Introduction - NPI VA-10X Operating Instructions Manual

VA-10X User Manual
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2. Introduction

Recently, electrochemical methods using carbon-fiber microelectrodes have been applied to
measure the release of oxidizable transmitter from single cells, and, even more impressively,
from single exocytotic vesicles. Transmitters that are oxidizable and which, therefore, can be
measured with this approach, include serotonin, dopamine, adrenaline, and noradrenaline. In
addition, some peptides or proteins such as insulin may be oxidizable owing to the presence
of oxidizable amino acids such as cysteine or tyrosine.
Cells that have been studied successfully with this technique include adrenal chromaffin cells,
sympathetic neurons, mast cells, pancreatic beta cells, carotid glomus cells and
melanotrophs, but the list is growing. In addition, in brain slices simultaneous intracellular
and voltammetric studies have been made to correlate intracellular electric signals with
transmitter release (10).
Two useful electrochemical approaches are amperometry and cyclic voltammetry. In
amperometry, a DC potential is applied to a carbon-fiber microelectrode. The applied
potential appears at the interface between the carbon and the mammalian ringer solution. If
the potential is much greater than the redox potential for a given transmitter, then molecules
of transmitter diffusing to the carbon surface are oxidized rapidly yielding a current that can
be measured. The sensitivity of the amperometric approach, in particular, has provided an
unprecedented look at the time course of transmitter release revealing distinct phases of
release. On the other hand, the amperometric approach provides little information about the
substance being oxidized or reduced.
Cyclic voltammetry provides a limited amount of information about the substance being
studied, at some expense to the time resolution. In this approach a cyclically repeating voltage
waveform, typically consisting of voltage ramps, is applied to the carbon-fiber electrode and
the resulting current is plotted as a function of the applied voltage (after subtraction of a
"background" record obtained in the absence of the redox species). Since different substances
have different potentials for oxidation and for reduction one can distinguish transmitters from
each other.
For more detailed information on the principles of electrochemical measurements at single
cells and the fabrication of carbon-fiber microelectrodes refer to several recent reviews (see
also chapter 7).
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