Hippocampal Cortical Imaging Window Implantation

Cortical excavation and imaging window implantation were performed >10 days after the initial virus injection, according to published protocols^{10 (link),48 (link)}. A craniotomy (diameter 3 mm) was made centered at A/P −1.5 mm and M/L −1.5 mm for CA1/DG imaging and A/P −1.5 mm, M/L −2.5 mm for CA2/3 recordings. For implantations over CA2/3, the head of the mouse was tilted by 20°, so that the implantation plane was parallel to the lateral part of the pyramidal cell layer. Parts of the somatosensory cortex as well as posterior parietal association cortex were gently aspirated while irrigating with chilled saline. We continued aspiration until the external capsule was exposed. The outer part of the external capsule was then gently peeled away using fine forceps leaving the inner capsule and the hippocampus itself undamaged. The imaging window implant consisted of a 3 mm diameter coverslip (CS-3R, Warner Instruments) glued to the bottom of a stainless-steel cannula (3 mm diameter 1.2–1.5 mm height). The window was gradually lowered into the craniotomy using a forceps until the glass was in contact with the external capsule. The implant was then affixed to the skull using cyanoacrylate. Mice were allowed to recover from window implantation for 2-3 days.