This article describes a method for analyzing the properties of the extracellular space in brain slices using ion-selective microelectrodes. The technique involves measuring the diffusion of tetramethylammonium ions to assess extracellular volume fraction and other parameters.
Begin with a recording chamber containing a secured thick brain slice immersed in artificial cerebrospinal fluid or aCSF.
This brain slice includes interstitial fluid and extracellular matrix, forming the interconnected extracellular space that surrounds brain cells.
Submerge the dual-channel ion-selective microelectrode and the iontophoresis microelectrode into the aCSF.
The dual-channel ion-selective microelectrode features a sensing channel for tetramethylammonium or TMA ions and a reference channel, while the iontophoresis microelectrode contains a TMA solution.
The iontophoresis electrode releases TMA ions, which diffuse toward the ion-selective microelectrode.
The sensing channel detects these TMA ions and generates an electrical signal. Record this as a baseline.
Insert both electrodes into the brain slice, keeping them slightly apart.
As the TMA ions diffuse through the extracellular space, they reach the ion-selective microelectrode.
Rerecord the signal and compare it with the baseline to analyze extracellular space properties, such as the volume fraction and the degree of twisting.
Place a 400-micrometer thick brain slice in the recording chamber, ensuring that it is fully submerged in the flowing ACSF. Next, move both the iontophoresis microelectrode and the ISM above the field of interest on the brain slice. Submerge both electrodes in the flowing ACSF but above the slice. Then, offset the voltage for both the reference and the ion-sensing channels to 0 millivolts. Wait for the voltage in both channels to stabilize.
On the chart recorder, Mark the voltage measured on the ion-sensing channel of the ISM. Next, place the ISM and iontophoresis microelectrodes 200 micrometers deep in the slice and 120 micrometers away from each other. Calculate the voltage difference between the TMA signals measured in the flowing ACSF and in the brain and input this value into the baseline volt millivolt field in the measuring electrode box of the Wanda GUI.
On the left side of the GUI, ensure that all the experimental parameters are correctly entered. Start the recording by clicking Acquire, and allow it to take a full recording. After moving both microelectrodes out of the slice, use the chart recorder to determine any change between the voltage measured now, and its measurement from before.
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