Bidirectional Regulation of Protein Kinase Mζ in the Maintenance of Long-Term Potentiation and Long-Term Depression

Long-term potentiation (LTP) and long-term depression (LTD) are persistent modifications of synaptic efficacy that may contribute to information storage in the CA1 region of the hippocampus. Persistently enhanced phosphorylation has been implicated in the maintenance phase of LTP. This hypothesis is supported by our previous observation that protein kinase Mζ (PKMζ), the constitutively active catalytic fragment of a single protein kinase C isoform (PKCζ), increases in LTP maintenance. In contrast, dephosphorylation may be important in LTD maintenance, because phosphatase inhibitors reverse established LTD, in addition to blocking its induction. Because phosphorylation is determined by a balance of phosphatases and kinases, both increases in phosphatase activity and decreases in kinase activity could contribute to LTD. We now report that the reduction of protein kinase activity by H7, as well as selective inhibition of PKC by chelerythrine, mimics and occludes the maintenance phase of homosynaptic LTD in rat hippocampal slices. Conversely, saturated LTD occludes the synaptic depression caused by chelerythrine. Biochemical analysis demonstrates a decrease of PKMζ, as well as PKCs γ and e, in LTD maintenance and a concomitant loss of constitutive PKC activity. LTD and the downregulation of PKMζ are prevented by NMDA receptor antagonists and Ca 2+ -dependent protease inhibitors. Both LTD and the downregulation of PKMζ are reversible by high-frequency afferent stimulation. Our findings indicate that the molecular mechanisms of LTP and LTD maintenance are inversely related through the bidirectional regulation of PKC.