Biomedical Engineering Reference
In-Depth Information
be distinguished from faradaic current by their cyclic voltammogram because
the change in current for such artifacts is in the same direction for the anodic
and cathodic scans (unlike for a redox couple). Ionic changes in the media
can also produce background shifts. These give current in opposite directions
for the anodic and cathodic scans (like genuine redox couples). Their cyclic
voltammograms have broad features, particularly at infl ection points of the
background current, and so they can be distinguished from electroactive
analytes which generate sharp peaks.
Once dopamine has been chemically verifi ed, the current at its peak oxidation
potential (approx +0.60 to 0.70 V vs Ag/AgCl) can be plotted against time to
reveal the temporal profi le of dopamine concentration changes (
Fig. 1E
). This
current is directly proportional to the concentration of dopamine at the electrode
and can be converted to concentration by a factor obtained by in vitro calibration
of the electrode with a dopamine stock solution (
see
Subheading 3.6.
).
Because of the requirement for background subtraction, fast-scan cyclic
voltammetry is a differential technique and therefore best suited for monitoring
chemical changes that take place over seconds rather than minutes or hours.
For this reason, it is not good at monitoring slow, tonic changes in dopamine
concentration (unlike microdialysis). However, owing to its exquisite temporal
resolution, along with its chemical resolving power, it is the foremost technique
for the measurement of fast, phasic changes in dopamine concentration.
Waveform generation and electrochemical data acquisition are carried
out on a personal computer with a multifunction input/output card using
software locally written in LabVIEW. The waveform is applied to the carbon
fi ber microelectrode via a potentiostat. Current generated at the carbon fi ber
microelectrode is converted to voltage (200 nA/V) by a head-mounted current-
to-voltage converter (headstage;
Fig. 2
), sent to the potentiostat where it is ampli-
fi ed (2-10
) and low-pass fi ltered (2 kHz), and then acquired (100-333 kHz)
on the personal computer. Connections between the potentiostat and the
headstage are made through an electrical swivel.
×
3.2. Electrical Stimulation
By electrically stimulating dopaminergic neurons, a “snapshot” of the
status of dopamine release and uptake can be taken. This strategy has been
particularly useful for determining the neurochemical mechanisms involved
in behavior following pharmacological manipulations
(4
,
5)
. In experiments in
which we wish to measure behaviorally evoked dopamine release, we employ
electrical stimulations for optimization of the carbon fi ber microelectrode
placement into a dopamine-rich region. In addition, by electrically stimulating
dopamine release before and after the experiment we not only get an in vivo
template of dopamine at the carbon fi ber microelectrode, but also verify that
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