Neurogastroenterology of tegaserod (HTF 919) in the submucosal division of the guinea-pig and human enteric nervous system X. FANG,* 1 S. LIU,* X-Y. WANG,* N. GAO,* H-Z. HU,* G-D. WANG,* C. H. COOK,  B. J. NEEDLEMAN,  D. J. MIKAMI,   Y. XIA,*, à G-J. FEI,* G. A. HICKS§ & J. D. WOOD* *Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, USA  Department of Surgery, College of Medicine, The Ohio State University, Columbus, OH, USA àDepartment of Anesthesiology, College of Medicine, The Ohio State University, Columbus, OH, USA §Novartis Pharmaceuticals Corp., East Hanover, NJ, USA Abstract Actions of the 5-HT 4 serotonergic receptor partial agonist, tegaserod, were investigated on mucosal secretion in the guinea-pig and human small intestine and on electrophysiological behaviour of secretomotor neurons in the guinea-pig small intesti- nal submucosal plexus. Expression of 5-HT 4 receptor protein and immunohistochemical localization of the 5-HT 4 receptor in the submucosal plexus in relation to expression and localization of choline acetyltransf- erase and the vesicular acetylcholine (ACh) transpor- ter were determined for the enteric nervous system of human and guinea-pig small intestine. Immunoreac- tivity for the 5-HT 4 receptor was expressed as ring-like fluorescence surrounding the perimeter of the neur- onal cell bodies and co-localized with the vesicular ACh transporter. Exposure of mucosal/submucosal preparations to tegaserod in Ussing chambers evoked increases in mucosal secretion reflected by stimula- tion of short-circuit current. Stimulation of secretion had a relative high EC 50 of 28.1 ± 1.3 lmol L )1 , was resistant to neural blockade and appeared to be a direct action on the secretory epithelium. Tegaserod acted at presynaptic 5-HT 4 receptors to facilitate the release of ACh at nicotinic synapses on secretomotor neurons in the submucosal plexus. The 5-HT 2B recep- tor subtype was not involved in actions at nicotinic synapses or stimulation of secretion. Keywords 5-HT 4 serotonergic receptors, enteric nervous system, intestine, secretion. INTRODUCTION Tegaserod is recognized as a partial agonist at the serotonergic 5-HT 4 receptor subtype as reflected by competition with [ 3 H]5-HT and highly selective 5-HT 4 antagonists (e.g. [ 3 H]GR113808) in receptor binding assays. It binds with essentially equal potency to the human 5-HT 4(a) and 5-HT 4(b) splice variants. 1 Tegas- erod binding to the receptor triggers a clustering of the human 5-HT 4(b) splice variant expressed in HEK293 cells and this terminates with internalization of the bound receptor. 2 Tegaserod action at 5-HT 4 receptors is suggested to account for its gastrointestinal prokinetic properties in animals and humans and its therapeutic efficacy in the improvement of the global symptoms of the constipation-predominant form of the irritable bowel syndrome (IBS) and chronic constipation. 3–7 Application of 5-HT to intestinal mucosal prepara- tions in Ussing chambers evokes chloride secretory responses that are mediated by actions at receptors on secretomotor neurons and directly on the epithe- lium. 8,9 In the guinea-pig small intestine, the involved receptors appear to be the 5-HT 3 and 5-HT 4 receptor subtypes. 10,11 Serotonergic receptors, which stimulate secretion, are expressed by both enteric secretomotor neurons and the mucosal epithelium. In human ileum and jejunum, a 5-HT 4 receptor mediates secretory responses to 5-HT, which persist after neural blockade Address for correspondence Jackie D. Wood PhD, Department of Physiology & Cell Biology, The Ohio State University, 304 Hamilton Hall, 1645 Neil Avenue, Columbus, OH 43210-1218, USA. Tel: +1 614 292 5449; fax: +1 614 292 4888; e-mail: wood.13@osu.edu 1 Present address: Division of Gastroenterology, Peking Union Medical College Hospital, Beijing, China Received: 15 March 2006 Accepted for publication: 19 June 2007 Neurogastroenterol Motil (2008) 20, 80–93 doi: 10.1111/j.1365-2982.2007.00983.x Ó 2007 The Authors Journal compilation Ó 2007 Blackwell Publishing Ltd 80