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Current Research in Neuroscience
Activation of Hilar Mossy Cells and Dentate Granule Cells During Sharp Wave/Ripples
Ayako Ouchi, Nobuyoshi Matsumoto and Yuji Ikegaya

Background and Objective: Sharp waves (SWs)/ripples are high-frequency oscillations emitted by the hippocampus mainly during slow-wave sleep or quiet rest states and contribute to offline memory consolidation. They are thought to be initiated in the CA3 subregion and propagate to the downstream regions, including the CA1 area, the subiculum and the entorhinal cortex. At the same time, they also propagate from the CA3 subregion back to the dentate gyrus. However, neither the role of the CA3-to-DG backpropagation nor its circuit mechanism has been fully understood. Hilar mossy cells receive direct synaptic inputs from CA3 pyramidal cells and make synaptic connections with granule cells in the dentate gyrus. Methodology: The synaptic inputs to mossy cells and granule cells in response to SWs were investigated by using whole-cell recordings and extracellular recordings from acute brain slices that spontaneously emit SWs, therefore, mossy cells or granule cells were patch-clamped, while recording the local field potentials from the CA3 stratum pyramidale. Results: Results of this study showed that the mossy cells exhibited excitatory synaptic responses to SWs earlier than granule cells. The time lags of EPSPs relative to the SW onsets were 7.3±3.9 msec in mossy cells and 15.7±7.8 msec in granule cells (SDs of 10 and 3 cells, respectively, p = 0.023, student’s t-test). Furthermore, the ratio of the SW responsive cells was significantly higher in the mossy cells than the granule cells (mossy cell: 10/17 cells, granule cell: 3/15 cells, p = 0.036, Fisher’s exact test). Conclusion: It can be concluded that the synaptic responses of mossy cells to SWs were likely stochastic but more reliable than those of granule cells. Our findings shed light on a novel mechanism underlying the SW backpropagation and provide a new perspective about memory consolidation.
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