Tachykinin NK1 and NK2 receptors mediate non-adrenergic non-cholinergic excitatory neuromuscular transmission in the guinea-pig stomach

Influence of a neurokinin-1 receptor antagonist (aprepitant) on gastric sensorimotor function in healthy volunteers

BACKGROUND

Substance P (SP) is a member of the neurokinin (NK) family and is one of the established neurotransmitters in the mammalian central and enteric nervous system. It is unclear whether NK1 receptors are involved in the control of gastric sensorimotor function in man.

METHODS

We studied the effects of aprepitant, an NK1 receptor antagonist used in the treatment of chemotherapy-induced emesis, on gastric sensorimotor function in healthy volunteers. Sixteen healthy volunteers (six males, 32.4 ± 2.7 years) were studied on three separate occasions after placebo, aprepitant 80 or 125 mg in randomized double-blind study to assess gastric compliance, perception to isobaric distensions, and gastric accommodation with a gastric barostat.

KEY RESULTS

Compared to placebo, both doses of aprepitant did not influence gastric compliance or sensitivity to gastric distension. Aprepitant 80 and 125 mg did not have any significant effects on gastric accommodation compared with placebo (mean postprandial gastric volume increase, respectively, 83.4 ± 28.4 vs 35.3 ± 16.2 vs 83.9 ± 30.4 mL, NS). Postprandial gastric compliance and sensitivity to distention were also not altered.

CONCLUSIONS & INFERENCES

In health, NK1 receptors do not appear to be involved in the control of gastric compliance, accommodation or sensitivity to distention in man.

Mutational analysis of TAC3 and TACR3 genes in patients with idiopathic central pubertal disorders

OBJECTIVE

To investigate the presence of variants in the TAC3 and TACR3 genes, which encode NKB and its receptor (NK3R), respectively, in a large cohort of patients with idiopathic central pubertal disorders.

SUBJECTS AND METHODS

Two hundred and thirty seven patients were studied: 114 with central precocious puberty (CPP), 73 with normosmic isolated hypogonadotropic hypogonadism (IHH), and 50 with constitutional delay of growth and puberty (CDGP). The control group consisted of 150 Brazilian individuals with normal pubertal development. Genomic DNA was extracted from peripheral blood and the entire coding region of both TAC3 and TACR3 genes were amplified and automatically sequenced.

RESULTS

We identified one variant (p.A63P) in NKB and four variants, p.G18D, p.L58L (c.172C>T), p.W275* and p.A449S in NK3R, which were absent in the control group. The p.A63P variant was identified in a girl with CPP, and p.A449S in a girl with CDGP. The known p.G18D, p.L58L, and p.W275* variants were identified in three unrelated males with normosmic IHH.

CONCLUSION

Rare variants in the TAC3 and TACR3 genes were identified in patients with central pubertal disorders. Loss-of-function variants of TACR3 were associated with the normosmic IHH phenotype.

Role for NK(1) and NK(2) receptors in the motor activity in mouse colon

The present study examined the effects induced by endogenous and exogenous activation of NK(1) and NK(2) receptors on the mechanical activity of mouse proximal colon. Experiments were performed in vitro recording the changes in intraluminal pressure from isolated colonic segments. Electrical field stimulation in the presence of atropine and guanethidine produced a small relaxation, followed by nonadrenergic noncholinergic (NANC) contraction. SR140333, NK(1) receptor antagonist, or SR48968, NK(2) receptor antagonist, significantly reduced the contraction, although SR48968 appeared more efficacious. The co-administration of SR140333 and SR48968 virtually abolished the NANC contraction. [Sar(9), Met(O(2))(11)]-substance P, selective NK(1) receptor agonist, induced a concentration-dependent biphasic effect, contraction followed by reduction of the mechanical spontaneous activity. Both effects were antagonized by SR140333, but not by SR48968. [beta-Ala(8)]-neurokinin A (4-10), selective NK(2) receptor agonist, evoked concentration-dependent contraction, which was antagonized by SR48968, but not by SR140333. The contraction induced by [Sar(9), Met(O(2))(11)]-substance P, but not by [beta-Ala(8)]-neurokinin A (4-10), was reduced by tetrodotoxin or atropine, and increased by N(omega)-nitro-L-arginine methyl ester (L-NAME), inhibitor of nitric oxide synthase. The inhibitory effects induced by [Sar(9), Met(O(2))(11)]-substance P were abolished by tetrodotoxin or L-NAME. The results of the present study suggest that in mouse colon both NK(1) and NK(2) receptors are junctionally activated by endogenous tachykinins to cause an additive response. NK(1) receptors appear to be located on cholinergic and on nitrergic neurons as well as on smooth muscle cells, whereas NK(2) receptors seem to be present exclusively on smooth muscle cells.

Fibre-free diet leads to impairment of neuronally mediated muscle contractile response in rat distal colon

Dietary fibre consumption is known to be beneficial to increase stool bulk and frequency. In contrast, it is unclear whether chronic dietary fibre deficiency affects colonic motor functions, especially neuronally mediated muscle contractions. In this study, rats were fed a fibre-free diet or diet containing dietary fibre (cellulose or guar gum) for 27 days. Furthermore, neurogenic and myogenic contractions were evaluated in circular and longitudinal muscle strips of the distal colon. Additionally, the number of enterochromaffin (EC) cells, which play important roles in the initiation of the peristaltic reflex, was also examined by immunohistochemistry for serotonin. Myogenic contractions induced by carbachol or substance P were examined in the presence of tetrodotoxin. Circular muscle was hyposensitive to carbachol, but longitudinal muscle was hypersensitive to substance P in the fibre-free group. Nerve-mediated circular (5-20 Hz) and longitudinal (1-2 Hz) muscle contractions evoked by electrical field stimulation were attenuated in the fibre-free group and the latter response was almost abolished by atropine, suggesting functional changes of cholinergic neurons. EC cell number was decreased in the fibre-free group. In conclusion, changes in neurogenic and myogenic contractions and a decrease in EC cell number observed may affect colonic motility of the fibre-free group.

Dual effects of neurokinin on calcium channel currents and signal pathways in neonatal rat nucleus tractus solitarius

Neurokinins, such as substance P (SP), modulate the reflex regulation of cardiovascular and respiratory function in the CNS, particularly in the nucleus tractus solitarius (NTS). There is considerable evidence of the action of SP in the NTS, but the precise effects have not yet been determined. Voltage-dependent Ca2+ channels (VDCCs) serve as crucial mediators of membrane excitability and Ca2+ -dependent functions such as neurotransmitter release, enzyme activity and gene expression. The purpose of this study was to investigate the effects of neurokinins on VDCCs currents (ICa) in the NTS using patch-clamp recording methods. In 142 of 282 neurons, an application of [Sar(9), Met(O(2)11]-substance P (SSP, NK(1) receptor agonist) caused facilitation of L-type I(Ba). Intracellular dialysis of the Galpha(q/11)-protein antibody attenuated the SSP-induced facilitation of I(Ba). In addition, phospholipase C (PLC) inhibitor, protein kinase C (PKC) inhibitor and PKC activator attenuated the SSP-induced the facilitation of I(Ba). In contrast, in 115 of 282 neurons, an application of SSP caused inhibition of N- and P/Q-types I(Ba). Intracellular dialysis of the Gbetagamma-protein antibody attenuated the SSP-induced inhibition of I(Ba). These results indicate that NK(1) receptor facilitates L-type VDCCs via Galpha(q/11)-protein involving PKC in NTS. On the other hand, NK(1) receptor inhibits N- and P/Q-types VDCCs via Galpha(q/11)-protein betagamma subunits in NTS.

Role of NK1 and NK2 receptors in mouse gastric mechanical activity

1. The aim of the present study was to examine the role of NK1 and NK2 receptors in the control of mechanical activity of mouse stomach. In this view, the motor effects induced by NK1 and NK2 receptor agonists and antagonists were analyzed, measuring motility as intraluminal pressure changes in mouse-isolated stomach preparations. In parallel, immunohistochemical studies were performed to identify the location of NK1 and NK2 receptors on myenteric neurons and smooth muscle cells. 2. Substance P (SP) induced biphasic effects: a contraction followed by relaxation; neurokinin A (NKA) and [beta-Ala8]-NKA(4-10), selective agonist of NK2 receptors, evoked concentration-dependent contractions, whereas [Sar9, Met(O2)11]-SP, selective agonist of NK1 receptors, induced concentration-dependent relaxation. 3. SR48968, NK2 receptor antagonist, did not modify the spontaneous activity and reduced the contractile effects induced by tachykinins without affecting the relaxation. SR140333, NK1 receptor antagonist, did not modify the spontaneous activity and antagonized the relaxant response to tachykinins, failing to affect the contractile effects. 4. The relaxation to SP or to [Sar9, Met(O2)11]-SP was abolished by tetrodotoxin (TTX) and significantly reduced by N(omega)-nitro-L-arginine methyl ester (L-NAME). 5. NK2-immunoreactivity (NK2-IR) was seen at the level of the smooth muscle cells of both circular and longitudinal muscle layers. NK1-immunoreactive (NK1-IR) neurons were seen in the myenteric ganglia and NK1/nNOS double labeling revealed that some neurons were both NK1-IR and nNOS-IR. 6. These results suggest that, in mouse stomach, NK1 receptors, causing relaxant responses, are present on nitrergic inhibitory myenteric neurons, whereas NK2 receptors, mediating contractile responses, are present at muscular level.

Mechanical activation of rectal intraganglionic laminar endings in the guinea pig distal gut

The rectum receives specialized extrinsic afferent innervation by stretch-sensitive, low threshold, slowly adapting mechanoreceptors, with transduction sites shown to correspond to rectal intraganglionic laminar endings (IGLEs). Rectal IGLEs are located in myenteric ganglia, surrounded by the longitudinal and circular smooth muscle layers; in this study we investigated the mechanical stimuli to which they respond. Mechanoreceptors had graded responses to highly focal transmural compression with von Frey hairs. They were activated when stretched circumferentially by imposed increases of both length and load. Under both conditions, firing typically occurred in bursts associated with phasic muscle contractions. However, many contractions did not evoke firing. Longitudinal stretch also evoked firing, again associated with contractile activity. Thus, mechanoreceptors did not show directional sensitivity. Two agonists that excited smooth muscle directly (0.1 microm [beta-Ala(8)]-neurokinin A (4-10) and 1 microm carbachol) activated rectal mechanoreceptors, but not in the presence of Ca(2)(+)-free solution or when preparations were kept entirely slack. We measured the dimensions, in both longitudinal and circumferential axes, of receptive fields during smooth muscle contractile activity, using video micrography. Contractile activity within the receptive field often differed significantly from the behaviour of the preparation as a whole, providing an explanation for many of the discrepancies between gross contractility and firing. Simultaneous contraction of both muscle layers within the receptive field was the strongest predictor of mechanoreceptor activation. Our results suggest that rectal mechanoreceptors do not act simply as tension receptors: rather they appear to detect mechanical deformation of myenteric ganglia - especially flattening - associated with stretch of the receptive field, or contractions of smooth muscle within the receptive field.

Protein kinase C is involved in neurokinin receptor modulation of N- and L-type Ca2+ channels in DRG neurons of the adult rat

Whole cell patch-clamp techniques were used to examine neurokinin receptor modulation of Ca2+ channels in small to medium size dorsal root ganglia neurons (<40 pF) that express mainly N- and L-type Ca2+ currents. Low concentrations of substance P enhanced Ca2+ currents (5-40%, <0.2 microM), while higher concentrations applied cumulatively reversed these enhancements (5-28% reductions, >0.5 microM). This apparent inhibition by high concentrations of substance P was blocked by the administration of the NK3 antagonist SB 235,375 (0.2 microM). The NK1 agonist, [Sar9,Met11]-substance P (0.05 to 1.0 microM) did not alter Ca2+ currents; whereas the NK2 agonist, [betaAla8]-neurokinin A (4-10), enhanced Ca2+ currents (5-36% increase, 0.05-0.5 microM). The enhancement was reversed by the NK2 antagonist MEN 10,376 (0.2 microM) but unaffected by the NK3 antagonist SB 235,375 (0.2 microM). The NK3 agonist [MePhe7]-neurokinin B (0.5-1.0 microM) inhibited Ca2+ currents (6-24% decrease). This inhibition was not prevented by the NK2 antagonist MEN 10,376 (0.2 microM) but was blocked by the NK3 antagonist SB 235,375 (0.2 microM). Both the enhancement and inhibition of Ca2+ currents by neurokinin agonists were reversed by the protein kinase C inhibitor bisindolylmaleimide I HCl (0.2-0.5 microM). Following inhibition of Ca2+ channels by [MePhe7]-neurokinin the facilitatory effect of BayK 8644 (5 microM) was increased and the inhibitory effect of the N-type Ca2+ channel blocker w -conotoxin GVIA (1 microM) was diminished, suggesting that the NK3 agonist inhibits N-type Ca2+ channels. Similarly, block of all but N-type Ca2+ channels, revealed that [betaAla8]-neurokinin A (4-10) enhanced the currents while [MePhe7]-neurokinin B inhibited the currents. Inhibition of all but L-type Ca2+ channels, revealed that [betaAla8]-neurokinin A (4-10) enhanced the currents while [MePhe7]-neurokinin B had no effect. Activation of protein kinase C with low concentrations of phorbol-12,13-dibutyrate enhanced Ca2+ currents, but high concentrations inhibited N- and L-type Ca2+ currents. In summary, these data suggest that in adult rat dorsal root ganglia neurons, NK2 receptors enhance both L- and N-type Ca2+ channels and NK3 receptors inhibit N-type Ca2+ channels and that these effects are mediated by protein kinase C phosphorylation of Ca2+ channels.

Interaction of the growth hormone-releasing peptides ghrelin and growth hormone-releasing peptide-6 with the motilin receptor in the rabbit gastric antrum

The structural relationship between the motilin and the growth hormone secretagogue receptor (GHS-R), and between their respective ligands, motilin and ghrelin, prompted us to investigate whether ghrelin and the GHS-R agonist growth hormone-releasing peptide-6 (GHRP-6), could interact with the motilin receptor. The interaction was evaluated in the rabbit gastric antrum with binding studies on membrane preparations and with contraction studies on muscle strips in the presence of selective antagonists under conditions of electrical field stimulation (EFS) or not. Binding studies indicated that the affinity (pK(d)) for the motilin receptor was in the order of ghrelin (4.23 +/- 0.07) < GHRP-6 (5.54 +/- 0.08) < motilin (9.13 +/- 0.03). The interaction of ghrelin with the motilin receptor requires the octanoyl group. Motilin induced smooth muscle contractile responses but ghrelin and GHRP-6 were ineffective. EFS elicited on- and off-responses that were increased by motilin already at 10(-9) M, but not by 10(-5) M ghrelin. In contrast, GHRP-6 also enhanced the on- and off-responses. The motilin antagonist Phe-cyclo[Lys-Tyr(3-tBu)-betaAla-] trifluoroacetate (GM-109) blocked the effect of GHRP-6 on the off-responses but not on the on-responses. Under nonadrenergic noncholinergic conditions, the effects of motilin and GHRP-6 on the on-responses were abolished; those on the off-responses were preserved. All responses were blocked by neurokinin (NK)(1) and NK(2) antagonists. In conclusion, ghrelin is unable to induce contractions via the motilin receptor. However, GHRP-6 enhances neural contractile responses, partially via interaction with the motilin receptor on noncholinergic nerves with tachykinins as mediator, and partially via another receptor that may be a GHS-R subtype on cholinergic nerves that corelease tachykinins.

5-HT4 receptors contribute to the motor stimulating effect of levosulpiride in the guinea-pig gastrointestinal tract

BACKGROUND

The dopamine D2 receptor antagonist levosulpiride is a substituted benzamide derivative, whose gastrokinetic properties are exploited clinically for the management of functional dyspepsia. However, for other benzamide derivatives, such as cisapride and mosapride, agonism towards serotonin 5-HT4 receptors is considered the main mechanism leading to gastrointestinal prokinesia.

AIMS

To assess whether levosulpiride is able to activate 5-HT4 receptors in the guinea-pig isolated gastrointestinal tract.

MATERIALS AND METHODS

Circular muscle strips from gastric antrum, and colonic longitudinal muscle strips were used to detect electrically stimulated neurogenic contractions. The effect of levosulpiride was assessed in the absence and presence of GR125487, a selective 5-HT4 receptor antagonist. Furthermore, potential interaction of levosulpiride with 5-HT3 receptors and tissue cholinesterases was assessed in unstimulated ileal longitudinal muscle-myenteric plexus preparations.

RESULTS

Antral and colonic strip contractions were cholinergic/tachykinergic in nature. Micromolar concentrations of levosulpiride potentiated submaximal responses, through a mechanism competitively antagonized by GR125487 (pKB=9.4). In LMMPs, levosulpiride slightly affected contractions caused by the 5-HT, receptor agonist 2-methyl-5-HT, and had no effect on contractions to exogenous acetylcholine.

CONCLUSIONS

Our results indicate that levosulpiride acts as a moderate agonist at the 5-HT4 receptor. This property, together with antagonism at D2 receptors, may contribute to its gastrointestinal prokinetic effect.

The promise of substance P inhibitors in fibromyalgia

The discovery of SP and its potent biological activities have lead to the discovery of other tachykinins and to receptors for them, including the NK1 receptor. Blockade of the NK1 receptor has a number of potentially beneficial effects in medical care including the management of drug-induced emesis and the treatment of depression. The analgesic potential of NK1 receptor antagonists that, in theory, seemed so promising has not met early expectations. However, there is still reason to predict valuable clinical uses for more potent NK1 receptor antagonists in a variety of medical conditions, including FMS.

Role of tachykininergic and cholinergic pathways in modulating canine gastric tone and compliance in vivo

Acetylcholine and tachykinins act as co-transmitters along excitatory pathways at different gut levels. Since cholinergic pathways are involved in maintaining gastric tone during fasting, our aim was to study the possible role of tachykininergic pathways in modulating canine gastric tone and compliance in vivo by using selective tachykinin receptor antagonists. In four fasting, conscious dogs, we characterized the pressure-volume relationship in the proximal stomach by using a barostat. We increased the pressure of the intragastric bag by 2 mmHg increments every 3 min, starting from a baseline value of 2 up to 12 mmHg. Drug effects were investigated by studying pressure-volume relationships before and 15 min after intravenous (i.v.) administration of SR140333, SR48968, or SR142801 (respectively, NK (1)-, NK (2)-, and NK (3)-receptor antagonist, each at the dose of 1 mg kg (-1)) or atropine (100 microg kg (-1)). Pressure-volume curves were fitted by nonlinear regression analysis. Before drug administration, the curve that best fitted the pressure-volume relationship was exponential. SR140333, SR48968 and SR142801 did not affect baseline gastric tone or the gastric pressure-volume curve at any distension level. At a distending pressure of 6 mmHg, the Delta volumes obtained after administration of SR140333, SR48968 or SR142801 vs control were 65 +/- 28, 27 +/- 26, 14 +/- 20 ml, respectively. The same was true even when all three antagonists were administered together to achieve simultaneous blockade of all three tachykinin receptor subtypes. Atropine increased baseline gastric volume (Delta volume = 237 +/- 15 ml; P< 0.01) and significantly (P< 0.0001) shifted the pressure-volume curve to the left. After atropine, a linear equation best fitted the pressure-volume curve. We conclude that tachykininergic pathways are not involved in modulating canine gastric tone and compliance during fasting, whereas cholinergic pathways play a major role not only in maintaining gastric tone, but also in modulating the compliance of the proximal stomach to a distending stimulus.

Immunohistochemical demonstration of the NK(1) tachykinin receptor on muscle and epithelia in guinea pig intestine

BACKGROUND AND AIMS

Previous immunohistochemical studies failed to reveal neurokinin (NK)(1) tachykinin receptors on intestinal muscle, despite convincing pharmacologic data indicating their presence. This study aimed to apply optimal immunohistochemical methods to reveal the receptors.

METHODS

NK(1)-receptor immunoreactivity was examined by confocal microscopy in tissue incubated with or without 10(-7) mol/L substance P (SP), 10(-7) mol/L SP plus 10(-6) mol/L NK(1) receptor antagonist (CP99994), or with fluorescent cyanine 3.18 (Cy3) SP.

RESULTS

Without incubation, NK(1)-receptor immunoreactivity was strong on muscle of the rectum and distal colon and weak in proximal colon and small intestine. NK(1) receptor was located on the surface of muscle cells in all gut regions. Exposure to SP increased the intensity of immunoreactivity, and the receptor moved into the cytoplasm. Mobilization of the receptor by SP was blocked by the NK(1)-receptor antagonist CP99994. Cy3-SP was internalized by muscle cells and colocalized with the receptor. NK(1)-receptor immunoreactivity occurred on crypt epithelial cells in the small intestine and the base of glands in the proximal colon.

CONCLUSIONS

The NK(1) receptor occurs on the external muscle throughout the small and large intestines. SP binds and triggers NK(1)-receptor aggregation and internalization in the muscle.

Role of NK2 receptors in gastric barosensitivity and in experimental ileus in rats

This study was performed to evaluate the role of tachykinin NK2 receptors in gastric barosensitivity and in postsurgical intestinal atony, using a selective NK2 antagonist (MEN 11420). Gastric distensions were performed in rats equipped with a gastric balloon and electrodes implanted in the neck muscles. Ileus was produced by laparotomy and caecum palpation in rats previously prepared with electrodes implanted on the proximal jejunum. Fifteen minutes before gastric distension or laparotomy, the animals received MEN 11420 (10, 100 or 200 microg kg-1 intravenously) or saline. The first distending pressure to increase the integrated neck electromyogram > 100% was considered the pain threshold. MEN 11420 (100 microg kg-1) increased significantly pain threshold (20.5 +/- 1.2 vs. 17.0 +/- 0.8 mm Hg) but did not modify gastric volumes at the three doses tested. Abdominal surgery was followed by a total inhibition of jejunal spiking activity lasting 80.4 +/- 18.7 min. MEN 11420 (10 and 100 microg kg-1) shortened the duration of motor inhibition by 36 and 39%, and induced a premature recovery of the phase III of migrating myoelectric complex at the lowest dose tested (130 +/- 32 vs. 192 +/- 28 min). We conclude that NK2 receptors, probably located on afferent fibres, are involved in gastric barosensitivity and in postsurgical intestinal atony.

Cellular expression of the neurokinin 1 receptor in the human antrum

The localization of the neurokinin 1 receptor in rat and guinea pig gastrointestinal tract has been extensively studied but not in human tissues. The present study used antibodies to characterize the cellular expression of neurokinin 1 receptors in human antrum. Cryostat sections (40-80 microm) were immunostained for the neurokinin 1 receptor double labeled with substance P, von Willebrand's factor, c-kit, fibronectin, S-100, serotonin, gastrin and somatostatin. Neurokinin 1 receptor-immunoreactivity was observed on neurons within the myenteric and submucosal plexuses surrounded by substance P-immunoreactive fibers and on von Willebrand's factor-immunoreactive endothelial cells lining blood vessels throughout the antral wall. c-Kit-immunoreactive interstitial cells of Cajal and gastrin cells were co-stained by the monoclonal neurokinin 1 receptor antibody. Finally, there was no evidence for the presence of the neurokinin 1 receptor on fibroblasts, Schwann, somatostatin, serotonin or smooth muscle cells. This study clearly demonstrates an expanded cellular expression of the neurokinin 1 receptor in the human antrum.

Tachykinin receptor and neutral endopeptidase gene expression in the rat uterus: characterization and regulation in response to ovarian steroid treatment

Tachykinin neuropeptides, such as substance P, are localized to a population of sensory fibers that innervate the mammalian female reproductive tract. In the present study, we have characterized tachykinin NK1 receptor (NK1R), NK2 receptor (NK2R), and NK3 receptor (NK3R) gene expression by semiquantitative RT-PCR in uteri from ovariectomized rats and studied their regulation in response to 17beta-estradiol (E2), progesterone (P4), or a combination of both. In addition, we analyzed the expression and regulation of the neutral endopeptidase 24.11 (NEP), the most important enzyme involved in tachykinin degradation in the rat uterus. In uteri from control (olive oil-treated) rats, RT-PCR assays revealed single bands corresponding to the expected product sizes encoding complementary DNA for NK1R (232 bp), NK2R (491 bp), NK3R (325 bp), and NEP (221 bp). The identity of the amplified fragments was confirmed by DNA sequence analysis. Compared with control rats, NK1R messenger RNA (mRNA) was increased by 2-fold in uteri from rats treated with E2, was decreased by 3.3-fold in rats treated with P4, and was decreased by 1.8-fold in rats treated with both E2 and P4. Uterine NK2R mRNA levels were not altered by any steroid treatment. E2 treatment decreased by 15-fold NK3R mRNA. P4 was without effect if administered alone and did not influence the E2-induced decrease in NK3R mRNA. NEP mRNA levels were about 4-fold lower in E2-treated than in P4-treated rats. Functional studies were carried out in uteri from E2- or P4-treated ovariectomized rats to characterize the contractile response evoked by the selective tachykinin receptor agonists [Sar9Met(O2)11]substance P (NK1R selective), [Nle10]NKA-(4-10) (NK2R selective), and [MePhe7]NKB (NK3R selective) in the presence of the NEP inhibitor phosphoramidon (1 microM). A marked correlation was observed between the magnitude of the contractile response to each agonist and the level of expression determined by RT-PCR for each tachykinin receptor. The present findings show that tachykinin NK1R, NK2R, NK3R, and NEP are expressed in the rat uterus and that ovarian steroids differentially regulate their expression.

Evidence that tachykinins are the main NANC excitatory neurotransmitters in the guinea-pig common bile duct

Application of electrical field stimulation (EFS; trains of 10 Hz, 0.25 ms pulse width, supramaximal voltage for 60 s) to the guinea-pig isolated common bile duct pretreated with atropine (1 microM), produced a slowly-developing contraction ('on' response) followed by a quick phasic 'off' contraction ('off peak' response) and a tonic response ('off late' response), averaging 16+/-2, 73+/-3 and 20+/-4% of the maximal contraction to KCl (80 mM), n=20 each, respectively. Tetrodotoxin (1 microM; 15 min before) abolished the overall response to EFS (n 8). Neither in vitro capsaicin pretreatment (10 microM for 15 min), nor guanethidine (3 microM, 60 min before) affected the excitatory response to EFS (n 5 each), showing that neither primary sensory neurons, nor sympathetic nerves were involved. Nomega-nitro-L-arginine (L-NOARG, 100 microM, 60 min before) or naloxone (10 microM, 30 min before) significantly enhanced the 'on' response (294+/-56 and 205+/-25% increase, respectively; n=6-8, P<0.01) to EFS. The combined administration of L-NOARG and naloxone produced additive enhancing effects (655+/-90% increase of the 'on' component, n = 6, P<0.05). The tachykinin NK2 receptor-selective antagonist MEN 11420 (1 microM) almost abolished both the 'on' and 'off late' responses (P<0.01: n=5 each) to EFS, and reduced the 'off-peak' contraction by 55+/-8% (n=5, P<0.01). The subsequent administration of the tachykinin NK1 receptor-selective antagonist GR 82334 (1 microM) and of the tachykinin NK3 receptor-selective antagonist SR 142801 (30 nM), in the presence of MEN 11420 (1 microM), did not produce any further inhibition of the response to EFS (P>0.05; n=5 each). At 3 microM, GR 82334 significantly reduced (by 68+/-9%, P<0.05, n=6) the 'on' response to EFS. The contractile 'off peak' response to EFS observed in the presence of both MEN 11420 and GR 82334 (3 microM each) was abolished (P<0.01; n=6) by the administration of the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM). PPADS (30 microM) selectively blocked (75+/-9 and 50+/-7% inhibition, n = 4 each) the contractile responses produced by 100 and 300 microM ATP. Tachykinin-containing nerve fibres were detected by using immunohistochemical techniques in all parts of the bile duct, being distributed to the muscle layer and lamina propria of mucosa. In the terminal part of the duct (ampulla) some labelled ganglion cells were observed. In conclusion, this study shows that in the guinea-pig terminal biliary tract tachykinins, released from intrinsic neuronal elements, are the main NANC excitatory neurotransmitters, which act by stimulating tachykinin NK2 (and possibly NK1) receptors. ATP is also involved as excitatory neurotransmitter. Nitric oxide and opioids act as inhibitory mediators/modulators in this preparation.

Pharmacological evidence for the existence of multiple P2 receptors in the circular muscle of guinea-pig colon

1. By using the sucrose gap technique, we have investigated the effect of the metabolically stable P2Y receptor agonist, adenosine 5'-O-2-thiodiphosphate (ADPbetaS), on the membrane potential and tension in the circular muscle of the guinea-pig proximal colon. All experiments were performed in the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM), nifedipine (1 microM), L-nitroarginine (L-NOARG, 100 microM) and of the tachykinin NK1 and NK2 receptor antagonists, SR 140333 (0.1 microM) and GR 94800 (0.1 microM), respectively. 2. ADPbetaS (100 microM for 15 s) evoked a tetrodotoxin- (1 microM) resistant hyperpolarization and contraction of the smooth muscle. In the presence of apamin (0.1 microM), the ADPbetaS-induced hyperpolarization was converted to depolarization and the contraction was potentiated while tetraethylammonium (TEA, 10 mM) did not affect significantly the response to ADPbetaS. The combined application of apamin and TEA reproduced the effect observed with apamin alone. 3. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acids (PPADS, 30 microM) slightly but significantly increased the ADPbetaS-induced hyperpolarization, while the contraction evoked by ADPbetaS was reduced by about 80%. Suramin (100 microM) did not affect the ADPbetaS-induced hyperpolarization but totally blocked the ADPbetaS-induced contraction. In the presence of suramin (100 microM), a small relaxation of the circular muscle was observed upon application of ADPbetaS. 4. The contraction and hyperpolarization evoked by ADPbetaS were abolished in Ca2+-free Krebs solution. The blocker of sarcoplasmic reticulum Ca2+ pump, cyclopiazonic acid (10 microM) reduced contraction and hyperpolarization induced by ADPbetaS by about 60 and 50%, respectively. 5. A comparison of our present and previous findings enables to conclude that at least 3 types of P2 receptors are present on the smooth muscle of the guinea-pig colon, as follows: (1) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by alpha,beta-methylene ATP (alpha,beta-meATP) and by endogenously released purines, sensitive to suramin and PPADS; (2) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by ADPbetaS and are resistant to suramin and PPADS; (3) excitatory P2 receptors, producing contraction, which are activated by ADPbetaS and are sensitive to suramin and PPADS. The data also support the idea of the existence of a restricted pool of specialized junctional P2 receptors producing the apamin-sensitive NANC inhibitory junction potential in response to endogenous ligand(s).