An invited review following the soujinkai award : Diversity of excitatory and inhibitory synapses onto CA1 pyramidal neurons : A possible mechanism of contextual learning

The hippocampus is processing temporal and spatial information in particular context. To examine the encoding rule creating contextual memory, 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 provide evidence that CA1 neurons transmit essential data of experienced context. 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 GABA_A receptor-mediated inhibitory synapses on CA1 neurons. Since the extent of synaptic strengthens are different in each CA1 neuron, each CA1 neuron expressed wide diversity of excitatory and inhibitory synaptic inputs after IA training. Here I propose a new hypothesis that the diversity of synaptic inputs on CA1 neurons depicts cell-specific outputs processing experienced episodes.

AMPA receptors

GABA_A receptors

synaptic plasticity

acetylcholine

learning and memory