Modulation of non-adrenergic non-cholinergic inhibitory transmission in rat duodenum: role of opiates and 5-hydroxytryptamine

: In rat duodenal segments in vitro, electrical field stimulation induced a TTX-sensitive relaxation in the presence of atropine and guanethidine. A correlation between the amplitude of the evoked response and stimulus frequency was observed. Opioid peptides DAGO, DPDPE and DYN caused a dose-dependent increase in the amplitude of the response to EFS. Naloxone shifted to the right the dose-response curves for each opioid peptide significantly enhancing the ED50 values. The amplitude of the response to EFS was enhanced, dose-dependently, also in the presence of 5-HT. Such an effect induced by 5-HT was prevented by 5-HT receptor desensitization, but persisted unchanged after naloxone pretreatment. Opioids failed to affect the response to EFS after 5-HT receptor desensitization. Results suggest that in rat duodenum opioids modulate NANC inhibitory neurotransmission, indirectly the release of 5-HT.