Calcium/calmodulin-dependent protein kinase II (CaMKII) is a leading candidate for a synaptic memory molecule because it is persis-tently activated after long-term potentiation (LTP) induction and because mutations that block this persistent activity prevent LTP and learning. Previous work showed that synaptic stimulation causes a rapidly reversible translocation of CaMKII to the synaptic region. We have now measured green fluorescent protein (GFP)–CaMKII␣ translocation into synaptic spines during NMDA receptor-dependent chemical LTP (cLTP) and find that under these conditions, translocation is persistent. Using red fluorescent protein as a cell morphology marker, we found that there are two components of the persistent accumulation. cLTP produces a persistent increase in spine volume, and some of the increase in GFP–CaMKII␣ is secondary to this volume change. In addition, cLTP results in a dramatic increase in the bound fraction of GFP–CaMKII␣ in spines. To further study the bound pool, immunogold electron microscopy was used to measure CaMKII␣ in the postsynaptic density (PSD), an important regulator of synaptic function. cLTP produced a persistent increase in the PSD-associated pool of CaMKII␣. These results are consistent with the hypothesis that CaMKII␣ accumulation at synapses is a memory trace of past synaptic activity.
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