Evidence for neurokinin-3 receptor-mediated tachykinin release in the guinea-pig ileum

Differential role of tachykinin NK3 receptors on cholinergic excitatory neurotransmission in the mouse stomach and small intestine

BACKGROUND AND PURPOSE

Tachykinin NK(3) receptors are widely expressed in the mouse gastrointestinal tract but their functional role in enteric neuromuscular transmission remains unstudied in this species. We investigated the involvement of NK(3) receptors in cholinergic neurotransmission in the mouse stomach and small intestine.

EXPERIMENTAL APPROACH

Muscle strips of the mouse gastric fundus and ileum were mounted in organ baths for tension recordings. Effects of NK(3) agonists and antagonists were studied on contractions to EFS of enteric nerves and to carbachol.

KEY RESULTS

EFS induced frequency-dependent tetrodotoxin-sensitive contractions, which were abolished by atropine. The cholinergic contractions to EFS in the stomach were enhanced by the NK(3) antagonist SR142801, but not affected by the NK(3) agonist senktide or neurokinin B. The cholinergic contractions to EFS in the small intestine were not affected by SR142801, but dose-dependently inhibited by senktide and neurokinin B. This inhibitory effect was prevented by SR142801 but not by hexamethonium. SR142801, senktide or neurokinin B did not induce any response per se in the stomach and small intestine and did not affect contractions to carbachol.

CONCLUSIONS AND IMPLICATIONS

NK(3) receptors modulate cholinergic neurotransmission differently in the mouse stomach and small intestine. Blockade of NK(3) receptors enhanced cholinergic transmission in the stomach but not in the intestine. Activation of NK(3) receptors inhibited cholinergic transmission in the small intestine but not in the stomach. This indicates a physiological role for NK(3) receptors in mouse stomach contractility and a pathophysiological role in mouse intestinal contractility.

Slow excitatory post-synaptic potentials in myenteric AH neurons of the guinea-pig ileum are reduced by the 5-hydroxytryptamine7 receptor antagonist SB 269970

Serotonin (5-HT) is a key modulator of neuronal excitability in the central and peripheral nervous system. In the enteric nervous system, 5-HT causes a slow depolarization in the intrinsic sensory neurons, but the receptor responsible for this has not been correlated with known gene products. The aim of this study was to determine whether the newly characterized 5-HT7 receptor may participate in the 5-HT-mediated depolarization of, and synaptic transmission to, the intrinsic sensory neurons of the guinea-pig ileum. Intracellular electrophysiological recordings were made from intrinsic sensory neurons identified as myenteric AH neurons from guinea-pig ileum. 5-HT (5 microM) applied to the cell body evoked both a fast depolarization (5-HT3 mediated) and/or a slow depolarization (5-HT1P-like). The 5-HT1/5/7 receptor agonist 5-carboxamidotryptamine (5-CT) (5 microM) evoked only a slow depolarization. When the fast depolarization evoked by 5-HT was blocked with granisetron (1 microM, 5-HT3 receptor antagonist), only a slow depolarization remained; this was abolished by the 5-HT7 receptor antagonist SB 269970 (1 microM, control: 14+/-2 mV, granisetron+SB 269970: -1+/-2 mV). The slow depolarization evoked by 5-CT was also significantly reduced by SB 269970 (control: 14+/-1 mV, SB 269970: 5+/-2 mV) suggesting a 5-HT7 receptor was activated by exogenous application of 5-CT and 5-HT. Slow excitatory postsynaptic potentials evoked by stimulating descending neural pathways (containing serotonergic fibers) were reduced by SB 269970 (control: 8+/-3 mV, SB 269970: 3+/-1 mV). However, SB 269970 had no effect on slow excitatory postsynaptic potentials evoked by stimulation of circumferential (tachykinergic) pathways (control: 7+/-1 mV, SB 269970: 6+/-1 mV). These data are consistent with the presence on enteric AH neurons of functional 5-HT7 receptors that participate in slow synaptic transmission.

Localization of neurokinin B receptor in mouse gastrointestinal tract

AIM: To observe the location of neurokinin receptor (NK3r) in the mouse gastrointestinal tract.

METHODS: The abdomens of 8 male Kunming mice were opened under anaesthesia with sodium pentobarbital. The exposed gut organs were kept moisture and temperature at the same time. Then the esophagus, jejulum, ileum, colon, etc were respectively cut and the segments from the stomach to the distal colon were opened along the mesenteric border. A circular 4 mm-6 mm enteric part(pieces of 1 cm² were to be prepared) and mucosa and submucosa were removed, then the longitudinal muscle layer was pulled off from the circular muscle layer under microphotograph. They were rinsed in 50 nmol·L^-1 potassium phosphate-buffered saline(PBS). Immunohistochemistry and immunoreactive fluorescence were used in the staining procedures.

RESULTS: There was not NK3r-Like(-Li) positive material on the smooth muscle cells of the esophagus, stomach, and intestines and other regions. The nerve cell bodies with immunoreactivity for NK3r were mainly distributed in the submucousal nerve plexus or myenteric nerve plexus of the gastrointestinal tract except for the esophagus, stomach and rectum. The reaction product was located on the surface of the nerve cell plasma. It was occasionally observed in the cell plasma endosomes, but was very weakly stained. Among the NK3-like positive neurons in the plexus,the morphological type in many neurons appeared like Dogiel II type cells. Some neuron cell bodies were big, having many profiles, some were long ones or having grading structure. Cell body diameter was about 10 μm-46 μm and 8 μm-42 μm in myenteric plexus and submucous plexus.

CONCLUSION: This study not only described the distribution of neurokinin B receptor in the mouse gut in detail, but also provided a morphological basis for deducing the functional identity of the NK3r-LI immunoreactivity neurons, suggesting the possibility that these neurons were closely related to gastrointestinal tract contraction and relaxing activity.

NK2 tachykinin receptors mediate contraction of the pig intravesical ureter: tachykinin-induced enhancement of non-adrenergic non-cholinergic excitatory neurotransmission

The current study was designed to characterize the functionally active tachykinin receptors involved in tachykinin-elicited contractions in the pig intravesical ureter, and to investigate the possible modulation exerted by the natural tachykinins substance P (SP) and neurokinin A (NKA) on the non-adrenergic non-cholinergic (NANC) excitatory ureteral neurotransmission. In pig intravesical ureteral strips pretreated with phosphoramidon (10(-5) mol/L) to block the endopeptidase activities, isometric force recordings showed that SP, NKA, and the NK2 receptor selective agonist [beta-Ala(8)]-NKA (4-10), all three induced contractions, with the following potency order: NKA > [beta-Ala(8) ]-NKA (4-10) > SP. [Sar(9), Met(O(2))(11)]-SP and senktide, selective agonists of the NK1 and NK3 receptors, respectively, failed to modify the ureteral tone. Urothelium removal and incubation with tetrodotoxin (10(-6) mol/L), phentolamine (10(-7) mol/L), propranolol (3 x 10(-6) mol/L), atropine (10(-7) mol/L) and indomethacin (3 x 10(-6) mol/L), did not alter the contraction induced by a submaximal (10(-7) mol/L) dose of [beta-Ala(8)]-NKA (4-10). MEN 10,376 (10(-8)-10(-7) mol/L), a NK2 receptor antagonist, reduced the contraction to 3 x 10(-8) mol/L NKA. GR 82334 (10(-6) -10(-5) mol/L) and SR 142801 (10(-8)-10(-7) mol/L), selective antagonists of the NK1 and NK3 receptors, respectively, did not modify that contraction. In pig intravesical ureteral strips in NANC conditions, SP and NKA induced a potentiation of the contractions to electrical field stimulation (EFS) and to exogenous ATP. The results suggest that the tachykinins evoke a direct contraction of pig intravesical ureteral strips through NK2 receptors located in the smooth muscle. SP and NKA exert an enhancement of the NANC excitatory neurotransmission of the pig intravesical ureter.

Contribution of NK3 tachykinin receptors to propulsion in the rabbit isolated distal colon

The role of NK3 receptors in rabbit colonic propulsion has been investigated in vitro with the selective agonist, senktide, and two selective antagonists, SR142801 and SB222200. Peristalsis was elicited by distending a rubber balloon with 0.3 and 1.0 mL of water leading to a velocity of 2.2 and 2.8 mm s-1, respectively. At concentrations of 1 nM, senktide inhibited propulsion evoked by both distensions (range 25-40%), whereas at 6 and 60 nmol L-1 facilitated 'submaximal' propulsion by 30%. In the presence of Nomega-nitro-L-arginine (L-NNA, 200 micromol L-1), which per se caused a slight prokinetic effect, 1 nmol L-1 senktide markedly accelerated propulsion (range 35-50%). Hexamethonium (200 micromol L-1) had minor effects on propulsion. In its presence, 60 nmol L-1 senktide significantly inhibited propulsion induced by both stimuli (range 20-50%). SR142801 (0.3, 3 nmol L-1) and SB222200 (30, 300 nmol L-1) facilitated 'submaximal' propulsion (range 20-40%). Conversely, higher antagonist concentrations (SR142801: 30, 300 nM; SB222200: 1, 10 micromol L-1) inhibited propulsion to both distensions by 20%. A combination of SR142801 (300 nmol L-1) plus hexamethonium (200 micromol L-1) induced an approximately four-fold greater inhibition of propulsion than that induced by SR142801 alone. In conclusion, in the rabbit-isolated distal colon, a subset of NK3 receptors located on descending pathways mediates an inhibitory effect on propulsion by activating a NO-dependent mechanism. Another subset of NK3 receptors, located on ascending pathways mediates a facilitative effect involving a synergistic interaction with cholinergic nicotinic receptors.

Tachykinin-dependent and -independent components of peristalsis in the guinea pig isolated distal colon

BACKGROUND & AIMS

In the intestine, tachykinins regulate motility by participating in neuromuscular and neuro-neuronal transmission. The aim of this study was to test the hypothesis that colonic propulsion is regulated by an interplay between tachykinergic and cholinergic transmission.

METHODS

Propulsion was elicited by intraluminal distention of a thin rubber balloon, which traveled from the oral to the anal end of guinea pig isolated distal colon segments. The overall contribution of endogenous tachykinins to colonic propulsion was examined by blocking NK1, NK2, and NK3 receptors simultaneously.

RESULTS

NK2-receptor blockade by MEN 11420 inhibited propulsion, whereas blockade of NK(1) by SR 140333 or of NK3 receptors by SR 142801 had minor effects on motility. Blockade of muscarinic or nicotinic receptors by hyoscine or hexamethonium decelerated peristalsis up to propulsion arrest. In the presence of partial muscarinic receptor blockade, the NK1-receptor antagonist SR 140333 and the NK2-receptor antagonist MEN 11420 markedly inhibited propulsion. Propulsion was also inhibited by the NK3-receptor antagonist SR 142801 in the presence of partial nicotinic receptor blockade. The simultaneous administration of the 3 tachykinin antagonists inhibited propulsion by 50%.

CONCLUSIONS

This study demonstrates the existence of an interplay between tachykinergic and cholinergic pathways during peristalsis and the importance of endogenous tachykinins acting at multiple receptor sites in the control of colonic propulsion.

Evidence for functional NK1-tachykinin receptors on motor neurones supplying the circular muscle of guinea-pig small and large intestine

The guinea-pig intestine was investigated to determine which neurones are excited via NK1 receptors. The specific NK1 receptor agonists [Sar9, Met(O2)11]-SP and septide contracted the circular muscle of all regions via a tetrodotoxin (TTX)-insensitive mechanism. In the proximal colon, they also evoked a TTX-sensitive relaxation; in the distal colon, the contractions were larger when nerve impulses were blocked with TTX, indicating that the agonists excited inhibitory motor neurones. In the duodenum and ileum, TTX reduced agonist-evoked contractions indicating that excitatory motor neurones were activated. In the presence of indomethacin, TTX enhanced contractions of ileal circular muscle evoked by these agonists suggesting that NK1 receptors were on inhibitory motor neurones. Blockade of nitric oxide synthase (NOS) enhanced NK1 receptor agonist evoked contractions of duodenal circular muscle, indicating that the agonists excite inhibitory motor neurones in duodenum. Neurones immunoreactive for NK1 receptors were studied in the duodenum and distal colon. As reported previously for the ileum, 1 some neurones were immunoreactive for NOS and had Dogiel type I morphology; features characteristic of inhibitory motor neurones. In conclusion, there are functional NK1 receptors on excitatory and inhibitory motor neurones in the guinea-pig small intestine and on inhibitory motor neurones in the colon.

Role of neurokinin 3 receptors on responses to colorectal distention in the rat: electrophysiological and behavioral studies

BACKGROUND & AIMS

Tachykinins contribute to the control of gastrointestinal motility and modulation of somatic and visceral pain. The role of neurokinin (NK) B and NK3 receptors in visceral pain and gastrointestinal disorders has not been determined.

METHODS

Using electromyographic recordings of both abdominal and colonic muscle and electrophysiological recordings of pelvic nerve afferent fibers, we studied drug effects on responses to colorectal distention.

RESULTS

In awake rats, intraperitoneal administration of the NK3-receptor antagonist SR 142,801 reduced, whereas the NK3-receptor agonist senktide increased, both the rectocolonic inhibitory reflex and abdominal contractions produced by colorectal distention. In contrast, intracerebroventricular administration of SR 142,801 increased the number of abdominal contractions without affecting the rectocolonic inhibitory reflex produced by colorectal distention. In a similar manner, intracerebroventricular injection of senktide diminished the number of abdominal contractions. In electrophysiological experiments, SR 142,801 decreased responses of pelvic nerve afferent fibers to colorectal distention. Responses of pelvic nerve fibers to urinary bladder distention, however, were unaffected by SR 142,801.

CONCLUSIONS

These results suggest that peripheral NK3 receptors are involved in the mediation of both visceral nociception and gastrointestinal disorders. Also, central NK3 receptors seem to play a role in the modulation of visceral nociception.

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.

Roles of neuronal NK1 and NK3 receptors in synaptic transmission during motility reflexes in the guinea-pig ileum

1. The role of NK1 and NK3 receptors in synaptic transmission between myenteric neurons during motility reflexes in the guinea-pig ileum was investigated by recording intracellularly the reflex responses of the circular muscle to distension or compression of the mucosal villi. Experiments were performed in a three-chambered organ bath that enabled drugs to be selectively applied to different sites along the reflex pathways. 2. When applied in the recording chamber, an NK1 receptor antagonist, SR140333 (100 nM), reduced by 40-50% the amplitudes of inhibitory junction potentials (i.j.ps) evoked in the circular muscle by activation of descending reflex pathways. This effect was abolished when synaptic transmission in the stimulus region was blocked with physiological saline containing 0.1 mM Ca2+ plus 10 mM Mg2+, leaving only the component of the descending reflex pathway conducted via long anally directed collaterals of intrinsic sensory neurons. 3. SR140333 (100 nM) had no effect on descending reflex i.j.ps when applied to the stimulus region. Ascending reflexes were also unaffected by SR140333 in the stimulus region or between the stimulus and recording sites. 4. Septide (10 nM), an NK1 receptor agonist, enhanced descending reflexes by 30-60% when in the recording chamber. [Sar9,Met(O2)11]substance P had no effect at 10 nM, but potentiated distension-evoked reflexes at 100 nM. 5. A selective NK3 receptor antagonist, SR142801 (100 nM), when applied to the stimulus region, reduced the amplitude of descending reflex responses to compression by 40%, but had no effect on responses to distension. SR142801 (100 nM) had no effect when applied to other regions of the descending reflex pathways. 6. SR142801 (100 nM) only inhibited ascending reflexes when applied at the recording site. However, after nicotinic transmission in the stimulus region was blocked, SR142801 (100 nM) at this site reduced responses to compression. 7. Contractions of the circular muscle of isolated rings of ileum evoked by low concentrations of septide, but not [Sar9,Met(O2)11]substance P, were potentiated by tetrodotoxin (300 nM). 8. Contractile responses evoked by an NK3 receptor agonist, senktide, were non-competitively inhibited by SR142801. After excitatory neuromuscular transmission was blocked, senktide produced inhibitory responses that were also antagonised by SR142801, but to a lesser extent and in an apparently competitive manner. 9. These results indicate that tachykinins acting via NK1 receptors partly mediate transmission to inhibitory motor neurons. NK3 receptors play a role in transmission from intrinsic sensory neurons and from ascending interneurons to excitatory motor neurons during motility reflexes.

Desensitization of the neurokinin-1 receptor (NK1-R) in neurons: effects of substance P on the distribution of NK1-R, Galphaq/11, G-protein receptor kinase-2/3, and beta-arrestin-1/2

Observations in reconstituted systems and transfected cells indicate that G-protein receptor kinases (GRKs) and beta-arrestins mediate desensitization and endocytosis of G-protein-coupled receptors. Little is known about receptor regulation in neurons. Therefore, we examined the effects of the neurotransmitter substance P (SP) on desensitization of the neurokinin-1 receptor (NK1-R) and on the subcellular distribution of NK1-R, Galphaq/11, GRK-2 and -3, and beta-arrestin-1 and -2 in cultured myenteric neurons. NK1-R was coexpressed with immunoreactive Galphaq/11, GRK-2 and -3, and beta-arrestin-1 and -2 in a subpopulation of neurons. SP caused 1) rapid NK1-R-mediated increase in [Ca2+]i, which was transient and desensitized to repeated stimulation; 2) internalization of the NK1-R into early endosomes containing SP; and 3) rapid and transient redistribution of beta-arrestin-1 and -2 from the cytosol to the plasma membrane, followed by a striking redistribution of beta-arrestin-1 and -2 to endosomes containing the NK1-R and SP. In SP-treated neurons Galphaq/11 remained at the plasma membrane, and GRK-2 and -3 remained in centrally located and superficial vesicles. Thus, SP induces desensitization and endocytosis of the NK1-R in neurons that may be mediated by GRK-2 and -3 and beta-arrestin-1 and -2. This regulation will determine whether NK1-R-expressing neurons participate in functionally important reflexes.

Characterization of receptors mediating contraction induced by tachykinins in the guinea-pig isolated common bile duct

1. We studied the effect of the natural tachykinins and of synthetic agonists selective for the tachykinin NK1, NK2 and NK3 receptors, on the motility of guinea-pig isolated common bile duct longitudinally-oriented smooth muscle. 2. All the tachykinins tested (both natural and synthetic) produced a concentration-dependent contractile response of the guinea-pig isolated common bile duct: these effects underwent a marked tachyphylaxis, especially the responses elicited by NK1 and NK3 receptor-selective agonists. 3. Among the natural tachykinins neurokinin B (EC50 = 3.2 nM; 95% c.l. = 2.0-5.1; n = 4) was the most potent, being about 40 and 25 fold more potent than substance P (EC50 = 121.6 nM; 95% c.l. = 94-157; P < 0.01; n = 4) and neurokinin A (EC50 = 83.4 nM; 95% c.l. = 62-112; P < 0.01; n = 4), respectively. Among the synthetic analogues the NK3 receptor-selective agonist senktide (EC50 = 1.1 nM; 95% c.l. = 0.7-1.8; n = 8) was the most potent, being about 120, 110 and 20 fold more potent than [Sar9]substance P sulfone (NK1 receptor-selective) (EC50 = 130.4 nM; 95% c.l. = 99-172; P < 0.01; n = 8), [beta Ala8]NKA (4-10) (NK2 receptor-selective) (EC50 = 120.1 nM; 95% c.l. = 95-151; P < 0.01; n = 8) and septide (NK1 receptor-selective) (EC50 = 22.6 nM; 95% c.l. = 18-28; P < 0.01; n = 8), respectively. All tachykinins (natural or synthetic receptor agonists) produced a similar Emax, averaging about 50% of that produced by KCl (80 mM). 4. Atropine (1 microM) did not affect the responses to either NK1 or NK2 receptor-selective agonists, whereas it reduced the Emax of senktide by about 50%, without affecting its potency (EC50). Tetrodotoxin (1 microM) totally blocked senktide-induced contractions, as did the combined pretreatment with atropine plus the tachykinin NK1 and NK2 receptor-selective antagonists GR 82334 and MEN 11420 (1 microM each), respectively. 5. GR 82334 (1 microM) blocked with apparent competitive kinetics septide- (apparent pKB = 7.46 +/- 0.10; n = 5) and [Sar9]substance P sulfone- (apparent pKB = 6.80 +/- 0.04; n = 4) induced contractions. MEN 11420 (30-300 nM), a novel potent NK2 receptor antagonist, potently antagonized [beta Ala8]NKA (4-10), with competitive kinetics (pKB = 8.25 +/- 0.08; n = 12: Schild plot slope = -0.90; 95% c.l. = -1.4; -0.35). The NK3 receptor-selective antagonist SR 142801 (30 nM) produced insurmountable antagonism of the senktide-induced contractions (Emax inhibited by 64%). None of the above antagonists, tested at the highest concentrations employed against tachykinins, affected the concentration-response curve to methacholine (0.1-300 microM). 6. We conclude that tachykinins produce contraction of the guinea-pig isolated common bile duct by stimulating NK1, NK2 and NK3 receptors. The responses obtained by activating NK1 and NK2 receptors are atropine-resistant. The contraction obtained by stimulating NK3 receptors is totally neurogenic, being mediated by the release of endogenous acetylcholine and tachykinins; the latter act, in turn, on postjunctional tachykinin NK1/NK2 receptors. The role of the NK3 receptor as prejunctional mediator of the excitatory transmission operated by tachykinins is discussed.

Sequential activation of the triple excitatory transmission to the circular muscle of guinea-pig colon

The aim of this study was to resolve the temporal relationships of the triple excitation of the circular muscle of guinea-pig colon that occurs in response to activation of the intrinsic excitatory nerves by using atropine and tachykinin NK1 and NK2 receptor selective antagonists to define the relative contribution of the transmitters involved. In organ bath experiments, performed in the presence of blockers of inhibitory innervation, a train of electrical pulses at 5 Hz for 300 s produced a sustained contraction of the circular muscle of guinea-pig colon: the sequential addition of atropine (1 microM), of the tachykinin NK1 receptor antagonist, SR 140333 (0.3 microM) and of the tachykinin NK2 receptor antagonist, MEN 11420 (1 microM) produced a cumulative inhibitory effect and progressively delayed the onset of the contractile response to nerve stimulation. In the presence of peptidase inhibitors, atropine was less effective in inhibiting the contractile response for prolonged periods of stimulation: however, the pattern of inhibition of the evoked response produced by the sequential addition of blocker drugs was not substantially affected. The selective tachykinin NK3 receptor agonist, senktide, produced a concentration-dependent contraction of guinea-pig colon. The sequential addition of atropine (1 microM), SR 140333 (0.3 microM) and MEN 11420 (1 microM) reproduced the effect of the same drugs on the response to electrical nerve stimulation. The peptide blocker of N-type voltage-dependent calcium channels, omega-conotoxin (0.1 microM) produced a partial inhibitory effect of the response to senktide. The omega-conotoxin-resistant response to 1 microM senktide was inhibited and delayed by the progressive application of atropine, SR 140333 and MEN 11420, similar to the effect observed in the absence of omega-conotoxin. In sucrose gap, single-pulse electrical field stimulation produced a fast excitatory junction potential which was largely (90%) inhibited by atropine; application of a low concentration of the potassium channel blocker, 4-aminopyridine (30 microM), markedly enhanced the atropine-resistant excitatory junction potential which was abolished by the NK1 receptor antagonist, GR 82334. We conclude that, during prolonged electrical or chemical stimulation of excitatory motorneurons, there is a sequential, time-dependent activation of the three excitatory mechanisms in the circular muscle of guinea-pig colon: the pattern of activation is relatively independent of the intensity of stimulation and/or the mechanisms of secretion of released transmitters. Postjunctional factors predominate in determining the relative contribution of the three transmitters, acetylcholine, substance P and neurokinin A, in producing excitation of the circular muscle.

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.

Characterisation of substance P-induced endocytosis of NK1 receptors on enteric neurons

Immunoreactivity for NK1 receptors is confined to specific nerve cell bodies in the guinea-pigileum, including inhibitory motor neurons and secretomotor neurons. In the present work, endocytosis of NK1 receptors in these enteric neurons was studied following addition of substance P (SP) to isolated ileum. NK1 receptors were localised with antibodies against the C-terminus of this receptor. Some preparations were incubated with SP tagged with the fluorescent label, Cy3.18, so that the fate of SP bound to receptors could be followed. Preparations were analysed by confocal microscopy. In tissue that was incubated at 4 degrees C in the absence of SP, most NK1 receptor immunoreactivity (IR) was confined to surface membranes of nerve cells. At 37 degrees C in the presence of 10(-7) M SP (plus 3 x 10(-7)M tetrodotoxin to prevent indirect activation via other neurons) the neuronal NK1 receptor was rapidly internalised. After 5 min, NK1 receptor IR was partially internalised, at 20 min NK1 receptor IR was throughout the cytoplasm and in perinuclear aggregates and at 30 min it was again at the cell surface. SP-induced NK1 receptor endocytosis was inhibited by the specific NK1 receptor antagonist, SR140333. Cy3-SP was colocalised with NK1 receptor IR and was internalised with the NK1 receptor. These results show that enteric neurons exhibit authentic NK1 receptors that are rapidly internalised when exposed to their preferred ligand.

Characterization of antisera specific to NK1, NK2, and NK3 neurokinin receptors and their utilization to localize receptors in the rat gastrointestinal tract

Understanding the physiological role of tachykinins requires precise cellular and subcellular localization of their receptors. We raised antisera by immunizing rabbits with peptides corresponding to portions of the intracellular tails of the rat neurokinin 1, 2, and 3 receptors (NK1-R, NK2-R, NK3-R). Receptors were localized by immunofluorescence and confocal microscopy. NK1-R, NK2-R, and NK3-R were detected at the plasma membrane of transfected cells with minimal intracellular stores. Staining was abolished by preabsorption of the antisera with the peptides used for immunization. Nontransfected cells were unstained. Each antiserum only stained cells transfected with the appropriate receptor and did not stain cells transfected with the other receptors. Therefore, the antisera are specific and do not cross-react with other neurokinin receptors. We examined the distribution of the neurokinin receptors in the gastrointestinal tract of the rat. NK1-R was detected in myenteric and submucosal neurons and in interstitial cells of Cajal. NK2-R was localized to circular and longitudinal muscle cells and to nerve endings in the plexuses. NK3-R was detected in numerous myenteric and submucosal neurons. Some neurons expressed both NK1-R and NK3-R. Receptors were detected at the plasma membrane and in endosomes. Cells expressing the receptors were closely associated with tachykinin-containing nerve fibers. Thus, NK1-R and NK3-R mediate neurotransmission by tachykinins within enteric nerve plexuses, and NK1-R and NK2-R mediate the effects of tachykinins on interstitial and smooth muscle cells, respectively.

Analysis of contributions of acetylcholine and tachykinins to neuro-neuronal transmission in motility reflexes in the guinea-pig ileum

1. The roles of acetylcholine (ACh) and tachykinins in neuro-neuronal transmission during ascending excitatory and descending inhibitory reflexes were studied by recording intracellular reflex responses of the circular muscle to physiological stimuli. Experiments were carried out in opened segments of guinea pig ileum in an organ bath that was partitioned so that three regions could be independently exposed to drugs. 2. Ascending excitatory reflexes evoked by either distension from the serosal side or compression of the mucosa were depressed by 55% and 85%, respectively, in the presence of hexamethonium (200 microM) and by 30% and 45%, respectively, by a desensitizing concentration of the selective NK3 receptor agonist, senktide (1 microM), in the chamber in which reflexes were initiated. Together, hexamethonium and senktide abolished responses to compression. A residual response to distension persisted. This was abolished by hyoscine (1 microM). 3. Hexamethonium (200 microM) abolished ascending reflexes when applied to the region between the stimulus and the recording sites, or to the recording chamber. 4. Descending reflex responses were reduced by 35% by synaptic blockade in the stimulus chamber with physiological saline containing 0.1 mM Ca2+ plus 10 mM Mg2+. Senktide (1 microM) in the stimulus chamber reduced distension reflexes to the same extent as synaptic blockade, whereas hexamethonium (200 microM) and hyoscine (1 microM) depressed responses by less than 20%. Responses to compression were reduced by 40% by senktide alone, while senktide and hexamethonium together reduced responses by 60%, an effect similar to synaptic blockade. Under these conditions, hyoscine in the stimulus chamber restored reflexes evoked by distension, but did not alter those evoked by mucosal compression. 5. Total synaptic blockade in the intermediate chamber, between stimulus and recording sites, reduced descending reflex responses by more than 90%. In contrast, hexamethonium (200 microM) had no effect and hyoscine (1 microM) reduced only the responses to distension (by 30%). Senktide (1 microM) depressed responses to both stimuli by approximately 80%. 6. Application of hexamethonium (200 microM) to the recording chamber depressed descending reflex responses to distension applied in the near stimulation chamber by 15%, but had no effect on responses to compression in the near chamber or to either stimulus applied in the far chamber. 7. Descending reflexes evoked by near chamber stimuli were unaffected by hyoscine (1 microM) or senktide (1 microM) applied to the recording chamber; hyoscine enhanced reflexes evoked by compression in the far chamber by 50%. 8. For the ascending excitatory reflex pathway, it is concluded that transmission from sensory neurones is mediated by ACh acting via both nicotinic and muscarinic receptors, and by tachykinins acting at NK3 receptors. Transmission from ascending interneurones appears to be predominantly via nicotinic receptors. The descending inhibitory pathways are more complex, and while transmission from sensory neurones involves nicotinic, muscarinic and NK3 receptor-dependent components, transmission from descending interneurones to inhibitory motor neurones is neither cholinergic nor due to tachykinins acting via NK3 receptors.

Localisation of NK1 receptor immunoreactivity to neurons and interstitial cells of the guinea-pig gastrointestinal tract

Tachykinins, including substance P, neurokinin A, and neuropeptides K and gama, are expressed widely in the peripheral nervous system where they affect smooth muscle contraction, exocrine gland secretion, vascular permeability, and neurotransmission. Substance P, the preferred ligand for the NK1 receptor, is found in high concentrations in the enteric nervous system. In the present study, the localisation and distribution of the NK1 receptor was studied throughout the gastrointestinal tract of the guinea-pig by using a polyclonal antiserum raised against the C-terminal 15 amino acids of the NK1 receptor. Co-localisation with other neuronal markers was examined in the ileum. Nerve cell bodies reactive for the NK1 receptor were found in the myenteric plexus of all regions and the submucous plexus of the small and large intestines. In the small intestine, the interstitial cells of Cajal were also immunoreactive. Immunoreactivity was largely confined to cell surfaces. Almost all immunoreactive myenteric nerve cells had Dogiel type I morphology, and most of these were immunoreactive for nitric oxide synthase, a transmitter of inhibitory neurons to the muscle and of descending interneurons. Neuropeptide Y-containing secretomotor neurons in the submucous and myenteric plexuses also exhibited NK1 receptor immunoreactivity. NK1 receptors were present on a minority of tachykinin immunoreactive neurons of submucous ganglia. The results suggest that receptors on the longitudinal muscle might not be conventional NK1 receptors, that excitation of the circular muscle of the ileum is indirect, perhaps via the interstitial cells of Cajal, and that enteric inhibitory neurons may be excited via NK1 receptors.

Investigation into the 5-hydroxytryptamine-induced atropine-resistant neurogenic contraction of guinea-pig proximal colon

1. The aim of this study was to characterize the receptors mediating the atropine-resistant neurogenic contraction to 5-hydroxytryptamine (5-HT) in the longitudinal muscle of the guinea-pig proximal colon and to determine the type of tachykinin receptors involved in the contractile response to 5-HT by the use of selective antagonists. 2. In the presence of atropine (0.3 microM), guanethidine (5 microM), hexamethonium (100 microM), ketanserin (0.1 microM) and indomethacin (3 microM), 5-HT (0.01-3 microM) produced concentration-dependent neurogenic contractions of colonic strips and at 0.3 microM produced a maximal effect (pEC50 = 7.39 +/- 0.09, n = 18). The 5-HT4 receptor stimulant, 5-methoxytryptamine (5-MeOT, 0.03-10 microM) also produced neurogenic contractions with similar maximum effect to those of 5-HT (pEC50 = 6.89 +/- 0.16). 3. The 5-HT4 receptor antagonist, DAU 6285 (3 microM) shifted the concentration-response curves to both 5-HT and 5-MeOT to the right without significant depression of the maximum, but the 5-HT1/5-HT2 receptor antagonist, metitepine (0.1 microM) and the 5-HT3 receptor antagonist, ondansetron (0.3 microM) had no effect on the control curves to 5-HT and 5-MeOT. 4. The selective NK1 receptor antagonist, FK 888 (1 microM) markedly attenuated the contractions to 5-HT and 5-MeOT. In contrast, the selective NK2 receptor antagonist, SR 48968 (10 nM) and the selective NK3 receptor antagonist, SR 142801 (10 nM) had no effect on the contractions to 5-HT and 5-MeOT. 5. These results indicate that the 5-HT-induced atropine-resistant neurogenic contraction of guinea-pig proximal colon is due to activation of 5-HT4 receptors, presumably located on excitatory motor neurones, innervating the longitudinal muscle. The contraction evoked by activation of the 5-HT4 receptors is mediated primarily via NK1 receptors but not NK2 or NK3, suggesting that the 5-HT4 receptor-mediated contraction is evoked indirectly via tachykinin release from tachykinin-releasing excitatory neurones.

Evidence for the presence of NK1 and NK3 receptors on cholinergic neurones in the guinea-pig ileum

In a guinea-pig ileum longitudinal muscle preparation, substance P (SP) (> or = 6 nM) caused an initial contraction followed by a sustained plateau contraction of about 20-50% of the initial response. This plateau contraction is caused by the SP-induced activation of cholinergic motoneurones which contract the smooth muscles by the released acetylcholine (ACh). We investigated the contribution of neurokinin NK1 and NK3 receptors during this 'plateau phase' of contraction. The plateau contraction induced by SP (60 nM) was significantly reduced by the NK1 receptor antagonist CP-96,345 (200 nM) added 5 min after SP, but was not affected by its inactive enantiomer CP-96,344 (200 nM). The NK1 receptor antagonist CP-99,994 (100 nM) significantly reduced the plateau contraction induced by SP (60 nM and 600 nM) and that induced by the NK1 receptor agonist substance P-O-methylester (SPOMe; 100 nM). CP-99,994 (100 nM), however did not affect the plateau contraction induced by the NK3 receptor agonist [Asp5,6, MePhe8]-SP(5-11) (100 nM). The plateau contraction induced by SP (600 nM) was not affected by the NK3 receptor antagonist SR-142,801 (100 nM), added 5 min after SP. Pre-incubation of the ileum with SR-142,801 (100 nM) 30 min prior to the addition of SP (600 nM) also had no significant effect on the plateau contraction. However, it significantly reduced the ileal contraction in the first minutes after the initial spasmogenic contraction. We suggest that SP induces the plateau contraction of the guinea-pig ileum longitudinal muscle mainly by the activation of NK1 receptors on cholinergic neurones.

Neuronal tachykinin NK2 receptors mediate release of non-adrenergic non-cholinergic inhibitory transmitters in the circular muscle of guinea-pig colon

The aims of this study were: (i) verify the usefulness of the recently described non-peptide antagonist, SR 142801, for blocking tachykinin NK3 receptors in the circular muscle of the guinea-pig colon and (ii) after occlusion of NK3 receptors by SR 142801, test the hypothesis that tachykinins may activate non-adrenergic non-cholinergic inhibitory neurons via non-NK3 receptors. In sucrose gap, we found that SR 142801 (0.1 microM) time-dependently inhibited the senktide-induced atropine (1 microM)-sensitive depolarization, action potentials and contractions of circular muscle of guinea-pig colon without affecting the cholinergic excitatory junction potential and contraction produced by single pulse electrical field stimulation. Likewise, SR 142801 (0.1 microM) time-dependently inhibited the senktide-induced non-adrenergic non-cholinergic hyperpolarization and relaxation of the circular muscle, without affecting the non-adrenergic non-cholinergic inhibitory junction potentials and relaxation produced by single pulse electrical field stimulation. Therefore, SR 142801 is a suitable tool to occlude neuronal NK3 receptors in guinea-pig colon. In the presence of SR 142801 (0.1 microM), atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM) and nifedipine (1 microM) superfusion with neurokinin A (0.3 microM) produced depolarization on which a series of inhibitory junction potentials were superimposed. The incidence, number and amplitude of the inhibitory junction potentials evoked by neurokinin A was partly reduced by pretreatment with either apamin (0.1 microM) or L-nitroarginine (30 microM) and was totally blocked by pretreatment with apamin plus L-nitroarginine or by tetrodotoxin (1 microM). None of these treatments affected the depolarization and contraction produced by neurokinin A. The NK1 receptor selective antagonist, GR 82,334 (3 microM), did not affect the responses to neurokinin A, which were abolished by the NK2 receptor-selective antagonist GR 94,800 (0.1 microM). Substance P (0.3 microM) produced a large depolarization of the membrane but was poorly effective in producing superimposed inhibitory junction potentials. The NK1 receptor-selective agonist [Sar9]substance P sulfone (0.3 microM) produced large depolarization without inducing superimposed inhibitory junction potentials, while the NK2 receptor-selective synthetic agonist [beta-Ala8]neurokinin A(4-10) (0.3 microM) produced depolarization and superimposed inhibitory junction potentials. We conclude that neurokinin A, in addition to direct excitation and contraction of circular muscle activates, via neuronal NK2 receptors, inhibitory non-adrenergic non-cholinergic motorneurons. Thus, neuronal NK2 receptors should be considered as targets for endogenous tachykinins in enteric circuitries leading to descending relaxation in guinea-pig colon.

Tachykinin receptors in gastrointestinal motility

For a long time research on the action of TKs on gastrointestinal tissue has been demonstrating the importance of the TKs as non-cholinergic stimulators of motility in most parts of the mammalian gastrointestinal tract. The past years witnessed the development of TK agonists and antagonists selective for the various receptor types, which prompted a wealth of new insight into the pharmacology and molecular biology of the TK receptors. This knowledge now allows a more specific elucidation of the role of TKs and their receptors in the various aspects of gastrointestinal motility, not only in normal tissue but also under pathological conditions.

Activity of SR 142801 at peripheral tachykinin receptors

The pharmacological profile of the novel tachykinin NK3 receptor antagonist SR 142801, ((S)-(N)-(1-(3-(1-benzoyl-3-(3,4-dichlorophenyl) piperidin-3-yl) propyl)-4-phenylpiperidin-4-yl)-N-methylacetamide), was studied at tachykinin NK1, NK2 and NK3 receptors, in several in vitro bioassays. In the guinea-pig isolated ileum longitudinal muscle preparation, SR 142801 (10 nM-1 microM) caused an insurmountable antagonism of tachykinin NK3 receptor-mediated contractions produced by senktide (apparent pKB = 9.27). The blockade induced by SR 142801 was essentially irreversible, since it was not removed by washout (up to 2 h) and was increased by prolonging the incubation from 15 to 120 min. SR 142801 showed similar antagonist potency at rat tachykinin NK3 receptors (portal vein) and rabbit tachykinin NK2 receptors (pulmonary artery) (pKB = 7.49 and 7.66, respectively), whereas it was distinctly less potent at hamster tachykinin NK2 receptors (trachea; pKB = 6.84) and inactive at guinea-pig tachykinin NK1 receptors (ileum, longitudinal muscle). In the guinea-pig whole ileum SR 142801 (100 nM) did not affect the contraction produced by capsaicin (1 microM). The combined SR 142801 pretreatment and tachyphylaxis of neuronal CGRP (calcitonin gene-related peptide) receptors produced a slight (about 25%), but significant reduction of the response to capsaicin, suggesting that tachykinin NK3 receptors play a minor role in capsaicin-induced neuronal excitation of afferent nerves in the guinea-pig ileum.

Tachykinin NK3 receptor mediates NANC hyperpolarization and relaxation via nitric oxide release in the circular muscle of the guinea-pig colon

In the presence of atropine (1 microM), guanethidine (3 microM) and of the tachykinin NK1 (SR 140,333 0.1 microM) and NK2 (GR 94,800 3 microM) receptor antagonists, the application of the tachykinin NK3 receptor selective agonist senktide, or that of neurokinin B, produced concentration-dependent sustained nonadrenergic noncholinergic (NANC) relaxation of mucosa-free circular muscle strips from the guinea-pig proximal colon. The maximal relaxant responses to senktide and neurokinin B were similar, approaching about 70% of the relaxation to 1 microM isoprenaline. Senktide (EC50 0.16 nM) was about 64-fold more potent than neurokinin B (EC50 10.3 nM). When tested in the presence of peptidase inhibitors (thiorphan 1 microM, captopril 1 microM and amastatin 10 microM), neurokinin B (EC50 0.24 nM) was equipotent to senktide (EC50 0.19 nM). At 1 nM, substance P and neurokinin A were ineffective in producing a NANC relaxation of the colon. At 1 microM substance P, neurokinin A and neurokinin B produced a NANC relaxation, which averaged 23, 40 and 79% of the maximal response to isoprenaline, respectively. In the presence of peptidase inhibitors, 1 nM substance P and neurokinin A produced threshold relaxant responses and, at 1 microM, the three natural tachykinins were equieffective (66 +/- 8, 72 +/- 5 and 75 +/- 5% of the relaxation to isoprenaline for substance P, neurokinin A and neurokinin B, respectively). The relaxant response to 1 nM senktide (producing about 70-80% of its maximal effect) was totally abolished by 1 microM tetrodotoxin and largely (> 90%) inhibited by 100 microM L-nitroarginine (L-NOARG). The inhibition by L-NOARG was partially reversed by L-arginine (3 mM) but not D-arginine. Apamin (1 microM) produced a slight (about 20%) inhibition of the response to senktide. The peptide blocker of N-type calcium channels, omega-conotoxin (0.1 microM) was ineffective. In sucrose gap electrophysiological experiments, superfusion with senktide (0.1 microM for 10 s) produced a slowly developing and prolonged hyperpolarization of the membrane and relaxation. Both effects were inhibited by L-NOARG while apamin had no effect. These findings indicate that a neuronal NK3 receptor mediates NANC hyperpolarization and relaxation of the circular muscle of the guinea-pig proximal colon, principally through the release of NO. NO generation/release in response to NK3 receptor stimulation does not require calcium influx through N-type calcium channels.

Effects of ageing on regional differences in the contractile responses to acetylcholine and neurokinin A in rabbit airway

1. Experiments were designed to study the effects of ageing on muscarine and NK2 receptor mechanisms in the three different regions of rabbit airway. 2. The pD2 value of acetylcholine changed with age in three different regions while that of carbamylcholine, which is resistant to acetylcholinesterase, did not. 3. The pD2 values of neurokinin A and the activity of protease, a degradative enzyme, changed with age. However, by the pretreatment with phosphoramidon, a protease inhibitor, the regional difference and age related change of the pD2 value of neurokinin A disappeared. 4. In conclusion, the observations about age related changes and regional differences of pD2 value of acetylcholine and neurokinin A were due to the difference of their degradative enzyme activities.

Septide but not substance P stimulates inhibitory neurons in guinea-pig ileum

Recent evidence suggests that [pGlu6,Pro9]substance P-(6-11) (septide) may act on unusual tachykinin receptors. Here, we investigated the mechanisms of action of septide and other agonists acting on tachykinin NK1 receptors, in the guinea-pig ileum. Responses to septide but not to substance P or [Sar9,Met(O2)11]substance P were significantly (P < 0.01) potentiated by 1 microM tetrodotoxin. The data suggest that septide not only acts directly on smooth muscle tachykinin receptors to produce contraction, but also stimulates another receptor subtype on myenteric neurons to release inhibitory substance(s).

Effect of longitudinal muscle-myenteric plexus removal and indomethacin on the response to tachykinin NK-2 and NK-3 receptor agonists in the circular muscle of the guinea-pig ileum

1. The effect of removal of the longitudinal muscle-myenteric plexus (LM-MP) and/or indomethacin (10 microM) on the response to the tachykinin NK-2 receptor selective agonist, [beta Ala8]NKA(4-10), or to the NK-3 receptor selective agonist, senktide, was investigated by measuring mechanical activity (isotonic recording) of circular muscle (ring preparation) of the guinea-pig ileum. 2. Indomethacin (10 microM) increased the percentage of ileal rings displaying spontaneous activity, either intact or LM-MP-free. The response to senktide (10 nM and 1 microM) was lower in LM-MP-free than in intact ileal rings, either in the absence or presence of indomethacin. The response to a low concentration (10 nM) of [beta Ala8] NKA (4-10) was enhanced in LM-MP-free rings and by indomethacin. 3. In intact ileal rings, the response to senktide was unaffected by atropine (3 microM) alone or by the tachykinin NK-2 receptor antagonist MEN 10,376 (10 microM) alone while it was reduced by the combined administration of the two antagonists. The response to senktide was greatly reduced by tetrodotoxin (TTX, 1 microM). Senktide-induced contractions (10 nM) were also reduced by the blocker of N-type voltage-sensitive calcium channels, omega-contoxin (CTX, 0.1 microM). 4. In about 30% of preparations tested, an inhibitory response (decrease in spontaneous activity) to 10 nM senktide, was disclosed in CTX-treated intact ileal rings. This inhibitory effect was TTX-sensitive. 5. In LM-MP-free ileal rings, the response to senktide was abolished or reduced by atropine and MEN 10,376, alone or in combination, and was also reduced or abolished by TTX and CTX. 6. The response to [beta Ala8]NKA (4-10) was inhibited by MEN 10,376, in both intact and LM-MP-free ileal rings while it was unaffected by atropine, TTX or CTX. 7. These results indicate that indomethacin pretreatment induces a regular background activity for studying the motor response to tachykinins in the circular muscle of the ileum, probably by blocking the formation of relaxant prostanoids. A further increase in sensitivity to direct smooth muscle stimulation (NK-2 receptor agonist) can be obtained by removal of the LM-MP. The response to NK-3 receptor stimulation is diminished but not abolished by removal of the LM-MP, suggesting that NK-3 receptors are located on neuronal bodies of myenteric neurons, but possibly also at other sites (possibly, nerve terminals).(ABSTRACT TRUNCATED AT 400 WORDS)

Nitric oxide is the mediator of tachykinin NK3 receptor-induced relaxation in the circular muscle of the guinea-pig ileum

The tachykinin NK3 receptor agonist, senktide, produces concentration-dependent contraction of the circular muscle of the guinea-pig ileum (EC50 2.59 nM). In the presence of the blocker of neuronal type of voltage-sensitive calcium channels, omega-conotoxin (0.1 microM), the contractile response to a low concentration of senktide was converted to an inhibitory effect on spontaneous activity of the ileum. This inhibitory effect was further enhanced in the presence of atropine (1 microM) and was abolished by tetrodotoxin (1 microM), indicating its neural origin. In the presence of atropine and omega-conotoxin, the inhibitory response to senktide (1 nM) was greatly inhibited or even abolished by L-nitroarginine (30 microM), its effect being prevented by L-arginine but not by D-arginine (300 microM in each case). Apamin (0.1 microM) failed to significantly affect the inhibitory response to senktide. Apamin enhanced spontaneous activity of the preparation while L-nitroarginine had no effect. Neither apamin nor L-nitroarginine affected the inhibitory response to isoprenaline. These findings indicate that inhibition of circular muscle activity produced through NK3 receptor stimulation in the guinea-pig ileum is mediated through a neuronal pathway involving nitric oxide or a nitric oxide-like substance(s) generation.

Pharmacological characterisation of neurokinin receptors mediating anion secretion in rat descending colon mucosa

Substance P (SP), neurokinin A (NKA), neurokinin B (NKB), [Sar9, Met (O2)11]-SP (SMSP), senktide, [beta Ala8]-NKA(4-10) and neuropeptide gamma (NP gamma) all stimulate secretory responses in rat descending colon mucosa under voltage clamp conditions. Secretory responses (measured as short circuit current under voltage clamp conditions) were transient and those evoked by SP, SMSP, NKA and senktide were significantly reduced by pretreating tissues with the chloride channel blocker, diphenylamine carboxylate (DPC). Concentration-response curves showed varying degrees of sensitivity to tetrodotoxin (TTX). Senktide-induced secretion was virtually abolished by TTX, while NP gamma and [beta Ala8]-NKA(4-10) were not significantly altered. Rightward shifts of concentration-response curves were observed for SMSP, NKA and SP in TTX treated preparations compared with controls. NKA response curves in the presence of TTX were further inhibited by MEN10,207 and CP-96,345. GR71251, GR82334 and CP-96,345 all inhibited SMSP secretory responses with pA2 values of 5.8, 6.5 and 6.9 respectively. In conclusion three types of neurokinin receptor exist in preparations of rat colon mucosa and their relative location within neuronal and epithelial surfaces are discussed.

Tachykinin receptors: a radioligand binding perspective

The last decade has witnessed major breakthroughs in the study of tachykinin receptors. The currently described NK-1, NK-2, and NK-3 receptors have been sequenced and cloned from various mammalian sources. A far greater variety of tachykinin analogues are now available for use as selective agonists and antagonists. Importantly, potent nonpeptide antagonists highly selective for the NK-1 and NK-2 receptors have been developed recently. These improved tools for tachykinin receptor characterization have enabled us to describe at least three distinct receptor types. Furthermore, novel antagonists have yielded radioligand binding and functional data strongly favoring the existence of putative subtypes of NK-1 and especially NK-2 receptors. Whether these subtypes are species variants or true within-species subtypes awaits further evidence. As yet undiscovered mammalian tachykinins, or bioactive fragments, may have superior potency at a specific receptor class. The common C terminus of tachykinins permits varying degrees of interaction at essentially all tachykinin receptors. Although the exact physiological significance of this inherent capacity for receptor "cross talk" remains unknown, one implication is for multiple endogenous ligands at a single receptor. For example, NP gamma and NPK appear to be the preferred agonists and binding competitors at some NK-2 receptors, previously thought of as exclusively "NKA-preferring." Current evidence suggests that tachykinin coexistence and expression of multiple receptors may also occur with postulated NK-2 and NK-1 receptor subtypes. Other "tachykinin" receptors may recognize preprotachykinins and the N terminus of SP. In light of these recent developments, the convenient working hypothesis of three endogenous ligands (SP, NKA, and NKB) for three basic receptor types (NK-1, NK-2, and NK-3) may be too simplistic and in need of amendment as future developments occur (Burcher et al., 1991b). In retrospect, the 1980s contributed greatly to our understanding of the structure, function, and regulation of tachykinins and their various receptors. The development of improved, receptor subtype-selective antagonists and radioligands, in addition to recent advances in molecular biological techniques, may lead to a more conclusive pharmacological and biochemical characterization of tachykinin receptors. The 1990s may prove to be the decade of application, where a better understanding of the roles played by endogenous tachykinins (at various receptor subtypes) under pathophysiological conditions will no doubt hasten the realization of clinically useful therapeutic agents.

Pharmacological characterization of the contractile responses to angiotensin analogues in guinea-pig isolated longitudinal muscle of small intestine

1. The contractile responses to angiotensin II, angiotensin III and two synthetic analogues, [Lys2]angiotensin II and [Sar1]angiotensin II, in the guinea-pig isolated longitudinal muscle preparation of small intestine have been characterized in vitro. 2. Tachyphylaxis to the angiotensin analogues was reduced by use of a Krebs-Henseleit solution containing a raised (sub-contractile) concentration of potassium (11.2 mM). Under these conditions. reproducible cumulative concentration-response curves to all agonists were established. The pD2 estimates for angiotensin II, [Lys2]angiotensin II, angiotensin III and [Sar1]angiotensin II were 9.15 +/- 0.14, 7.42 +/- 0.06, 7.69 +/- 0.18 and 9.50 +/- 0.15 respectively and the maximum responses achieved were not significantly different. 3. The contractile responses to angiotensin II, angiotensin III and [Sar1]angiotensin II were reduced by greater than 80% by tetrodotoxin (TTX; 0.1 microM). However, the responses to [Lys2]angiotensin II were reduced by only 63 +/- 5%. Atropine (0.1 microM) also reduced the responses to angiotensin II, angiotensin III and [Lys2]angiotensin II, although its effect was less than that produced by TTX. Furthermore, while responses to these agonists were not significantly modified by the NK1 receptor antagonist (+/-)-CP-96,345 (30 nM) alone, the combined pre-incubation with both atropine and (+/-)-CP-96,345 reduced maximum agonist responses to a level not significantly different from those produced by TTX. 4. Indirect and direct contractile responses to angiotensin II and [Lys2]angiotensin II (in the presence of TTX) respectively were characterized by use of the selective AT1 receptor antagonist, losartan and the AT2 receptor antagonist, PD123177. Losartan produced parallel rightward displacement of the concentration-response curve to angiotensin II and [Lys2]angiotensin II, with an estimated pKB of 8.56(8.42-8.68) and 9.18 (8.63-9.50) respectively. The AT2 receptor antagonist, PD123177 (3 microM) failed to modify the contractile responses to either angiotensin II or [Lys2]angiotensin II.5. We conclude that two populations of angiotensin II receptors exist in the guinea-pig longitudinal muscle of small intestine, one located neuronally mediating the release of both acetylcholine and substance P and the other located on the smooth muscle mediating direct contractile responses. The neuronal component provides the major contribution to the agonist responses. Both receptor populations are of the AT1 receptor subtype.

Regional differences of the contractile responses to acetylcholine and neurokinin A in rabbit airway: heterogeneous distribution of the metabolic enzymes

1. Experiments were designed to determine whether differences exist in the sensitivity to muscarinic and tachykinin agonists in rabbit airways. 2. The rank order of sensitivity (pD2 value) to acetylcholine was: trachea > proximal bronchus > distal bronchus, whereas no regional difference was observed in the sensitivity to carbamylcholine which is resistant to acetylcholinesterase. 3. Acetylcholinesterase activity was greater in the distal than in the proximal airway. 4. In the absence of the peptidase inhibitor, phosphoramidon, the pD2 values of neurokinin A (NKA) and substance P (SP) in trachea were significantly greater than that in bronchus, whereas no regional difference was observed in the NK1 selective agonist, substance P methyl ester (SPOMe). 5. Application of phosphoramidon (10 microM) to avoid peptide degradation abolished the regional difference of the pD2 values of SP. 6. In conclusion, regional differences in sensitivities to acetylcholine and NKA in the rabbit airway were suggested to be due to distribution to the metabolic enzymes of these drugs.

Tachykininergic transmission to the circular muscle of the guinea-pig ileum: evidence for the involvement of NK2 receptors

1. The effect of newly developed, receptor-selective tachykinin antagonists (GR 71,251 for NK1 receptors, MEN 10,376 and L 659,877 for NK2 receptors) on noncholinergic transmission to the circular muscle of the guinea-pig ileum has been investigated. 2. In circular muscle strips of the ileum, electrical field stimulation in the presence of atropine (2 microM) and apamin (0.1 microM) evoked a complex motor response. The tonic primary contraction in this response was reduced by GR 71,251 (10 microM) and MEN 10,376 (3-10 microM) but not by L 659,877 (up to 10 microM). The presence of apamin was necessary in this experimental arrangement to unmask an atropine-resistant primary contraction, sensitive to tachykinin antagonists. The motor response was abolished by tetrodotoxin. 3. In circular strips of the ileum GR 71,251 (10 microM) inhibited the tonic contraction produced by [Sar9] substance P sulphone, a selective NK1 receptor agonist but not that produced by [beta Ala8] neurokinin A (4-10), a selective NK2 receptor agonist. By contrast, MEN 10,376 antagonized the effect of the NK2 agonist while leaving the response to the NK1 agonist unaffected. 4. In whole segments of the ileum, distension of the gut wall by an intraluminal balloon placed at about 1 cm from the point of recording of mechanical activity of the circular muscle produced atropine-sensitive phasic contractions (ascending enteric reflex). In the presence of atropine (2 microM), a noncholinergic response was elicited, which required larger volumes of distension that the cholinergic one. The atropine-resistant ascending enteric reflex was enhanced by apamin (0.1 microM) and abolished by tetrodotoxin, either in the presence or absence of apamin.5. MEN 10,376 (3-lOmicroM) inhibited the atropine-resistant ascending enteric reflex in the presence of apamin while GR 71,251 or L 659,877 (10 microM each) were ineffective. MEN 10,376 inhibited the atropine-resistant ascending enteric reflex to a larger extent in the absence than in the presence of apamin and also slightly inhibited the ascending enteric reflex in the absence of atropine.6. These findings provide evidence for an involvement of NK2 tachykinin receptors in excitatory transmission to the circular muscle of the guinea-pig ileum. NK2 receptors are also involved in the physiological-like circular muscle activation produced by stimulation of intramural neuronal pathways which subserve the atropine-resistant ascending enteric reflex.

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,.

Tachykinin receptors in the circular muscle of the guinea-pig ileum

1. We have studied the mechanical response of circular strips of the guinea-pig ileum to tachykinins and characterized the receptors involved by means of receptor-selective agonists. 2. The strips responded to both substance P (SP) and neurokinin A (NKA), as well as to [Pro9]-SP sulphone (selective NK1-receptor agonist), [beta Ala8]-NKA(4-10) (selective NK2-receptor agonist) and [MePhe7]-neurokinin B (selective NK3-receptor agonist). The ED50s of the various peptides (calculated as the concentration of agonist which produced 50% of the response to 10 microM carbachol) were similar, in the range of 40-200 nM, i.e. no clearcut rank order of potency was evident. 3. The response to a submaximal (10 nM) concentration of SP or NKA was unaffected in the presence of peptidase inhibitors (thiorphan, captopril and bestatin, 1 microM each). 4. The response to the NK1-agonist was totally atropine-resistant, but was reduced (about 30% inhibition) by tetrodotoxin. The response to the NK3-receptor agonist was halved by atropine and abolished by tetrodotoxin. The response to the NK2-agonist was unaffected by either atropine or tetrodotoxin. 5. The response to the selective NK2-agonist was unchanged after desensitization of NK1- or NK3-receptors. 6. The response to the NK2-selective agonist was strongly inhibited by [Tyr5, D-Trp6,8,9, Arg10]-NKA(4-10) (MEN 10,207) a selective NK2-receptor antagonist which did not modify the response to the NK1-selective agonist. 7. Our findings indicate that all the three known types of tachykinin receptors mediate the contractile response of the circular muscle of the guinea-pig ileum to peptides of this family. The response to activation of NK3-receptors is totally neurogenic and partially mediated by endogenous acetylcholine, the response to activation of NK1-receptors is partly neurogenic and largely myogenic and the response to activation of NK2-receptors is totally myogenic.

Characterization of tachykinin receptors in urinary bladder from guinea pig

The contractile responses to substance P (SP), neurokinin A (NKA), Tyro-neurokinin B (Tyr-NKB), senktide (NK3 receptor selective agonist) and SP methyl ester (SPOMe, NK1 receptor selective agonist) were investigated in detrusor strips from guinea pigs. Except for senktide, all drugs induced a concentration-related contraction with the following rank order of potency: SPOMe greater than SP greater than NKA greater than or equal to Tyr-NKB. After desensitization of NK1 receptors with SPOMe, the rank order of potency was NKA greater than or equal to Tyr-NKB greater than SP greater than SPOMe. Both NK1 and NK2 receptors exist in the detrusor strip from guinea pigs.

Pharmacological analysis of [3H]-senktide binding to NK3 tachykinin receptors in guinea-pig ileum longitudinal muscle-myenteric plexus and cerebral cortex membranes

1. The binding properties and pharmacological specificity of the selective NK3 tachykinin receptor agonist [3H))-senktide [( 3H]-succinyl[Asp6,MePhe8] substance P (6-11] have been examined in homogenates of guinea-pig ileum longitudinal muscle-myenteric plexus (LM/MP) and cerebral cortex. 2. Scatchard analysis of saturation binding studies in guinea-pig ileum LM/MP and cerebral cortex membranes indicated that [3H]-senktide bound to a single site with apparent high affinity, KD = 2.21 +/- 0.65 nM; Bmax = 13.49 +/- 0.04 fmol mg-1 protein in ileum and KD = 8.52 +/- 0.45 nM; Bmax = 76.3 +/- 1.6 fmol mg-1 protein in cortex (values are means +/- ranges; n = 2). 3. The pharmacological profile for tachykinins and analogues in displacing [3H]-senktide from ileum membranes was: [MePhe7] neurokinin B greater than neurokinin B (NKB) congruent to senktide greater than eledoisin greater than substance P (SP) greater than neurokinin A(NKA) greater than physalaemin greater than [Sar9,Met(O2)11]SP greater than [Nle10]NKA(4-10) = [Glp6,L-Pro9]-SP(6-11) greater than substance P methyl ester, consistent with [3H]-senktide binding to an NK3 subtype of tachykinin receptor. A similar rank order of affinity was obtained for these peptides in displacing [3H]-senktide from cortex membranes. 4. Several tachykinin receptor agonists were tested for their ability to displace [3H]-senktide from ileal and cortical NK3 binding sites and were found to be either weak displacers (pIC50 less than 5.00) or inactive. 5. The binding of [3H]-senktide to cortex membranes was inhibited by GTP (p1C,0 = 6.49)and GTP-gamma- S (p1C,0 = 6.67) with ATP being at least three orders of magnitude less potent (pIC50 = 3.55). 6. These results indicate that both central and peripheral NK3 receptors share a similar pharmacological specificity and that they may be labelled selectively with the NK3 receptor agonist [3H]-senktide.