Alzheimer's disease amyloid consists of amyloid beta-peptides (Abeta) derived from the larger precursor amyloid precursor protein (APP). Non-amyloidogenic APP processing involves regulated cleavage within the Abeta domain followed by secretion of the ectodomain (APPs). APPs secretion can be stimulated by muscarinic acetylcholine receptors coupled to phospholipases and kinases. To determine whether other receptor classes can regulate APP processing, we examined the relation between serotonin receptors and APPs secretion. Serotonin increased APPs release 3-4-fold in 3T3 cells stably overexpressing 5-HT2aR or 5-HT2cR. The increase was dose-dependent and was blocked by serotoninergic antagonists. Phorbol esters also increased APPs secretion, but neither kinase inhibitors nor down-regulation of PKC blocked the serotonin-induced increase in APPs secretion. Thus PKC is not necessary to stimulate APPs secretion. Phospholipase A2 (PLA2) inhibitors blocked the 5-HT2aR-mediated increase in APPs secretion, suggesting a role of PLA2 in coupling 5-HT2aR to APP processing. In contrast, coupling of 5-HT2cR to APPs secretion involved both PKC and PLA2. Serotonin also stimulated the release of the APLP2 ectodomain, suggesting that additional members of the APP multigene family are processed via similar regulated pathways. Inasmuch as generation of APPs precludes the formation of amyloidogenic derivatives, serotonin receptors provide a novel pharmacological target to reduce these derivatives in Alzheimer's disease.