Distributed encoding of hippocampal information in mossy cells

In neural information processing, the nervous system transmits neuronal activity across 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. In vivo/vitro patch-clamp recordings revealed that MCs were reliably depolarized in response to sharp-wave ripples (SWRs), synchronous neuronal events transmitted from the CA3 region to the dentate gyrus via the MC layer. Machine-learning algorithms predicted the waveforms of SWRs in the CA3 region, based on the MC depolarization waveforms, suggesting that CA3 neural information is indeed transmitted to the MC layer. However, the prediction accuracy varied; i.e., a particular MC showed a more robust association with a particular SWR cluster, and the SWR cluster associated with one MC rarely overlapped with the SWR clusters associated with other MCs. Thus, CA3 network activity is distributed across MC ensembles with pseudo-orthogonal neural representations, allowing the small MC layer to effectively compress hippocampal information.