We have reported that arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), act in synergy to increase release of glutamate from synaptosomes prepared from rat dentate gyrus. The observation that prior induction of LTP in perforant path-granule cell synapses occluded this synergism suggested that the interaction between arachidonic acid and ACPD might trigger the increase in glutamate release that accompanies LTP in dentate gyrus. Our objective was to identify the mechanism underlying the synergism between arachidonic acid and ACPD in LTP. The data indicate that both agents activate phospholipase C(PLC); the arachidonic acid-induced increase in phospholipase C activation was inhibited by the tyrosine kinase inhibitor, genistein, suggesting that PLCgamma, which is stimulated by tyrosine phosphorylation may be activated by arachidonic acid. The ACPD-induced increase was inhibited by neomycin, indicating the involvement of a G-protein and suggesting that PLCbeta may be activated by ACPD. We report that arachidonic acid stimulated phosphorylation of the specific tyrosine kinase substrate, poly(Glu80,Tyr20) and direct analysis indicated that arachidonic acid increased phosphorylation of PLCgamma. PLCgamma phosphorylation was assessed in control dentate gyrus and dentate gyrus in which LTP was induced in vivo. We report that the tyrosine kinase inhibitor, genistein, blocked expression of LTP and also blocked the associated increase in phosphorylation of PLCgamma. The data presented here indicate that tyrosine phosphorylation of PLCgamma was significantly enhanced following induction of LTP, but in separate experiments, in which LTP was inhibited by intraventricular injection of genistein, phosphorylation of PLCgamma was inhibited. The evidence presented is consistent with the hypothesis that PLC acts as a coincidence detector in LTP. The data indicate that PLCbeta is activated by ACPD, PLCgamma is activated by arachidonic acid, and coincident activation of both isoforms is necessary to stimulate an increase in glutamate release.