We're interested in understanding how the brain operates at a network level. This method is our attempt to explore the development of brain networks to identify developmental alterations and age-dependent diseases like autism, schizophrenia, or bipolar disorder. The silicon probe technology provides a simpler, more consistent method to record network activity in vivo, but despite this, chronic tetrode recordings do offer some advantages over silicon probes, such as simultaneously recording over a broader spatial distribution of brain regions.
Chronic recordings pose a unique challenge to in vivo electrophysiology due to several factors, including gliosis at the recording sites, movement of the recording site over time, or failure of the attachment method. Our recent work demonstrated that the sweeps encoded by the theta oscillation of the hippocampus during active movement of the animal every 100 milliseconds or so iteratively cycle forward, prospectively evaluating possible future states, and backwards, retrospectively evaluating prior actions. Performing these in vivo recordings in juvenile mice poses several engineering challenges due to the small size of the mice, their relative weakness, and the lack of development in their skull.
Our methodology overcomes these limitations and allows us to chronically record network-level activity daily in the developing mouse brain. Besides recording chronically in juvenile mice, our method allows us to record from up to 16 distinct bilateral brain regions, irrespective of the spatial relationship of those regions. These developments will allow us to identify how networks establish functional communication across developments, both in the healthy brain and in mouse models, of neurodevelopmental disorders like autism spectrum disorders.
After digitally designing and printing the micro-drive, attach two screw attachments to each tetrode advancing screw, one above and one below the ridge. Hold the screw attachments together with gel cyanoacrylate. Ensure that the screw attachments do not move in the longitudinal axis of the screw, but can rotate freely with minimal resistance.
Extend each polyamide section through the output holes on the micro-drive cannula by a few millimeters. Secure the polyamide to the cannula using a clean 30 gauge needle in small amounts of liquid cyanoacrylate, being careful not to let it enter the inside of the polyamide tube. Retrieve the large polyamide tubes from the top of the micro-drive cannula and pass them through the large polyamide holes in the micro-drive body.
Gently push the micro-drive cannula and micro-drive body together until they are adjacent and the cannula body attachment tabs interlock. Secure the micro-drive body and the micro-drive cannula with cyanoacrylate. Using a new sharp razor blade, severe the large polyamide tube ends extruding from the bottom of the cannula output holes, ensuring the cut is precisely at the cannula's base, making the tubes and cannula bottom flush.
With sharp scissors, cut the large polyamide tubing just above the edge of the inner rim of the drive body at a 45 degree angle. Cut small polyamide tubing into four centimeter long sections. Pass the small polyamide sections through the large polyamide tubing already mounted in the micro-drive and ensure that excess small polyamide tubing protrudes from the top and bottom.
Secure the small polyamide tubes to the screw attachments with cyanoacrylate. Avoid letting any cyanoacrylate enter the large or small polyamide tubes. Using a new sharp razor blade, sever the small polyamide tube ends extruding from the bottom of the cannula holes.
Ensure the cut is at the cannula's base and clean with nothing blocking the polyamide tube hole. Then use sharp scissors to cut the top of the small polyamide a few millimeters above the top of the screw attachment at a 45 degree angle, ensuring the cut is clean. Using ceramic or rubber tipped forceps, carefully pass a tetrode through one of the small polyamide tubes, leaving approximately two centimeters protruding from the top of the small polyamide tube.
Secure the tetrode to the top of the small polyamide tube with liquid cyanoacrylate. Retract the screw until it is near the top of the drive. Grab the tetrode wire protruding from the bottom of the drive and gently kink it at the point where it emerges from the cannula.
Fully advanced the screw into the drive. Using sharp scissors, cut the tetrode wire just above the kink, then inspect it under a microscope to ensure the cut is clean and the metal of all four tetrodes is exposed. Finally, connect each electrode of each tetrode to the appropriate port on the electronic interface board.
Tightly wind one bone screw with a thin, highly conductive wire that will be ground and attached to the electronic interface board. After preparing the mouse for surgery and exposing the skull, place the bone screws in the extreme lateral, rostral, or caudal portions of the skull where the bone is thickest and the bone screws are sufficiently far away from the micro-drive implant. Use a scalpel blade or drill bit to score the skull near the bone screw hole locations, providing a rough surface for the liquid cyanoacrylate to bind.
Then thread each bone screw into place, careful not to pierce the underlying dura. Using a sterile 30 gauge needle, apply liquid cyanoacrylate around each bone screw, thickening the skull where the bone screws have been attached. Ensure that no cyanoacrylate enters the exposed dura above the recording sites.
To begin, set up the counterbalance system by connecting a 0.75 inch diameter PVC pipe. Insert one arm of the system through the holes in the cage lid. Place the second arm on top of the cage lid and extend the third arm above and beyond the cage.
Finish by capping the topmost arm. After implanting the micro-drive on the mouse skull, attach the micro-drive to the counterbalance system using a counterbalance weight equal to the weight of the micro-drive and bone screws. Secure a strong thread or fishing line from the electronic interface board to the counterbalance weight, which hangs over the topmost arm.
Finally, ensure the counterbalance is firmly connected to the micro-drive electronic interface board and that the mouse has full access to the cage.
