TRANSDUCTION OF A PROTEIN-KINASE C-GENERATED SIGNAL INTO THE LONG-LASTING FACILITATION OF GLUTAMATE RELEASE

The present study investigated the delayed and persistent effects of 4 beta-phorbol 12,13-dibutyrate (PDBu) on the K+-evoked release of endog enous glutamate and dynorphin B-like immunoreactivity from a subcellul ar fraction (P3) that is enriched in hippocampal mossy fiber synaptoso mes. It is demonstrated that the alpha, beta, gamma, epsilon, and zeta isoforms of protein kinase C (PKC) are present in the P3 fraction obt ained using the guinea pig hippocampus as starting tissue. The K+-evok ed release of glutamate was found to be selectively enhanced when moss y fiber-enriched synaptosomes were preincubated with PDBu for 15 minut es and extensively washed with a PDBu-free medium. The persistent enha ncement of glutamate release observed under this condition was not rev ersed by the protein kinase inhibitor staurosporine and was desensitiz ed to the potentiating effects of an acute reexposure to PDBu. The ove rall content and activity of PKC was not substantially altered during the initial 15 minutes of treatment with PDBu (10 muM). More prolonged pretreatments with PDBu altered the substrate specificity of PKC and decreased the content of all PKC isoforms, but did not reverse the fac ilitation of glutamate release that followed preincubation in the pres ence of PDBu. It is concluded that the persistent activation of PKC en hances K +-evoked glutamate release from hippocampal mossy fiber-enric hed synaptosomes and that, once established, this presynaptic facilita tion is sustained by a process that is no longer directly dependent on continued PKC phosphotransferase activity.