The organization of capsaicin-sensitive visceral afferent systems

It has become clear that a number of neuropeptides are found in sensory nerves, some of which have been identified in visceral afferents. The best studied peptide is substance P, which has been localized in a population of capsaicin-sensitive visceral afferents. It has been established that there are a varied proportion of substance P-containing afferents in different visceral structures. In general, the peripheral termination of these nerves is around blood vessels. The central terminations of visceral afferents are in laminae I and V in the dorsal horn of the spinal cord. Substance P has been localized in these laminae and appears to be capsaicin-sensitive and therefore of sensory origin. Recently, substance K, which is derived from the same gene as substance P, has been found in visceral structures. Calcitonin gene-related peptide has been found in certain viscera to be contained in capsaicin-sensitive nerves. The contribution that other peptides make to visceral afferent innervation is not known.     

Area postrema: part of the autonomic circuitry of caloric homeostasis

Investigations in which lesion techniques are used suggest a role for the area postrema (AP) in caloric homeostasis. Ablations of the AP in rat are associated with temporary hypophagia, hypodipsia, and rapid body weight loss. This is followed by a steady period of relatively normal eating and drinking and body weight gain. This steady period is characterized specifically by lowered body weight maintenance levels, overingestion of palatable foods, and attenuated taste aversion learning and glucoprivic feeding. These effects cannot be attributed simply to lesions of central terminations of gustatory and visceral afferents. The AP may be involved in feeding behaviors that are triggered by chemical signals in the blood or cerebrospinal fluid. In addition, the AP along with the adjacent nucleus tractus solitarii (NTS) seems to be part of the central autonomic system subserving caloric homeostasis; this system includes the lateral hypothalamus, which has a well-documented role in energy balance. The contribution of the AP along with the NTS must be considered with respect to their relationship to other structures within this system.     

Localization of unmyelinated axons in rat skin and mucocutaneous tissue utilizing the isolectin GS-I-B

The a-D-galactose specific isolectin I-B4 from Griffonia simplicifolia (GS-I-B4) labels CNS microglia and certain peripheral neurons, including a subpopulation of small dark, type B dorsal root ganglion cells, some post-ganglionic sympathetic axons, and nearly all peripheral gustatory axons. The innervation patterns of GS-I-B4 reactive sensory ganglion cells are unknown for many peripheral target tissues, including their probable primary target, the skin. The present study describes the distribution of GS-I-B4 reactive axons in hairy and glabrous hindpaw skin and in the glans penis of rats, using both single and double-labelling histochemical techniques. Neuronal processes were identified using (1) histochemistry with horseradish peroxidase conjugated GS-I-B4 or (2) immunohistochemistry against PGP 9.5 to identify all axons, and biotinylated lectin histochemistry with avidin-FITC to identify the subpopulation of GS-I-B4 reactive axons. GS-I-B4 strongly labelled unmyelinated cutaneous sensory afferents, as well as some sympathetic efferents and visceral afferents. Lectin reactive axons were seen to innervate the upper hair shaft epidermis in hairy skin, and were abundant in the shallow dermis in hairy and glabrous skin and glans penis. Lectin reactive axons were also abundant in the lamina propria and distal urethral epithelium of the penis. These results provide new evidence for the cutaneous sensory role of GS-I-B4 reactive primary afferents, as well as evidence to support the contention that the lectin is a specific marker for a subpopulation of unmyelinated axons and not simply a marker for the myelination state of an axon.     

The chorda tympani and glossopharyngeal nerves in the adult hamster

The numbers and diameters of axons in the intact chorda tympani(CT) and lingual branch of the glossopharyngeal nerve (GN) arequantified with the use of electron microscopic photomontages.The cross-sectional diameters of the CT and GN average 68 and86 microns, respectively. The intact CT contains {small tilde}1050 fibers, 63% are unmyelinated and 37% are myelinated. TheGN contains {small tilde} 1600 fibers, 79% are unmyelinatedand 21% are myelinated. Both nerves are made up of relativelysmall unmyelinated and myelinated fibers, although the GN showsa broader distribution of diameters for its myelinated fibersdue to the presence of general somatosensory fibers. Followingde-efferentation, there is a 48% reduction in the number ofunmyelinated fibers in the CT. Fifty-two per cent of the unmyelinatedfibers are sensory. The number of myelinated fibers is not significantlyreduced and nearly all of the myelinated fibers are sensory.Sixty-seven per cent of the fibers within the CT are sensory.The de-efferented CT contains an equal number of unmyelinatedand myelinated axons and a total of {small tilde} 700 fibers.Comparable data in the rat indicate that its intact and de-efferentedCT are organized differently in regards to the numbers of sensoryand motor, and myelinated versus unmyelinated fibers. The findingsof the present study, together with the available data fromother species, suggest that anatomical differences in the make-upof the major gustatory nerves do not contribute in any obviousway to the known differences in the response properties betweenthe rat and hamster CT, and that the number of myelinated fibersin the visceral motor component of the CT varies considerablyacross species.     

NMDA and non-NMDA receptors mediate responses in the primary gustatory nucleus in goldfish

Primary gustatory afferents from the oropharynx of the goldfish, Carassius auratus, terminate in the vagal lobe, a laminated structure in the dorsal medulla comparable to the gustatory portion of the nucleus of the solitary tract in mammals. We utilized an in vitro brain slice preparation to test the role of different ionotropic glutamate receptor subtypes in synaptic transmission of gustatory information by recording changes in field potentials after application of various glutamate receptor antagonists. Electrical stimulation of the vagus nerve (NX) evokes two short-latency postsynaptic field potentials from sensory layers of the vagal lobe. 6,7-Dinitroquinoxaline-2,3-dione and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione, two non-N-methyl-D-aspartate (NMDA) ionotropic receptor antagonists, blocked these short-latency potentials. Slower potentials that were revealed under Mg2+ -free conditions, were abolished by the NMDA receptor antagonist, D(-)-2-amino-5-phosphonovaleric acid (APV). Repetitive stimulation produced short-term facilitation, which was attenuated by application of APV. These results indicate that the synaptic responses in the vagal lobe produced by stimulation of the gustatory roots of the NX involve both NMDA and non-NMDA receptors. An NMDA receptor-mediated facilitation may serve to amplify incoming bursts of primary afferent activity.     

The Central Projections of the Monkey Tooth Pulp Afferent Neurons

Transganglionic transport of horseradish peroxidase conjugated to wheatgerm agglutinin (HRP:WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the upper and lower tooth pulps within the trigeminal sensory nuclear complex (TSNC) of the monkey. HRP:WGA injections were also made into the lower incisors and molars, in order to examine the topographic arrangement of pulpal afferent projections. HRP-labeled pulpal afferents innervating lower and upper teeth projected ipsilaterally to the rostral subnucleus dorsalis (Vpd) and caudal subnucleus ventralis (Vpv) of the nucleus principalis (Vp); the rostrodorsomedial (Vo.r) and dorsomedial (Vo.dm) subdivisions of the nucleus oralis (Vo); the dorsomedial subdivision of the nucleus interpolaris (Vi); and laminae I—II and/or V of the nucleus caudalis (Vc) at its rostralmost level. The HRP-labeled terminals from upper and lower pulpal afferents formed a rostrocaudal column from the midlevel of Vp to the rostral tip of Vc. The label in Vp and Vo was considerably dense, but the column of terminals was interrupted at the Vpd-Vpv transition. The label in Vi and Vc was much less dense compared to that in the rostral nuclei, and the column of terminals was interrupted frequently. The representation of the upper and lower teeth in TSNC was organized in a somatotopic fashion that varied from one subdivision to the next, though their terminal zones overlapped within Vpd. The upper and lower teeth were represented in Vpv, Vo.r, Vo.dm, Vi, and Vc in a ventrodorsal, dorsoventral, lateromedial, lateromedial, and lateromedial sequence, respectively. Topographic arrangement was also noticed for the projections of pulpal afferents from the lower incisors and molars: The representations of the lower incisors and molars in Vpv, Vo.r, Vo.dm, Vi, and Vc were organized in a lateromedial, dorsoventral, ventrodorsal, ventrodorsal, and lateromedial sequence, respectively. The present results indicating sparse projections from pulpal afferents in the monkey's Vc are in good correspondence with a clinical report that trigeminal tractotomy just rostral to the obex has no significant effect on dental pain perception in patients. Furthermore, the present study indicates that projection patterns of pulpal afferents—which include the termination sites, the density of terminations between nuclei, and topographic arrangement—differ among animal species.     

Vasomotor control by subretrofacial neurones in the rostral ventrolateral medulla

In this paper we review our recent work in the rabbit and cat on the role of the rostral ventrolateral medulla in cardiovascular regulation. Microinjection of neuroexcitatory amino acids into a highly circumscribed region, located just ventral to the retrofacial nucleus at the level of the rostral part of the inferior olive, leads to an increase in blood pressure, owing to sympathetic vasoconstriction. Bilateral destruction of this region, which we have termed the subretrofacial nucleus, leads to a profound fall in blood pressure. Anatomical studies show that the subretrofacial nucleus contains a compact group of bulbospinal neurones that project to sympathetic preganglionic nuclei in the thoracolumbar spinal cord. Single-unit recording studies have shown that these bulbospinal neurons are spontaneously active and are powerfully inhibited by baroreceptor inputs. These observations indicate that the subretrofacial bulbospinal cells are sympathoexcitatory and play a major role in the tonic and phasic control of the cardiovascular system. Some important unresolved questions regarding the subretrofacial neurones will be discussed. (i) Are they functionally homogenous, or are they viscerotopically organized with respect to particular end organs? (ii) What are their afferent inputs? (iii) What are their histochemical properties? Specifically, are they part of the group of adrenaline-synthesizing cells, or alternatively, substance P cells?     

Functional organization of autonomic neural pathways

There is now abundant functional and anatomical evidence that autonomic motor pathways represent a highly organized output of the central nervous system. Simplistic notions of antagonistic all-or-none activation of sympathetic or parasympathetic pathways are clearly wrong. Sympathetic or parasympathetic pathways to specific target tissues generally can be activated tonically or phasically, depending on current physiological requirements. For example, at rest, many sympathetic pathways are tonically active, such as those limiting blood flow to the skin, inhibiting gastrointestinal tract motility and secretion, or allowing continence in the urinary bladder. Phasic parasympathetic activity can be seen in lacrimation, salivation or urination. Activity in autonomic motor pathways can be modulated by diverse sensory inputs, including the visual, auditory and vestibular systems, in addition to various functional populations of visceral afferents. Identifying the central pathways responsible for coordinated autonomic activity has made considerable progress, but much more needs to be done.     

Brief exposure to sensory cues elicits stimulus-nonspecific general sensitization in an insect

The effect of repeated exposure to sensory stimuli, with or without reward is well known to induce stimulus-specific modifications of behaviour, described as different forms of learning. In recent studies we showed that a brief single pre-exposure to the female-produced sex pheromone or even a predator sound can increase the behavioural and central nervous responses to this pheromone in males of the noctuid moth Spodoptera littoralis. To investigate if this increase in sensitivity might be restricted to the pheromone system or is a form of general sensitization, we studied here if a brief pre-exposure to stimuli of different modalities can reciprocally change behavioural and physiological responses to olfactory and gustatory stimuli. Olfactory and gustatory pre-exposure and subsequent behavioural tests were carried out to reveal possible intra- and cross-modal effects. Attraction to pheromone, monitored with a locomotion compensator, increased after exposure to olfactory and gustatory stimuli. Behavioural responses to sucrose, investigated using the proboscis extension reflex, increased equally after pre-exposure to olfactory and gustatory cues. Pheromone-specific neurons in the brain and antennal gustatory neurons did, however, not change their sensitivity after sucrose exposure. The observed intra- and reciprocal cross-modal effects of pre-exposure may represent a new form of stimulus-nonspecific general sensitization originating from modifications at higher sensory processing levels.     

A star in the brainstem reveals the first step of cortical magnification

A fundamental question in the neurosciences is how central nervous system (CNS) space is allocated to different sensory inputs. Yet it is difficult to measure innervation density and corresponding representational areas in the CNS of most species. These measurements can be made in star-nosed moles (Condylura cristata) because the cortical representation of nasal rays is visible in flattened sections and afferents from each ray can be counted. Here we used electrophysiological recordings combined with sections of the brainstem to identify a large, visible star representation in the principal sensory nucleus (PrV). PrV was greatly expanded and bulged out of the brainstem rostrally to partially invade the trigeminal nerve. The star representation was a distinct PrV subnucleus containing 11 modules, each representing one of the nasal rays. The 11 PrV ray representations were reconstructed to obtain volumes and the largest module corresponded to ray 11, the mole's tactile fovea. These measures were compared to fiber counts and primary cortical areas from a previous investigation. PrV ray volumes were closely correlated with the number of afferents from each ray, but afferents from the behaviorally most important, 11(th) ray were preferentially over-represented. This over-representation at the brainstem level was much less than at the cortical level. Our results indicate that PrV provides the first step in magnifying CNS representations of important afferents, but additional magnification occurs at higher levels. The early development of the 11(th), foveal appendage could provide a mechanism for the most important afferents to capture the most CNS space.     

Receptors and transmission in the brain-gut axis: potential for novel therapies. I. Receptors on visceral afferents

Visceral afferents are the information superhighway from the gut to the central nervous system. These sensory nerves express a wide range of membrane receptors that can modulate their sensitivity. In this themes article, we concentrate on those receptors that enhance the excitability of visceral afferent neurons. Some receptors are part of a modality-specific transduction pathway involved in sensory signaling. Others, which are activated by substances derived from multiple cellular sources during ischemia, injury, or inflammation, act in a synergistic fashion to cause acute or chronic sensitization of the afferent nerves to mechanical and chemical stimuli. Such hypersensitivity is the hallmark of conditions such as irritable bowel syndrome. Accordingly, these receptors represent a rational target for drug treatments aimed at attenuating both the inappropriate visceral sensation and the aberrant reflex activity that are the foundation for alterations in bowel function.     

Area postrema and adjacent solitary nucleus in water and energy balance

Lesions of the area postrema (AP) and immediately adjacent nucleus tractus solitarii cause a syndrome of permanent weight loss with little disruption to behavioral controls of body weight regulation. There is also a permanent polydipsia and elevated salt appetite, which appear to be secondary to deficits in renal conservation of water and sodium. Neuroanatomical studies indicate that the immediately adjacent solitary nucleus and AP are sites of termination of visceral afferents from abdominal organs and the destination and origin of distant central afferent and efferent connections.     

GABAergic modulation of primary gustatory afferent synaptic efficacy

Modulation of synaptic transmission at the primary sensory afferent synapse is well documented for the somatosensory and olfactory systems. The present study was undertaken to test whether GABA impacts on transmission of gustatory information at the primary afferent synapse. In goldfish, the vagal gustatory input terminates in a laminated structure, the vagal lobes, whose sensory layers are homologous to the mammalian nucleus of the solitary tract. We relied on immunoreactivity for the GABA-transporter, GAT-1, to determine the distribution of GABAergic synapses in the vagal lobe. Immunocytochemistry showed dense, punctate GAT-1 immunoreactivity coincident with the layers of termination of primary afferent fibers. The laminar nature and polarized dendritic structure of the vagal lobe make it amenable to an in vitro slice preparation to study early synaptic events in the transmission of gustatory input. Electrical stimulation of the gustatory nerves in vitro produces synaptic field potentials (fEPSPs) predominantly mediated by ionotropic glutamate receptors. Bath application of either the GABA(A) receptor agonist muscimol or the GABA(B) receptor agonist baclofen caused a nearly complete suppression of the primary fEPSP. Coapplication of the appropriate GABA(A) or GABA(B) receptor antagonist bicuculline or CGP-55845 significantly reversed the effects of the agonists. These data indicate that GABAergic terminals situated in proximity to primary gustatory afferent terminals can modulate primary afferent input via both GABA(A) and GABA(B) receptors. The mechanism of action of GABA(B) receptors suggests a presynaptic locus of action for that receptor.     

Integration of gastric distension and gustatory responses in the parabrachial nucleus

Palatable gustatory stimuli promote feeding, whereas gastric distension generally inhibits this behavior. We explored a neural basis for integration of these opposing sensory signals by evaluating the effect of gastric distension on gustatory responses in the parabrachial nucleus (PBN) of anesthetized rats. Sixteen percent of 92 taste cells were coactivated; they responded to independent taste or gastric distension stimulus application. Modulation of taste responses by distension was more prevalent; taste responses declined 37% in response to distension in 25% of the cells and increased by 46% in 10% of cells. Across the whole population, however, the suppressive effect of distension on taste responses was small (6%). The incidence of modulation did not vary as a simple hedonic function of gustatory sensitivity, i.e., similar proportions of sucrose-, citric-acid-, and QHCl-best, but not NaCl-best, neurons were modulated by gastric distension. Coactivated, modulated, and nonmodulated gustatory-responsive cells were intermingled in the gustatory zone of the caudal PBN. The suppression of PBN taste responses by visceral stimulation may reflect a mechanism for satiation and further implicates the PBN in the control of ingestive function.     

Expression of Fos during sham sucrose intake in rats with central gustatory lesions

For humans and rodents, ingesting sucrose is rewarding. This experiment tested the prediction that the neural activity produced by sapid sucrose reaches reward systems via projections from the pons through the limbic system. Gastric cannulas drained ingested fluid before absorption. For 10 days, the rats alternated an hour of this sham ingestion between sucrose and water. On the final test day, half of them sham drank water and the other half 0.6 M sucrose. Thirty minutes later, the rats were killed and their brains immunohistochemically stained for Fos. The groups consisted of controls and rats with excitotoxic lesions in the gustatory thalamus (TTA), the medial (gustatory) parabrachial nucleus (PBN), or the lateral (visceral afferent) parabrachial nucleus. In controls, compared with water, sham ingesting sucrose produced significantly more Fos-positive neurons in the nucleus of the solitary tract, PBN, TTA, and gustatory cortex (GC). In the ventral forebrain, sucrose sham licking increased Fos in the bed nucleus of the stria terminalis, central nucleus of amygdala, and the shell of nucleus accumbens. Thalamic lesions blocked the sucrose effect in GC but not in the ventral forebrain. After lateral PBN lesions, the Fos distributions produced by distilled H(2)O or sucrose intake did not differ from controls. Bilateral medial PBN damage, however, eliminated the sucrose-induced Fos increase not only in the TTA and GC but also in the ventral forebrain. Thus ventral forebrain areas associated with affective responses appear to be activated directly by PBN gustatory neurons rather than via the thalamocortical taste system.     

Endomorphin-2 is an endogenous opioid in primary sensory afferent fibers

Evidence is presented that the recently discovered endogenous mu-selective agonist, endomorphin-2, is localized in primary sensory afferents. Endomorphin-2-like immunoreactivity was found to be colocalized in a subset of substance P- and mu opiate receptor-containing fibers in the superficial laminae of the spinal cord and spinal trigeminal nucleus. Disruption of primary sensory afferents by mechanical (deafferentation by dorsal rhizotomy) or chemical (exposure to the primary afferent neurotoxin, capsaicin) methods virtually abolished endomorphin-2-like immunoreactivity in the dorsal horn. These results indicate that endomorphin-2 is present in primary afferent fibers where it can serve as the endogenous ligand for pre- and postsynaptic mu receptors and as a major modulator of pain perception.     

3H] muscimol receptors sites in the carp (Cyprinus carpio L.) brain: binding assay and autoradiographic distribution

Autoradiographic and binding techniques were used to study the presence of [(3)H]muscimol receptors sites in the carp brain. The radioligand was distributed with an high degree of anatomical selectivity. We found abundant labelling in the cerebellum, in the nucleus diffusus lobi inferioris, and in the torus longitudinalis. No labelling was detected within the epithalamus, thalamus and hypothalamus, while the telencephalon and the rhombencephalon displayed a low density of [(3)H]muscimol receptors sites. Binding assay showed the highest concentration of receptor sites in the nucleus diffusus lobi inferioris and the lowest in the medulla oblongata. Presence of [(3)H]muscimol binding sites within the visceral sensory area was noted. The rank order of displacement efficacy of unlabelled ligands observed, suggested that in brain membranes of carp the receptor binding of [(3)H]muscimol has the same pharmacological specificity previously reported in a large number of experiments with tissue homogenates. A general agreement in binding and autoradiography was observed. The present findings suggested that muscimol receptor could be involved in neuronal pathway controlling basic central actions like gustatory signal processing or spatial learning acquisition and retention.     

Parametric analysis of gastric distension responses in the parabrachial nucleus

The parabrachial nucleus (PBN) is regarded as an important locus for the processing and integration of sensory inputs from oral, gastrointestinal, and postabsorptive receptor sites and is thus thought to play an important role in regulating food intake. Gastric distension is an important satiation cue; however, such responses have been qualitatively characterized only over a limited area of the PBN. To more fully characterize gastric distension responses throughout the PBN, the responses of single units to gastric distension were tested using computer-controlled balloon inflation (3-18 ml air) in pentobarbital sodium- and/or urethan-anesthetized male rats. Distension-responsive neurons were indeed distributed throughout the nucleus from rostral areas typically considered to be visceral to more caudal areas associated with gustatory function, providing further anatomical support for the hypothesis that the PBN integrates taste and visceral signals that control feeding. Most PBN neurons had thresholds of 6 ml or less, similar to vagal afferent fibers. However, in contrast to the periphery, there were both excitatory and inhibitory responses. Increases in volume were associated with two distinct effects. First, as volume increased, the response rate increased; second, the duration of the response increased. In fact, in a subset of cells, responses to gastric distension lasted well beyond the stimulation period, particularly at larger volumes. Prolonged gastric distension responses are not common in the periphery and may constitute a central mechanism that contributes to satiation processes.