Modulation of N-methyl-D-aspartate (NMDA) receptor-mediated ion currents by serotonin was investigated with a two-electrode voltage clamp technique in Xenopus oocytes injected with rat brain RNA. After a 1-min application of 200 nM serotonin a transient potentiation of the NMDA receptor-mediated ion currents was observed. The serotonin-induced enhancement was mimicked by the protein kinase C activators 1-oleoyl-2-acetyl-sn-glycerol (100 microM) and phorbol 12-myristate 13-acetate (10 nM), whereas the inactive phorbol ester 4-alpha-phorbol 12-myristate 13-acetate (10 nM) had no effect. From these observations it was concluded that protein kinase C was involved in the enhancement of NMDA-induced currents. In agreement with this conclusion, it was found that the serotonin effect was inhibited by the protein kinase C inhibitors sphingosine (1 microM) or staurosporine (1 microM) added 20 min before NMDA application and by oocyte injection of protein kinase C (PKC)-inhibitor peptide (500 ng/oocyte) 1 hr prior to recordings. The serotonin receptor involved was identified as a 5-HT2 receptor subtype by the finding that 200 nM of the selective 5-HT2 receptor agonist alpha-methyl-5-hydroxytryptamine mimicked the potentiation of NMDA-induced ion currents by serotonin. Furthermore, the observed potentiation was significantly reduced by co-application of serotonin with 100 microM of the selective 5-HT2 receptor antagonist ketanserin. These results indicate that 5-HT2 receptors enhance NMDA receptor function via phosphoinositol hydrolysis and subsequent stimulation of PKC.