Implanting an Electrode into the Subthalamic Nucleus of a Rat

Prepare an anesthetized rat, secured on a stereotaxic unit.

Make a midline incision.  

Remove the periosteum, exposing the skull sutures.  

Mark the right subthalamic nucleus, or STN coordinates for electrode implantation.

Drill a hole and retract the membrane.

Drill additional holes and fix bone screws.

Align the monopolar electrode with the STN coordinates and gently insert it into the region.

Connect the electrode to the recording system. Ground the stereotaxic unit and cover the rat with aluminum foil to prevent electromagnetic interference.

Advance the electrode, monitoring neural activity, to target the STN identified by its irregular firing pattern.

Secure the implants with dental cement.

Disconnect the recording system.

Attach the stimulator connector to the electrode and ground it. Secure the assembly with dental cement.

Close the incision.

Connect the stimulator plug to the swivel-fixed wire for STN stimulation.

Return the rat to its cage and allow recovery.

Use a scalpel to make a midline incision beginning between the ears and extending forward for about 2 centimeters. Ensure that the periosteum is also incised, and then expose the skull with four clamps. Use a cotton bud to gently remove the periosteum until the coronal and sagittal sutures are exposed. Stanch any blood with cotton wool. Fix a needle onto a probe holder, and then mark the tip of the needle with a black felt-tip pen.

Using the anterior-posterior, midline lateral, and dorsoventral drive screws, position the tip of the needle directly over bregma. Take the AP and ML vernier scale readings. Subtract 3.6 millimeters from the AP reading and 2.5 millimeters from the ML reading for electrode implantation into the right STN.

Lower the tip of the needle to the surface of the skull to mark the injection site with the dye from the felt-tip pen. Clamp the dental drill onto the large probe holder of the stereotaxic instrument. Move the dental drill to the marked point on the skull, and while looking through the microscope, drill a hole of about 1-millimeter diameter through the skull until the dura is visible. This is the hole for the electrode.

Retract the dura using microdissection forceps or a sterile needle as the dura is tough enough to destroy the tip of the electrode. Drill a hole with the dental drill in each frontal squama, and in the parietal squama opposite to the hole for the electrode. Thereafter, drill a hole in each interparietal squama. Screw a bone screw into each of the five holes. Avoid threading the screws in too deep, which will put pressure on the brain. The number of turns will depend on the pitch of the screw. Two to three turns of each screw is used here.

Disconnect the probe holder from the stereotaxic instrument and clamp the probe holder with the electrode in the micromanipulator. Using the AP, ML, and DV drive screws, move the probe holder with the electrode until the tip is almost touching bregma. Note the AP, ML, and DV vernier scale readings at bregma. Then, raise the electrode a few millimeters to prevent the electrode from scraping the skull during movement.

To determine the coordinates of the position where the electrode has to be inserted into the hole, add 3.6 millimeters to the AP reading, and add 2.5 millimeters to the ML reading. Using the AP and ML drive screws, move the electrode to the calculated position. At this point, the electrode tip should be situated directly over the drilled electrode hole.

Next, while looking through the microscope, lower the electrode to the level of the dura. Note this as 0 in the DV direction. Then, gently insert the tip of the electrode into the brain while looking through the microscope. Connect the electrode pin to the connector of the recording system. Ground the stereotaxic instrument with the counterpoise of the room.

Then, place a Faraday cage over the rat in the stereotaxic instrument. In place of a Faraday cage, you can also use aluminum foil as shown here since its effect is the same as that of a Faraday cage. Start the recording system while recording the electric activity during the advancement of the electrode. The specific electrical activity of the STN is usually detectable at a depth of between 7.5 and 8.1 millimeters from the dura.

The typical activity of neurons in the STN is characterized by an irregular firing pattern and a high firing rate. Now, swab away any blood or cerebrospinal fluid that was displaced at the surface of the skull when lowering the electrode. Then, mix up a small amount of dental cement, and then use a small spatula to apply it around the electrode and around four of the five screws.

When the dental cement has hardened, disconnect the electrode pin from the electrode holder and connector of the recording system. Then, unscrew the fifth screw that was not fixed by dental cement. Put the plug on the electrode pin and fix the ground wire of the plug with the fourth screw. Apply dental cement around the plug. As the cement thickens, mold it around the plug to form a cap. Avoid sharp edges of the dental cement that may harm the animal and remove them during hardening.

Debride the wound edges and close them with a suture behind the cap and at the front. Disinfect the wound edges. Connect the head plug to the wire that is fixed on a swivel. Then, remove the rat from the stereotaxic instrument. Administer appropriate postoperative analgesia. 12.5 milligrams per kilogram tramadol is used here. Place the rat in a clean cage with thermal support, and fix the swivel on this cage. Observe the recovery of the animal.