Different ion channel mechanisms between low concentrations of capsaicin and high concentrations of capsaicin and nicotine regarding peptide release from pulmonary afferents

Carcinogenesis and Metastasis: Focus on TRPV1-Positive Neurons and Immune Cells

Both sensory neurons and immune cells, albeit at markedly different levels, express the vanilloid (capsaicin) receptor, Transient Receptor Potential, Vanilloid-1 (TRPV1). Activation of TRPV1 channels in sensory afferent nerve fibers induces local effector functions by releasing neuropeptides (most notably, substance P) which, in turn, trigger neurogenic inflammation. There is good evidence that chronic activation or inactivation of this inflammatory pathway can modify tumor growth and metastasis. TRPV1 expression was also demonstrated in a variety of mammalian immune cells, including lymphocytes, dendritic cells, macrophages and neutrophils. Therefore, the effects of TRPV1 agonists and antagonists may vary depending on the prominent cell type(s) activated and/or inhibited. Therefore, a comprehensive understanding of TRPV1 activity on immune cells and nerve endings in distinct locations is necessary to predict the outcome of therapies targeting TRPV1 channels. Here, we review the neuro-immune modulation of cancer growth and metastasis, with focus on the consequences of TRPV1 activation in nerve fibers and immune cells. Lastly, the potential use of TRPV1 modulators in cancer therapy is discussed.

Itch Beyond the Skin-Mucosal Itch

Itch is a nociceptive sensation linked with reflexes and cognitive motor actions. We traditionally think of itch as a sensation of the skin related to allergy, an insect sting or interestingly, anxiety and frustration. Less understood and considered are the physiological processes involved in the itching sensation that occurs at mucosal and junctional dermal sites, which is extraordinary as from an evolutionary point of view these sites serve important guardian roles, rich in sensory nerves and inflammatory cells. Despite itch being an ancient reflex and evolutionarily conserved phenomenon, better clinical understanding of the nuances between sites of itch sensation may lead to improved clinical outcomes. This review invites readers to appreciate itch beyond the skin by highlighting several specific itch patterns-nasal, oral, auricular, vulvovaginal, anal, and perineal itch-the pathophysiological mechanisms that underlie them, the clinical patterns these may cause, and some unique treatments.

The effect of capsaicin on expression patterns of CGRP in trigeminal ganglion and trigeminal nucleus caudalis following experimental tooth movement in rats

Objectives

The aim of this study was to explore the effect of capsaicin on expression patterns of calcitonin gene-related peptide (CGRP) in the trigeminal ganglion (TG) and trigeminal subnucleus caudalis (Vc) following experimental tooth movement.

Material and Methods

Male Sprague-Dawley rats were used in this study and divided into small-dose capsaicin+force group, large-dose capsaicin+force group, saline+force group, and no force group. Closed coil springs were used to mimic orthodontic forces in all groups except for the no force group, in which springs were inactivated. Capsaicin and saline were injected into periodontal tissues. Rats were euthanized at 0 h, 12 h, 1 d, 3 d, 5 d, and 7 d following experimental tooth movement. Then, TG and Vc were obtained for immunohistochemical staining and western blotting against CGRP.

Results

Immunohistochemical results indicated that CGRP positive neurons were located in the TG, and CGRP immunoreactive fibers were distributed in the Vc. Immunohistochemical semiquantitative analysis and western blotting analysis demonstrated that CGRP expression levels both in TG and Vc were elevated at 12 h, 1 d, 3 d, 5 d, and 7 d in the saline + force group. However, both small-dose and large-dose capsaicin could decrease CGRP expression in TG and Vc at 1 d and 3 d following experimental tooth movement, as compared with the saline + force group.

Conclusions

These results suggest that capsaicin could regulate CGRP expression in TG and Vc following experimental tooth movement in rats.

Activation of transient receptor potential ankyrin 1 induces CGRP release from spinal cord synaptosomes

Transient receptor potential ankyrin 1 (TRPA1) is a sensor of nociceptive stimuli, expressed predominantly in a subpopulation of peptidergic sensory neurons which co-express the noxious heat-sensor transient receptor potential vanilloid 1. In this study, we describe a spinal cord synaptosome-calcitonin gene-related peptide (CGRP) release assay for examining activation of TRPA1 natively expressed on the central terminals of dorsal root ganglion neurons. We have shown for the first time that activation of TRPA1 channels expressed on spinal cord synaptosomes by a selection of agonists evokes a concentration-dependent release of CGRP which is inhibited by TRPA1 antagonists. In addition, our results demonstrate that depolarization of spinal cord synaptosomes by a high concentration of KCl induces CGRP release via a T-type calcium channel-dependent mechanism whilst TRPA1-induced CGRP release functions independently of voltage-gated calcium channel activation. Finally, we have shown that pre-treatment of synaptosomes with the opioid agonist, morphine, results in a reduction of depolarization-induced CGRP release. This study has demonstrated the use of a dorsal spinal cord homogenate assay for investigation of natively expressed TRPA1 channels and for modulation of depolarizing stimuli at the level of the dorsal spinal cord.

Functional expression and axonal transport of α7 nAChRs by peptidergic nociceptors of rat dorsal root ganglion

In recent pain studies on animal models, α7 nicotinic acetylcholine receptor (nAChR) agonists demonstrated analgesic, anti-hyperalgesic and anti-inflammatory effects, apparently acting through some peripheral receptors. Assuming possible involvement of α7 nAChRs on nociceptive sensory neurons, we investigated the morphological and neurochemical features of the α7 nAChR-expressing subpopulation of dorsal root ganglion (DRG) neurons and their ability to transport α7 nAChR axonally. In addition, α7 receptor activity and its putative role in pain signal neurotransmitter release were studied. Medium-sized α7 nAChR-expressing neurons prevailed, although the range covered all cell sizes. These cells accounted for one-fifth of total medium and large DRG neurons and <5% of small ones. 83.2% of α7 nAChR-expressing DRG neurons were peptidergic nociceptors (CGRP-immunopositive), one half of which had non-myelinated C-fibers and the other half had myelinated Aδ- and likely Aα/β-fibers, whereas 15.2% were non-peptidergic C-fiber nociceptors binding isolectin B4. All non-peptidergic and a third of peptidergic α7 nAChR-bearing nociceptors expressed TRPV1, a capsaicin-sensitive noxious stimulus transducer. Nerve crush experiments demonstrated that CGRPergic DRG nociceptors axonally transported α7 nAChRs both to the spinal cord and periphery. α7 nAChRs in DRG neurons were functional as their specific agonist PNU282987 evoked calcium rise enhanced by α7-selective positive allosteric modulator PNU120596. However, α7 nAChRs do not modulate neurotransmitter CGRP and glutamate release from DRG neurons since nicotinic ligands affected neither their basal nor provoked levels, showing the necessity of further studies to elucidate the true role of α7 nAChRs in those neurons.

Transient receptor potentials (TRPs) and anaphylaxis

The transient receptor potential (TRP) superfamily consists of 28 members in mammals (27 in human) that act as polymodal sensors and ion channels. They regulate cellular calcium influx, generate depolarization thereby triggering voltage dependent cellular processes, and in turn they are critical in inducing the metabolic activities of cells. It is increasingly apparent that many of the inflammatory mediators released in allergic reactions involve at least two of these ion channels, the 'Vanilloid' TRPV1 and the 'Ankyrin" TRPA1. This review mainly focuses on TRPV1 and TRPA1 and the role they have in the allergic response and how these receptors may be influenced in exercise-induced anaphylaxis. The threshold to react to an allergen for mast cells and lymphocytes can be reduced by activating the melastatin channel TRPM4. This channel is briefly discussed in the context of allergy.

Contribution of TRPV1 to microglia-derived IL-6 and NFkappaB translocation with elevated hydrostatic pressure

PURPOSE

The authors investigated the contributions of the transient receptor potential vanilloid-1 receptor (TRPV1) and Ca(2+) to microglial IL-6 and nuclear factor kappa B (NFkappaB) translocation with elevated hydrostatic pressure.

METHODS

The authors first examined IL-6 colocalization with the microglia marker Iba-1 in the DBA/2 mouse model of glaucoma to establish relevance. They isolated microglia from rat retina and maintained them at ambient or elevated (+70 mm Hg) hydrostatic pressure in vitro and used ELISA and immunocytochemistry to measure changes in the IL-6 concentration and NFkappaB translocation induced by the Ca(2+) chelator EGTA, the broad-spectrum Ca(2+) channel inhibitor ruthenium red, and the TRPV1 antagonist iodo-resiniferatoxin (I-RTX). They applied the Ca(2+) dye Fluo-4 AM to measure changes in intracellular Ca(2+) at elevated pressure induced by I-RTX and confirmed TRPV1 expression in microglia using PCR and immunocytochemistry.

RESULTS

In DBA/2 retina, elevated intraocular pressure increased microglial IL-6 in the ganglion cell layer. Elevated hydrostatic pressure (24 hours) increased microglial IL-6 release, cytosolic NFkappaB, and NFkappaB translocation in vitro. These effects were reduced substantially by EGTA and ruthenium red. Antagonism of TRPV1 in microglia partially inhibited pressure-induced increases in IL-6 release and NFkappaB translocation. Brief elevated pressure (1 hour) induced a significant increase in microglial intracellular Ca(2+) that was partially attenuated by TRPV1 antagonism.

CONCLUSIONS

Elevated pressure induces an influx of extracellular Ca(2+) in retinal microglia that precedes the activation of NFkappaB and the subsequent production and release of IL-6 and is at least partially dependent on the activation of TRPV1 and other ruthenium red-sensitive channels.

Mechanisms of potassium- and capsaicin-induced axonal calcitonin gene-related peptide release: involvement of L- and T-type calcium channels and TRPV1 but not sodium channels

We have previously shown that capsaicin, noxious heat, protons and potassium ions (K(+)) induce a graded, calcium- and receptor-dependent increase of immunoreactive calcitonin gene-related peptide (iCGRP) release from isolated rat sciatic axons. Morphological evidence for axonal vesicular exocytosis has also been presented. Here we determine the differential contribution of voltage-gated calcium and sodium channels to high extracellular potassium and capsaicin-induced iCGRP secretion. Blockade of L-type calcium channels significantly decreased the K(+)-induced axonal response (nimodipine (10 microM) by 66% and methoxyverapamil, D600 (50 microM), by 77%). Interestingly, however, D600 was unable to reduce the capsaicin-induced iCGRP release. Omega-Conotoxin GVIA (1 microM), a N-type blocker, and omega-agatoxin TK (0.1 microM), a P/Q-type blocker, had no significant effect. Also the anticonvulsant gabapentin (50 microM and 100 microM), reported to impede calcium channels, was ineffective. Inhibition of low threshold T-type calcium channels by mibefradil (10 microM) significantly reduced potassium (by 47%) but not capsaicin-stimulated iCGRP release. Reduction of total sodium channel conductance by tetrodotoxin (1 microM), lidocaine (10 microM, 50 microM or 500 microM) or by replacement of extracellular sodium with choline-chloride did not result in a reduction of either potassium- or capsaicin-induced axonal iCGRP release. These results suggest that slow depolarization by high extracellular potassium activates axonal low threshold (T-type) as well as high threshold-activated (L-type) voltage-gated calcium channels to mediate iCGRP release, and that capsaicin-induced release is largely dependent on calcium influx through TRPV1. Action potential generation and propagation are not required for axonal release mechanisms.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Potentiation of evoked calcitonin gene-related peptide release from oral mucosa: a potential basis for the pro-inflammatory effects of nicotine

Inflammation of the buccal mucosa, gingiva and periodontal tissues is a significant problem in users of nicotine-containing tobacco products; however, the potential role of nicotine in the development of this inflammation is unclear. In many tissues, nicotine, acting through nicotinic acetylcholine receptors (nAChRs), has been shown to increase the release of the pro-inflammatory mediator calcitonin gene-related peptide (CGRP) thereby potentially contributing to neurogenic inflammation. The purpose of the present studies was to determine the effects of nicotine and other nAChR agonists on capsaicin-evoked immunoreactive CGRP (iCGRP) release from rat buccal mucosa and to identify a potential cellular basis for these effects. Using a previously validated model of in vitro superfusion, we show that the nAChR agonists nicotine (EC50 557 micro m), epibatidine (EC50 317 pm) and cytisine (EC50 4.83 nm) potentiated capsaicin-evoked iCGRP release in a concentration-dependent manner by 123, 70 and 76%, respectively. The expression and distribution patterns of the mRNA transcripts encoding the alpha3, alpha4 and alpha6 nAChR subunits and their colocalization with CGRP and the capsaicin receptor VR1 were examined in rat trigeminal ganglion using combined in situ hybridization and immunohistofluorescence. Of all trigeminal neurons counted, mRNA encoding the alpha3, alpha4 and alpha6 subunits was found, respectively, in 14.45, 9.2 and 19.21% of neurons. The cell body diameter of most neurons containing any nAChR subunit was in the 30-40 micro m range with slightly fewer in the 20-30 micro m range. Co-localization of these alpha subunit transcripts with either CGRP or VR1 immunoreactivity ranged from approximately 5 to 7% for alpha4 and over 8% for alpha3 to 18% for alpha6. These data support the hypothesis that nicotinic agents, acting at nAChRs contained on primary sensory neurons, are capable of directly modulating the stimulated release of iCGRP. In the case of users of nicotine-containing tobacco products, this modulation could contribute to inflammatory processes within the oral cavity.

Role of voltage-gated cation channels and axon reflexes in the release of sensory neuropeptides by capsaicin from isolated rat trachea

In order to reveal the role of axon reflexes and sensory receptors in sensory neuropeptide release in response to capsaicin, liberation of substance P, calcitonin gene-related peptide and somatostatin from isolated rat tracheae was investigated in the presence of voltage-sensitive Na(+) and Ca(2+) channel blocking agents. Neuropeptide release induced by capsaicin (10 nM) remained unchanged in the presence of 25 mM lidocaine, 1 microM tetrodotoxin or the N-type Ca(2+) channel inhibitor, omega-conotoxin GVIA (100-300 nM). Peptide release by 100 pulses of 2 Hz field stimulation was prevented by lidocaine or tetrodotoxin. Omega-agatoxin TK (250 nM) significantly inhibited and Cd(2+) (200 microM) prevented capsaicin-induced neuropeptide release. These results suggest that chemical stimulation-induced neuropeptide release does not involve activation of fast Na(+) channels or N- and P-type voltage-dependent Ca(2+) channels, but contribution of Q-type Ca(2+) channels is possible. Sensory neuropeptides are released by capsaicin from sensory receptors without axon reflexes.

Endothelial nitric oxide modulates perivascular sensory neurotransmission in the rat isolated mesenteric arterial bed

1. A possible role of nitric oxide (NO) as a modulator of capsaicin-sensitive sensory neurotransmission in blood vessels was investigated in the rat isolated mesenteric arterial bed. 2. Electrical field stimulation (EFS) of methoxamine-preconstricted mesenteric beds elicited frequency-dependent vasorelaxation mediated by capsaicin-sensitive sensory nerves. N(G)-nitro-L-arginine methyl ester (L-NAME, 10 and 300 microM) and 7-nitroindazole (7-NI, 100 microM), inhibitors of nitric oxide synthase (NOS), augmented sensory neurogenic vasorelaxation. D-NAME (300 microM), 6-aminoindazole (100 microM) and N(omega)-propyl-L-arginine (50 nM), a selective inhibitor of neuronal NOS, were without effect. The effect of 10 microM L-NAME was reversed by L-arginine (1 mM), the substrate for NOS. 3. L-NAME (300 microM) and 7-NI (100 microM) had no significant effect on vasorelaxations to calcitonin gene-related peptide (CGRP), the principal motor neurotransmitter of capsaicin-sensitive sensory nerves in rat mesenteric arteries, or to capsaicin, indicating a prejunctional action. The inhibitors of NOS had no effect on vasorelaxation to forskolin, but augmented vasorelaxation to sodium nitroprusside (SNP). 4. Removal of the endothelium augmented sensory neurogenic vasorelaxation, but did not affect vasorelaxation to CGRP, indicating a prejunctional action of endothelial NO. 5. In the absence of endothelium, L-NAME (300 microM) inhibited, and 7-NI (100 microM) caused no further augmentation of sensory neurotransmission. 6. SNP (100 nM), a nitric oxide donor, attenuated sensory neurogenic relaxations to EFS. 7. In rat isolated thoracic aortic rings, L-NAME (100 microM) and 7-NI (100 microM) attenuated concentration-dependent relaxations to acetylcholine. 8. These data show that NO modulates sensory neurotransmission evoked by EFS of the rat isolated mesenteric arterial bed, and that when NO synthesis is blocked sensory neurogenic relaxation is augmented. The source of NO is the vascular endothelium.

Nicotine induces calcium spikes in single nerve terminal varicosities: a role for intracellular calcium stores

While nicotine is known to act at neuronal nicotinic acetylcholine receptors (nAChRs) to facilitate neurotransmitter release, the mechanisms underlying this action are poorly understood. Some of its effects are known to be mediated by presynaptic receptors. In the mouse vas deferens nicotine (10-30 microM) transiently increased the force of neurogenic contraction by 135+/-25%, increased the amplitude of excitatory junction potentials by 74+/-6% and increased the frequency of spontaneous excitatory junction potentials in four out of six preparations. Confocal microscopy and the calcium indicator Oregon Green 488 BAPTA-1 dextran were used to measure calcium concentration changes in the nerve terminals. Nicotine did not affect the action potential-evoked calcium transient but instead triggered small, random fluctuations ("calcium spikes") in intra-varicosity calcium concentrations at an average frequency of 0.09+/-0.02 Hz. These were insensitive to tetrodotoxin at a concentration that blocked action-potential evoked calcium transients (300 nM). They were abolished by the nAChR blocker hexamethonium (100 microM) and by both ryanodine (100 microM) and caffeine (3 mM), agents that modify calcium release from intracellular stores. We propose a novel mechanism whereby nicotine's action at nAChRs triggers calcium-induced calcium release from a ryanodine-sensitive calcium store in nerve terminals. This primes neurotransmitter release mechanisms and enhances both spontaneous and action potential-evoked neurotransmitter release.

Riot control agents: pharmacology, toxicology, biochemistry and chemistry

The desired effect of all riot control agents is the temporary disablement of individuals by way of intense irritation of the mucous membranes and skin. Generally, riot control agents can produce acute site-specific toxicity where sensory irritation occurs. Early riot control agents, namely, chloroacetophenone (CN) and chlorodihydrophenarsazine (DM), have been replaced with 'safer' agents such as o-chlorobenzylidene malononitrile (CS) and oleoresin of capsicum (OC). Riot control agents are safe when used as intended: however, the widespread use of riot control agents raises questions and concerns regarding their health effects and safety. A large margin exists between dosages that produce harassment and dosages likely to cause adverse health effects for modern riot control agents such as CS and dibenz[b,f]1 : 4-oxazepine (CR). Yet, despite the low toxicity of modern riot control agents, these compounds are not entirely without risk. The risk of toxicity increases with higher exposure levels and prolonged exposure durations. Ocular, pulmonary and dermal injury may occur on exposure to high levels of these substances, and exposure to riot control agents in enclosed spaces may produce significant toxic effects. Reported deaths are few involving riot control agents, and then only under conditions of prolonged exposure and high concentrations. Recently, concern has focused on the deaths resulting from law enforcement use of OC, a riot control agent generally regarded as safe because it is a natural product. As with other xenobiotics, not enough is known concerning the long-term/chronic effects of riot control agents. Clearly, there is considerable need for additional research to define and delineate the biological and toxicological actions of riot control agents and to illuminate the full health consequences of these compounds as riot control agents.

Different ion channel control pH6-induced bronchoconstriction and calcitonin gene-related peptide release in the guinea-pig lung

We have studied the bronchoconstriction and the release of calcitonin gene-related peptide-like immunoreactivity induced by perfusion of pH6 buffer in the isolated guinea-pig perfused lung. Both bronchoconstriction and peptide release were completely abolished after systemic capsaicin pretreatment. Ca(2+)-free pH6 buffer infusion also completely inhibited the bronchial response, whereas the calcitonin gene-related peptide-like immunoreactivity overflow was significantly reduced. omega-Conotoxine and omega-agatoxin IVA known as N-, L- and P-type Ca2+ channel blocker, respectively, and the Na+ channel blocker tetrodotoxin decreased significantly the pH6-induced bronchial response and calcitonin gene-related peptide like immunoreactivity overflow. Nifedipine was without influence suggesting the involvement of both P- and N-type Ca2+ channel as well as the activation of an axon reflex. Ruthenium red had a more pronounced reduction effect on the functional response than on the peptide release. Ryanodine and caffeine are both agents known to influence Ca2+ release from sarcoplasmic reticulum. Ryanodine significantly reduced both bronchoconstriction and calcitonin gene-related peptide-like immunoreactivity overflow. Caffeine as well as theophylline and the Na(+)-H+ blocker, dimethylamiloride, largely depressed the functional response while producing a significant increase of calcitonin gene-related peptide-like immuno-reactivity basal value. The pH6-induced peptide overflow was slightly inhibited after caffeine and dimethylamiloride pre-treatment whereas no significant change was observed after theophylline. It is concluded that multiple ion channels including different type of Ca2+ channels appear to participate in pH6-induced bronchoconstriction and calcitonin gene-related peptide-like immunoreactivity release in the guinea-pig lung.

A comparison of sensory nerve function in human, guinea-pig, rabbit and marmoset airways

We have investigated the role of sensory nerves in regulating airway smooth muscle function in the guinea-pig, marmoset, rabbit and man. Tissue levels of the sensory neuropeptides CGRP and substance P in the airways of the guinea-pig were significantly greater compared with the rabbit and marmoset. The relative order of tissue content was guinea-pig >>> rabbit = marmoset. Marmoset bronchial and tracheal preparations responded weakly to exogenously administered substance P and neurokinin A but contracted to methacholine and demonstrated atropine-sensitive cholinergic responses. In marmoset, rabbit and human airway preparations, capsaicin mediated weak contractile responses to exogenously administered capsaicin. However, high concentrations of capsaicin elicited a relaxation response that was epithelium-independent, cyclo-oxygenase-insensitive, not involving nitric oxide and not dependent on the activation of capsaicin-sensitive afferents. These results suggest that rabbit and marmoset airways respond functionally in a similar way to human airway preparations and maybe more relevant than guinea-pig airways with regard to understanding the role of sensory neuropeptides in airways.

Ion channels involved in the release of calcitonin gene-related peptide by low pH, prostacyclin and capsaicin in the isolated guinea-pig heart

The aim of this study was to characterise the release of calcitonin gene-related peptide (CGRP) by capsaicin, low pH and prostacyclin in terms of Ca2+ channel dependence, interactions with K(ATP) channels and the role of action potential propagation, in the isolated, perfused guinea-pig heart. The Ca2+ channel blocker omega-conotoxin reduced CGRP release evoked by 10(-7) M capsaicin, as well as CGRP release evoked by pH 7. CGRP release caused by capsaicin at low (10(-7) M) but not high (10(-6) M) concentrations was also attenuated by tetrodotoxin, indicating partial dependence on action potential propagation. CGRP release caused by prostacyclin was not altered by any of the tested drugs. The K(ATP) channel activator cromakalim and the K(ATP) channel blocker glibenclamide had no effect on CGRP release. Previous findings that low pH and capsaicin stimulate capsaicin-sensitive afferents in the isolated heart at least partly through common mechanisms are thus supported. Attenuation of capsaicin-evoked release of CGRP by tetrodotoxin suggests recruitment of additional nerve terminals by a local axon reflex.

Neuronal nicotinic receptor expression in sensory neurons of the rat trigeminal ganglion: demonstration of alpha3beta4, a novel subtype in the mammalian nervous system

The identification of a family of neuronal nicotinic receptor subunit genes establishes the potential for multiple subtypes with diverse physiological functions. Virtually all of the high affinity nicotinic receptors measured to date in the rodent CNS are composed of alpha4 and beta2 subunits only. However, the demonstration of other subunit transcripts in a variety of central and peripheral nervous tissues suggests a greater degree of receptor subtype heterogeneity than so far has been elucidated. The purpose of the present studies was to determine at the mRNA and protein levels which neuronal nicotinic receptor subunits are expressed by sensory neurons of the rat trigeminal ganglion and in what combinations these gene products associate to form neuronal nicotinic receptor subtypes in this tissue. Radioreceptor binding analysis indicated that in the adult rat trigeminal ganglion there exist at least two nicotinic receptor binding sites with differing affinities for [3H]-epibatidine. In situ hybridization histochemical studies revealed the existence of mRNA encoding the alpha3, alpha4, alpha5, beta2, and beta4 subunits, but not the alpha2 subunit. Immunoprecipitation with subunit-specific antisera demonstrated that each of the subunits present in the ganglion at the mRNA level is a constituent of nicotinic receptors capable of binding 3H-epibatidine. Various applications of these approaches yielded strong evidence that, in addition to alpha4beta2, which is thought to be the predominant neuronal nicotinic receptor subtype in the rodent CNS, trigeminal sensory neurons express as the principal subtype alpha3beta4, which has not been demonstrated previously in mammalian nervous tissue.

Multiple capsaicin-evoked currents in isolated rat sensory neurons

The response to capsaicin in functional assays suggests multiple sites of capsaicin action. This hypothesis is supported by the results of the present patch-clamp study of isolated dorsal root ganglion cells of the rat. The response to a prolonged application of capsaicin of different concentrations in an external solution with different ion compositions was investigated. Capsaicin evoked up to three distinct current components. The first and second current components could be activated independently. The third component occurred only in the presence of sodium and only in cells in which the second component was also elicited. In an extracellular solution with a physiological composition of ions and 300 nM capsaicin, the peaks of the three components, when evoked, occurred at 10.1 +/- 1.35 s (mean +/- S.E.M., n = 9), 44.0 +/- 2.64 s (n = 16) and 79.0 +/- 8.10 s (n = 5). The activation of the first and/or second current component depended on the concentration of capsaicin. A low concentration predominantly elicited the second component, while a high concentration activated the first and suppressed the second one. The third component seems to be a secondary response of the cell and was not investigated in detail. The activation and decay phases of the first two current components could be fitted by single exponential functions, whereas those of the third component could not. The first and second current components were carried by sodium and calcium. After tachyphylaxis, if the extracellular medium was then acidified to a pH of 6.3, the second component alone could then be elicited by capsaicin. The results demonstrate that capsaicin can elicit different current components that are distinguishable by their time-course, by the effects of acidification of the extracellular solution and by the concentration of capsaicin required to activate these currents. We postulate two distinct binding sites of capsaicin causing two distinct current components. This may account for the variety of physiological responses evoked by capsaicin and the variations in these responses between species.

NANC neural control of airway smooth muscle tone

1. This review addresses the functional role of the nonadrenergic, noncholinergic (NANC) neural response in the control of airway smooth muscle tone. 2. Functional data from guinea pig airways in vitro indicate that the level of basal smooth muscle tone determines the direction and magnitude of the NANC neural response such that it can stabilise tone. 3. The NANC stabilising effect on tone is adjustable through variation in impulse frequency and the NANC stabilising effect is also powerful; it can abolish near-maximum differences in tone. 4. Cholinergic activation increases the level towards which the NANC responses tend to adjust tone. 5. Adrenergic activation reduces the level towards which the NANC responses tend to adjust tone via beta-adrenoceptors. 6. NANC neural activation, with or without simultaneous adrenergic or cholinergic activation, can stabilise tone at low, intermediate or high levels with a high degree of accuracy. 7. Evidence from other investigators on effects of putative NANC neurotransmitters supports the idea of functional interactions within the NANC system in the airways. 8. It remains to be confirmed whether or not NANC responses play a stabilising role in the control of airway smooth muscle tone in vivo and in higher mammals.

Capsaicin-induced biphasic oxygen uptake in rat muscle: antagonism by capsazepine and ruthenium red provides further evidence for peripheral vanilloid receptor subtypes (VN1/VN2)

Previous studies with the vanilloid spice principle capsaicin have demonstrated a biphasic VO2 response, with vasoconstriction, in the perfused rat hindlimb that has led to suggestions of vanilloid receptor subtypes (VN1/VN2) in this preparation (1). In the present study, the known competitive vanilloid antagonist capsazepine inhibited the above capsaicin-mediated effects in a manner that was indicative of binding at specific vanilloid recognition sites. Low concentrations of capsazepine selectively inhibited the increased VO2 produced by the putative VN1 receptor at submicromolar concentrations of capsaicin, while the inhibition of VO2 produced by high concentrations of capsaicin (putative VN2) was enhanced. These observations, showing different susceptibilities to blockade by capsazepine, further support the presence of two vanilloid receptor subtypes in the rat hindlimb. Schild plots of the data yielded variable slopes that approach unity at greater responses to capsaicin (mean KB = 8.44 +/- 2.08 microM and 7.28 +/- 0.78 microM for VO2 and perfusion pressure curves, respectively). Low concentrations of the capsaicin antagonist ruthenium red selectively blocked the putative VN2 receptor-mediated effects produced by high concentrations of capsaicin. The noncompetitive nature of this inhibitor suggests an operation through separate receptor-coupled ion channel complexes at high and low concentrations of the vanilloid. Tetrodotoxin failed to attenuate any changes produced by capsaicin, suggesting that the mechanism of action of capsaicin in the rat hindlimb may differ from other tissues.

Tachykinins, sensory nerves, and asthma--an overview

Tachykinin peptides, substance P (SP) and neurokinin A (NKA), are released from airway sensory nerves upon exposure to irritant chemicals and endogenous agents including bradykinin, prostaglandins, histamine, and protons, The released neuropeptides are potent inducers of a cascade of responses, including vasodilatation, mucus secretion, plasma protein extravasation, leukocyte adhesion--activation, and bronchoconstriction. Neurokinin 1 receptors (preferably activated by SP) seem to be most important for inflammatory actions, while neurokinin 2 receptors (preferably activated by NKA) mediate bronchoconstriction. Species differences exist whereby rat and guinea-pig have a more developed neurogenic inflammation response than normal human airways. However, disease states such as inflammation or viral infections lead to enhanced peptide synthesis and (or) increased sensory nerve excitability. Together with increased neurokinin 1 receptor synthesis and loss of major tachykinin-degrading enzymes such as neutral endopeptidase in airway inflammation, this suggests that recently developed, orally active nonpeptide neurokinin receptor antagonists could have a therapeutic potential in asthmatic patients.

Functional and metabolic evidence for two different vanilloid (VN1 and VN2) receptors in perfused rat hindlimb

Vanilloid spice principles, including capsaicin, stimulate vasoconstriction in the rat hindlimb perfused at constant flow and, depending on dose, either stimulate or inhibit oxygen consumption by this vascular bed. We now present metabolic and functional evidence for two different vanilloid (VN1 and VN2) receptor types. These receptors can be distinguished on the basis of their differing agonist affinity for capsaicin, their different calcium and oxygen dependencies for inducing vasoconstriction, and whether they stimulate, or inhibit, oxygen consumption. The higher affinity vanilloid receptor, VN1 can be distinguished on the basis of initiating vasoconstriction at low doses of capsaicin and simultaneously stimulating oxygen consumption. Its apparent biological function is dependent on the presence of oxygen and external calcium. In contrast, the lower affinity receptor, VN2 induces vasoconstriction associated with inhibition of oxygen consumption. Its vasoconstriction action can occur independently of either external calcium ions, or the presence of oxygen in the perfusate.

Characterization of ruthenium red as an inhibitor of neurogenic inflammation in the rat trachea

1. We investigated the ability of ruthenium red, an inorganic dye with capsaicin antagonist properties, to inhibit capsaicin-induced plasma extravasation in the rat trachea. 2. The amount of plasma extravasation produced by intravenous capsaicin was reduced dose-dependently by i.v. ruthenium red. Complete inhibition was achieved with a dose of 5 mumol/kg. 3. The inhibitory effect of ruthenium red persisted for at least 16 hr after its administration, but was not present 24 hr later. 4. Ruthenium red did not reduce the amount of plasma extravasation produced by electrical stimulation of the vagus nerve, nor the amount produced by intravenous substance P or platelet-activating factor. 5. Prior exposure to a high dose of capsaicin reduced the amount of plasma extravasation produced by a second capsaicin exposure 48 hr later. However, giving ruthenium red 30 min before the initial capsaicin exposure largely prevented this loss of sensory nerve function. 6. We conclude that systemic administration of ruthenium red produces long-lasting, selective, and reversible inhibition of capsaicin-induced plasma extravasation in the rat trachea. Moreover, ruthenium red attenuates the long-term, desensitizing effect of capsaicin on sensory nerves.

Cigarette smoke-induced bronchoconstriction and release of tachykinins in guinea pig lungs

Two series of experiments were carried out to determine whether the release of tachykinins is involved in the bronchoconstriction induced by inhalation of cigarette smoke in guinea pigs. In the first series, cigarette smoke consistently induced bronchoconstriction (delta RL = +203% and delta Cdyn = -46%) in anesthetized guinea pigs, and the response was only partially blocked by bilateral cervical vagotomy. However, the smoke-induced bronchial constriction was completely abolished in animals receiving a systemic capsaicin pretreatment to destroy the tachykinin-containing C-fiber afferents. In the second series, the bronchoconstrictive effect of cigarette smoke was increased by approx. three times in isolated perfused guinea pig lungs when phosphoramidon (3 x 10(-6) M) was added to the perfusate to prevent the degradation of tachykinins after their release. Moreover, the enhanced bronchomotor response to smoke was accompanied by an overflow of neurokinin A-like immunoreactivity (LI) and calcitonin gene-related peptide -LI in the pulmonary effluent. These studies showed that cigarette smoke triggers the release of tachykinins in the lungs, which plays an important role in the smoke-induced bronchoconstrictive effect in guinea pigs.

Differential effects of omega-conotoxin GVIA and tetrodotoxin on vasoconstrictions evoked by electrical stimulation and nicotinic receptor stimulation in canine isolated, perfused splenic arteries

1. The effects of omega-conotoxin GVIA (omega-CgTX) and tetrodotoxin (TTX) on vasoconstrictions induced by acetylcholine (ACh) and nicotine were investigated and compared with those induced by periarterial electrical stimulation in the isolated and perfused canine splenic arteries. 2. ACh and nicotine at doses of 0.01 to 1 mumol constricted the splenic artery, dose-dependently. ACh induced consistent responses, but the vasoconstrictor responses to nicotine became significantly smaller with repeated administration of nicotine. 3. Periarterial electrical stimulation produced a vasoconstriction that was abolished by either TTX (30 nmol) or omega-CgTX (3 nmol), but the vasoconstrictor response to nicotine was not significantly affected by the same doses of TTX and omega-CgTX. Inhibitions by TTX and omega-CgTX of ACh-induced vasoconstrictions were small but statistically significant, showing that the percentage inhibition was less than 15%. TTX and omega-CgTX did not affect the vasoconstrictor responses to exogenous noradrenaline (NA). 4. ACh did not produce any vasoconstriction in the preparations treated either with alpha-adrenoceptor antagonists (10 microM bunazosin and 10 microM midaglizole) or with 30 microM guanethidine. NA-induced responses were abolished by alpha-adrenoceptor antagonists, but not affected by guanethidine treatment. 5. Vascular responses to ACh were completely inhibited by 1 mumol hexamethonium. In the preparations treated with 100 nmol nicotine, ACh did not produce any vasoconstriction. However, the NA-induced vasoconstriction was affected by neither hexamethonium nor nicotine treatment. 6. Atropine (1 microM) significantly inhibited but did not abolish the vasoconstrictor responses to ACh. The vascular responses to nicotine and NA were also significantly inhibited by atropine treatment. 7. These results indicate that (1) ACh constricts the splenic artery through the activation of presynaptic nicotinic receptors present on the sympathetic nerves; (2) differential effects of TTX and omega-CgTX on the vascular responses to ACh and nicotine, and to electrical stimulation suggest that the receptor-operated ion channels are mainly responsible for NA release induced by nicotinic receptor stimulation, but N-type VOCCs are responsible for that by electrical stimulation; (3) atropine may have an inhibitory action on nicotine-related responses, in addition to its inhibitory action on NA.

Sensory neuropeptide release by bradykinin: mechanisms and pathophysiological implications

Bradykinin (BK) and related kinins excite primary sensory neurons, thus leading to the activation of sensory impulses. More recently, both functional and neurochemical evidence have been accumulated that BK evokes release of neuropeptides, including calcitonin gene-related peptide and the tachykinins substance P and neurokinin A, from peripheral terminals of capsaicin-sensitive primary afferents. The present article will review the mechanisms and the pathophysiological implications of the ability of BK to release sensory neuropeptides at the peripheral level. An account of the clinical studies performed on this subject will be also given.

Different effects of the K+ channel blockers 4-aminopyridine and charybdotoxin on sensory nerves in guinea-pig lung

In the isolated guinea-pig bronchus, the potassium channel blocking agent 4-aminopyridine (10(-4) M) caused a contraction which was abolished by capsaicin tachyphylaxis, suggesting involvement of sensory neuropeptides. Charybdotoxin (10(-8), 5 x 10(-8) M), which is a potent blocker of the high-conductance Ca(2+)-activated K+ channel in smooth muscle, caused slowly developing and long lasting bronchoconstriction, which was resistant to capsaicin tachyphylaxis. Neither 4-aminopyridine (10(-3), 10(-4) M) nor charybdotoxin (10(-8), 5 x 10(-8) M) had any significant effect on the bronchoconstriction induced by electrical field stimulation. Furthermore, charybdotoxin had no significant influence on the inhibitory effect of the alpha 2-adrenoceptor agonist SKF 35886 (5 x 10(-7) M) on the bronchoconstriction induced by electrical field stimulation. In the isolated perfused guinea-pig lung, 4-aminopyridine (3 x 10(-5) -10(-3) M) caused bronchoconstriction and enhanced both basal and (at 3 x 10(-5) M) vagal nerve stimulation-evoked calcitonin gene-related peptide outflow from pulmonary sensory nerves. In conclusion, 4-aminopyridine stimulated capsaicin-sensitive sensory neurons and enhanced the sensory activation induced by vagal nerve stimulation in guinea-pig lung. Charybdotoxin, on the other hand, caused bronchial contraction independently of capsaicin-sensitive nerves.