Biochemical characterization of phosphoinositide hydrolysis stimulated by 5-HT3 receptor agonists.

Serotonin (5-HT) stimulates phosphoinositide (PI) turnover in rat fronto-cingulate cortical slices and is probably mediated through the activation of both 5-HT2 and 5-HT3 receptors. We have extended these findings and have assessed whether the increased stimulation of PI turnover is secondary to 5-HT stimulated arachidonate metabolism or to the release of another neurotransmitter. Incubation of the cortical slices by the two 5-HT3 receptor agonists, 2-methyl-serotonin (2-Me-5-HT) and phenylbiguanide (PBG), significantly decreases serotonin-stimulated phosphoinositide turnover, indicating that activation of 5-HT3, receptors by 2-Me-5-HT and PBG caused the desensitized PI hydrolysis to 5-HT. Indomethacin did not affect the increased PI hydrolysis induced by 5-HT, 2-Me-5-HT and PBG, suggesting that neither cyclooxygenase nor lipoxgenase activity is required for the PI response and that it is independent of arachidonic acid metabolism. The stimulation in PI turnover induced by 5-HT and the 5-HT3 receptor agonists was not potentiated by proteinase inhibitors suggesting that the release of a peptide neurotransmitter is not involved in the PI response. In addition, the effects of 5-HT, 2-Me-5-HT and PBG on PI turnover are additive to the effect of KCl and veratrine. In conclusion, our results indicate that the action of 2-Me-5-HT and PBG on PI turnover is direct.