Chamber Implant for Chronic Optical Recordings from the Cerebral Cortex of Marmosets

We describe a novel imaging chamber for chronic optical recordings from the marmoset cerebral cortex, together with the surgery needed for its implantation and that of an associated headpost. MRI data allow optimizing positioning on the skull. The chamber is implanted into a precisely matched craniotomy, improving mechanical stability. For maximal biocompatibility, chamber and headpost are made out of titanium. It consists of an outer cylinder into which screws adjustably an inner one, called “well”, with a glass window on its bottom. This allows easy opening and closing whenever access to the cortex is needed, while preserving a sterile seal when closed. Moreover, simply rotating the well allows fine adjustment of the window’s distance with respect to the underlying cortex. Together with using a curved - rather than flat - glass window, this allows achieving gentle but continuous contact with the underlying cortex, which helps to delay tissue regrowth, notably of a neomembrane that must otherwise be surgically removed. The chamber’s sealing system combines a silicone elastomer and O-ring, minimizing infection risk and CSF leakage. Data from longitudinal two-photon imaging using genetically encoded fluorescent calcium sensors reveal high optical quality over months, with excellent resolution of neurons and their activity. We also demonstrate the feasibility of two-photon imaging of genetically encoded voltage sensors in the marmoset cortex in-vivo. Finally, although the chamber provides a durable, adaptable solution for long-term imaging studies in marmosets already in its current version, we discuss a few straightforward modifications that could likely improve its performance even further.