Inhibition of cholinergic neurotransmission in isolated guinea-pig main bronchi by SR 48968

Blockade of tachykinin NK1/NK2 receptors in the brain attenuates the activation of corticotrophin-releasing hormone neurones in the hypothalamic paraventricular nucleus and the sympathoadrenal and pituitary-adrenal responses to formalin-induced pain in the rat

Evidence from pharmacological studies has implicated substance P (SP), a natural ligand of tachykinin NK(1) receptors which can also interact with NK(2) receptors, in the generation of pressor and tachycardic responses to stress. Using selective blockade of brain NK(1) and NK(2) receptors, we tested in conscious rats the hypothesis that SP initiates, within the neuronal brain circuits, the sympathoadrenal, hypothalamic-pituitary-adrenal (HPA) and behavioural responses to noxious stimuli. Formalin injected s.c. through a chronically implanted catheter in the area of the lower leg was used as a pain stimulus. Rats were pretreated i.c.v. with vehicle or the selective, nonpeptide antagonists of tachykinin NK(1) and NK(2) receptors, RP 67580 and SR 48968, respectively. Ten minutes thereafter, formalin was injected s.c. and the cardiovascular responses were recorded, plasma concentrations of catecholamines, adrenocorticotrophic hormone (ACTH) and corticosterone were determined and the expression of the inducible transcription factor c-Fos in the paraventricular (PVN) and supraoptic nuclei was detected to identify neurones which were activated during pain stimulation. Blockade of NK(1) and NK(2) receptors attenuated the formalin-induced increases in mean arterial pressure and heart rate, adrenaline and ACTH concentrations in plasma, and completely abolished the pain-induced c-Fos expression in corticotrophin-releasing hormone neurones localised in the parvocellular division of the PVN. The results obtained provide pharmacological evidence that tachykinins, most probably SP, act as mediators within the neuronal circuits linked to the initiation and control of the cardiovascular, sympathoadrenal, HPA and behavioural responses to pain stimuli and provide an excitatory input to corticotrophin-releasing hormone neurones in the PVN to activate the HPA axis. Our data demonstrating the inhibition of the complex response pattern to noxious stimuli and stress are consistent with the proposed anxiolytic and antidepressant activity of NK(1) and NK(2) receptor antagonists.

Tachykinin NK2 receptor antagonists for the treatment of irritable bowel syndrome

Tachykinin NK2 receptors are expressed in the gastrointestinal tract of both laboratory animals and humans. Experimental data indicate a role for these receptors in the regulation of intestinal motor functions (both excitatory and inhibitory), secretions, inflammation and visceral sensitivity. In particular, NK2 receptor stimulation inhibits intestinal motility by activating sympathetic extrinsic pathways or NANC intramural inhibitory components, whereas a modulatory effect on cholinergic nerves or a direct effect on smooth muscle account for the NK2 receptor-mediated increase in intestinal motility. Accordingly, selective NK2 receptor antagonists can reactivate inhibited motility or decrease inflammation- or stress-associated hypermotility. Intraluminal secretion of water is increased by NK2 receptor agonists via a direct effect on epithelial cells, and this mechanism is active in models of diarrhoea since selective antagonists reverse the increase in faecal water content in these models. Hyperalgesia in response to intraluminal volume signals is possibly mediated through the stimulation of NK2 receptors located on peripheral branches of primary afferent neurones. NK2 receptor antagonists reduce the hyper-responsiveness that occurs following intestinal inflammation or application of stressful stimuli to animals. Likewise, NK2 receptor antagonists reduce intestinal tissue damage induced by chemical irritation of the intestinal wall or lumen. In healthy volunteers, the selective NK2 antagonist nepadutant reduced the motility-stimulating effects and irritable bowel syndrome-like symptoms triggered by intravenous infusion of neurokinin A, and displayed other characteristics that could support its use in patients. It is concluded that blockade of peripheral tachykinin NK2 receptors should be considered as a viable mechanism for decreasing the painful symptoms and altered bowel habits of irritable bowel syndrome patients.

Effect of a tachykinin NK(2) receptor antagonist, nepadutant, on cardiovascular and gastrointestinal function in rats and dogs

The effect of the tachykinin NK(2) receptor antagonist, nepadutant (MEN 11420 or (c[[(beta-D-GlcNAc)Asn-Asp-Trp-Phe-Dpr-Leu]c(2beta-5beta)])) was assessed on cardiovascular function (unanaesthetized rats and anaesthetized dogs) and gastrointestinal motor activity (fasted unanaesthetized dogs). The selective tachykinin NK(2) receptor agonist, [betaAla(8)]neurokinin A (4-10), up to 100 nmol/kg, i.v., did not produce changes on mean blood pressure or heart rate in unanaesthetized rats. Nepadutant did not affect blood pressure and heart rate up to 10 micromol/kg, whereas saredutant (SR 48968 or ((S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl] benzamide), a nonpeptide antagonist, produced a transient reduction of mean blood pressure and heart rate. Nepadutant up to 20 micromol/kg, i.v. neither caused changes of cardiovascular and respiratory parameters in anaesthetized dogs nor induced any changes in left ventricular systolic pressure, left ventricular dP/dt or of electrocardiogram (lead II) waveforms. Intravenous administration of neurokinin A (9 nmol/kg) in unanaesthetized dogs stimulated gastrointestinal motility for 20-25 min. Nepadutant at 0.1 micromol/kg suppressed the stimulant effects of neurokinin A but, up to a dose of 10 micromol/kg, did not produce significant changes in the basal migrating motor complexes. We conclude that tachykinin NK(2) receptors do not participate in the physiologic regulation of resting cardiovascular and respiratory functions and that they do not regulate the fasted pattern of gastrointestinal motility. The cardiovascular changes induced by the nonpeptide tachykinin NK(2) receptor antagonist, saredutant, likely arise from nonspecific effects unrelated to tachykinin NK(2) receptor blockade.

The involvement of tachykinin NK2 and NK3 receptors in central sensitization of a spinal withdrawal reflex in the decerebrated, spinalized rabbit

Repetitive electrical stimulation of the sural nerve at a strength sufficient to excite C-fibres results in prolonged (> 20 min) increases in the reflex responses of gastrocnemius motoneurones evoked by stimulation of myelinated axons in the sural nerve. We have tested the susceptibility of this effect to blockade of tachykinin NK2 and NK1 receptors. In the control state, iterative stimulation of sural nerve C fibres increased gastrocnemius reflexes to a peak of between 200 and 400% of pre-stimulus levels, an effect that recovered to baseline values over 23-30 min. Pre-treatment with selective antagonists for NK2 (SR 48968, 1 mg/kg i.v.) or NK3 (SR 142801, 0.1 and 1 mg/kg i.v.) receptors failed to alter the peak facilitation resulting from sural nerve stimulation. However, both drugs significantly reduced the duration of enhancement of reflexes, to 18 min after SR 48968 and to 5 min after SR 142801. The inactive enantiomers of these compounds (SR 48965 and SR 142806, both 1 mg/kg i.v.) did not reduce facilitation of reflexes after sural nerve stimulation. We conclude that activation of tachykinin NK3 receptors is a major factor in maintaining central sensitization of the sural-gastrocnemius reflex after electrical stimulation of sural nerve C-fibres. The effects of SR 48968 were surprisingly weak and may have resulted from an effect of this compound at NK3 receptors.

Functional characterisation of tachykinin receptors mediating ion transport in porcine jejunum

In the present study, tachykinin receptors (designated NK 1, NK2 and NK3) involved in regulation of ion transport in porcine jejunum were characterised. Stripped tissue preparations were mounted in Ussing chambers and short-circuited. Substance P produced a concentration dependent increase in short-circuit current, the relationship showing a double sigmoidal form. The non-peptide NK1 receptor antagonist, CP 99,994 ((2S,3S)-3-(2-methoxybenzyl)amino-2-phenylpiperidine), caused a dextral shift of the first sigmoidal response, indicating the involvement of an NK1 receptor. This was further supported by a concentration-dependent response of the NK1 receptor agonist [Sar9Met(O2)11]substance P with an EC50 value of 235.0+/-53.9 nM. Increasing concentrations of CP 99,994 (0.1, 0.3 and 1 microM) produced a parallel dextral shift of the [Sar9Met(O2)11]substance P curve with a slope of the Schild regression significantly different from unity (1.59). The neurokinin A concentration-response curve, with an EC50 value of 68.87+/-16.23 nM, was not significantly changed by the non-peptide NK2 receptor antagonist SR 48,968 ((S)-N-methyl-N-(4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophe nyl)butyl)bezamide). In additional studies, the peptide NK2 receptor antagonists, GR 94,800 (PhCO-Ala-Ala-DTrp-Phe-DPro-Pro-NleNH2) and PD 147,714 ((2,3-diOMeZ)-(S)Trp(S)alphaMePheGlyNH2), did not change the response to neurokinin A. However, CP 99,994 totally inhibited neurokinin A responses at 0.5 microM and above. The NK2 receptor agonist, [beta-Ala8]neurokinin A-(4-10), caused only an increase in short-circuit current in microM concentrations, whereas the NK3 receptor agonist, senktide, did not elicit a response. These results indicate, that substance P and neurokinin A mediate ion transport in porcine jejunum through NK1 receptors. However, tachykinins seem to activate another receptor. Two active conformers of the NK1 receptor might be present.

In vitro characterization of tachykinin NK2-receptors modulating motor responses of human colonic muscle strips

1. Human in vitro preparations of transverse or distal colonic circular smooth muscle were potently and dose-dependently contracted by neurokinin A (EC50, 4.9 nM), the tachykinin NK2-receptor selective agonist [beta-Ala8]neurokinin A (4-10) ([beta-Ala8]NKA (4-10)) (EC50, 5.0 nM), neurokinin B (EC50, 5.3 nM) and substance P (EC50, 160 nM), but not by the tachykinin NK1-receptor selective agonist [Sar9Met(O2)11] substance P, or the NK3-receptor selective agonists, senktide and [MePhe7] neurokinin B. No regional differences between transverse and distal colon were observed in response to [beta-Ala8]NKA (4-10). 2. Atropine (1 microM) and tetrodotoxin (1 microM) did not significantly inhibit responses to [beta-Ala8]NKA (4-10), neurokinin A, substance P or neurokinin B. 3. The newly developed non-peptide antagonists for tachykinin NK2-receptors SR 48968, SR 144190 and its N-demethyl (SR 144743) and N,N-demethyl (SR 144782) metabolites, were used to challenge agonist responses, as appropriate. SR 144190 and the metabolites all potently and competitively antagonized the response to [beta-Ala8]NKA (4-10), with similar potency (Schild plot pA2 values 9.4, 9.4 and 9.3, slope = 1). SR 48968 antagonism was not competitive: the Schild plot slope was biphasic with a high (X intercept approximately 9.3) and a low (X intercept 8.4, slope 1.6) affinity site. Co-incubation of SR 48968 (10, 100 nM) and SR 144782 (10 nM) produced additive effects; in this experimental condition, SR 48968 apparent affinity (pKB) was 8.2. In addition, SR 144782 (0.1 microM) antagonized responses to neurokinin A, substance P and neurokinin B, with pKB consistent with its affinity for tachykinin NK2-receptors. The potent and selective NK1 and NK3-receptor antagonists, SR 140333 and SR 142801 (both 0.1 microM), failed to inhibit contractions induced by SP or NKB. 4. In conclusion, the in vitro mechanical responses of circular smooth muscle preparations from human colon are strongly consistent with the presence of non-neuronal tachykinin NK2-receptors, but not tachykinin NK1- or NK3-receptors. Our findings with SR 48968 suggest the existence of two tachykinin NK2-receptor subtypes, that it seems to distinguish, unlike SR 144190 and its metabolites. However, the precise nature of SR 48968 allotopic antagonism remains to be elucidated, since allosteric effects at the tachykinin NK2-receptor might well account for the complexity of the observed interaction.

The role of tachykinin NK1 and NK2 receptors in atropine-resistant colonic propulsion in anaesthetized guinea-pigs

1. The role of endogenous tachykinins on guinea-pig colonic propulsion was investigated by using potent and selective tachykinin NK1 and NK2 receptor antagonists. Colonic propulsion and contractions were determined by means of a balloon-catheter device, inserted into the rectum of guanethidine (68 micromol kg(-1), s.c., 18 and 2 h before)-pretreated, urethane-anaesthetized guinea-pigs. Propulsion of the device (dynamic model) was determined by measuring the length of the catheter expelled during 60 min filling of the balloon (flow rate 5 microl min(-1)). 2. In control conditions the tachykinin NK1 receptor antagonist SR 140333 (1 micromol kg(-1), i.v.) did not affect either colonic propulsion or the amplitude of contractions. The tachykinin NK2 receptor antagonists MEN 10627 and MEN 11420 (1 micromol kg(-1), i.v.) increased colonic propulsion at 10 min (+120% and 150%, respectively) but at 60 min the effect was significant only for MEN 10627 (+84%). SR 48968 (1 micromol kg(-1), i.v.) did not significantly enhance the colonic propulsion. None of these tachykinin NK2 receptor antagonists modified the amplitude of colonic contractions. In contrast, both atropine (6 micromol kg(-1), i.v., plus infusion of 1.8 micromol h(-1)) and hexamethonium (55 micromol kg(-1), i.v., plus infusion of 17 micromol h(-1)) abolished propulsion (81% and 87% inhibition, respectively) and decreased the amplitude of contractions (68% inhibition for either treatment). 3. In atropine-treated animals (6 micromol kg(-1), i.v., plus infusion of 1.8 micromol h(-1)), apamin (30 nmol kg(-1), i.v.) restored colonic propulsion (+416%) and increased the amplitude of contractions (+367% as compared to atropine alone). Hexamethonium (55 micromol kg(-1), i.v., plus infusion of 17 micromol h(-1)) abolished the apamin-induced, atropine-resistant colonic propulsion (97% inhibition) and reduced the amplitude of the atropine-resistant contractions (52% inhibition). 4. The apamin-induced, atropine-resistant colonic propulsion was inhibited by SR 140333 (-69% at 1 micromol kg(-1)), SR 48968 (-78% at 1 micromol kg(-1)), MEN 11420 (-59% at 1 micromol kg(-1)) and MEN 10627 (-50% at 1 micromol kg(-1)), although the latter effect was not statistically significant. The combined administration of SR 140,333 and MEN 10,627 (1 micromol kg(-1) for each antagonist) almost completely abolished colonic propulsion (90% inhibition). The amplitude of colonic contractions was also reduced by SR 140333 (-42%), SR 48968 (-29%), MEN 11420 (-45%) but not by MEN 10627 (-16%). The combined administration of SR 140333 and MEN 10,627 reduced the amplitude of contractions by 47%. SR 140603 (1 micromol kg(-1), i.v.), the less potent enantiomer of SR 140333, was inactive. 5. In control animals, apamin (30 nmol kg(-1), i.v.) enhanced colonic propulsion (+84%) and increased the amplitude of contractions (+68%), as compared to the vehicle. Hexamethonium (55 micromol kg(-1), i.v. plus infusion of 17 micromol h(-1)) inhibited propulsion (86% inhibition) and decreased the amplitude of contractions (49% inhibition). SR 140333, SR 48968, MEN 11420, MEN 10627, or the coadministration of SR 140333 and MEN 10627 had no effect. 6. In a separate series of experiments, the mean amplitude of colonic contractions was also recorded under isovolumetric conditions through the balloon-catheter device kept in place at 75 mm from the anal sphincter (static model). In control conditions, neither SR 140333 nor MEN 11420 modified the amplitude of contractions. In atropine-pretreated guinea-pigs, SR 140333 and MEN 11420 (0.1-1 micromol kg(-1)) dose-dependently decreased the amplitude of contractions. In apamin- and atropine-pretreated animals, only the highest (1 micromol kg(-1)) dose of SR 140333 or MEN 11420 significantly decreased the amplitude of contractions. The inhibitory potency of atropine (0.3-1 micromol kg(-1)) was similar in apamin-pretreated animals and in controls. 7. It was concluded that, in anaesthetized guinea-pigs, endogenous tachykinins, acting through both NK(1) and NK(2) receptors, act as non-cholinergic excitatory neurotransmitters in promoting an apamin-evoked reflex propulsive activity of the distal colon.

Role of tachykinins in castor oil diarrhoea in rats

1. We set out to ascertain the role of tachykinins, neurokinin A and substance P, in castor oil-induced diarrhoea in rats as disclosed by the inhibitory effect of the non-peptide NK1- and NK2-receptor antagonists. SR 140333 and SR 48968, respectively. 2. SR 48968 (0.02 to 20 micrograms kg-1, s.c. or p.o.), and the opioid receptor agonist loperamide (1-10 mg kg-1, p.o.), dose-dependently prevented castor oil effects: % inhibition vs castor oil, diarrhoea 0 to 100, increase in faecal mass 7 to 90 and water content 16 to 90. SR 140333 (0.02 to 20 micrograms kg-1, s.c.) and the platelet activating factor antagonist SR 27417 (5 to 500 micrograms kg-1, p.o.) did not prevent the increase in faecal water content, but reduced faecal mass (35 to 66%) and diarrhoea (0 to 57%). 3. The R-enantiomers of tachykinin NK1 and NK2 receptor antagonists, SR 140603 and SR 48605 (both at 2 or 20 micrograms kg-1, s.c.) had no effect other than reducing faecal mass at the highest dose tested. 4. SR 48968 (20 micrograms kg-1, p.o.) but not loperamide (10 mg kg-1, p.o.) given 24 h before castor oil, still slightly but significantly reduced by 30% the increase of faecal mass output; both treatments significantly reduced (30 to 70%) the effect of castor oil on faecal water content, although the incidence of diarrhoea was only slightly less than in controls. 5. In castor oil-treated rats, naloxone (2 mg kg-1, s.c.) completely blocked the antidiarrhoeal action of loperamide (10 mg kg-1, p.o.) but not of SR 48968 (20 micrograms kg-1, p.o.): a similar result was obtained on faecal mass and water content. 6. Castor oil strongly increased the occurrence of manometrically recorded propulsive giant contractions (500 to 1000% over control values) of transverse and distal colon, this effect being significantly prevented (80 to 100%) by SR 48968 and loperamide and partially by SR 140333 (35% distal colon, 70% transverse colon). 7. In castor oil free rats, loperamide but not SR 48968 or SR 140333 significantly reduced by 50% the gastrointestinal transit of a charcoal test meal, as well as 24 h faecal mass output. Consistently, loperamide, unlike the tachykinin receptor antagonists, had a dramatic effect on manometric recordings of intestinal motility, reducing all kinds of colonic contractions. 8. Our findings suggest that castor oil diarrhoea in rats entails activation of NK1 and NK2 receptors by endogenous tachykinins, whose antagonists may have a potential as antidiarrhoeal agents free from the constipating action of opioids.

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.

Antitussive effect of SR 48968, a non-peptide tachykinin NK2 receptor antagonist

The antitussive effects of SR 48968, a non-peptide tachykinin NK2 receptor antagonist, were investigated on citric acid-induced cough in the unanesthetized guinea-pig and compared with the effects of codeine. SR 48968 (0.01-0.3 mg/kg i.p.) inhibited in a dose-dependent manner the number of coughs induced by inhalation of an aqueous solution of citric acid with an ED50 of 0.1 mg/kg (0.17 mumol/kg). Under similar conditions, the codeine ED50 was 8 mg/kg (27 mumol/kg). Naloxone, an opioid receptor antagonist, abolished the effects of codeine but did not modify the effects of SR 48968. These data suggest that NK2 receptor stimulation might play an important role in the regulation of the cough reflex and that SR 48968 could be a potential antitussive agent.