We investigated the effect of changes in membrane-voltage on intracellular sodium concentration ([Na+]i) of dopamine-sensitive neurons of the substantia nigra pars compacta in a slice preparation of rat mesencephalon. Whole-cell patch-clamp techniques were combined with microfluorometric measurements of [Na+]i using the Na+-sensitive probe, sodium-binding benzofuran isophthalate (SBFI). Hyperpolarization of spontaneously
active dopamine neurons (recorded in current-clamp mode) caused the cessation of action potential firing accompanied by an elevation in [Na+]i. In dopamine neurons voltage-clamped at a holding potential of −60 mV elevations of [Na+]i were induced by long-lasting (45–60 s) voltage jumps to more negative membrane potentials (–90 to −120 mV) but not by corresponding voltage jumps to −30 mV. These hyperpolarization-induced elevations of [Na+]i were depressed during inhibition of Ih, a hyperpolarization-activated inward current, by Cs+. Hyperpolarization-induced elevations in [Na+]i might occur also in other cell types which express a powerful Ih and might signal lack of postsynaptic activity.