greatest proportion of nerve endings was in submucosa (32%), circular muscle (25%) and myenteric ganglia (22%). Two morphologically distinct classes innervated myenteric ganglia. These were most commonly a novel class of intraganglionic varicose endings (IGVEs) and occasionally rectal intraganglionic laminar endings (rIGLEs). Three distinct classes of varicose nerve endings were found to innervate the submucosa and circular muscle, while one class innervated internodal strands, blood vessels, crypts of lieberkuhn, the mucosa and the longitudinal muscle. Distinct populations of sensory endings were CGRP-positive. We present the first complete characterization of the different types of spinal afferent nerve endings in a mammalian visceral organ. The findings reveal an unexpectedly complex array of different types of primary afferent endings that innervate specific layers of the large intestine. Some of the novel classes of nerve endings identified must underlie the transduction of noxious and/or innocuous stimuli from the large intestine.

A novel anterograde neuronal tracing technique to selectively label spinal afferent nerve endings that encode noxious and innocuous stimuli in visceral organs.

BACKGROUND: One major weakness in our understanding of pain perception from visceral organs is the lack of knowledge of the location, morphology and neurochemistry of all the different types of spinal afferent nerve endings, which detect noxious and innocuous stimuli. This is because we lack techniques to selectively label only spinal afferents. Our aim was to develop an anterograde tracing technique that labels only spinal afferent nerve endings in visceral organs, without also labeling all other classes of extrinsic afferent and efferent nerves. METHODS: Mice were anesthetized with isoflurane and dextran-biotin injected, via glass micropipettes (diameter 5 µm), into L6 and S1 dorsal root ganglia. Mice recovered for 7 days, were then euthanized and the colon removed. KEY RESULTS: Anterograde labeling revealed multiple unique classes of afferent endings that terminated within distinct anatomical layers of the colon and rectum. We characterized a particular class of intramuscular ending in the circular muscle (CM) layer of the colon that consists of multiple varicose axons that project circumferentially. CONCLUSIONS & INFERENCES: We demonstrate a technique for selective anterograde labeling of spinal afferent nerve endings in visceral organs. This approach facilitates selective visualization of the precise morphology and location of the different classes of spinal afferent endings, without visual interference caused by indiscriminant labeling of other classes of afferent and efferent nerve axons which also innervate internal organs. We have used this new technique to identify and describe the details of a particular class of intramuscular spinal afferent ending in the CM layer of mouse large intestine.

Taste representation in the human insula.

The sense of taste exists so that organisms can detect potential nutrients and toxins. Despite the fact that this ability is of critical importance to all species there appear to be significant interspecies differences in gustatory organization. For example, monkeys and humans lack a pontine taste relay, which is a critical relay underlying taste and feeding behavior in rodents. In addition, and of particular relevance to this special issue, the primary taste cortex appears to be located further caudally in the insular cortex in humans compared to in monkeys. The primary aim of this paper is to review the evidence that supports this possibility. It is also suggested that one parsimonious explanation for this apparent interspecies differences is that if, as Craig suggests, the far anterior insular cortex is newly evolved and unique to humans, then the human taste cortex may only appear to be located further caudally because it is no longer the anterior-most section of insular cortex. In addition to discussing the location of taste representation in human insular cortex, evidence is presented to support the possibility that this region is better conceptualized as an integrated oral sensory region that plays role in feeding behavior, rather than as unimodal sensory cortex.

Efferent connections of an auditory area in the caudal insular cortex of the rat: anatomical nodes for cortical streams of auditory processing and cross-modal sensory interactions.

In the rat cortex, the two non-primary auditory areas, posterodorsal and ventral auditory areas, may constitute the two streams of auditory processing in their distinct projections to the posterior parietal and insular cortices. The posterior parietal cortex is considered crucial for auditory spatial processing and directed attention, while possible auditory function of the insular cortex is largely unclear. In this study, we electrophysiologically delineated an auditory area in the caudal part of the granular insular cortex (insular auditory area, IA) and examined efferent connections of IA with anterograde tracer biocytin to deduce the functional significance of IA. IA projected to the rostral agranular insular cortex, a component of the lateral prefrontal cortex. IA also projected to the adjacent dysgranular insular cortex and the caudal agranular insular cortex and sent feedback projections to cortical layer I of the primary and secondary somatosensory areas. Corticofugal projections terminated in auditory, somatosensory and visceral thalamic nuclei, and the bottom of the thalamic reticular nucleus that could overlap the visceral sector. The ventral part of the caudate putamen, the external cortex of the inferior colliculus and the central amygdaloid nucleus were also the main targets. IA exhibited neural response to transcutaneous electrical stimulation of the forepaw in addition to acoustic stimulation (noise bursts and pure tones). The results suggest that IA subserves diverse functions associated with somatosensory, nociceptive and visceral processing that may underlie sound-driven emotional and autonomic responses. IA, being potentially involved in such extensive cross-modal sensory interactions, could also be an important anatomical node of auditory processing linked to higher neural processing in the prefrontal cortex.

Age-related changes in vagal afferents innervating the gastrointestinal tract.

Recent progress in understanding visceral afferents, some of it reviewed in the present issue, serves to underscore how little is known about the aging of the visceral afferents in the gastrointestinal (GI) tract. In spite of the clinical importance of the issue-with age, GI function often becomes severely compromised-only a few initial observations on age-related structural changes of visceral afferents are available. Primary afferent cell bodies in both the nodose ganglia and dorsal root ganglia lose Nissl material and accumulate lipofucsin, inclusions, aggregates, and tangles. Additionally, in changes that we focus on in the present review, vagal visceral afferent terminals in both the muscle wall and the mucosa of the GI tract exhibit age-related structural changes. In aged animals, both of the vagal terminal types examined, namely intraganglionic laminar endings and villus afferents, exhibit dystrophic or regressive morphological changes. These neuropathies are associated with age-related changes in the structural integrity of the target organs of the affected afferents, suggesting that local changes in trophic environment may give rise to the aging of GI innervation. Given the clinical relevance of GI tract aging, a more complete understanding both of how aging alters the innervation of the gut and of how such changes might be mitigated should be made research priorities.

Ion channel and receptor mechanisms of bladder afferent nerve sensitivity.

Sensory nerves of the urinary bladder consist of small diameter A(delta) and C fibers running in the hypogastic and pelvic nerves. Neuroanatomical studies have revealed a complex neuronal network within the bladder wall. Electrophysiological recordings in vitro and in vivo have revealed several distinct classes of afferent fibers that may signal a wide range of bladder stimulations including physiological bladder filling, noxious distension, cold, chemical irritation and inflammation. The exact mechanisms that underline mechanosensory transduction in bladder afferent terminals remain ambiguous; however, a wide range of ion channels (e.g., TTX-resistant Na(+) channels, Kv channels and hyperpolarization-activated cyclic nucleotide-gated cation channels) and receptors (e.g., TRPV1, TRPM8, TRPA1, P2X(2/3), etc) have been identified at bladder afferent terminals and implicated in the generation and modulation of afferent signals. Experimental investigations have revealed that expression and/or function of these ion channels and receptors may be altered in animal models and patients with overactive and painful bladder disorders. Some of these ion channels and receptors may be potential therapeutic targets for bladder diseases.

The intermedius nucleus of the medulla: a potential site for the integration of cervical information and the generation of autonomic responses.

The intermedius nucleus of the medulla (InM) is a small perihypoglossal brainstem nucleus, which receives afferent information from the neck musculature and also descending inputs from the vestibular nuclei, the gustatory portion of the nucleus of the solitary tract (NTS) and cortical areas involved in movements of the tongue. The InM sends monosynaptic projections to both the NTS and the hypoglossal nucleus. It is likely that the InM acts to integrate information from the head and neck and relays this information on to the NTS where suitable autonomic responses can be generated, and also to the hypoglossal nucleus to influence movements of the tongue and upper airways. Central to the integratory role of the InM is its neurochemical diversity. Neurones within the InM utilise the amino acid transmitters glutamate, GABA and glycine. A proportion of these excitatory and inhibitory neurones also use nitric oxide as a neurotransmitter. Peptidergic transmitters have also been found within InM neurones, although as yet the extent of the pattern of co-localisation between peptidergic and amino acid transmitters in neurones has not been established. The calcium binding proteins calretinin and parvalbumin are found within the InM in partially overlapping populations. Parvalbumin and calretinin appear to have complementary distributions within the InM, with parvalbumin being predominantly found within GABAergic neurones and calretinin being predominantly found within glutamatergic neurones. Neurones in the InM receive inputs from glutamatergic sensory afferents. This glutamatergic transmission is conducted through both NMDA and AMPA ionotropic glutamate receptors. In summary the InM contains a mixed pool of neurones including glutamatergic and GABAergic in addition to peptidergic neurones. Neurones within the InM receive inputs from the upper cervical region, descending inputs from brain regions involved in tongue movements and those involved in the coordination of the autonomic nervous system. Outputs from the InM to the NTS and hypoglossal nucleus suggest a possible role in the coordination of tongue movements and autonomic responses to changes in posture.

Exploring relationships for visceral and somatic pain with autonomic control and personality.

The autonomic nervous system (ANS) integrates afferent and motor activity for homeostatic processes including pain. The aim of the study was to compare hitherto poorly characterised relations between brainstem autonomic control and personality in response to visceral and somatic pain. Eighteen healthy subjects (16 females, mean age 34) had recordings during rest and pain of heart rate (HR), cardiac vagal tone (CVT), cardiac sensitivity to baroreflex (CSB), skin conductance level (SC), cardiac sympathetic index (CSI) and mean blood pressure (MBP). Visceral pain was induced by balloon distension in proximal (PB) and distal (DB) oesophagus and somatic pain by nail-bed pressure (NBP). Eight painful stimuli were delivered at each site and unpleasantness and intensity measured. Personality was profiled with the Big Five inventory. (1) Oesophageal intubation evoked "fight-flight" responses: HR and sympathetic (CSI, SC, MBP) elevation with parasympathetic (CVT) withdrawal (p<0.05). (2) Pain at all sites evoked novel parasympathetic/sympathetic co-activation with elevated HR but vasodepression (all p<0.05). (3) Personality traits correlated with slope of distal oesophageal pain-related CVT changes wherein more neurotic-introvert subjects had greater positive pain-related CVT slope change (neuroticism r 0.8, p<0.05; extroversion r -0.5, p<0.05). Pain-evoked heart rate increases were mediated by parasympathetic and sympathetic co-activation - a novel finding in humans but recently described in mammals too. Visceral pain-related parasympathetic change correlated with personality. ANS defence responses are nuanced and may relate to personality type for visceral pain. Clinical relevance of these findings warrants further exploration.

Inside information: the unique features of visceral sensation.

Most of what is written and believed about pain and nociceptors originates from studies of the "somatic" (non-visceral) sensory system. As a result, the unique features of visceral pain are often overlooked. In the clinic, the management of visceral pain is typically poor, and drugs that are used with some efficacy to treat somatic pain often present unwanted effects on the viscera. For these reasons, a better understanding of visceral sensory neurons-particularly visceral nociceptors-is required. This review provides evidence of functional, morphological, and biochemical differences between visceral and non-visceral afferents, with a focus on potential nociceptive roles, and also considers some of the potential mechanisms of visceral mechanosensation.

Brainstem sites controlling the lower esophageal sphincter and crural diaphragm in the ferret: a neuroanatomical study.

The lower esophageal sphincter (LES) and the crural diaphragm (CD) surrounding the esophagogastric junction are key components of the gastroesophageal reflex mechanism, which engages the vago-vagal brainstem circuitry. Although both components work in conjunction to prevent gastroesophageal reflux, little is known about the brain area(s) where this integration takes place. The aims of this study were to: (1) trace the brainstem circuitry associated with the CD and the LES, and (2) determine possible sites of convergence. Experiments were done in adult male ferrets. Under isoflurane anesthesia, recombinant strains of the transneuronal pseudorabies virus (PRV-151 or PRV-Bablu) or the monosynaptic retrograde tracer cholera toxin beta-subunit (CTb) were injected into either the CD or the LES. Following a survival period of 5-7 days, animals were euthanized, perfused and their brains removed for dual-labeling immunofluorescence processing. In animals injected with recombinants of PRV into the CD and the LES, distinct labeling was found in various brainstem nuclei including: area postrema, DMV, nucleus tractus solitarius (NTS), medial reticular formation (MRF) and nucleus ambiguous (NA). Double-labeled cells were only evident in the DMV, NTS and MRF. Injections of CTb into the CD or the LES resulted in retrograde labeling only in the DMV. These findings demonstrate the presence of a direct projection from the DMV to the CD. They further suggest that the neuronal connections responsible for CD or LES function are contained in circuitries that, though largely independent, may converge at the level of DMV, NTS and MRF.

Interactions of carotid sinus or aortic input with emetic signals from gastric afferents and vestibular system.

This study investigated how baro- and chemoreceptor afferents interact with emetic signals from gastric afferents and the vestibular system, and how these interactions modulate emetic and prodromal responses. We performed splanchnic denervation and abdominal vagotomy in anesthetized shrews (Suncus murinus), and then induced emetic responses by gastric distension. Next, we investigated the effects of these gastric afferent sections on cardiovascular and emetic responses induced by electrical stimulation of the aortic depressor nerve (ADN) and the carotid sinus nerve (CSN) with or without gastric distension. Splanchnic denervation abolished the prodromal response before retching and aortic baroreflex inhibition caused by gastric distension, but had no effects on the emetic response. In contrast, abdominal vagotomy abolished the emetic response induced by gastric distension with or without CSN stimulation, but without affecting gastric distension-induced or CSN stimulation-induced vascular and respiratory responses. In conscious animals, CSN denervation significantly suppressed veratrine- and motion-induced emetic responses, whereas ADN denervation had no significant effects. These results suggest that aortic baroreflex inhibition via the activation of splanchnic afferents contributes to the prodromal response before retching and circulatory homeostasis. In contrast, carotid sinus inputs, which are usually non-emetic signals, interact with vagal and vestibular inputs, and modulate the development of retching and vomiting.

Vaginocervical stimulation enhances social recognition memory in rats via oxytocin release in the olfactory bulb.

The ability of vaginocervical stimulation (VCS) to promote olfactory social recognition memory at different stages of the ovarian cycle was investigated in female rats. A juvenile social recognition paradigm was used and memory retention tested at 30 and 300 min after an adult was exposed to a juvenile during three 4-min trials. Results showed that an intact social recognition memory was present at 30 min in animals with or without VCS and at all stages of the estrus cycle. However, whereas no animals in any stage of the estrus cycle showed retention of the specific recognition memory at 300 min, those in the proestrus/estrus phase that received VCS 10 min before the trial started did. In vivo microdialysis studies showed that there was a significant release of oxytocin after VCS in the olfactory bulb during proestrus. There was also increased oxytocin immunoreactivity within the olfactory bulb after VCS in proestrus animals compared with diestrus ones. Furthermore, when animals received an infusion of an oxytocin antagonist directly into the olfactory bulb, or a systemic administration of alpha or beta noradrenaline-antagonists, they failed to show evidence for maintenance of a selective olfactory recognition memory at 300 min. Animals with vagus or pelvic nerve section also showed no memory retention when tested after 300 min. These results suggest that VCS releases oxytocin in the olfactory bulb to enhance the social recognition memory and that this may be due to modulatory actions on noradrenaline release. The vagus and pelvic nerves are responsible for carrying the information from the pelvic area to the CNS.

Characterization of mouse lumbar splanchnic and pelvic nerve urinary bladder mechanosensory afferents.

Sensory information from the urinary bladder is conveyed via lumbar splanchnic (LSN) and sacral pelvic (PN) nerves to the spinal cord. In the present report we compared the mechanosensitive properties of single afferent fibers in these two pathways using an in vitro mouse bladder preparation. Mechanosensitive primary afferents were recorded from the LSN or PN and distinguished based on their response to receptive field stimulation with different mechanical stimuli: probing (160 mg to 2 g), stretch (1-25 g), and stroking of the urothelium (10-1,000 mg). Four different classes of afferent were recorded from the LSN and PN: serosal, muscular, muscular/urothelial, and urothelial. The LSN contained principally serosal and muscular afferents (97% of the total sample), whereas all four afferent classes of afferent were present in the PN (63% of which were muscular afferents). In addition, the respective proportions and receptive field distributions differed between the two pathways. Both low- and high-threshold stretch-sensitive muscular afferents were present in both pathways, and muscular afferents in the PN were shown to sensitize after exposure to an inflammatory soup cocktail. The LSN and PN pathways contain different populations of mechanosensitive afferents capable of detecting a range of mechanical stimuli and individually tuned to detect the type, magnitude, and duration of the stimulus. This knowledge broadens our understanding of the potential roles these two pathways play in conveying mechanical information from the bladder to the spinal cord.

The effects of viscero-somatic interactions on thalamic mast cell recruitment in cystitic rats.

Mast cells accessing the brain parenchyma through the blood-brain barrier in healthy animals are limited to pre-cortical sensory relays - the olfactory bulb and the thalamus. We have demonstrated that unilateral repetitive stimulation of the abdominal wall generates asymmetry in midline thalamic mast cell (TMC) distribution in cyclophosphamide-injected rats, consisting of contralateral side-prevalence with respect to the abdominal wall stimulation. TMC asymmetry 1) was generated in strict relation with cystitis, and was absent in disease-free and mesna-treated animals, 2) was restricted to the anterior portion of the paraventricular pars anterior and reuniens nuclei subregion, i.e., the rostralmost part of the paraventricular thalamic nucleus, the only thalamic area associated with viscero-vagal and somatic inputs, via the nucleus of the solitary tract, and via the medial contingent of the spinothalamic tract, respectively, and 3) originated from somatic tissues, i.e., the abdominal wall where bladder inflammation generates secondary somatic hyperesthesia leading to referred pain in humans. Present data suggest that TMCs may be involved in thalamic sensory processes, including some aspects of visceral pain and abnormal visceral/somatic interactions.

Urinary bladder irritation alters efficacy of vagal stimulation on rostral medullary neurons in chronic T8 spinalized rats.

The presence of pelvic visceral inputs to neurons in the rostral medulla that are responsive to electrical stimulation of the abdominal branches of the vagus nerve (VAG-abd) was investigated in a complete chronic T8 spinal transection rat model. Using extracellular electrophysiological recordings from single medullary reticular formation (MRF) neurons, 371 neurons in 15 rats responsive to pinching the ear (search stimulus) were tested for somato-visceral and viscero-visceral convergent responses to stimulation of the following nerves/territories: VAG-abd, dorsal nerve of the penis, pelvic nerve, distention of urinary bladder and colon, penile stimulation, urethral infusion, and touch/pinch of the entire body surface. In addition to these mechanical and electrical stimuli, a chemical stimulus applied to the bladder was assessed as well. Of the total neurons examined, 205 were tested before and 166 tested beginning 20 min after application of a chemical irritant (2% acetic acid) to the urinary bladder (same rats used pre/post irritation). As with intact controls, many ear-responsive MRF neurons responded to the electrical stimulation of VAG-abd. Although MRF neuron responses failed to be evoked with direct (mechanical and electrical nerve) pelvic visceral stimuli, acute chemical irritation of the urinary bladder produced a significant increase in the number of MRF neurons responsive to stimulation of VAG-abd. The results of this study indicate a central effect that potentially relates to some of the generalized below level pelvic visceral sensations that have been documented in patients with complete spinal cord injury.

Similarities of the neuronal circuit for the induction of fictive vomiting between ferrets and dogs.

Previous studies suggested that the following neuronal circuit participates in the induction of vomiting by afferent vagal stimulation in decerebrated paralyzed dogs: (1) afferent fibers of the vagus nerve, (2) neurons of the solitary nucleus (NTS), (3) neurons of the prodromal sign center near the semicompact part of the nucleus ambiguus (scAMB), (4) neurons of the central pattern generator in the reticular area adjacent to the compact part of nucleus ambiguus (cAMB), (5) respiratory premotor neurons in the caudal medulla, (6) motor neurons of the diaphragm and abdominal muscles. However, the commonality of this neuronal circuit in different species has not yet been clarified. Thus, this study was conducted to clarify this point. This study clarified for the first time that fictive vomiting in decerebrated paralyzed ferrets could be induced by vagal stimulation, and could be identified by centrifugal activity patterns of the phrenic and abdominal muscle nerves. The distributions of c-Fos immunoreactive neurons in the NTS, scAMB and cAMB areas in ferrets that exhibited fictive vomiting were denser than those in ferrets that did not. Application of the nonNMDA receptor antagonist into the 4th ventricle produced the reversible suppression of fictive vomiting. The NK1 receptor immunoreactive puncta were found in the reticular area adjacent to the scAMB. Microinjections of NK1 receptor antagonist into the reticular areas on both sides abolished fictive vomiting. All these results in the ferrets are identical with results previously obtained in dogs and cats. Therefore, this suggests that the above neuronal circuit commonly participates in the induction of emesis in these animal species.

Sensory transmission in the gastrointestinal tract.

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to 'sense' the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways.

The same dorsal root ganglion neurons innervate uterus and colon in the rat.

The purpose of this study was to determine whether primary sensory neurons that innervate the uterus receive convergent input from the colon. To test this, in the rat, cell bodies of colonic and uterine dorsal root ganglia were retrogradely labeled with fluorescent tracer dyes microinjected into the colon/rectum and bilaterally into the uterine horns. Ganglia were harvested, cryoprotected and cut into 20 microm slices to identify positively stained cells for fluorescent microscopy. Up to 5% of neurons were colon-specific or uterus-specific, and 10-15% of labeled ganglion neurons innervate both viscera in the L1, L2, L6 and S1-S3 levels. These results suggest a novel form of visceral sensory integration in the dorsal root ganglion that may underlie comorbidity of many functional pain syndromes.

Response properties of the pharyngeal branch of the glossopharyngeal nerve for umami taste in mice and rats.

Many studies have reported the mechanism underlying umami taste. However, there are no investigations of responses to umami stimuli taste originating from chemoreceptors in the pharyngeal region. The pharyngeal branch of the glossopharyngeal nerve (GPN-ph) innervating the pharynx has unique responses to taste stimulation that differs from responses of the chorda tympani nerve and lingual branch of the glossopharyngeal nerve. Water evokes robust response, but NaCl solutions at physiological concentrations do not elicit responses. The present study was designed to examine umami taste (chemosensory) responses in the GPN-ph. Response characteristics to umami taste were compared between mice and rats. In mice, stimulation with compounds eliciting umami taste (0.1M monosodium L-glutamate (MSG), 0.01M inosine monophosphate (IMP) and the mixture of 0.1M MSG+0.01M IMP) evoked higher responses than application of distilled water (DW). However, synergistic response of a mixture of 0.1M MSG+0.01M IMP was not observed. In rats, there is no significant difference between the responses to umami taste (0.1M MSG, 0.01M IMP and the mixture of 0.1M MSG+0.01M IMP) and DW. Monopotassium glutamate (MPG) was used in rats to examine the contribution of the sodium component of MSG on the response. Stimulation with 0.1M MPG evoked a higher response when compared with responses to DW. The present results suggest that umami taste compounds are effective stimuli of the chemoreceptors in the pharynx of both mice and rats.

Chemical stimulation of visceral afferents activates medullary neurones projecting to the central amygdala and periaqueductal grey.

Cholecystokinin (CCK) stimulates gastrointestinal vagal afferent neurones that signal visceral sensations. We wished to determine whether neurones of the nucleus of the solitary tract (NTS) or ventrolateral medulla (VLM) convey visceral afferent information to the central nucleus of the amygdala (CeA) or periaqueductal grey region (PAG), structures that play a key role in adaptive autonomic responses triggered by stress or fear. Male Sprague-Dawley rats received a unilateral microinjection of the tracer cholera toxin subunit B (CTB, 1%) into the CeA or PAG followed, 7 days later, by an injection of CCK (100 microg/kg, i.p.) or saline. Brains were processed for detection of Fos protein (Fos-IR) and CTB. CCK induced increased expression of Fos-IR in the NTS and the VLM, relative to control. When CTB was injected into the CeA, CTB-immunoreactive (CTB-IR) neurones were more numerous in the rostral NTS ipsilateral to the injection site, whereas they were homogeneously distributed throughout the VLM. Double-labelled neurones (Fos-IR+CTB-IR) were most numerous in the ipsilateral NTS and caudal VLM. The NTS contained the higher percentage of CTB-IR neurones activated by CCK. When CTB was injected into the PAG, CTB-IR neurones were more numerous in the ipsilateral NTS whereas they were distributed relatively evenly bilaterally in the rostral VLM. Double-labelled neurones were not differentially distributed along the rostrocaudal axis of the NTS but were more numerous in this structure when compared with the VLM. NTS and VLM neurones may convey visceral afferent information to the CeA and the PAG.