Ruthenium red antagonism of the effect of capsaicin on the motility of the isolated guinea-pig ileum

Ruthenium red (1 microM), an inorganic dye which blocks transmembrane calcium (Ca) fluxes in neural tissues, selectively reduced the capsaicin (1 microM)-induced contraction of the guinea-pig ileum and protected the sensory fibers from capsaicin-induced desensitization. The ruthenium red (0.5-1 microM) antagonism of capsaicin-induced inhibition of responses to mesenteric nerve stimulation or field stimulation in the isolated guinea-pig ileum was an example of a similar antagonism of the effect of capsaicin. In view of the known action of ruthenium red on the depolarization-coupled entry of Ca into synaptosomes and the release of transmitter, our results support the proposal that ruthenium red could antagonize the action of capsaicin on the peripheral terminals of sensory nerves by a similar mechanism, thereby suppressing transmitter release and preventing the establishment of desensitization.     

Ruthenium red selectively prevents capsaicin-induced nociceptor stimulation

The effect of Ruthenium Red on capsaicin-induced stimulation of polymodal nociceptors was investigated in the isolated perfused rabbit ear with an intact neuronal connection. Ruthenium Red, 20 microM, completely prevented the response to capsaicin but not that to acetylcholine or bradykinin. It is therefore suggested that capsaicin activates sensory nerve terminals by a mechanism which is different from that for bradykinin or acetylcholine, and which can be blocked selectively by Ruthenium Red.     

The ‘efferent’ function of capsaicin-sensitive nerves: Ruthenium Red discriminates between different mechanisms of activation

We have investigated the ability of Ruthenium Red, an inorganic dye with calcium entry blocking properties, to interfere with the ‘efferent’ function of capsaicin-sensitive sensory nerves. These nerves were activated in the guinea-pig isolated bronchus (atropine in the bath) or left atria (reserpine-pretreated animals, atropine in the bath) by electrical field stimulation or with capsaicin. Both stimuli produced a contraction of the bronchus and a positive inotropic response in the atria, responses which are mediated by endogenous neuropeptides (tachykinins in the bronchus, calcitonin gene-related peptide in the atria) released from sensory nerves. Ruthenium Red (10 μM for 20 min in the cases) selectively inhibited the responses produced by the administration of capsaicin, while leaving the responses to electrical field stimulation unaffected. Likewise, the bronchonconstrictor response to exogenous neurokinin A and the atrial positive inotropic response to calcitonin gene-related peptide were unaffected by Ruthenium Red. A prejunctional site of action Ruthenium Red was confirmed in release experiments where the dye strongly inhibited the capsaicin-evoked outflow of calcitonin gene-related peptide, which is taken as a marker of activation in sensory nerves. Together with other observations, these findings support the concept that there are two independent mechanisms for activating the ‘efferent’ function of sensory nerves, one of which is activated by capsaicin and is Ruthenium Red-sensitive but ω-conotoxin-resistant, while the other is activated by propagated action potentials (field stimulation) and is ω-conotoxin-sensitive and Ruthenium Red-resistant.     

The effect of calcium free medium and nifedipine on the release of substance P-like immunoreactivity and contractions induced by capsaicin in the isolated guinea-pig and rat bladder

1. Capsaicin produced a prompt release of substance P-like immunoreactivity (SP-LI) from superfused mucosa-free muscle strips excised from the guinea-pig urinary bladder. A second application of capsaicin had no further effect, indicating desensitization. 2. Neither tetrodotoxin (1 microM) or nifedipine (10 microM) had any inhibitory effect on SP-LI release by capsaicin nor influenced the establishment of the desensitized state. Nifedipine produced per se some SP-LI release. 3. SP-LI release by capsaicin was abolished by incubation in a Calcium(Ca)-free medium containing EDTA (1.0 mM) which also afforded a partial protection toward desensitization. A lower EDTA concentration (0.1 mM) did not suppress SP-LI release by capsaicin but still inhibited desensitization. 4. When the concentration of CaCl2 in the medium was lowered to 1/10-1/100 of that present in normal Krebs solution, capsaicin still evoked a marked SP-LI release and desensitization occurred. In a nominally Ca free medium (maximal Ca concentration due to impurities was 6.7 microM) SP-LI release was still observed and desensitization was incomplete. 5. In a nominally Ca free medium, removal of Mg ions enhanced the SP-LI release induced by capsaicin and enhanced desensitization. 6. In functional studies, nifedipine greatly reduced or abolished the capsaicin- or SP-induced contraction of the rat or guinea-pig isolated bladder but did not prevent desensitization. Likewise, SP-LI depletion in the rat bladder following systemic capsaicin desensitization was not prevented by nifedipine pretreatment. On the other hand, the protective action of Ca free media (containing EDTA) was confirmed in organ bath studies (guinea-pig bladder). 7. These findings indicate that: (a) the requirements of extracellular calcium for activation of neuropeptide release from sensory nerves by capsaicin are very low; (b) both excitation of sensory fibers (SP-LI release) and desensitization are dependent upon the presence of extracellular calcium and (c) L-type voltage-sensitive Ca channels are not likely to be involved in the actions of capsaicin on sensory nerve terminals.     

Effect of ruthenium red on responses mediated by activation of capsaicin-sensitive nerves of the rat urinary bladder

Summary (1) Topical administration of Ruthenium Red (10–100 M in saline) to the serosal surface of the urinary bladder in urethane-anesthetized rats prevented the motor response of the urinary bladder to topical administration of capsaicin and protected the sensory fibers from capsaicin desensitization, but had no effect on the volume-evoked contractions (micturition reflex). At 1 mM increased bladder capacity and decreased amplitude of micturition contraction were observed. (2) At 100 M, topical Ruthenium Red prevented the blood pressure rise produced by topical administration of capsaicin onto the bladder but did not affect the blood pressure rise produced by sudden bladder distension in spinal rats. (3) After intrathecal administration, Ruthenium Red (80–800 ng/rat) produced a long lasting inhibition of the micturition reflex in urethane-anesthetized rats, this effect being evident in both vehicleor capsaicin- (50 mg/kg s. c. 4 days before) pretreated rats. At 800 ng/rat, intrathecal Ruthenium Red did not affect the blood pressure rise produced by topical administration of capsaicin onto the rat bladder nor that produced by bladder distension. (4) These findings provide further evidence that Ruthenium Red acts quite selectively as a capsaicin antagonist preventing both reflex and efferent responses activated by peripherally administered capsaicin. By contrast, sensory impulse generation by a natural stimulus such as bladder distension is apparently unaffected by Ruthenium Red. The marked inhibition of the micturition reflex observed after intrathecal administration of Ruthenium Red does probably not involve an interaction with primary afferents in the spinal cord.     

Inhibitory effect of capsaicin on the ascending pathway of the guinea-pig ileum and antagonism of this effect by ruthenium red.

A segment of guinea-pig ileum, which was continuous with a strip of longitudinal muscle-myenteric plexus (LM-MP) at the anal end, was used to examine the effect of capsaicin on ascending excitatory pathways. Electrical field stimulation of the LM-MP caused an ascending contraction of the segment. After initially causing contraction capsaicin (3 microM) inhibited the ascending contraction. This inhibitory effect of capsaicin exhibited rapid desensitization and was abolished after extrinsic (mesenteric) denervation. Desensitization to calcitonin gene-related peptide (CGRP) prevented the capsaicin-induced inhibition without affecting the ascending contraction. Neither naloxone nor alpha- and beta-adrenoceptor antagonists affected the capsaicin-induced inhibition. CGRP (25 nM) also inhibited the ascending contraction, mimicking the inhibition induced by capsaicin. Ruthenium red (0.1-3 microM) antagonized the capsaicin-induced inhibition in a concentration-related manner, but did not affect the CGRP-induced inhibition. These findings suggest that the inhibitory effect of capsaicin on the ascending pathways might be mediated via the release of CGRP from extrinsic nerve terminals, and that the site of the antagonism of the action of capsaicin by ruthenium red is prejunctional.     

Use and limitations of three TRPV-1 receptor antagonists on smooth muscles of animals and man: a vote for BCTC

Specificity of the effect is a crucial factor in using antagonists for detecting the physiological/pathophysiological roles of receptors. Here we examined the capsaicin receptor antagonist effects of three commercially-available substances, capsazepine, iodo-resiniferatoxin (I-RTX) and BCTC, on isolated smooth muscle preparations, including the human intestine. Care was taken to observe possible non-specific effects, to find out safe and effective concentrations. Capsazepine appeared to have a low margin of safety. I-RTX (up to 1μM) specifically inhibited capsaicin-induced contractions in the guinea-pig ileum and urinary bladder. I-RTX showed agonist activity on the rat urinary bladder. BCTC (1μM) abolished the contractile effects of capsaicin (1 or 2μM) on all preparations tested (guinea-pig ileum, bladder, trachea, as well as rat and mouse bladder), and on the guinea-pig renal pelvis, where it failed to influence capsaicin-sensitive, sensory neuron-mediated positive inotropy in response to field stimulation. On human intestinal preparations BCTC prevented the relaxant effect of capsaicin. It is concluded that of the three antagonists tested BCTC seems the safest one for inhibiting TRPV-1 receptors. The effect of capsazepine may be complicated by non-specific inhibition of smooth muscle contractility and that of I-RTX by agonist activity. The "local efferent" function of capsaicin-sensitive sensory neurons is not influenced by BCTC, as shown by the results obtained in the renal pelvis. In conclusion, of the TRPV-1 receptor antagonists studied, BCTC (1μM) seems the most reliable in isolated organ experiments. This substance is also effective in the human intestine.     

Possible mechanism of ruthenium red antagonism of capsaicin-induced action in the isolated guinea pig ileum.

Ruthenium red (3-5 microM) antagonism of the inhibitory effect of capsaicin (1 microM) on the contractile response to mesenteric nerve stimulation in the presence of hexamethonium (50 microM) and guanethidine (2 microM) was reversed significantly by sialic acid (2 mM) or neuraminidase (0.1 U/ml). These results suggested that ruthenium red at low concentrations inhibits the capsaicin-induced desensitization of activated Ca2+ influx into sensory nerves at least in part by binding to sialic acid residues.     

The effects of ruthenium red on the response of guinea-pig ileum to capsaicin

Ruthenium red has recently been found to inhibit the effects of capsaicin on peripheral terminals of sensory neurones. Thus the effects of ruthenium red on the responses of the guinea-pig isolated ileum to capsaicin, acetylcholine (ACh), substance P (SP) and nicotine were investigated. Ruthenium red, 5 mumol/l, abolished responses to capsaicin 1.5 mumol/l and nicotine 2 mumol/l, and shifted the concentration-response lines to ACh and SP to the right. Pretreatment of ileum preparations with ruthenium red, 12.5 mumol/l for 2 min, prevented desensitization of ileum responses to capsaicin tested 30 min later. Tetrodotoxin, 1 mumol/l, abolished the response to capsaicin on control preparations and those pretreated with atropine, 5 mumol/l, ruthenium red, 12.5 mumol/l or spantide, 10 mumol/l. It is proposed that capsaicin acts via a specific receptor coupled to a receptor-operated membrane calcium channel, and that ruthenium red binds irreversibly to the calcium channel part of the complex but reversibly to some other site which prevents the action or binding of capsaicin at its specific receptor.     

Prejunctional modulatory action of neuropeptide Y on responses due to antidromic activation of peripheral terminals of capsaicin-sensitive sensory nerves in the isolated guinea-pig ileum.

1. The effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated ileum was determined by use of capsaicin itself and electrical mesenteric nerve stimulation as stimuli. 2. NPY inhibited or suppressed the cholinergic contractile response produced by electrical mesenteric nerve stimulation while leaving the contractile response to a threshold concentration of capsaicin. 3. NPY had no effect on motor responses produced by a submaximal concentration of substance P, the putative endogenous mediator of the 'efferent' function of sensory fibres in this preparation. 4. It is concluded that NPY exerted a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves at interneuronal synapses.     

Nociceptin/orphanin FQ inhibits capsaicin-induced guinea-pig airway contraction through an inward-rectifier potassium channel

Nociceptin/orphanin FQ (N/OFQ), an endogenous opioid-like orphan receptor (NOP receptor, previously termed ORL1 receptor) agonist, has been found to inhibit capsaicin-induced bronchoconstriction in isolated guinea-pig lungs and in vivo. The underlying mechanisms are not clear. In the present studies, we tested the effect of N/OFQ on VR1 channel function in isolated guinea-pig nodose ganglia cells. Capsaicin increased intracellular Ca(2+) concentration in these cells through activation of vanilloid receptors. Capsaicin-induced Ca(2+) responses were attenuated by pretreatment of nodose neurons with N/OFQ (1 microM). N/OFQ inhibitory effect on the Ca(2+) response in nodose ganglia cells was antagonized by tertiapin (0.5 microM), an inhibitor of inward-rectifier K(+) channels, but not by verapamil, a voltage gated Ca(2+) channel blocker, indicating that an inward-rectifier K(+) channel is involved in N/OFQ inhibitory effect. In isolated guinea-pig bronchus, N/OFQ (1 microM) inhibited capsaicin-induced airway contraction. Tertiapin (0.5 microM) abolished the N/OFQ inhibition of capsaicin-induced bronchial contraction. Capsaicin (10 microg) increased pulmonary inflation pressure in the isolated perfused guinea-pig lungs. This response was significantly attenuated by pretreatment with N/OFQ (1 microM). Tertiapin also abolished the N/OFQ inhibitory effect on capsaicin-induced bronchoconstriction in perfused lungs. Capsaicin increased the release of substance P and neurokinin A from isolated lungs. N/OFQ (1 microM) blocked the capsaicin-induced tachykinin release. These results indicate that N/OFQ-induced hyperpolarization of tachykinin containing airway sensory nerves, through an inward-rectifier K(+) channel activation, accounts for the inhibition of capsaicin-evoked broncoconstriction.     

Several neuropeptides determine the visceromotor response to capsaicin in the guinea-pig isolated ileal longitudinal muscle

Capsaicin (1 microM) produced, after an initial contraction, a depression of the field stimulation-induced contraction of the guinea-pig isolated ileal longitudinal muscle. Both effects exhibited prompt desensitization, indicating the involvement of a specific action on sensory nerves. The initial contraction was inhibited by [D-Pro4,D-Trp7,9,Phe11]SP-(4-11), a substance P (SP) antagonist, which did not affect the inhibitory component of the response. Incubation of the strips with antiCGRP (CGRP = calcitonin gene-related peptide) serum did not modify the amplitude of the capsaicin-induced contraction but inhibited the twitch depression induced by capsaicin. AntiCGRP serum blocked the effects of exogenous CGRP but not the inhibitory response induced by baclofen. These findings provide evidence that the release of several neuropeptides from sensory nerves determines the visceromotor response to capsaicin in this preparation. In particular, a CGRP-like peptide could be responsible for the inhibitory phase which follows the initial contraction which is due to release of SP and/or related peptides.     

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.     

Analysis of the Mechanisms Underlying the Contractile Response Induced by the Hydroalcoholic Extract of Phyllanthus urinaria in the Guinea-pig Urinary Bladder In-vitro

The hydroalcoholic extract of Phyllanthus urinaria (Euphorbiaceae), substance P and substance P methyl ester all caused graded contractions in the guinea-pig urinary bladder. Responses to hydroalcoholic extract and substance P were markedly inhibited in calcium-free Krebs solution, this effect being reversed by reintroduction of calcium in the medium. The contraction in response to hydroalcoholic extract was unaffected by atropine, propranolol, prazosin, yohimbine, tetrodotoxin, w-conotoxin, nicardipine, HOE 140, guanethidine, staurosporine, phorbol ester or indomethacin, excluding the involvement of nervous mediated responses, or action via cholinergic, adrenergic, kinins, cyclo-oxygenase metabolites, protein kinase C or activation of L or N-type calcium channels. The selective NK₁ tachykinin antagonist (FK 888), but not NK₂ (SR 48968) antagonized substance P-induced contraction, but both drugs failed to effect Phyllanthus urinaria-induced contraction. Prolonged desensitization of guinea pig urinary bladder with capsaicin (10 μM) or preincubation of guinea-pig urinary bladder with capsazepine did not affect contraction caused by hydroalcoholic extract. Ruthenium red almost completely abolished capsaicin-induced contraction, but had no effect on hydroalcoholic extract-mediated contraction. Substance P and the hydroalcoholic extract caused marked potentiation of the twitch response in the preparations field stimulated. The facilitatory effect of substance P, but not that of hydroalcoholic extract, was prevented by the NK₁ (FK 888), but not by NK₂ (SR 48968) antagonist. We concluded that contraction induced by hydroalcoholic extract of Phyllanthus urinaria in the guinea pig urinary bladder involves direct action on smooth muscle and relies on the mobilization of extracellular calcium influx unrelated to activation of L- and N-type calcium channels or activation of protein kinase C mechanisms. In addition contraction caused by the hydroalcoholic extract of Phyllanthus urinaria in guinea-pig urinary bladder does not involve the activation of tachykinin or vanilloid receptors.     

Regional differences in the effects of capsaicin and tachykinins on motor activity and vascular permeability of the rat lower urinary tract

Summary 1. The effects of capsaicin, substance P (SP) and neurokinin A (NKA) on motor activity and vascular permeability was investigated in the rat lower urinary tract (bladder dome and neck, proximal urethra and ureters). 2. Capsaicin produced contractions of the rat bladder dome and neck and of the proximal urethra in vitro, which were unaffected by tetrodotoxin and abolished by ganglionectomy. SP and NKA were almost equipotent in producing a contraction of the rat isolated bladder dome or neck and urethra. However, the maximal response to NKA was about twice that of SP on the urethra and bladder neck. 3. Capsaicin did not affect motility of the unstimulated rat isolated ureter, while NKA or SP activated rhythmic contractions, NKA being about 850 times more potent than SP. Either capsaicin or field stimulation produced a transient inhibition of the NKA-activated rhythmic contractions of the rat isolated ureter which was prevented by capsaicin-desensitization. 4. The capsaicin-(1 M) or field stimulation-induced inhibition of NKA-activated rhythmic contractions of the rat isolated ureter were unaffected by removal of pelvic ganglia but abolished by cold storage (72 h at 4°C). 5. Intravenous capsaicin induced an inflammatory response (Evans blue leakage) in the bladder, proximal urethra and ureters in vivo. Plasma extravasation was greater in the ureters, urethra and bladder neck than in the dome. SP, NKA and histamine produced a dose-dependent dye leakage in all segments of the rat urinary tract, the response being slightly greater in the bladder neck than in the dome. 6. The capsaicin-induced inflammatory response was abolished by systemic capsaicin-desensitization and reduced, to a variable extent, by pelvic ganglionectomy, in the various tissues examined. Topical application of tetrodotoxin on the bladder dome failed to affect the capsaicin-induced plasma extravasation in the urinary bladder. 7. These findings indicate that chemoceptive, capsaicin-sensitive nerves are present throughout the whole rat lower urinary tract and their activation determines a variety of visceromotor responses and an increase of vascular permeability. In various instances the response to capsaicin may be explained by the action of tachykinins but some effects may involve other sensory neuropeptides. Send offprint requests to C. A. Maggi at the above address     

Evidence for two independent modes of activation of the 'efferent' function of capsaicin-sensitive nerves

Field stimulation (10 Hz for 10 s, 0.5 ms pulse width, 60 V) of the guinea-pig isolated main bronchi (atropine plus indomethacin in the bath) produced reproducible contractions which were abolished by tetrodotoxin or in vitro capsaicin desensitization. These responses were almost abolished by omega-conotoxin GVIA (CTX), a peptide modulator of neuronal calcium channels which, however, did not affect the bronchial contraction due to neurokinin A or to capsaicin. Field stimulation (10 Hz for 2.5 s, 1 ms, 60 V) of the electrically driven, isolated guinea-pig left atria excised from reserpine-pretreated animals (atropine in the bath) produced a delayed positive inotropic response which was abolished by tetrodotoxin or in vitro capsaicin desensitization. This response was abolished by CTX, which did not affect the response to exogenous calcitonin gene-related peptide nor that to capsaicin. These findings indicate that CTX-sensitive mechanisms (presumably Ca channels regulating the release of transmitters) are activated upon antidromic invasion of sensory terminals and consequent production of the 'efferent' response while the activation of sensory nerve endings by capsaicin occurs through CTX-resistant mechanisms.     

Prostacyclin activates tachykinin release from capsaicin-sensitive afferents in guinea-pig bronchi through a ruthenium red-sensitive pathway.

1. We have investigated the ability of prostacyclin (PGI2) to contract guinea-pig isolated bronchi and the possible involvement of capsaicin-sensitive primary afferents in the response to PGI2. 2. PGI2 (0.1-100 microM) produced concentration-dependent contractions of the guinea-pig isolated bronchi. In vitro capsaicin desensitization (10 microM for 30 min followed by washing) significantly reduced the PGI2-induced contraction at all concentrations tested. A capsaicin-resistant component of contraction (40-60% of the overall response) was also evident. 3. Ruthenium red (3 microM), an inorganic dye which acts as a selective functional antagonist of capsaicin, significantly decreased PGI2-induced contractions, without affecting the response to substance P, neurokinin A or acetylcholine. 4. MEN 10, 207, (Tyr5, D-Trp6,8,9, Arg10)-neurokinin A (4-10) (3 microM), a selective antagonist of NK2-tachykinin receptors, significantly decreased PGI2-induced contractions and neurokinin A-induced contractions, without affecting the response to acetylcholine. 5. The effect of ruthenium red and MEN 10,207 on the one hand, and that of ruthenium red and capsaicin on the other was non additive. 6. These results indicate that PGI2-induced contraction of the guinea-pig isolated bronchi involves two distinct mechanisms, one of which involves transmitter (tachykinins) release from peripheral endings of capsaicin-sensitive primary afferents. In as much as PGI2-activation of primary afferents is sensitive to ruthenium red, we suggest that PGI2 shares a common mechanism of tachykinin release with that activated by capsaicin.     

Effects of carbonyl cyanide p-trichloromethoxyphenylhydrazone (CCCP) and of ruthenium red (RR) on capsaicin-evoked neuropeptide release from peripheral terminals of primary afferent neurones

Summary In the superfused isolated rat urinary bladder, capsaicin as well as electrical field stimulation evoked the release of calcitonin gene-related peptide-like immunoreactivity (CGRP-IR). Carbonyl cyanide p-trichloromethoxyphenylhydrazone (CCCP, threshold 2 M) reduced both, the capsaicin- and the electrical field stimulation-evoked release of CGRP-IR while a low concentration of Ruthenium Red (RR, 0.6 M and 2 M) selectively attenuated the capsaicin-evoked release of CGRP-IR but did not influence the effect of electrical field stimulation. 20 M RR nearly abolished the capsaicin-evoked release, but also attenuated the effect of electrical field stimulation.In the isolated guinea-pig bronchus, electrical field stimulation and capsaicin induced non-cholinergic contractions which are known to be caused by tachykinin release from afferent nerve terminals. CCCP (0.6 M) only reduced the response to field stimulation; a ten-fold higher concentration of CCCP attenuated field stimulation as well as capsaicin-induced contractions. This is in contrast to the reported selective inhibition of capsaic-ininduced contractions by RR.The present data demonstrate that CCCP generally inhibits evoked neuropeptide release, regardless of the kind of stimulation used while low concentrations of RR preferentially inhibit capsaicin-evoked neuropeptide release.Send offprint requests to: R. Amann at the above address     

Calcium dependence of baclofen- and GABA-induced depression of responses to transmural stimulation in the guinea-pig isolated ileum

Both baclofen and gamma-aminobutyric acid (GABA) induced a dose-dependent depression of cholinergic twitch contractions in the transmurally stimulated guinea-pig isolated ileum. Over a range of 0.6-2.4 mM Ca2+, the degree of depression was inversely related to the Ca2+ concentration, with an increased sensitivity and sinistral shift of the dose-response curve at lower Ca2+ concentrations. Partial occupation of Ca2+ channels by Ruthenium Red (0.1 microM) also potentiated the depressive responses to baclofen and GABA. It is concluded that these agonists, acting through GABAB receptors, limit the availability of Ca2+ required for neurotransmitter release in myenteric motor nerves.     

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.