Differential coupling of cytokine receptors to sensory neurons in health and post inflammation

Abstract

INTRODUCTION: In post infectious or diarrhoea predominant Irritable Bowel Syndrome (IBS) there are increased levels of proinflammatory cytokines released from immune cells (1). These products induce mechanical hypersensitivity of healthy colonic afferents (2). We showed in a mouse model of chronic post inflammatory visceral hypersensitivity (PIVH) that colonic afferents are mechanically hypersensitive (3). AIMS: Determine if proinflammatory cytokines exacerbate or ameliorate colonic mechanical hypersensitivity. Determine mechanisms of any sensitizing or direct action. METHODS: Afferent mechano- and chemosensitivity was assessed in mouse colo-rectum In Vitro. High-threshold nociceptive afferents were characterised by responses to mechanical stimuli (3). Cytokine receptor abundance was assessed using PCR of Dorsal Root Ganglia (DRG) (L6-S1) and specifically in laser-captured colonic DRG neurons. Healthy and PIVH (TNBS 130g/mL with 28 day recovery) mice were compared (3). RESULTS: In healthy mice TNF-(100ng/mL) induced potent mechanical hypersensitivity (P<0.0001) that was blocked by ruthenium red and selective TRPA1 antagonism (HC-030031), but not by selective antagonism of TRPV1 (capsazepine). TNF-had no direct chemosensory effects. A switch in effect occurred in PIVH whereby TNF-inhibited mechanical hypersensitivity. This inhibition was blocked by iberiotoxin, a selective potassium channel (BKCa) antagonist. TNF1 or 2 receptor mRNA abundance in DRG was unaltered. In healthy mice IL-1(20ng/mL) induced modest mechanical hypersensitivity but caused a direct chemosensory effect in 4/7 afferent recordings, that was inhibited by tetrodotoxin (TTX). Similar responses occurred in PIVH, and IL-1R1 mRNA abundance was unaltered. IL-2 (100ng/mL) had no effect in healthy mice (n=7), but induced a profound TTX-sensitive chemosensory effect in PIVH (7/10 afferents), that was greater than observed for IL-1, and a significant (P<0.05) increase in IL-2 receptor mRNA. Data are shown for pelvic serosal afferents, but were broadly similar in splanchnic serosal afferents. DISCUSSION: IL-1and IL- 2 induce direct chemosensory effects in colonic afferents via sodium channels without altering mechanosensory function. In contrast, TNF-exclusively induced mechanical hypersensitivity, predominately via TRPA1. In PIVH, IL-1effects are unchanged, IL-2 effects are increased, and TNF-effects are in the opposite direction whereby TNF-now causes inhibition via BKCa channels. Therefore cytokines interact with visceral afferents via different mechanisms, which undergo markedly different changes in PIVH. 1: Leibregts Gastro 2007. 2: Hughes Gut 2009. 3: Hughes Gut 2009.