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Reference Section
norepinephrine. The MultiClamp 700B, like the Axopatch 200B, can be used to
measure the electrical signals generated by the presence of these chemicals.
To make electrochemical measurements, a voltage is typically applied to the sensor.
This results in the oxidation or reduction of the electro-active species in solution
near the tip of the sensor. The current that is derived from the measurement is a
complex combination of chemical kinetics and molecular diffusion that is relatively
specific for different chemical classes of compounds. In short, the technique
generates a chemical fingerprint for each compound of interest. Furthermore, the
current that is derived from the oxidation (or reduction) of these compounds is
directly proportional to the concentration.
Two methods are used for making electrochemical measurements, cyclic
voltammetry and amperometry.
Cyclic voltammetry typically involves applying an episodic voltage ramp to the
sensor while the resultant current is measured under voltage clamp. The potential
at which dopamine (and other catechol-containing species such as epinephrine and
norepinephrine) oxidizes is approximately 0.15 V
cf.
the silver/silver chloride
reference potential. In order to accurately measure the voltammetric response of
dopamine in solution, the sensor is poised at a reducing potential between
measurements and ramped to more oxidizing potentials to generate the
electrochemical fingerprint. In a typical experiment, the ramp may last 100 ms;
this, then, is the resolution of the measurement. Cyclic voltammetry is most often
used to make relatively slow, volume-averaged measurements of the concentrations
of electro-active compounds.
Amperometry involves voltage clamping the sensor at the oxidation/reduction
potential of the compound of interest while measuring the resultant current. Sudden
changes in the concentration of the compound are registered as blips of current.
Amperometry is typically used for measuring quantal release of electro-active
chemicals from vesicles. The temporal resolution is determined only by the
response times of the sensor and the voltage clamp.
MultiClamp 700B Theory and Operation, Copyright 2005 Axon Instruments / Molecular Devices
