Tachykinins: effects on gastric secretion and emptying in rats

To test the effects on gastric acid secretion and gastric emptying of non-mammalian (eledoisin, physalaemin, kassinin), mammalian (substance P. neurokinin A, neurokinin B) and synthetic tachykinins (septide, senktide), intracerebroventricular injections of these peptides were given to conscious pylorus-ligated rats (gastric secretory study) or to animals fed with a phenol red meal (gastric emptying study). The tachykinins able to cause central inhibition of acid output were those active on NK2 and NK3 receptors (eledoisin, kassinin, neurokinin A, neurokinin B and senktide). Tachykinins active on NK1 receptors (substance P. physalaemin and septide) were devoid of this activity. The most potent inhibitor was the synthetic NK3 agonist, senktide. All tested natural tachykinins significantly inhibited gastric emptying, the most potent being those active on NK2 receptors. Septide, the synthetic NK1 agonist, had lowest effect and senktide, the synthetic NK3 agonist, was far less active than eledoisin, kassinin and neurokinin A. Thus, specific tachykinin receptors exist in the rat brain that mediate gastric functions: NK3 receptors participate in the control of gastric secretion, while NK2 receptors seem to be involved in the regulation of gastric emptying by activating inhibitory pathways.     

Effects of tachykinins on inositol phospholipid hydrolysis in slices of hamster urinary bladder.

Tachykinin-stimulated inositol phospholipid hydrolysis was examined in slices of hamster urinary bladder. In the presence of lithium, to inhibit inositol monophosphatase activity, substance P, eledoisin and related tachykinins induced large, dose-dependent increases in [3H]-inositol monophosphate accumulation. The responses to substance P and eledoisin were not antagonized by the cholinoceptor antagonist, atropine. The rank order of potency for various tachykinins was kassinin greater than neurokinin A greater than neurokinin B greater than eledoisin greater than physaelamin greater than substance P greater than substance P methyl ester. The synthetic analogue [p-Glu6, D-Pro9]SP (6-11) was considerably more potent than its L-prolyl stereoisomer at stimulating inositol phospholipid hydrolysis. These results suggest that in the hamster urinary bladder, tachykinin-induced inositol phospholipid breakdown is mediated via tachykinin receptors of the SP-E type, as opposed to the SP-P type.     

NK-1 receptor mediation of neurogenic plasma extravasation in rat skin.

1. Plasma extravasation was induced by electrical nerve stimulation and by perfusion of tachykinins over a vacuum-induced blister base on rat footpad. 2. Stimulation of the sciatic nerve (18 V, 15 Hz, 0.5 ms) for 20 min produced a significant increase in the protein content of the perfusate. The response in capsaicin pretreated rats was only 4% of the control response. This indicates that the electrically-induced plasma extravasation response was mediated by capsaicin-sensitive sensory fibres. 3. Exogenous perfusion of the mammalian tachykinins substance P, neurokinin A and neurokinin B and the non-mammalian tachykinins physalaemin, kassinin and eledoisin was used to determine the tachykinin receptor type mediating the plasma extravasation response. Dose-response curves of the tachykinins (10(-9) M-10(-4) M) gave a rank order of potency of substance P = physalaemin greater than eledoisin greater than or equal to kassinin greater than neurokinin B = neurokinin A. 4. In addition, specific agonists of neurokinin receptors were perfused. Perfusion of [Glp6, D-Pro9] SP6-11 and [Glp6, L-Pro9]SP6-11 demonstrated that the L-Pro isomer was much more potent than the D-Pro isomer. 5. The rank order of potency and the greater potency of [Glp6, L-Pro9]SP6-11 over its D-isomer indicate an NK-1 neurokinin receptor mediates plasma extravasation in rat footpad skin.     

Tachykinin receptors in smooth muscles: a study with agonists (substance P, neurokinin A) and antagonists

Four preparations, sensitive to tachykinins, the guinea-pig urinary bladder, the rat duodenum, the hamster and dog urinary bladders have been investigated and compared with four other preparations described before: the guinea-pig ileum and trachea, the dog carotid artery and the rabbit mesenteric vein. On the basis of the order of potency of agonists, evaluated with substance P, physalaemin, eledoisin, kassinin and neurokinin A, the preparations can be separated into three groups, the guinea-pig urinary bladder and the dog carotid artery, in which substance P is the most potent and neurokinin A the weakest tachykinin, the rabbit mesenteric vein, the guinea-pig trachea and the rat duodenum, in which the opposite order is observed and the hamster and dog urinary bladders, in which kassinin is the most potent agonist. The guinea-pig ileum shows similar sensitivity to the five tachykinins. C-terminal partial sequences appear to be weaker than SP-(1-11) in three of the four new preparations, SP-(6-11) being first in the rat duodenum and slightly weaker than SP-(1-11) in the hamster and dog urinary bladders. Studies performed with antagonists or inhibitors of endogenous agents suggest that substance P and neurokinin A act directly on specific receptors. The effects of the two peptides are reduced by antagonists analogues of the sequence SP-(4-11). One of the antagonists, [D-Pro4,Lys6,D-Trp7,9,10, Phe11]SP-(4-11) has been shown to be competitive against substance P and neurokinin A in the guinea-pig ileum, the guinea-pig urinary bladder and the rat duodenum. This compound, shows definitely higher activity against neurokinin A and kassinin, compared to substance P in various preparations. [D-Tyr4,D-Trp7,9,Nle11]SP-(4-11) is the most potent tachykinin antagonist in the hamster and dog urinary bladders. In these preparations, the antagonists act also against substance P, but with lower affinity. These findings with antagonists support the indication, emerged from the order of potency of agonists, that tachykinins may act on two and possibly three different receptor types.     

Comparison of the effects of substance P and substance K on blood pressure, salivation and urinary bladder motility in urethane-anaesthetized rats

The effects of substance P and substance K on blood pressure, salivation and urinary bladder motility were investigated in urethane-anaesthetized rats. On intravenous administration, both peptides produced dose-related reductions in blood pressure, substance P being 25-30 times more potent than substance K. Substance P induced a dose-dependent increase of salivation, while substance K was half as active and induced only 40% of the maximum effect of substance P. The two tachykinins induced contractions of the urinary bladder, substance K being about twice as effective as substance P. Substance K was more effective than substance P in activating micturition. These findings provide further evidence that multiple receptor types are involved in mediating the effects of tachykinins in peripheral organs.     

Evidence for multiple tachykinin receptor subtypes on the rabbit iris sphincter muscle

The actions of mammalian tachykinins (substance P, substance K/neurokinin a, neuromedin K/neurokinin b) and non-mammalian tachykinins (eledoisin, kassinin, physalaemin) were compared on the rabbit pupillary sphincter. All acted as direct spasmogens with potencies in the order: eledoisin greater than physalaemin = neurokinin b = substance P greater than kassinin greater than neurokinin a. However, their actions could be divided into at least two categories on the basis of similar kinetics of contractions, differential sensitivity to the tachykinin antagonist (D-Arg1, D-Pro2, D-Trp7,9, Leu11) substance P and specific cross-protection against phenoxybenzamine inactivation by structurally related tachykinins. The relationship between these observations and the suggested "P" and "E" subtypes of tachykinin receptors is discussed.     

Contractile effects of substance P and neurokinin A on the rat stomach in vivo and in vitro.

Substance P and neurokinin A (substance K) were infused into the coeliac artery of anaesthetized rats at doses of 0.06-20 nmol min-1. Both tachykinins caused contractions of the stomach, the threshold dose of neurokinin A being 10 times lower than of substance P. The dose-response curve for substance P was flatter than that for neurokinin A. On circular muscle strips from the rat gastric corpus in vitro, the dose-response curves for both tachykinins were parallel, neurokinin A being 10 times more potent than substance P. The contractions in response to 10 microM neurokinin A and to 30 microM substance P were 58 and 54%, respectively, of the maximal contraction to bethanechol (1 mM). The effect of substance P was reduced by atropine both in vivo and in vitro. In vitro, the contractions to substance P were also reduced by tetrodotoxin but left unaffected by methysergide. The action of neurokinin A was not affected by these drugs. It is concluded that neurokinin A contracts rat stomach by a direct action on the circular smooth muscle, whereas the action of substance P is mediated, at least in part, by cholinergic interneurones.     

In vivo and in vitro actions of mammalian tachykinins

Summary Recently, two kassinin-like tachykinins have been isolated from mammalian nervous tissue. The potencies of these peptides, substance K (neurokinin ) and neuromedin K (neurokinin ) were compared with those of substance P, eledoisin, and kassinin in various pharmacological systems in vivo and in vitro.In contracting the isolated guinea-pig ileum and rabbit jejunum the potencies of eledoisin, kassinin, substance K, and neuromedin K were 13–80% that of substance P. In the rat vas deferens substance K and neuromedin K potentiated the electrically induced contractions with potencies similar to those of eledoisin and kassinin; they were 46–236 times as potent as substance P.In stimulating salivation in the rat after intravenous injection, eledoisin, kassinin, and substance K were respectively 2.3, 1.3 and 0.33 times as potent as substance P. In contrast, neuromedin K exhibited negligible activity. Each peptide tested led to a short fall in blood pressure after intravenous injection in the rabbit, substance P being 12–250 times as potent as the other peptides. Substance P was 20 times as potent as substance K or neuromedin K in inducing vasodilatation in the rat hind paw in vivo.Of the peptides tested, only substance P (10 nmol/min) significantly increased the release of histamine from the rat isolated hindquarter preparation.The results are discussed with respect to several theories of tachykinin receptor heterogeneity.The work was supported by the Austrian Scientific Research Funds, grant No. P5616, by the Austrian National Bank, grant No. 2216, and the Pain Research Commission of the Austrian Academy of Sciences     

Role of supraspinal tachykinins for micturition in conscious rats with and without bladder outlet obstruction

In order to clarify the role of supraspinal tachykinins in volume-induced micturition and in bladder hyperactivity secondary to bladder outlet obstruction, conscious, normal, female Sprague-Dawley rats were investigated cystometrically before and after intracerebroventricular administration of RP 67,580, a selective antagonist of neurokinin (NK)-1 receptors and/or SR 48,968, a selective antagonist of NK-2 receptors. In normal rats, RP 67,580 or SR 48,968, at a dose of 2 nmol, caused no marked changes in cystometric parameters. Higher doses (up to 20 nmol) caused dose-dependent decreases in micturition pressure and increased bladder capacity, micturition volume and residual urine. A combination of the two drugs, each at a dose of 2 nmol, significantly decreased micturition pressure and increased bladder capacity. In rats with bladder outlet obstruction, the antagonists suppressed micturition dose-dependently, producing urinary retention in two out of eight rats already at a dose of 2 nmol. At a dose of 20 nmol, dribbling incontinence, due to urinary retention, was seen in five out of ten rats. A combination of the two drugs (2 nmol of each drug) caused urinary retention in three out of nine animals and significantly increased bladder capacity, micturition volume and residual volume. The results suggest that outflow obstruction in rats increases the effects of tachykinins in supraspinal structures involved in micturition, and that antagonism of supraspinal NK-receptors may depress the micturition reflex. Whether or not this implies that supraspinal NK-receptors can be targets for drugs aimed for inhibiting bladder hyperactivity in humans should be explored.     

Interactions between the tachykinins and calcitonin gene-related peptide lead to the modulation of oedema formation and blood flow in rat skin.

1. The mechanisms involved in tachykinin-induced oedema were investigated in rat skin and interactions between the tachykinins and calcitonin gene-related peptide (CGRP) were studied. 2. Intradermal injections of the tachykinins, substance P, neurokinin A and neurokinin B, stimulated local oedema formation which was in each case potentiated by co-injection of the vasodilator CGRP. Oedema induced by substance P, in the presence and absence of CGRP, was significantly inhibited by pretreatment of rats with a combination of the histamine H1 antagonist, mepyramine, and the 5-hydroxytryptamine antagonist, methysergide. Oedema induced by neurokinin A or B was not inhibited by this pretreatment. 3. Intradermally-injected CGRP induced a long lasting increase in local blood flow, which was measured with a laser Doppler blood flow meter. The simultaneous injection of substance P, but not of the structurally-related neurokinins, caused a loss of the prolonged vasodilator activity of CGRP. 4. These results show that oedema induced by substance P is partially dependent on mast cell amines and that only substance P causes a loss of the prolonged vasodilator activity of CGRP. 5. We suggest that the ability of substance P to prevent the persistent vasodilator activity of CGRP may be a direct consequence of substance P-induced activation of mast cells.     

Effects of natural tachykinins on ovine lower urinary tract smooth muscle

1 Numerous studies have demonstrated that the urinary bladder is particularly sensitive to tachykinins; rat, rabbit and guinea pig bladders, besides human detrusor, have been the most extensively studied, whereas very little is known about most large animal detrusors. The aim of this work was to study natural tachykinin activity on the lower urinary tract of ovine to make a comparison with data obtained in laboratory animals. 2 As in other animal species, tachykinins are also able to contract ovine bladder smooth muscle. 3 The results reported in this study indicate that in ovine bladder, neurokinin 2 (NK2) receptors are expressed most. In fact, on lamb and sheep bladder neurokinin A (NKA), a NK2‐ almost selective peptide, was shown to be > 100% more active than the natural tachykinins kassinin (KASS) and eledoisin (ELED). Eledoisin was shown to be 50% less active than KASS, which is typical behaviour for an almost exclusively NK2 receptor population. Moreover, NK1‐ preferential peptides, namely substance P (SP) and physalaemin (PHYS), showed a lack of activity even when applied at high concentrations. 4 The results reported in this study show that lamb and sheep detrusor represent a good alternative model for the characterization of NK2‐selective tachykinins.     

Interaction of tachykinins with an adrenergic receptor in the rat urinary bladder

The rat urinary bladder was examined as a model for studying tachykinin receptors. The order of potency, the maximal effect and the slope of the dose-response curve were examined with six tachykinins - substance P (SP), physalaemin, phyllomedusin, uperolein, eledoisin, kassinin - and several substance P fragments - SP-(2-11), SP-(3-11), SP-(4-11) and SP-(6-11). The tachykinin receptor on the rat urinary bladder was shown to bind preferentially tachykinins having a hydrophilic amino acid residue in position 5-6, as occurs with physalaemin, phyllomedusin, eledoisin and kassinin. The N-terminal of the tachykinins and in particular substance P is suggested to play a major role in regulating affinity, intrinsic activity and the slope of the dose-response curve. The tachykinins are thought to exert a direct action on smooth muscle. An accessory binding site associated with the tachykinin receptor on rat urinary bladder was also identified. This accessory site binds the N-terminal amino acids of the tachykinins as well as some alpha-adrenergic compounds (phentolamine, prazosin, noradrenaline or adrenaline in the presence of propranolol). When the accessory binding site is occupied by adrenergic compounds, the affinity of the tachykinins is markedly reduced. This observation is interpreted to mean that catecholamines may have a modulatory influence on tachykinin activity on the rat urinary bladder.     

Actions of tachykinins on the rabbit mesenteric artery: substance P and [Glp6,L-Pro9]SP6-11 are potent agonists for endothelial neurokinin-1 receptors.

1. Identification of the subtype of neurokinin receptor on the endothelium of the rabbit mesenteric artery was demonstrated by comparing the relative potencies of the naturally occurring tachykinins, substance P, neurokinin A and neurokinin B and the highly selective agonists [Glp6,L-Pro9]SP6-11 (L-Pro), [Glp6,D-Pro9]SP6-11 (D-Pro) and N-succinyl-[Asp6,MePhe8]SP6-11 (senktide). 2. Relaxations of the rabbit mesenteric artery to substance P, neurokinin A and neurokinin B were concentration-dependent and were abolished by the removal of the endothelium. 3. Substance P was more potent than neurokinin A or neurokinin B and L-Pro was more potent than D-Pro or senktide. 4. Substance P, neurokinin A and neurokinin B all significantly reduced the nerve-mediated contractile response in the presence of the endothelium at a concentration of 0.1 microM, with a rank order of potency substance P greater than neurokinin A greater than neurokinin B. 5. At a concentration of 0.1 microM, L-Pro also significantly reduced the nerve-mediated contractile response, unlike D-Pro and senktide. 6. It is concluded that the relaxation of the rabbit mesenteric artery produced by substance P is mediated by neurokinin-1 receptors (NK-1) located on the endothelium. Furthermore, of the analogues, L-Pro was particularly potent for these receptors.     

NK-1 receptors mediate the tachykinin stimulation of salivary secretion: selective agonists provide further evidence

The relative contribution of NK-1, NK-2 and NK-3 receptors to the sialogogic response to i.v. tachykinins was investigated in urethane-anesthetized rats. [Pro⁹,Met(O₂)¹¹]substance P (SP), a selective NK-1 receptor agonist, was about 10 times more potent than SP itself and its action was unaffected by pretreatment. On the other hand [Nle¹⁰]neurokinin A (NKA)-(4–10) and [MePhe⁷]neurokinin B (NKB), two selective agonist for NK-2 and NK-3 receptors, respectively, were ineffective.     

Tachykinins in the lungs

Tachykinins are neuropeptides which regulate various biological responses, some of which are potentially important in the pathogenesis of pulmonary diseases such as asthma. Tachykinins produce their biological effects by stimulating specific tachykinin receptors (NK(1), NK(2) and NK(3)). Tachykinins have a variety of effects in the lungs. They are among the most potent bronchoconstrictor agents known and have potent effects on airway blood vessel caliber, causing vasodilation by an endothelium-dependent mechanism. Exogenously administered tachykinins (substance P, neurokinin A and neurokinin B) induce mucus secretion in most species, including humans. In addition to having effects on airway secretion, tachykinins also modulate the mucociliary clearance mechanisms of the airway. Tachykinin receptors are found on pulmonary/bronchial C fibers, and both excitatory and inhibitory effects of tachykinins on neural discharge and neurotransmitter release from these nerves have been described. Tachykinins are also involved in several reflex responses, particularly the cough reflex. Tachykinins have been implicated in the inflammatory response in the lungs and they also participate in the regulation of the immune system. A considerable body of evidence implicates tachykinins as important mediators of the neurogenic inflammatory response in a variety of pulmonary diseases. It is thus expected that tachykinin receptor antagonists will prove useful in the therapy of diseases such as asthma, allergic rhinitis and chronic bronchitis.     

Effects of natural tachykinins on porcine lower urinary tract smooth muscle

1. The aim of our study was to ascertain the possible differences and/or similarities in natural tachykinin activity in vitro on lower urinary tract of large-sized animals as compared with data obtained in laboratory animals. 2. Besides tachykinins normally present in mammals, namely substance P (SP), neurokinin A (NKA) and neurokinin B (NKB), we tested non-mammalian tachykinins, such as eledoisin (ELED), physalaemin (PHYS), kassinin (KASS) and PG-kassinin II (PG-KASS II). 3. NKA, KASS and ELED were found to be the most potent peptides in contracting detrusor strips from porcine bladder. In particular, NKA showed a pD2 of 7.14, whereas KASS and ELED showed pD2 values of 7.20 and 7.22, respectively. The activity of NKB and PG-KASS II corresponded to 72.4 and 55.0% respectively of that of NKA. SP and PHYS activity corresponded to only 2% of that of NKA. 4. NKA (pD2 7.92) was the most active peptide in contracting bladder neck tissues as well. ELED and KASS were found to have lower, similar pD2 values (7.62 and 7.70, respectively), whereas NKB and PG-KASS II were much less active (pD2 7.12 and 6.74, respectively). Moreover, SP and PHYS showed an activity range lower than 2% of that of NKA. 5. The reported results confirm that, on pig vesical neck and detrusor, NK1 receptors represent a minority as compared with NK2 and NK3 receptors. By contrast, the presence of NK2 receptors is demonstrated by a greater potency of NKA. The presence of NK3 receptors both on detrusor and neck is evidenced by NKB activity and by results achieved with PG-KASS II.     

Tachykinin receptors of the NK2 type involved in the acetylcholine release by nicotine in guinea-pig bladder.

1. The effects of guanethidine and tachykinins on nicotine- and electrical stimulation-induced cholinoceptor responses were studied in isolated urinary bladder from the guinea-pig. 2. Acetylcholine release and the contractile response stimulated by nicotine were partially reduced by a sympathetic nerve blocker, guanethidine. Neurokinin A (but not substance P methyl ester or senktide) enhanced both acetylcholine release and contraction by nicotine in the presence of guanethidine. 3. Frequency-contraction curves (1 to 50 Hz) for electrical field stimulation (EFS) were partially reduced by atropine (1 microM), and after desensitization to alpha,beta-methylene adenosine 5'-triphosphate, the atropine-resistant contraction to EFS was completely abolished. Guanethidine, the tachykinin antagonist [D-Arg1, D-Pro2, Trp7,9, Leu11]-substance P and application of neurokinin A or substance P did not change the contractile response to EFS. Preganglionic nerve stimulation (5 Hz and 20 Hz) also evoked a similar response to EFS and was not influenced at all by guanethidine or neurokinin A. 4. We conclude that the ability of nicotine to release acetylcholine is enhanced both by endogenous tachykinins (probably released from sympathetic nerves) and by exogenously applied tachykinins as a result of interaction with NK2 receptors in the urinary bladder.     

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.     

The rat isolated portal vein: a preparation sensitive to neurokinins, particularly neurokinin B

The rat isolated portal vein is a pharmacological preparation more sensitive to neurokinin B than to any other neurokinin or tachykinin. The preparation is more sensitive to C-terminal partial sequences of substance P (SP) particularly SP-(6-11) than to the whole undecapeptide. The order of potency of neurokinins is as follows: neurokinin B greater than neurokinin A greater than substance P. The preparation shows high sensitivity also to kassinin and eledoisin. Comparative tests performed with strips of the rat portal vein suspended in a microbath under continuous perfusion (system 1) or in ordinary baths for isolated smooth muscles (system 2) have given similar results and have shown that the myotropic effect of neurokinin B is not modified by a variety of antagonists of endogenous agents as well as by inhibitors of the arachidonic acid cascade. The present results suggest that neurokinin B contracts the rat portal vein by activating specific receptors, presumably located on the smooth muscle membrane, different from those of biologically active amines and peptides which are active stimulants of the vein. Neurokinin B is ten times more active than neurokinin A and at least 100 times more than substance P. Such an order of potency of agonists suggests the existence of a new neurokinin receptor type, particularly sensitive to neurokinin B.     

Comparison of the responses to the sensory neuropeptides, substance P, neurokinin A, neurokinin B and calcitonin gene-related peptide and to trigeminal nerve stimulation in the iris sphincter muscle of the rabbit

Three mammalian tachykinins (substance P, neurokinin A and B) and two non-mammalian ones (eledoisin and physalaemin) produced potent contractions of the isolated rabbit iris sphincter muscle. The rank order of potencies was eledoisin greater than neurokinin B = physalaemin greater than substance P greater than neurokinin A. The maximum efficacy was much the same. The contractile responses to neurokinin A and eledoisin developed more rapidly than did those to the other tachykinins used and were selectively attenuated by [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP. Electrical transmural stimulation produced a contraction consisting of cholinergic and tachykininergic components. The tachykininergic component was abolished by pretreatment with capsaicin or by trigeminal denervation (Fujiwara et al., 1984). [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP attenuated the tachykininergic component, but not the cholinergic one. KCl and capsaicin also produced a tachykininergic contraction which was inhibited by [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP. Calcitonin gene-related peptide affected neither the iris sphincter muscle nor the response to electrical transmural stimulation. These results suggest that the tachykininergic responses induced by electrical transmural stimulation, KCl and capsaicin are predominantly mediated by neurokinin A, probably released from the peripheral endings of trigeminal nerves.