Effect of capsaicin as a neuropeptide-releasing substance on sneezing reflex in a type I allergic animal model.

To elucidate the effect of capsaicin-sensitive nerve fibers, which are known to contain substance P (SP) and other sensory neuropeptides, on the sneezing reflex, we have investigated the effect of capsaicin on this reflex provoked in guinea pigs passively sensitized with anaphylactic antibody followed by specific antigen challenge. It has already been established that histamine released from mast cells is a reliable inducer of the sneezing reflex in type I allergy. Our experimental results indicated that the frequency of sneezing provoked by antigen challenge as well as histamine application was significantly reduced by pretreatment with capsaicin in a dose-dependent fashion. SP is considered to be one of the main neurotransmitters in sensory nerves. When the amount of SP in animal nasal mucosa was measured 12 h after capsaicin treatment, a marked reduction was noted. However, the histamine content in the nasal mucosa was not changed by capsaicin treatment. These data suggest that neuropeptides, especially SP, which are released or depleted from sensory nerves by capsaicin treatment, probably play an important role as neurotransmitters of the stimulant histamine in the development of sneezing in type I allergy.     

Evidence for substance P-immunoreactive spinal afferents that mediate bronchoconstriction

The origin and functional role of capsaicin-sensitive substance P-(SP-) immunoreactive (IR) nerve fibres in the lower airways were studied in the guinea-pig. Stellatectomy caused a significant reduction of SP-IR in the lung and pulmonary artery. Immunohistochemical analysis, however, did not reveal any clear-cut change in the number and distribution of SP-IR fibres in the lung of these animals. After combined stellatectomy plus local capsaicin treatment of the vagal nerves, most SP-IR nerves disappeared in the lower airways. The bronchoconstriction induced by capsaicin was significantly reduced after stellatectomy and abolished after stellatectomy plus capsaicin pretreatment of the vagal nerves. Ether inhalation caused bronchoconstriction, which was not influenced by stellatectomy but markedly reduced by combined capsaicin treatment of vagal nerves and stellatectomy. Stellate ganglion stimulation in animals that had been chemically sympathectomized by 6-OH-dopamine caused bronchoconstriction, which was resistant to cholinergic or adrenergic receptor blockade. This response was absent after systemic capsaicin pretreatment, suggesting that it was due to antidromic stimulation of afferent fibres traversing the stellate ganglion. In conclusion, the present data suggest that the lower airways receive SP-IR capsaicin-sensitive C-fibre afferents of both vagal and spinal origin. These sensory fibres seem to have branches both within the bronchial smooth muscle and around blood vessels.     

Neonatal capsaicin pretreatment suppresses intramedullary inflammation in adjuvant-induced spondylitis.

In order to investigate the proposed involvement of neuropeptides in musculoskeletal inflammation we pretreated rats, in an adjuvant spondylitis model, with capsaicin, a neurotoxin. Immunohistochemistry showed that administration of capsaicin to newborn rats depleted irreversibly the neuropeptide, substance P. Elimination of capsaicin-sensitive fibres by the neonatal injection of capsaicin did not suppress the peridiscitis of rats in which adjuvant spondylitis was induced at 7 weeks of age. However, elimination of capsaicin-sensitive fibres did suppress the inflammation usually seen in the bone marrow. We speculate that this intramedullary inflammation is normally induced or sustained by capsaicin-sensitive fibres.     

Substance P as a potent stimulator of sneeze responses in experimental allergic rhinitis of guinea pigs

Substance P was examined for sneeze-inducing activity and its involvement of sneeze responses in experimental allergic rhinitis. Substance P, dripped into a nostril of guinea pigs, at concentrations of 100 pM and above induced sneezing in a dose-dependent fashion. The activity of substance P was not affected by the previous subcutaneous injections of capsaicin that depleted substance P in nerve fibers. Histamine induced sneezing at concentrations of 30 mM and above and the activity was reduced by capsaicin treatment. The frequency of antigen-induced sneezing was proportional to the substance P content in nasal mucosa of sensitized guinea pigs treated with increasing doses of capsaicin; correlation coefficient 0.91. These results suggest that substance P plays an important role as a stimulator of sneeze responses in experimental allergic rhinitis in guinea pigs.     

The effect of intra-articular capsaicin on nerve fibres within the synovium of the rat knee joint

The aim of this study was to establish the effects of intra-articular capsaicin (pelargonic acid vallinylamide) on synovial innervation of the rat knee. Rats were sacrificed 1, 2, 4 and 7 days after intra-articular injection of capsaicin and joint tissues stained with either conventional haematoxylin and eosin (H and E) or with specific antibodies to the calcitonin gene-related peptide (CGRP), substance P (both of which are markers for primary afferent fibres), the C-flanking peptide of neuropeptide Y (CPON) (localised in postganglionic sympathetic fibres), or protein gene product 9.5 (a pan-neuronal marker). At lower concentrations (0.1% and 0.25%), capsaicin produced no change in peptide staining pattern or histological appearance. At 0.5% capsaicin, there was complete loss of nerve fibres showing positive staining for CGRP and substance P at all time points. Staining for CPON and protein gene product 9.5 was still present, but decreased, 1 and 2 days after treatment and virtually absent at 4 and 7 days. These findings provide evidence for partially selective denervation induced by 0.5% capsaicin, in contrast to 1% capsaicin which abolished staining for all peptide markers, indicating a total ablation of nerve fibres. A consistent but unexpected finding was the presence of a severe inflammatory response in joints treated with 0.5% and 1% capsaicin. An influx of polymorphonuclear leucocytes was found to occur within 4 h of injection, with progressive appearance of mononuclear cells after this time. We conclude that it is difficult to specifically deplete sensory nerve fibres from the synovium by means of local capsaicin injection. Although selective loss of staining for sensory nerve fibres could be achieved by injection of 0.5% capsaicin, there was progressive non-specific loss of post-ganglionic autonomic fibres which may be related to the severe inflammatory response provoked by the higher doses of capsaicin.     

Capsaicin-sensitive peptidergic neurons are involved in the zosteriform spread of herpes simplex virus infection.

The intraneuronal transport of herpes simplex virus (HSV) is an essential component in disease pathogenesis. Capsaicin, a neuropharmacologic agent lacking direct antiviral activity, has been shown to protect animals against HSV-induced disease. It has been hypothesized that capsaicin acts by interfering with the intraneuronal transport of virus. Since animal models have been useful in studying the spread of virus, we used two guinea pig models of zosteriform herpes to examine the effect of capsaicin on HSV spread. Capsaicin was subcutaneously administered to Hartley guinea pigs prior to intravaginal or cutaneous HSV-2 inoculation. Treatment did not prevent the development of herpetic vesicles at the site of inoculation but significantly reduced the zosteriform spread of lesions in male and female animals. Further, after recovery from primary infection, capsaicin-treated male guinea pigs were observed to have fewer days with recurrent herpetic lesions. These results suggest that capsaicin-sensitive nerve fibers play a role in the pathogenesis of primary and recurrent HSV infections. Capsaicin appears to reduce the severity of cutaneous HSV infections by interfering with the spread of virus.     

Depletion by capsaicin of substance P-immunoreactivity and acetylcholinesterase activity from nerve fibres in the guinea-pig heart

The sensitivity to capsaicin of substance P-immunoreactive nerve fibres in the hearts of guinea-pigs was examined. Capsaicin decreased considerably the substance P-immunoreactive material in nerve fibres of the parietal pericardium, atria, bicuspid and tricuspid valves. Pericardial and valvar nerve fibres localized by acetylcholinesterase (AChE) histochemistry had a distribution and density pattern similar to that of the substance P immunoreactive fibres. Capsaicin treatment also decreased the number of visible AChE-positive nerve fibres. It is known that capsaicin has a selective action on those substance P-immunoreactive fibres that are of sensory origin; thus, these results imply that substance P-immunoreactive fibres in the heart are sensory. Moreover, the results suggest that some of the nerve fibres localized in the heart by AChE histochemistry are substance P-containing sensory fibres.     

Enkephalin and capsaicin-resistant substance P-like immunoreactivities in intra-ocular grafts of different fetal spinal cord areas

Fetal spinal cord tissue was grafted to the anterior chamber of the eye of adult rats in order to evaluate survival and distribution of substance P- and enkephalin-immunoreactive neurons. Capsaicin treatment was used to evaluate any possible contribution of host iris-derived substance P fibres to the innervation of the grafts and to check for capsaicin sensitivity of graft substance P-positive systems. Substance P- and enkephalin-immunoreactive nerve fibres were present in grafts of half-transverse segments of the spinal cord and were clearly co-distributed throughout the grafts. Areas with a high density of substance P- and enkephalin-positive fibres resembling substantia gelatinosa were seen. Grafts of the dorsal horn alone had a moderate to high density of substance P- and enkephalin-positive fibres, while ventral horn grafts contained a low amount of such fibres. Capsaicin eliminated the substance P innervation of the host iris and the dorsal root ganglion-derived substance P innervation of the host spinal cord, while sparing the intrinsic substance P innervation of both host spinal cord and spinal cord grafts. These experiments show that intra-ocular grafts of defined spinal cord areas express relatively organotypic amounts of substance P- and enkephalin-positive nerve fibres, and thus emphasize the importance of intrinsic genetic determinants for spinal cord development.     

Effects and distribution of vagal capsaicin-sensitive substance P neurons with special reference to the trachea and lungs

The origin of substance P (SP)-immunoreactive neurons in the lower respiratory tract, esophagus and heart of guinea-pigs was demonstrated by surgical denervation or capsaicin pretreatment with subsequent determination of the tissue levels of SP by radioimmunoassay. In other experiments the effect of vagal nerve stimulation on the SP levels in these tissues was studied. The effects of capsaicin-sensitive afferents in the respiratory tract mucosa and bronchial smooth muscle was also studied by analysis of vascular permeability to Evans blue and insufflation-pressure changes. Our present data indicate that all SP nerves in the trachea and lung are afferent and capsaicin-sensitive. The trachea and stem bronchi receive SP afferents mainly from the right vagus nerve with cell bodies located in both the nodose and jugular ganglia. The SP innervation of the lung seems to have a dual origin: 1. Afferents from both vagal nerves with a crossed type of innervation pattern. 2. A non-vagal source which consists of about 40% of the SP nerves in the lung. These nerves probably originate from thoracic spinal ganglia. The effects of ether and capsaicin on insufflation pressure and increase in vascular permeability were dependent on the integrity of capsaicin-sensitive afferents of both vagal and non-vagal origin. In the guinea pig, systemic capsaicin pretreatment to adult animals seemed to result in irreversible changes in the respiratory tract, while in the rat a successive recovery of the functional response of capsaicin-sensitive afferents occurred. Different regimes of systemic capsaicin pretreatment induced different effects on the cholinergic (atropine-sensitive) insufflation-pressure response. Capsaicin pretreatment, using multiple injections over two days, depressed the cholinergic insufflation-pressure increase, while the cholinergic vagal component was unaffected in animals which received a single dose of capsaicin or local pretreatment with capsaicin on the vagal nerves. The local treatment was more effective with regard to SP depletion in target areas when using alcohol as solvent than when capsaicin was dissolved in paraffin oil, while the functional deficits were similar. The SP nerves in the esophagus were mainly of vagal afferent origin, while the heart atrium seemed to have a dual innervation by both vagal and non-vagal SP nerves.     

Studies of vasoactive intestinal polypeptide expression in injured peripheral neurons using capsaicin, sympathectomy and mf mutant rats

The increased expression of vasoactive intestinal polypeptide (VIP) in injured peripheral neurons was studied. In contrast to substance P, there was a marked increase, and maintained fast axonal transport, of VIP in rat sciatic nerve after peripheral axotomy. Local capsaicin application to the nerve trunk failed to inhibit the injury-induced VIP increase, and capsaicin even increased VIP levels when applied locally to uninjured nerves. Pharmacological sympathectomy showed that some of the peripheral VIP increase may occur in post-ganglionic sympathetic fibres. The VIP increase after injury appeared unaffected in the mf mutant rat, in spite of its loss of lumbar dorsal root ganglion cells. VIP-staining fibres in the epi- and peri-neurium and perivascular plexuses of sciatic nerve showed an increase in number in parallel with the changes of the nerve VIP content. These findings suggest that sensory and sympathetic nerve fibres expressing VIP after injury play a role in the regulation of blood flow to nerves, and in the pathophysiological processes in nerve and dorsal spinal cord which follow peripheral nerve injury.     

The nature of the substance P-containing nerve fibres in taste papillae of the rat tongue

The nature of the association of substance P (SP) with taste buds in the rat tongue was investigated by immunohistochemical and radioimmunoassay techniques. Both the circumvallate and fungiform papillae were found to receive a rich innervation by substance P-containing fibres. Although these fibres were closely associated with the taste buds in these structures, they assumed a perigemmal rather than an intragemmal location. Bilateral lesions of the glossopharyngeal nerve resulted in the depletion of taste buds from the vallate papilla and a large reduction in substance P immunoreactive fibres in this area. Lesions of the chorda tympani, which led to the degeneration of taste buds in fungiform papillae, had no effect on the immunohistochemical appearance of substance P in these papilla or on the substance P levels in the anterior part of the tongue. Lesions of the mandibular division of the trigeminal nerve or neonatal capsaicin treatment had no effect on the structural integrity of taste buds in fungiform papillae but led to the depletion of substance P-immunoreactive fibres from these papillae. Both of these procedures caused a 71% reduction in the substance P content of the anterior tongue, ipsilaterally after the nerve lesion and bilaterally after capsaicin treatment. The results are discussed in relation to the possible functional role of substance P-containing fibres within nerves supplying taste structures of the tongue.     

Naloxone-precipitated contractions of the guinea pig ileum made tolerant to a synthetic opioid peptide: evidence against the involvement of capsaicin-sensitive neurones

In segments of guinea pig ileum made tolerant-dependent to the synthetic opioid peptide [D-Met2,Pro5]enkephalinamide, naloxone (1 microM) caused half-maximal longitudinal contractions. These were strongly reduced by hyoscine or desensitization to serotonin but not by hexamethonium, desensitization to substance P or pretreatment with capsaicin. Contractions of similar size evoked by capsaicin (0.3 microM) in the presence of naloxone were abolished by capsaicin pretreatment, reduced by substance P desensitization or hyoscine but not by desensitization to serotonin. Thus, the two types of contraction clearly have different mechanisms; enteric capsaicin-sensitive neurones or substance P-containing motor neurones seem not to be involved in the naloxone-precipitated contraction while serotonin may play a role in this response.     

Substance P content of the skin, neurogenic inflammation and numbers of C-fibres following capsaicin application to a cutaneous nerve in the rabbit

The long-term effects of applying capsaicin briefly to a cutaneous nerve in the rabbit have been assessed 10 days after treatment. No changes in C-fibre numbers were seen in treated saphenous nerves and the average ratio of C-fibres to A-fibres was close to 7 in both control and treated nerves. However, the substance P content of the skin innervated by the saphenous nerve fell by 46% after capsaicin treatment compared with a fall of 65% after nerve section. Plasma extravasation in response to 7.5% mustard oil applied to the skin was also reduced following capsaicin treatment (by 43%) and following denervation (by 47%), although the response to 25% mustard oil was unaffected by previous capsaicin treatment. Thus, as in other species that have been examined (notably the rat), brief capsaicin treatment of rabbit skin nerves leads to a reduction in a neurogenic inflammatory response and in substance P content of the skin. However, unlike the rat, there is no degeneration of C-fibres. In the rabbit it is therefore possible to separate the neurotoxic, degenerative action of capsaicin from its ability to deplete substance P. The question of whether a similar dissociation between neurotoxic and other actions could be achieved in the rat by using lower capsaicin concentrations remains to be answered.     

Capsaicin suppresses substance P-induced joint inflammation in the rat

Intra-articular injection of 20 micrograms substance P in rat knee joints results in a pronounced inflammatory response. However, prior intra-articular injection of 1% capsaicin solution (1-5 weeks previously) virtually abolishes this response. This is not a neurotoxic effect of capsaicin on nerve fibres as denervation of the knee produces no alteration of the response to injected substance P. The potent effect of capsaicin on substance P-mediated inflammation cannot be attributed to depletion of mast cells by this treatment as the mast cell degranulator compound 48/80 injected into capsaicin pre-treated knees still gives rise to a marked inflammatory response. Compound 48/80 does not activate nerve fibres to cause release of substance P as it is equally effective in eliciting an inflammatory response in the presence of 100 micrograms of the substance P antagonist D-Pro4, D-Trp7,9,10 SP(4-11) in the synovial cavity. The results suggest that capsaicin may act by depleting substance P receptors in joint tissue.     

Capsaicin-sensitive afferents are involved in signalling transneuronal effects between cutaneous sensory nerves

The aim of the present study was to investigate changes in contralateral nerves associated with peripheral nerve injuries. Transection and subsequent regeneration of the saphenous nerve on one side caused a suppression of the ability of the contralateral saphenous nerve to produce a neurogenic plasma extravasation response. This effect was transient, and was first evident two weeks after injury, reaching its maximum at four weeks, but was no longer detectable at eight weeks. This change was paralleled by a decrease in the content of substance P, a neuropeptide involved in neurogenic plasma extravasation, in the contralateral nerve. The neurotoxin capsaicin was used to deplete the nerve of a subclass of C-fibres, namely the polymodal nociceptor afferents. Pretreatment of the nerve to be lesioned with capsaicin was sufficient to significantly attenuate the changes in the plasma extravasation response and substance P content observed on the contralateral side. The effectiveness of the capsaicin treatment was confirmed by histological examination. These results strongly suggest that changes observed at a site distant from the location of the nerve injury are dependent on the integrity of capsaicin-sensitive C-fibre afferents within the injured nerve. Furthermore, given that the contralateral nerve has commonly been used as the control for an injury conducted on the homologous nerve or muscle on the opposite side of the body, the underlying assumption being that the contralateral nerve remained unchanged, the present findings emphasize the need for separate groups of control animals which have undergone no surgical procedures.     

Vagal origin of substance P-containing nerves in the guinea pig lung

This study reports on the possible origin and putative sensory nature of substance P-containing fibres in the lung. Following vagal ligation experiments, carried out on adult guinea pigs, the vagi, nodose ganglia and lungs were investigated by immunocytochemistry and radioimmunoassay. After ligation the lung showed a marked depletion of substance P-immunoreactivity; concomitantly there was an accumulation of immunoreactive material proximal to the ligature, indicating a peripheral direction of the peptide axonal flow. Several substance P-immunoreactive neuronal cell bodies were observed in the nodose ganglion. These results suggest that substance P-immunoreactive fibres in the lung might originate from primary sensory neurones in the nodose ganglion, which is compatible with other evidence that substance P has a sensory role.     

Transganglionic transport of choleragenoid by capsaicin-sensitive C-fibre afferents to the substantia gelatinosa of the spinal dorsal horn after peripheral nerve section

Choleratoxin B subunit-binding thick myelinated, A-fibre and unmyelinated, capsaicin-sensitive nociceptive C-fibre primary afferent fibres terminate in a strict topographic and somatotopic manner in the spinal cord dorsal horn. Injection of choleratoxin B subunit-horseradish peroxidase conjugate into injured but not intact peripheral nerves produced transganglionic labelling of primary afferents not only in the deeper layers (Rexed's laminae III-IV), but also in the substantia gelatinosa (Rexed's laminae II) of the spinal dorsal horn. This was interpreted in terms of a sprouting response of the Abeta-myelinated afferents and suggested a contribution to the pathogenesis of neuropathic pain [Nature 355 (1992) 75; J Comp Neurol 360 (1995) 121]. By utilising the selective neurotoxic effect of capsaicin, we examined the role of C-fibre sensory ganglion neurons in the mechanism of this phenomenon. Elimination of these particular, capsaicin-sensitive C-fibre afferents by prior intrathecal or systemic capsaicin treatment inhibited transganglionic labelling by the choleratoxin B subunit-horseradish peroxidase conjugate of the substantia gelatinosa evoked by chronic sciatic nerve section. More importantly, prior perineural capsaicin treatment of the transected nerve proximal to the anticipated site of injection of choleragenoid 12 hours later prevented the labelling of the substantia gelatinosa, but not that of the deeper layers. Electron microscopic examination of the dorsal roots revealed no significant difference in the proportion of labelled myelinated fibres relating to the intact (54.4+/-5.5%) and the transected (62.4+/-5.4%) sciatic nerves. In contrast, the proportion of labelled unmyelinated dorsal root axons relating to the transected, but not the intact nerves showed a significant, six-fold increase after sciatic nerve transection (intact: 4.9+/-1.3%; transected: 35+/-6.7%).These observations indicate that peripheral nerve lesion-induced transganglionic labelling of the substantia gelatinosa by choleratoxin B subunit-horseradish peroxidase may be primarily accounted for by the uptake and transganglionic transport of choleragenoid by injured capsaicin-sensitive C-fibre afferents rather than a sprouting response of A-fibre afferents. The present findings suggest an essential role of capsaicin-sensitive primary sensory neurons in lesion-induced spinal neuroplastic changes and provide further support for C-fibre nociceptor neurons being promising targets for the development of new strategies in pain management.     

Vanilloid receptor 1 expression in the rat urinary tract

Previous findings have shown that the capsaicin sensitivity of sensory fibres is due to the expression of a specific membrane protein, the vanilloid receptor type 1 (VR1). In the present work we studied the distribution, morphology and the neurochemical content of nerve fibres expressing this receptor in the rat urinary tract. Immunolabelling was performed against the VR1 and the positive fibres were examined by light and electron microscopy. Colocalisation of VR1 and substance P or calcitonin gene-related peptide immunoreactivities, and isolectin B4 binding, was evaluated under the confocal microscope. In addition, the effect of intravesical administration of resiniferatoxin, an ultra-potent vanilloid receptor agonist, in the receptor expression in the bladder was also studied. Numerous VR1-immunoreactive fibres were found in the mucosa and muscular layer of the entire urinary tract except the kidney. In the bladder, most fibres were also substance P- or calcitonin gene-related peptide-immunoreactive but did not bind isolectin B4. Under the electron microscope VR1 immunoreactivity was confined to unmyelinated axons and varicosities containing small clear and large dense-core synaptic vesicles. They occurred beneath or among epithelial cells or closely apposed to smooth muscle cells. Intravesical resiniferatoxin decreased VR1 immunoreactivity transiently. These data indicate that primary sensory fibres expressing VR1 are extremely abundant in the rat urinary tract and that, in contrast to the skin, they belong almost exclusively to the peptide-containing sub-population of primary afferents. As capsaicin-sensitive bladder afferents are involved in nociception and reflex micturition control, the numerous free terminal nerve endings expressing VR1 in the mucosa seem more adequate to accomplish the former function. However, the close apposition between VR1-expressing fibres and smooth muscle cells suggests that they may also encode the tonus of the muscular layer.     

Cardiac vanilloid receptor 1-expressing afferent nerves and their role in the cardiogenic sympathetic reflex in rats

Myocardial ischaemia causes the release of metabolites such as bradykinin, which stimulates cardiac sensory receptors to evoke a sympathoexcitatory reflex. However, the molecular identity of the afferent neurons and fibres mediating this reflex response is not clear. In this study, we tested the hypothesis that the cardiogenic sympathoexcitatory reflex is mediated by capsaicin-sensitive afferent fibres. Enhanced immunofluorescence labelling revealed that vanilloid receptor 1 (VR1)-containing afferent nerve fibres were present on the epicardial surface of the rat heart. Resiniferatoxin (RTX), a potent analogue of capsaicin, was used to deplete capsaicin-sensitive afferent fibres in rats. Depletion of these fibres was confirmed by a substantial reduction of VR1 immunoreactivity in the epicardium and dorsal root ganglia. The thermal sensitivity was also diminished in RTX-treated rats. Renal sympathetic nerve activity (RSNA) and blood pressure were recorded in anaesthetized rats during epicardial application of bradykinin or capsaicin. In vehicle-treated rats, epicardial bradykinin (10 μg ml⁻¹) or capsaicin (10 μg ml⁻¹) application produced a significant increase in RSNA and arterial blood pressure. The RSNA and blood pressure responses caused by bradykinin and capsaicin were completely abolished in RTX-treated rats. Furthermore, epicardial application of iodo-RTX, a highly specific antagonist of VR1 receptors, blocked capsaicin- but not bradykinin-induced sympathoexcitatory responses. Thus, these data provide important histological and functional evidence that the heart is innervated by VR1-expressing afferent nerves and these afferent nerves are essential for the cardiogenic sympathoexcitatory reflex during myocardial ischaemia.     

Ruthenium red prevents capsaicin-induced neurotoxic action on sensory fibers of the guinea pig ileum

Capsaicin, a neurotoxin which impairs some primary afferent nerve fibers, was locally applied to the mesenteric nerve trunks of adult guinea pigs. The effect of capsaicin was studied on contractile responses of the ileum to mesenteric nerve stimulation (20 Hz). Exposure of the mesenteric nerve to capsaicin (1 microM) irreversibly abolished the contractile response to mesenteric nerve stimulation, without influencing the resting tension and spontaneous motility of the ileum. However, if the mesenteric nerve had been incubated in a Ca-free medium containing EGTA (0.1 mM) or pretreated with Ruthenium red (10 microM), capsaicin only partially inhibited (about 30%), but did not abolish, the contractile response to the mesenteric nerve stimulation, thus indicating protection from desensitization.