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Recent studies indicated that oxytocin (OT) induced the antinociception in the spinal cord.Oxytocin might be a neuropeptide or hormone in the gastrointestinal tract that involved in the modulation the GI motility, but its effect on the visceral perception has not been identified.The purpose of this study was to investigate the effect of OT on the chemical sensitivity of afferent mesenteric nerve of the jejunum of the rat.The mesenteric nerve was prepared from a 2-cm segment of jejunum in rat in vitro and the spikes were recorded by a polygraph.Both of 5-HT(250 μM) and bradykinin (BK, 0.5 μM) induced a increase of the afferent spikes.Administration OT (10-6/10-7/10-8M) did not influence the spontaneous afferent activity and afferent sensitivity to 5-HT but significantly decreased that to BK.With the pretreatment of OT (10-6/10-7 M) for 2 min, the maximal response of the afferent spikes to BK was decreased (59 ± 14 vs.22 ± 7 spikes/s;75 ± 16 vs.28 ± 10 spikes/s;P < 0.05).The pretreatment of OT receptor antagonist atosiban, ω-conotoxin GVIA, iberiotoxin, NG-Nitro-L-arginine Methyl Ester (L-NAME), Nω-Propyl-L-arginine hydrochloride before OT (10-6/10-7 M) respectively partial reversed the maximal response of the afferent spikes to BK (P < 0.05).Oxytocin receptor (OTR) in duodenum enteric nervous system (ENS) was located by immunofluorescence staining.Because BK excites the endings of the spinal afferent fibers which encode the noxious stimulation in the GI tract, we believed that OT might downregulate the visceral nociception through releasing NO.