Hippocampal tangential insertions of high-density silicone probes in head-fixed mice enhance spatial sampling

The hippocampus is a key structure within the medial temporal lobe which plays a central role in the formation and consolidation of declarative memories. Despite technological progress, a full understanding of hippocampal rhythms, neuronal population activity, and the underlying connectivity remain elusive, partly due to its complex morphology in a croissant-like shape which limits coronal and sagittal approaches. To address this limitation, we propose two different tangential insertions of high-density electrode probes (Neuropixels 1.0) aligned to either the dorsal or ventral sections of the hippocampus. Each approach enables the placement of up to 384 recording sites within the hippocampal formation. Both insertions are performed in head-fixed condition, which further allows for detailed monitoring of the animals behavioral states, including spontaneous running and facial movements. The two proposed tangential dorsal and ventral insertions capture almost continuously the physiology of the hippocampus across 3.84 mm of tissue. When placed across the pyramidal layer, such insertions can harness ripple oscillations over 120 channels, corresponding to approximately [~]1.2 mm of tissue, with a coverage of 2 channels every 20 {micro}m. Furthermore, such insertions enabled us to record up to 400 hippocampal neurons simultaneously from a single probe, endowing us with the capability to capture within a single recording several hundreds of functionally connected neuronal pairs. Lastly, these insertions were performed in head-fixed mice on a running band, which can be combined with high frequency sampling camera, allowing us to precisely monitor facial movements, and their hippocampal correlates. An orthogonal/vertical approach is a more standard way to record electrophysiological activity from most brain regions, including the hippocampus. Vertical insertions thereby miss the antero-posterior axis of the brain in the hippocampus and therefore putative propagating events. Furthermore, vertical insertions have a lower reach in terms of number of channels, single units, and spatial density being recorded - even with multi-shank high-density probes. Tangential insertions therefore substantially increase spatial reach and resolution, which leads to a higher number of cells recorded in a single insertion along the antero-posterior axis of the hippocampus. Furthermore, compared to freely moving approaches, head-fixed conditions enable precise monitoring of facial movements. Dorsal and ventral head-fixed tangential insertions of Neuropixels probes in the hippocampus increase the resolution and spatial extent of recordings, allowing more precise and extensive monitoring of network activity and its underlying connectivity.