Many animal and plant tissues have electrical properties, which can be measured with electrodes. Electrophysiology can be used to study sensory organs, muscles (in particular the heart), cells of the endocrine system, the central nervous system. Recording of voltage change or electric current may be measured on anything from an ion channel protein to an entire organ. The study of ion channels shows great promise for the development of rationally designed drugs. Brain slice preparations may be used to study neurons. In this case, artificial cerebrospinal fluid (ACSF) must be used in order to keep them viable.
Classical electrophysiology techniques involve placing electrodes into cells or biological tissue. In optical electrophysiology, voltage sensitive dyes and fluorescent proteins are used, which allow the measurement of changes in potential without disruption of the tissue studied.
Various techniques are being used in electrophysiology for intracellular recording, such as:
Voltage clamp The voltage clamp is used to measure the ion currents across a cell membrane while holding the membrane voltage at a constant level. The membrane voltage can be manipulated independently of the ionic currents, allowing the current-voltage relationships of membrane channels to be studied.
Current clamp The current clamp records the voltage difference across a cell membrane by injecting current into the cell through the recording electrode (usually a sharp micropipette).
Patch-clamp Unlike conventional intracellular recordings, which involve impaling a cell with a fine electrode, the patch clamp uses a relatively large microelectrode that is placed next to the cell. Gentle suction is applied on the small area of membrane ("patch") covered by the pipette. This technique allows the study of single or multiple ion channels in cells, and is often used to study the mechanism of action of drugs.
Many variations of these methods are being used. New technologies offer higher throughput than conventional techniques, such as planar patch clamp technology (instead of positioning the pipette on an adherent cell, a cell suspension is pipetted on a chip containing a microstructured aperture). The emergence of automated electrophysiology platforms (IonWorks, PatchXpress systems, etc.) has had a major impact on the capacity and speed of information-rich assays that can be performed. These methods are used extensively to study ion channels, which have become important targets for drug discovery. Ion channels play a critical role in nerve and muscle function. Drugs that modulate ion channels have been investigated in therapeutic areas such as neuropathic pain, cardiac arrhythmia, hypertension, local anaesthesia, stroke, Parkinson’s, obesity, epilepsy, diabetes and depression.
Extracellular recordings can also be performed, using techniques such as single unit recording, a method used to measure the activity of a single neuron in the brain of animals in vivo. Brain slices may also be used for the study of electrophysiology of neurons and local brain circuits.
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