Differential Actions of Endogenous and Iontophoretic Dopamine in Rat Striatum

We have voltammetrically monitored dopamine release in the rat striatum evoked either by stimulation of the median forebrain bundle or by iontophoresis. Neuronal spike activity was recorded with the same microelectrode alternately with voltammetric scans. Low levels of stimulation-evoked dopamine (< 1 microM) excited all striatal units tested. However, this effect could be converted into an inhibition at higher levels of release. In contrast to the results with neuronally released dopamine, excitation was never seen when dopamine was applied by iontophoresis. Inhibition was, however, observed at high concentrations (> 1 - 5 microM) of exogenous dopamine. These data show that endogenous dopamine at low levels may be predominantly an excitatory transmitter in the striatum, but exogenous (iontophoretic) dopamine does not have the same effect on striatal unit activity as the endogenous material.