Transcript Cyclic Voltammetry and the Detection of Neurotransmitters

Cyclic Voltametry for the
Detection of Dopamine in vivo
Dopamine
• Neurotransmitter
– small molecule chemical messenger
• Important for motor and
cognitive functions
– Deficts in dopamine levels cause
Parkinson Disease
• Regulates reward
– Dopamine increases after drugs of
abuse like cocaine
Structure of Dopamine.
4-(2-aminoethyl)
benzene-1,2-diol
Dopaminergic Neurons
• Dopamine is synthesized in dopaminergic neurons
and packaged into membrane bound vesicles
• Electrical action potential initiates the release of
dopamine
• Dopamine vesicles undergo exocytosis
– Spills out into the extracellular space
– Can be detected
The dompaminergic neuron can
release dopamine into the
extracellular space where its
contents can be detected by
target neurons.
Detection of Dopamine
• Exocytosis of dopamine from vesicle occurs on
a millisecond time scale
• Sensor must be fast, sensitive, and selective
since dopamine concentrations are low
• Fast scan cyclic voltametry is the dominant
electrochemical technique used
Dopamine
HO
HO
N H2
Dopamine-o-quinone
- 2e
-
O
N H2
+ 2 H+
O
Carbon Fiber Microelectrodes
• Carbon Fiber
Microelectrodes (CFME)
are electrodes in which a
carbon fibers serve as the
electroactive area
• Advantages:
– Carbon fibers are biological
compatible to cells
– Small size (less than 10um
in diameters) allows for
implantation in vivo
• Commonly used with cyclic
voltametry
Carbon Fiber Microelectrode
Voltammetry
• Electrochemical technique in which the
current (I) is measured as a function of voltage
(Eapp).
• There are several kinds:
– Linear Scan voltametry (Polarography)
– Differential pulse polarography
– Square-wave voltametry
– Cyclic Voltametry
Fast-scan Cyclic Voltametry
1.3 V
400 V/s
-0.4 V
100 ms
8.5 ms
Scan electrode from a holding potential to a switching potential and back at a
high scan rate. Repeat these scans every 100 ms.
Dopamine
HO
HO
Dopamine-o-quinone
N H2
- 2e
-
O
N H2
+ 2 H+
O
As potential is ramped up, dopamine is oxidized to dopamine-o-quinone.
As potential is ramped down, dopamine-o-quinone reduced back to dopamine.
Fast-Scan Cyclic Voltametry
Cyclic voltammograms
No dopamine
Dopamine present
Background-subtracted
cyclic voltammogram of dopamine
oxidation
reduction
Fast scan rate cause a large background
charging current due to charging
the double layer.
The background current is stable and can
be subtracted out to obtain a backgroundsubtracted cyclic voltammogram for dopamine.
When dopamine is added (red line),
the differences are small.
The position of the peaks help identify the
molecule being detected.
Fast-scan cyclic voltametry
E EvsvsAg/AgCl
Ag/AgCl
-0.4VV
-0.4
10ss
10
dopamine
reduction
33
00
1.0VV
1.0
i, nA
dopamine
oxidation
-0.4VV
-0.4
-5
-5
i, nA
dopamine present
•Data can also be depicted as a color plot to show many CVs over time.
•Current is in color and shows when dopamine is present.
Results
Electrically-Stimulated release in a rat
•Carbon-fiber microelectrode implanted in nucleus accumbens
•Nucleus accumbens regulates reward
•Electrically stimulate cell bodies in ventral tegmental area
•Size of dopamine evoked depends on frequency of stimulation
• Color plots look like dopamine
Results
Spontaneous dopamine transients in a rat
Spontaneous dopamine transients in a rat after cocaine
•Cocaine increases the concentration and the length of time for dopamine
signaling.
Advantages
• Allows real-time detection of dopamine in
behaving animals
• Can determine during what behaviors
dopamine is released
• Can determine how drugs affect dopamine
signaling in the brain