Characterization of a Barium-sensitive Outward Current following Glutamate Application on Rat Midbrain Dopaminergic Cells

Using intracellular electrophysiological recordings in dopaminergic (principal) neurons of the rat mesencephalon maintained in vitro, we studied a postexcitatory amino acid response (PEAAR). Under current-clamp mode, bath application of glutamate produced a depolarization that was followed by a hyperpolarization when the perfusion of the excitatory amino acid was discontinued. Under single-microelectrode voltage-clamp mode, an outward current followed the glutamate-induced inward current. The PEAAR was associated with an increase in membrane conductance and reversed polarity at about -85 mV (2.5 mM extracellular K⁺). The null potential for the PEAAR was independent of the intracellular loading of chloride ions and was shifted towards less negative values (?23 mV) by increasing extracellular K⁺ from 2.5 to 8.5 mM. The PEAAR was present in neurons treated with tetraethylammonium (5–10 mM), apamin (1 μM) or glibenclamide (1–300 μM). However, it was strongly depressed or blocked by extracellular barium (300 μM to 1 mM), by low-calcium (0.5 mM) plus cadmium (100 μM) or magnesium (10 mM), and by low-sodium solutions. An outward response was also generated after an inward current induced by the perfusion of the specific agonists for the ionotropic excitatory amino acid receptors NMDA, a-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and kainate. The PEAAR was not affected by tetrodotoxin (1 μM), saclofen (100–300 μM), bicuculline (30 μM), sulpiride (1 μM) or strychnine (1 μM). In addition, the inhibition of the ATP-dependent Na⁺-K⁺ pump by ouabain and strophanthidin (1–10 μM) prolonged the glutamate-induced membrane depolarization/inward current while the subsequent PEAAR was reduced or not observed. Our data indicate that the PEAAR mainly results from the activation of a barium-sensitive potassium current. This response might limit the excitatory and eventually neurotoxic effects of glutamate.