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

Modifications of capsaicin-sensitive neurons in isolated guinea pig ileum by [6]-gingerol and lafutidine

A segment of guinea pig ileum was used to confirm the hypothesis that [6]-gingerol and lafutidine interact with capsaicin-sensitive neurons. Addition of 30 and 100 microM [6]-gingerol (a pungent constituent of ginger) induced contraction of the ileum immediately. Like capsaicin, [6]-gingerol-induced contraction was inhibited by antagonists of the vanilloid receptor (capsazepine and ruthenium red), tetrodotoxin, and atropine. Treatment with [6]-gingerol up to 0.3 microM, which alone had no effect, enhanced 3 microM capsaicin-induced contraction, but greater than 3 microM [6]-gingerol significantly inhibited capsaicin-induced contraction. Treatment with lafutidine (a new type of antagonist of the histamine H(2) receptor), which was suggested to interact with capsaicin-sensitive neurons in vivo, also showed both stimulatory and inhibitory effects on capsaicin-induced contraction depending on the concentrations. Lafutidine alone had no effect. The enhanced contraction induced by capsaicin in the [6]-gingerol- or lafutidine-treated ileum was also inhibited by antagonists of the vanilloid receptor, tetrodotoxin, and atropine. Capsaicin and [6]-gingerol, but not lafutidine, at 30 microM stimulated [(3)H]choline release from the prelabeled slices of the ileum. These findings suggest that [6]-gingerol and lafutidine act on capsaicin-sensitive cholinergic neurons and modulate the contraction in isolated guinea pig ileum.

The involvement of calcitonin gene-related peptide (CGRP) and substance P in feline pial artery diameter responses evoked by capsaicin

The effects of capsaicin and selective neuropeptide antagonists on pial artery diameter were measured using an on-line image analyser in anaesthetised cats, in order to monitor the effects of mediators released in response to activation of trigeminal nerves. Perivascular injection of CGRP (10(-8) M) and the neurokinin-1 (NK1) receptor agonist substance P methyl ester, SPOMe (10(-6) M) produced an increase in pial artery diameter. The vasodilatory action of these agonists was significantly and selectively inhibited using the CGRP receptor antagonist, CGRP8-37 (10(-6) M), and the NK1 receptor antagonist, CP99994 (10(-6) M) respectively. Capsaicin (10(-8)-10(-5) M) produced a biphasic response upon perivascular injection that was concentration dependent. At 10(-6) M capsaicin an initial transient vasoconstriction was observed followed by a longer-lasting vasodilatation. The vasodilator component was significantly reduced by CGRP8-37 (10(-6) M) or CP99994 (10(-6) M). These results show that chemical (capsaicin) activation of trigeminal nerves leads to vasodilatation of feline arteries in situ. This vasodilatation is mediated via the activation of CGRP and NK1 receptors probably via the efferent release of CGRP and a substance P-like peptide.

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

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

An investigation into the mechanism of capsaicin-induced oedema in rabbit skin

1. Oedema formation induced by intradermal capsaicin has been studied in rabbit skin. The effect of the anti-inflammatory steroid dexamethasone and also of a range of known inhibitors of oedema formation have been investigated in order to elucidate mechanisms involved in capsaicin-induced oedema formation. 2. Oedema formation, in response to intradermally-injected test agents, was measured by the local extravascular accumulation of intravenously injected 125I-labelled albumin. In separate experiments skin blood flow was assessed by the clearance of intradermally-injected 133xenon. 3. Oedema formation induced by intradermal histamine (3 nmol) and bradykinin (1 nmol), when in the presence of vasodilator doses of calcitonin gene-related peptide (CGRP) (3 pmol) or prostaglandin E1, (PGE1) (10 pmol), was significantly inhibited (P < 0.01) in rabbits pretreated with intravenous dexamethasone (3 mg kg-1, -4 h). In contrast dexamethasone had no effect on capsaicin (3 mumol)-induced oedema formation or, on capsaicin (30-100 nmol)-induced blood flow. 4. Oedema formation observed in response to intradermal capsaicin (3 mumol) was significantly inhibited (P < 0.01) when the selective capsaicin antagonist, ruthenium red (3 nmol) was co-injected. This suggests that the mechanism of capsaicin-induced oedema involves activation of sensory nerves. However, oedema was not inhibited when capsaicin was co-injected with the neurokinin NK1 receptor antagonist, RP67580 (10 nmol), the NK2 antagonist SR48960 (10 nmol) or the CGRP antagonist CGRP8-37 (300 pmol). 5. Oedema formation induced by capsaicin was not inhibited when co-injected with the histamine HI receptor antagonist, mepyramine (3 nmol), the PAF antagonist, WEB 2086 (100 nmol), the bradykinin B2 receptor antagonist, Hoel4O (1 nmol), or the cyclo-oxygenase inhibitor, indomethacin (10 nmol),suggesting that these mediators do not play a major role in the capsaicin-induced response.6. Histological analysis of capsaicin-treated skin sites revealed undamaged, intact microvessels and lack of haemorrhage. Further, co-injection of capsaicin with the hydrogen peroxide remover, catalase(2,200 u), had no effect on oedema formation. This suggests that capsaicin does not induce oedema formation secondary to free radical-induced damage.7. These results indicate that capsaicin-induced oedema in rabbit skin involves activation of sensory nerves. However, the oedema is not inhibited by pretreatment with the anti-inflammatory steroid,dexamethasone. Further the mechanisms which lead to the oedema formation observed after intradermal capsaicin remain unknown.

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.