We report here the procedure for in vivo bilateral extracellular recording, along with analyzing the synchronization of dual-region LFP signals, which is both flexible and easy to conduct for estimating brain hemisphere lateralization, as well as the connectivity, directionality or coupling between neural activities of two brain areas. This can be widely used to reveal not only group-neuronal activities, but also some basic properties of interregional electrophysiology, especially for labs which are interested in screening oscillatory activities or labs which do not have systems for multi-channel recording in behaving animals23.
In general, a series of techniques are available to monitor brain activities, including electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI). Such methods have relatively lower temporal and spatial resolution in comparison with our presented recordings. For example, EEG is one of the oldest and most commercially available instruments for investigating extracellular activity of the brain. Although there are studies using "high density" EEG in freely moving rodents to improve the insufficient spatial resolution24,25,26, the skull always generates more noise and thus reduces the signal-to-noise ratio of cortical gamma oscillation, especially for small-sized mice. Our method with glass microelectrodes would be a good choice to prevent researchers from that "distorting noise" since microelectrodes could be inserted into the brain structure directly. Moreover, the recording glass pipettes used here are inexpensive, highly maneuverable, and can be applied to explore deeper brain areas not limited to cortical areas.
Close attention should be paid to the following. First, it is mandatory to carry out anesthesia strictly based on the body weight, and to test the depth of anesthesia hourly. This is because the physiological state of the mouse plays an important role in the quality of the LFP recorded, and any movement of the referencing sites caused by, e.g., sudden awakening of the animal, could generate background electrophysiological noise that would depreciate the availability. Second, because microelectrode resistance varies with the shape and diameter of the glass pipette tip, the heating must be carefully adjusted within the range for appropriate impedance when pulling microelectrodes. As described earlier in the protocol section, we found that the electrodes with impedance ranging from 1 to 2 MΩ captured high qualitied cortical oscillatory activities.
Gamma oscillations reflect the neuronal synchronization of different brain regions when animals are engaged in learning or stimulation-cued tasks27,28,29. The synchronization of gamma-band modulates excitation rapidly to activate postsynaptic neurons effectively30. It is worth noting that although the gamma oscillation was defined in the present study as oscillatory activity with frequency in the range 25-80 Hz as shown by several groups28,31,32, there are studies that describe 30-70 Hz as low gamma and 70-100 Hz as high gamma33,34,35. Regardless of the definition, the principles for data analysis remain similar. In signal processing, cross-correlation is used for determining the time delay between electrical signals of two brain regions36. For signals under stimulation conditions, the durations selected for cross-correlation analysis could be shorter37.
Though there are limitations in the use of LFP recording for the evaluation of neural activities; for instance, it can neither distinguish between pre- and post-synaptic activities nor detect resting membrane potentials of the neurons recorded23, the approach introduced here serves as a useful tool for the measurement of activities of a group of neurons from different brain areas of mice, allowing the investigation of brain-area functional connectivity and the coupling of electrical signals before and after drug infusion.
Several explanations have been proposed for the emergence of hemispheric asymmetry, e.g., asymmetry enhances an individual's ability to perform two different tasks at the same time38; or asymmetry increases neural capacity, avoiding unnecessary duplication of neural networks39; or two different cognitive processes may be more readily performed simultaneously if they are lateralized to different hemispheres40. Hemisphere lateralization is assumed to provide cognitive advantages, but it changes with age12,41. Neuroimaging studies have shown consistently that prefrontal activation tends to be less lateralized in older adults than in younger individuals42,43. AD patients with early unilateral or bilateral pathological changes develop brain abnormalities, including lateralization associating with forgetfulness, slow responses to sound stimulation and cognitive decline11,44. We observed, in the present study, a disrupted level of hemisphere lateralization between left and right M2 of APP/PS1 mice at 3-5 months, which is the period when such mice do not aggregate apparent deposition of beta amyloid plaques45,46, implying that toxicity induced by soluble beta amyloid oligomers may contribute, at least in part, to aberrant cortical hemisphere lateralization, which could accelerate brain deterioration in AD pathogenesis16,47.