The dendritic layer-specific persistent enhancement of synaptic transmission induced by repetitive activation of...

Synaptic plasticity, the cellular basis of brain memory, is established through at least two phases: short-term and long-term plasticity. It is assumed that the short-term plasticity instantaneously provoked in pre-existing synapses, as represented by a long-term potentiation (LTP) in the mammalian hippocampus, is converted to the long-term plasticity that develops slowly accompanying the formation of new synapses. However, this conversion has scarcely been analyzed primarily because of the lack of the model system. Recently, we found that a repeated activation of protein kinase A (PKA), but not a single activation of PKA, led to a slowly-developing long-lasting enhancement of synaptic strength coupled with synaptogenesis in cultured rat hippocampus and proposed that this phenomenon would serve as the required model system. In the present study, we investigated the geographical aspect of this phenomenon using a high-speed voltage-sensitive dye (VSD) imaging methodology. Before doing this, we had to overcome the difficulties in applying this methodology to the quantitative analysis on the cultured hippocampal slices. Those difficulties are multiple types of signal decay and a large variance in the number of cells among specimens. After resolving these problems we found that the enhancement of synaptic efficacy in the CA1 stratum radiatum occurred predominantly in the proximal dendritic layer.