Distributed subthreshold representation of sharp wave-ripples by hilar mossy cells

In neural information processing, the nervous system transmits neuronal activity between layers of neural circuits, occasionally passing through small layers composed only of sparse neurons. Hippocampal hilar mossy cells (MCs) constitute such a typical bottleneck layer. However, how efficient information encoding is achieved within such constrained layers remains poorly understood. To address this, we focused on sharp wave-ripples (SWRs) - synchronous neural events originating in the CA3 region - and investigated functional diversity within MC populations using in vivo/in vitro patch-clamp recordings in mice. By combining machine learning algorithms, we developed a model to predict CA3 SWR waveforms based on the synaptic response waveforms of MCs, suggesting that SWR-related information is indeed encoded in their subthreshold activity. While individual MCs were generally associated with specific SWR clusters, partial overlap across some MCs was also observed, indicating that CA3 activity is distributed across the MC population. Our findings suggest that CA3 SWR activity is represented in a pseudo-orthogonal manner across MC populations, allowing the small MC layer to effectively compress and relay hippocampal information.