Acute effects of capsaicin on gastrointestinal vagal afferents

Capsaicin is an important tool for investigation of thin afferent fibres, b ut its acute effects on subtypes of vagal afferent endings are unknown. In the gastrointestinal tract, these subtypes are: muscle endings (thought to be purely tension sensitive), mucosal endings (sensitive to stroking and ch emical stimuli) and endings in the oesophagus with both properties. Acute c apsaicin sensitivity was investigated in ferrets using in vivo and in vitro methods. Single-fibre activity was recorded from 63 vagal afferents: 12 A delta-fibres, 15 C-fibres and 36 unclassified fibres with endings in the oe sophagus (n = 42), stomach (n = 19) and duodenum (il = 2). Responses to cap saicin occurred independently of motility changes and were therefore due to direct activation of the receptor ending. In the oesophagus in vivo, two o f 10 tension receptors and one of one mucosal receptor responded to intralu minal application of 3.25 mM capsaicin. In the stomach and duodenum, five o f 14 tension receptors and two of four mucosal receptors responded to close -systemic (32-164 nmol) capsaicin. In an in vitro gastro-oesophageal prepar ation, three of five tension, four of 21 mucosal and two of eight tension/m ucosal receptors responded to topical application of 1 mM capsaicin. Occurr ence of responses was therefore unrelated to location of endings and isolat ion of tissue. Responsiveness was also unrelated to conduction velocity. Ca psaicin caused desensitization of responses to further capsaicin applicatio n in 37% of afferents. It additionally caused cross-desensitization to mech anical stimuli, which was also seen in efferents that did not respond direc tly to capsaicin. In conclusion, capsaicin acutely activates all subtypes of gut vagal affere nts in vivo and in vitro, although responsiveness is restricted to 30% of f ibres and follows no specific pattern. Acute desensitization may be induced with or without a response. (C) 2000 IBRO. Published by Elsevier Science L td.