Simultaneous electrochemical and unit recording measurements: Characterization of the effects of d-amphetamine and ascorbic acid on neostriatal neurons

Simultaneous electrochemical measurements and recordings of single-unit activity were obtained from the neostriatum of immobilized, locally anesthetized rats. Electrical cross-talk between the two systems was minimized by the use of carbon-fiber voltammetric electrodes, which generate very small currents during electrolysis, and by the careful isolation of the recording apparatus. An intraperitoneal (i.p.) injection of 2.5 mg/kg d-amphetamine (d-AMPH) typically inhibited unit activity and produced a corresponding mirror-image increase in oxidation current. Both the depression of firing rate and the elevated electrochemical signal were maximal approximately 25 min after d-AMPH administration. In contrast, both unit activity and oxidation current were elevated after administration of 7.5 mg/kg d-AMPH with the maximum change in firing rate occurring significantly sooner than the peak electrochemical response. In view of evidence that the d-AMPH-induced voltammetric signal is similar to that for ascorbic acid (AA), but not dopamine (DA), a separate group of 12 animals received 1000 mg/kg AA (i.p.). In all cases, AA increased the electrochemical signal and in 7 animals this response was accompanied by an increase in firing rate. Our results, which demonstrate the feasibility of combining in vivo electrochemistry with single-unit electrophysiology, suggest that a d-AMPH-induced release of DA cannot completely explain the dose-dependent biphasic shift in neuronal activity produced by this drug in the neostriatum. Moreover, AA appears to be an effective modulator of neostriatal activity.