This article details a procedure for recording local field potentials (LFP) and electroencephalogram (EEG) signals from the cerebral cortex of an anesthetized rat. The method involves precise electrode placement and stimulation of the whisker pads to measure neuronal activity.
Secure an anesthetized rat to a stereotaxic frame with its skull exposed.
The rat has a burr hole in the skull with the dura pierced, a neck incision, and stimulating electrodes attached to its whisker pads.
Fix an electroencephalogram, or EEG, spider electrode smeared with conductive paste on the skull, keeping the burr hole clear.
The spider electrode measures surface EEG, reflecting overall neuronal activity from the cortical area beneath it.
Mount a linear multi-channel microelectrode onto a micromanipulator.
Insert the microelectrode into the cerebral cortex through the burr hole to a depth of 1500 micrometers.
The microelectrode measures local field potential, or LFP, reflecting cortical neuronal activity.
Place the EEG and LFP reference electrodes inside the neck incision.
Connect the recording and reference electrodes to a preamplifier linked to a data acquisition unit.
Apply electrical stimuli to the whisker pads and adjust the microelectrode depth to maximize the evoked LFP amplitude. Record the co-localized EEG and LFP signals.
In this procedure, clean and dry the skull surrounding the burr hole using a cotton swab. Carefully apply the conductive EEG paste on a flat side of an EEG spider electrode. Leave a small hole clear of EEG paste on the spider electrode to allow a multilaminar microelectrode to pass through the hole without contacting the paste and the spider electrode.
Align the spider electrode with the burr hole in the skull with the EEG paste facing the skull. Carefully press the spider electrode onto the skull, making firm contact with the skull via the EEG paste.
Remove any paste obscuring the burr hole using a needle on a syringe and remove excessive EEG paste beyond the periphery of the spider electrode, so that the contact between the spider electrode and the skull is spatially constrained to the size of the electrode. Then, smear EEG paste onto the reference electrode for the EEG, and place it securely inside the incision at the back of the rat's neck.
Next, connect the EEG electrodes to the pre-amplifier via a passive signal splitter for low-impedance signals. At this stage, test the resistance of the EEG probe to make sure it is below five kiloohms. If not, add more EEG paste and make sure the spider electrode makes a good contact with the skull.
Subsequently, mount a micromanipulator arm on the stereotactic frame. Connect a linear 16-channel microelectrode to a 16-channel acute head stage clipped securely onto the micromanipulator arm. Then, smear EEG paste onto the reference electrode for the microelectrode, and then place it securely inside the incision next to the reference electrode for the EEG. Adjust the angle of the micromanipulator arm so that the microelectrode is perpendicular to the cortical surface.
Now, lower the microelectrode under a microscope so that the tip of the microelectrode is aimed at the tiny opening at the bottom of the burr hole, until the uppermost electrode just penetrates the cortical surface. Care must be taken to avoid forcing the microelectrode onto the surface of the dura, as this would break the electrode.
Insert the microelectrode to the cortical surface at a depth of 1500 micrometers. Microadjust the depth by applying a train of stimulus to the whisker pad, and observing the 16-channel evoked LFP on a PC monitor. Carefully turn the z-axis knob on the micromanipulator until the highest amplitude of the evoked LFP occurs as this coincides with the layer 4 in the cortex.
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