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

Distribution, projections, and association with calbindin baskets of motor neurons, interneurons, and sensory neurons in guinea-pig distal colon

Normal gut function relies on the activity of the enteric nervous system (ENS) found within the wall of the gastrointestinal tract. The structural and functional organization of the ENS has been extensively studied in the guinea pig small intestine, but less is known about colonic circuitry. Given that there are significant differences between these regions in function, observed motor patterns and pathology, it would be valuable to have a better understanding of the colonic ENS. Furthermore, disorders of colonic motor function, such as irritable bowel syndrome, are much more common. We have recently reported specialized basket-like structures, immunoreactive for calbindin, that likely underlie synaptic inputs to specific types of calretinin-immunoreactive neurons in the guinea-pig colon. Based on detailed immunohistochemical analysis, we postulated the recipient neurons may be excitatory motor neurons and ascending interneurons. In the present study, we combined retrograde tracing and immunohistochemistry to examine the projections of circular muscle motor neurons, myenteric interneurons, and putative sensory neurons. We focused on neurons with immunoreactivity for calbindin, calretinin and nitric oxide synthase and their relationship with calbindin baskets. Retrograde tracing using indocarbocyanine dye (DiI) revealed that many of the nerve cell bodies surrounded by calbindin baskets belong to motor neurons and ascending interneurons. Unique functional classes of myenteric neurons were identified based on morphology, neuronal markers and polarity of projection. We provide evidence for three groups of ascending motor neurons based on immunoreactivity and association with calbindin baskets, a finding that may have significant functional implications.

Substance P and vasoactive intestinal peptide are reduced in right transverse colon in pediatric slow-transit constipation

BACKGROUND

Slow-transit constipation (STC) is recognized in children but the etiology is unknown. Abnormalities in substance P (SP), vasoactive intestinal peptide (VIP) and nitric oxide (NO) have been implicated. The density of nerve fibers in circular muscle containing these transmitters was examined in colon from children with STC and compared to other pediatric and adult samples.

METHODS

Fluorescence immunohistochemistry using antibodies to NO synthase (NOS), VIP and SP was performed on colonic biopsies (transverse and sigmoid colon) from 33 adults with colorectal cancer, 11 children with normal colonic transit and anorectal retention (NAR) and 51 with chronic constipation and slow motility in the proximal colon (STC). The percentage area of nerve fibers in circular muscle containing each transmitter was quantified in confocal images.

KEY RESULTS

In colon circular muscle, the percentage area of nerve fibers containing NOS > VIP > SP (6 : 2 : 1). Pediatric groups had a higher density of nerve fibers than adults. In pediatric samples, there were no regional differences in NOS and VIP, while SP nerve fiber density was higher in sigmoid than proximal colon. STC children had lower SP and VIP nerve fiber density in the proximal colon than NAR children. Twenty-three percent of STC children had low SP nerve fiber density.

CONCLUSIONS & INFERENCES

There are age-related reductions in nerve fiber density in human colon circular muscle. NOS and VIP do not show regional variations, while SP nerve fiber density is higher in distal colon. 1/3 of pediatric STC patients have low SP or VIP nerve fiber density in proximal colon.

Electrical stimulation - an evolving concept in the treatment of colonic motor dysfunctions

Electrical stimulation of digestive organs is a new approach for the treatment of dismotility-based diseases affecting the gastrointestinal (GI) tract. The most significant advancement in this field has been obtained with stomach stimulation. As a result, a fully implantable stimulation system to treat gastroparesis - the 'Enterra' system - is now commercially available. Similarly, electrical stimulation of the colon may become a valuable alternative to drug therapy and surgical procedures in the treatment of colonic motor dysfunctions. Over the past decade, several stimulation patterns to modulate colon motility have been tested in animal and human models. The results of these studies are reviewed here in connection with aspects regarding physiological mechanisms activated by electrical stimulation of the colon.

The role of tachykinins in circular muscle contractility of the murine ileum: A functional investigation

We investigated the participation of different tachykinin receptors in contractility of circular muscle strips of the mouse ileum using selective NK receptor agonists and antagonists. The NK1 receptor agonist septide (1-100 nM) induced dose-dependent contractions which were reduced by atropine and augmented by L-NNA. L-NNA increased and TTX consecutively reduced contractions to the NK2 receptor agonist beta-A-NKA (1-100 nM). Senktide, agonist of NK3 receptors, failed to induce contractions. NANC contractions to EFS were decreased after NK1 receptor blockade with RP67580. This inhibitory effect was more pronounced after additional blockade of NK2 and NK3 receptors. NK3 receptor antagonism alone reduced contractions at higher frequencies of stimulation. When the duration of the EFS stimulus was increased, the participation of all NK receptor subtypes became more evident. Our results suggest that excitatory NANC transmission in the circular muscle layer of the mouse ileum is mediated by tachykinins acting principally on NK1 receptors on cholinergic nerves and smooth muscle cells. Also NK2 receptors, located on smooth muscle cells and nitrergic neurons, and NK3 receptors on enteric neurons are involved.

Functional characterisation of tachykinin receptors in the circular muscle layer of the mouse ileum

OBJECTIVES

Tachykinins are important mediators in neuromuscular signalling but have not been thoroughly characterised in the mouse gut. We investigated the participation of tachykinin receptors in contractility of circular muscle strips of the mouse ileum.

RESULTS

Electrical field stimulation (EFS) of excitatory nonadrenergic noncholinergic (NANC) nerves induced frequency-dependent contractions which were mimicked by substance P (SP). Desensitisation of SP and NK(1), NK(2) or NK(3) receptors significantly reduced contractions to EFS. The NK(1) receptor blocker RP67580 significantly inhibited NANC contractions to EFS. The NK(2) and NK(3) receptor blockers nepadutant and SR142801 did not affect NANC contractions per se but increased the RP67580-induced inhibition of NANC contractions to EFS. Contractions to SP were significantly reduced by RP67580 but not affected by nepadutant or SR142801. The NK(1) and NK(2) receptor agonists, septide and [beta-ala(8)]-NKA 4-10 (beta-A-NKA), respectively, but not the NK(3) receptor agonist senktide-induced dose-dependent contractions. Atropine inhibited and l-NNA augmented contractions to septide. Contractions to beta-A-NKA were insensitive to atropine but augmented by l-NNA.

CONCLUSIONS

Tachykinins mediate NANC contractions to EFS in the mouse small intestine. Endogenously released tachykinins activate mainly NK(1) receptors, located on cholinergic nerves and smooth muscle cells and, to a lesser degree, NK(2) and NK(3) receptors, most likely located presynaptically.

Tachykinin NK2 receptor antagonists for the treatment of irritable bowel syndrome

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

Otilonium bromide inhibits muscle contractions via L-type calcium channels in the rat colon

The aim of this study is to evaluate in vitro the effect of otilonium bromide (OB) on the mechanical and electrical activities of the rat colonic smooth muscle using muscle bath, microelectrodes and patch-clamp techniques. Otilonium bromide dose dependently inhibited the spontaneous activity (logIC(50) +/- SE: -5.31 +/- 0.05). This effect was not modified by TTX (10(-6) mol L(-1)). Cyclic depolarizations were abolished by OB (10(-4) mol L(-1)). Electrical field stimulation induced inhibitory junction potentials (IJPs) followed by a depolarization with superimposed spikes causing a contraction. In the presence of OB (10(-4) mol L(-1)) IJPs were recorded, but spikes and contractions were abolished. Otilonium bromide (3 x 10(-6) mol L(-1)) inhibited inward current obtained in isolated cells (amphotericin perforated patch technique). The otilonium-sensitive current amplitude was maximal (75pA) around 0 mV. The effect of different doses of OB was tested by depolarizing cells from -70 mV to 0 mV. OB dose dependently inhibited the inward current with an EC(50) of 885 nmol L(-1). Abolishment of the otilonium-sensitive current by 3 x 10(-6) mol L(-1) nifedipine confirmed that it was an L-type Ca(2+) current. Our results show that OB inhibits the spontaneous and triggered muscular contractions. This effect is produced by the inhibition of muscular action potentials carried by L-type calcium current, confirming the spasmolytic properties of OB.

Peristalsis regulation by tachykinin NK1 receptors in the rabbit isolated distal colon

In the gastrointestinal tract, tachykinin NK1 receptors are widely distributed in a number of neuronal and nonneuronal cells involved in the control of gut motor activity. In particular, in the rabbit isolated distal colon, which is a suitable model system to investigate the contribution of tachykinins as noncholinergic excitatory transmitters, the influence of NK1 receptors in the regulation of peristalsis is not known. The selective NK1-receptor antagonists SR-140333 (0.3 and 1 nM) and MEN-10930 (0.3-10 nM) significantly enhanced the velocity of rabbit colonic propulsion to submaximal stimulation. The prokinetic effect of SR-140333 was prevented by N(omega)-nitro-L-arginine (L-NNA), a nitric oxide synthase inhibitor, indicating that NK1 receptors located on nitrergic innervation exert a functional inhibitory restraint on the circular muscle and probably on descending excitatory and inhibitory pathways during propulsion. Conversely, the selective NK1-receptor agonist septide (3-10 nM) significantly inhibited colonic propulsion. In the presence of L-NNA, the inhibitory effect of septide was reverted into a prokinetic effect, which is probably mediated by the activation of postjunctional excitatory NK1 receptors.

Peripheral tachykinin receptors as targets for new drugs

Tachykinins are widely distributed in the peripheral nervous system of the respiratory, urinary and gastrointestinal tract, stored in enteric neurons and in peripheral nerve endings of capsaicin-sensitive primary afferent neurons from which are released by stimuli having both pathological and physiological relevance. The most studied effects produced by tachykinins in these systems are smooth muscle contraction, plasma protein extravasation, mucus secretion and recruitment/activation of immune cells. The use of tachykinin receptor-selective antagonists and knockout animals has enabled to identify the involvement of tachykinin NK(1), NK(2) and NK(3) receptors as mediators of peripheral effects of tachykinins in different systems/species. The bulk of data obtained in experimental animal models suggests that tachykinins could contribute to the genesis of symptoms accompanying various human diseases including asthma/bronchial hyperreactivity, cystitis of various aetiology, inflammatory bowel diseases and irritable bowel syndrome. Tachykinin receptor antagonists are expected to afford therapeutically relevant effects.

Tachykinin NK(2) receptors and enhancement of cholinergic transmission in the inflamed rat colon: an in vivo motility study

In the gastrointestinal tract, tachykinin NK(2) receptors are localized both on smooth muscle and nerve fibres. NK(2) receptor antagonists reduce exaggerated intestinal motility in various diarrhoea models but the site of action contributing to this effect is unknown. In this study we investigated the effects of atropine (1.4 micromol kg(-1), i.v.), hexamethonium (13.5 micromol kg(-1), i.v.), and nepadutant (0.1 micromol kg(-1), i.v.), a selective tachykinin NK(2) receptor antagonist, on distension (0.5 and 1 ml)-, or irritation (acetic acid, 0.5 ml of 7.5% v v(-1))-induced motility in the rat distal colon in vivo. The effects of atropine, hexamethonium or N(omega)-nitro-L-argininemethylester (L-NAME, 1.85 micromol kg(-1), i.v.) on [betaAla(8)]NKA(4-10) (10 nmol kg(-1), i.v.)-induced colonic contractions were also investigated. When the colonic balloon was filled with a subthreshold volume (0.5 ml), the intraluminal instillation of acetic acid triggered a high-amplitude phasic colonic motility which was partially reduced by nepadutant and suppressed by either hexamethonium or atropine. Filling of the balloon with 1 ml evoked reflex (hexamethonium-sensitive), atropine-sensitive phasic colonic motility: nepadutant had no significant effect on the distension-evoked motility. Neither hexamethonium nor atropine significantly reduced [betaAla(8)]NKA(4-10)-induced colonic contractions, whereas nepadutant suppressed them. Following L-NAME pretreatment, [betaAla(8)]NKA(4-10)-induced colonic contractions were inhibited by both atropine and hexamethonium. In hexamethonium-pretreated animals, an atropine-sensitive component of [betaAla(8)]NKA(4-10)-induced colonic contractions was also evident. These results indicate that the application of irritants onto the colonic mucosa induces the release of endogenous tachykinins which enhance excitatory cholinergic mechanisms through the stimulation of NK(2) 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.

Peripheral actions of tachykinins

Tachykinins mediate a variety of physiological processes in the gastrointestinal, pulmonary and genito-urinary tract mainly through the stimulation of NK1 and NK2 receptors. Preclinical evidence obtained through the use of selective tachykinin receptor antagonists indicates that endogenous tachykinins are involved in augmented smooth muscle contraction, vasodilatation, chemotaxis and activation of immune cells, mucus secretion, water absorption/secretion. Recent evidence also suggests that endogenous tachykinins released at the peripheral level may play a role in visceral inflammation, hyperreflexia and hyperalgesia. Possible mechanisms underlying the stimulation of primary afferent neurons by tachykinins may involve a direct excitation of these neurons and the release of mediators which sensitise or stimulate sensory nerves. Tachykinin receptor antagonists could have a clinical utility in several human diseases such as irritable bowel syndrome, asthma, and in micturition disturbances characterized by a hyperactive bladder.

Involvement of tachykinin NK2 receptors in the modulation of spontaneous motility in rat proximal colon

The role of endogenous tachykinins and the mechanisms whereby they act on NK2 receptors, modulating spontaneous motility, were investigated in rat isolated proximal colon. The mechanical activity was detected as changes in intraluminal pressure. The NK2 receptor antagonist, MEN 10627, produced a concentration-dependent reduction of the contraction amplitude. [beta-Ala8]-neurokinin A(4-10), an NK2 receptor agonist, and [Sar9, Met(O2)11]-Substance P ([Sar9, Met(O2)11]-SP), an NK1 receptor agonist, induced a concentration-dependent contractile response, characterized by an increase in basal tone with superimposed phasic contractions. MEN 10627 antagonized the response to [beta-Ala8]-neurokinin A(4-10), without affecting that to [Sar9, Met(O2)11]-SP. Tetrodotoxin (TTX), hexamethonium and Nomega-nitro-L-arginine methyl ester (L-NAME) significantly reduced the response to MEN 10627. The NK3 receptor agonist, senktide, was able to activate the nitrergic inhibitory pathway, as it induced a TTX-and L-NAME-sensitive inhibitory effect. [beta-Ala8]-neurokinin A(4-10) was able to antagonize the inhibitory response to senktide. These findings suggest that tachykinins acting on NK2 receptors play a role in the modulation of the spontaneous mechanical activity. The mechanism of this action would be, in part, acting directly on the smooth muscle cells, and, in part neurogenic, sustained by nicotinic inputs, and possibly due to inhibition of NO tonic release.

Co-distribution of NK2 tachykinin receptors and substance P in nerve endings of guinea-pig ileum

The distribution of NK2 tachykinin receptors-immunoreactivity (NK2r-IR) in the guinea-pig ileum and the co-distribution of NK2r-IR with substance P-immunoreactivity (SP)-IR were investigated. NK2r-IR was detected in varicose fibers of myenteric and submucous ganglia and nerve strands, in the longitudinal and circular muscle layers and at the deep muscular plexus (DMP). Except for the submucous plexus, some of the NK2r-IR varicose fibers were co-distributed with SP-IR ones and quantitative analysis showed significant regional differences in the percentages of these fibers. These results demonstrate that presynaptic NK2 receptors are located at varicose fibers likely originating from motor neurons projecting to muscle layers and DMP, and from interneurons. Furthermore, the NK2r/SP-IR co-distribution suggests that some of these receptors are autoreceptors on SP nerve endings.

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

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

Contribution of NK(2) tachykinin receptors to propulsion in the rabbit distal colon

The role of the tachykinin neurokinin (NK)(2) receptors on rabbit distal colon propulsion was investigated by using two selective NK(2)-receptor antagonists, MEN-10627 and SR-48968. Experiments on colonic circular muscle strips showed that contractile responses to [beta-Ala(8)]NKA-(4-10) (1 nM-1 microM), a selective NK(2)-receptor agonist, were competitively antagonized by MEN-10627 (1-100 nM), whereas SR-48968 (0.1-10 nM) caused an insurmountable antagonism, thus confirming the difference in the mode of action of the two compounds. Colonic propulsion was elicited by distending a mobile rubber balloon with 0.3 ml (submaximal stimulus) or 1.0 ml (maximal stimulus) of water. The velocity of anal displacement of the balloon (mm/s) was considered the main propulsion parameter. At low concentrations (1.0-100 nM and 0.1-10 nM, respectively), MEN-10627 and SR-48968 facilitated the velocity of propulsion, whereas at high concentrations (100 nM and 1 microM, respectively) they decelerated propulsion. The excitatory and inhibitory effects of both antagonists were observed only with submaximal stimulus. We focused on the hypothesis that the facilitatory effect on propulsion may result from blockade of neuronal NK(2) receptors and the inhibitory effect from suppression of the excitatory transmission mediated by NK(2) receptors on smooth muscle cells. In the presence of N(G)-nitro-L-arginine (300 microM), a nitric oxide synthase inhibitor, MEN-10627, at a concentration (10 nM) that was found to accelerate propulsion in control experiments inhibited the velocity of propulsion. In the presence of threshold (1-10 nM) or full (1 microM) concentration of atropine, which inhibited to a great extent the velocity of propulsion, the inhibitory effect of MEN-10627 (1 microM) was markedly increased. In conclusion, in the rabbit distal colon NK(2) receptors may decelerate propulsion by activating a nitric oxide-dependent neuronal mechanism and may accelerate it by a postjunctional synergistic interaction with cholinergic muscarinic receptors.

Tachykinin NK1 and NK2 receptor-mediated control of peristaltic propulsion in the guinea-pig small intestine in vitro

The tachykinins substance P and neurokinin A are excitatory cotransmitters of cholinergic enteric neurons, their actions being mediated by NK1, NK2 and NK3 receptors. This study examined which of these receptors are part of the neural circuitry of peristalsis. Peristaltic propulsion in luminally perfused segments of the guinea-pig isolated ileum was elicited by a rise of the intraluminal pressure. The pressure threshold at which peristaltic contractions were triggered was used to quantify drug effects on peristalsis, inhibition of peristalsis being reflected by an increase in the pressure threshold. The NK1, NK2 and NK3 receptor antagonists SR-140333, SR-48968 and SR-142 801 (each at 0.1 microM), respectively, had little effect on peristaltic activity as long as cholinergic transmission was left intact. However, both the NK1 and NK2 receptor antagonist (each at 0.1 microM) abolished peristalsis after cholinergic transmission via muscarinic receptors had been blocked by atropine (1 microM) and peristalsis rescued by naloxone (0.5 microM). When cholinergic transmission via nicotinic receptors was suppressed by hexamethonium (100 microM) and peristalsis restored by naloxone (0.5 microM), only the NK2 receptor antagonist (0.1 microM) was able to attenuate peristaltic performance as deduced from a rise of the peristaltic pressure threshold by 106%. The NK3 receptor antagonist (0.1 microM) lacked a major influence on peristalsis under any experimental condition. It is concluded that tachykinins acting via NK1 and NK2 receptors sustain intestinal peristalsis when cholinergic neuroneuronal and neuromuscular transmission via muscarinic receptors has been suppressed. NK2 receptors help maintaining peristalsis once cholinergic neuroneuronal transmission via nicotinic receptors has been blocked, whereas NK3 receptors play little role in the neural pathways of peristalsis.

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

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

Involvement of nitric oxide in the substance P-induced inhibition of intestinal peristalsis

Although considered as an intestinal motor stimulant, substance P can inhibit intestinal peristalsis via stimulation of tachykinin NK1 receptors. Since NK1 receptors are present on enteric nitrergic neurones, the contribution of nitric oxide (NO) to the peristaltic motor inhibition caused by tachykinins was examined in luminally perfused segments of isolated guinea-pig ileum. Substance P (100 nM) and the NK1 receptor agonist substance P methyl ester (100 nM) increased the intraluminal pressure threshold at which peristaltic contractions were elicited. This inhibitory influence on peristalsis was prevented by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (300 microM) in an enantiomer-selective manner. It is concluded that the substance P/NK1 receptor-mediated depression of intestinal peristalsis involves inhibitory motor pathways utilizing NO as a transmitter.

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.