CHARACTERIZATION OF AMPEROMETRY FOR IN-VIVO MEASUREMENT OF DOPAMINE DYNAMICS IN THE RAT-BRAIN

Constant potential amperometry with Nafion-coated carbon-fiber electro des has been evaluated as a technique for in vivo detection of the neu rotransmitter dopamine. The results of this technique have been compar ed to results obtained with fast-scan cyclic voltammetry at the same e lectrode during release of dopamine into the extracellular space of th e brain during electrical stimulation of neurons. The data indicate th at constant potential amperometry is a viable technique for detecting low concentrations of dopamine. Dopamine permeates the film more quick ly with constant-potential amperometry than with repeated fast-scan cy clic voltammetry as predicted by diffusion equations. For the case of cyclic voltammetry, it is demonstrated that the temporal delay caused by diffusion through Nafion film can be removed by deconvolution proce dures. Despite the suitability of constant potential amperometry as an in vivo monitoring technique, it does have several disadvantages when compared to fast-scan cyclic voltammetry. The diffusion layer extends outside of the Nafion film making determination of concentration base d on in vitro calibrations more difficult to interpret. The reported c oncentrations are larger than obtained by cyclic voltammetry, a techni que with the diffusion layer restricted to the Nafion film, and this r esult is likely an underestimation of the effect of the catalytic reac tion between the o-quinone of dopamine and ascorbate. Amperometry was found to provide only slightly improved signal-to-noise ratios than cy clic voltammetry despite the use of greater filtering. This was becaus e the advantage of dopamine accumulation in the film was lost. In addi tion, the small magnitude of the amperometric signal makes it more sus ceptible to electrical interference.