Diffusion processes measured at microvoltammetric electrodes in brain tissue

Voltammetric electrodes fabricated from carbon fibers have been employed as sensors of fluctuations in the concentration of easily oxidized species in the brains of anesthetized rats. The factors which affect access of these compounds to the electrode surface have been examined. While these electrodes show virtually time-independent chronoamperometric currents in vitro during a 2.5-s potential step, the current shows an increased time dependence when the electrode is implanted in the brain, suggesting an altered diffusion profile obtains in vivo. Values of brain diffusion coefficients for ascorbic acid, dihydroxyphenylacetic acid, and α-methyldopamine were estimated by measuring the time for these species to diffuse to an electrode through brain tissue from a very small injection volume (100 nl). Diffusion coefficients in the brain as measured by this method are found to be approximately one third of the value in pH 7.4 buffer. These data suggest that chronoamperometric currents during long potential steps in vivo involve two types of diffusion: at very short times (approximately 100 ms), molecules are electrolyzed which diffuse to the electrode through a pool of extracellular fluid at the tip of the electrode in which diffusion coefficients are close to those measured in aqueous solution. With longer potential step times, the effects of the reduced diffusion coefficient of these substances through brain tissue is apparent. The utility of these concepts is demonstrated by the rapid response time of the electrode to concentration changes induced by electrical stimulation of dopamine-containing neurons.