The overall goal of this experimental procedure is to record long-term electroencephalograms with stereotaxic epidural, and intracerebral electrode placement in different brain regions of freely moving rodents using non-restraining EEG radiotelemetry. This method can have answered key questions in the systemic integrative and translation in neuro-science field, such as dysrhythmia and neurological and neuropsychiatric diseases, as well as epilepsy. The main advantage of this technique is that it allows for epidural and deep intracerebral EEG recordings from freely moving rodents that serve as animal models of human diseases.
Julien Soos, a graduate student from our laboratory will be demonstrating the technique. To prepare the epidural electrode, pull out of the silicone isolation, the tip of the transmitter lead with the forceps so it will be bent to a 90 degree angle. Shorten the tip of the lead.
To prepare the deep electrode expose the stainless steel helix of the transmitter lead by pulling it out of the silicone isolation. Mechanically insert the deep electrode into the lead of the transmitter. Rewire the transmitter lead to the deep brain electrode to ensure a stable connection.
Then bend the electrode to a 90 degree angle and shorten it to the required length. To begin this procedure, shave the head of a fully anesthetized mouse. Then disinfect the shaved area using 70%ethanol and an iodin based scrub.
Place the animal in a prone position on a heating blanket to maintain it's body temperature during anesthesia. Using a scalpel, make a mid-line incision on the scalp from the forehead to the neck. Then open a subcutaneous pouch starting from the nuchal incision site along the lateral flank of the animal by blunt dissection with surgical scissors.
Subsequently, inject one milliliter of 0.9%sodium chloride in the subcutaneous pouch. Afterward, place a transmitter with the sensing leads oriented crainially into the subcutaneous pocket at the flank close to the ventral abdominal region. In this procedure, place the anesthetized animal on the stereotaxic frame.
Carefully position it's head with ear bars and a nose clamp so that the brahma and lambda cranium metric landmarks of the skull are at the same level. Next, clean the periosteum with cotton tips without damaging the temporal and occipital muscles. After that, apply hydrogen peroxide to the skull to exemplify the cranial suture, brahma and lambda.
Prepare the 3D stereotaxic system for target coordinates. Drill the target holes at maximum velocity without applying pressure. Place the recording electrode on the motor cortex surface for recording EEG.
Then fix the electrodes with dental cement to provide strong adhesion to the underlining neurocranium and leave the cement to dry for five minutes. Place the epidural reference electrode on the cerebellar cortex as it is an electroencephalographically silent region and fix it with cement as well. After having drilled the holes at the coordinates of interest, using the stereotaxic system attach the deep electrode to the vertical arm of the stereotaxic device.
Place the deep electrode in the CA1 region. Subsequently, fix the electrode with dental cement to provide strong adhesion to the underlying neurocranium. Place the epidural reference electrode on the cerebellar cortex and fix it with dental cement as well.
Leave the cement to dry for five minutes before closing the incision with sutures. This schematic shows that one epidural differential electrode is placed on the motor cortex. An additional intrahippocampal differential electrode is placed in the CA1 region of the hippocampus.
Both pseudo reference electrodes are localized on the cerebellum. This coronal section illustrates the localization of the deep intracranial electrode for recording the electrohipppocampagram. And shown here, is the close up of the deep EEG electrode, the sensing lead of the radio frequency transmitter and their arrangement on top of the mirroring skull.
These intrahippocampal CA1 recordings are taken from mice following acute treatment with kainate acid at day one, three, and five post-injection. This figure illustrates the examples of simultaneous epidural and deep EEGs in the pilocarpine model of mesial temporal lobe epilepsy in rats. Once mastered, this technique can be done within 30 to 45 minutes, depending on the number of electrodes to be implanted and the target regions if it is performed properly.
While attempting this procedure, special attention has to be paid to the vital signs of the anesthetized animals. As transgenic and pharmacological mouse and rat models of human diseases can exhibit differences in anesthesia susceptibilities. Following this procedure, other behavioral, cognitive and electrophysiological masses, such as, T maze, regular maze or evoke potential recordings can be performed in order to address additional questions like, how rhythmicity depends on different functional stages or changes upon repetitive stimulus exposure.
After watching this video, you should have a good understanding of how to perform accurate surface and deep EEG electrode placement in mice and rats using a three dimensional stereotaxic device and implantable EEG radiotelemetry.
