Serotonin 5-hydroxytryptamine2A receptor-coupled phospholipase C and phospholipase A2 signaling pathways have different receptor reserves

NIH3T3 cells stably expressing the rat 5-hydroxytryptamine(2A) (5-HT2A) receptor (5500 fmol/mg) were used to explore further the capacity of structurally distinct ligands to elicit differential signaling through the phospholipase C (PLC) or phospholipase A(2) (PLA(2)) signal transduction pathways. Initial experiments were designed to verify that 5-HT2A receptor-mediated PLA(2) activation in NIH3T3 cells is independent from, and not a subsequent result of, 5-HT2A receptor-mediated PLC activation. In addition, we also explored the extent of receptor reserve for the endogenous ligand, 5-HT, for both PLC and PLA(2) activation. Finally, we employed structurally diverse ligands from the tryptamine, phenethylamine, and ergoline families of 5-HT2A receptor agonists to test the hypothesis of agonist-directed trafficking of 5-HT2A receptor-mediated PLC and PLA(2) activation. To measure agonist-induced pathway activation, we determined the potency and intrinsic activity of each compound to activate either the PLA(2) pathway or the PLC pathway. The results showed that a larger receptor reserve exists for 5-HT-induced PLA(2) activation than for 5-HT-induced PLC activation. Furthermore, the data support the hypothesis of agonist-directed trafficking in NIH3T3-5HT(2A) cells because structurally distinct ligands were able to induce preferential activation of the PLC or PLA(2) signaling pathway. From these data we conclude that structurally distinct ligands can differentially regulate 5-HT2A receptor signal transduction.