Long-term Transverse Imaging of the Hippocampus with Glass Microperiscopes

The hippocampus consists of a stereotyped neuronal circuit repeated along
the septal-temporal axis. This transverse circuit contains distinct
subfields with stereotyped connectivity that support crucial cognitive
processes, including episodic and spatial memory. However, comprehensive
measurements across the transverse hippocampal circuit in vivo are
intractable with existing techniques. Here, we developed an approach for
two-photon imaging of the transverse hippocampal plane in awake mice via
implanted glass microperiscopes, allowing optical access to the major
hippocampal subfields and to the dendritic arbor of pyramidal neurons.
Using this approach, we tracked dendritic morphological dynamics on CA1
apical dendrites and characterized spine turnover. We then used calcium
imaging to quantify the prevalence of place and speed cells across
subfields. Finally, we measured the anatomical distribution of spatial
information, finding a non-uniform distribution of spatial selectivity
along the DG-to-CA1 axis. This approach extends the existing toolbox for
structural and functional measurements of hippocampal circuitry.