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Contextual Learning Requires Functional Diversity at Excitatory and Inhibitory Synapses onto CA1 Pyramidal Neurons

Department of Systems Neuroscience, Yamaguchi University Graduate School of Medicine, Minami-kogushi Ube, Yamaguchi 755-8505, JAPAN

Special Issues: What hypotheses can be supported as alternatives (or parallels) to synaptic plasticity as substrates for learning and consolidation of memory in the brain?

Although the hippocampus is processing temporal and spatial information in particular context, the encoding rule creating memory is completely unknown. To examine the mechanism, we trained rats on an inhibitory avoidance (IA) task, a hippocampus-dependent rapid one-trial contextual learning paradigm. By combining Herpes virus-mediated in vivo gene delivery with in vitro patch-clamp recordings, I reported contextual learning drives GluR1-containing AMPA receptors into CA3-CA1 synapses. The molecular event is required for contextual memory, since bilateral expression of delivery blocker in CA1 successfully blocked IA learning. Moreover, I found a logarithmic correlation between the number of delivery blocking cells and learning performance. Considering that one all-or-none device can process 1-bit of data per clock (Nobert Wiener 1961), the logarithmic correlation may provides evidence that CA1 neurons transmit essential data of contextual information. Further, I recently reported critical role of acetylcholine as an intrinsic trigger of learning-dependent synaptic plasticity. IA training induced ACh release in CA1 that strengthened not only AMPA receptor-mediated excitatory synapses, but also GABAA receptor-mediated inhibitory synapses on each CA1 neuron. More importantly, IA-trained rats showed individually different excitatory and inhibitory synaptic inputs with wide variation on each CA1 neuron. Here I propose a new hypothesis that the diversity of synaptic inputs on CA1 neurons may depict cell-specific outputs processing experienced episodes after training.
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