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

Affinity, potency, efficacy, and selectivity of neurokinin A analogs at human recombinant NK2 and NK1 receptors

A series of peptide NK2 receptor agonists was evaluated for affinity, potency, efficacy, and selectivity at human recombinant NK2 and NK1 receptors expressed in CHO cells to identify compounds with the greatest separation between NK2 and NK1 receptor agonist activity. Binding studies were performed using displacement of [¹²⁵I]-NKA binding to NK2 receptors and displacement of [³H]-Septide binding to NK1 receptors expressed in CHO cells. Functional studies examining the increase in intracellular calcium levels and cyclic AMP stimulation were performed using the same cell lines. A correlation was demonstrated between binding affinities (Ki) and potency to increase intracellular calcium (EC₅₀) for NK2 and NK1 receptors. Ranking compounds by their relative affinity (Ki) or potency (EC₅₀) at NK2 or NK1 receptors indicated that the most selective NK2 agonists tested were [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA(4-10) (NK1/NK2 Ki ratio = 674; NK1/NK2 EC₅₀ ratio = 105) and [Arg⁵,MeLeu⁹,Nle¹⁰]-NKA(4-10) (NK1/NK2 Ki ratio = 561; NK1/NK2 EC₅₀ ratio = 70). The endogenous peptide, NKA, lacked selectivity with an NK1/NK2 Ki ratio = 20 and NK1/NK2 EC₅₀ ratio = 1. Of the compounds selected for evaluation in cyclic AMP stimulation assays, [β-Ala⁸]-NKA(4–10) had the greatest selectivity for activation of NK2 over NK1 receptors (NK1/NK2 EC₅₀ ratio = 244), followed by [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA(4-10) (ratio = 74), and NKA exhibited marginal selectivity (ratio = 2.8).

Prokinetic effects of neurokinin-2 receptor agonists on the bladder and rectum of rats with acute spinal cord transection

The suitability of various neurokinin-2 (NK2) receptor agonists and routes of administration to elicit on-demand voiding of the bladder and bowel, as future therapy for individuals with spinal cord injury, was examined using a rat model. The current study examined the feasibility of alternative routes of administration, which are more practical for clinical use than intravenous (IV) administration. Voiding and isovolumetric cystometry were recorded in anesthetized, acutely spinalized, female rats after IV, subcutaneous (SC), intramuscular (IM), intranasal (IN), or sublingual (SL) administration of [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA_(4-10) (LMN-NKA). Administration of LMN-NKA (1-10μg/kg IV; 10-300μg/kg SC or IM; 15-1000μg/kg IN or 300-1500μg/kg SL) elicited rapid-onset, short-duration, dose-related increases in bladder pressure and voiding with the rank order for time of both onset and duration being IV < IN < SC = IM < SL. The incidence of voiding was dependent on the dose and route, with all routes resulting in a high voiding efficiency (~ 70%). Like LMN-NKA, neurokinin A (NKA 1-100μg/kg IV) and GR 64349 (0.1-30μg/kg IV or 1-300μg/kg SC) produced rapid-onset, short-duration increases in bladder pressure, as well as colorectal pressure. Administration of vehicle never produced bladder or rectal contractions or voiding. Transient hypotension was observed after IV injection of LMN-NKA, which was less pronounced after SC injection. Hypotension was not apparent with GR 64349. In conclusion, selective NK2 receptor agonists, administered through various non-IV routes of administration, may provide a safe, convenient, and efficacious method for inducing voiding.

Pharmacodynamic evaluation of Lys5, MeLeu9, Nle10-NKA(4-10) prokinetic effects on bladder and colon activity in acute spinal cord transected and spinally intact rats

The purpose of this study was to determine feasibility of a novel therapeutic approach to drug-induced voiding after spinal cord injury (SCI) using a well-characterized, peptide, neurokinin 2 receptor (NK2 receptor) agonist, Lys5, MeLeu9, Nle10-NKA(4-10) (LMN-NKA). Cystometry and colorectal pressure measurements were performed in urethane-anesthetized, intact, and acutely spinalized female rats. Bladder pressure and voiding were monitored in response to intravenous LMN-NKA given with the bladder filled to 70% capacity. LMN-NKA (0.1-300 μg/kg) produced dose-dependent, rapid (<60 s), short-duration (<15 min) increases in bladder pressure. In intact rats, doses above 0.3-1 μg/kg induced urine release (voiding efficiency of ~70% at ≥1 μg/kg). In spinalized rats, urine release required higher doses (≥10 μg/kg) and was less efficient (30-50%). LMN-NKA (0.1-100 μg/kg) also produced dose-dependent increases in colorectal pressure. No tachyphylaxis was observed, and the responses were blocked by an NK2 receptor antagonist (GR159897, 1 mg/kg i.v.). No obvious cardiorespiratory effects were noted. These results suggest that rapid-onset, short-duration, drug-induced voiding is possible in acute spinal and intact rats with intravenous administration of an NK2 receptor agonist. Future challenges remain in regard to finding alternative routes of administration that produce clinically significant voiding, multiple times per day, in animal models of chronic SCI.

Targeting tachykinin receptors for the treatment of functional gastrointestinal disorders with a focus on irritable bowel syndrome

BACKGROUND

Tachykinins (TKs) are a family of endogenous peptides widely expressed in the central and in the peripheral nervous systems as well as in the gastrointestinal (GI) tract. They act as full agonists at three different membrane receptors neurokinin (NK) 1, NK2, and NK3, which are G protein-coupled receptors and in the GI tract are expressed both on neurons and effector cells.

PURPOSE

This article reviews the literature concerning the role of TKs in the GI tract function in physiological and pathological conditions and their potential relevance in the treatment of functional GI disorders with particular reference to irritable bowel syndrome (IBS). The efficacy of NK1 antagonists in chemotherapy-induced and postoperative nausea and vomiting is well established. While pharmacodynamic studies have reported conflicting and negative results concerning the effects of NK1 and of NK3 antagonists, respectively, on the GI tract function in humans, clinical studies applying the NK3 antagonist talnetant in IBS-D were negative. Pharmacodynamic studies applying NK2 antagonists have suggested a role for antagonism of NK2 receptors in modulation of GI chemical-induced altered motility and of stress-induced altered bowel habits. Clinical studies and in particular a recently completed Phase 2 study have reported that the NK2 antagonist ibodutant is effective and safe in treating symptoms of D-IBS, especially in females.

Chronic treatment with otilonium bromide induces changes in L-type Ca²⁺ channel, tachykinins, and nitric oxide synthase expression in rat colon muscle coat

BACKGROUND

Otilonium bromide (OB) is a quaternary ammonium derivative used for the treatment of intestinal hypermotility and is endowed with neurokinin2 receptor (NK2r) antagonist and Ca²⁺ channel blocker properties. Therefore, the possibility that OB might play a role in the neurokinin receptor/Substance-P/nitric oxide (NKr/SP/NO) circuit was investigated after chronic exposition to the drug.

METHODS

Rats were treated with OB 2-20 mg kg⁻¹ for 10 and 30 days. In the proximal colon, the expression and distribution of muscle NOsynthase 1 (NOS1), NK1r, NK2r, SP and Cav 1.2 subunit (for L-type Ca²⁺ channel) and the spontaneous activity and stimulated responses to NK1r and NK2r agonists were investigated.

KEY RESULTS

Immunohistochemistry showed a redistribution of NK1r and L-type Ca²⁺ channel in muscle cells with no change of NK2r at 30 days, a significant increase in muscle NOS1 expression at 10 days and a significant decrease in the SP content early in the ganglia and later in the intramuscular nerve fibers. Functional studies showed no change in spontaneous activity but a significant increase in maximal contraction induced by NK1r agonist.

CONCLUSIONS & INFERENCES

Chronic exposition to OB significantly affects the NKr/SP/NO circuit. The progressive decrease in SP-expression might be the consequence of the persistent presence of OB, the increase of NOS1 expression in muscle cells at 10 days in an attempt to guarantee an adequate NO production, and, at 30 days, the redistribution of the L-type Ca²⁺ channel and NK1r as a sign to compensate the drug channel block by re-cycling both of them. The physiological data suggest NK1r hypersensitivity.

Investigational agents for the irritable bowel syndrome

IMPORTANCE OF THE FIELD

Irritable bowel syndrome (IBS) is a common disorder with significant health and economic consequences. The etiology of IBS is complex and appears to be multifactorial. Traditional IBS therapies have been directed primarily at the relief of individual symptoms but have been largely disappointing. This has triggered the search for newer treatment strategies with improved patient outcomes.

AREAS COVERED IN THIS REVIEW

Enhanced knowledge about the putative pathophysiology of IBS has allowed the identification of new mechanistic targets for treatment. Our aim is to review emerging and promising drugs in the treatment of IBS based on disease pathophysiology. Data were extracted using Medline and PubMed search engines until January 2010. Abstracts were identified through 'Web of Science' and abstract supplements of major gastrointestinal scientific meetings. Drugs were classified according to mechanism of action and those with efficacy in trials involving human subjects examined.

WHAT THE READER WILL GAIN

Additional insight into the pathophysiology as well as current and prospective treatments of IBS.

TAKE HOME MESSAGE

A multitude of putative drug targets have been identified and some novel treatments have progressed through to human clinical trials, but very few will be approved for the market in the near future. Moreover, and in keeping with the complex and multifactorial nature of this syndrome, it is unlikely that there will be one dominant and universally effective form of therapy for all IBS patients.

Nitrergic inhibition of tachykininergic neuro-muscular transmission via cyclic GMP in the hamster ileum

The present study was designed to explore the inhibitory mechanism by nitric oxide (NO) of the tachykininergic neuro-muscular transmissions in the hamster ileum. In the presence of guanethidine (1 µM), atropine (0.5 µM), nifedipine (0.1 µM) and apamin (100 nM), electrical field stimuli (EFS; 0.5 ms duration, 15 V) evoked non-adrenergic, non-cholinergic excitatory junction potentials (EJPs) in circular smooth muscle cells. The EJPs were markedly inhibited by the tachykinin NK1 receptor antagonists [D-Pro(4), D-Trp(7,9)]-SP(4-11) (3 µM). Both the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 200 µM) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 µM), did not affect on the resting membrane potentials, but enhanced the tachykininergic EJPs. In the presence of L-NAME (200 µM), exogenously applied NO (10 µM) and the membrane permeable analogue of guanosine 3',5'-cyclic monophosphate (cGMP), 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP, 3 mM), significantly inhibited the tachykininergic EJPs. Application of EFS (0.5 msec duration, 15 V) with trains of 20 pulses at 20 Hz increased amount of released substance P (SP). The release of SP was further increased by the treatment of L-NAME or ODQ, but markedly reduced by exogenously applied NO and 8-Br-cGMP. These results suggest that the endogenous NO may inhibit the tachykininergic neuro-muscular transmissions by the decrease of SP release from the tachykininergic neurons, possibly through a guanylate cyclase-cGMP-dependent mechanism in the hamster ileum.

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.

Therapeutic advances in functional gastrointestinal disease: irritable bowel syndrome

Reported prevalence rates of irritable bowel syndrome (IBS) are between 8% to 20% in the US general population with an average medical expenditure of US$1.35 billion direct and US$205 million indirect costs. Current pathophysiologic theories are based on abnormalities of both the brain and gut, thus setting a new stage for current and future therapeutic approaches. There are numerous treatment options in IBS acting centrally and peripherally by influencing motility and visceral sensitivity. Clinical evidence is variable; however, newer emerging treatments are being evaluated using better-designed clinical trials. Accurate assessment of IBS drug efficacy is still hampered by heterogeneity of the IBS population. Novel methods such as pharmacogenomics or brain imaging may be helpful in the future to better understand and characterize IBS patient subtypes, and this in turn will lead to more specific and efficient therapeutic options. Patient subpopulation measurement of side effects is also a clinical challenge and further understanding could improve treatment efficacy enhancing the patient compliance.

Chronic inflammation alters the contribution of neurokinin receptor subtypes to epithelial function in rat colon

We have previously shown that neurokinin-1 (NK1) receptors predominantly mediate substance P-induced secretion of the non-inflamed rat colonic mucosa in vitro with a gradient in the magnitude of these responses. The aim of this study was to examine the effects of chronic inflammation on the contributions of different neurokinin receptor subtypes to colonic mucosal secretion. Colitis was induced by the intracolonic administration of 2,4,6-trinitrobenzene sulfonic acid in rats, reactivated 6 weeks later. Segments of proximal, mid- and distal colon were stripped of muscularis propria and mounted in Ussing chambers for measurement of short-circuit current. Use of selective agonists suggests that in the chronically inflamed rat colon NK1 receptors play a greater role in neurokinin-mediated mucosal secretion than do either NK2 or NK3. Selective antagonism implies that this is region-specific, with the inflammatory process altering the relative contribution of the neurokinin receptor subtypes within each region of the rat colon.

Altered tachykinergic influence on gastric mechanical activity in mdx mice

This study investigated whether alterations in gastric activity in dystrophic mdx mouse can be attributed to dysfunctions of tachykinins. Endoluminal pressure was recorded and the expression of neuronal nitric oxide synthase (nNOS), NK1 and NK2 neurokinin receptors was investigated by immunohistochemistry. SR48968, NK2 receptor antagonist, but not SR140333, NK1 receptor antagonist, decreased the tone only in mdx gastric preparations. In the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME), inhibitor of NOS, SR48968 reduced the tone also in normal stomach. [Sar(9), Met(O(2))(11)]-SP, agonist of NK1 receptors, caused tetrodotoxin-sensitive relaxations, antagonized by SR140333 or l-NAME, with no difference in the potency or efficacy between normal and mdx preparations. [beta-Ala(8)]-NKA(4-10), an NK2 receptor agonist, induced SR48968-sensitive contractions in both types of preparations, although the maximal response of mdx tissues was significantly lower than normal preparations. Immunohistochemistry demonstrated a consistent reduction of nNOS and NK2 receptor expression in mdx stomach smooth muscle cells and no change in nNOS and NK1 receptor expression in neurones. In conclusion, in mdx stomach the activation of NK2 receptors plays a role in the development of the tone, associated with a reduced NO production by muscular nNOS. The hypo-responsiveness to NK2 receptors could depend on the reduced expression of these receptors.

Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human

Tachykinins (TKs), substance P (SP), neurokinin A (NKA) and B (NKB) are important peptide modulators of intestinal motility in animal species studied so far, including humans. Modulation of motility by TKs can occur at various levels, since these peptides are expressed in cholinergic excitatory motor neurons projecting to both circular and longitudinal muscle, interneurons, and intramural and extramural sensory neurons. The effects of SP, NKA and NKB are preferentially mediated through the stimulation of NK1, NK2 and NK3 receptors, respectively; however, the selectivity of natural TKs for their preferred receptors is relative. In addition, SP and NKA are expressed in similar quantities in the human intestine and adequate stimuli can release similar amount of these TKs from enteric nerves. Furthermore, a single anatomical substrate can express more than one TK receptor type, so that the blockade of a single receptor type may not reveal functional effects in integrated models of motility. In isolated human small intestine and colon circular muscle strips, both NK1 and NK2 receptors mediate contractile effects. Indeed, in the human small intestine, smooth muscle electrical and motor events induced by electrical field stimulation (EFS) can involve either or both NK1 and NK2 receptors or these latter receptors predominantly, depending on the experimental conditions. In contrast, in the human colonic smooth muscle, only the NK2 receptor-mediated component of the response to EFS is prominent and some evidence would suggest that this component is the main excitatory motor mechanism at this level. Furthermore, a NK2 receptor-mediated secretory component in the human colonic mucosa has been recently demonstrated. Thus, it could be speculated that the blockade of both NK1 and NK2 receptors will be necessary to antagonise motor effects induced by exogenous administration or endogenous release of TKs in the small intestine, whereas the blockade of the NK2 receptors would be sufficient to disrupt physiological motor and, possibly, secretory activity at the colonic level. Available evidence indicates that, in healthy volunteers, the infusion of NKA (25 pmol/kg/min i.v.) stimulated small intestine motility and precipitated a series of intestinal and non-intestinal adverse events. Nepadutant (8 mg i.v.), a selective NK2 receptor antagonist, antagonised small intestine motility induced by NKA and prevented associated intestinal adverse events. In another study, the same dose of nepadutant increased colo-rectal compliance during isobaric balloon distension in healthy volunteers pretreated with a glycerol enema, disclosing a NK2 receptor-mediated component in the regulation of colonic smooth muscle tone. However, the prolonged blockade of NK2 receptors by nepadutant (16 mg i.v. b.i.d. for 8 days) did not affect bowel habits, neither in term of movements nor of stool consistency. Altogether, these results indicate that, even when there is a significant redundance in the effects of TKs and in the role of their receptors, the selective blockade of tachykinin NK2 receptors can have functional consequences on human intestinal motility and perception, but this can occur without the disruption of the physiological functions.

Regional variations in neurokinin receptor subtype contributions to muscularis mucosae and epithelial function in rat colon

It is known that the muscularis mucosae and mucosa are not pharmacologically homogeneous throughout the rat colon. The aim of this study was to simultaneously characterize all three neurokinin (NK) receptors in the muscularis mucosae and mucosa along the length of the rat colon. Strips of proximal, mid, and distal colonic muscularis mucosae were prepared for isometric recording or sheets of muscle-free mucosa were mounted in Ussing chambers for measurement of short-circuit current. In both muscularis mucosae and mucosa the greatest responses to substance P were found in the proximal region. Use of selective agonists revealed the presence of all three NK receptors in both structures, however, selective antagonism suggests that only NK2 receptors in the muscularis mucosae and NK1 receptors in the mucosa are physiologically relevant. In conclusion, substance P-induced responses in the rat colon are region-specific and not mediated by a single NK receptor subtype common to both structures.

Increase in neurokinin-1 receptor-mediated colonic motor response in a rat model of irritable bowel syndrome

AIM: Irritable bowel syndrome (IBS) is a functional bowel disorder. Its major symptom is bowel dysmotility, yet the mechanism of the symptom is poorly understood. Since the neurokinin-1 receptor (NK₁R)-mediated signaling in the gut is important in the control of normal bowel motor function, we aimed to investigate whether the NK₁R-mediated bowel motor function was altered in IBS, using a rat IBS model that was previously reported to show colonic dysmotility in response to restraint stress.

METHODS: IBS symptoms were produced in male Sprague-Dawley rats by inducing colitis with acetic acid. Rats were left to recover from colitis for 6 d, and used for experiments 7 d post-induction of colitis. Motor activities of distal colon were recorded in vitro.

RESULTS: The contractile sensitivity of isolated colon to a NK₁R agonist [Sar⁹,Met(O₂)¹¹]-substance P (1-30 nmol/L) was higher in IBS rats than that in normal rats. After the enteric neurotransmission was blocked by tetrodotoxin (TTX, 1 μmol/L), the contractile sensitivity to the NK₁R agonist was increased in normal colon but not in IBS rat colon. The NK₁R agonist-induced contraction was not different between the two groups when the agonist was challenged to the TTX-treated colon or the isolated colonic myocytes. A nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME, 100 μmol/L) augmented the NK₁R agonist-induced contraction only in normal rat colon.

CONCLUSION: These results suggest that the NK₁R-meidated colonic motor response is increased in IBS rats, due to the decrease in the nitrergic inhibitory neural component.

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.

Involvement of nitric oxide and tachykinins in the effects induced by protease-activated receptors in rat colon longitudinal muscle

(1) The aim of the present study was to verify a possible involvement of nitric oxide (NO) and of tachykinins in the contractile and relaxant effects caused by the activation of protease-activated receptor (PAR)-1 and PAR-2 in the longitudinal muscle of rat colon. (2) Mechanical responses to the PAR-1 activating peptides, SFLLRN-NH(2) (10 nM-10 micro M) and TFLLR-NH(2) (10 nM-10 micro M), and to the PAR-2-activating peptide, SLIGRL-NH(2) (10 nM-10 micro M), were examined in vitro in the absence and in the presence of different antagonists. (3) The relaxation induced by SFLLRN-NH(2), TFLLR-NH(2) and SLIGRL-NH(2) was antagonised by the inhibitor of NO synthase L-N(omega)-nitroarginine methyl ester (300 micro M), or by the inhibitor of the guanylyl cyclase, 1-H-oxodiazol-[1,2,4]-[4,3-a]quinoxaline-1-one (10 micro M). (4) The contractile responses to PAR-1 and PAR-2 activation were concentration-dependently attenuated by SR140333 (0.1-1 micro M), NK(1) receptor antagonist, or by SR48968 (0.1-1 micro M), NK(2) receptor antagonist. The combined pretreatment with SR140333 (1 micro M) and SR48968 (1 micro M) produced additive suppressive effects on the contractile responses to PAR activation. Pretreatment of the preparation with capsaicin (10 micro M) markedly reduced the contractions evoked by SFLLRN-NH(2), TFLLR-NH(2) and SLIGRL-NH(2), while omega-conotoxin GVIA (0.2 micro M) had no effect. (5) The present results suggest that in rat colonic longitudinal muscle, PAR-1 and PAR-2 activation can evoke (i) relaxation through the production of NO or (ii) contraction through the release of tachykinins, likely, from sensory nerves. These actions may contribute to motility disturbances during intestinal trauma and inflammation.

Intraileal capsaicin inhibits gastrointestinal contractions via a neural reflex in conscious dogs

BACKGROUND & AIMS

The aim of the present study was to determine the effect of intraileal administration of capsaicin on gastrointestinal motility.

METHODS

Mongrel dogs equipped with strain gauge force transducers on the stomach, small intestine, and colon were used. We studied the effects of intraileal capsaicin on gastrointestinal contractions with or without pharmacologic antagonists. The effects of capsaicin administration into the lumen of innervated and extrinsically denervated ileal Thiry loops were also studied.

RESULTS

Intraileal capsaicin dose dependently inhibited postprandial contractions at all sites and interdigestive contractions in the upper gastrointestinal tract. Intraileal capsaicin-induced inhibition of gastrointestinal contractions was partially reversed by a nitric oxide (NO) synthase inhibitor, a 5 hydroxytryptamine-3 receptor antagonist (5-HT(3)), and an opiate antagonist. Administration of capsaicin into the innervated ileal Thiry loop had inhibitory effects on gastrointestinal contractions, but gastrointesinal motor activity was not affected by capsaicin administered into the extrinsically denervated Thiry loop.

CONCLUSIONS

Stimulation of ileal afferent fibers by capsaicin inhibits gastrointestinal contractions via an extrinsic neural reflex.

Effects of GABA on circular smooth muscle spontaneous activities of rat distal colon

GABAergic regulation of intestinal motility through the modulation of non-adrenergic non-cholinergic (NANC) neurons remains poorly understood especially in rat colon where very few studies have been undertaken. Therefore, the effects of GABA on circular preparations of rat distal colon were investigated using classical organ bath chambers to record spontaneous mechanical activities (SMA). SMA was characterized by the occurrence of rhythmic phasic contractions (type-I) or by spontaneously occurring large contractions superimposed on small rhythmic contractions (type-II). In the presence of atropine and guanethidine (NANC conditions), these large contractions were inhibited by bicuculline, a GABA(A)-receptor antagonist as well as by TTX, L-NAME and apamin together, or L 732-138, a NK1-receptor antagonist. In NANC conditions, GABA induced a transient monophasic relaxation or a biphasic effect characterized by a relaxation followed by a tonic contraction in both type-I and -II preparations. Both the inhibitory and excitatory effects of GABA were blocked by TTX and L-NAME + apamin; the GABA-induced contraction was also sensitive to L 732-138. The responses to GABA were mimicked by the GABA(A)-receptor agonist, muscimol, whereas baclofen and CACA, respectively GABA(B) and GABA(C)-receptors agonists showed no effect. These results demonstrated that only GABA(A)-receptors seem to be involved in the regulation of SMA in rat distal colon in NANC conditions. Release of NANC inhibitory transmitter (NO and probably ATP) and NANC excitatory transmitter (maybe substance P) might be involved.