CA2-INDUCED PERSISTENT PROTEIN-KINASE-C ACTIVATION IN RAT HIPPOCAMPALHOMOGENATES()

Protein kinase C (PKC) is thought to play an important role in neurona l function by mediating changes in synaptic strength. Specifically, it has been argued that persistent PKC activation underlies the maintena nce of long-term potentiation (LTP) of synaptic transmission in the hi ppocampus, a model widely used to study mammalian learning and memory. Because the induction of LTP is known to be dependent upon Ca2+ influ x into the postsynaptic neuron, we investigated Ca2+-dependent mechani sms that operate to elicit persistent PKC activation in the hippocampu s. Hippocampal homogenates were incubated with Ca2+ for a brief period and subsequently assayed for persistent changes in basal (Ca2+-indepe ndent) PKC activity, using the selective PKC substrate neurogranin((28 -43)) (NG((28-43))). After Ca2+ incubation, basal PKC phosphorylation of NG((28-43)) was increased and expression of the increased activity could be inhibited by PKC(19-36), a selective peptide inhibitor of PKC . These data indicate the presence of a persistently activated form of PKC in Ca2+-pretreated hippocampal homogenates. The persistently acti vated PKC was localized to the soluble fraction of homogenates. Genera tion of the soluble, persistently activated form of PKC was blocked by the calpain inhibitor, leupeptin, suggesting a proteolytic activation of PKC. Column chromatography and Western blots indicated the presenc e of PKM, a proteolytic fragment of PKC that is active in the absence of calcium, diacylglycerols, or phospholipid cofactors. Thus, Ca2+ ind uces proteolytic activation of PKC in hippocampal homogenates. This su ggests that proteolytic activation is a plausible candidate as a mecha nism underlying the persistent activation of PKC associated with LTP.