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Contextual learning rapidly promotes synaptic plasticity at CA1 synapses
Yuya Sakimoto(Dai Mitsushima) Yamaguchi University Graduate School of Medicine

Although contextual learning requires plasticity at both excitatory and inhibitory synapses in CA1 neurons, the temporal dynamics across the neuronal population are poorly understood. Using an inhibitory avoidance task, we analyzed the dynamic changes in learning-induced excitatory/inhibitory synaptic plasticity. The training strengthened GABAA  receptor–mediated synapses within 1 min, peaked at 10 min, and lasted for over 60 min. The intracellular loop (Ser408−409) of GABAA  receptor β3  subunit was also phosphorylated within 1 min of training. As the results of strengthening of AMPA receptor–mediated synapses, CA1 pyramidal neurons exhibited broad diversity of excitatory/inhibitory synaptic currents within 5 min. Moreover, presynaptic glutamate release probability at basal dendrites also increased within 5 min. To further quantify the diversified excitatory/inhibitory synaptic currents, we calculated self-entropy (bit) for individual neurons. The neurons showed individual levels of the parameter, which rapidly increased within 1 min of training and maintained for over 60 min. These results suggest that learning-induced synaptic plasticity is critical immediately following encoding rather than during the retrieval phase of the learning. Understanding the temporal dynamics along with the quantification of synaptic diversity would be necessary to identify a failure point for learning-promoted plasticity in cognitive disorders

Temporal dynamics of learning-promoted synaptic diversity in CA1 pyramidal neurons.
Sakimoto Y, Kida H, Mitsushima D. FASEB Journal, 33: 14382-14393, 2019.

<Figure Legends>
Schematic image of CA1 pyramidal neurons. IA training rapidly strengthened GABAA receptor-mediated inhibitory synapses within 1 min, while the training strengthened AMPA receptor-mediated excitatory synapses within 5 min. CA1 pyramidal neurons exhibited broad diversity of excitatory/inhibitory synaptic currents within 5 min, and the neuron-specific synaptic diversity was sustained for more than 60 min.

Department of Physiology, Graduate School of Medicine, Yamaguchi University, Japan