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

Excitatory cholinergic responses in mouse colon intramuscular interstitial cells of Cajal are due to enhanced Ca2+ release via M3 receptor activation

Colonic intramuscular interstitial cells of Cajal (ICC-IM) are associated with cholinergic varicosities, suggesting a role in mediating excitatory neurotransmission. Ca²⁺ release in ICC-IM activates Ano1, a Ca²⁺ -activated Cl^- conductance, causing tissue depolarization and increased smooth muscle excitability. We employed Ca²⁺ imaging of colonic ICC-IM in situ, using mice expressing GCaMP6f in ICC to evaluate ICC-IM responses to excitatory neurotransmission. Expression of muscarinic type 2, 3 (M₂ , M₃ ), and NK₁ receptors were enriched in ICC-IM. NK₁ receptor agonists had minimal effects on ICC-IM, whereas neostigmine and carbachol increased Ca²⁺ transients. These effects were reversed by DAU 5884 (M₃ receptor antagonist) but not AF-DX 116 (M₂ receptor antagonist). Electrical field stimulation (EFS) in the presence of L-NNA and MRS 2500 enhanced ICC-IM Ca²⁺ transients. Responses were blocked by atropine or DAU 5884, but not AF-DX 116. ICC-IM responses to EFS were ablated by inhibiting Ca²⁺ stores with cyclopiazonic acid and reduced by inhibiting Ca²⁺ influx via Orai channels. Contractions induced by EFS were reduced by an Ano1 channel antagonist, abolished by DAU 5884, and unaffected by AF-DX 116. Colonic ICC-IM receive excitatory inputs from cholinergic neurons via M₃ receptor activation. Enhancing ICC-IM Ca²⁺ release and Ano1 activation contributes to excitatory responses of colonic muscles.

G protein-coupled receptor trafficking and signaling: new insights into the enteric nervous system

G protein-coupled receptors (GPCRs) are essential for the neurogenic control of gastrointestinal (GI) function and are important and emerging therapeutic targets in the gut. Detailed knowledge of both the distribution and functional expression of GPCRs in the enteric nervous system (ENS) is critical toward advancing our understanding of how these receptors contribute to GI function during physiological and pathophysiological states. Equally important, but less well defined, is the complex relationship between receptor expression, ligand binding, signaling, and trafficking within enteric neurons. Neuronal GPCRs are internalized following exposure to agonists and under pathological conditions, such as intestinal inflammation. However, the relationship between the intracellular distribution of GPCRs and their signaling outputs in this setting remains a "black box". This review will briefly summarize current knowledge of agonist-evoked GPCR trafficking and location-specific signaling in the ENS and identifies key areas where future research could be focused. Greater understanding of the cellular and molecular mechanisms involved in regulating GPCR signaling in the ENS will provide new insights into GI function and may open novel avenues for therapeutic targeting of GPCRs for the treatment of digestive disorders.

Heterogeneous localization of muscarinic cholinoceptor M1 in the salivary ducts of adult mice

We hypothesize variation in expression and localization, along the course of the glandular tubule, of muscarinic cholinergic receptor M₁ which plays as a distinct contribution, though minor in comparison with M₃ receptor, in saliva secretion. Localization of the M₁ receptor was examined using immunohistochemistry in three major salivary glands. Although all glandular cells were more or less M₁-immunoreactive, acinar cells were weakly immunoreactive, while ductal cells exhibited substantial M₁-immunoreactivity. Many ductal cells exhibited clear polarity with higher immunoreactivity in their apical/supra-nuclear domain. However, some exhibited indistinct polarity because of additional higher immunoreactivity in their basal/infra-nuclear domain. A small group of cells with intense immunoreactivity was found, mostly located in the intercalated ducts or in portions of the striated ducts close to the intercalated ducts. In immuno-electron microscopy, the immunoreactive materials were mainly in the cytoplasm including various vesicles and vacuoles. Unexpectedly, distinct immunoreactivity on apical and basal plasma membranes was infrequent in most ductal cells. The heterogeneous localization of M₁-immunoreactivity along the gland tubular system is discussed in view of possible modulatory roles of the M₁ receptor in saliva secretion.

Substance P and the neurokinin-1 receptor expression in dog ileum with and without inflammation

In the gastrointestinal tract, the tachykinin Substance P (SP) is involved in motility, fluid and electrolyte secretion, and blood flow and regulation of immunoinflammatory response. SP exerts its biological activity on target cells by interacting mainly with the neurokinin-1 receptor (NK₁R). The present study aims to quantify the percentage of SP-immunoreactive (SP-IR) enteric neurons and the density of SP-IR nerve fibers in the ileum of control dogs (CTRL-dogs; n=7) vs dogs with spontaneous ileal inflammation (INF-dogs; n=8). In addition, the percentage of enteric neurons bearing NK₁R, and nitrergic neurons (nNOS-IR) expressing NK₁R immunoreactivity were evaluated in both groups. The percentages of SP-IR neurons were similar in CTRL- and INF-dogs, in either the myenteric (MP) (15±8% vs. 16±7%, respectively) and submucosal plexus (SMP) (26±7% vs. 24±14%, respectively). In INF-dogs, the density of SP-IR mucosal nerve fibers showed a trend to decrease (P=0.07). Myenteric neurons of CTRL- and INF-dogs expressed similar percentages of NK₁R-immunoreactivity (39±5% vs. 38±20%, respectively). Submucosal NK₁R-IR neurons were occasionally observed in a CTRL-dog. MP nitrergic neurons bearing NK₁R showed a trend to decrease in INF-dogs vs. CTRL- dogs (41±22% vs. 65±10%, respectively; P=0.11). In INF-dogs, muscle cells and immune cells overexpressed NK₁R immunoreactivity. These findings should be taken as a warning for possible intestinal motility disorders, which might occur during administration of NK₁R-antagonist drugs. Conversely, the strong expression of NK₁R immunoreactivity observed in muscle and mucosal immune cells of inflamed tissues may provide a rationale for the use of NK₁R antagonist drugs in the treatment of intestinal inflammation.

Xenin Augments Duodenal Anion Secretion via Activation of Afferent Neural Pathways

Xenin-25, a neurotensin (NT)-related anorexigenic gut hormone generated mostly in the duodenal mucosa, is believed to increase the rate of duodenal ion secretion, because xenin-induced diarrhea is not present after Roux-en-Y gastric bypass surgery. Because the local effects of xenin on duodenal ion secretion have remained uninvestigated, we thus examined the neural pathways underlying xenin-induced duodenal anion secretion. Intravenous infusion of xenin-8, a bioactive C-terminal fragment of xenin-25, dose dependently increased the rate of duodenal HCO₃^- secretion in perfused duodenal loops of anesthetized rats. Xenin was immunolocalized to a subset of enteroendocrine cells in the rat duodenum. The mRNA of the xenin/NT receptor 1 (NTS1) was predominantly expressed in the enteric plexus, nodose and dorsal root ganglia, and in the lamina propria rather than in the epithelium. The serosal application of xenin-8 or xenin-25 rapidly and transiently increased short-circuit current in Ussing-chambered mucosa-submucosa preparations in a concentration-dependent manner in the duodenum and jejunum, but less so in the ileum and colon. The selective antagonist for NTS1, substance P (SP) receptor (NK1), or 5-hydroxytryptamine (5-HT)₃, but not NTS2, inhibited the responses to xenin. Xenin-evoked Cl^- secretion was reduced by tetrodotoxin (TTX) or capsaicin-pretreatment, and abolished by the inhibitor of TTX-resistant sodium channel Nav1.8 in combination with TTX, suggesting that peripheral xenin augments duodenal HCO₃^- and Cl^- secretion through NTS1 activation on intrinsic and extrinsic afferent nerves, followed by release of SP and 5-HT. Afferent nerve activation by postprandial, peripherally released xenin may account for its secretory effects in the duodenum.

Substance P as a neuronal factor in the enteric nervous system of the porcine descending colon in physiological conditions and during selected pathogenic processes

The present investigation pertains to changes in substance P-like immunoreactive (SP-LI) nerve structures of the enteric nervous system (ENS) in the porcine descending colon, caused by chemically-induced inflammation and nerve injury (axotomy). The distribution pattern of SP-LI structures was studied using the double immunofluorescence technique in the myenteric (MP), outer submucous (OSP) and inner submucous (ISP) plexuses, as well as in the circular muscle and mucosal layers. Under physiological conditions, SP-LI neurons have been shown to constitute 4.13 ± 0.24%, 3.36 ± 0.26%, and 7.92 ± 0.16% in the MP, OSP, and ISP, respectively. Changes in SP-immunoreactivity depended on the pathological factor studied. The numbers of the SP-LI perikarya amounted to 7.89 ± 0.34, 5.56 ± 0.30, and 19.96 ± 0.57 in chemically-induced colitis, and 4.28 ± 0.13%, 7.18 ± 20%, and 11.62 ± 0.48% after axotomy in MP, OSP, and ISP, respectively. The both studied processes generally resulted in an increase in the number of SP-LI nerve fibers in the circular muscle and mucosal layers. The obtained results suggest that SP-LI nerve structures of the ENS may participate in various pathological processes in the porcine descending colon and exact functions of SP probably depend on the type of the pathological factor.

Roles of substance P and ATP in the subepithelial fibroblasts of rat intestinal villi

The ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes and also villous movement in the gut. In the intestinal villi, subepithelial fibroblasts under the epithelium form contractile cellular networks and closely contact to the varicosities of substance P and nonsubstance P afferent neurons. Subepithelial fibroblasts of the duodenal villi possess purinergic receptor P2Y1 and tachykinin receptor NK1. ATP and substance P induce increase in intracellular Ca(2+) and cell contraction in subepithelial fibroblasts. They are highly mechanosensitive and release ATP by mechanical stimuli. Released ATP spreads to form an ATP "cloud" with nearly 1μM concentration and activates the surroundings via P2Y1 and afferent neurons via P2X receptors. These findings suggest that villous subepithelial fibroblasts and afferent neurons interact via ATP and substance P. This mutual interaction may play important roles in the signal transduction of mechano reflex pathways including a coordinate villous movement and also in the maturation of the structure and function of the intestinal villi.

Enhanced nerve-stimulated muscarinic and neurokinin contractions of ileum from streptozotocin guinea-pigs

Diabetes mellitus can lead to neuropathy of enteric neurons, resulting in abnormal gut motility. These studies investigated voltage-dependent contributions of muscarinic M₃ receptor activation by acetylcholine and neurokinin NK₁ receptor activation by neurokinins to nerve-stimulated contractions of longitudinal ileal strips from STZ guinea-pigs, a type 1 diabetic model with insulin deficiency, but mild hyperglycaemia. Contractions to bethanechol, substance P methyl ester, and nerve stimulation were greater in diabetic as compared to control ileum. The muscarinic M₃ receptor antagonist 4-DAMP at lower voltages and the neurokinin NK₁ receptor antagonist SR140333 at higher voltages, but not the neurokinin NK₁ receptor antagonist CP-96,345, were more effective at inhibiting nerve-stimulated immediate peak contractions and total areas of contraction of ileum from diabetic as compared to control animals. For diabetic ileum, voltage-dependent increases in the areas of nerve-stimulated contraction were observed in the presence of 4-DAMP and CP-96,345 but not SR140333. At low voltages only, nerve-stimulated release of acetylcholine was greater from diabetic as compared to control ileum. Fluorescence intensity of tachykinin-like immunoreactivity was increased in ileal myenteric ganglia from diabetic as compared to control animals. In diabetic guinea-pigs, stronger ileal nerve-stimulated contractions reflected increased release of acetylcholine at lower voltages and tachykinins at higher voltages, as well as increased sensitivity of smooth muscle M₃ and NK₁ receptors to acetylcholine and tachykinins. Hypoinsulinaemia may be a primary contributor to intestinal motility dysfunction in type 1 diabetes mellitus.

Subepithelial trypsin induces enteric nerve-mediated anion secretion by activating proteinase-activated receptor 1 in the mouse cecum

Serine proteases are versatile signaling molecules and often exert this function by activating the proteinase-activated receptors (PAR(1)-PAR(4)). Our previous study on the mouse cecum has shown that the PAR(1)-activating peptide (AP) and PAR(2)-AP both induced electrogenic anion secretion. This secretion mediated by PAR(1) probably occurred by activating the receptor on the submucosal secretomotor neurons, while PAR(2)-mediated anion secretion probably occurred by activating the receptor on the epithelial cells. This present study was aimed at using trypsin to further elucidate the roles of serine proteases and PARs in regulating intestinal anion secretion. A mucosal-submucosal sheet of the mouse cecum was mounted in Ussing chambers, and the short-circuit current (I(sc)) was measured. Trypsin added to the serosal side increased I(sc) with an ED(50) value of approximately 100 nM. This I(sc) increase was suppressed by removing Cl(-) from the bathing solution. The I(sc) increase induced by 100 nM trypsin was substantially suppressed by tetrodotoxin, and partially inhibited by an NK(1) receptor antagonist, by a muscarinic Ach-receptor antagonist, and by 5-hydroxytryptamine-3 (5-HT(3)) and 5-HT(4) receptor antagonists. The I(sc) increase induced by trypsin was partially suppressed when the tissue had been pretreated with PAR(1)-AP, but not by a pretreatment with PAR(2)-AP. These results suggest that the serine protease, trypsin, induced anion secretion by activating the enteric secretomotor nerves. This response was initiated in part by activating PAR(1) on the enteric nerves. Serine proteases and PARs are likely to be responsible for the diarrhea occurring under intestinal inflammatory conditions.

Visceral hypersensitivity in symptomatic diverticular disease and the role of neuropeptides and low grade inflammation

BACKGROUND

  Recurrent abdominal pain is reported by a third of patients with diverticulosis, particularly those with previous episodes of acute diverticulitis. The current understanding of the etiology of this pain is poor. Our aim was to assess visceral sensitivity in patients with diverticular disease and its association with markers of previous inflammation and neuropeptides.

METHODS

  Patients with asymptomatic and symptomatic diverticular disease underwent a flexible sigmoidoscopy and biopsy followed 5-10 days later by visceral sensitivity testing with barostat-mediated rectal distension. Inflammation was assessed by staining of serotonin (5HT) and CD3 positive cells. mRNA levels of tumor necrosis factor alpha (TNF α) and interleukin-6 (IL-6) were quantitated using RT-PCR. Neuropeptide expression was assessed from percentage area staining with substance P (SP) and mRNA levels of the neurokinin 1 & 2 receptors (NK1 & NK2), and galanin 1 receptor (GALR1).

KEY RESULTS

  Thirteen asymptomatic and 12 symptomatic patients were recruited. The symptomatic patients had a lower first reported threshold to pain (28.4 mmHg i.q.r 25.0-36.0) than the asymptomatic patients (47 mmHg i.q.r 36.0-52.5, P < 0.001). Symptomatic patients had a higher median overall pain rating for the stimuli than the asymptomatic patients (P < 0.02). Symptomatic patients had greater median relative expression of NK1 and TNF alpha mRNA compared with asymptomatic patients. There was a significant correlation between barostat VAS pain scores and NK 1 expression (Figure 4, r(2) 0.54, P < 0.02).

CONCLUSIONS & INFERENCES

  Patients with symptomatic diverticular disease exhibit visceral hypersensitivity, and this may be mediated by ongoing low grade inflammation and upregulation of tachykinins.

Gender differences in reduced substance P (SP) in children with slow-transit constipation

PURPOSE

Adult slow-transit constipation (STC) occurs predominantly in females and is associated with low numbers of substance P (SP)-containing nerves in colonic circular muscle.

AIM

To determine if reduced SP nerves is female predominant in paediatric STC.

METHODS

Children with STC were identified from records of more than 600 nuclear transit studies (NTS) and intestinal biopsies done for intractable chronic constipation between November 1998 and March 2009. Colonic seromuscular biopsies collected from hepatic and splenic flexures, and sigmoid colon were processed for immunohistochemistry. Nerve fibre density in circular muscle containing SP was measured qualitatively by a pathologist.

RESULTS

Eighty-eight children with chronic constipation had both NTS and intestinal biopsies. Seventy-eight children (52 M; age 2-15.5 years; mean 7.7 years) had STC diagnosed by NTS. SP was reduced in 10/26 girls, but only 11/52 boys.

CONCLUSION

In this sample, STC was more common in boys than girls. However, in girls with STC, SP deficiency occurred in 40%, when compared with 20% of boys. During puberty, the percentage of girls with reduced SP decreased, whilst the percentage of boys increased. These results suggest that STC is heterogeneous and that there are some gender differences, the implication of which requires further investigation.

Contractile effect of tachykinins on rabbit small intestine

AIM

To study the role of the tachykinin receptors in spontaneous contractions of longitudinal and circular smooth muscle from rabbit small intestine and to determine the mechanism of action of Substance P (SP).

METHODS

Rabbit duodenum, jejunum and ileum segments were prepared. The spontaneous contractions of longitudinal and circular smooth muscle were recorded using a computer via an isometric force transducer. The specific agonists and antagonists of tachykinin receptors were added into the organ bath.

RESULTS

The agonists of tachykinin NK1 receptor (SP and [Sar9] SP), NK2 receptor (NKA and (β-Ala8)-NKA), and NK3 receptor (NKB and Senktide) all induced contractions in the small intestine. The contractions were diminished by NK1 receptor antagonist L-733,060, NK2 receptor antagonist GR-94800, and NK3 receptor antagonist SB 218795. Contractions caused by SP were also reduced by atropine, verapamil, PKC inhibitor staurosporine, and PLC inhibitor U73122.

CONCLUSION

Ttachykinin NK1, NK2, and NK3 receptors mediate the contractions of the smooth muscle in rabbit intestine. Furthermore, SP acts directly on smooth muscle cells through the tachykinin NK1 receptor.

Localization of NK1 receptors and roles of substance-P in subepithelial fibroblasts of rat intestinal villi

Subepithelial fibroblasts of the intestinal villi, which form a contractile cellular network beneath the epithelium, are in close contact with epithelial cells, nerve varicosities, capillaries, smooth muscles and immune cells, and secrete extracellular matrix molecules, growth factors and cytokines, etc. Cultured subepithelial fibroblasts of the rat duodenal villi display various receptors such as endothelins, ATP, substance-P and bradykinin, and release ATP in response to mechanical stimulation. In this study, the presence of functional NK1 receptors (NK1R) was pharmacologically confirmed in primary culture by Ca(2+) measurement, and the effects of substance-P were measured in an acute preparation of epithelium-free duodenal villi from 2- to 3-week-old rats using a two-photon laser microscope. Substance-P elicited an increase in the intracellular Ca(2+) concentration and contraction of the subepithelial fibroblasts in culture and the isolated villi. The localization of NK1R and substance-P in the villi was examined by light and electron microscopic immunohistochemistry. NK1R-like immunoreactivity was intensely localized on the plasma membrane of villous subepithelial fibroblasts in 10-day- to 4-week-old rats and mice and was decreased or absent in adulthood. The pericryptal fibroblasts of the small and large intestine were NK1R immuno-negative. These villous subepithelial fibroblasts form synapse-like structures with both substance-P-immunopositive and -immunonegative nerve varicosities. Here, we propose that the mutual interaction between villous subepithelial fibroblasts and afferent neurons via substance-P and ATP plays important roles in the maturation of the structure and function of the small intestine.

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.

The mechanisms underlying the generation of the colonic migrating motor complex in both wild-type and nNOS knockout mice

Colonic migrating motor complexes (CMMCs) propel fecal contents and are altered in diseased states, including slow-transit constipation. However, the mechanisms underlying the CMMCs are controversial because it has been proposed that disinhibition (turning off of inhibitory neurotransmission) or excitatory nerve activity generate the CMMC. Therefore, our aims were to reexamine the mechanisms underlying the CMMC in the colon of wild-type and neuronal nitric oxide synthase (nNOS)(-/-) mice. CMMCs were recorded from the isolated murine large bowel using intracellular recordings of electrical activity from circular muscle (CM) combined with tension recording. Spontaneous CMMCs occurred in both wild-type (frequency: 0.3 cycles/min) and nNOS(-/-) mice (frequency: 0.4 cycles/min). CMMCs consisted of a hyperpolarization, followed by fast oscillations (slow waves) with action potentials superimposed on a slow depolarization (wild-type: 14.0 +/- 0.6 mV; nNOS(-/-): 11.2 +/- 1.5 mV). Both atropine (1 microM) and MEN 10,376 [neurokinin 2 (NK2) antagonist; 0.5 microM] added successively reduced the slow depolarization and the number of action potentials but did not abolish the fast oscillations. The further addition of RP 67580 (NK1 antagonist; 0.5 microM) blocked the fast oscillations and the CMMC. Importantly, none of the antagonists affected the resting membrane potential, suggesting that ongoing tonic inhibition of the CM was maintained. Fecal pellet propulsion, which was blocked by the NK2 or the NK1 antagonist, was slower down the longer, more constricted nNOS(-/-) mouse colon (wild-type: 47.9 +/- 2.4 mm; nNOS(-/-): 57.8 +/- 1.4 mm). These observations suggest that excitatory neurotransmission enhances pacemaker activity during the CMMC. Therefore, the CMMC is likely generated by a synergistic interaction between neural and interstitial cells of Cajal networks.

Spatial and temporal expression of tachykinins and NK1- and NK2-receptor gene during TNB induced colitis in rats

The distribution and modification of the tachykinins Substance P (SP) and neurokinin A (NKA), their precursor beta-preprotachykinins (beta-PPT) and the receptors involved in their activity, NK-1 and NK-2, were studied in trinitrobenzensulphonic acid (TNB) colitis. Rats were intrarectally treated with a 120 mg/ml of TNB solution and sacrificed at various times after colitis induction. During the acute phases of colitis, a marked decrease in tissue SP and NKA levels were observed along with an increased transcription of beta-PPT mRNA in the neurons of the myenteric plexus and an increased myeloperoxidase activity, which is an index of the tissue's inflammatory status. De novo expression of both NK(1) and NK(2) receptor mRNA was observed during the acute phase of TNB-colitis in mesenchymal cells around dilated submucosal vessels but their expression in smooth muscle cells of the muscularis mucosae and propria was moderately down-regulated. The peptide levels, myeloperoxidase activity and gene expression of tachykinin receptors were then restored during the late phases (2-4 weeks after the apten administration) while beta-PPT mRNA remained highly expressed in the myenteric plexus ganglia showing that SP and NKA are involved in repair processes. These results point to the enhanced release of tachykinins during the initial phase of colitis and a restoration of this neuropeptide pool in the healing of the tissue.

Role of tachykinin NK(1) and NK(2) receptors in colonic sensitivity and stress-induced defecation in gerbils

The pharmacology of tachykinin NK receptors varies greatly among species. The aim of the present study was to assess the role of NK(1) and NK(2) receptors in mediating colorectal distension-evoked nociception and psychological stress-induced defecation in gerbils, a species with human-like NK receptor pharmacology. The effects of the selective NK(1) and NK(2) receptor antagonists, aprepitant and saredutant, on acute (1 h) restraint stress-evoked defecation and plasma adenocorticotropin (ACTH) levels in gerbils were assessed. The effects of antagonists alone or in combination on colorectal distension-evoked visceral pain in conscious gerbils were evaluated using the visceromotor response as a surrogate marker of pain. Restraint stress increased fecal pellet output 2-3-fold and plasma ACTH levels 9-fold. Aprepitant inhibited the defecatory and endocrine responses to stress by 50%, while saredutant completely normalized the same parameters. Visceral pain responses during colorectal distension were attenuated by both compounds, but aprepitant (19+/-6% inhibition, P<0.01) was slightly more effective than saredutant (10+/-9% inhibition, P<0.05). A combination of both compounds resulted in an additive effect (30+/-10% inhibition, P<0.01). The results demonstrate that NK(1) and NK(2) receptors are involved in stress-related colonic motor alterations and visceral pain responses in gerbils and that combined antagonism provides enhanced inhibition of visceral pain responses. This suggests that for therapeutic use in for instance functional gastrointestinal disorders, dual NK(1)/NK(2) receptor antagonists may provide better clinical outcome than selective compounds.

Older versus newer antidepressants: Substance P or calcium antagonism?

Substance P (SP) is possibly involved in the pathophysiology of depression and anxiety. We investigated interactions between antidepressants on SP-induced effects and their potential calcium-blocking activity in the isolated guinea pig ileum. All the antidepressants tested, except pargyline, moclobemide, mianserin, and reboxetine, were able to inhibit in a concentration-dependent manner the contraction induced by 100 nmol/L SP. Clomipramine, fluoxetine, maprotiline, and amitriptyline (all at 3 μmol/L) flattened the concentration–response curves to SP, resulting in a reduction of up to 59%, 63%, 32%, and 23%, respectively, of the maximum contractile effect. All the antidepressants tested (3 μmol/L), except pargyline, moclobemide, and mianserin, produced a rightward parallel shift of the concentration–response curve to CaCl2. The L-type selective calcium blocker nifedipine and the T-type selective mibefradil showed similar behaviour against both agonists used, SP and CaCl2. The relative order of potency was nifedipine (pA2, 7.6 ± 0.1) > clomipramine (pA2, 7.0 ± 0.1) > fluoxetine (pKB, 6.5 ± 0.1) = mibefradil (pKB, 6.6 ± 0.1) > amitriptyline (pKB, 6.3 ± 0.1) = maprotiline (pKB, 6.2 ± 0.1) > fluvoxamine (pKB, 5.9 ± 0.1). The data reported in the present study suggest that the antidepressants tested did not behave as competitive antagonists versus NK1-receptor subtypes, but their inhibitory action seems to be related to their calcium-blocking properties.

Immunohistochemical localisation of cholinergic muscarinic receptor subtype 1 (M1r) in the guinea pig and human enteric nervous system

Little is known regarding the location of cholinergic muscarinic receptor 1 (M1r) in the ENS, even though physiological data suggest that M1rs are central to cholinergic neurotransmission. This study localised M1rs in the ENS of the guinea pig ileum and human colon using fluorescence immunohistochemistry and RT-PCR in human colon. Double labelling using antibodies against neurochemical markers was used to identify neuron subytpes bearing M1r. M1r immunoreactivity (IR) was present on neurons in the myenteric and submucosal ganglia. The two antibodies gave similar M1r-IR patterns and M1r-IR was abolished upon antibody preabsorption. M1r-IR was present on cholinergic and nNOS-IR nerve cell bodies in both guinea pig and human myenteric neurons. Presynaptic M1r-IR was present on NOS-IR and VAChT-IR nerve fibres in the circular muscle in the human colon. In the submucosal ganglia, M1r-IR was present on a population of neurons that contained cChAT-IR, but did not contain NPY-IR or calretinin-IR. M1r-IR was present on endothelial cells of blood vessels in the submucosal plexus. The localisation of M1r-IR in the guinea pig and human ENS shown in this study agrees with physiological studies. M1r-IR in cholinergic and nitrergic neurons and nerve fibres indicate that M1rs have a role in both cholinergic and nitrergic transmission. M1r-IR present in submucosal neurons suggests a role in mediating acetylcholine's effect on submucosal sensory and secretomotor/vasodilator neurons. M1r-IR present on blood vessel endothelial cells suggests that M1rs may also mediate acetylcholine's direct effect on vasoactivation.

Substance P regulates migration in rat intestinal epithelial cells

OBJECTIVE

The current study examined the effect of substance P (SP) upon intestinal epithelial cells and the mechanistic details of this interaction.

SUMMARY BACKGROUND DATA

Intestinal epithelial cells must be capable of migration to reseal mucosal wounds for several vital intestinal functions. This process is incompletely understood; however, recent evidence implicates the neurotransmitter SP in this process.

METHODS

Normal rat intestinal epithelial cells (IEC-6 cells) were studied to identify the presence of the SP receptor (NK-1 subtype) and then exposed to physiologic doses of SP and antagonists to assess for increased migration.

RESULTS

Examination IEC-6 cells revealed the presence of the SP receptor. Wounding of these cells followed by subsequent exposure to SP (10 mol/L) resulted in increased migration. Similarly, SP-induced increases in intracellular calcium concentration and actomyosin stress fiber formation. These effects were all blocked through specific NK-1 receptor antagonists.

CONCLUSIONS

These results indicate that SP stimulates intestinal epithelial migration and increases in calcium concentration. These data support a beneficial role for SP in the maintenance of intestinal mucosal homeostasis.

Plasticity of rat small intestine after removal of a chronic mechanical obstruction

Chronic intestinal obstruction is associated with morphological changes and functional disorders clinically reported and experimentally documented in laboratory animals. In contrast, little is known about the properties of the hypertrophied intestine after removal of the obstruction. In the present study, we removed the ileal obstruction previously applied to the ileum of rats and, after 1 or 2 weeks, studied in vitro the motor responses of de-obstructed segments of intestine to pharmacological or electrical field stimulation (EFS). By 2 weeks after de-obstruction, maximal contractile responses to receptor (acetylcholine) and non-receptor (K(+)) mediated stimuli were comparable in operated and control tissues; furthermore, the loss of sensitivity to nitric oxide (NO) unmasked in obstructed tissues was, after de-obstruction, replaced by supersensitivity to exogenous NO and vasoactive intestinal polypeptide, probably acting through cyclic nucleotide-independent pathways. Despite the complete recovery of smooth muscle responses, neurogenic contractions remained impaired in de-obstructed tissue; however, the equal contribution of cholinergic/peptidergic components to EFS responses could represent a sign of gradual but delayed recovery of enteric neurotransmission.

Muscarinic M(2) acetylcholine receptor distribution in the guinea-pig gastrointestinal tract

In the enteric nervous system, acetylcholine is the most common neurotransmitter to induce gastrointestinal smooth muscle contractions. Cholinergic signaling is mediated by muscarinic acetylcholine receptors on the surface of smooth muscle cells. Five different muscarinic receptor subtypes (M(1)-M(5)) have been identified and characterized, all of which belong to the superfamily of the G-protein-coupled receptor. The muscarinic M(2) acetylcholine receptor is the major muscarinic receptor subtype expressed by smooth muscle tissues in the gastrointestinal tract, where it is coexpressed with a smaller population of M(3) receptor. In this study, we examined the immunohistochemical distribution of the M(2) receptor using a specific antibody in the guinea-pig gastrointestinal tract. M(2) receptor-like immunoreactivity was mainly observed as associated with smooth muscle cells in the gastrointestinal tract. M(2) receptor-like immunoreactivity in smooth muscle cells was distributed throughout the cell membrane associated with caveolae. In the proximal colon, M(2) receptor-like immunoreactivity in the smooth muscle cells was weak. In the small intestine, interstitial cells of Cajal that possessed neurokinin 1 receptor-like immunoreactivity had intense M(2) receptor-like immunoreactivity. In the proximal colon, intramuscular and myenteric interstitial cells of Cajal exhibited M(2) receptor-like immunoreactivity. These findings indicate that, in the gastrointestinal musculature, M(2) receptors are distributed both in the smooth muscle cells and interstitial cells of Cajal, suggesting that the M(2) receptor elicits smooth muscle cell contraction and the interstitial cells of Cajal are the sites of innervation by enteric cholinergic neurons.

Relationships between neurokinin receptor-expressing interstitial cells of Cajal and tachykininergic nerves in the gut

The so-called interstitial cells of Cajal (ICC) are distributed throughout the muscle coat of the alimentary tract with characteristic intramural location and species-variations in structure and staining. Several ICC sub-types have been identified: ICC-DMP, ICC-MP, ICC-IM, ICC-SM. Gut motility is regulated by ICC and each sub-type is responsible for the electrical activities typical of each gut region and/or muscle layer. The interstitial position of the ICC between nerve endings and smooth muscle cells has been extensively considered. Some of these nerve endings contain tachykinins. Three distinct tachykinin receptors (NK1r, NK2r and NK3r) have been demonstrated by molecular biology. Each of them binds with different affinities to a series of tachykinins (SP, NKA and NKB). In the ileum, SP-immunoreactive (SP-IR) nerve fibers form a rich plexus at the deep muscular plexus (DMP), distributed around SP-negative cells, and ICC-DMP intensely express the SP-preferred receptor NK1r; conversely a faint NK1r-IR is detected on the ICC-MP and mainly after receptor internalization was induced by agonists. ICC-IM are never stained in laboratory mammals, while those of the human antrum are NK1r- IR. RT-PCR conducted on isolated ileal ICC-MP and gastric ICC-IM showed that these cells express NK1r and NK3r. Colonic ICC, except those in humans, do not express NK1r-IR, at least in resting conditions. Outside the gut, NK1r-IR cells were seen in the arterial wall and exocrine pancreas. In the mouse gut only, NK1r-IR is present in non-neuronal cells located within the intestinal villi, so-called myoid cells, which are c-kit-negative and alpha-smooth muscle actin-positive. Immunohistochemistry and functional studies confirmed that ICC receive input from SP-IR terminals, with differences between ICC sub-types. In the rat, very early after birth, NK1r is expressed by the ICC-DMP and SP by the related nerve varicosities. Studies on pathological conditions are few and those on mutant strains practically absent. It has only been reported that in the inflamed ileum of rats the NK1r-IR ICC-DMP disappear and that at the peak of inflammatory conditions ICC-MP are NK1r-IR. In the ileum of mice with a mutation in the W locus, ICC-DMP were seen to express c-kit-IR but not NK1-IR, and SP-IR innervation seems unchanged. In summary, there are distinct ICC populations, each of them under a different tachykininergic control and, likely, having different functions. Further studies are recommended at the aim of understanding ICC involvement in modulating/transmitting tachykininergic inputs.

Expression of the neurokinin type 1 receptor in the human colon

The distribution of the neurokinin type 1 receptor (NK1r) in human intestine, mapped in a few immunohistochemical investigations in the antrum and the duodenum, is comparable to that widely studied in rodents. Importantly, despite pharmacological evidence of their presence in mammalian intestinal muscle, their immunohistochemical visualization in smooth muscle cells remains to be determined in human digestive tract. In the present work, we studied the distribution of NK1r in the human colon, with a particular view to visualize their expression in muscle cells. With this aim, part of colonic segments were incubated with nicardipine and TTX in order to induce accumulation of the NK1r on cell membrane. NK1r were visualized by using immunohistochemistry combined with fluorescence and confocal microscopy. Without incubation, NK1r-IR was clearly observed on the membrane and the cytoplasm of myenteric and submucous neurons and interstitial cells of Cajal, but could not be clearly determined in the longitudinal and circular muscle. NK1r-IR-expressing neurons and interstitial cells were closely surrounded by substance P (SP) immunoreactive nerves. Incubation of colonic segments with nicardipine and TTX at 4 degrees C for 1 h with SP allowed to reveal a strong NK1r-IR at the surface of muscle cells. Incubation with SP (10(-6) M) at 37 degrees C for 1 min induced a relocation of NK1r-IR into the cytoplasm of muscle. This is interpreted as an internalization of NK1r induced by the binding of SP on muscular NK1r. The present data contribute to emphasize the role of NK1r in tachykinin-mediated neuronal processes regulating intestinal motility.

Immunohistochemical localization of substance P NK1 receptor in guinea pig distal colon

BACKGROUND

Neurokinin receptors facilitate tachykinin mediated intestinal motility and secretion. Distribution of Substance P (SP) neurokinin 1 receptor (NK1r) immunoreactivity (IR) has been previously characterized in guinea pig ileum, but not colon. This study localizes NK1rs in guinea pig distal colon.

METHODS

Neurons were double labelled for NK1r and either acetylcholine transferase (ChAT), calbindin (calb), neuropeptide Y (NPY), nitric oxide synthase (NOS) or SP. The NK1r endocytosis was induced by 10(-5) mol L(-1) SP, septide, [SarMet] SP or neurokinin A.

RESULTS

In guinea pig distal colon, NK1r-IR was present on 70% of submucosal neurons. Sixty-threepercent of the NK1r-IR submucosal neurons were ChAT-IR, 16% calb/SP-IR, 19% NPY-IR and 14% NOS-IR neurons. The NK1r-IR was present on 5% of myenteric neurons. Of these 63% were ChAT-IR, 16% calb-IR neurons and 25% NOS-IR. The NK1rs were also on myenteric plexus interstitial cells of Cajal and on circular muscle.

CONCLUSION

In guinea pig distal colon, NK1rs were on 70% of submucosal neurons including all three secretomotor neuron subtypes and sensory neurons, suggesting NK1rs have a major role in neuronal control of mucosal reflexes. The NK1rs were on few myenteric neurons but were dense on muscle cells, suggesting NK1rs affect motility through neuro-muscular rather than neuro-neuronal transmission.

Substance P and the neurokinin-1 receptor in relation to eosinophilia in ulcerative colitis

Substance P (SP) has been implicated in the pathophysiology of ulcerative colitis (UC) and it has been suggested that blocking of its effect would be advantageous in this disease. Eosinophils have also been implicated in the pathophysiology of UC. In the present study, specimens from the sigmoid colon of UC patients were investigated by the use of antisera against SP and the neurokinin-1 receptor (NK-1R) and staining for demonstration of eosinophils. The degrees of SP innervation and NK-1R immunoreaction, as well as the levels of eosinophil infiltration, varied between different patients. Interestingly, NK-1R immunoreaction in the epithelium was often seen to be the most marked where there were numerous eosinophils in the underlying mucosa and where the mucosa showed a marked morphologic derangement. The observations suggest that there are marked fluctuations in effects of SP and eosinophils during the disease. The infiltrating eosinophils may be involved in the destruction of the mucosal tissue. Furthermore, for the majority of cases where there is marked derangement of the mucosa, it is apparent that there is an upregulation of the NK-1 receptor in the epithelium in parallel with the infiltration of the eosinophils.

Chronic constipation in children: organic disorders are a major cause

Diagnostic tools for paediatric chronic constipation have been limited, leading to over 90% of patients with treatment-resistant constipation being diagnosed with chronic idiopathic constipation, with no discernible organic cause. Work in our institution suggests that a number of children with intractable symptoms actually have slow colonic transit leading to slow transit constipation. This paper reviews recent data suggesting that a significant number of the children with chronic treatment-resistant constipation may have organic causes (slow colonic transit and outlet obstruction) and suggests new approaches to the management of children with chronic treatment-resistant constipation.

Effects of TAK-637 on NK(1) receptor-mediated mechanisms regulating colonic secretion

This study investigates the effect of a selective NK(1) receptor antagonist TAK-637 on enteric mechanisms involved in regulation of epithelial secretion in the colon. Mucosal sheets isolated from guinea-pig colon were placed in modified Ussing chambers and the net active transport of electrolytes was measured as short-circuit current (Isc). GR-73632, a selective NK(1) receptor agonist, induced an increase in basal Isc, which was inhibited by TAK-637 (IC(50) of 21 nM). The increase in Isc induced by GR-73632 was significantly attenuated by tetrodotoxin (TTX, 1 microM), indicating that TAK-637 inhibits neuronal NK(1) receptors. Moreover, TAK-637 reduced the TTX-resistant component of the response to GR-73632 suggesting that NK(1) receptors expressed by epithelial cells are inhibited by TAK-637. In separate experiments, TAK-637 partially inhibited the submaximal Isc induced by electrical field stimulation (EFS, 0.5 ms, 15 Hz) of enteric nerves or by activation of primary afferent fibers using capsaicin (50 microM). TAK-637 had no significant effect on the basal Isc or on responses induced by neurokinin A (NKA), senktide, or forskolin. The results imply that inhibition of peripheral NK(1) receptors may reduce autonomic epithelial secretion in response to activation of autonomic secretomotor pathways, while having no significant effect on basal epithelial transport.

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.

Regulation of intestinal secretion involved in the interaction between neurotransmitters and prostaglandin E2

In this short review, it will be described that neurotransmitter-induced secretion in the intestine may be influenced by the tissue level of prostaglandin E2 (PGE2). In the normal condition, vasoactive intestinal polypeptide (VIP) and acetylcholine (ACh) are the predominant neurotransmitters of secretomotor neurones. VIP and ACh activate distinct second messenger systems in epithelial cells, i.e. adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium ion (Ca2+), respectively. An increase in intracellular cAMP induces a small amount of chloride (Cl-) secretion in epithelial cells, while simultaneous increases in intracellular Ca2+ and cAMP greatly enhances the cAMP-induced Cl- secretion. When the concentration of prostaglandins reaches a high level in the intestinal tissue substance P, which is a neurotransmitter of sensory neurones, can also induce a massive Cl- secretion by cross-potentiation of cAMP and Ca2+ in epithelial cells. In conclusion, it is considered that the concentration of tissue PGE2 may indicate tissue alert level, and when this level elevates, PGE2 enhances ACh and SP-induced Cl- secretion, thus mediating massive fluid secretion for host defence.

Expression of functional neurokinin-1 receptors in regenerative glands during gastric wound healing in rodents

BACKGROUND & AIMS

Although functions of the neurokinin-1 receptor have been well explored in neurogenic inflammation and immunoinflammatory responses, little is known about neurokinin-1 receptors during gastric wound healing. The aim of this study was to assess whether neurokinin-1 receptors play a role in gastric wound healing.

METHODS

In vitro neurokinin-1 receptor autoradiography and immunohistochemistry were performed to identify, locate, and quantify neurokinin-1 receptors during wound healing in rodents with cryoulcers in the gastric corpus and antrum. Moreover, to assess the functionality of these receptors, the effect of the neurokinin-1 receptor antagonist NKP608 on gastric wound healing was quantified in vivo in wild-type and cyclooxygenase-2(-/-) mice.

RESULTS

Regenerative glands of the mucosal ulcer margin of rat cryoulcers of the gastric corpus showed strong expression of neurokinin-1 receptors in binding studies between days 3 and 22, with little expression on days 29-84. In addition, strong immunoreactivity for neurokinin-1 receptors was detected on the cell membrane of these regenerative glands. Expression of neurokinin-1 receptors in regenerative glands was confirmed in the rat antrum and the mouse gastric corpus. Moreover, in vivo functional tests during gastric ulcer healing showed that cell proliferation in the regenerative epithelia of the ulcer margin was significantly decreased by NKP608 compared with placebo; furthermore, gastric ulcer healing was significantly delayed by NKP608 both in wild-type and cyclooxygenase-2(-/-) mice.

CONCLUSIONS

This report shows the time-limited overexpression of neurokinin-1 receptors in the mucosal repair tissue of the corpus and antrum. Our in vitro and in vivo data suggest that neurokinin-1 receptors are involved in gastric wound healing.

Interstitial cells of Cajal are functionally innervated by excitatory motor neurones in the murine intestine

Recent studies have demonstrated that intramuscular interstitial cells of Cajal (ICC) are preferential targets for neurotransmission in the stomach. Terminals of enteric motor neurones also form tight, synaptic-like contacts with ICC in the small intestine and colon, but little is known about the role of these cells in neurotransmission. ICC at the deep muscular plexus (ICC-DMP) of the small intestine express neurokinin 1 receptors (NK1R) and internalize these receptors in response to exogenous substance P. We used NK1R internalization as an assay of functional innervation of ICC-DMP in the murine small intestine. Under basal conditions NK1R-like immunoreactivity (NK1R-LI) was mainly observed in ICC-DMP (519 cells counted, 100% were positive) and myenteric neurones. ICC-DMP were closely apposed to substance P-containing nerve fibres. Of 338 ICC-DMP examined, 65% were closely associated with at least one substance P-positive nerve fibre, 32% were associated with at least two, 2% were associated with more than two nerve fibres and 1% with none. After electrical field stimulation (EFS, 10 Hz; 1 min) NK1R-LI was internalized in more than 80% of ICC-DMP, as compared to 10% of cells before EFS. Internalization of NK1R was not observed in myenteric ICC or smooth muscle cells in response to nerve stimulation. Internalization of NK1R-LI was blocked by the specific NK1 receptor antagonist WIN 62577 (1 microm) and by tetrodotoxin (0.3 microm), suggesting that internalization resulted from stimulation of receptors with neurally released neurokinins. These data suggest that ICC-DMP are primary targets for neurokinins released from enteric motor neurones in the intestine.

Cholinergic transmission to colonic circular muscle of children with slow-transit constipation is unimpaired, but transmission via NK2 receptors is lacking

Tachykinins (TKs) colocalize with acetylcholine in excitatory motor neurones supplying human colonic circular muscle (CCM). Some children with slow-transit constipation (STC) have reduced TK-immunoreactivity in nerve terminals in CCM suggesting a deficit in neuromuscular transmission. This study aimed to test this possibility. Seromuscular biopsies of transverse colon were obtained laparoscopically from STC children (37, 17 with low density of TK-immunoreactivity). Specimens of transverse (17) and sigmoid colon (20) were obtained from adults undergoing colonic resection for cancer. CCM contractions were measured isotonically and responses to carbachol, neurokinin A (NKA) and electrical field stimulation (EFS) recorded. Carbachol and NKA-evoked contractions in adult and STC colon. Hyoscine (2 micromol L-1) significantly depressed responses to EFS in all preparations. Blockade of NK2 receptors (SR 48968, 2 micromol L-1) significantly depressed EFS-evoked contractions of adult transverse CCM, but had no effect on STC preparations. Thus, neuromuscular transmission in both adults and STC children is predominantly cholinergic and this component is unimpaired in the latter, indicating that reduced TK-immunoreactivity is not a marker for depressed cholinergic responses. Although pharmacologically responsive TK receptors are present in STC colon, we did not detect neuromuscular transmission mediated by release of TKs in these preparations.

The third intracellular loop and carboxyl tail of neurokinin 1 and 3 receptors determine interactions with beta-arrestins

Tachykinins interact with three neurokinin receptors (NKRs) that are often coexpressed by the same cell. Cellular responses to tachykinins depend on the NKR subtype that is activated. We compared the colocalization of NK1R and NK3R with beta-arrestins 1 and 2, which play major roles in receptor desensitization, endocytosis, and signaling. In cells expressing NK1R, the selective agonist Sar-Met-substance P induced rapid translocation of beta-arrestins 1 and 2 from the cytosol to the plasma membrane and then endosomes, indicative of interaction with both isoforms. In contrast, the NK3R interacted transiently with only beta-arrestin 2 at the plasma membrane. Despite these differences, both NK1R and NK3R similarly desensitized, internalized, and activated MAP kinases. Because interactions with beta-arrestins can explain differences in the rate of receptor resensitization, we compared resensitization of agonist-induced Ca2+ mobilization. The NK1R resensitized greater than twofold more slowly than the NK3R. Replacement of intracellular loop 3 and the COOH tail of the NK1R with comparable domains of the NK3R diminished colocalization of the NK1R with beta-arrestin 1 and accelerated resensitization to that of the NK3R. Thus loop 3 and the COOH tail specify colocalization of the NK1R with beta-arrestin 1 and determine the rate of resensitization.

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.

Cellular localization and distribution of neurokinin-1 receptors in the rat stomach

In the stomach, the majority of substance P's effects are mediated by the activation of neurokinin-1 (NK1) receptors. The gastric cellular distribution of these receptors in Wistar and Sprague-Dawley rats was determined using immunocytochemistry. The localization of the NK1 receptors with respect to von Willebrand's factor, protein gene product 9.5, substance P, vasoactive intestinal peptide, and calcitonin gene-related peptide was also determined. Results show that NK1 receptor immunoreactivity was dependent on the duration of fixation. In corpus and antrum tissues that were fixed in 4% paraformaldehyde for 30 min, the presence of NK1 receptor immunoreactivity was demonstrated on nerve fibers throughout the stomach, on the surface and in the cytoplasm of myenteric cell bodies, on circular smooth muscle cells, and on vascular endothelial cells. This was observed in tissues from both rodent strains. Overnight fixation in the same fixative, however, demonstrated the presence of NK1 receptor immunoreactivity only on nerve fibers and cell bodies of the myenteric plexus, and on circular smooth muscle cells. In 30-min fixed tissues, the localization of NK1R immunoreactivity on vascular endothelial cells and nerve fibers was confirmed by co-localization with von Willebrand's factor and protein gene product 9.5 immunoreactivity, respectively. In both rodent strains, NK1 receptor immunoreactivity was co-localized with substance P immunoreactivity on nerve fibers of the longitudinal and circular muscle. In the Wistar rat, NK1 receptor immunoreactivity was co-localized with vasoactive intestinal peptide immunoreactivity or calcitonin gene-related peptide immunoreactivity throughout the stomach. However, in the Sprague-Dawley rat, NK1 receptor immunoreactivity was only co-localized with calcitonin gene-related peptide immunoreactivity in a minority of fibers of the circular muscle. The overall results of this study show that the antigenic epitopes of the NK1 receptor are sensitive to overfixation. When tissues were not overfixed, NK1 receptor immunoreactivity was distributed more extensively throughout the rat stomach than has been described previously. The results of this study provide the anatomical basis for many of the actions of substance P in the rat stomach.

Effect of nepadutant, a neurokinin 2 tachykinin receptor antagonist, on immediate-early gene expression after trinitrobenzenesulfonic acid-induced colitis in the rat

Tachykinins have been implicated in inflammatory responses such as those occurring in inflammatory bowel disease. Accordingly, we investigated the effect of a selective neurokinin (NK) 2 receptor antagonist, nepadutant, on proto-oncogene expression in the L(6)-S(1) spinal cord as well as in dorsal root ganglion (DRG) neurons after either non-noxious colorectal distension (CRD) or trinitrobenzenesulfonic acid (TNBS)-induced colitis in the adult rat. In both preparations, c-fos was expressed in similar spinal cord regions, including medial and lateral dorsal horn, dorsal commissure (DCM; laminae X above the central canal), and the sacral parasympathetic nucleus (SPN, laminae V-VII). However, TNBS-induced colitis produced significantly larger numbers (8-10-fold increase over control) of Fos-positive spinal cord neurons. In addition, there was also a significant increase (3-4-fold) in the number of Jun-positive colon DRG neurons after colitis compared with CRD. Nepadutant had no significant effect on proto-oncogene expression induced by CRD in either spinal cord neurons or DRG neurons. In contrast, nepadutant significantly decreased (70%) the number of Fos-positive neurons in dorsal horn, DCM, and SPN spinal cord regions and significantly decreased (75%) the number of Jun-positive DRG neurons after TNBS-induced irritation of the colon. These findings indicate that nepadutant suppresses the responses of colonic afferent neurons to nociceptive stimuli and that NK2 receptor antagonists may be beneficial in the treatment of sensory symptoms of colitis.

Substance P-evoked Cl(-) secretion in guinea pig distal colonic epithelia: interaction with PGE(2)

Interaction between substance P (SP) and PGE(2) on Cl(-) secretion in the guinea pig distal colonic epithelia was investigated. A short-circuit current (I(sc)) was measured as an index of ion transport. Mucosa preparations deprived of muscle and submucosa of distal colon were mounted in the Ussing flux chamber and treated with TTX and piroxicam to remove the influences of neuronal activity and endogenous PG synthesis, respectively. Although SP (10(-7) M) itself evoked little increase in I(sc), exogenous PGE(2) concentration dependently enhanced the response of SP. The effect of PGE(2) on the SP-evoked response was mimicked by forskolin and 8-bromoadenosine cAMP. Depletion of Ca2+ from the bathing solution reduced the PGE(2)-dependent response of SP. Effects of PGE(2), SP, and SP in the presence of PGE(2) on intracellular Ca2+ concentration ([Ca2+](i)) in isolated crypt cells were measured by the confocal microscope fluorescence imaging system. SP, but not PGE(2), temporally evoked an increase in [Ca2+](i) but declined to the baseline within 3 min. A return of the SP-evoked increase in [Ca2+](i) was slower in the presence of PGE(2) than SP alone. These results suggest that PGE(2) synergistically enhances SP-evoked Cl(-) secretion via an interaction between the intracellular cAMP and [Ca2+](i) in the epithelial cells. In conclusion, SP and PGE(2) could cooperatively induce massive Cl(-) secretion in guinea pig distal colon at epithelial levels.

Roles of substance P receptors in human colon circular muscle: alterations in diverticular disease

The characteristics of [(125)I]Bolton-Hunter[Sar(9),Met(O(2))(11)]substance P ([(125)I]BH-SarSP) binding were investigated in membranes of human ascending, transverse, distal, and sigmoid colon circular muscle. Binding of [(125)I]BH-SarSP was of high affinity (K(D) = 68 nM) and low capacity (B(max) = 0.31 fmol/mg of wet weight tissue), and showed no regional differences. [(125)I]BH-SarSP binding was inhibited by SP approximately equal to [Pro(9)]SP > or = (2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine (CP99994) >> neurokinin (NK) A > or = neuropeptide gamma > [Lys(5),MeLeu(9),Nle(10)]-NKA(4-10) approximately (S)-N-methyl-N[4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl) butyl]benzamide (SR48968) >> senktide, suggesting binding to NK-1 sites. Most agonists seemed to bind to two sites. In autoradiographic studies, dense binding for [(125)I]BH-SarSP was associated with submucosal and longitudinal muscle blood vessels, and the submucosal margin of circular muscle (corresponding to interstitial cells of Cajal), with moderate binding over most of the circular muscle. In normal colon circular muscle strips, [Pro(9)]SP was almost ineffective, and SP caused contractions with pD(2) values of 5.3 to 5.7. No regional differences were observed in potency or efficacy. Responses to SP were inhibited by the NK-2 receptor antagonist SR48968, but not by NK-1 antagonist CP99994, indicating the involvement of NK-2 rather than NK-1 receptors. Atropine significantly inhibited contractions induced by SP, indicating a minor cholinergic component. Contractile responses to SP were considerably reduced in preparations from patients with diverticular disease, and marginally reduced in ulcerative colitis compared with control. This study clearly demonstrates an NK-1 binding site on human colon circular muscle, but its role in this tissue remains unclear and may not involve contractile mechanisms. The attenuated contractility in specimens with diverticular disease may reflect disease-related alterations of the tachykinin receptor system.

Alterations of neurokinin receptors and interstitial cells of Cajal during and after jejunal inflammation induced by Nippostrongylus brasiliensis in the rat

Substance P (SP) and its receptors NK1 and NK2 are widely expressed in the intestinal wall by neurones, interstitial cells of Cajal (ICC) and smooth muscle cells. Changes in SP and/or its NK receptors have been documented during experimental inflammation in animals or inflammatory bowel diseases in humans, but the data concern the acute phase of the inflammatory process. We determined immunohistochemically whether NK receptors and SP were altered in the muscle coat during jejunal inflammation induced by the nematode Nippostrongylus brasiliensis and whether these alterations persisted when inflammation had spontaneously resolved 30 days postinfection. An ultrastructural analysis was also conducted on ICC, nerves and muscle. At day 14, when inflammation peaked, there was a reduction in NK1 receptors in myenteric neurones and in SP-immunoreactive nerve endings. There were also ultrastructural anomalies in synaptic vesicles and NK2 receptor loss in the circular muscle layer. The SP decrease persisted at day 30, whereas neurones and circular muscle cells re-expressed NK1 and NK2 receptors, respectively. The ICC at the deep muscular plexus, located near to the inflammatory site, underwent alterations leading to their complete loss at day 30. These morphological changes are probably associated with impairment in tachykinergic control of jejunal functions leading to the alterations of motility and sensitivity to distension already described in these animals.

Tachykinin receptors in the gut: physiological and pathological implications

The tachykinins substance P and neurokinin A participate in the regulation of gastrointestinal motility, secretion, vascular permeability and pain sensitivity. Advances made during the past two years corroborate a causal involvement of tachykinins in inflammation-induced disturbances of gut function, such as dysmotility, secretory diarrhoea, oedema and hyperalgesia. It would therefore appear that tachykinin receptors, which in the digestive system are expressed in a cell-specific manner, represent attractive targets for novel therapeutics in gastroenterology.