Activation of the epsilon isoform of protein kinase C in the mammalian nerve terminal

Activation of protein kinase C (PKC) by phorbol ester facilitates hormonal secretion and transmitter release, and phorbol ester-induced synaptic potentiation (PESP) is a model for presynaptic facilitation. A variety of PKC isoforms are expressed in the central nervous system, but the isoform involved in the PESP has not been identified. To address this question, we have applied immunocytochemical and electrophysiological techniques to the calyx of Held synapse in the nrl nucleus of the trapezoid body (MNTB) of rat auditory brainstem. Western blot analysis indicated that both the Ca2+-dependent conventional PKC and Ca2+-independent novel PKC isoforms are expressed in the MNTB. Denervation of afferent fibers followed by organotypic culture, however, selectively decreased novel epsilon PKC isoform expressed in this region. The afferent calyx terminal was clearly labeled with the epsilon PKC immunofluorescence. On stimulation with phorbol ester, presynaptic epsilon PKC underwent autophosphorylation and unidirectional translocation toward the synaptic side. Chelating presynaptic Ca2+, by using membrane permeable EGTA analogue or high concentration of EGTA directly loaded into calyceal terminals, had only a minor attenuating effect on the PESP. We conclude that the Ca2+-independente epsilon PKC isoform mediates the PESP at this mammalian central nervous system synapse.