Endogenous dopamine modifies electroresponsiveness of pars compacta cells in the guinea pig substantia nigra in vitro

In the nigrostriatal pathway, dopamine is released not only from striatal nerve terminals, but also locally from the dendrites of nigrostriatal neurones within the substantia nigra itself. Exogenous dopamine is known to inhibit the firing of these neurones when applied directly to the substantia nigra in micromolar concentrations: but the amounts used are probably much higher than the endogenous concentration of the transmitter. Moreover, the direct, local blockade of nigral dopamine autoreceptors has not been reported to affect the firing rates of these neurones. The electrophysiological effects of endogenous dopamine were therefore examined using intracellular recordings from the substantia nigra in vitro. When slices of the midbrain were pharmacologically depleted of endogenous dopamine, selective membrane properties of nigrostriatal neurones were altered in a manner consistent with the effects of the exogenous transmitter. Similar changes were observed in control slices on exposure to the dopamine antagonist haloperidol. It is concluded that endogenous dopamine normally exerts a tonic influence on the electrical properties of nigrostriatal neurones.