Ginap_1998_Neurochem.Int_32_487

The effects of nitric oxide (NO) on the spontaneous release of 5-hydroxytryptamine (5-HT) were studied in the in vitro vascularly perfused guinea-pig small intestine. The NO donor SIN-1 concentration-dependently decreased 5-HT release with an EC50 of 1.34 microM, whereas the NO synthase inhibitor N(G)-nitro-L-arginine (100 microM) was without effect. The inhibition by SIN-1 of 5-HT release was enhanced by superoxide dismutase (150 U/ml) and antagonized by the selective inhibitor of soluble guanylyl cyclase, ODQ (1 microM). Tetrodotoxin (1 microM) prevented the inhibition by SIN-1 of 5-HT release, which suggests that the effect of SIN-1 is indirectly mediated via release of an inhibitory neurotransmitter. Substance P could be excluded as inhibitory transmitter because the effect of SIN-1 remained unchanged in the presence of the NK1 receptor antagonist CP 99994 (100 nM). The cyclic GMP analogue, 8-bromo cyclic GMP (300 microM), also decreased basal release of 5-HT, but this decrease was not tetrodotoxin-sensitive. It is concluded that NO inhibits the release of 5-HT from enterochromaffin cells via release of an enteric neurotransmitter. Acetylcholine (via nicotinic receptors) and substance P (via NK1 receptors) are not involved in the NO-mediated inhibition. The inhibition of 5-HT outflow by NO is due to the activation of soluble guanylyl cyclase. 8-Bromo cyclic GMP inhibited 5-HT release by a direct effect on the enterochromaffin cells.