Respiratory actions of vanilloid receptor agonists in the nucleus of the solitary tract: comparison of resiniferatoxin with non-pungent agents and anandamide

1. Activation of vanilloid receptors on sensory nerve terminals in the commissural nucleus of the solitary tract (cNTS) of rats with capsaicin, produces respiratory slowing. In this study, we used microinjection techniques employing pungent and non-pungent vanilloids to further characterize vanilloid receptors in the cNTS. 2. Microinjection of the pungent vanilloid, resiniferatoxin (RTX), into the cNTS of urethane-anaesthetized rats, dose-dependently reduced respiratory rate without affecting tidal volume. RTX was 20 fold more potent at slowing respiration ( approximately ED(50), 100 pmol) than capsaicin ( approximately ED(50), 2 nmol). Doses of RTX greater than 100 pmol caused either irregular (dyspnoeic) breathing or terminal apnoea (>250 pmol). The respiratory slowing response to RTX (75 pmol), was dose-dependently attenuated by injecting RTX (but not vehicle) into the same site 60 min earlier. 3. The non-pungent phorbol derivative of RTX, phorbol 12-phenylacetete 13-acetate 20-homovanillate (PPAHV, 0.1-1 nmol), also slowed respiration (ED(50), approximately 1 nmol) and almost abolished response to RTX (75 pmol) injected into the same site 60 min later. 4. In contrast to RTX, PPAHV and capsaicin, the putative endogenous vanilloid receptor agonist, arachidonyl ethanolamide (AEA), and non-pungent capsaicin derivative, olvanil, had no direct effect on respiration. However, both AEA and olvanil dose-dependently reduced the respiratory response to injection of RTX (75 pmol) 60 min later into the same site (EC(50)s, for AEA and olvanil, approximately 2 and 0.2 nmol, respectively). 5. These studies suggest that both pungent and non-pungent vanilloids interact with vanilloid receptors in the cNTS. However, whereas RTX and PPAHV activate and subsequently desensitize vanilloid receptors on sensory nerve terminals in the cNTS, olvanil and AEA fail to activate despite readily desensitizing responses to RTX in this region.     

Effect of olvanil on the afferent and efferent function of capsaicin-sensitive C-fibers in guinea pig airways

The aim of the present study was to examine the ability of the nonpungent vanilloid VR1 receptor agonist, olvanil, to activate the afferent and efferent function of capsaicin-sensitive C-fibers in guinea pig airways. We found that while capsaicin (10 nM-10 microM) and resiniferatoxin (0.1 nM-1.0 microM) evoked a robust contraction of the guinea pig trachea in vitro, olvanil (10 nM-10 microM) was a weak spasmogen. In addition, pretreatment with olvanil caused only a minor reduction of subsequent responses to capsaicin or resiniferatoxin. Using single fiber recording from guinea pig airway C-fibers, we found that olvanil (10 microM) did not evoke action potential discharge although these fibers responded vigorously to capsaicin after prolonged treatment with olvanil (10 microM). These findings are indicative of significant differences in the relative sensitivity of vanilloid VR1 receptor-transfected cells and the peripheral terminals of airway C-fibers to pungent and nonpungent vanilloid VR1 receptor agonists.     

Effect of olvanil and anandamide on vagal C-fiber subtypes in guinea pig lung

Certain fatty acid amides such as anandamide (AEA) and olvanil are agonists for the transient receptor potential, vanilloid-1 (TRPV1) receptor, but have been found to activate TRPV1-containing C-fibers in some tissues but not others. We used extracellular recording and whole-cell patch clamp techniques to investigate the effect of olvanil and AEA on different types of vagal C-fibers innervating the same tissue, namely jugular and nodose vagal C-fibers in guinea pig lungs. A 30 s exposure to AEA and olvanil caused action potential discharge in all nodose C-fiber innervating lung but failed to activate jugular C-fibers innervating lung and airways. The activation of nodose C-fibers was blocked by the TRPV1 antagonist iodo-resiniferatoxin. In whole-cell patch clamp recordings of dissociated nodose and jugular capsaicin-sensitive neurons labeled from lungs and airways, olvanil induced large TRPV1-dependent inward currents in cell bodies of both nodose and jugular ganglion neurons. Prolonged exposure (up to 5 min) to olvanil caused action potential discharge in jugular C-fiber innervating lung but the onset latency was four times longer in jugular than in nodose C-fibers. The onsets of capsaicin response in nodose and jugular C-fibers were not different. Decreasing the tissue temperature to 25 degrees C increased the onset latency of olvanil-induced activation of nodose C-fibers 2-3-fold, but did not effect the latency of the capsaicin response. Capsaicin, olvanil, and AEA stimulate jugular C-fibers leading to tachykinergic contractions of isolated bronchi. The time to reach half-maximum is more than four times longer for olvanil and AEA, as compared to capsaicin in evoking contractions. We conclude that brief exposure to certain fatty acid amides, such as AEA and olvanil activate nodose but not jugular C-fiber terminals in the lungs. We hypothesize that this is because the nodose C-fiber terminals are equipped with a temperature-dependent mechanism for effectively and rapidly transporting the TRPV1 agonists so that they gain access to the intracellular binding sites on TRPV1. This transport mechanism may be differently expressed in two distinct subtypes of pulmonary C-fiber terminals innervating the same tissue.     

Selective sensory denervation by capsaicin aggravates adriamycin-induced cardiomyopathy in rats

Capsaicin-sensitive sensory nerves that contain calcitonin gene-related peptide (CGRP) contribute significantly to cardioprotective mechanisms. In this study, the possible role of capsaicin-sensitive afferent nerves in the development of congestive heart failure was examined in an established model of adriamycin-induced experimental cardiomyopathy in rats. Systemic treatment with capsaicin was utilized to deplete sensory neuropeptides from cardiac afferent nerves. Echocardiography was applied to assess the cardiac function in adriamycin-treated rats pretreated with capsaicin or its vehicle. In control rats, adriamycin treatment produced a reduction in the fractional shortening of the left ventricle and an increase in the ratio of the left atrial diameter and the aortic diameter, indicative of a decreased myocardial contractility and heart failure only at 3–4 weeks post-treatment. In contrast, in capsaicin-pretreated rats, a deterioration of the cardiac function was already evident 1 week after the cessation of adriamycin administration, while the clinical signs associated with cardiomyopathy were more severe and displayed a significantly more rapid progression. Immunohistochemistry revealed a complete depletion of calcitonin gene-related peptide from cardiac sensory nerves after systemic capsaicin treatment. This study has demonstrated that elimination of capsaicin-sensitive afferent nerves promotes the development and progression of adriamycin-induced myocardial dysfunction. The results suggest that interfering with capsaicin/vanilloid receptor function and/or perturbation of the myocardial CGRP metabolism may open up new perspectives concerning prevention and/or alleviation of the pathological changes that follow adriamycin treatment.     

Pharmacology of vanilloids at recombinant and endogenous rat vanilloid receptors

This study compared the actions of members of five different chemical classes of vanilloid agonists at the recombinant rat vanilloid VR1 receptor expressed in HEK293 cells, and at endogenous vanilloid receptors on dorsal root ganglion cells and sensory nerves in the rat isolated mesenteric arterial bed. In mesenteric beds, vanilloids elicited dose-dependent vasorelaxation with the rank order of potency: resiniferatoxin>capsaicin=olvanil>phorbol 12-phenyl-acetate 13-acetate 20-homovanillate (PPAHV)>isovelleral. Scutigeral was inactive. Responses were abolished by capsaicin pretreatment and inhibited by ruthenium red. In VR1-HEK293 cells and dorsal root ganglion neurones, Ca(2+) responses were induced by resiniferatoxin>capsaicin=olvanil>PPAHV; all four were full agonists. Isovelleral and scutigeral were inactive. The resiniferatoxin-induced Ca(2+) response had a distinct kinetic profile. Olvanil had a Hill coefficient of approximately 1 whilst capsaicin, resiniferatoxin and PPAHV had Hill coefficients of approximately 2 in VR1-HEK293 cells. The capsaicin-induced Ca(2+) response was inhibited in a concentration-dependent manner by ruthenium red>capsazepine>isovelleral. These data show that resiniferatoxin, capsaicin, olvanil and PPAHV, but not scutigeral and isovelleral, are agonists at recombinant rat VR1 receptors and endogenous vanilloid receptors on dorsal root ganglion neurones and in the rat mesenteric arterial bed. The vanilloids display the same relative potencies (resiniferatoxin>capsaicin=olvanil>PPAHV) in all of the bioassays.     

Mechanisms underlying tissue selectivity of anandamide and other vanilloid receptor agonists

Anandamide acts as a full vanilloid receptor agonist in many bioassay systems, but it is a weak activator of primary afferents in the airways. To address this discrepancy, we compared the effect of different vanilloid receptor agonists in isolated airways and mesenteric arteries of guinea pig using preparations containing different phenotypes of the capsaicin-sensitive sensory nerve. We found that anandamide is a powerful vasodilator of mesenteric arteries but a weak constrictor of main bronchi. These effects of anandamide are mediated by vanilloid receptors on primary afferents and do not involve cannabinoid receptors. Anandamide also contracts isolated lung strips, an effect caused by the hydrolysis of anandamide and subsequent formation of cyclooxygenase products. Although capsaicin is equally potent in bronchi and mesenteric arteries, anandamide, resiniferatoxin, and particularly olvanil are significantly less potent in bronchi. Competition experiments with the vanilloid receptor antagonist capsazepine did not provide evidence of vanilloid receptor heterogeneity. Arachidonoyl-5-methoxytryptamine (VDM13), an inhibitor of the anandamide membrane transporter, attenuates responses to olvanil and anandamide, but not capsaicin and resiniferatoxin, in mesenteric arteries. VDM13 did not affect responses to these agonists in bronchi, suggesting that the anandamide membrane transporter is absent in this phenotype of the sensory nerve. Computer simulations using an operational model of agonism were consistent, with differences in intrinsic efficacy and receptor content being responsible for the remaining differences in agonist potency between the tissues. This study describes differences between vanilloid receptor agonists regarding tissue selectivity and provides a conceptual framework for developing tissue-selective vanilloid receptor agonists devoid of bronchoconstrictor activity.     

Capsaicin-sensitive sensory neurons regulate myocardial nitric oxide and cGMP signaling

We studied whether tissue levels of nitric oxide (NO) and cGMP are regulated by sensory nerves in normoxic and ischemic hearts. Wistar rats were treated with capsaicin to deplete neurotransmitters from capsaicin-sensitive sensory nerves. In separate experiments, capsaicin was applied perineurally to both vagus nerves for selective chemodenervation of vagal cardiac afferent nerves. Systemic capsaicin administration significantly decreased basal myocardial NO content assessed by electron spin resonance (ESR) spectroscopy, whereas, local treatment of vagus nerves did not change it. Both systemic and local capsaicin treatment decreased cardiac cGMP content measured by radioimmunoassay. In separate experiments, isolated hearts from control and systemic capsaicin-treated rats were subjected to 30-min global ischemia. NO signal intensity increased 10-fold after ischemia, whereas, cardiac cGMP decreased. Capsaicin pretreatment did not influence ischemic NO or cGMP content. These results suggest a major role for capsaicin-sensitive sensory neurons in the maintenance of basal but not ischemic myocardial NO and cGMP content. Vagal sensory nerves may be involved in the regulation of basal myocardial cGMP but not basal NO level. Consequently, basal NO content in the heart is regulated primarily by spinal afferent nerves.     

Neuropharmacological mechanisms of capsaicin and related substances.

Capsaicin activates poorly myelinated primary afferent neurons, many of which are polymodal nociceptors. Activation is accompanied by membrane depolarization and the opening of a unique, cation-selective, ion channel which can be blocked by the polyvalent dye ruthenium red. The capsaicin-induced activation is mimicked by resiniferatoxin, a potent analogue, and by low pH. Activation is mediated by a specific membrane receptor which can be selectively and competitively antagonized by capsazepine. Repetitive administration of capsaicin produces a desensitization and an inactivation of sensory neurons. Several mechanisms are involved including receptor inactivation, block of voltage activated calcium channels, intracellular accumulation of ions leading to osmotic changes, and activation of proteolytic enzyme processes. Systemic and topical capsaicin produces a reversible antinociceptive and anti-inflammatory action after an initial undesirable algesic effect. Capsaicin analogues, such as olvanil, have similar properties with minimal initial algesic activity. Antinociception produced by capsaicin does not involve neurotoxicity, sensory neuropeptide depletion or activity at peripheral receptors; rather, systemic capsaicin produces antinociception by activating capsaicin receptors on afferent nerve terminals in the spinal cord. Spinal neurotransmission is blocked by a prolonged inactivation of sensory neurotransmitter release. However, local or topical applications of capsaicin block C-fibre conduction and inactive neuropeptide release from peripheral nerve endings. These mechanisms account for localized antinociception and the reduction of neurogenic inflammation, respectively.     

Capsaicin sensitive nerves in the jejunum of Nippostrongylus brasiliensis-sensitized rats participate in a cardiovascular depressor reflex

Summary Superfusion of capsaicin onto the serosal surface of jejunum of Nippostrongylus brasiliensis-sensitized rats induces a short-lasting (1–3 min), dose-dependent (2 to 20 g) decrease in blood pressure which ranges from –5.3±1.40% to –22.6±2.20%. The hypotension evoked by capsaicin was more marked in sensitized rats than in unsensitized animals, which responded only to the highest dose (20 mg) of capsaicin tested. The hypotensive effects of capsaicin were not affected by intravenous injections of mepyramine (10 mg/kg), a histamine receptor antagonist, or by the cycloxygenase inhibitor indomethacin (10 mg/kg). However, an intravenous injection of a platelet-activating factor (PAT) antagonist, BN 52021 (20 mg/kg), or an intraperitoneal injection of guanethidine (8 mg/kg) 18 h prior to experimentation, to functionally impair the sympathetic nerves, abolished the capsaicin-induced drop in blood pressure. Treatment of neonatal rats with capsaicin reduced by 75% the hypotensive effects of capsaicin, whereas the capsaicin antagonist, ruthenium red, reduced non-significantly the hypotensive action of capsaicin. It is concluded that the activation of jejunal sensory nerves in N. brasiliensis-sensitized rats by capsaicin induced a reflex hypotension that is dependent upon PAF release from mast cells and functional sympathetic nerves. In addition, the afferent function of the sensory nerves are not totally blocked by ruthenium red as capsaicin elicits the reflex hypotension in the presence of this blocker of sensory nerve efferent function.Correspondence to: R. Mathison     

Biphasic response of cutaneous blood flow induced by passive cutaneous anaphylaxis in rats

In the immediate phase of passive cutaneous anaphylaxis, sensitized skin mast cells release various mediators when activated by antigen. The present study investigated the effects of the mediators on cutaneous blood flow at the antigen-antibody reaction site. Induction of passive cutaneous anaphylaxis produced a biphasic response consisting of an initial decrease, followed by a sustained increase, in the cutaneous blood flow. The initial phase was almost eliminated by the 5-hydroxytryptamine receptor antagonist methysergide, whereas the second phase was sensitive to the histamine H(2) receptor antagonist ranitidine. The histamine H(1) receptor antagonist chlorpheniramine, the denervation of sensory nerves with capsaicin, the cyclooxygenase inhibitor indomethacin, or the bradykinin B(2) receptor antagonist D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl-L-(2alpha,3beta,7abeta)-octahydro-1H-indole-2-carbonyl-L-arginine (HOE140) did not affect the blood-flow changes caused by the anaphylaxis. These results suggest that 5-hydroxytryptamine and histamine H(2) receptors mediate the initial decrease and the subsequent increase in cutaneous blood flow, respectively, induced by passive cutaneous anaphylaxis in rats.     

Multiple impairments of cutaneous nociceptor function induced by cardiotoxic doses of Adriamycin in the rat

Besides their deleterious action on cardiac muscle, anthracycline-type cytostatic agents exert significant neurotoxic effects on primary sensory neurons. Since cardiac sensory nerves confer protective effects on heart muscle and share common traits with cutaneous chemosensitive nerves, this study examined the effects of cardiotoxic doses of adriamycin on the function and morphology of epidermal nerves. Sensory neurogenic vasodilatation, plasma extravasation, and the neural CGRP release evoked by TRPV1 and TRPA1 agonists in vitro were examined by using laser Doppler flowmetry, the Evans blue technique, and ELISA, respectively. Carrageenan-induced hyperalgesia was assessed with the Hargreaves method. Immunohistochemistry was utilized to study cutaneous innervation. Adriamycin treatment resulted in profound reductions in the cutaneous neurogenic sensory vasodilatation and plasma extravasation evoked by the TRPV1 and TRPA1 agonists capsaicin and mustard oil, respectively. The in vitro capsaicin-, but not high potassium-evoked neural release of the major sensory neuropeptide, CGRP, was markedly attenuated after adriamycin treatment. Carrageenan-induced inflammatory hyperalgesia was largely abolished following the administration of adriamycin. Immunohistochemistry revealed a substantial loss of epidermal TRPV1-expressing nociceptive nerves and a marked thinning of the epidermis. These findings indicate impairments in the functions of TRPV1 and TRPA1 receptors expressed on cutaneous chemosensitive nociceptive nerves and the loss of epidermal axons following the administration of cardiotoxic doses of adriamycin. Monitoring of the cutaneous nociceptor function in the course of adriamycin therapy may well be of predictive value for early detection of the deterioration of cardiac nerves which confer protection against the deleterious effects of the drug.     

Protection by capsaicin against attenuated endothelium-dependent vasorelaxation due to lysophosphatidylcholine

Previous studies have shown that pretreatment with calcitonin gene-related peptide (CGRP), a principal transmitter in sensory nerves, can protect the endothelial cell. We therefore evaluated whether in vivo capsaicin treatment prevents endothelial damage elicited by lysophosphatidylcholine (LPC) in the rat aorta. Acute treatment or repeated pretreatment with capsaicin resulted in stimulation of neurotransmitter release from sensory nerves or depletion of their transmitter content respectively. Vasodilator responses to acetylcholine (ACh) were examined in the aorta of these animals. Acute application of capsaicin (50 mg/kg) increased the plasma concentration of CGRP-like immunoreactivity (CGRP-LI) concomitantly with a reversal of the inhibition by LPC of endothelium-dependent ACh-induced relaxation in the isolated rat aorta. After repeated pretreatment with capsaicin to deplete sensory nerve neurotransmitter content the effects of capsaicin were absent as shown by the plasma CGRP-LI concentration and the vasodilator response to ACh. The results demonstrate that systemic capsaicin treatment, which evokes the release of CGRP from sensory nerves, protects the endothelial cell. The present study also suggests that CGRP may be an endogenous vascular protective substance. Received: 16 January 1997 / Accepted: 28 April 1997     

Topical capsaicin in the treatment of cutaneous disorders

Capsaicin, the primary pungent principle in hot peppers, produces marked alterations in the function of unmyelinated sensory afferent fibers, which are believed to signal pain and to initiate inflammatory responses. Capsaicin first excites and then desensitizes these nerves to subsequent stimulation both by itself and by a variety of physiocochemical stimuli. Accordingly, capsaicin is finding increasing use as a topical therapy for a variety of cutaneous disorders that involve pain and inflammation. Although topical capsaicin has shown therapeutic potential, the utility of capsaicin appears to be limited at present primarily by its irritant properties and secondarily by less than optimal therapeutic response, perhaps resulting from insufficient drug delivery. A capsaicin analogue with diminished irritant properties that retained the critical pharmacologic activities might present a reasonable alternative.     

Olopatadine hydrochloride inhibits capsaicin-induced flare response in humans

Capsaicin, a vanilloid, has the potential for releasing substance P (SP) from sensory nerves. Topical application of capsaicin induces a flare response in the skin. However, it has not been clarified whether the release of SP is involved in the process of flare response or not. A potent antihistamine drug, olopatadine hydrochloride, is known to have inhibitory action against the release of SP. We examined the effects of olopatadine (at a dose of 5 mg) on skin reaction induced by topical application of capsaicin in 10 healthy subjects. The scores of capsaicin-induced flare responses after olopatadine administration were significantly lower at 30 min than at baseline. Our findings suggest that olopatadine hydrochloride could inhibit capsaicin-induced flare responses.     

Modulatory action of galanin on responses due to antidromic activation of peripheral terminals of capsaicin-sensitive sensory nerves

Galanin inhibited, in a concentration-dependent manner (EC50 7.2 nM), the positive inotropic response produced by field stimulation of isolated left atria from reserpine-pretreated guinea-pigs (in the presence of atropine). These responses were shown to involve antidromic activation of capsaicin-sensitive primary afferents. On the other hand, galanin did not affect the inotropic response to capsaicin or calcitonin gene-related peptide, the putative endogenous mediator released from sensory nerves. Galanin-(1-10) was at least 10,000 times less potent than the parent peptide, while galanin-(25-29) was ineffective. Likewise, galanin inhibited the non-cholinergic contraction produced by field stimulation of the isolated guinea-pig bronchus but not the contraction produced by exogenous neurokinin A. These findings indicate a prejunctional neuromodulatory action of galanin on the excitability of peripheral terminals of capsaicin-sensitive sensory nerves.     

白三烯拮抗剂ONO-1078对化学物质诱导大鼠皮肤微血管渗漏的抑制作用

大鼠皮内注射组胺、辣椒素和甲醛诱导皮肤微血管渗漏,白三烯拮抗剂ONO-1078剂量依赖性抑制这一反应,ID₅₀分别为1.98,1.78,2.23mg·kg^-1。与扑尔敏相比,对组胺的作用较弱,对辣椒素和甲醛的作用较强,地塞米松的作用强于ONOl078和扑尔敏。ONO-l078还抑制LTD4的作用,对大剂量组胺、缓激肽和P物质无明显作用。ONO-l078的作用可能与抑制感觉神经肽释放有关。     

Capsaicin-sensitive nerves modulate resting blood flow and vascular tone in rat gut

Summary Acute and chronic treatments with capsaicin were used to evaluate the role of afferent neurons in the regulation of intestinal blood flow. Experiments were performed on anesthetized rats, in which mean intestinal blood flow was determined with a pulsed Doppler flowmeter mean systemic arterial pressure was determined with a transducer, and intenstinal vascular conductance (C) was calculated from these measurements. Acute administration of periarterial capsaicin (0.5 mg) induced biphasic intestinal vascular responses. An early hyperemic response occurred with a maximal increase in blood flow of 31 % at 5 min, followed by a decrease in blood flow of 17% at 30 min. Arterial pressure was decreased by the application of capsaicin, initially by 10%. There was an early increase of 49% in conductance, followed by a 15% decrease, compared with control values. When 1 or 4 mg capsaicin was instilled into the lumen of the jejunum there was a response pattern similar to that observed after periarterial application of capsaicin. Intrajejunal capsaicin (4 mg) increased blood flow by 51%, followed by a decrease of 16%. Mean mesenteric artery conductance was increased by 32% initially and subsequently was decreased by 21%, in response to acute intrajejunal administration of capsaicin. Both mean blood flow and conductance were increased (44% and 76%, respectively) in adult rats chronically pretreated with capsaicin (170 mg total dose) when compared with vehicle-treated controls.However, in rats pretreated neonatally with capsaicin (50 mg/kg) and allowed to mature, basal flood flow was lower than in control animals but C was not different from control littermates. These findings suggest that the hyperemic effect of acute administration of capsaicin may be related to the stimulation of afferent sensory nerves and release of vasodilatory peptides. The late vasoconstrictor effect could be due to neuronal depletion of vasodilator peptides in perivascular nerves. However, changes observed in the intestinal circulation after chronic pretreatment with capsaicin in the adult rat, in contrast to the observations in the neonatally treated rat, cannot be explained entirely by simple depletion of peptides from the sensory nerves. We conclude that capsaicin-sensitive, afferent nerves in the small intestine modulate the resting vascular tone in rat gut. Send offprint requests to O. D. Hottenstein at the above address     

Calcitonin gene-related peptide but not substance P mimics capsaicin-induced coronary vasodilation in the pig

The vasodilator effects of the human calcitonin gene-related peptides alpha (hCGRP alpha) and beta (hCGRP beta) were studied in vitro and in vivo in relation to the effects of substance P (SP) and capsaicin on coronary vascular tone in the pig. Both hCGRP alpha and -beta induced a concentration-dependent, long-lasting relaxation of precontracted small (diameter 0.5 mm) pig coronary arteries in vitro. SP was slightly more potent but caused a transient relaxation with a smaller maximal response than CGRP. The relaxation induced by hCGRP alpha and -beta as well as SP was resistant to propranolol and atropine. Capsaicin also induced a long-lasting relaxation of potassium and PGF2 alpha-precontracted coronary arteries. After tachyphylaxis to SP had developed the relaxant effects of CGRP and capsaicin were unchanged. Rubbing the vessels to remove the endothelium completely abolished the relaxant effects of SP while the vasodilation induced by hCGRP alpha as well as capsaicin remained unchanged. Injections of hCGRP alpha, SP or capsaicin into the constantly perfused left anterior descending coronary artery of the pig in vivo caused a dose-dependent decrease in perfusion pressure, suggesting coronary vasodilation. In conclusion, the vasodilator effects of SP in vitro differ from the response to CGRP both with regard to their transient nature, the development of tachyphylaxis and endothelium dependence. The capsaicin-induced coronary vasodilation is therefore more likely to depend on release of CGRP rather than tachykinins from sensory nerves since neither endothelium removal nor SP-tachyphylaxis influenced the capsaicin and CGRP responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nociceptin protects capsaicin-sensitive afferent fibers in the rat urinary bladder from desensitization

Chemical stimulation of primary afferent nerves in the rat urinary bladder in vivo with topical capsaicin (1 microg in 50 microl saline) determines a dual motor response, consisting of a contractile effect mediated by tachykinins released from sensory nerves in the bladder wall and a transient activation of a bladder-to-bladder micturition reflex organized at the supraspinal level (chemoceptive micturition reflex). Both responses undergo complete desensitization upon repeated applications of capsaicin. The i.v. administration of the novel neuropeptide nociceptin (100 nmol/kg) produced a long-lasting protection from capsaicin desensitization of afferent nerves which mediate the chemoceptive micturition reflex. In fact a chemoceptive micturition reflex could be repeatedly evoked by topical capsaicin in nociceptin-pretreated rats. In sharp contrast, nociceptin did not influence the development of desensitization of the local response to capsaicin, corresponding to the 'efferent' function of capsaicin-sensitive afferent neurons. These results suggest that the afferent and 'efferent' function of capsaicin-sensitive primary afferent neurons (CSPANs) in the rat bladder are differentiated by nociceptin. Alternative mechanisms underlying this phenomenon are discussed.     

Evidence that toluene diisocyanate activates the efferent function of capsaicin-sensitive primary afferents

Isocyanates are an important cause of occupational asthma. The mechanism of isocyanate-induced asthma is still unknown. To determine whether toluene diisocyanate stimulates the 'efferent' function of peripheral endings of capsaicin-sensitive sensory nerves, we investigated the effect of toluene diisocyanate in the rat isolated urinary bladder, a preparation in which the action of capsaicin has been well characterized. Toluene diisocyanate (0.03-3 mM) produced a concentration-dependent contraction of the bladder strips. Its maximal effect was about 50% of the response to capsaicin (1 microM). Previous exposure of the strips to capsaicin followed by washing out produced complete unresponsiveness, both to the first exposure to toluene diisocyanate and to a second exposure of capsaicin. Further, the response to both toluene diisocyanate and capsaicin was completely prevented by extrinsic bladder denervation, achieved by bilateral removal of pelvic ganglia (72 h before). Repeated exposure of the rat bladder to toluene diisocyanate reduced the capsaicin-evoked release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI), taken as biochemical marker of activation of these sensory nerves. These experiments provide the first evidence that toluene diisocyanate activates directly or indirectly the efferent function of capsaicin-sensitive primary sensory nerves.