A technology widely used in neuropharmacology research is whole-cell recordings. This technique is a modification of the patch clamp technique. The membrane patch underlying the electrode is ruptured, typically by applying a small suction through the pipette, which results in the interior of the electrode becoming continuous with the intracellular fluid. The combination of a tight giga-ohm seal around the electrode-cell membrane junction and the very low resistance of the electrode-intracellular patch has several major advantages: the recordings are very low noise, and the low resistance of the electrodes can minimize errors that may arise during voltage clamp recordings or distort the recorded voltage response to a current input.
When combined with infrared video microscopy techniques, which allow both the large electrode tip and the cell to which it is to be attached to be visualized (Figure 5-6), recordings can be made from specific cell types or from cells labeled with a retrogradely transported fluorescent dye. Although such a preparation has many unique and powerful advantages, this low-resistance junction is also subject to the introduction of artifacts. This is particularly true in cases in which the response to be measured is mediated by diffusible second messengers: the large bore of the attached electrode has been reported to dialyze intracellular constituents from the cell into the electrode. When this process happens, the experimenter often observes a rundown of the response, in which the current gradually decreases with time as a result of loss of the intracellular milieu. To test for this possibility, investigators often rely on a "perforated patch" technique, in which the patch pipette is filled with the ionophore nystatin, gramicidin, or others. When a patch pipette of this type is attached to the cell surface, these channel ionophores are incorporated into the membrane section adjacent to the bore of the electrode. As a result, a low-resistance access to the intracellular space is obtained without the need for rupturing the membrane.
FIGURE 5-6. Patch clamp electrophysiology and calcium imaging.
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