A presynaptic role for protein kinase C in hippocampal mossy fiber synaptic transmission

It has been suggested that the maintenance of long-term potentiation (LTP) in the hippocampal mossy fiber (MF) synapse involves a presynaptic mechanism that does not require the activation of protein kinase C (PKC), since this enzyme appears to be absent in the MF presynaptic terminals. In the present study the authors evaluated this proposal by directly comparing the metabolic properties of hippocampal MF synaptosomes and a conventional P2B synaptosomal preparation prepared from the same hippocampal tissue. Protein kinase C-dependent histone phosphotranferase activity was found to be comparable in MF and P2B synaptosomes. Western blot analysis was performed using antisera prepared against four of the PKC isoforms, and the results demonstrate that the alpha, beta, and gamma PKC isoforms are present in relatively equivalent amounts in these two subcellular fractions. However, the cytosolic fraction derived from the hippocampal MF synaptosomes appeared to contain a greater amount of the PKC-epsilon isoform when compared to the P2B synaptosomal preparation. Four distinct endogenous substrates present in the MF synaptosomes are shown to be phosphorylated in response to PKC activation. A functional role for PKC in the hippocampal MF nerve endings seems to be indicated by the finding that 4 beta-phorbol 12,13-dibutyrate (PDBu) and 4 beta-phorbol 12,13-diacetate produce a dose-dependent potentiation of the K(+)-evoked release of endogenous glutamate and dynorphin B, while the inactive 4-alpha-phorbol was without effect. The PDBu-induced enhancement of transmitter release was blocked by the PKC inhibitor, staurosporine. In addition, PDBu significantly facilitated the rise in cytosolic free calcium that immediately followed depolarization of the MF synaptosomal membrane. It is concluded that hippocampal MF presynaptic terminals possess a variety of PKC isoforms and that their activation may have an important facilitory influence on MF synaptic transmission and plasticity.