This method helps establish the direct relationship of electrocortical signals recorded from both ECoG and the depths of wires to answer key questions about the neuronal contributions of laser-evoke potentials. The main advantage of this technique is that electrode implantation can be protected by polylactic shell, thus the recording can be applied to our free moving rats. This method can be applied in recording brain responses evoked by a stimuli of different sensations or brief features of psychiatric diseases which promote the investigation of their respective neuromagnetisms.
Begin this procedure with anesthetization of the rat as detailed in the text protocol. Then, use a stereotaxic apparatus to fix the head of the rat with it's nose placed into the anesthetic mask. Surgical tolerance is achieved when then rat fails to respond to toe pinching.
Apply ophthalmic ointment to the eyes to avoid corneal drying. Shave the top of the rat's scalp using a standard shaver. Then, sterilize the scalp using the medical iodophor disinfectant solution and 75%alcohol to remove the iodine.
Inject 2%lidocaine into the scalp for local analgesia. Then, administer atropine to inhibit respiratory hypersecreation. Make a midline incision of approximately two to three centimeters on the scalp using a scalpel.
Cut and remove part of the scalp along the midline and expose the cranium. Mark the locations of the ECoG electrodes based on the predefined stereotaxic coordinates. Also, mark the locations of the reference and ground electrodes on the midline.
Drill holes for the ECoG screws using an electric cranial drill on the skull at the marked sights without destroying the dura. Drive a stainless screw, which connects to the insulation coated copper wire, into the hole for an approximate one millimeter depth. Avoid penetrating the underlying dura.
Place a protective shell base on the cranium. Fix the base with it's adjacent screws on the cranium using dental acrylic. Mark the locations of the depth wire electrodes based on the predefined stereotaxic coordinates.
Now, drill small holes on the skull around the mark sights for wire implantation and carefully remove the bone flap to expose the dura. Wash the craniotomy frequently with normal saline. Using a needle, lift and cut the dura without damaging the pia mater, vessels and the surface of the neocortex.
Lower the depth wire electrodes to the surface of the neocortex. Then, slowly penetrate the brain to the target depth. Seal the craniotomy with a mixture of wax and paraffin oil to ensure that the depth wire electrodes can be moved for subsequent experimental manipulations.
Fix the electrode apparatus using dental acrylic on the skull. Weld each copper wire that connects to the ECoG screw to the corresponding channel on the connector module. Assemble the protective shell wall to the base and weld the reference and ground electrodes to the corresponding channels.
Fix the cap to the protective shell using tapes to avoid contamination. After injecting the rat with penicillin, single house the rat in a temperature and humidity controlled cage after the surgery as detailed in the text protocol. Place the rat in the behavior chamber for at least one hour before the experiment to ensure that the rat acclimatizes to the recording environment.
Gently connect the recording head stage with the electrode module to avoid scaring the rat and damaging the electrode module. Set up the laser generator. Connect the optic fiber and adjust the spot size of the laser according to the equipment operators manual.
Now, connect the digital output from the trigger generator to the digital input port of the recording board. Set the video camera beneath the corner of the experimental chamber to continuously record the nociceptive behaviors of the rat when it's paw receives nociceptive laser stimulation. Deliver ongoing white noise via a loud speaker at the top of the chamber.
Collect the electrophysiological data from both the ECoG and the depth wire electrodes using the recording system according to the equipment operators manual. Now, deliver the laser pulses to the plantar of the rat's forepaw through the gaps in the bottom of the chamber by first positioning the laser generator. Then, press the switch to deliver laser stimulation.
Shown here is the raw electrophysiological data from bilateral primary motor cortices, bilateral primary somatosensory cortices and ECoG. A clear laser-evoked potential, or LEP response, is detectable after the onset of the laser stimulus. Shown here is the group level averaged LEP wave forms from six electrodes of five rats.
The LEP responses consist of a dominant negative deflection called N1 Wave. Note the the N1 Wave of bilateral primary somatosensory cortices shows a shorter latency and higher amplitude than that of bilateral primary motor cortices. This figure shows wavelet transformed coherence between LEPs sampled from the ECoG and depth wires in bilateral S1 and M1, with the laser delivered on the plantar of the rat's right forepaw.
Note that the left S1 and M1 showed a higher coherence than the right S1 and M1 at the gamma frequency band. After it's development this technique will pave the way for researchers to concurrently recall both ECoG and intercortical local fill potentials in free moving rats to expose the neural contributions of laser-evoked potentials.
