Ruthenium red selectively inhibits capsaicin-induced release of calcitonin gene-related peptide from the isolated perfused guinea pig lung

Additive inhibitory effects of calcitonin and capsaicin on voltage activated calcium channel currents in nociceptive neurones of rat

Calcitonin, a peptide hormone expressed in C-cells of the thyreoid gland, as well as capsaicin, isolated from chili, both, modify intracellular signalling in nociceptive neurones. The pathways triggered by the activation of either of these receptors results in a modulation of the intracellular calcium ([Ca(2+)](i)) concentration. While the regulation of [Ca(2+)](i) depends on many factors, voltage activated calcium channels (VACCs) are a major gate for the calcium entry into neurones. Here we describe the changes of voltage gated calcium channel currents (I(Ca(V))) induced by calcitonin and/or capsaicin. Currents were recorded using adequate solutions and voltage protocols with the whole cell patch-clamp technique. When the channels were opened by a depolarisation to 0 mV, both substances reduce the peak I(Ca(V)) (calcitonin (10nM): 29.3 ± 3.9%; capsaicin (0.5 μM): 41.1 ± 7.7%). While the effect of calcitonin was voltage dependent, capsaicin shifted the largest current to the more hyperpolarizing range (peak current from -10 to -20 mV). A subsequent co-application of either of the two substances (with a pre-application of either 3 min or 60 min) results in an additive reduction of the currents, and prevents the capsaicin-induced shift of the current-voltage relation. Therefore, we hypothesize, that the activation of either of the two receptors reduces I(Ca(V)) by different cellular binding sites of the channel protein triggering channel opening. These findings may be useful to understand cellular mechanisms of pain modulation and might help to find better treatments for neuropathic pain.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

Licking for taste solutions by potassium-deprived rats: specificity and mechanisms

There has been little work on the specificity and mechanisms underlying the appetite of potassium (K(+)) deprived rats, and there are conflicting results. To investigate the contribution of oral factors to changes in intake induced by K(+) deficiency, we conducted two experiments using 20-s "brief access" tests. In Experiment 1, K(+)-deprived rats licked less for water than did replete rats. After adjusting for this difference, K(+)-deprived rats exhibited increased licking for 100 mM CaCl(2), 100 mM MgCl(2), and 100 mM FeCl(2) compared with K(+)-replete rats. In Experiment 2, which used larger rats, the K(+)-deprived and replete groups licked equally for water, 500 mM Na.Gluconate, 350 mM KCl, 500 mM KHCO(3), and 1 mM quinine.HCl, but the K(+)-deprived rats licked more for 500 mM KCl, 500 mM CsCl, and 500 mM NaCl than did the replete rats. Licking was unaffected by addition to NaCl of 200 muM amiloride, an epithelial Na(+) channel (ENaC) blocker, or 100 muM ruthenium red, a vanilloid receptor 1 (VR-1) antagonist, or by addition to KCl of 50 muM 4-aminopyridine, a K(+) channel blocker. These findings suggest that K(+)-deprivation produces a non-specific appetite that is guided by oral factors. We found no evidence that this response was mediated by ENaC, VR-1, or K(+) channels in taste receptor cells.

Involvement of spinal calcitonin gene-related peptide in the development of acute visceral hyperalgesia in the rat

This study aimed to characterize the role of the neuropeptide calcitonin gene-related peptide (CGRP) in the development of mechanically induced visceral hyperalgesia. Tonic colorectal distension (CRD) was performed in fasted, conscious male Sprague-Dawley rats. The visceromotor reflex associated with noxious CRD was determined as the number of contractions during each of two consecutive tonic distensions (10 min at 60 mmHg), which were separated by a series of phasic distensions (repeated 15-s distensions to 80 mmHg at 30-s intervals). The effect of the CGRP receptor antagonist h-CGRP8-37 given intrathecally (i.t.) (0.03-3 nmol rat-1) or intravenously (i.v.) (20 microg kg-1 bodyweight [bw]) on the visceromotor response was evaluated. The dose for i.v. administration was chosen based on previous results from similar studies. In addition, the effect of a CGRP monoclonal antibody (6 mg kg-1 bw) given intravenously was evaluated. Compared to the baseline response, a significant increase in the number of abdominal contractions was observed during the second tonic distension. The i.t. application of h-CGRP8-37 dose-dependently reduced the numbers of abdominal contractions both during the first and the second tonic distension period, with a maximum effect observed at a peptide concentration of 3 nmol. Intravenous administration of h-CGRP8-37 or of the CGRP antiserum produced a small reduction of the visceromotor response induced by the second tonic distension and had no effect on colonic compliance. The development of mechanically induced colorectal hyperalgesia by repeated tonic distension involves the spinal release of CGRP, while peripheral release of CGRP plays only a minor role.

Inflammation-induced changes in primary afferent-evoked release of substance P within trigeminal ganglia in vivo

Substance P (SP) is synthesized in a subset of nociceptive sensory neurons and is released from their peripheral and central terminals. Here we demonstrate with the use of in vivo microdialysis and radioimmunoassay techniques that SP is also released within trigeminal ganglia following intraganglionic application of KCl, veratridine or capsaicin, and after electrical stimulation of peripheral afferent fibers. Both the basal and KCl-evoked release of SP are shown to be dependent on extracellular calcium. Using the turpentine-induced model of unilateral orofacial inflammation we also show that both the basal and KCl-evoked release of SP within trigeminal ganglia are greatly increased on the inflamed side 48 h after induction of inflammation. Coupled with previous demonstrations of excitatory effects of SP on sensory neurons, these results suggest that SP fulfils the role of a non-synaptically released diffusible chemical messenger that may modulate the somatic excitability of neurons within sensory ganglia in inflammatory pain states.

Bradykinin-evoked sensitization of neuropeptide release from afferent neurons in the guinea-pig lung

1. It has been shown that bradykinin (BK) causes sensitization of airway sensory neurons and an enhancement of the cough reflex in guinea-pigs. In the present study, the guinea-pig isolated perfused lung was used to investigate the possible enhancement by BK of histamine-evoked neuropeptide release from peripheral terminals of primary afferent neurons, and to determine the contribution of cyclooxygenase products of arachidonate metabolism to this effect. 2. The lung was perfused with oxygenated physiological salt solution containing peptidase inhibitors (thiorphan, bestatin and captopril, 1 microM each). BK and histamine were added to the perfusate for 10 and 5 min, respectively. 3. BK alone (0.1 microM) evoked the release of 10.35+/-2.4 fmol immunoreactive calcitonin gene-related peptide (CGRP), histamine alone (100 microM) evoked the release of 12.7+/-1.6 fmol CGRP. Stimulation with 100 microM histamine in the presence of 0.1 microM BK (added 5 min before histamine and present during histamine) evoked the release of 67.1+/-5.3 fmol CGRP. 4. Prostaglandin (PG) release was stimulated by BK (418+/-71 pmol 15-keto-13,14-dihydro-PGF2alpha and 345+/-59 pmol 6-keto-PGF1alpha), and, to a lesser extent, by histamine (36.1+/-7.4 pmol 15-keto-13,14-dihydro-PGF2alpha, and 24.6+/-3.9 pmol 6-keto-PGF1alpha). Prostaglandin release induced by histamine in the presence of BK was not significantly higher than with BK alone. 5. Indomethacin (5 microM) as well as the bradykinin B2 receptor antagonist HOE140 (icatibant, 1 microM) inhibited prostaglandin release following stimulation with histamine in combination with BK. CGRP release evoked by histamine in combination with BK was attenuated by indomethacin and HOE140 to 22.1+/-7.8 fmol and 16.4+/-3.8 fmol, respectively, significantly less than the value obtained in control experiments (67.1+/-5.3 fmol). 6. The results suggest that BK-induced stimulation of prostaglandin synthesis results in facilitation of histamine-evoked release of pro-inflammatory neuropeptides from afferent neurons, a mechanism that probably becomes relevant during inflammation, and that can be blocked by a bradykinin B2 receptor antagonist.

Mechanism of prostaglandin E2-induced substance P release from cultured sensory neurons

Hyperalgesia (tenderness) is a prominent feature of the inflammatory response. It is thought to be mediated, in part, by humoral factors such as prostaglandin E2, which act directly to sensitize primary afferent nociceptors. Prostaglandin E2 also interacts with nociceptors to induce a release of substance P, which can feed back to enhance the inflammatory response and also induce a long-lasting hyperalgesia. This study examined the mechanism of prostaglandin E2-induced substance P release from cultured adult rat dorsal root ganglion cells. Release studies were performed by bathing cultures with Tyrode solution +/- test agents and substance P was measured by radioimmunoassay. Substance P release induced by 100 nM prostaglandin E2 was inhibited by the prostaglandin antagonist, SC19220, and modulated by the guanine nucleotide analogs, guanosine-5'-[gamma-thio]triphosphate and guanosine-5'-[beta-thio]diphosphate, which stimulate and inhibit, respectively, stimulatory G-proteins. Substance P release was found to be Ca(2+)-dependent, requiring an influx of Ca2+ via N-type voltage-sensitive Ca2+ channels, since it was blocked by omega-conotoxin, but not nifedipine. The results suggest that prostaglandin E2 acts via a G-protein-coupled binding site on dissociated dorsal root ganglion cells to induce a Ca(2+)-dependent release of substance P, and provide further insight into the possible mechanisms underlying hyperalgesia associated with inflammation.

Acute joint inflammation--mechanisms and mediators

1. This review discusses factors contributing to acute joint inflammation, particularly sensory neuropeptides. 2. Mediators known to contribute importantly to the inflammatory process include cytokines, eicosanoids, complement and the kinin systems, histamine and 5-hydroxytryptamine and sensory neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP). 3. The pro-inflammatory neurokinins, SP and CGRP, are present in nerves innervating joints and could significantly contribute to the increased vascular permeability and hyperaemia occurring in acute arthritis. 4. Although perhaps contributing to the pathogenesis of chronic inflammatory joint disease, there is little evidence for involvement of the sympathetic nervous system in acute models of inflammation.

Profile of capsaicin-induced mouse ear oedema as neurogenic inflammatory model: comparison with arachidonic acid-induced ear oedema

1. We have investigated the mechanism of capsaicin-induced mouse ear oedema compared with that of arachidonic acid (AA)-induced ear oedema, and evaluated the possible involvement of neuropeptides in the development of capsaicin-induced oedema. 2. Topical application of capsaicin (0.1-1.0 mg per ear) to the ear of mice produced immediate vasodilatation and erythema followed by the development of oedema which was maximal at 30 min after the treatment. This oedema was of shorter duration with less swelling than AA-induced oedema (2.0 mg per ear). 3. Capsaicin-induced ear oedema was unaffected when inhibitors of arachidonate metabolites including platelet activating factor (PAF) were administered before capsaicin (250 micrograms per ear) application, while these agents significantly prevented AA-induced oedema. Dexamethasone, histamine H1 and/or 5-hydroxytryptamine (5-HT) antagonists, and substance P (SP) antagonists were effective in inhibiting both models. Furthermore, a Ca(2+)-channel blocker and the capsaicin inhibitor, ruthenium red, were effective inhibitors of capsaicin oedema but had no effect on AA-induced oedema. 4. Phosphoramidon (50 micrograms kg-1, i.v.), an endopeptidase inhibitor, markedly (P < 0.001) enhanced only capsaicin-induced ear oedema, but bestatin (0.5 mg kg-1, i.v.), an aminopeptidase, failed to enhance oedema formation. 5. Neuropeptides (1-100 pmol per site) such as rat calcitonin gene-related peptide (CGRP), SP, neurokinin A (NKA), and vasoactive intestinal peptide (VIP), which are released from capsaicin-sensitive neurones, caused ear oedema by intradermal injection. Furthermore, a synergistic effect of CGRP (10 fmol per site) and SP (10 pmol per site) on oedema formation was observed. 6. The oedema induced by neuropeptides was significantly (P<0.05 or P<0.001) inhibited when cyproheptadine (20 mg kg-1, p.o.), a histamine H, and 5-HT antagonist, was administered before injection. In contrast, nifedipine (50 mg kg-1, p.o.), a Ca2+-channel blocker, and indomethacin(10 mg kg-1, p.o., except for NKA), a cyclo-oxygenase inhibitor, had little effect on neuropeptide induced oedema.7. These results suggest that the mechanism of capsaicin-induced ear oedema is different from that of AA-induced oedema and suggest that the development of capsaicin-induced ear oedema is primarily mediated by neuropeptides. The neuropeptides released after activation of sensory nerves cause an increase of vascular permeability by interactions with endothelial cells and by histamine (and 5-HT)release from mast cells.

Effects of ruthenium red and capsazepine on C-fibres in the rabbit iris

1. We have investigated the effects of ruthenium red and capsazepine on a C-fibre-smooth muscle preparation (the rabbit isolated iris sphincter muscle). 2. Like capsaicin, ruthenium red and capsazepine were found to produce contractions in a concentration-dependent manner. C-fibre activation was held to be responsible since the contractions could be inhibited by tachykinin receptor blockade. 3. Both ruthenium red and capsazepine inhibited capsaicin-induced contractions concentration-dependently; the pIC50 values were 5.1 and 4.9, respectively. The contractions induced by bradykinin, which, like capsaicin, acts by releasing tachykinins from C-fibres, were also inhibited by ruthenium red and capsazepine in a concentration-dependent manner; the pIC50 values were 4.1 and 4.6, respectively. 4. Electrically evoked, tachykinin-mediated contractions were inhibited by ruthenium red and capsazepine in a concentration-dependent manner; the pIC50 values were 4.3 and 4.5, respectively. 5. The contractile response to neurokinin A (NKA) was inhibited by capsazepine (and by capsaicin), but not by ruthenium red, in a concentration-dependent manner; the pIC50 value was 4.3. 6. The results suggest that, besides their ability to antagonize capsaicin, ruthenium red and capsazepine possess a weak capsaicin-like effect. Conceivably, capsazepine interacts with binding sites for capsaicin, acting as a partial agonist/antagonist, while ruthenium red interacts with capsaicin-operated cation channels. The inhibition of electrically evoked- or bradykinin-induced responses by capsazepine and ruthenium red suggests that capsaicin/capsazepine binding sites and capsaicin-operated cation channels play a role in the process of transmitter release in response not only to capsaicin but also to other C-fibre stimuli. In addition, capsazepine (and capsaicin) may affect smooth muscle non-specifically since the response to NKA was also inhibited by this drug. The fact that ruthenium red did not affect the response to NKA provides further evidence that ruthenium red acts in a mode different from that ofcapsazepine.

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.

Involvement of calcium channels in depolarization-evoked release of adenosine from spinal cord synaptosomes

The potential involvement of L- and N-type voltage-sensitive calcium (Ca2+) channels and a voltage-independent receptor-operated Ca2+ channel in the release of adenosine from dorsal spinal cord synaptosomes induced by depolarization with K+ and capsaicin was examined. Bay K 8644 (10 nM) augmented release of adenosine in the presence of a partial depolarization with K+ (addition of 6 mM) but not capsaicin (1 and 10 microM). This augmentation was dose dependent from 1 to 10 nM and was followed by inhibition of release from 30 to 100 nM. Nifedipine and nitrendipine inhibited the augmenting effect of Bay K 8644 in a dose-dependent manner, but neither antagonist had any effect on release of adenosine produced by K+ (24 mM) or capsaicin (1 and 10 microM). omega-Conotoxin inhibited K(+)-evoked release of adenosine in a dose-dependent manner but had no effect on capsaicin-evoked release. Ruthenium red blocked capsaicin-induced release of adenosine but had no effect on K(+)-evoked release. Although L-type voltage-sensitive Ca2+ channels can modulate release of adenosine when synaptosomes are partially depolarized with K+, N-type voltage-sensitive Ca2+ channels are primarily involved in K(+)-evoked release of adenosine. Capsaicin-evoked release of adenosine does not involve either L- or N-type Ca2+ channels, but is dependent on a mechanism that is sensitive to ruthenium red.

Calcitonin gene-related peptide stimulates active Na(+)-K+ transport in rat soleus muscle

Calcitonin gene-related peptide (CGRP) is found in a wide variety of tissues, including sensory and motor nerve endings in skeletal muscle. After intense electrical stimulation or K(+)-induced depolarization, CGRP can be released from nerve terminals and bound to receptors on sarcolemma. We show here that CGRP (rat and human) and salmon calcitonin stimulate 22Na extrusion and the influx of 86Rb and 42K in isolated rat soleus muscle. This leads to a pronounced (up to 56%) decrease in intracellular Na+, a minor increase in intracellular K+, and hyperpolarization. All these effects were blocked by ouabain or cooling, indicating that they reflect an acute stimulation of active electrogenic Na(+)-K+ transport. Capsaicin, which induces release of CGRP from sensory nerve endings, was found to exert similar effects on Na(+)-K+ transport. Various Na(+)-K+ pump-stimulating agents have been shown to counteract the inhibitory effect of a high extracellular concentration of K+ ([K+]o) on muscle contractility (4, 20). CGRP and capsaicin were likewise found to improve contractile performance of muscles inhibited by high [K+]o, and these effects were blocked by ouabain. CGRP might play a role in the maintenance of Na(+)-K+ gradients and excitability during intensive muscle work, known to be associated with an acute rise in the interstitial K+ concentration.

Capsaicin-desensitization to the human nasal mucosa selectively reduces pain evoked by citric acid

1. Kallidin (5-500 nmol), hypertonic saline (0.9-20% NaCl) or low pH medium (citric acid: pH 2.5-1) applied (50 microliters) to the human nasal mucosa produced a pain response (evaluated by a visual analogue scale) that was related to the concentration of the peptide, NaCl or hydrogen ions, respectively. 2. Application (50 microliters) of capsaicin (50 nmol) to the human nasal mucosa produced overt pain. After repeated administrations (once a day for 5-7 days) to one nostril this effect underwent almost complete desensitization, while in the contralateral nostril, treated with the vehicle, the response to capsaicin was unaffected. 3. The pain response produced in the human nasal mucosa by topical application (50 microliters) or kallidin (50-500 nmol), NaCl (10-20%) or citric acid (pH 1.5-1) solutions was then studied before and after local capsaicin desensitization. 4. The pain response to pH 1.5 or 1 citric acid was markedly reduced (by 60% and 75%, respectively) in the capsaicin-treated nostril. However, the pain response to 10% or 20% NaCl or the mild pain response to 50 or 500 nmol kallidin were unaffected by capsaicin pre-treatment. 5. The present results suggest that prolonged topical capsaicin treatment to the human nasal mucosa may lead to selective desensitization to certain algesic stimuli such as capsaicin itself and hydrogen ions.

Effect of bradykinin and prostaglandins on the release of calcitonin gene-related peptide-like immunoreactivity from the rat spinal cord in vitro

1. The release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) from the dorsal horn of the rat spinal cord in vitro in response to dorsal root stimulation was measured by radioimmunoassay. 2. Stimulation of the dorsal roots (3 or 4 roots on each side) at 10 Hz for 5 min evoked a mean release (R1) of 134.3 +/- 17.5 (n = 10) fmol CGRP-LI; the release (R2) evoked by a second stimulation period 30 min later under control conditions was 77 +/- 10% (n = 10) of R1. Test compounds were applied to the preparation following release R1, and their effect calculated from the value of R2/R1. 3. Bradykinin (0.01-10 microM) had no significant effect on the basal release of CGRP-LI, but at 0.1-10 microM it increased 2-3 fold the release evoked by dorsal root stimulation. 4. This effect of bradykinin was prevented by indomethacin (10 microM), or by the B2-receptor antagonist, Hoe140 (1-10 microM). In the presence of Hoe140, bradykinin significantly reduced R2/R1; the explanation for this is not clear. 5. The B1-receptor agonist, Des-Arg9-bradykinin (10 microM), did not affect CGRP-LI release nor was the effect of bradykinin blocked by the B1-receptor antagonist, Des-Arg9-Leu8-bradykinin (10 microM). 6. Various prostaglandins were found to mimic the effect of bradykinin on CGRP-LI release. Their approximate order of potency was prostaglandin D2 (PGD2) = PGE1 > PGF2 alpha = PGE2; PGI2 was ineffective at 10 microM.7. Forskolin (30 muM) and 3-isobutyl l-methylxanthine (IBMX; 10 fM) also increased the evoked release of CGRP-LI.8. It is concluded that bradykinin acts on B2-receptors in the spinal cord, causing the formation ofprostanoids, which in turn cause an enhancement of neuropeptide release from primary afferent nerve terminals in the dorsal horn. This effect may be secondary to activation of adenylate cyclase. Because B2-receptors are mainly associated with primary afferent nerve terminals, it is likely that prostanoid production is also a function of these structures. Whether this action of bradykinin has any physiological function in nociceptive transmission remains unclear..

Release of calcitonin gene-related peptide from sensory neurons

CGRP is released from capsaicin-sensitive sensory neurons in a Ca(2+)-dependent manner in a variety of peripheral organs as well as from central terminals. The mechanisms for CGRP release by low concentrations of capsaicin, electrical antidromic nerve stimulation, and bradykinin have several similar characteristics regarding sensitivity to TTX, CTX, and alpha 2-adrenoceptor activation. High capsaicin concentration and nicotine evoke CGRP release via other mechanisms. The effects of capsaicin, resiniferatoxin, and SO2 are blocked by RR, which probably inhibits ion fluxes associated with capsaicin receptor activation. CGRP released upon irritation of peripheral branches of primary afferents may evoke a variety of cardiovascular actions and influence motility in the gastrointestinal and urogenital tracts.

Time-course of capsaicin-evoked release of calcitonin gene-related peptide from rat spinal cord in vitro. Effect of concentration and modulation by Ruthenium Red

The capsaicin-evoked release of calcitonin gene-related peptide (CGRP) from rat superfused dorsal spinal cord slices was investigated during sustained capsaicin exposure thought to represent equilibrium conditions. The dose-effect relationship for total peptide release evoked by single capsaicin doses (26 min exposure) was very steep with a threshold at 0.06 microM and a maximum at 0.3 microM capsaicin. With concentrations of capsaicin within this range the slow decline of the peptide release in the presence of capsaicin was not a consequence of exhaustion of an available peptide pool nor of neuronal impairment because potassium depolarization was still able to release CGRP. In contrast, with concentrations of capsaicin at 1.5 microM and above, there was a much faster decay of the release after the peak, most probably due to a loss of the secretion capacity caused by neuronal inactivation. When cumulative dose regimens for capsaicin were employed, release of CGRP could be stimulated only up to a dose of 1-1.5 microM capsaicin; further increase in capsaicin concentration was ineffective. This was also most probably due to a loss of the secretion capacity caused by neuronal inactivation and not caused by depletion of a releaseable peptide pool. Release of CGRP evoked by capsaicin concentrations in the range of 0.1-0.3 microM in either dosage protocol was reduced in the presence of Ruthenium Red (RR, 2.5 microM). RR did not reduce neuropeptide release evoked by capsaicin concentrations at or above 1-1.5 microM, nor did it affect the inactivation of the release process at such high capsaicin concentrations. The results demonstrate that, upon sustained exposure to capsaicin, different ranges of concentration can be established at which either only stimulatory or a mixture of stimulatory and inhibitory effects determine the amount of neuropeptides released.

Variable α2-adrenoceptor-mediated inhibition of bronchoconstriction and peptide release upon activation of pulmonary afferents

In the present study, we evaluated the possible regulation by alpha 2-receptor agonists (SKF 35886 and UK 14304) of peptide release and functional responses upon sensory nerve activation in the guinea-pig lung. The peptide release and bronchoconstriction caused by antidromic vagal nerve stimulation (low frequency, 1 Hz), and a low concentration of capsaicin (10(-8) M) and resiniferatoxin (3 x 10(-10) M) were attenuated by alpha 2-adrenoceptor agonists. The effects of capsaicin and nicotine in high concentrations and high frequency nerve stimulation (10 Hz) were influenced to a much smaller extent by alpha 2-adrenoceptor stimulation. The calcitonin gene-related peptide release evoked by bradykinin but not the functional effects was inhibited by alpha 2-adrenoceptor activation. It is concluded that alpha 2-adrenoceptor stimulation mainly inhibits the release of mediator and/or the bronchoconstriction caused by moderate activation of sensory nerves. It is necessary to measure mediator release directly to reveal prejunctional effects and not to rely only on indirect functional evidence.

Ruthenium red and capsaicin induce a neurogenic inflammatory response in the rabbit eye: effects of omega-conotoxin GVIA and tetrodotoxin

The effects of ruthenium red, an inorganic dye with known capsaicin antagonist properties, was investigated in the rabbit eye. At a dose of 0.24 nmol ruthenium red inhibited the inflammatory effects of capsaicin (1 or 8 nmol). Unexpectedly, when the dye was injected in doses ranging from 0.24 to 7.4 nmol, it caused an inflammatory response with constriction of the pupil (miosis) and a breakdown of the blood-aqueous barrier, leading to a rise intraocular pressure. Tetrodotoxin (30 nmol) inhibited the ruthenium red-induced rise in intraocular pressure but had less effect on the miotic response. The tachykinin antagonist spantide inhibited the miosis but had no effect on the rise in intraocular pressure. Ruthenium red induced an increase in substance P-like immunoreactivity and calcitonin gene-related peptide-like immunoreactivity in the aqueous humor. These levels were positively correlated with the rise in aqueous humor protein concentration. The ruthenium red-induced miosis and, to a less extent, the rise in intraocular pressure were inhibited by the Ca2+ channel-blocking agent omega-conotoxin GVIA (CTX), indicating a partial dependence on an influx of extracellular Ca2+. CTX also attenuated the miotic effect of capsaicin but had no effect on the capsaicin-induced rise in intraocular pressure. It is concluded that, in the rabbit eye, ruthenium red induces a neurogenic inflammatory response besides its capsaicin antagonist effects.

Different pathways by which extracellular Ca2+ promotes calcitonin gene-related peptide release from central terminals of capsaicin-sensitive afferents of guinea pigs: effect of capsaicin, high K+ and low pH media

Different modes by which Ca2+, entering the nerve terminal, promotes transmitter secretion as well as the ability of protons to release neuropeptides, have been shown in peripheral endings of capsaicin-sensitive afferents. We have studied these two aspects in the central endings of these neurons by measuring the release of calcitonin-gene related peptide-like immunoreactivity (CGRP-LI) from slices of the dorsal half of the guinea pig spinal cord. Although capsaicin (1 microM) released both CGRP-LI and substance P-like immunoreactivity (SP-LI), CGRP-LI was chosen as the sole suitable marker of peptides released from central terminals of capsaicin-sensitive afferents, since after in vitro desensitization to capsaicin (1 microM capsaicin for 20 min), high K+ (80 mM) failed to evoke CGRP-LI release, whereas SP-LI release was still observed. The capsaicin (1 microM)-evoked CGRP-LI release was entirely dependent on extracellular Ca2+. It was unaffected by 0.3 microM tetrodotoxin (TTX), slightly reduced by 0.1 microM omega-conotoxin (CTX) and blocked by 10 microM Ruthenium red (RR). The Ca(2+)-dependent K+ (80 mM)-evoked CGRP-LI release was unaffected by TTX, markedly reduced by CTX and only moderately inhibited by RR. Low pH (pH 5) produced a remarkable increase in CGRP-LI outflow that was abolished after exposure to capsaicin, reduced by about 50% in Ca(2+)-free medium and unaffected by TTX (0.3 microM). The Ca(2+)-dependent component of the proton-evoked CGRP-LI release was abolished in the presence of RR (10 microM) and slightly inhibited by CTX (0.1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Ruthenium red antagonism of the effects of capsaicin mediated by extrinsic sensory nerves on myenteric plexus neurons of the isolated guinea-pig ileum

The effects of Ruthenium red and its antagonism of capsaicin-induced action on the electrophysiological behavior of myenteric neurons were investigated with intracellular recording techniques in the isolated guinea-pig ileum. Ruthenium red antagonized dose-dependently (1-10 microM) a capsaicin-induced marked long-lasting slow depolarizing action associated with increased input resistance, during which the cells spiked repeatedly or displayed anodal break excitation. This action of capsaicin has been found to be mediated via a release of substance P from sensory nerve endings. The slow depolarizing response to exogenous substance P applied by pressure microejection, which mimicked the capsaicin-induced action, was not affected by Ruthenium red. Therefore, present results indicate that Ruthenium red antagonizes the specific effect of capsaicin on myenteric neurons by acting on the presynaptically located peripheral nerve terminals of sensory neurons and inhibiting the release of substance P. Electron-microscopic examination showed that the neurotoxic action of capsaicin towards extrinsic sensory nerve fibers was also dose-dependently (1-10 microM) protected by pretreatment of ruthenium red. Present results suggest that Ruthenium red inhibits a capsaicin-induced activation of cation channels at the cell membrane of sensory nerves.

Selectivity of ruthenium red in inhibiting bronchoconstriction and CGRP release induced by afferent C-fibre activation in the guinea-pig lung

In the present study we evaluated the effects of ruthenium red, a blocker of transmembrane Ca2+ fluxes, on bronchoconstriction and the release of calcitonin gene-related peptide-like immunoreactivity induced by different stimuli in the isolated perfused guinea-pig lung. Vagal stimulation (1 Hz, 1 min), capsaicin (10(-8) M, 10(-6) M), resiniferatoxin (3 x 10(-10) M), nicotine (10(-4) M), bradykinin (5 x 10(-6) M) and histamine (10(-5) M) evoked bronchoconstriction and calcitonin gene-related peptide-like immunoreactivity overflow. Ruthenium red (5 x 10(-6) M) almost completely inhibited the bronchoconstriction and calcitonin gene-related peptide-like immunoreactivity overflow induced by capsaicin and resiniferatoxin but did not influence the effects induced by vagal nerve stimulation, nicotine, bradykinin or histamine. The 20-deacetylated derivative of resiniferatoxin (ROPA), which lacks the homovanillyl ester group, did not evoke release or bronchoconstriction. Ruthenium red (3 x 10(-4) M) aerosol attenuated the cough induced by nebulized citric acid in conscious guinea-pigs. Citric acid-induced coughing is mediated via capsaicin-sensitive neurons. However, cigarette smoke-induced coughing, which involves capsaicin-resistant mechanisms, was not affected by ruthenium red. In conclusion, ruthenium red selectively inhibits the capsaicin, resiniferatoxin and citric acid-induced excitation of the sensory nerves as revealed by calcitonin gene-related peptide-like immunoreactivity release, bronchoconstriction and coughing, suggesting that these agents share a common mechanism of action.

Ruthenium red as a capsaicin antagonist

Definition of the physiological and pharmacological properties of primary afferent neurons by the use of capsaicin and its analogues (e.g. resiniferatoxin) has represented one of the most active areas of research of the last decade (1-4 for reviews). In the past 3 years many important advancements have been made in this field, dealing with: a) discovery of the capsaicin (or 'vanilloid' receptor (5); b) discovery of capsazepine as a competitive receptor antagonist at the vanilloid receptor (6); c) definition of the cation channel coupled with the vanilloid receptor and the ionic basis for excitation and "desensitization" of primary afferents by capsaicin and related substances (7,8) and d) discovery of ruthenium red as a functional capsaicin antagonist. The aim of the present article is to briefly review the pharmacology of ruthenium red as a capsaicin antagonist and attempting to define the usefulness and the limits of this substance as a tool in sensory neuron research.

Low pH-induced release of calcitonin gene-related peptide from capsaicin-sensitive sensory nerves: mechanism of action and biological response

Protons can release in a Ca(2+)-dependent manner, calcitonin gene-related peptide (CGRP)-like immunoreactivity from peripheral endings of capsaicin-sensitive afferents. Here we have studied the mechanism by which proton promotes CGRP-like immunoreactivity release and whether the neuropeptide released might exert a biological action. In muscle slices of guinea-pig urinary bladder high pH (pH 8 or 9) media neither enhanced CGRP-like immunoreactivity outflow nor affected the capsaicin-evoked CGRP-like immunoreactivity release. The CGRP-like immunoreactivity release evoked by superfusion with pH 5 medium was not affected by tetrodotoxin (0.3 microM) indomethacin (10 microM) or the protein kinase C inhibitor H-7 (30 microM). However, it was reduced by 35% in the presence of the voltage-sensitive Ca(2+)-channel antagonists nifedipine (1 microM) and omega-conotoxin (0.1 microM) and by 80% in presence of the capsaicin "antagonist" Ruthenium Red (10 microM). The CGRP-like immunoreactivity release by capsaicin (10 microM) was reduced by 80% in the presence of Ruthenium Red, and not affected by voltage-sensitive Ca(2+)-channel blockers, while that evoked by 80 mM K+ was decreased by 82% in the presence of nifedipine and omega-conotoxin. The Ca(2+)-channel agonist Bay K 8644 enhanced the high K(+)-evoked CGRP-like immunoreactivity release but not that induced by capsaicin or pH 5 medium. Exposure to pH 6 solution of one half of the neck of guinea-pig urinary bladder induced a slowly developing inhibition of electrically evoked contractions, that was absent in the half pre-exposed in vitro to a desensitizing dose of capsaicin.(ABSTRACT TRUNCATED AT 250 WORDS)

Capsaicin desensitization in vivo is inhibited by ruthenium red

The effect of systemic administration of Ruthenium Red on the excitatory and desensitizing effect of capsaicin was investigated in rats. Ruthenium Red was injected s.c. 30 min before capsaicin was administered. The excitatory effect of capsaicin on corneal, perivascular and visceral afferents was not influenced by treatment with Ruthenium Red. However, determination of the neuropeptide content and evoked neuropeptide release in peripheral organs and dorsal spinal cord 48 h after treatment showed that Ruthenium Red attenuated the 'desensitizing' effect of capsaicin at peripheral, but not at central, endings of primary afferents. On the other hand, a capsaicin-elicited autonomic reflex mediated by visceral afferents was still obtained in 9 of 14 rats that had received Ruthenium Red and capsaicin. The results indicate that a single dose of Ruthenium Red, which does not reduce the acute excitatory effect of capsaicin, reduces the desensitizing effect of capsaicin on peripheral endings of primary afferents in vivo. This long-lasting protective effect of Ruthenium Red suggests that it is possible to pharmacologically differentiate between the acute and chronic effects of capsaicin.

Pharmacological modulation of the contractile response to toluene diisocyanate in the rat isolated urinary bladder

1. Toluene diisocyanate produced concentration-dependent contractions of the rat isolated urinary bladder. 2. The contractions were tetrodotoxin-resistant and were abolished by previous exposure of the strips to capsaicin. 3. Indomethacin (5 microM) and ruthenium red (30 microM) inhibited toluene diisocyanate-induced contractions. Responses expressed as a percentage of the response obtained with substance P, 30 nM, were respectively 141.6 +/- 24.8% and 20.1 +/- 5.1% in control and indomethacin-treated strips (P less than 0.005); 123.0 +/- 30.2% and 14.0 +/- 6.5% in control and ruthenium red-treated strips (0.01 less than P less than 0.05). 4. These results suggest that toluene diisocyanate-induced contractions of the rat isolated bladder are the result of the release of cyclo-oxygenase products which may act by activating the capsaicin receptor.

Which mechanisms account for the sensory neuron blocking action of capsaicin on primary afferents in the rat urinary bladder?

In the rat isolated bladder, capsaicin produced a concentration-dependent contraction, shown previously to depend upon transmitter release from peripheral endings of primary afferents. When using low concentrations (30-300 nM) of capsaicin, exposure to a second and third dose of capsaicin produced smaller responses than the first application, although a subsequent challenge with 10 microM capsaicin still elicited a contraction which was not reduced as compared to the response produced by the first exposure to a low dose of capsaicin. Capsaicin also evoked a prompt outflow of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI), taken as a marker for sensory nerve activation. A second or third application of a submaximal concentration of the drug was ineffective, although a subsequent challenge with 1 microM capsaicin was effective. These findings indicate that neuropeptide depletion does not necessarily account for the early stage of capsaicin 'desensitization' of primary afferents.

Ruthenium red blocks the capsaicin-induced increase in intracellular calcium and activation of membrane currents in sensory neurones as well as the activation of peripheral nociceptors in vitro

In a number of sensory neuron preparations, Ruthenium red (RR) selectively attenuated the response to capsaicin. First, RR (100 nM) reversibly abolished capsaicin but not bradykinin induced increases in [Ca2+]i measured in single DRG neurons from neonatal rats, using the calcium sensitive dye Fura-2. Second, RR completely but reversibly abolished capsaicin-activated single ion channel currents measured in membrane patches from rat DRG neurons. This effect of RR differed from that produced by lanthanum. Finally, in a neonatal rat spinal cord-tail preparation maintained in vitro, RR selectively attenuated the activation of peripheral nociceptors produced by capsaicin but not by bradykinin or noxious heat. These data indicate that RR inhibits capsaicin mediated effects on sensory neurons by an action on the plasma membrane to prevent opening of capsaicin activated ion channels.

Similarities and differences in the action of resiniferatoxin and capsaicin on central and peripheral endings of primary sensory neurons

We have compared the ability of capsaicin and resiniferatoxin, a natural diterpene present in the latex of plants of the Euphorbia family to excite and desensitize capsaicin-sensitive primary afferents in a variety of models. Both capsaicin and resiniferatoxin inhibited the twitch contractions of the rat isolated vas deferens and prevented, in a concentration-related manner, the effect of a subsequent challenge with 1 microM capsaicin (desensitization). Resiniferatoxin was 1000-10,000 times more potent than capsaicin in both cases. The time course of action of resiniferatoxin was much slower than that of capsaicin, suggesting a slower penetration rate in the tissue. The action of resiniferatoxin was blocked by Ruthenium Red, a proposed antagonist at the cation channel coupled to the capsaicin receptor. Both capsaicin and resiniferatoxin produced a contraction of the rat isolated urinary bladder. Resiniferatoxin was about as potent as capsaicin in this assay although it was 500-1000 times more potent than capsaicin in desensitizing the primary afferents to a subsequent challenge with capsaicin itself. Resiniferatoxin did not affect motility in the isolated vasa deferentia or urinary bladder from capsaicin-pretreated rats. After topical application onto the rat urinary bladder both resiniferatoxin (10 nM) and capsaicin (10 microM) increased bladder capacity as assessed in a volume-evoked micturition reflex model in rats without affecting micturition contraction. Intrarterial injection of resiniferatoxin or capsaicin in the ear of anesthetized rabbits evoked a systemic depressor reflex due to activation of paravascular nociceptors, resiniferatoxin being about three times more potent than capsaicin.(ABSTRACT TRUNCATED AT 250 WORDS)

Bradykinin-induced release of calcitonin gene-related peptide from capsaicin-sensitive nerves in guinea-pig atria: mechanism of action and calcium requirements

The mechanism of neuropeptide secretion induced by bradykinin from capsaicin-sensitive afferents was studied in guinea-pig atria. Both the inotropic response induced by bradykinin (0.1 microM) in the electrically driven isolated guinea-pig left atria and the bradykinin (10 microM)-induced release of calcitonin gene-related peptide calcitonin gene-related peptide-like immunoreactivity from slices of guinea-pig atria were abolished in vitro by capsaicin pretreatment or in the presence of indomethacin. Bradykinin-induced calcitonin gene-related peptide-like immunoreactive release was unaffected by tetrodotoxin (0.3 microM), the protein kinase C inhibitor, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (30 microM), nefedipine (1 microM) or Ruthenium Red (10 microM). It was significantly reduced by 79% in a Ca2(+)-free medium and by 52% in the presence of 0.1 microM omega-conotoxin (fraction GVIA). It is proposed that bradykinin releases calcitonin gene-related peptide from capsaicin-sensitive afferents in guinea-pig atria, via prostanoid generation. This mode of activation of the "efferent" function of capsaicin-sensitive nerves appears to be distinct from those produced by capsaicin or electrical field stimulation as they have been characterized in previous works. In fact, the bradykinin activation of capsaicin-sensitive afferents is not affected by tetrodotoxin and Ruthenium Red, but is partially sensitive to the selective blocker of N-type Ca2(+)-channels, omega-conotoxin.

Capsaicin-induced stimulation of polymodal nociceptors is antagonized by ruthenium red independently of extracellular calcium

The dual effect of capsaicin on primary afferent neurons, excitation and stimulation of transmitter release, its dependence on extracellular calcium and its modulation by Ruthenium Red have been investigated in the rabbit ear. Injection of capsaicin into the central artery of the isolated perfused ear with intact neuronal connection induced a reflex fall in systemic arterial blood pressure of the anaesthetized rabbit. Addition of Ruthenium Red (0.6-20 microM) to the perfusate of the ear reversibly attenuated this response in a dose-dependent manner. Perfusion of the ear with a Ca2+-free, 3 mM EGTA-containing physiological salt solution enhanced the capsaicin-evoked depressor reflex but did not prevent the inhibitory action of Ruthenium Red. Perfusion of the isolated rabbit ear with capsaicin (10 microM)-containing physiological salt solution induced the release of substance P-like immunoreactivity which was inhibited by Ruthenium Red (0.6-20 microM) and by omission of extracellular Ca2+. The results demonstrate that capsaicin-evoked transmitter release is dependent on extracellular calcium while capsaicin-evoked excitation is not reduced in a Ca2+-free perfusate. Both effects of capsaicin are potently inhibited by Ruthenium Red. The fact that capsaicin-induced excitation of primary afferents is antagonized by Ruthenium Red also in the absence of extracellular Ca2+ suggests this inhibitory action of Ruthenium Red is not only mediated by inhibition of transmembrane Ca2+ fluxes.