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
NH2HO
HO
-2e-
O
NH2O
Dopamine Dopamine-o-quinone
+ 2 H+
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
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are needed to see this picture.
1.3 V
-0.4 V8.5 ms 100 ms
400 V/s
Scan electrode from a holding potential to a switching potential and back at a high scan rate. Repeat these scans every 100 ms.
NH2HO
HO
-2e-
O
NH2O
Dopamine Dopamine-o-quinone
+ 2 H+
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
Fast scan rate cause a large backgroundcharging current due to chargingthe double layer.
When dopamine is added (red line),the differences are small.
The background current is stable and canbe subtracted out to obtain a background-subtracted cyclic voltammogram for dopamine.
The position of the peaks help identify themolecule being detected.
oxidation
reduction
Cyclic voltammograms
Dopamine presentNo dopamine
Background-subtracted cyclic voltammogram of dopamine
Fast-scan cyclic voltametry
-0.4 V
1.0 V
-0.4 V10 s
-5
0
3
i, nA
E vs
Ag/
AgCl
-0.4 V
1.0 V
-0.4 V10 s
-5
0
3
i, nA
E vs
Ag/
AgCl
•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.
dopamineoxidation
dopaminereduction
dopamine 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
ResultsSpontaneous 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