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

QuickTime™ and a decompressor

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