Neurokinin A. A pharmacological study

Affinity, potency, efficacy, and selectivity of neurokinin A analogs at human recombinant NK2 and NK1 receptors

A series of peptide NK2 receptor agonists was evaluated for affinity, potency, efficacy, and selectivity at human recombinant NK2 and NK1 receptors expressed in CHO cells to identify compounds with the greatest separation between NK2 and NK1 receptor agonist activity. Binding studies were performed using displacement of [¹²⁵I]-NKA binding to NK2 receptors and displacement of [³H]-Septide binding to NK1 receptors expressed in CHO cells. Functional studies examining the increase in intracellular calcium levels and cyclic AMP stimulation were performed using the same cell lines. A correlation was demonstrated between binding affinities (Ki) and potency to increase intracellular calcium (EC₅₀) for NK2 and NK1 receptors. Ranking compounds by their relative affinity (Ki) or potency (EC₅₀) at NK2 or NK1 receptors indicated that the most selective NK2 agonists tested were [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA(4-10) (NK1/NK2 Ki ratio = 674; NK1/NK2 EC₅₀ ratio = 105) and [Arg⁵,MeLeu⁹,Nle¹⁰]-NKA(4-10) (NK1/NK2 Ki ratio = 561; NK1/NK2 EC₅₀ ratio = 70). The endogenous peptide, NKA, lacked selectivity with an NK1/NK2 Ki ratio = 20 and NK1/NK2 EC₅₀ ratio = 1. Of the compounds selected for evaluation in cyclic AMP stimulation assays, [β-Ala⁸]-NKA(4–10) had the greatest selectivity for activation of NK2 over NK1 receptors (NK1/NK2 EC₅₀ ratio = 244), followed by [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA(4-10) (ratio = 74), and NKA exhibited marginal selectivity (ratio = 2.8).

Neurokinin A engages neurokinin-1 receptor to induce NF-κB-dependent gene expression in murine macrophages: implications of ERK1/2 and PI 3-kinase/Akt pathways

Neurokinin A (NKA) belongs to the tachykinin neuropeptide family. Its biological functions are primarily mediated by the neurokinin (NK)-2 receptor. NKA has been implicated in several inflammatory conditions. However, there are limited data about the mechanism of its pathogenetic action. Here, we investigated proinflammatory effects of NKA on peripheral immune cells using the mouse macrophage/monocyte cell line RAW 264.7 and primary peritoneal macrophages. The signaling mechanistic pathways involved were also studied. In mouse macrophages with no detectable NK-2 receptors, NKA induces the upregulation of NK-1 but not NK-2 receptor expression. Furthermore, NKA engages this NK-1 receptor, resulting in inflammatory-like responses involving activation of the transcription factor nuclear factor (NF)-κB and induction of NF-κB-responsive proinflammatory chemokine expression. NKA activates NF-κB as evidenced by induced phosphorylation (leading to degradation) of its inhibitory protein IκBα, increased cellular levels of the transactivation-active phospho(Ser276)-p65 and its nuclear translocation, as well as enhanced DNA-binding activity of NF-κB. These responses are specifically inhibited by selective NK-1 receptor antagonists but not NK-2 receptor antagonists, thereby excluding the role of NK-2 receptor. Further investigation on the upstream signaling mechanisms suggests that two NF-κB-activating pathways (extracellular signal-regulated kinase 1/2 and phosphatidylinositol 3-kinase/protein kinase B) are activated by NKA. Specific inhibitors of the two pathways block NF-κB-dependent chemokine expression. The inhibitory effects are mediated through regulation of nuclear translocation, DNA-binding activity, and/or transactivation activity of NF-κB. Together, we provide novel evidence that NKA engages NK-1 receptors on mouse macrophages to elicit NF-κB-dependent cellular responses. The findings reveal cellular mechanisms that may underlie NKA-mediated inflammatory and immunological conditions.

Topological analysis of the complex formed between neurokinin A and the NK2 tachykinin receptor

Neurokinin A stimulates physiological responses in the peripheral and central nervous systems upon interacting primarily with the tachykinin NK2 receptor (NK2R). In this study, the structure of NKA bound to the NK2R is characterised by use of fluorescence resonance energy transfer. Four fluorescent NKA analogues with Texas red introduced at amino acid positions 1, 4, 7 and 10 were prepared. When bound to a NK2R carrying enhanced green fluorescent protein at the N-terminus, all peptides reduce green fluorescent protein fluorescence from 10% to 50% due to energy transfer. The derived donor-acceptor distances are 46, 55, 59 and 69 A for the fluorophore linked to positions 1-10, respectively. The monotonic increase in distance clearly indicates that the peptide adopts an extended structure when bound to its receptor. The present data are used, in combination with rhodopsin structure, fluorescence studies, photoaffinity labelling and site-directed mutagenesis data to design a computer model of the NKA-NK2R complex. We propose that the N-terminus of NKA is exposed and accessible to the extracellular medium. Subsequent amino acids of the NKA peptide become progressively more buried residues up to approximately one-third of the transmembrane-spanning domain.

Specificity of a Vibrio vulnificus aminopeptidase toward kinins and other peptidyl substrates

Recently, phosphoglucose isomerase with a lysyl aminopeptidase (PGI-LysAP) activity was identified in Vibrio vulnificus. In this paper, we demonstrate the proteolytic cleavage of human-derived peptides by PGI-LysAP of V. vulnificus using three approaches: (i) a quantitative fluorescent ninhydrin assay for free lysine, (ii) matrix-assisted laser desorption ionization-two-stage time of flight mass spectrometry (MALDI-TOF-TOF), and (iii) Tricine gel electrophoresis. PGI-LysAP hydrolyzed bradykinin, Lys-bradykinin, Lys-(des-Arg9)-bradykinin, neurokinin A, Met-Lys-bradykinin, histatin 8, and a myosin light chain fragment. We detected the proteolytic release of free L-lysine from peptide digests using a rapid, simple, sensitive, and quantitative fluorescent ninhydrin assay, and results were confirmed by MALDI-TOF-TOF. The use of the fluorescent ninhydrin assay to quantitatively detect free lysine hydrolyzed from peptides is the first application of its kind and serves as a paradigm for future studies. The visualization of peptide hydrolysis was accomplished by Tricine gel electrophoresis. Proteolytic processing of kinins alters their affinities toward specific cellular receptors and initiates signal transduction mechanisms responsible for inflammation, vasodilation, and enhanced vascular permeability. By applying novel approaches to determine the proteolytic potential of bacterial enzymes, we demonstrate that PGI-LysAP has broad exopeptidase activity which may enhance V. vulnificus invasiveness by altering peptides involved in signal transduction pathways.

Developmental expression of neurokinin A and functional neurokinin-2 receptors in lung

Peribronchial smooth muscle constriction causes airway stretch, an important mechanical force in developing lung. Little is known about factors influencing these spontaneously active muscle elements. We measured contractile activity of neurokinin (NK) receptors on fetal intrapulmonary smooth muscle by tracheal perfusion assay ( n = 11). Injecting either capsaicin or the NK2 receptor agonist [NLE10]NKA resulted in significant ( P < 0.05) bronchoconstriction. A specific NK2 receptor antagonist inhibited constriction caused by endogenous tachykinins released by capsaicin. We then examined NK2 receptor ( n = 44) and NKA ( n = 23) ontogeny in human lung. NKA immunostaining was identified in peribronchial nerves in samples with gestational age >12 wk. NK2 receptor protein was identified in peribronchial and perivascular smooth muscle. These results indicate that endogenous tachykinins released by the developing lung act via NK2receptors to cause smooth muscle constriction. We speculate that tachykinins could modulate lung development.

Structure-activity relationships of neurokinin A (4-10) at the human tachykinin NK(2) receptor: the role of natural residues and their chirality

A structure-activity study of neurokinin A (NKA) (4-10) was performed to investigate the importance of residue and chirality for affinity and efficacy at the NK(2) receptor in human colon circular muscle. Two series of NKA(4-10) analogues were produced with either L-alanine or the D-enantiomer substituted. Their activities were determined in vitro by means of radioligand binding and isolated smooth muscle pharmacology. NKA was more potent than NKA(4-10) at the human, unlike the rabbit, NK(2) receptor. The contractile response of NKA(4-10) was unaffected by N-terminal acetylation. L-Ala substitution of Asp(4), Val(7), Leu(9), and Met(10) caused an 8- to 80-fold decrease, and substitution of Phe(6) caused a 5000-fold decrease in binding affinity (P < 0.01). Positions Ser(5) and Gly(8) were not significantly affected. In functional studies, a similar pattern was observed. The replacement of residues with their respective D-enantiomer drastically reduced binding affinity and functional potency, particularly at positions 6 and 7 (P < 0.05). NKA(4-10) analogues L-Ala(6), L-Ala(8), D-Phe(6), D-Val(7), and D-Met(10) were partial agonists. An excellent correlation was observed between binding and functional data (r = 0.95). A retro-inverso analogue of NKA(4-10) was inactive. In conclusion, the side chains of Asp(4), Phe(6), Val(7), Leu(9), and Met(10) are structurally important features of NKA(4-10) for agonist activity, and changes in amino acid chirality are detrimental to binding affinity and functional activity. Overall, our data are broadly similar to those of previous studies in the rat. However, at the human NK(2) receptor, unlike the rat, [Ala(8)]NKA(4-10) was an antagonist.

Importance of the aromatic residue at position 6 of [Nle(10)]neurokinin A(4-10) for binding to the NK-2 receptor and receptor activation

Steric and electrostatic requirements at position 6 of [Nle(10)]NKA(4-10), a full agonist of NK-2 receptors, for molecular recognition by the receptor were studied. Two series of peptide analogues, (a) p-substituted analogues, [p-X-Phe(6), Nle(10)]NKA(4-10), where X = F, Cl, Br, I, NH(2), NO(2), and (b) [D-Phe(6),Nle(10)]NKA(4-10), [Trp(6),Nle(10)]NKA(4-10), and [Chex-Ala(6),Nle(10)]NKA(4-10), were synthesized, and their biological activity was examined. Competition binding experiments with [(3)H]NKA were performed using cloned human NK-2 receptors expressed in CHO cells. Antagonistic and agonistic properties of the analogues were studied using an in vitro functional assay with hamster tracheal rings. The rank order of potency of agonists was [Nle(10)]NKA(4-10) approximately [p-F-Phe(6),Nle(10)]NKA(4-10) > [p-NH(2)-Phe(6),Nle(10)]NKA(4-10) > [p-Cl-Phe(6),Nle(10)]NKA(4-10) > [p-NO(2)-Phe(6),Nle(10)]NKA(4-10) > [Trp(6),Nle(10)]NKA(4-10). Size and planarity of the aromatic side chain were crucially important for the biological activity, whereas electron-donating and electron-withdrawing properties of the para-substituent were less important. The results favor the hypothesis that weakly polar pi-pi interactions exist between the aromatic group and the receptor.

Rat gastrointestinal motor responses mediated via activation of neurokinin receptors

Natural neurokinins (NK) and their specific receptor agonists, including substance P (SP), neurokinin A (NKA), neurokinin B (NKB), septide, [NIe10]-NKA4-10 and senktide, were used to assess whether they could activate established NK receptors in rat gastrointestinal tract and central nervous system to alter gastric emptying or intestinal transit. Fasting rats were intubated with an orogastric catheter to feed them liquid radiochromium. Neurokinins and analogues (at 10(-10), 10(-9), 10(-8) and 10(-7) mol/kg) and vehicle (saline + 0.1% bovine serum albumin) were injected via an intraperitoneal route. Rats were killed 15 min later and the whole gut was removed. The radioactivity of the stomach and 10 equally divided small intestinal segments was counted to determine gastric emptying and the geometric centre of intestinal transit. Septide treatment at 10(-8) and 10(-7) mol/kg markedly delayed gastric emptying. All doses of NKA inhibited gastric emptying. However, other peptides did not influence gastric emptying. Both septide and NKB treatment at 10(-8) and 10(-7) mol/kg enhanced intestinal transit. Substance P or senktide treatment (10-(-7) mol/kg) also enhanced intestinal transit. Stasis of remaining radioactivity in the proximal intestine was found following SP, septide, NKA and NKB treatment, whereas accelerated transit in the distal intestine was induced following NKA, NKB and senktide treatment. In conclusion, the in vivo study of NK and their specific agonists manifests a selective influence of these compounds on rat gastrointestinal tract. This selective activation of stomach NK1 and NK2 receptors delays gastric emptying, whereas activation of intestinal NK1 and NK3 receptors enhances intestinal transit.

Role of tachykinins in distilled water-induced bronchoconstriction in guinea-pigs

BACKGROUND

An inhalation of ultrasonically nebulized distilled water (UNDW) induces bronchoconstriction only in asthmatics, but the mechanism underlying the response is not fully understood. We recently showed that bronchoconstriction occurs immediately after UNDW is inhaled 20min after an aerosolized antigen challenge in passively sensitized guinea-pigs.

OBJECTIVE

This study was conducted to examine the role of tachykinins in this response.

METHODS

Passively sensitized animals were anaesthetized and artificially ventilated, and changes in pressure at the airway opening (Pao) were measured as an overall index of airway narrowing. A tachykinin NK1 and NK2 dual receptor antagonist, FK224, and a tachykinin NK1 selective antagonist, FK888, were intravenously administered 15 min after the antigen challenge. The effects of capsaicin desensitization and a neutral endopeptidase inhibitor, phosphoramidon, were also examined.

RESULTS

FK224 and FK888 significantly (P < 0.05 and P < 0.05, respectively) reduced the time course curve of the increase in Pao caused by UNDW inhalation in a dose-dependent manner. The percentage increase in Pao from the preantigen challenge value at 1 min after the UNDW inhalation was 267.4+/-17.1, 358.0+/-33.7 and 412.4+/-27.6% with 10 mg/kg of FK224, 1.0 mg/kg of FK224 and vehicle, respectively, (P<0.01 between 10 mg/kg of FK224 and vehicle) and the value was 254.4+/-48.5% with 10 mg/kg of FK888, 327.1+/-57.6% with 1.0 mg/kg of FK888 and 418.5+/-39.0% with vehicle, respectively (P < 0.05 between 10 mg/kg of FK888 and vehicle). The capsaicin desensitization, but not phosphoramidon, significantly reduced the UNDW-induced increase in Pao.

CONCLUSION

These results suggest that tachykinins, at least substance P, are involved in a part of the UNDW-induced bronchoconstriction in our guinea-pig model.

Potencies of agonists acting at tachykinin receptors in the oestrogen-primed rat uterus: effects of peptidase inhibitors

The uterotonic potencies of the naturally occurring mammalian tachykinins and the synthetic subtype-selective agonist analogues of these agents [Lys5,MeLeu9,Nlel0]neurokinin A-(4-10) and [Nle10]neurokinin A-(4-10) (tachykinin NK2 receptor-selective), [Sar9,Met(O2)11]substance P (tachykinin NK1 receptor-selective) and senktide (tachykinin NK3 receptor-selective) were determined using preparations from oestradiol-treated rats. The endopeptidase 24.11 inhibitor, N-[N-[1-(S)-carboxyl-3-phenylpropyl]-(S)-phenyl-alanyl-(S)-isoserine+ ++ (SCH 39370), potentiated responses to neurokinin A, neurokinin B and substance P, but not to [Lys5,MeLeu9,Nle10)]neurokinin A-(4-10) or senktide. [Nle10]neurokinin A-(4-10) effects were potentiated by SCH 39370 with amastatin and those to [Sar9,Met(O2)11]substance P were potentiated by SCH 39370 and captopril in combination. In the presence of optimal concentrations of peptidase inhibitors the relative order of agonist potency was: neurokinin A > substance P > neurokinin B for the naturally occurring mammalian tachykinins and [Lys5,MeLeu9,Nle10]neurokinin A-(4-10) > [Nle10]neurokinin A-(4-10) > [Sar9,Met(O2)11]substance P > senktide for the synthetic tachykinin analogues. Thus, while a tachykinin NK2 receptor predominates in the oestrogen-primed uterus, a tachykinin NK1 receptor may also be present. The non-peptide tachykinin NK3 receptor antagonist, SR 142801, did not antagonise the effects of senktide suggesting that tachykinin NK3 receptors do not mediate its relatively minor effect on the uterus of the oestrogen-primed rat.

Identification of residues involved in ligand binding to the neurokinin-2 receptor

Several residues of the human neurokinin-2 receptor have been identified to be critical for the binding of peptide agonists and non-peptide antagonists. Amino acid substitutions in the first and second extracellular segments and the second transmembrane segment led to substantial reduction in peptide affinity without affecting the affinity of antagonist SR48968. These effects are identical to those observed for homologous residues in the neurokinin-1 receptor, suggesting that these three regions are involved in high-affinity peptide binding to both receptor subtypes. On the other hand, some conserved residues in the fourth to seventh transmembrane segments are required for peptide binding to only one receptor subtype but not both. The conserved nature and location of these receptor residues suggest that the distance between bound peptide and helices 4-7 varies depending on the receptor subtype. It is likely that the conformational compatibility between a ligand and a given receptor determines the magnitude of binding affinity, and thus receptor subtype selectivity. While many single-residue substitutions did not affect the binding affinity of the antagonist SR48968, two double mutants in the sixth and seventh transmembrane segments were found to reduce its affinity substantially. Therefore, receptor residues participate cooperatively in the binding of SR48968. These results demonstrate the usefulness of combining single-residue substitutions in studying and confirming the role of receptor residues in ligand binding. Finally, the overlapping nature of agonist and antagonist binding sites is consistent with the observation that substitutions of some residues modify the binding affinities of both peptide agonists and non-peptide antagonists.

In vitro characterization of the non-peptide tachykinin NK1 and NK2-receptor antagonists, SR140333 and SR48968 in different rat and guinea-pig intestinal segments

We investigated the potent non-peptide tachykinin receptor antagonists SR140333 and SR48968 for their ability to prevent the contraction of isolated intestinal tissues elicited by the non-selective agonists substance P (SP) and neurokinin A (NKA), or by [Sar9,Met(O2)11]SP and [beta-Ala8]NKA-(4-10) that are selective agonists for NK1 and NK2 receptors, respectively. In guinea-pig ileum, containing mainly NK1-receptors: SR140333 caused a pseudo-irreversible blockade of contractions induced by either SP (KB, 0.01 nM) or [Sar9,Met(O2)11]SP (KB, 0.03 nM); SR140333 but not SR48968, dose-dependently (IC50, 0.06 nM) antagonized the contractions elicited by capsaicin. In rat duodenum, containing mainly NK2 receptors, SR48968 caused a parallel rightward shift of the concentration-response curves of [beta-Ala8]NKA-(4-10) (pA2, 9.5), but not of NKA. In rat esophageal tunica muscularis mucosae, SR48968 non-competitively antagonized [beta-Ala8]NKA-(4-10) and NKA. SR48968 and SR140333 thus appear to be potent tachykinin receptor antagonists, selective for intestinal receptors respectively of the NK2 and NK1 type. The results also suggest that rat esophagus might contain a NK2-receptor subtype different from that of rat duodenum.

SR 48968 selectively prevents faecal excretion following activation of tachykinin NK2 receptors in rats

We tested the ability of SR 48968, (S)-N-methyl-N(4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichloropheny l)butyl ) benzamide, a non-peptide antagonist highly selective for tachykinin NK2 receptors, to prevent defecation induced in rats by several agents. The tachykinin agonists substance P, [MePhe7]neurokinin B and [beta-Ala8]neurokinin A (4-10) all promoted defecation and increased faecal water content, the last compound being over ten times more potent than the other two (intraperitoneal dose inducing the excretion of 1 g faeces dry weight = 6.7 micrograms kg-1). SR 48968 given either orally (p.o.) or subcutaneously (s.c.) was similarly potent in dose-dependently inhibiting faecal output stimulated by the selective NK2-agonist [beta-Ala8]neurokinin A (4-10) (doses causing 50% inhibition 0.4 microgram kg-1, p.o. and 0.3 microgram kg-1, s.c.). This inhibition was long-lasting (more than 18 h after 1 microgram kg-1 SR 48968 either s.c. or p.o.). At the higher doses tested, SR 48968 also significantly prevented the increase in faecal water content produced by [beta-Ala8]neurokinin A (4-10). In rats treated with SR 48968, stimulation of faecal output by the alpha 2-adrenergic antagonist idazoxan and by salmonella endotoxin (LPS), but not by the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin, 5-HT, carbachol or platelet-activating factor, was partially prevented. The present results suggest that activation of intestinal NK2 receptors, either directly by the selective agonist [beta-Ala8]neurokinin A (4-10) or indirectly through the release of endogenous neurokinin A (by idazoxan or LPS), promotes defecation, presumably as a consequence of increased gut motility or secretion, or both. SR 48968 should therefore be useful for studying the role of neurokinin A-dependent mechanisms in health and disease, including those of the gastrointestinal system, and possibly for developing new therapeutic agents.

Depression of primary afferent-evoked responses by GR71251 in the isolated spinal cord of the neonatal rat

1. The pharmacological profile of GR71251, a new tachykinin receptor antagonist, and its effect on the responses evoked by stimulation of primary afferent fibres were studied in isolated spinal cord preparations of neonatal rats. Potential changes were recorded extracellularly from a lumbar ventral root (L3-L5). 2. Bath-application of substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) at 0.01-3 microM to the spinal cord induced depolarization of the ventral root in normal artificial cerebrospinal fluid (CSF). The NK1 agonist, acetyl-Arg6-septide, and the NK3 agonist, senktide, at 0.01-3 microM, also had potent depolarizing actions whereas two NK2 agonists, beta-Ala8NKA4-10 and Nle10NKA4-10, showed little depolarizing effects at 1 microM. 3. GR71251 (0.3-3 microM) caused a rightward shift of the concentration-response curves for SP, acetyl-Arg6-septide and NKA with pA2 values of 6.14, 6.75 or 6.70, respectively. The effects of GR71251 were readily reversible within 15-30 min after its removal. By contrast, GR71251 (1-5 microM) had little effect on the depolarizing responses to NKB and senktide. 4. GR71251 (1-3 microM) did not depress the depolarizing responses to bombesin, neuromedin B and gastrin-releasing peptide in normal artificial CSF. 5. Application of capsaicin to the spinal cord induced a depolarizing response, which was partially depressed by GR71251 (3-10 microM). 6. In the isolated spinal cord preparation, intense electrical stimulation of a dorsal root evoked a slow depolarizing response of the contralateral ventral root of the same segment. A similar slow ventral root depolarization was evoked by electrical stimulation of the ipsilateral saphenous nerve in an isolated spinal cord-saphenous nerve preparation. GR71251 (0.3-10 microM) dose-dependently depressed these slow ventral root potentials.7. In the spinal cord-peripheral nerve preparation, conditioning stimulation of the saphenous nerve evoked an inhibition of the muscle nerve-evoked monosynaptic reflex lasting about 20 s. The late part of the inhibition was markedly depressed by GR71251 (1-3 microM).8. The present results indicate that GR71251 is a potent and specific antagonist for tachykinin receptors in the spinal cord. The present study further provides evidence for the involvement of SP and NKA in the slow ventral root depolarization and the prolonged inhibition of monosynaptic reflex that are evoked by primary afferent stimulation.

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.

Effects of the tripeptide substance P antagonist, FR113680, on airway constriction and airway edema induced by neurokinins in guinea-pigs

FR113680 is a newly developed tripeptide substance P (SP) receptor antagonist. The effects of FR113680 on airway constriction and airway edema induced by neurokinins were investigated in guinea-pigs. In in vitro experiments, FR113680 inhibited the contraction of isolated guinea-pig trachea induced by SP and neurokinin A (NKA) in a dose-dependent manner with IC50 values of 2.3 x 10(-6) and 1.5 x 10(-5) M, respectively. The tracheal contraction induced by histamine and acetylcholine was not affected by FR113680. FR113680 (5 x 10(-5) M) also significantly inhibited the atropine-resistant contraction of isolated guinea-pig bronchi induced by electrical field stimulation. In in vivo experiments, FR113680 given i.v. inhibited SP-induced airway constriction in guinea-pigs at doses of 1 and 10 mg kg-1. However, FR113680 only inhibited NKA- and capsaicin-induced airway constriction by 40-50% even at a dose of 10 mg kg-1. FR113680 also inhibited SP-induced airway edema in guinea-pigs with the same potency as it inhibited SP-induced airway constriction. Histamine-induced airway constriction and airway edema were not affected at a dose of 10 mg kg-1. These results suggest that FR113680 preferentially inhibits responses induced by NK1 receptor activation (SP-induced airway constriction and airway edema), but is less effective on a NK2 receptor-induced response (airway constriction by NKA and neurogenic stimulation).

A potent and selective non-peptide antagonist of the neurokinin A (NK2) receptor

SR 48968 is a potent and selective non-peptide antagonist of the neurokinin A (NK2) receptor. SR 48968 selectively inhibited neurokinin A binding to its receptor and was a competitive antagonist of neurokinin A-mediated contraction of different isolated smooth muscle preparations from various species including human. In vivo, the compound inhibited the bronchoconstriction induced by neurokinin A in guinea pigs. SR 48968 can be used to study the physiological or pathological role of neurokinin A and may be useful in the treatment of neurokinin A-dependent pathology.

Contrasting effects of tachykinins and guanethidine on the acetylcholine output stimulated by nicotine from guinea-pig bladder [corrected]

1. Contractile responses and acetylcholine release evoked by nicotine in guinea-pig detrusor strips were determined by isotonic transducer and radioimmunoassay, respectively. Nicotine stimulated acetylcholine release and a contractile response in guinea-pig detrusor strips treated with the cholinesterase inhibitor, methanesulphonyl fluoride (MSF). Both actions evoked by nicotine were antagonized by the nicotinic receptor antagonist, hexamethonium but were insensitive to tetrodotoxin. 2. A sympathetic nerve blocker, guanethidine and a tachykinin antagonist, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-substance P (rpwwL-SP) partially inhibited the acetylcholine release evoked by nicotine to much the same degree. The inhibitory effects of guanethidine and rpwwL-SP on acetylcholine release were significantly greater than corresponding effects on the contraction evoked by nicotine. 3. In preparations treated with rpwwL-SP to block the tachykinin receptors, guanethidine had no effect on the response to nicotine. Conversely, after treatment with guanethidine to block release of a mediator from sympathetic nerve endings, nicotine-induced responses were not affected by rpwwL-SP. 4. Nicotine-induced contraction was reduced to 30% by the muscarinic cholinoceptor antagonist, atropine and completely abolished after desensitization of P2-purinoceptors with alpha,beta-methylene ATP in the presence of atropine. 5. A concentration-contractile response curve to neurokinin A (NKA) was shifted to the left after cholinesterase inhibition with MSF. Atropine abolished the facilitatory effect of MSF and partially inhibited contractions induced by NKA at 100 nM to 1 microM. The contractile responses to substance P methyl ester (SPOMe) and Tyr0-neurokinin B (Tyr0-NKB) were not influenced by MSF or atropine. 6. After desensitization of NK, tachykinin receptors with SPOMe or preincubation with senktide, the cholinergic component of the nicotine-induced contraction was the same as the control value (100%). 7. Our findings give further support to our previous results: nicotine stimulates acetylcholine release in a tetrodotoxin-resistant manner in guinea-pig bladder and acetylcholine release evoked by nicotine is increased by the coordinated action of sympathetic nerves and tachykinin(s). It is suggested that the tachykinin receptor subtype involved in acetylcholine release is NK,.

Pharmacology of neurokinin receptors

The neurokinins are a group of naturally occurring peptides with the common C-terminal sequence Phe-X-Gly-Leu-Met.NH2. They include substance P (SP), neurokinin A (NKA), and neurokinin B (NKB). SP and NKA are coded on the same gene, the PPT-A, while NKB is coded on a separate gene, the PPT-B. Neurokinins are present in the central nervous system and in peripheral organs where they exert various actions. They act on three receptors--NK-1, NK-2, and NK-3--characterized through pharmacological, biochemical, and histochemical studies. Selective agonists for each neurokinin receptor were developed and evaluated on isolated smooth muscle preparations containing only one neurokinin receptor type. All three neurokinin receptors were cloned and expressed in Xenopus oocytes. Relative affinities of those receptors to neurokinins are the same as in their respective smooth muscle preparation. Finally, the mechanism of action of SP on histamine release from rat peritoneal mast cell has been studied and a direct activation of G proteins by peptides with basic amino acids is proposed as a working hypothesis.