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 remains 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 conditions, which can be easily combined with 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. We show that when placed across the pyramidal layer, such insertions can harness ripple oscillations from approximately 130 channels, corresponding to about [~]1.3 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 putative functionally connected neuronal pairs. Although the conventional orthogonal/vertical approach is optimal for recording the laminar organization of hippocampal activity, we demonstrate how, for the longitudinal axis of hippocampus, tangential insertions can substantially increase spatial reach and sampling resolution, enabling a more detailed description of activity along this axis.