Altered prejunctional modulation of intestinal cholinergic and noradrenergic pathways by α2-adrenoceptors in the presence of experimental colitis

This study investigates the influence of intestinal inflammation on: (1) the control of intestinal neurotransmission and motility by prejunctional α2-adrenoceptors and (2) the expression of intestinal α2-adrenoceptors. Experimental colitis was induced by intrarectal administration of 2,4-dinitrobenzenesulphonic acid (DNBS) to rats. UK-14,304 inhibited atropine-sensitive electrically evoked contractions of ileal and colonic longitudinal muscle preparations. UK-14,304 acted with similar potency, but higher efficacy, on tissues from DNBS-treated animals; its effects were antagonized with greater potency by phentolamine than rauwolscine. Electrically induced [3H]noradrenaline release from ileal preparations was reduced in the presence of colitis. Tritium outflow was decreased by UK-14,304 and stimulated by rauwolscine or phentolamine: these effects were enhanced in preparations from animals with colitis. Reverse transcription–polymerase chain reaction and Western blot assay demonstrated the protein expression of α2A-adrenoceptors in mucosal and muscular tissues isolated from ileum and colon. The induction of colitis increased α2A-adrenoceptor expression in both ileal and colonic muscular layers, without concomitant changes in mucosal tissues. Induction of colitis reduced gastrointestinal propulsion of a charcoal suspension in vivo. In this setting, the gastrointestinal transit was inhibited by intraperitoneal (i.p.) UK-14,304 and stimulated by i.p. rauwolscine. After pretreatment with guanethidine, the stimulant action of rauwolscine no longer occurred, and UK-14,304 exerted a more prominent inhibitory effect that was antagonized by rauwolscine. The present results indicate that, in the presence of intestinal inflammation, prejunctional α2-adrenoceptors contribute to an enhanced inhibitory control of cholinergic and noradrenergic transmission both at inflamed and noninflamed distant sites. Evidence was obtained that such modulatory actions depend on an increased expression of α2A-adrenoceptors within the enteric nervous system. Keywords: α2-Adrenoceptors, acetylcholine, noradrenaline, enteric nervous system, intestinal motility, intestinal inflammation Introduction The gastrointestinal tract is extensively innervated by extrinsic noradrenergic neurons, whose projections end in close proximity to several cellular targets (i.e. epithelial cells, neurons, blood vessels). Locally, through their interaction with α2-adrenoceptors, endogenous catecholamines modulate a variety of digestive functions, including secretions and motility (Del Tacca et al., 1982; De Ponti et al., 1996; Liu & Coupar, 1997), gastro-colonic sensation (Malcom et al., 2000), and epithelial cell proliferation (Schaak et al., 2000). Both sympathetic and cholinergic axon terminals of the enteric nervous system are equipped with prejunctional α2-adrenoceptors, the activation of which inhibits noradrenaline and acetylcholine release, respectively (Blandizzi et al., 1993; Funk et al., 1995; Colucci et al., 1998). Inflammatory diseases of the human gut (e.g. inflammatory bowel disease, coeliac disease, infectious gastroenteritis) are commonly associated with alterations of enteric functions, including increased secretion and motor abnormalities, which are thought to contribute to the generation of digestive symptoms (Collins, 1996, Giaroni et al., 1999; Sharkey & Kroese, 2001). For this reason, there is currently great interest in understanding the pathophysiological changes occurring in the enteric nervous system as a consequence of intestinal inflammation (Sharkey & Kroese, 2001). Such alterations occur throughout the gut, at both inflamed and noninflamed sites, and they depend, at least in part, on mutual interactions between enteric nerves and the digestive immune system (Collins, 1996). The role played by adrenoceptors in intestinal inflammation remains poorly understood. Experimental colitis in rats was associated with downregulation of β3-adrenoceptors in colonic circular smooth muscle (Zhao et al., 2001). An increased density of α2-adrenergic binding sites was detected in cell membrane preparations obtained from jejunal tissues of guinea-pigs with small bowel inflammation (Martinolle et al., 1993). However, data on the possible influence exerted by prejunctional α2-adrenoceptors on enteric neurotransmission and digestive motility in the presence of intestinal inflammation are lacking. Accordingly, the present study was designed to assess the influence of experimentally induced intestinal inflammation on: (1) the control of enteric neurotransmission and intestinal motility by prejunctional α2-adrenoceptors and (2) the expression pattern of intestinal α2-adrenoceptors.