Tachykinin receptors mediating responses to sensory nerve stimulation and exogenous tachykinins and analogues in the rabbit isolated iris sphincter

New evidence on the mechanisms underlying bradykinin-mediated contraction of the pig iris sphincter in vitro

We have reported previously that bradykinin (BK) induces potent and reproducible concentration-dependent contractions of the pig iris sphincter (PIS) muscle in vitro through the activation of BK B(2) receptors. Here we attempted to investigate additional mechanisms by which BK induces contraction of the PIS in vitro. BK-mediated contraction of the PIS relied largely on the external Ca2+ influx by a mechanism sensitive to the L-, N- and P-type of Ca2+ channel selective blockers. Likewise, BK-induced contraction of the PIS was greatly inhibited by the CGRP-(8-37), NK(2) or NK(3) receptor antagonists (SR 48968, SR 142801), and to a lesser extent by the NK(1) antagonist (FK 888). Capsaicin desensitization of PIS or capsazepine pre-incubation also significantly reduced BK-mediated contraction in the PIS. Furthermore, KT 5720 or GF 109203X (the protein kinase A and C inhibitors, respectively) also significantly inhibited BK-mediated contraction. Taken together, these results indicate that BK-mediated contraction of the PIS seems to be mediated primarily by the release of CGRP and tachykinins from sensory nerve fibers, and relies largely on extracellular Ca2+ influx via activation of L-, N- and P-type of Ca2+ channels. Finally, these responses are mediated by activation of both protein kinase A- and C-dependent mechanisms.

Principles of tachykininergic co-transmission in the peripheral and enteric nervous system

The tachykinins substance P (SP) and neurokinin A (NKA) are synthesized and released from nerves in the peripheral and enteric nervous system (PNS and ENS). They act as nonadrenergic noncholinergic (NANC) excitatory transmitters in mammalian airways, and the genitourinary and gastrointestinal tract. At the postjunctional level, both NK(1) (SP-preferring) and NK(2) (NKA-preferring) receptors are often co-expressed by target cells innervated by TKergic nerves. Thus an issue of duplication seems to exists with regard to peripheral tachykininergic co-transmission, the duplication involving both messengers (the peptides) and effectors (the receptors). By using receptor selective antagonists it has been possible to dissect the relative contribution of different receptors to TKergic co-transmission: the available results indicate that multiple arrangements exist involving both summation, cooperation and specialization of different messengers/effectors in producing the overall response.

In vitro and in vivo characterization of NK3 receptors in the rabbit eye by use of selective non-peptide NK3 receptor antagonists

1. Inhibition of NK3 receptor agonist-induced contraction in the rabbit isolated iris sphincter muscle was used to assess the in vitro functional activity of three 2-phenyl-4-quinolinecarboxamides, members of a novel class of potent and selective non-peptide NK3 receptor antagonists. In addition, an in vivo correlate of this in vitro response, namely NK3 receptor agonist-induced miosis in conscious rabbits, was characterized with some of these antagonists. 2. In vitro senktide (succinyl-[Asp9,MePhe8]-substance P (6-11) and [MePhe7]-neurokinin B ([MePhe7]-NKB) were potent contractile agents in the rabbit iris sphincter muscle but exhibited quite different profiles. Senktide produced monophasic log concentration-effect curves with a mean pD2=9.03+/-0.06 and mean nH=1.2+/-0.02 (n=14). In contrast, [MePhe7]-NKB produced shallow log concentration-effect curves which often appeared biphasic (nH=0.54+/-0.04, n=8), preventing the accurate determination of pD2 values. 3. The contractile responses to the NK3 receptor agonist senktide were antagonized in a surmountable and concentration-dependent manner by SB 223412 ((-)-(S)-N-(alpha-ethylbenzyl)-3-hydroxy-2-phenylquinoline-4-ca rboxamide; 3-30 nM, pA2=8.4, slope=1.8+/-0.3, n=4). SB 222200 ((-)-(S)-N-(alpha-ethylbenzyl)-3-methyl-2-phenylquinoline-4-car box amide; 30-300 nM, pA2=7.9, slope=1.4+/-0.06, n=4) and SB 218795 ((-)-(R)-N-(alpha-methoxycarbonylbenzyl)-2-phenylquinoline-4-carboxamide; 0.3 and 3 microM apparent pKB=7.4+/-0.06, n=6). 4. Contractile responses to the NK3 receptor agonist [MePhe7]-NKB in the rabbit iris sphincter muscle were unaffected by SB 218795 (0.3 and 3 microM, n=8). In contrast, SB 223412 (30 and 300 microM n=4) and SB 222200 (0.3 and 3 microM, n=4) inhibited responses to low concentrations (< or = 1 nM), to a greater extent than higher concentrations (> 1 nM) of [MePhe7]-NKB. Furthermore, log concentration-effect curves to [MePhe7]-NKB became steeper and monophasic in the presence of each antagonist. 5. SB 218795 (3 microM, n=4) had no effect on contractions induced by transmural nerve stimulation (2 Hz) or substance P, exemplifying the selectivity of this class of antagonist for functional NK3 receptors over NK1 receptors in the rabbit. 6. In vivo, senktide (1, 10 and 25 microg i.v., i.e. 1.2, 11.9 and 29.7 nmol, respectively) induced concentration-dependent bilateral miosis in conscious rabbits (maximum pupillary constriction=4.25+/-0.25 mm; basal pupillary diameter 7.75+/-0.48 mm; n=4). The onset of miosis was within 2-5 min of application of senktide and responses lasted up to 30 min. Responses to two i.v. administrations of 25 microg senktide given 30 min apart revealed no evidence of tachyphylaxis. Topical administration of atropine (1%) to the eye enhanced pupillary responses to 25 microg senktide. This was probably due to the mydriatic effect of atropine since it significantly increased baseline pupillary diameter from 7.0+/-0.4 mm to 9.0+/-0.7 mm (n=4), thereby increasing the maximum capacity for miosis. Senktide-induced miosis was inhibited by SB 222200 (1 and 2 mg kg[-1], i.v., i.e. 2.63 and 5.26 micromol kg[-1]; maximum inhibition 100%; n=3-4), SB 223412 (0.5 and 1 mg kg[-1], i.v., i.e. 1.31 and 2.61 micromol kg[-1]; maximum inhibition 100%; n=3), SB 218795 (0.5 and 1 mg kg[-1] i.v., i.e. 1.26 and 2.52 micromol kg-1; maximum inhibition 78%; n=3), and the structurally distinct NK3 receptor antagonist SR 142801 ((S)-(N)-(1-(3-(1-benzoyl-3-(3,4-dichlorophenyl)piperidin-3-yl)propyl)-4-phenylepipiperidin-4-yl)-N-methylacetamide; 1.5mg kg-1, i.v., i.e. 2.47micromol kg-1, maximum inhibition 92%; n=3). 7. Topical administration of senktide (25microg; 29.7nmol) to the eye induced unilateral miosis in the treated eye only. At this dose there was no significant difference (P<0.05) between pupillary constriction obtained by topical or i.v. senktide, and topically administered atropine had no significant effect on responses to topical senktide (n=4). 8. [MePhe7]-NKB (125, 250 and 500microg, i.v., i.e. 98.31, 196.62 and 393.24nmol, respectively) also induced bilateral miosis in conscious rabbits (maximum pupillary constriction=4.13+/-0.30mm; n=4), but in contrast to in vitro studies this agonist was approximately 100 fold less potent than senktide. [MePhe7]-NKB-induced miosis was inhibited by SB 222200 (5mg kg-1, i.v., i.e. 13.14micromol kg-1; maximum inhibition 69%; n=3). 9. In summary, SB 223412, SB 222200 and SB 218795 are potent and selective antagonists of NK3 receptor-mediated contraction in the rabbit isolated iris sphincter muscle. In addition, NK3 receptor agonist-induced miosis in conscious rabbits is a good in vivo correlate of the in vitro rabbit iris sphincter muscle preparation and appears to be a useful model for characterizing the pharmacodynamic profile and efficacy of structurally distinct NK3 receptor antagonists, such as SB 222200, SB 223412, SB 218795 and SR 142801.

Sensory neuropeptides: their role in inflammation and wound healing

Vasoactive neuropeptides including substance P and calcitonin gene-related peptide (CGRP) are localised in sensory nerves which innervate blood vessels. These are the major vasoactive neuropeptides released from sensory nerve endings and both have been suggested to have roles in inflammatory and cardiovascular disease. The neuropeptides have potent effects on microvascular tone and permeability, which are seen soon after release from perivascular nerves. There is also evidence that neuropeptides can affect various activities of inflammatory cells and that sensory nerves play a role in the recovery of the healthy microcirculation during wound healing phases. This review concentrates on evidence that the neuropeptides substance P, acting via tachykinin NK1 and NK2 receptors, and CGRP, acting via CGRP1 receptors, play a pro-inflammatory role in disease and a beneficial role in wound healing. In addition, results from clinical trials of recently developed neuropeptide antagonists are discussed.

Relaxation of the ovine isolated iris sphincter by adenosine receptor agonists: lack of effect of adenosine A1 and A2 receptor antagonists

The effects of adenosine receptor ligands on the tone of the ovine isolated iris sphincter were investigated, and adenosine analogues were found to relax the carbachol-contracted tissue in a concentration-dependent manner with an order of potency of 5'-N-ethylcarboxamidoadenosine > or = 2-(p-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamidoadenos ine (CGS 21680) > or = N6-cyclopentyladenosine > adenosine, consistent with activation of an adenosine A2A receptor. However, these responses were not inhibited by the non-selective adenosine A1/A2 receptor antagonist 8-p-sulphophenyltheophylline (50 microM), the selective adenosine A2A/A1 receptor antagonist N-[2-(dimethylamino)ethyl]-N-methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3- dipropyl-1H-purin-8-yl)benzenesulphonamide (PD 115,199) (0.1 microM) or the non-xanthine adenosine A2A receptor antagonist (4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol) (ZM 241385) (0.1 microM). The relaxations cannot therefore be mediated by activation of adenosine A1, A2A or A2B receptors.

Ocular inflammation induced by electroconvulsive treatment: contribution of nitric oxide and neuropeptides mobilized from C-fibres

1. Electroconvulsive treatment (ECT) of rabbits produced ocular inflammation consisting of conjunctival hyperaemia, miosis and protein extravasation into the aqueous humour, reflected by the so-called aqueous flare response (AFR): the maximal reduction in pupil size was 3.8 +/- 0.1 mm (s.e. of mean, n = 16) while the maximal AFR was 28.1 +/- 2.8 (arbitrary units). 2. ECT also caused release of substance P (SP), pituitary adenylate cyclase-activating peptide (PACAP)-27, -38 and calcitonin gene-related peptide (CGRP). The concentrations of SP and CGRP in the aqueous humour of normal, untreated eyes were 10.6 +/- 1.4 and 117.4 +/- 12.4 pmol l-1, respectively, while the concentrations of PACAP-27 and -38 were below the detection limit. After ECT the concentrations of SP, PACAP-27, -38 and CGRP were 65.0 +/- 9.6, 46.9 +/- 8.4, 50.2 +/- 5.4 and 1109.9 +/- 133.1 pmol l-1, respectively (s.e. of mean, n = 12). Conceivably, ECT evoked an antidromic activation of sensory neurones in the trigeminal ganglion with the consequent release of neuropeptides from C-fibres in the uvea and the development of neurogenic inflammation. 3. Rabbits received the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 200 mg kg 1, i.v.). This pretreatment inhibited the ECT-evoked conjunctival hyperaemia, miosis and AFR: under these circumstances the maximal reduction in pupil size was 1.9 +/- 0.1 mm while the maximal AFR was 2.7 +/- 0.9 (n = 16). L-NAME also inhibited the ECT-evoked release of SP, PACAP-27, -38 and CGRP into the aqueous humour; the concentrations of SP and CGRP were 13.2 +/- 1.5 and 204.8 +/- 33.5 pmol l-1, respectively, while PACAP-27 and -38 were below the detection limit (n = 12). 4. The ECT-evoked miosis was also inhibited by pretreatment with the tachykinin receptor antagonist D-Pal9 spantide 11 (90 nmol, intravitreal injection); under these circumstances the maximal reduction in pupil size was only 0.7 +/- 0.03 mm, indicating an important role for SP in the miotic response. Pretreatment of the eye with capsaicin, which is known to cause functional ablation of C-fibres, inhibited the conjunctival hyperaemia, miosis and AFR by 40-50%; the maximal reduction in pupil size being 2.2 +/- 0.2 mm and the maximal AFR 13.8 +/- 2.1 (arbitrary units) (n = 8). 5. The results suggest (1) that ECT evokes ocular inflammation through antidromic C-fibre activation; (2) that SP contributes to the ECT-evoked miosis; and (3) that NO contributes to the antidromic C-fibre activation and possibly to the vascular responses mediated by the C-fibre transmitters.

Characterization of NK3 receptors in rabbit isolated iris sphincter muscle

1. Tachykinin NK3 receptors were characterized in the rabbit isolated iris sphincter muscle by use of autoradiography and in vitro functional studies. 2. [125I]-[MePhe7]-neurokinin B (NKB) (1nM), a selective NK3 receptor agonist, specifically labelled a population of NK3 receptors that were uniformly distributed throughout the rabbit iris sphincter muscle. This labelling was inhibited by unlabelled [MePhe7]-NKB (1 microM) but not by the NK1 receptor antagonist CP 99994 (1 microM). 3. In the presence of CP 99994 (1 microM), the selective NK3 receptor agonists senktide (n = 14) and [Pro7]-NKB (n = 4), and the natural preferred ligand for the NK3 receptor, NKB (n = 8), were potent contractile agents in the rabbit iris sphincter muscle. They all produced monophasic concentration-effect curves with pD2 values of 9.53 +/- 0.08, 8.56 +/- 0.09 and 9.75 +/- 0.09, and nH values of 0.93 +/- 0.03, 1.53 +/- 0.17 and 0.76 +/- 0.06, respectively. [MePhe7]-NKB (n = 12) was also a potent agonist, but produced shallow concentration-effect curves which appeared biphasic (nH = 0.45 +/- 0.04). 4. Contractile responses to senktide were surmountably antagonized in a concentration-dependent manner by the selective non-peptide NK3 receptor antagonist, SR 142801 (3-30 nM; pA2 = 8.9; slope = 0.99) and the non-peptide NK2/NK3 receptor antagonist, SR 48968 (3-30 microM; pA2 = 6.1; slope = 1.5). These pA2 values were consistent with functional rabbit NK3 receptors more closely resembling guinea-pig and human NK3 receptors, than rat NK3 receptors. SR 142801 (10-100 nM) and SR 48968 (3 and 30 microM) inhibited responses to low (< or = 1 nM) but not higher (> 1 nM) concentrations of [MePhe7]-NKB, and concentration-effect curves to [MePhe7]-NKb became steeper and monophasic in the presence of either antagonist. 5. SR 142801 (3-30 nM) and SR 48968 (3-30 microM) also surmountably antagonized concentration-effect curves to [Pro7]-NKB and NKB, although results were more difficult to interpret, since the relationship between log concentration-ratios and the concentration of antagonist used did not adhere to the Schild equation. However, analysis of data with the lowest concentration of SR 142801 (3 nM) tested against NKB, and SR 48968 (3 microM) tested against [Pro7]-NKB and NKB, yielded apparent pA2 estimates of 9.3, 6.8 and 6.4, respectively, consistent with blockade of NK3 receptors. 6. SR 142801 (100 nM) had no effect on contractions induced by transmural nerve stimulation (2 Hz, 0.3 ms, 20 V for 30 s), whereas CP 99994 (1 microM) abolished these responses. 7. Phenoxybenzamine pretreatment (20 microM, 10 min) markedly reduced maximum responses to [MePhe7]-NKB (from 101 +/- 6.2% to 38 +/- 9.5% reference contraction, n = 4) and induced a marked (10 fold) rightward shift in the concentration-effect curve. The residual responses to [MePhe7]-NKB after phenoxybenzamine pretreatment were unaffected by 1 microM CP 99994 (maximum response = 41 +/- 9.4%, n = 4). 8. These results demonstrate autoradiographically and functionally, the presence of NK3 receptors in rabbit iris sphincter muscle that mediate contractile responses to NK3 receptor agonists, but not to sensory trigeminal nerve simulation. The present data with senktide and selective NK3 receptor antagonists suggest that functional rabbit NK3 receptors more closely resemble human and guinea-pig NK3 receptors than rat NK3 receptors. However, the pharmacological profiles of [MePhe7]-NKB, SR 142801 and SR 48968 suggest the presence of an 'atypical' NK3 receptor or a heterogeneous population of NK3 receptors in this tissue.

Sensory nerves play an efferent role in the function of the arterioles, but not the dilator muscle, of the rat iris

We have studied the expression, distribution and function of receptors for the sensory neurotransmitters, substance P (SP) and calcitonin gene-related peptide (CGRP) in the dilator muscle and arterioles of the rat iris. Immunohistochemical studies showed that the sensory fibres containing these peptides are distributed throughout the connective tissue stroma of the iris and in association with the larger arterioles, but do not come into close association with the dilator muscle cells. Using reverse-transcription polymerase chain reaction, we have shown that both NK1 and NK3 receptor message is expressed by iris tissue, comprising both dilator muscle and stromal tissue. Binding sites for the NK1 agonist, [Sar9, Met(O2)11]-substance P (SarSP), and for CGRP are confined to the stromal layer and to the larger arterioles within that layer and do not appear to be associated with the dilator muscle itself. Application of either SarSP or CGRP produced both a vasodilatation and an inhibition of sympathetic nerve-induced vasoconstriction of the larger arterioles. Neither SarSP nor CGRP altered the resting tone of the dilator nor were they capable of modulating the contractions due to sympathetic nervous activity. These results suggest that the sensory fibres perform an efferent role in the larger irideal arterioles while their presence in the irideal stroma appears not to modulate the activity of the dilator muscle.

Inhibition of bradykinin-evoked trigeminal nerve stimulation by the non-peptide bradykinin B2 receptor antagonist WIN 64338 in vivo and in vitro

1. This study investigated the effect of the recently described non-peptide bradykinin B2 receptor antagonist, WIN 64338 ([[4-[[2- [[bis(cyclohexylamino)methylene]amino]-3-(2-naphthalenyl)-1-oxopropyl] amino]phenyl]methyl]tributylphosphoniumchloride monohydrochloride), in experimental models of bradykinin-evoked sensory nerve stimulation. 2. In the rabbit isolated iris sphincter in vitro, bradykinin-evoked contractile responses are mediated via tachykinins released from peripheral endings of the trigeminal sensory nerve. WIN 64338 (1-10 microM) competitively antagonised contractile responses to bradykinin with a pKB estimate of 6.6 +/- 0.1 (n = 11). The antagonism was selective since WIN 64338 (10 microM) did not significantly inhibit submaximal contractile responses to the direct-acting spasmogens substance P (10 nM), neurokinin A (3 nM), substance P methyl ester (10 nM) or senktide (100 nM); nor by sensory non-adrenergic non-cholinergic nerve stimulation evoked by capsaicin (10 microM), or electrical field-stimulation (3, 10, 30 Hz) (P > 0.05; n = 3-11). 3. Topical application of bradykinin to the conjunctiva and to the nasal mucosa of the guinea-pig in vivo causes plasma extravasation predominantly via the release of tachykinins from peripheral endings of the trigeminal nerve. The increases in plasma extravasation (measured by extravasation of Evans blue dye) induced by bradykinin in the guinea-pig conjunctiva (20 nmol) and nasal mucosa (50 nmol) were markedly reduced (by 81 +/- 3% and 69 +/- 5%, respectively) following pretreatment with WIN 64338 (30 nmol kg-1, i.v.) (n = 5-6; P < 0.05), with almost complete inhibition at a higher dose of WIN 64338 (300 nmol kg-1, i.v.; n = 5-6). This inhibition was selective since at 300 nmol kg-1, WIN 64338 did not inhibit plasma extravasation evoked by substance P in the conjunctiva (5 nmol; P > 0.05; n = 6) or in the nasal mucosa (50 nmol; P > 0.05; n = 5). 4. This study demonstrates that WIN 64338 is a selective and competitive bradykinin B2 receptor antagonist and can be useful for analysing bradykinin-evoked trigeminal nerve stimulation both in vitro and in vivo.

The mammalian tachykinin receptors

The tachykinins (TKs) are a family of small peptides which share the common C-terminal sequence Phe-X-Gly-Leu-MetNH2. Three peptides of this family, substance P, neurokinin A and neurokinin B, have an established role as neurotransmitters in mammals. 2. Three receptors for TKs have been cloned: they are G-protein coupled receptors with seven putative transmembrane spanning segments and have been termed NK1 (substance P-preferring), NK2 (neurokinin A-preferring) and NK3 (neurokinin B-preferring). 3. Synthetic agonists are available to selectively stimulate only one receptor, while natural TKs can act as full agonist at each one of the three receptors, albeit at different concentrations. 4. A number of potent and selective antagonists, both peptide and nonpeptide in nature, have recently been developed. 5. The introduction of these ligands has revealed an unforeseen pharmacological heterogeneity of NK1, NK2 and NK3 receptors which appears largely, if not exclusively, linked to the existence of species homologues of the three receptors.

Inhibition by SR 140333 of NK1 tachykinin receptor-evoked, nitric oxide-dependent vasodilatation in the hamster cheek pouch microvasculature in vivo

1. This study investigated tachykinin-evoked vasodilatation in the microvasculature of the hamster cheek pouch in vivo. Arterioles and venules were observed by intravital microscopy with video recording, and vasodilatation and constriction, defined as changes in blood vessel diameter, measured by image analysis. All agents were applied topically by superfusion. None of the agents tested had a significant effect on venule diameter. 2. When arterioles were preconstricted (by ca. 50%) with endothelin-1 present in the superfusing medium, substance P (0.3-30 nM) was a potent vasodilator, being 10 fold more active than both neurokinin A and the NK1 receptor-selective agonist, substance P methyl ester. The NK2 receptor-selective agonist, [beta-Ala8]-NKA(4-10)(0.1-10 microM) was active only at high concentrations, and the NK3 receptor-selective agonist senktide (0.1-10 microM) was virtually inactive (n = 8 hamsters). Dilatation evoked by tachykinins and analogues was rapid in onset (< 0.5 min) and readily reversible. 3. At low concentrations (1-10 nM), the non-peptide tachykinin NK1 receptor antagonist SR140333 ((S)1-(2-[3(3,4-dichlorophenyl)-1-(3-iso-propoxyphenylacetyl)pi peridin-3- yl]ethyl)-4-phenyl-1-azoniabicyclo[2.2.2]octone, chloride) had no effect on the diameter of preconstricted arterioles per se, but potently inhibited dilator responses to substance P methyl ester (apparent pKB 9.9 +/- 0.2; n = 5 hamsters, n = 10 estimates). SR140333 (10 nM) did not inhibit submaximal dilator responses evoked by human alpha calcitonin gene-related peptide (alpha CGRPh; 1.0 nM; P > 0.05; n = 5). 4 The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 microM) caused a51.3 +/- 5.4% arteriolar constriction. In the presence of L-NAME, submaximal vasodilator responses to substance P (10-I00 nM) and carbachol (0.1-1.0 microM) were significantly attenuated (n = 5 hamsters;P<0.05) as compared to responses obtained in preparations that were preconstricted to a similar extent by endothelin-l (48.0 +/- 5.6%). L-NAME (10 M) was without effect on submaximal vasodilator responses to alpha CGRPh (0.1 nM) or sodium nitroprusside (1O nM) (n = 5 hamsters; P> 0.05).5 We conclude that tachykinin-evoked arteriolar vasodilatation in the hamster cheek pouch is mediated via NK, receptor activation and depends, at least in part, on the release of nitric oxide. The NKI receptors mediating vasodilatation can be blocked by topical application of SR140333; which may therefore be useful in the investigation of the role of NK1 receptors in neurogenic inflammation in the microvasculature.

Typical and atypical NK1 tachykinin receptor characteristics in the rabbit isolated iris sphincter

1. A contraction of the rabbit isolated iris sphincter smooth muscle follows activation of either tachykinin NK1 or NK3 receptors. We have here characterized the pharmacological activity profiles of various tachykinin receptor agonists considered to have NK1-receptor-preferring activity in this preparation. 2. Two groups of NK1-receptor-preferring agonists could be distinguished in terms of a common pharmacological profile. The first group (Group 1) included [Glp6,L-Pro9]-SP(6-11) (septide), [Glp6]-SP (6-11), substance P methyl ester, delta-aminovaleryl-[L-Pro9, N-MeLeu10]-SP(7-11) (GR73632), and [Apa9-10]-SP. The second group (Group 2) included [Pro9]-SP, substance P, physalaemin and ranamargarin. 3. Under control conditions, the responses to Group 1 agonists were relatively fast in offset (time for reversal of maximal responses, 11.2-18.2 min), and were antagonized by NK1-receptor-selective antagonists (range of pKB estimates vs various agonists; GR82334, 7.1-8.2; (+/-)-CP-96,345, 8.9-9.5; RP67580, 7.0-7.4). Following incubation of the tissue with phenoxybenzamine (20 microM, 10 min), the affinity of GR82334, tested against the Group 1 agonists, substance P methyl ester and septide, was not significantly different (P < 0.05; n = 7-18) to that determined in untreated tissues (substance P methyl ester pKB 7.5 +/- 0.1 and 7.2 +/- 0.2, respectively; septide 7.7 +/- 0.2 and 7.9 +/- 0.2, respectively). Further, response offset times (5.0-8.5 min) were little reduced as compared to those observed in untreated tissues. 4. Under control conditions, the response to Group 2 agonists was markedly slow in offset (times for reversal of maximal responses, 51.4-70.4min), and was not attenuated significantly by the NK1-receptor-selective antagonists GR82334 (I MicroM), (+/-)-CP-96,345 (0.1 MicroM) or RP67580 (1 MicroM). In contrast,after phenoxybenzamine pretreatment, responses to Group 2 agonists reversed rapidly (times for reversal of maximal responses, 13.1-24.2 min), and were now antagonized by GR82334 (pKB estimates, 6.4-7.1).5. The responses to the NK3-receptor-selective agonist Succ-[Asp6,Me-Phe8]-SP(6-l1) (senktide) were relatively fast in offset (time for reversal of maximal response was 18.6 +/- 1.7 min) and were not inhibited by GR82334 (10 MicroM; n = 5). The contractile response resulting from co-application of the Group 1 agonist, septide together with senktide, did not exhibit prolonged response offset kinetics.6. Assuming simple competition at equilibrium, these data from the rabbit iris smooth muscle could be explained either by interaction of the various ligands with two separately-existing NK1 receptor-subtypes or -isoforms; or alternatively by a preferential interaction of the two agonist groups with different binding domains on a common NK1 receptor.

Investigation of the specificity of FK 888 as a tachykinin NK1 receptor antagonist

1. A recently described peptide tachykinin (NK1) receptor antagonist, FK 888, was found to inhibit the electrically-evoked, tachykinin-mediated contractile responses of the rabbit iris sphincter in a concentration-dependent manner; the pIC50 value was 6.6 +/- 0.08. 2. Contractions induced by a selective NK1 receptor agonist, [Sar9,Met(O2)11]substance P, were inhibited competitively by FK 888; the pKB value was 7.1. 3. FK 888 (1 nM-100 microM) was without effect on the electrically-evoked, cholinergic response of the rabbit iris sphincter and the electrically-evoked, sympathetic response of the guinea-pig vas deferens. The contractions of the rabbit iris sphincter, induced by either carbachol (10 nM-30 microM) or noradrenaline (0.1-100 microM), were not affected by 10 microM FK 888. 4. FK 888 (1-30 microM) did not induce histamine release from rat peritoneal mast cells. 5. FK 888 (33 and 333 microM) was without effect on the electrically-evoked action potentials of the frog sciatic nerve. Thus, FK 888 is a moderately high affinity and selective tachykinin (NK1) receptor antagonist.

Non-specific actions of the non-peptide tachykinin receptor antagonists, CP-96,345, RP 67580 and SR 48968, on neurotransmission

1. Three non-peptide tachykinin receptor antagonists, CP-96,345, RP 67580 and SR 48968, were found to inhibit the electrically-evoked, tachykinin-mediated contractile responses of the rabbit iris sphincter in a concentration-dependent fashion; the pIC50 values were 5.6 +/- 0.01, 5.4 +/- 0.07 and 4.8 +/- 0.03, respectively. 2. These antagonists also inhibited the electrically-evoked, parasympathetic response of the rabbit iris sphincter and the sympathetic response of the guinea-pig vas deferens in a concentration-dependent manner; the pIC50 values were 0.3-1.2 log units lower than those recorded for the tachykinin-mediated responses. 3. Two local anaesthetics, bupivacaine and oxybuprocaine, were also found to inhibit the tachykinin-mediated, cholinergic and sympathetic contractile responses in these tissues in a concentration-dependent manner; the concentration ranges for producing the inhibition were similar to those of the non-peptide tachykinin receptor antagonists. 4. On the sciatic nerves of frogs, the tachykinin receptor antagonists inhibited action potentials in a concentration-dependent manner; the potency of the three drugs was similar to that of bupivacaine. 5. Our results suggest that, in addition to blocking tachykinin receptors, the non-peptide tachykinin receptor antagonists, CP-96,345, RP 67580 and SR 48968, may exert non-specific inhibitory effects on neurotransmission.