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Voltage-gated sodium channel NaV 1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of NaV 1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor-specific NaV 1.7 knockout mouse (NaV 1.7(Nav1.8) ) and selective small-molecule NaV 1.7 antagonist PF-5198007. NaV 1.7(Nav1.8) mice showed normal nociceptive behaviours to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both NaV 1.7(Nav1.8) and littermate controls. Loss, or blockade, of NaV 1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve-gut preparations in mouse, or following antagonism of NaV 1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage-gated sodium channel α subunits revealed NaV 1.7 mRNA transcripts in nearly all retrogradely-labelled colonic neurons suggesting redundancy in function. By contrast, using comparative somatic behavioral models we identify that genetic deletion of NaV 1.7 (in NaV 1.8-expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective NaV 1.7 antagonist PF-5198007. Our data demonstrates that NaV 1.7 (in NaV 1.8-expressing neurons) contributes to defined pain pathways in a modality-dependent manner, modulating somatic noxious heat pain but is not required for visceral pain processing, and advocates that pharmacological block of NaV 1.7 alone in the viscera may be insufficient in targeting chronic visceral pain. This article is protected by copyright. All rights reserved.

Background: 5-HT2B receptors are localized within the myenteric nervous system, but their functions on motor/sensory neurons are unclear. To explore the role of these receptors, we further characterized the 5-HT2B receptor antagonist RS-127445 and studied its effects on peristalsis and defecation. Experimental approach: Although reported as a selective 5-HT2B receptor antagonist, any interactions of RS-127445 with 5-HT4 receptors are unknown; this was examined using the recombinant receptor and Biomolecular Interaction Detection technology. Mouse isolated colon was mounted in tissue baths for isometric recording of neuronal contractions evoked by electrical field stimulation (EFS), or under an intraluminal pressure gradient to induce peristalsis; the effects of RS-127445 on EFS-induced and on peristaltic contractions were measured. Faecal output of rats in grid-bottom cages was measured over 3 h following i.p. RS-127445 and separately, validation of the effective doses was achieved by determining the free, unbound fraction of RS-127445 in blood and brain. Key results: RS-127445 (up to 1 µmol·L−1) did not interact with the 5-HT4 receptor. RS-127445 (0.001–1 µmol·L−1) did not affect EFS-induced contractions of the colon, although at 10 µmol·L−1 the contractions were reduced (to 36 ± 8% of control, n= 4). RS-127445 (0.1–10 µmol·L−1) concentration-dependently reduced peristaltic frequency (n= 4). RS-127445 (1–30 mg·kg−1), dose-dependently reduced faecal output, reaching significance at 10 and 30 mg·kg−1 (n= 6–11). In blood and brain, >98% of RS-127445 was protein-bound. Conclusions and implications: High-protein binding of RS-127445 indicates that relatively high doses are required for efficacy. The results suggest that 5-HT2B receptors tonically regulate colonic motility.

BACKGROUND: Acute physical stress causes alteration in gut autonomic function and visceral hypersensitivity in patients with irritable bowel syndrome (IBS). We have developed a model to measure this stress response. AIM: To assess whether treatment with a drug effective in treating IBS (amitriptyline) alters the response to acute stress in IBS patients. METHODS: Nineteen patients with IBS were given amitriptyline 25-50 mg. Patients underwent physical stress (cold pressor) test at baseline and after 3 months of treatment. Physiological parameters measured were: stress perception; systemic autonomic tone [heart rate (HR) and blood pressure (BP)]; gut specific autonomic innervation [rectal mucosal blood flow (RMBF)] and visceral sensitivity (rectal electrosensitivity). RESULTS: Fourteen of 19 (74%) patients improved symptomatically after 3 months of amitriptyline. Acute stress induced increased perception of stress and systemic autonomic tone and reduced RMBF in symptomatic responders and nonresponders (P > 0.05 for all). All nonresponders but only 3 of 14 responders continued to exhibit stress-induced reduced pain threshold at 3 months (change from baseline -31% vs. +2%, P < 0.03 respectively). CONCLUSION: In this open study, amitriptyline appears to decrease stress-induced electrical hypersensitivity; this effect is independent of autonomic tone. The gut response to acute stress deserves further study as a model to study drug efficacy in IBS.

Ghrelin has been shown to decrease firing of gastric vagal afferents at doses comparable with cir- culating levels in the fasted state. This raises the possibility that ghrelin may have a hormonal action on other vagal afferent populations. The aim of this study was to determine the effects of ghrelin on jej- unal afferent activity; including responses to disten- sion, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT) and cholecystokinin (CCK) in both naive and vago- tomized rats. Ghrelin significantly augmented the afferent response to distension. No effect was observed on baseline afferent discharge, or the response to 2-methyl-5-HT and CCK. The effect of ghrelin was more pronounced at lower ramp distending pressures (0-30 mmHg). Similarly, ghrelin augmented the jej- unal afferent responses to phasic distension at 10- 30 mmHg, but had no effect at higher pressures. Chronic subdiaphragmatic vagotomy and adminis- tration of the growth hormone secretagogue receptor (GHS-R) antagonist (D-Lys3)-GHRP-6 prevented the augmentation of the afferent responses to distension indicating ghrelin is acting through the GHS-R on vagal afferent fibres. Ghrelin augments the mechano- sensation of jejunal vagal afferents and hence may lead to increased perception of hunger contractions.

The aim of this study was to investigate the contribution of the TRPV1 receptor to jejunal afferent sensitivity in the murine intestine. Multiunit activity was recorded in vitro from mesenteric afferents supplying segments of mouse jejunum taken from wild-type (WT) and TRPV1 knockout (TRPV1−/−) animals. In WT preparations, ramp distension of the gut (up to 60 mmHg) produced biphasic changes in afferent activity so the pressure–response curve had an initial rapid increase in afferent discharge followed by a second phase of slower increase in activity. Afferent response to distension was significantly lower in TRPV1−/− than in WT mice. Single-unit analysis revealed three functional types of afferent fibres: (1) low-threshold fibres (2) wide dynamic range fibres and (3) high-threshold fibres. There was a marked downward shift of the pressure–response curve for wide dynamic range fibres in the TRPV1−/− mice as compared to the WT controls. The afferent response to intraluminal hydrochloric acid (20 mm) was also attenuated in the TRPV1−/− mice. In contrast, the response to bath application of bradykinin (1 μm, 3 ml) was not significantly different between the two groups. The TRPV1 antagonist capsazepine (10 μm) significantly attenuated the nerve responses to distension, intraluminal acid and bradykinin, as well as the spontaneous discharge in WT mice. The WT jejunal afferents responded to capsaicin with rapid increases in afferent activity, whereas TRPV1−/− afferents were not at all sensitive to capsaicin. Previous evidence indicates that TRPV1 is not mechanosensitive, so the results of the present study suggest that activation of TRPV1 may sensitize small intestinal afferent neurones.

Background GSK962040, a small molecule motilin receptor agonist, was identified to address the need for a safe, efficacious gastric prokinetic agent. However, as laboratory rodents lack a functional motilin system, studies in vivo have been limited to a single dose, which increased defecation in rabbits. Motilin agonists do not usually increase human colonic motility, so gastric prokinetic activity needs to be demonstrated. Methods The effect of intravenous GSK962040 on gastro-duodenal motility was assessed in fasted dogs implanted with strain gauges. Activity was correlated with blood plasma concentrations of GSK962040 (measured by HPLC-MS/MS) and potency of GSK962040 at the dog recombinant receptor [using a Fluorometric Imaging Plate Reader (Molecular Devices, Wokingham, UK) after expression in HEK293 cells]. Key Results GSK962040 activated the dog motilin receptor (pEC50 5.79; intrinsic activity 0.72, compared with [Nle13]-motilin). In vivo, GSK962040 induced phasic contractions, the duration of which was dose-related (48 and 173 min for 3 and 6 mg kg−1), driven by mean plasma concentrations >1.14 μmol L−1. After the effects of GSK962040 faded, migrating motor complex (MMC) activity returned. Migrating motor complex restoration was unaffected by 3 mg kg−1 GSK962040 but at 6 mg kg−1, MMCs returned 253 min after dosing, compared with 101 min after saline (n = 5 each). Conclusions & Inferences The results are consistent with lower potency for agonists at the dog motilin receptor, compared with humans. They also define the doses of GSK962040 which stimulate gastric motility. Correlation of in vivo and in vitro data in the same species, together with plasma concentrations, guides further studies and translation to other species.