Neuronal NK3-receptors in guinea-pig ileum and taenia caeci: in vitro characterization by their first non-peptide antagonist, SR142801

High Throughput 96-Well Plate Assay for Receptor-Mediated Phosphatidylinositol Turnover

The G-protein coupled receptor family represents a large number of neurotransmitter receptors. Among the diverse signal transduction pathways mediated via G-proteins, phospholipase C mediated phosphatidylinositol hydrolysis represents one of the best characterized signal transduction mechanisms. Accordingly, the measurement of agonist-induced phosphatidylinositol turnover has been used as a convenient functional assay for receptor activation. Assays currently used for this purpose, however, are not suitable for high throughput screening. In this article, an improved technique using 96-well microtiter plate format for measuring phosphatidylinositol turnover is introduced. Anion exchange columns were prepared on fiber glass 96-well multiscreen filter plate. Separation and detection of released inositol phosphates were conducted in a 96-well format. Cells expressing certain neurotransmitter receptors were challenged with agonists and the receptor-mediated PI turnover was measured by the new technique and the results obtained were compared to that obtained from traditional assays. The results indicate that the 96-well assay is 10 to 20 times more efficient than the traditional method and is, furthermore, suitable for high throughput drug screening. Our data also indicate that this method is particularly useful for characterizing multiple antagonists by Schild analysis.

Electrical stimulation of the mucosa evokes slow EPSPs mediated by NK1 tachykinin receptors and by P2Y1 purinoceptors in different myenteric neurons

Slow excitatory postsynaptic potentials (EPSPs) in enteric neurons arise from diverse sources, but which neurotransmitters mediate specific types of slow EPSPs is unclear. We investigated transmitters and receptors mediating slow EPSPs in myenteric neurons evoked by electrical stimulation of the mucosa in guinea pig small intestine. Segments of ileum or jejunum were dissected to allow access to the myenteric plexus adjacent to intact mucosa, in vitro. AH and S neurons were impaled with conventional intracellular electrodes. Trains of stimuli delivered to the mucosa evoked slow EPSPs in AH neurons that were blocked or depressed by the neurokinin-1 (NK1) tachykinin antagonist SR140333 (100 nM) in 10 of 11 neurons; the NK3 tachykinin receptor antagonist SR142801 (100 nM) had no effect on slow EPSPs in seven of nine AH neurons. Single pulses to the mucosa evoked fast EPSPs and slow depolarizations in S neurons. The depolarizations were divided into intermediate (durations 300-900 ms) or slow (durations 1.3-9 s) EPSPs. The slow EPSPs were blocked by pyridoxal phosphate-6-axophenyl-2-4-disulfonic acid (30 microM, N = 3) or the specific P2Y(1) antagonist MRS 2179 (10 microM, N = 6) and were predominantly in anally projecting S neurons that were immunoreactive for nitric oxide synthase (NOS). In contrast, intermediate EPSPs were predominantly evoked in NOS-negative neurons; these were abolished by MRS 2179 (N = 8). Thus activation of pathways running from the mucosa excites three different types of slow EPSP in myenteric neurons, which are mediated by either a tachykinin (NK1, AH neurons) or a purine nucleotide (P2Y(1), S neurons).

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.

Participation of GABAA receptors in the modulation of experimental anxiety by tachykinin agonists and antagonists in mice

Mice were acutely intraperitoneally treated with diazepam (DZP), pentylenetetrazol (PTZ) or NaCl 0.9% (control group), and 15 min later, the DZP-treated group received substance P (SP), neurokinin A (NKA; NK1 and NK2 natural preferential agonists), [Trp7 beta-Ala8] NKA(4-10) (Trp-7; NK3 antagonist) or vehicle intracerebroventricularly, whereas the PTZ-treated group was intracerebroventricularly administered with FK 888, SR 48968 (NK1 and NK2 antagonists, respectively) or senktide (SENK--[succinil-Asp6, MePhe8] substance P(6-11); NK3 agonist), or vehicle immediately before they were submitted to the elevated plus-maze (EPM) test. Another group of animals was repeatedly treated with increasing doses of DZP or NaCl 0.9% intraperitoneally for 28 days, and 3 days after the last injection (test day), animals received DZP, FK 888, SR 48968, SENK or vehicle intracerebroventricularly, or DZP (NaCl 0.9%) intraperitoneally before the EPM evaluation. The anxiolytic action of the acute treatment with DZP was inhibited by the central administration of NKA and Trp-7 but not by SP. NK1 and NK2 antagonists, but not NK3 agonist, blocked the anxiogenic action of PTZ, as evaluated in the plus-maze test. Flumazenil (FLM), a benzodiazepine antagonist, was not able to inhibit the anxiolytic profile of action induced by the NK2 antagonist. Central administration of FK 888 and SR 48968 promoted anxiolytic effects in both control and DZP-withdrawn animals, suggesting a clear relationship between the GABAergic and the tachykinergic systems, mostly involving NK1 and NK2 receptors, in the modulation of experimental anxiety in mice.

Peripheral activity of a new NK1 receptor antagonist TAK-637 in the gastrointestinal tract

Pathways controlling gastrointestinal function involve the activation of neurokinin NK1 receptors by substance P (SP) under normal and pathological conditions. Our aim was to pharmacologically characterize the effect of a nonpeptide NK1 receptor antagonist TAK-637 [(aR,9R)-7-[3,5-bis(trifluoromethyl)benzyl]-8,9,10,11-tetrahydro-9-methyl-5-(4-methylphenyl)-7H-[1,4]diazocino[2,1-g] [1,7]naphthyridine-6,13-dione] and determine key mechanisms of TAK-637 action in the gastrointestinal tract. Experiments were performed using intestinal preparations isolated from the guinea pig. The selective agonists of NK1 receptors, [Sar9,Met(O2)11]-SP and GR 73632 [H2N-(CH2)4-CO-Phe-Phe-Pro-NMe-Leu-Met-NH2], induced contractions in colonic longitudinal muscle pretreated with atropine. TAK-637 (1-100 nM) caused a rightward shift of the concentration-response curves showing nanomolar affinity against [Sar9,Met(O2)11]-SP (Kb = 4.7 nM) and GR 73632 (K(b) = 1.8 nM). This antagonist effect remained unchanged by tetrodotoxin. Furthermore, neither the contractions of colonic circular muscle induced by selective activation of NK2 receptors by GR 64349 (Lys-Asp-Ser-Phe-Val-Gly-R-gamma-lactam-Leu-Met-NH2) nor the responses of taenia coli induced by the selective NK3 receptor agonist senktide were affected by TAK-637 (100 nM). Studies of electrically induced neurogenic contractions showed that TAK-637 had no effect on cholinergic responses to single-pulse (0.5 ms) stimulation or stimulation with increasing frequency (1-16 Hz, 0.5 ms, 5-s train duration). In contrast, TAK-637 significantly reduced nonadrenergic, noncholinergic contractions of colonic longitudinal muscle evoked at frequencies of 8 to 16 Hz and prevented the development of capsaicin-induced contractions in isolated segments of terminal ileum. Our results indicate that TAK-637 is a selective antagonist of smooth muscle NK(1) receptors that activate intestinal muscle contraction. Additionally TAK-637 inhibits neuronal NK1 receptors involved in the "local" motor response to stimulation of capsaicin-sensitive primary afferents.

Tachykinin NK(3) receptor agonists induced microvascular leakage hypersensitivity in the guinea-pig airways

Microvascular leakage hypersensitivity is a main component of neurogenic inflammation and of tachykinin effects. The aim of this study was to examine the ability of neurokinin B and of the tachykinin NK(3) receptor agonists, [MePhe(7)]neurokinin B or senktide, to potentiate when given by aerosol the microvascular leakage induced by histamine in guinea-pig airways and to compare their effects to those of tachykinin NK(1) (substance P, [Sar(9),Met(O(2))(11)]substance P) or tachykinin NK(2) (neurokinin A, [betaAla(8)]neurokinin A (4-10)) receptor agonists. Guinea-pigs were pretreated successively for 10 min with aerolized salbutamol and phosphoramidon; 15 min later, they were exposed for 30 min to an aerosolized solution of tachykinin receptor agonists; 24 h later, the animals were anaesthetized and vascular permeability was quantified by extravasation of Evans blue dye. Neurokinin B, [MePhe(7)]neurokinin B and senktide (3 x 10(-6)-3 x 10(-5)M) induced a potentiation of the effects of histamine on the vascular permeability in the trachea and main bronchi. Compared to other tachykinin NK(1) and NK(2) receptor agonists, the order of potency was: senktide>neurokinin B=[Sar(9),Met(O(2))(11)]substance P=[betaAla(8)]neurokinin A (4-10)=[MePhe(7)]neurokinin B>neurokinin A>substance P. The potentiation by [MePhe(7)]neurokinin B of histamine-induced microvascular leakage was abolished by the tachykinin NK(1) receptor antagonist SR140333 ([(S)1-(2-[3-(3,4-dichlorophenyl)-1-(3-iso-propoxyphenylacetyl)piperidin-3-yl]etyl)-4-phenyl-1-azoniabicyclo[2.2.2]octane, chloride]) or the tachykinin NK(3) receptor antagonists SR 142801 ([(R)-(N)-(1-(3-(l-benzoyl-3-(3,4-dichlorophenyl)piperidin-3-yl) propyl)-4-phenylpiperidin-4-yl)-N-methylacetamide]) and SB 223412 ([(S)-(-)-N-(alpha-ethylbenzyl)-3-hydroxy-2-phenylquinoline-4-carboxamide]). In conclusion, these results suggest that tachykinin NK(3) receptors might be involved in the potentiation of histamine-induced increase in microvascular permeability.

Intestinal motor stimulation by the 5-HT4 receptor agonist ML10302: differential involvement of tachykininergic pathways in the canine small bowel and colon

5-Hydroxytryptamine (5-HT)4 receptor agonists stimulate gut motility through cholinergic pathways, although there are data suggesting that noncholinergic (tachykininergic) excitatory pathways may also be involved. Differences may exist between the small bowel and colon. Our aims were: (i) to compare the prokinetic effect exerted by the 5-HT4 receptor agonist ML10302 in the canine small bowel and colon in vivo; and (ii) to investigate the role of tachykininergic pathways in mediating this response. In fasting, conscious dogs, chronically fitted with electrodes and strain-gauge force transducers along the small bowel and colon, intravenous injection of ML10302 (35 microg kg-1) immediately stimulated spike activity and significantly increased propagated myoelectrical events at both intestinal levels. In the small bowel, the effects of ML10302 were unchanged by previous administration of the selective NK1 tachykinin receptor antagonist SR140333, the NK2 tachykinin receptor antagonist SR48968, or the NK3 tachykinin receptor antagonist SR142801. In the colon, all tachykinin receptor antagonists significantly inhibited stimulation of spike and mechanical activity by ML10302, without affecting ML10302-induced propagated myoelectrical events. Atropine (100 microg kg-1 i.v.) suppressed the stimulatory effect of ML10302 at both intestinal levels. In conclusion, the 5-HT4 receptor agonist ML10302 induced significant prokinesia both in the small bowel and colon through activation of cholinergic pathways. Tachykininergic pathways are not involved in the ML10302-induced prokinesia in the small bowel, but they play an important role in mediating the colonic motor response to ML10302.

Idiopathic chronic constipation: tachykinins as cotransmitters in colonic contraction

BACKGROUND

Tachykinins (TKs) have been shown to be involved in the excitatory enteric motor pathway. This study aimed to examine the direct and nerve-mediated effect of specific NK1, NK2 and NK3 receptor agonists and antagonists in colonic preparations from control subjects and patients with idiopathic chronic constipation (ICC).

MATERIALS AND METHODS

Cumulative concentrations of Sar9Met(O2)11 substance P (selective NK1 receptor agonist), [Ala5,beta-Ala8]-neurokinin A (4-10) (selective NK2 receptor agonist) and [MePhe7]-neurokinin B (selective NK3 receptor agonist) were tested on colonic circular muscle strips to evaluate the direct drug effects. In addition, in the presence of atropine, the role of TKs in the off-contraction that follows the typical inhibitory response evoked by low frequencies of electrical field stimulation (0.5--10 Hz, 20 V, 1 ms pulse trains lasting 1 min) was investigated.

RESULTS

In control preparations, the rank order of potency was: NK2 receptor-selective agonist > NK3 receptor-selective agonist > NK1 receptor-selective agonist. The off-contraction was found to be reduced by about 30--40% in colonic circular muscle from ICC patients with respect to controls. Incubation with the NK1 receptor agonist did not modify the off-contraction measurements in either control or ICC preparations. Conversely, both NK2 and NK3 receptor agonists significantly (P < 0.01) increased the off-contraction in ICC preparations only. This increased response was fully antagonized by MEN-10627, a NK2 and NK3 receptor antagonist depending on the dose.

CONCLUSIONS

We may conclude that ICC is hyporesponsive to TKs and that the contractile reflex to distension is greatly reduced in ICC disease, but can be restored by incubation with NK2 and NK3 receptor agonists.

Neurokinin B- and specific tachykinin NK(3) receptor agonists-induced airway hyperresponsiveness in the guinea-pig

1. The aim of this study was to determine whether neurokinin B (NKB) or specific agonists of tachykinin NK(3) receptors, [MePhe(7)]NKB and senktide, were able to induce airway hyperresponsiveness in guinea-pigs. The effects of these compounds were compared to those of substance P (SP), neurokinin A (NKA) and the preferential tachykinin NK(1) ([Sar(9), Met(0(2))(11)]SP) or NK(2) ([betaAla(8)]NKA (4-10)) receptor agonists. 2. In guinea-pigs pretreated with phosphoramidon (10(-4) M aerosol for 10 min) and salbutamol (8.7x10(-3) M for 10 min), all tachykinins administrated by aerosol (3x10(-7) to 10(-4) M) induced airway hyperresponsiveness 24 h later, displayed by an exaggerated response to the bronchoconstrictor effect of acetylcholine (i.v.). The rank order of potency was: [betaAla(8)]NKA (4-10)>NKA=NKB=senktide=[MePhe(7)]NKB=[Sar(9),Met(0(2))(11)]SP>SP. 3. Airway hyperresponsiveness induced by [MePhe(7)]NKB was prevented by the tachykinin NK(3) (SR 142801) and NK(2) (SR 48968) receptor antagonists. 4. Bronchoconstriction induced by tachykinins administered by aerosol was also determined. SP, NKA, NKB and the tachykinin NK(1) and NK(2) receptor agonist induced bronchoconstriction. The rank order of potency was: NKA=[betaAla(8)]NKA (4-10)>NKB=SP=[Sar(9), Met(0(2))(11)]SP. Under similar conditions, and for concentrations which induce airway hyperresponsiveness, senktide and [MePhe(7)]NKB failed to induce bronchoconstriction. 5. It is concluded that tachykinin NK(3)-receptor stimulation can induce airway hyperresponsiveness and that this effect is not related to the ability of tachykinins to induce bronchoconstriction.

Pharmacologic characterization of receptor types mediating coronary vasodilator actions of sensory neuropeptides in the guinea pig

The receptor types mediating sensory neuropeptide-induced coronary vasodilatation were elucidated on isolated guinea pig hearts perfused with isotonic buffer containing 20 mM KCl. Substance P and the selective neurokinin-1 (NK1) receptor agonist [Sar9, Met(O2)11]-substance P produced dose-dependent reductions in perfusion pressure, but the selective NK2 receptor agonist [Nle10]-neurokinin A4-10 and the selective NK3 receptor agonist [MePhe7]-neurokinin B produced no change. The vasorelaxant effects of substance P and the NK1 receptor agonist were abolished by the selective NK1 receptor antagonist FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-prolyl]-N-methy l-N-phenylmethyl-3-(2-naphthyl)-L-alaninamide), whereas the selective NK2 receptor antagonist SR48968 ((S)-N-methyl-N-[4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl )-butyl] benzamide) and the selective NK3 receptor antagonist SR142801 ((S)-(N)-( 1-(3-(1-benzoyl-3-(3,4-dichlorophenyl)piperidin-3-yl)propyl)4-p henylpiperidin-4-yl)-N-methylacetamide) produced partial inhibition on their responses. Calcitonin gene-related peptide (CGRP) produced dose-dependent vasodilatation on the guinea pig coronary blood vessels, which was significantly (p = 0.0067) inhibited by the selective CGRP1 receptor antagonist hCGRP8-37. The selective CGRP2 receptor agonist [Cys(acetomethoxy)2,7]CGRP had no effect on perfusion pressure. These results demonstrate that the sensory neuropeptides substance P and CGRP are effective vasodilators of the guinea pig coronary vascular bed. The receptor types mediating their vasorelaxant effects were identified to be the NK1 receptors and CGRP1 receptors, respectively.

Differences in circular muscle contraction and peristaltic motor inhibition caused by tachykinin NK1 receptor agonists in the guinea-pig small intestine

The tachykinin NK1 receptor agonist substance P methyl ester (SPOME) impedes intestinal peristalsis by releasing nitric oxide (NO) from inhibitory motor neurones. Since NK1 receptor agonists differ in their receptor interaction, we set out to compare a range of NK1 receptor agonists including SPOME, septide and GR-73 632 in their effects on propulsive peristalsis and circular muscle activity in the guinea-pig isolated small intestine. SPOME (100-300 nM) inhibited peristalsis by a rise of the pressure threshold at which peristaltic waves were triggered, whereas septide and GR-73 632 (30-300 nM) interrupted peristalsis by causing circular muscle spasms. Separate experiments showed that all three NK1 receptor agonists caused contraction of the circular muscle, which was enhanced by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (300 mM) and the P2X purinoceptor antagonist suramin (300 mM). In contrast, tetrodotoxin (300 nM) augmented the contractile effect of septide and GR-73 632 but not that of SPOME. It is concluded that the motor response to NK1 receptor agonists involves release of NO and adenosine triphosphate from inhibitory motor neurones. However, the NK1 receptor agonists differ in the mechanism by which they cause inhibitory transmitter release, which corresponds to differences in their antiperistaltic action.

Antisecretory and relaxatory effects of tachykinin antagonists in the guinea-pig intestinal tract

Existing models used to study the mechanism of action and antagonism of tachykinergic effects on intestinal contraction and secretion suffer from technical problems and have not been fully characterized using specific tachykinin antagonists. Contraction of ileal segments by substance P, colonic circular muscle by beta-alanine-neurokinin A, and longitudinal muscle by senktide were used as models for neurokinin-induced contraction in the guinea-pig. Guinea-pig colonic epithelial tissue was stimulated by substance P and senktide to assess NK1- and NK3-mediated secretion. Using these models the potency of therapeutically useful compounds was determined. NK1 and NK2 activation directly contracted smooth muscle, while NK1-mediated secretion was nerve-mediated. NK3 stimulation of contraction and secretion was neurally mediated, involving cholinergic nerves and 5-HT release. NK1-mediated contraction and secretion were antagonized by SR140333 (pD'2 = 9.29 and pKb = 8.53); NK2-mediated contraction was antagonised by SR48968 (pD'2 = 8.35) and NK3-mediated contraction and secretion were antagonized by SB223412 (pKb = 8.97 and 8.79). The mixed antagonist MDL103392 blocked NK1- and NK2-mediated contraction with pKb values of 7.92 and 6.71 respectively and NK1-mediated secretion with a pKb value of 6.57. This data characterizes existing tachykinin antagonists, and should orientate the development of improved compounds as therapies for intestinal disease.

Connections between P2 purinoceptors and capsaicin-sensitive afferents in the intestine and other tissues

Relations between P2 purinoceptors and capsaicin-sensitive sensory neurons include an excitatory action of P2 purinoceptor agonists on spinal afferent neurons, as well as release of ATP from afferents at their central and peripheral endings, and a possible participation of ATP in nociception and/or in 'local efferent' responses mediated by sensory nerves at the periphery. The present paper briefly summarizes available evidence on these interrelations. Ample evidence shows that ATP and other P2 purinoceptor agonists can activate primary afferent neurons, through P2X3 receptors and probably other purinoceptors as well, but evidence for an involvement of P2 purinoceptors in nociception or in 'local efferent' responses due to activation of primary afferents is, at best, circumstantial. The possibility is also dealt with that P2 purinoceptor activation may cause small intestinal contraction with the mediation of capsaicin-sensitive sensory neurons and that the motor response to capsaicin in this tissue may involve the release of a P2 purinoceptor stimulant from sensory nerves. Our data show that cholinergic contractions of the guinea-pig ileum in response to the P2 purinoceptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) are blocked by atropine, but not by in vitro capsaicin pretreatment (which completely blocks the contractile action of capsaicin). Cholinergic ileum contractions due to capsaicin (2 microM) are insensitive to suramin (a P2 purinoceptor antagonist; 100 microM). In the presence of antagonists acting at tachykinin NK1 and NK2 receptors, however, suramin (100 microM) causes a significant inhibition of the capsaicin-evoked contraction. These data indicate that capsaicin-sensitive nerves are not involved in the excitatory effect of alpha,beta-methylene ATP on myenteric neurons. On the other hand, ATP is probably involved in the 'non-tachykininergic' component of the capsaicin-induced excitatory response of the small intestine. ATP may originate from sensory neurons and probably acts as activator of myenteric nerves.

Tachykinin receptors are involved in the "local efferent" motor response to capsaicin in the guinea-pig small intestine and oesophagus

The sensory neuron stimulant drug capsaicin stimulates primary afferent nerve endings in the guinea-pig small intestine, which in turn activate myenteric cholinergic neurons by an unknown mechanism. The tachykinins substance P and neurokinin A are present in primary afferent neurons. This study was performed to assess the possible involvement of endogenous tachykinins acting via neurokinin-1, neurokinin-2 and neurokinin-3 receptors in the contractile effect of capsaicin in the isolated guinea-pig ileum and oesophagus by using the receptor-specific antagonists GR 82334 (3 microM) for neurokinin-1 receptors, MEN 10627 (3 microM; ileum) or MEN 11420 (1 microM; oesophagus) for neurokinin-2 receptors and SR 142801 (0.1 microM) for neurokinin-3 receptors. In the ileum, the peak contraction evoked by capsaicin (2 microM) was not reduced when tachykinin neurokinin-1, neurokinin-2 or neurokinin-3 receptors were blocked separately, whereas an inhibition of neurokinin-3 receptors diminished the area under the curve of the capsaicin response. A combined blockade of neurokinin-1 and neurokinin-3 receptors significantly depressed the effect of capsaicin; the amplitude of the contractile response was 53.3+/-3.7% of the maximal longitudinal spasm in control preparations, whereas in the presence of GR 82334 plus SR 142801 it reached only 27.6+/-5% (P<0.001, Kruskal-Wallis test; n=9 and 10, respectively). Also, the area under the curve of the contractile response to capsaicin was more than 85% lower in the group of preparations treated with GR 82334 plus SR 142801 than in the control group (P<0.001). Including a neurokinin-2 blocker in the combination did not produce any further inhibition. A concomitant tachyphylaxis to substance P (natural neurokinin-1 receptor stimulant) and the neurokinin-3 receptor agonist senktide (5 and 1 microM, respectively) also reduced the contractile effect of capsaicin. In the oesophagus, capsaicin (1 microM) induced biphasic contractions which were strongly inhibited by atropine (1 microM) or capsaicin pretreatment (1 microM for 10 min). Here again, a blockade of tachykinin neurokinin-1, neurokinin-2 or neurokinin-3 receptors separately failed to inhibit the response to capsaicin, whereas a combined blockade of any two tachykinin receptors caused a partial inhibition. The reduction of the contractile effect of capsaicin was strongest when all three tachykinin receptors were blocked. In seven control preparations, peaks for the first and second phases of contraction reached 35.3+/-3.7% and 20+/-3.2% of maximal longitudinal spasm; the corresponding values in the presence of a combination of GR 82334, MEN 11420 and SR 142801 were 7.5+/-0.8% and 9.1+/-2.2%, respectively (n=6, P<0.001 and 0.05, respectively). Tetrodotoxin (0.5 microM) practically abolished the contractile effect of capsaicin in both tissues studied. It is concluded that an interplay of neuronal tachykinin neurokinin-1 and neurokinin-3 receptors (ileum) and neurokinin-1, neurokinin-2 and neurokinin-3 receptors (oesophagus) is involved in the contractile action of capsaicin, probably in mediating excitation of myenteric neurons by tachykinins released from primary afferents. In both tissues, there also seems to be a non-tachykininergic component of the capsaicin-induced contraction.

Mechanism of tachykinin NK3 receptor-mediated colonic ion transport in the guinea pig

The guinea pig colon was used to elucidate the mechanism of tachykinin-induced secretion. Increased short-circuit current was observed in response to natural and synthetic tachykinins with rank orders of potency of substance P > neurokinin A = neuropeptide K>> neuropeptide gamma; and senktide (tachykinin NK3 receptor agonist)> Sar-substance P (tachykinin NK1 receptor agonist)> betaAlaneurokinin A (tachykinin NK2 receptor agonist)). A functional role of tachykinin NK1 receptors was confirmed as substance P and neurokinin A responsiveness was blocked by the tachykinin NK1 receptor antagonist GR82334. The tachykinin NK3 receptor antagonist SB222200 had no effect, leaving in doubt the identity of the natural tachykinin NK3 receptor ligand in the colon. The response to tachykinin NK3 receptor activation was abolished by tetrodotoxin and predominantly due to atropine sensitive cholinergic activation. The non-cholinergic component resulted from stimulation of tachykinin NK 1 and 5-HT receptors as the response to senktide was blocked by GR82334 and tropisetron. In conclusion, tachykinin NK3 receptor activation stimulates cholinergic and non-cholinergic (tachykinin NK1-receptor and serotonin-mediated) secretory pathways.

Tachykinin-induced activation of non-specific cation conductance via NK3 neurokinin receptors in guinea-pig intracardiac neurones

1. Whole mount preparations from guinea-pig hearts were used to characterize the receptors and ionic mechanisms mediating the substance P (SP)-induced depolarization of parasympathetic postganglionic neurones of the cardiac ganglion. 2. Measurement of the amplitude of depolarization in response to superfusion of different tachykinin agonists (neurokinins A (NKA) and B (NKB), SP, and senktide) gave a rank-order potency of NKB = senktide > NKA > SP, indicating involvement of an NK3 receptor. The use of the selective tachykinin receptor antagonists SR 140333, SR 48986, and SR 142801 demonstrated that only the NK3 receptor antagonist SR 142801 inhibited the SP-induced depolarization. 3. The SP-induced depolarization was not inhibited by Ba2+, TEA, or niflumic acid, or altered by reduced Cl- solutions, but was attenuated in reduced Na+ solutions. Single electrode voltage clamp studies demonstrated that the SP-induced inward current increased in amplitude at more negative potentials, had a reversal potential of approximately 0 mV, and was reduced in amplitude in reduced Na+ solutions. 4. We conclude that the SP-induced depolarization in guinea-pig postganglionic parasympathetic neurones of the cardiac ganglion is due to NK3-mediated activation of a non-selective cation conductance.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

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.