Persistent enhancement of sustained calcium-dependent glutamate release by phorbol esters: role of calmodulin-independent serine/threonine phosphorylation and actin disassembly.

The phorbol ester 4 beta-phorbol 12,13-dibutyrate increases the final extent of Ca(2+)-dependent glutamate release during the continuous depolarization of the synaptosomal plasma membrane. Based on this finding, we suggested that the sustained activation of protein kinase C has a positive influence on the efficiency of synaptic vesicle recycling in the presence of saturating concentrations of Ca2+. Previous work from our laboratory demonstrated that this 4 beta-phorbol 12,13-dibutyrate-dependent enhancement of synaptic vesicle recycling persists following the removal of 4 beta-phorbol 12,13-dibutyrate, requires localized Ca2+ entry through voltage-regulated channels, and is insensitive to the protein kinase inhibitor staurosporine. In the present study, we examined the possibility that the facilitation of glutamate release may be propagated through interactions between the protein kinase C- and multifunctional Ca2+/calmodulin-dependent protein kinase pathways. However, our data argue strongly against the involvement of such a mechanism in the persistent enhancement of sustained glutamate release. We observed that 4 beta-phorbol 12,13-dibutyrate did not increase the availability of cytosolic free calmodulin or the level of autonomous Ca2+/calmodulin-dependent protein kinase activity. In addition, we determined the effects of various serine/threonine kinase and phosphatase inhibitors on the phorbol ester-dependent enhancement of sustained glutamate release and found that protein kinase C increased the extent, but not the duration, of Ca(2+)-dependent glutamate release through a kinase-independent mechanism. Given our finding that the actin-depolymerizing agent cytochalasin D totally occluded the eb1ect of 4 beta-phorbol 12,13-dibutyrate on release, we postulate that protein kinase C signals may be transduced through direct interactions between protein kinase C isoforms and cytoskeletal protein kinase C binding proteins.