Experience-specific plasticity at excitatory and inhibitory synapses onto granule cells in the dentate gyrus

The hippocampal dentate gyrus has been identified to play a critical role in maintaining contextual memory in many mammalian species. To evaluate learninginduced synaptic plasticity of granule cells, we subjected male rats to an inhibitory avoidance (IA) task and prepared acute hippocampal slices. In the presence of 0.5 μM tetrodotoxin, we recorded miniature excitatory post synaptic currents (mEPSCs) and inhibitory post synaptic currents (mIPSCs) in male rats experiencing four groups: untrained, IA-trained, unpaired, and walk-through. Compared with the untrained, IA-trained, unpaired, and walk-through groups, the unpaired group significantly enhanced mean mEPSC amplitudes, suggesting the experience-induced plasticity at AMPA receptor-mediated excitatory synapses. For inhibitory synapses, both unpaired and walk-through groups significantly decreased mean mIPSC amplitudes, showing the experience-induced reduction of postsynaptic GABAA receptor-mediated currents. Unlike the plasticity at CA1 synapses, it was difficult to explain the learning- specific plasticity at the synapses. However, overall multivariate analysis using four variables of mE(I)PSC responses revealed experience-specific changes in the diversity, suggesting that the diversity of excitatory/inhibitory synapses onto granule cells differs among the past experience of animals include the learning. In comparison with CA1 pyramidal neurons, granule cells consistently showed greater amplitude and frequency of mE(I)PSCs. Fluctuation analysis further revealed that granule cells provide more postsynaptic AMPA receptor channels and greater single-channel current of GABAA receptors of than CA1 pyramidal neurons. These findings show functional differences between two types of principal cells in the hippocampus.