Motor response of the human isolated colon to capsaicin and its relationship to release of vasoactive intestinal polypeptide

Extrinsic Primary Afferent Neurons Link Visceral Pain to Colon Motility Through a Spinal Reflex in Mice

BACKGROUND & AIMS

Proper colon function requires signals from extrinsic primary afferent neurons (ExPANs) located in spinal ganglia. Most ExPANs express the vanilloid receptor TRPV1, and a dense plexus of TRPV1-positive fibers is found around myenteric neurons. Capsaicin, a TRPV1 agonist, can initiate activity in myenteric neurons and produce muscle contraction. ExPANs might therefore form motility-regulating synapses onto myenteric neurons. ExPANs mediate visceral pain, and myenteric neurons mediate colon motility, so we investigated communication between ExPANs and myenteric neurons and the circuits by which ExPANs modulate colon function.

METHODS

In live mice and colon tissues that express a transgene encoding the calcium indicator GCaMP, we visualized levels of activity in myenteric neurons during smooth muscle contractions induced by application of capsaicin, direct colon stimulation, stimulation of ExPANs, or stimulation of preganglionic parasympathetic neuron (PPN) axons. To localize central targets of ExPANs, we optogenetically activated TRPV1-expressing ExPANs in live mice and then quantified Fos immunoreactivity to identify activated spinal neurons.

RESULTS

Focal electrical stimulation of mouse colon produced phased-locked calcium signals in myenteric neurons and produced colon contractions. Stimulation of the L6 ventral root, which contains PPN axons, also produced myenteric activation and contractions that were comparable to those of direct colon stimulation. Surprisingly, capsaicin application to the isolated L6 dorsal root ganglia, which produced robust calcium signals in neurons throughout the ganglion, did not activate myenteric neurons. Electrical activation of the ganglia, which activated even more neurons than capsaicin, did not produce myenteric activation or contractions unless the spinal cord was intact, indicating that a complete afferent-to-efferent (PPN) circuit was necessary for ExPANs to regulate myenteric neurons. In TRPV1-channel rhodopsin-2 mice, light activation of ExPANs induced a pain-like visceromotor response and expression of Fos in spinal PPN neurons.

CONCLUSIONS

In mice, ExPANs regulate myenteric neuron activity and smooth muscle contraction via a parasympathetic spinal circuit, linking sensation and pain to motility.

Acute administration of capsaicin increases resting energy expenditure in young obese subjects without affecting energy intake, appetite, and circulating levels of orexigenic/anorexigenic peptides

Although capsaicin has been reported to reduce energy intake and increase energy expenditure in an adult (normal weight or overweight) population, thus resulting in a net negative energy balance and weight loss, these beneficial effects have not been investigated in young obese subjects. We hypothesize that capsaicin acutely administered in young obese subjects exerts the same effects on energy balance and that these effects are mediated by changes in gastrointestinal peptides regulating appetite. Thus, the aim of the present study was to evaluate the acute effects of capsaicin (2 mg) or placebo on energy intake, hunger, and satiety in obese adolescents and young adults (female-male ratio: 4:6, age: 21.0 ± 5.8 years; body mass index: 41.5 ± 4.3 kg/m²) provided an ad libitum dinner. Furthermore, circulating levels of some orexigenic (ghrelin) and anorexigenic (glucagon-like peptide 1 and peptide YY) peptides were measured after a meal completely consumed (lunch), together with the evaluation of hunger and satiety and assessment of resting energy expenditure (REE) through indirect computerized calorimetry. When compared to placebo, capsaicin did not significantly change either energy intake or hunger/satiety 6 hours after its administration (dinner). No differences in circulating levels of ghrelin, glucagon-like peptide 1, and peptide YY and in hunger/satiety were found in the 3 hours immediately after food ingestion among obese subjects treated with capsaicin or placebo (lunch). By contrast, the meal significantly increased REE in the capsaicin- but not placebo-treated group (capsaicin: from 1957.2 ± 455.1 kcal/d up to 2342.3 ± 562.1 kcal/d, P < .05; placebo: from 2060.1 ± 483.4 kcal/d up to 2296.0 ± 484.5 kcal/d). The pre-post meal difference in REE after capsaicin administration was significantly higher than that observed after placebo (385.1 ± 164.4 kcal/d vs 235.9 ± 166.1 kcal/d, P < .05). In conclusion, although capsaicin does not exert hypophagic effects, these preliminary data demonstrate its ability as a metabolic activator in young obese subjects.

Capsaicin inhibits the spontaneous pacemaker activity in interstitial cells of cajal from the small intestine of mouse

Background/Aims

Capsaicin (8-methyl-N-vanillyl-6-ninenamide), a compound found in hot peppers, has been reported to have different physiological actions on different cell types. Not much work has been done about the effect of capsaicin on the function of interstitial cells of Cajal (ICC). In the present study, we examined the action of external application of capsaicin on pacemaker activity in the cultured ICC from the small intestine of mouse.

Methods

We investigated the effect of capsaicin on pacemaker currents in cultured ICC from the small intestine of mouse using a whole cell patch-clamp technique and Ca²⁺-imaging analysis.

Results

When capsaicin was applied externally to the pacemaker generating ICC, it completely inhibited the pacemaker potential under current-clamp mode (I = 0) and the pacemaker current under voltage-clamp mode at a -70 mV of holding potentials. The effect of capsaicin on pacemaker activity in ICC was shown dose dependently. The effect of capsaicin was not through the transient receptor potential of the vanilloid type 1 (TRPV1) channel as capsazepine did not block the effect of capsaicin. L-NAME, an inhibitor of nitric oxide synthase, also did not block the capsaicin-induced effects. When the action of capsaicin was examined in the intracellular calcium oscillation, it completely abolished the calcium oscillation.

Conclusions

These results prove that the capsaicin has the inhibitory effects on the ICC which is carried out neither through TRPV channel nor the nitric oxide production. Intracellular Ca²⁺ was also an important target for actions of capsaicin on ICC.

Localization of TRPV1 and contractile effect of capsaicin in mouse large intestine: high abundance and sensitivity in rectum and distal colon

We investigated immunohistochemical differences in the distribution of TRPV1 channels and the contractile effects of capsaicin on smooth muscle in the mouse rectum and distal, transverse, and proximal colon. In the immunohistochemical study, TRPV1 immunoreactivity was found in the mucosa, submucosal, and muscle layers and myenteric plexus. Large numbers of TRPV1-immunoreactive axons were observed in the rectum and distal colon. In contrast, TRPV1-positive axons were sparsely distributed in the transverse and proximal colon. The density of TRPV1-immunoreactive axons in the rectum and distal colon was much higher than those in the transverse and proximal colon. Axons double labeled with TRPV1 and protein gene product (PGP) 9.5 were detected in the myenteric plexus, but PGP 9.5-immunoreactive cell bodies did not colocalize with TRPV1. In motor function studies, capsaicin induced a fast transient contraction, followed by a large long-lasting contraction in the rectum and distal colon, whereas in the transverse and proximal colon only the transient contraction was observed. The capsaicin-induced transient contraction from the proximal colon to the rectum was moderately inhibited by an NK1 or NK2 receptor antagonist. The capsaicin-induced long-lasting contraction in the rectum and distal colon was markedly inhibited by an NK2 antagonist, but not by an NK1 antagonist. The present results suggest that TRPV1 channels located on the rectum and distal colon play a major role in the motor function in the large intestine.

The acute effects of a lunch containing capsaicin on energy and substrate utilisation, hormones, and satiety

Background

Addition of capsaicin to the diet has been shown to increase satiety and thermogenesis. The effects of capsaicin on ghrelin, peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), in relation to changes in hunger and satiety are unknown.

Aim

To test the acute effects of a lunch containing capsaicin on gut derived hormones (GLP-1, ghrelin, and PYY), energy expenditure (EE), substrate oxidation and satiety at lunch in the postprandial state.

Methods

Thirty subjects (age: 31 ± 14 years, BMI: 23.8 ± 2.8 kg/m²) were studied twice in a crossover design. After 30 min resting on a bed, resting metabolic rate was measured by a ventilated hood system. Subsequently lunch (35% of daily energy intake) was served. The two lunch conditions were: (1) lunch without capsaicin and (2) lunch with capsaicin (CAPS). The macronutrient composition (energy percentage) of the lunches was 60% carbohydrates, 10% protein and 30% fat. During 3 h after the lunch diet-induced thermogenesis was measured. Furthermore, anchored 100 mm visual analogue scales on the appetite profile were collected (t = 0, 30, 60, 120, 150, 180 and 240) and blood samples were taken for analysis of GLP-1, PYY, and ghrelin concentrations (t = 0, 45, 60, 120, and 180).

Results

Satiety and EE were not different after CAPS lunch as compared to the control lunch. Fifteen minutes after lunch CAPS lunch increased GLP-1 (p < 0.05) and tended to decrease ghrelin (p = 0.07) as compared to the control lunch. PYY responses were not different between the CAPS lunch and the control lunch.

Conclusions

An acute lunch containing capsaicin had no effect on satiety, EE, and PYY, but increased GLP-1 and tended to decrease ghrelin.

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.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

Impaired capsaicin and neurokinin-evoked colonic motility in inflammatory bowel disease

BACKGROUND

Inflammatory bowel disease (IBD) is associated with altered sensory and motor function in the human colon. The aim of the present study was to compare neuromuscular function in normal and IBD-affected colon in vitro, with emphasis on inhibitory enteric nerves, sensory neuropeptides and stimulation of axon collaterals.

METHODS

Strips of longitudinal and circular muscle were prepared following colectomy from six patients with intestinal carcinoma (mean age 64.2 +/- 4.8 years) and six patients with IBD (Crohn's disease, n = 3; ulcerative colitis, n = 3: mean age 35.8 +/- 5.7 years). Responses were measured to electrical field stimulation, potassium chloride, 1,1-dimethyl-4-phenylpiperazinium iodide, isoprenaline, calcitonin gene-related peptide (CGRP), capsaicin and neurokinin (NK)-1 and -2 receptor subtype-specific agonists, alone or after muscle precontraction.

RESULTS

The NK-1 and CGRP receptor-mediated relaxation was reduced in the circular (by 44%, P < 0.05) and longitudinal (by 61%, P < 0.05) muscle from IBD-affected colon, respectively. Maximal NK-2 receptor-mediated contraction was also significantly decreased in both longitudinal (71%, P < 0.001) and circular (51%, P < 0.01) muscle. Capsaicin evoked relaxation in precontracted colonic longitudinal and circular muscle; this was significantly diminished in the IBD-affected colon (by 63%, P < 0.001 and 76%, P < 0.01, respectively). Responses evoked by stimulation of enteric inhibitory nerves were not significantly altered.

CONCLUSIONS

Colonic muscle strips from patients with IBD exhibited impaired CGRP and NK-1 receptor-mediated relaxation and NK-2 receptor-mediated contraction. Capsaicin-activated relaxation of colonic smooth muscle is deficient in IBD-affected colon. These results suggest a discrete effect of IBD on sensory-motor coupling and tachykinin-mediated effects on colonic motility.

Inhibition of nitric oxide synthesis blocks the inhibitory response to capsaicin in intestinal circular muscle preparations from different species

Moderate concentrations of the sensory stimulant drug capsaicin caused relaxation in human and animal intestinal circular muscle preparations (guinea-pig proximal, mouse distal colon, human small intestine and appendix) in vitro. With the exception of the guinea-pig colon, the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NOARG; 10(-4) M) strongly inhibited the relaxant effect of capsaicin. Tetrodotoxin, an inhibitor of voltage-sensitive Na+ channels failed to significantly reduce the inhibitory effect of capsaicin in the guinea-pig colon, human ileum and appendix; it caused an approximately 50% reduction in the mouse colon. The relaxant effect of capsaicin was strongly reduced in colonic preparations from transient receptor potential vanilloid type (TRPV1) receptor knockout mice as compared to their wildtype controls. It is concluded that nitric oxide, possibly of sensory origin, is involved in the relaxant action of capsaicin in the circular muscle of the mouse and human intestine.

Neurotransmitter coding of enteric neurones in the submucous plexus is changed in non-inflamed rectum of patients with Crohn's disease

Knowledge of the neurochemical coding of submucosal neurones in the human gut is important to assess neuronal changes under pathological conditions. We therefore investigated transmitter colocalization patterns in rectal submucosal neurones in normal tissue (n=11) and in noninflamed tissue of Crohn's disease (CD) patients (n=17). Neurone-specific enolase (NSE), choline acetyltransferase (ChAT), vasoactive intestinal polypeptide (VIP), substance P (SP), nitric oxide synthase (NOS) and calcitonin gene-related peptide (CGRP) were detected immunohistochemically in whole-mount preparations from rectal biopsies. The neuronal marker NSE revealed no differences in the number of cells per ganglion (controls 5.0; CD 5.1). Four cell populations with distinct neurochemical codes were identified. The sizes of the populations ChAT/VIP (58% vs. 55%), ChAT/SP (8% vs. 8%), and ChAT/- (22% vs. 22%) were similar in control and CD. The population VIP/- was significantly increased in CD (12% vs. 2% in controls). Unlike in controls, all NOS neurones colocalized ChAT in CD. Thickened CGRP-fibres occurred in CD. We identified neurochemically distinct populations in the human submucous plexus. The increase in the VIP/- population, extensive colocalization of ChAT and NOS and hypertrophied CGRP fibres indicated adaptive changes in the enteric nervous system in noninflamed rectum of CD patients.

Circular muscle contraction, messenger signalling and localization of binding sites for neurokinin A in human sigmoid colon

1. Neurokinin (NK)A is the endogenous ligand for the tachykinin NK2 receptor. In the present study, tachykinins and selective receptor agonists were tested as contractile agonists in human colon circular muscle and [125I]-NKA was used to localize binding sites in human colon. 2. In strips of circular muscle, removal of mucosa and submucosa significantly (P < 0.05) increased the potency and the maximum response achieved by NKA. 3. The rank order of potency of tachykinin and selective receptor agonists in contracting circular muscle strips was NKA > or = [Lys5,MeLeu9,Nle10]NKA(4-10) > or = neuropeptide (NP)gamma > or = [betaAla8]NKA(4-10) >> NKB > substance P (SP) >> senktide approximate to [Pro9]SP. 4. Specific binding sites for [125I]-NKA were densely localized over circular muscle and muscularis mucosae. Weak specific binding was seen on longitudinal muscle and taenia coli, whereas no binding sites were seen on mucosa, ganglia or blood vessels. 5. In circular muscle, the selective NK2 receptor agonist [LysS,MeLeu9,Nle10]NKA(4-10) produced weak increases (maximum 37%) in inositol monophosphate formation with a pD2 of 6.8+/-0.51 (n = 3). Carbachol (100 micromol/L) was also a weak stimulant (maximum 45%). These agonists were over 10-fold more efficacious in stimulation of inositol monophosphate in rat urinary bladder. 6. In conclusion, [125I]-NKA binding sites localized on human colon circular muscle were characterized as NK2 receptors. Functionally, the tachykinin NK2 receptor is mediating circular smooth muscle contraction. Although the human NK2 receptor is coupled to the phosphatidylinositol pathway, other second messenger mechanisms may also operate in this tissue.

Capsaicin increases gastric emptying rate in healthy human subjects measured by 13C-labeled octanoic acid breath test

The role of capsaicin-sensitive primary afferent sensory nerves in the regulation of gastrointestinal motility in human is not clarified yet. In this study, we investigated the effect of 400 microg capsaicin given intragastrically on gastric emptying measured by 13C-octanoic acid breath test in ten healthy human subjects. Four parameters of gastric emptying curves were taken into consideration: 1) maximum value of the curve, 2) time belonging to this maximum, 3) slope of the rising part of the curve and 4) time belonging to the 50% of the area under the curve. Administration of 400 microg capsaicin significantly increased the slope of gastric emptying curve (from 0.1 +/- 0.01 to 0.139 +/- 0.014 U x min(-1), P < 0.05) and significantly decreased the time belonging to the maximum value of emptying curve (from 150 +/- 18 to 75 +/- 12 min, P < 0.05) and the time belonging to the 50% of the area under the curve (from 112 +/- 15 to 99 +/- 14 min, P < 0.05). According to our results 400 microg capsaicin enhances gastric emptying rate in healthy human subjects.

Neuroimmune appendicitis

BACKGROUND

15-25% of appendices removed from patients with suspected appendicitis appear normal on histological examination. The cause of pain in such patients is unknown. Since the content of neuropeptides seems to be altered in chronic inflammation, we investigated possible changes in peptidergic innervation for substance P (SP), vasoactive intestinal peptide (VIP), and growth-associated protein-43 (GAP-43).

METHODS

Appendices classified as showing acute appendicitis, non-acute appendicitis (clinical signs of acute appendicitis, but histologically not inflamed), or normal were processed for SP, VIP, and GAP-43 immunocytochemistry. The density of SP immunostaining was assessed by digitised morphometry.

FINDINGS

31 appendix specimens were studied (16 acute, 15 non-acute). 16 specimens were used as controls. Expression of GAP-43 was increased in the non-acute appendices. We observed larger amounts of SP-immunoreactive and VIP-immunoreactive nerves in the mucosal layer of the appendix in patients with non-acute appendicitis than in controls and patients with acute appendicitis (mean % area SP-immunoreactive 0.0496 [SD 0.0113] non-acute, 0.0221 [0.0049] acute, 0.0229 [0.0068] controls). In addition, a close spatial relation between SP-immunoreactive and VIP-immunoreactive nerve fibres and lymphoid cells was detected in the outer zone of lymph follicles.

INTERPRETATION

Neuroproliferation in the appendix, in association with an increase in neurotransmitters SP and VIP, may be involved in the pathophysiology of acute right abdominal pain in the absence of an acute inflammation of the appendix. Our data, together with increasing knowledge about the way in which the nervous system and immune cells interact, suggest that neuroimmune appendicitis is a distinct pathological entity.

Characterization of the [125I]-neurokinin A binding site in the circular muscle of human colon

Neurokinin A (NKA) is a potent contractile agonist of human colon circular muscle. These responses are mediated predominantly through tachykinin NK2 receptors. In the present study, the NK2 receptor radioligand [125I]-NKA has been used to characterize binding sites in this tissue, using tachykinin agonists and antagonists. 125INKA labelled a single, high affinity binding site. Specific binding (95% of total binding) of [125I]-NKA was saturable (K(D) 0.47+/-0.05 nM), of high capacity (Bmax 2.1+/-0.1 fmol mg(-1) wet weight tissue) and reversible (kinetically derived K(D) 0.36+/-0.07 nM). The rank order of agonists competing for the [125I]-NKA binding site was neuropeptide gamma (NPgamma) > or = NKA > or = [Lys5, MeLeu9,Nle10]NKA (4-10) (NK2 agonist) >> substance P (SP) > neurokinin B (NKB) > or = [Pro9]SP (NK1 agonist) >> senktide (NK3 agonist), indicating binding to an NK2 site. The nonpeptide selective NK2 antagonist SR48968 showed higher affinity for the [125I]-NKA site than selective peptide NK2 antagonists. The rank order of potency for NK2 antagonists was SR48968 > or = MEN11420 > GR94800 > or = MEN10627 > MEN10376 > or = R396. The NK1 antagonist SR140333 was a weak competitor. The competition curve for SP could be resolved into two sites. When experiments were repeated in the presence of SR140333 (0.1 microM), the curve for SP became monophasic and showed a significant shift to the right, whereas curves to NKA and NKB were unaffected. In conclusion, binding of the radioligand [125I]-NKA to membranes from circular muscle is predominantly to the NK2 receptor. There may be a small component of binding to the NK1 receptor. The NK2 receptor mediates circular muscle contraction, whereas the role of the NK1 receptor in circular muscle is unclear.

Characterization of tachykinin NK2 receptor on dog proximal colon. Antagonism by MEN 10,627 and SR 48,968

The nature of the tachykinin receptors involved in the contraction of the circular muscle of dog colon has been investigated. The following rank order of potency for agonists was obtained: [Sar9,Met(O2)11]substance P > or = neurokinin A > [beta-Ala8]neurokinin A-(4-10) >> [MePhe7]neurokinin B. The efficacy of the tachykinin NK2 receptor agonists was significantly greater than that of the tachykinin NK1 receptor agonists and of carbachol. A concentration-dependent rightward shift of the motor response to neurokinin A (obtained in the presence of (+/-)-CP 96,345) was induced by peptide and non-peptide tachykinin NK2 receptor antagonists with this rank order: MEN 10,627 = SR 48,968 >> L 659,877 > MEN 10,376 > MDL 28,564. MEN 10,627 and SR 48,968 affinities were similar to those measured in human tissues. In conclusion, the tachykinin NK2 receptor plays a predominant role in tachykinin-induced contraction of the canine colonic circular muscle and this tissue could be useful to predict the pharmacological actions of MEN 10,627 and SR 48,968 in human colon.

Tachykinin receptors in gastrointestinal motility

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

Intramural neural pathways between the duodenum and sphincter of Oddi in the Australian brush-tailed possum in vivo

1. Balloon distension of the duodenum 2 cm oral or anal to the sphincter of Oddi-duodenal junction elevated the amplitude of spontaneous sphincter of Oddi phasic contractions by 37.7 +/- 8.5 or 120.1 +/- 79.8%, respectively (mean +/- S.E.M., both n = 6, P < 0.05, Wilcoxon test). To further investigate this response, this study aimed to determine if: (i) electrical field stimulation (EFS) of the duodenum influences sphincter of Oddi activity; (ii) intramural nerves mediate the response; and (iii) nicotinic and/or muscarinic receptors are involved. 2. Electrical field stimulation (70 V, 0.5 ms; 5-60 Hz, 10-20 s) of the duodenal anterior serosal surface 2-4 cm oral or anal to the sphincter of Oddi-duodenal junction, produced excitatory responses in the sphincter of Oddi in anaesthetized Australian brush-tailed possums (n = 45). 3. These responses were frequency dependent, maximal at 30 Hz (n = 4) and abolished by tetrodotoxin (9 micrograms kg-1 I.A.; n = 6), or by crushing the duodenum (n = 3). Hexamethonium bromide (30 mg kg-1 I.V.) did not significantly alter the response to duodenal EFS either oral (n = 6) or anal (n = 8) to the sphincter of Oddi-duodenal junction. Atropine sulphate (30 micrograms kg-1 I.V.) reduced the response to duodenal EFS oral and anal to the sphincter of Oddi-duodenal junction to 11.2 +/- 5.8 (n = 6) and 45.0 +/- 26.8% (n = 8), respectively (both P < 0.05). 4. Bilateral cervical vagotomy and guanethidine infusion (10 mg kg-1 over 15 min I.V.) did not significantly alter the responses to duodenal EFS (n = 7). 5. Excitatory intramural neural pathways between the sphincter of Oddi and the segment of duodenum 4 cm oral and anal to the sphincter of Oddi-duodenal junction have been demonstrated. These postganglionic pathways may involve muscarinic receptors.

Regional specificities in the distribution, chemical phenotypes, and coexistence patterns of neuropeptide containing nerve fibres in the human anal canal

Despite the pivotal clinical significance of the human anal canal, little is known about its total and specific innervation. This study assessed the comparative distribution and histotopology of nerve fibres immunoreactive for neural markers and a variety of regulatory active neuropeptides in the human anal canal by light microscopic immunohistochemistry. Depending on the epithelial zone and region of the anal canal, the neural elements were differentially immunoreactive for the pan-neural marker protein gene product 9.5, the catecholamine marker tyrosine hydroxylase, the neuroendocrine marker chromogranin A, and various neuropeptides. Protein gene product 9.5-immunoreactive nerve fibres were ubiquitously abundant in the anal canal. In the anal transitional zone, ectopic epithelial types were supplied by the same pattern of peptidergic nerves as the respective type of epithelium in normotopic location. In the dermis of the squamous zone and in the perianal epidermis, unusual distribution patterns of nerve fibres, referred to as areas of high nerve fibre density, were encountered. Double immunohistochemistry revealed region-specific coexistence patterns of neuropeptidergic nerve fibres, and novel peptide coexistence patterns were detected in anal nerve fibres. Subsets of nerve fibres formed close spatial relationships with chromogranin A-positive neuroendocrine cells, most frequently in the anal transitional zone. Chromogranin-A positive cells were shown to be present in the epithelium of perianal eccrine sweat glands. The differential distribution, peptide phenotypes and coexistence patterns of different nerve fibre populations in the human anal canal may reflect topospecific regulatory functions of neurally released neuropeptides in health and disease.

Capsaicin releases calcitonin gene-related peptide from the human iris and ciliary body in vitro

Slices of human iris or ciliary body, obtained post-mortem (8-12 h after death, n = 5), were superfused in vitro with capsaicin (10 microM) and the immunoreactivity for substance P (SP-LI) or calcitonin gene-related peptide (CGRP-LI) was measured in the effluent. In the iris and in the ciliary body CGRP-LI was 3.71 +/- 0.74 pmol/g and 3.01 +/- 0.55 pmol/g and SP-LI was 6.68 +/- 0.75 pmol/g and 6.55 +/- 0.84 pmol/g, respectively. A first exposure to capsaicin increased the CGRP-LI outflow from the ciliary body (427 +/- 46 fmol/g/30 min), whereas a second challenge with the drug 30 min later, failed to significantly enhance the CGRP-LI outflow (21.8 +/- 15.6 fmol/g/30 min). Likewise, the capsaicin-evoked increase in CGRP-LI outflow from the iris slices (472 +/- 62 fmol/g/30 min) was no longer observed at the second drug administration (38.4 +/- 12.8 fmol/g/30 min). Capsaicin failed to increase the SP-LI outflow from either the iris or the ciliary body. Reverse phase HPLC analysis of CGRP-LI indicated that authentic CGRP was contained in the tissue and in the superfusate collected during exposure to capsaicin. The present results show that in the human iris and ciliary body, capsaicin releases CGRP possibly contained in terminals of sensory nerves.

The autonomic innervation of the large intestine of the toad (Bufo marinus)

1. A study was made of the pelvic and the splanchnic nerve supplies to the toad large intestine. 2. Stimulation of pelvic nerve fibres in the 9th and 10th spinal nerves caused a series of contractions of the circular muscle, only the first of which was abolished by hyoscine. The entire response was blocked by d-tubocurarine. The response was not affected by capsaicin treatment. 3. Stimulation of the splanchnic nerves caused a rapid contraction followed by a prolonged relaxation. The relaxation was abolished by bretylium. The contraction was selectively antagonised by prolonged exposure to capsaicin. Splanchnic nerve stimulation also caused a slow, prolonged excitation that was abolished by bretylium. 4. Application of adrenaline caused relaxation of circularly cut strips of large intestinal wall, whereas substance P, acetylcholine, 5-hydroxytryptamine, somatostatin and galanin caused contraction. 5. The results suggest that stimulation of the pelvic nerves releases acetylcholine and a non-cholinergic co-transmitter from peripheral postganglionic neurons. Both the inhibitory response to splanchnic nerve stimulation and the subsequent slow excitation appear to be mediated by adrenergic nerves. The rapid capsaicin-sensitive excitation is likely to be due to release of substance P from antidromically activated afferent nerve fibres in the splanchnic outflow.

Tachykinins and calcitonin gene-related peptide as co-transmitters in local motor responses produced by sensory nerve activation in the guinea-pig isolated renal pelvis

Electrical field stimulation of circular muscle strips from the guinea-pig isolated renal pelvis produces a frequency-dependent positive inotropic effect of the spontaneous contractions which is unaffected by atropine and guanethidine and abolished by tetrodotoxin or in vitro capsaicin desensitization. Omega conotoxin fraction GVIA markedly inhibited the response to low frequencies of stimulation but had only a partial or minor inhibitory effect at higher frequencies. Tachykinins produce a concentration-dependent positive inotropic effect, neurokinin A being more potent than substance P. On the other hand, rat alpha calcitonin gene-related peptide (CGRP) inhibited spontaneous contractions of the renal pelvis. MEN 10,376 a neurokinin A (4-10) analog, antagonized the positive inotropism produced by neurokinin A, without affecting the response to KCl, and suppressed the positive inotropic response produced by electrical field stimulation. In the presence of MEN 10,376, a negative inotropic response was produced by electrical field stimulation which was antagonized by the C-terminal fragment (8-37) of human alpha calcitonin gene-related peptide (hCGRP). hCGRP (8-37) antagonized the negative inotropic effect of exogenously administered CGRP without affecting inhibition by isoprenaline. Application of capsaicin (10 microM) produced a marked increase in the outflow of substance P-, neurokinin A- and CGRP-like immunoreactivities from the superfused guinea-pig renal pelvis. Substance P-, neurokinin A- and CGRP-like immunoreactivities were also detected in tissue extracts of the renal pelvis by radioimmunoassay. These experiments indicate that peptide release from peripheral endings of capsaicin-sensitive primary afferents represents the major type of nerve-mediated response affecting motility of the guinea-pig isolated renal pelvis. Tachykinins and CGRP act as physiological antagonists and the excitatory action of tachykinins prevails over the inhibitory action of CGRP. Local modulation of renal pelvis motility by sensory nerves could facilitate removal of irritants present in the urine, protecting the kidney during obstruction and ureteral antiperistalsis.

The role of nitric oxide in mediating non-adrenergic non-cholinergic relaxation in longitudinal muscle of human taenia coli

Electrical field stimulation (EFS) of isolated longitudinal muscle of human taenia coli at 4Hz produced relaxation which was abolished by tetrodotoxin but not adrenergic and cholinergic blockade (NANC-relaxation). NG-nitro L-arginine (L-NOARG; 1-100 microM), an NO synthesis inhibitor, produced a concentration-dependent partial inhibition of the NANC response; 10 microM L-NOARG inhibited EFS-induced relaxation by 48.6 +/- 5.20% and 100 microM L-NOARG by 54.2 +/- 10.1%. L-Arginine (1mM), but not D-arginine (1mM) partially reversed the inhibitory effect and this was inversely proportional to the concentration of L-NOARG used. Cumulative administration of NO (acidified sodium nitrite solution; 1-100 microM) produced a concentration-dependent relaxation of the strips. L-NOARG (1 mM) did not affect either NO or isoprenaline-induced relaxations. These results provide the first preliminary evidence that NO is partially responsible for the NANC inhibitory transmission in the longitudinal muscle of the taenia coli of human colon.

NK2 tachykinin receptors and contraction of circular muscle of the human colon: characterization of the NK2 receptor subtype

The contractile effect of substance P, neurokinin A, receptor selective agonists for tachykinin receptors and NK2 tachykinin receptor antagonists was investigated in mucosa-free circular strips of the human isolated colon. Neurokinin A and substance P produced concentration-dependent contractions which approached 80-90% of the maximal response to carbachol. Neurokinin A was about 370 times more potent than substance P. The action of neurokinin A and substance P was not modified by peptidase inhibitors (bestatin, captopril and thiorphan, 1 microM each). The NK2 receptor selective agonist, [beta-Ala8]neurokinin A-(4-10) closely mimicked the response to neurokinin A while NK1 and NK3 receptor selective agonists were active only at microM concentrations. The pseudopeptide, MDL 28,564, which is one of the most selective NK2 ligands available, behaved as a full agonist. Responses to [beta-Ala8]neurokinin A were antagonized by NK2 receptor selective antagonists, with the rank order of potency MEN 10,376 greater than L 659,877 much greater than R 396. These data indicate that NK2 tachykinin receptors play a dominant role in determining the contraction of the circular muscle of the human colon to peptides of this family. The NK2 receptor subtype responsible for this effect belongs to the same subtype (NK2A) previously identified in the rabbit pulmonary artery and guinea-pig bronchi.

Ruthenium red selectively antagonizes capsaicin-induced release of vasoactive intestinal polypeptide (VIP) from the human colon

Ruthenium red (RR) is an inorganic dye that has shown to block the actions of capsaicin on primary sensory neurons in different animal models. The aim of this study was to assess whether RR is able to antagonize the release of vasoactive intestinal polypeptide (VIP) evoked by capsaicin in the human colon. Samples of descending colon were collected from patients undergoing colectomy for carcinoma of the colon. Tissue slices from the muscle of human colon were exposed to either 10 microM capsaicin or an isotonic high K+ medium (KCl 80 mM), in the absence or presence of 10 microM RR. Either capsaicin or high K+ produced a prompt release of VIP. RR (10 microM) completely antagonized the capsaicin-induced release of VIP from muscle of human colon. This effect was elective, since VIP release evoked by high K+ was unaffected by the presence of RR. These findings indicate that RR acts as a selective antagonist of capsaicin in human tissue and that the mechanism underlying peptide release by capsaicin is preserved across species. Multiple mechanisms leading to VIP release exist in the human colon.

Ruthenium red as a selective capsaicin antagonist of the motor response to capsaicin in the human isolated ileum

We have investigated the effect of ruthenium red, omega-conotoxin fraction GVIA (CTX) and tetrodotoxin (TTX) on the relaxation produced in the circular muscle of the human isolated ileum by capsaicin, electrical field stimulation (EFS) or vasoactive intestinal polypeptide (VIP). Ruthenium red (10 microM) selectively blocked the capsaicin-evoked relaxation while leaving the response to EFS or VIP unaffected. CTX had no significant effect on the various stimuli. TTX blocked the relaxation due to EFS but not that due to capsaicin or VIP. It is concluded that capsaicin excitation of primary afferents in the human ileum, leading to VIP release and muscle relaxation, occurs with mechanisms similar to those operating in animal tissues and that ruthenium red acts as a selective capsaicin antagonist in the human ileum.