NK1- and NK3-receptor mediated inhibition of 5-hydroxytryptamine release from the vascularly perfused small intestine of the guinea-pig

The effects of tachykinins on the spontaneous release of 5-hydroxytryptamine (5-HT) from the enterochromaffin cells into the portal circulation was investigated in vitro using the vascularly perfused isolated guinea-pig small intestine. 5-HT was determined by HPLC with electrochemical detection. Test substances were applied intraarterially. Substance P (SP) caused a concentration-dependent decrease in 5-HT outflow with an EC50 of 50pmol/l. Similarly, the selective NK1 receptor agonist SP methyl ester (1nmol/l) significantly inhibited 5-HT outflow (to 51±3%). When tetrodotoxin (1μmol/l) was added to the arterial perfusion medium, the inhibition by SP of 5-HT outflow was not affected. The selective NK1 receptor antagonist CP 99994 [(+)-(2S,3S)-3-(2-methoxyben-zylamino)-2-phenylpiperidine] (0.1μmol/l) prevented the inhibitory effect of SP (0.1μmol/l). Neither GR 94800 (PhCO-Ala-Ala-DTrp-Phe-DPro-Pro-NleNH2) (0.1μmol/l) nor SR 142801 [(S)-(N)-(1-(3-(1-benzoyl-3-(3, 4-dichlorophenyl)piperidin-3-yl)propyl)-4-phenyl-piperidin-4-yl)-N-methylacetamide] (10nmol/l), which are selective NK2 and NK3 receptor antagonists, changed the SP-mediated inhibition. The selektive NK3 receptor agonist senktide (10nmol/l) also decreased the 5-HT outflow (to 57±5%). This inhibition was prevented by SR 142801 (10nmol/l) and by tetrodotoxin. CP 99994 (0.1μmol/l) significantly antagonized the senktide-mediated inhibition of 5-HT outflow. The outflow of 5-HT was unaffected when CP 99994, GR 94800 or SR 142801 alone were added to the perfusion medium. It is concluded that the release of 5-HT from enterochromaffin cells is directly inhibited by NK1 receptors, and indirectly by neuronal NK3 receptors whose stimulation leads to the release of SP.