The maintenance of long-term potentiation (LTP) in the CA1 region of the hippocampus has been reported to require both a persistent increase in phosphorylation and the synthesis of new proteins. The increased activity of protein kinase C (PKC) during the maintenance phase of LTP may result from the formation of PKMzeta, the constitutively active fragment of a specific PKC isozyme. To define the relationship among PKMzeta, long-term EPSP responses, and the requirement for new protein synthesis, we examined the regulation of PKMzeta after sub-threshold stimulation that produced short-term potentiation (STP) and after suprathreshold stimulation by single and multiple tetanic trains that produced LTP. We found that, although no persistent increase in PKMzeta followed STP, the degree of long-term EPSP potentiation was linearly correlated with the increase of PKMzeta. The increase was first observed 10 min after a tetanus that induced LTP and lasted for at least 2 hr, in parallel with the persistence of EPSP enhancement. Both the maintenance of LTP and the long-term increase in PKMzeta++ were blocked by the protein synthesis inhibitors anisomycin and cycloheximide. These results suggest that PKMzeta is a component of a protein synthesis-dependent mechanism for persistent phosphorylation in LTP.

We isolated cDNA clones from an Aplysia sensory-cell library encoding two isoforms of protein kinase C (PKC). Several isozyme-specific regions are conserved in the Aplysia kinases, notably the variable regions V5 in the Ca(2+)-dependent PKC (Apl I) and V1 in the Ca(2+)- independent PKC (Apl II). Neuronal proteins with the properties expected of these two isoforms can be identified with antibodies raised against peptides synthesized from the amino acid sequences deduced from the clones. Sacktor and Schwartz (1990) measured the proportion of kinase activity that can be translocated to membrane in Aplysia sensory neurons and ganglia by stimuli that produce the presynaptic facilitation underlying behavioral sensitization. Much less Apl I and Apl II are translocated, suggesting that still other isoforms of PKC exist in these cells.

1Activity-dependent modifications of synaptic efficacies are a cellular substrate of 2 learning and memory. Experimental evidence shows that these modifications are 3 synapse-specific and that the long-lasting effects are associated with the sustained 4 increase in concentration of specific proteins like PKMζ. However, such proteins are 5 likely to diffuse away from their initial synaptic location and spread out to neighbor-6 ing synapses, potentially compromising synapse-specificity. In this paper we address 7 the issue of synapse-specificity during memory maintenance. Assuming that the 8 long-term maintenance of synaptic plasticity is accomplished by a molecular switch 9 we perform simulations using the reaction-diffusion package in NEURON and analyt-10 ical calculations to determine the limits of synaptic-specificity during maintenance. 11Moreover, we explore the effects of the diffusion and degradation rates of proteins 12 and of the geometrical characteristics of dendritic spines on synapse specificty. We 13 conclude that the necessary conditions for synaptic specificity during maintenance 14 require that protein synthesis occurs in dendritic spines and that the activated den-15 dritic spines exhibit small neck diameters. 16Overwhelming experimental evidence indicates that activity-dependent modification 18 of synaptic efficacies is the cellular substrate of learning and memory (Morris et 19 al.