Evaluation of Hemisphere Lateralization with Bilateral Local Field Potential Recording in Secondary Motor Cortex of Mice

This technique can be used to review some basic properties of interregional electrophysiology for hemisphere lateralization, as well as the connectivity, directionality, and coupling. Electrophysiological measurement is a sensitive and effective method of evaluating in the animals, neuronal activities. This protocol provides a better way to prop into the synchronization of electrical signals.

The understanding of the underlying mechanism of possible altered brain lateralization in Alzheimer’s disease pathogenesis, may provide new insights into potential biomarkers for Alzheimer’s treatment. Prior to surgery, confirm the depth of anesthesia of the mouse, by performing a tail or toe pinch with forceps. Next, position the mouse in the stereotaxic apparatus, and fix its head.

Apply ointment on both eyes to keep them moist. Then, shave the head, and sterilize the area. Make a small incision of 12 to 15 millimeters, in the middle of the shaved area.

Using forceps, gently pull the scalp away from the midline. Afterward, separate the skin gently, and remove the residual tissue. Clean the skull, using hydrogen peroxide coated cotton buds.

Under a stereo microscope, drill two small holes, of one to 1.5 millimeters radii, on both the left and right sides of the skull, to allow insertion of the recording microelectrodes into the M2 regions. Carefully remove the dura mater with a tungsten needle. Then, insert two separate recording microelectrodes, filled with 0.5 molar sodium chloride, into the holes, at an angle of 60 degrees, using mechanical micromanipulators.

For LFP recording, slowly lower the left and right glass electrodes to the M2 coordinates. For quality control, test the resistance of each electrode, using the differential amplifier. Next, set the recording process at 0.1 hertz high pass, and 1000 hertz low pass, with 1000 times amplification.

Collect the digitized raw LFP data in stable state, for at least 60 seconds, with the mouse breathing evenly at two breaths per second, under anesthesia. After recording, slowly raise the electrodes out of the brain. Save the data and analyze offline, with the analysis software.

To perform cross-correlation analysis, in the analysis software, click analysis, then waveform correlation, and import the data. Next, assign one waveform channel signal to be the first channel, and the other as the reference. Set width as two, and offset as one.

Subsequently, set the duration of both LFPs to 100 seconds, by selecting the start time and end time. Then, press the process button to perform cross-correlation analysis. Click file, export as, then save the cross-correlation results corresponding to the resulting pop-up chart in text format.

Afterward, remove the correlation values at time lags ranged at zero plus and minus 01 second then deal with the rest of the cross-correlation data. To perform coherence analysis, run the data in the analysis software. Next, arrange the two LFP signals as the first and second waveform channels, then set the block size value as 4096.

Block size means the number of data points used in the first for real transformation. The larger the block size, the better the frequency resolution. Move the dotted lines manually, to ensure the time accuracy for signals in both channels are being set as the same period.

Press the add area button to load the area and perform coherence analysis. Afterward click file and save as, to save the coherence results corresponding to the resulting pop-up chart in text format. To see whether early Alzheimer’s disease pathology impairs the capacity of hemisphere lateralization, extracellular LFPs were recorded in the left and right M2 of APP/PS1 mice and wild type controls, and their cross correlation was analyzed.

In wild type mice, the results demonstrated that the mean correlation between the left and right LFPs at positive time lags differed significantly from that at negative time lags, implicating the existence of hemispheric asymmetries in the M2 areas of the wild type controls. In comparison, the left and right LFPs of APP/PS1 mice showed higher synchronized in time domain, suggesting a reduction of asymmetry between the left and right M2.Gamma oscillations were then filtered from the LFPs and a coherence analysis was performed to measure the similarity of electrical signals in the gamma frequency range. The result showed that the gamma coherence between the left and right M2 in APP/PS1 mice was significantly higher than that in the wild type mice, indicating a higher synchronization, and consequently reduced lateralization between the left and right M2 in APP/PS1 mice.

Urethane is toxic and carcinogenic, so please always be careful, and follow the safety regulations when handling it. It’s very important to test the depth of anesthesia hourly, in order to ensure stable LFPs being recorded. The recording and analysis process can be applied to other brain pathways, especially for labs which do not have systems for multi-channel recording in freely moving animals.