Opioid-receptor-mediated excitation of rat mesenteric afferent fibres supplying the rat jejunum

The aim of the present study was to examine the sensitivity to opioid-recep tor agonists of mesenteric afferents supplying the small intestine and to c haracterize the subpopulations of any responsive fibres. Mesenteric afferen t discharge was recorded electrophysiologically in response to cumulative d oses (1-400 mug kg(-1)) of the mu -receptor agonist [D-ala,(2) N- me-Phe(4) ,Gly(5)-ol]-enkephalin (DAMGO), the delta -receptor agonist [D-ala,(2) D-le u(5)]-enkephalin (DADLE) and the kappa -receptor agonist U-50488. DAMGO and DADLE, but not U-50488, markedly stimulated whole nerve mesenteric afferen t discharge (P < 0.05) that was unrelated to intestinal motor events. Subpo pulations of afferent fibres responding to DAMGO were examined using wavefo rm analysis to identify single units from within the whole mesenteric nerve bundles. One population was CCK-sensitive (15/15 fibres) and the other was a subpopulation of mechanosensitive afferents that responded to distension (17/28). 5-HT-sensitive afferents did not respond to DAMGO (0/11). We conc lude that specific subpopulations of mesenteric afferents respond to mu- an d possibly delta- but not kappa -receptor agonists. This sensitivity to opi oids may contribute to the antinociceptive property of vagal afferents.