Characterization of a barium-sensitive outward current following glutamate application on rat midbrain dopaminergic...

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 (approximately 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 microM) or glibenclamide (1-300 microM). However, it was strongly depressed or blocked by extracellular barium (300 microM to 1 mM), by low-calcium (0.5 mM) plus cadmium (100 microM) 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, alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and kainate. The PEAAR was not affected by tetrodotoxin (1 microM), saclofen (100-300 microM), bicuculline (30 microM), sulpiride (1 microM) or strychnine (1 microM). In addition, the inhibition of the ATP-dependent Na(+)-K+ pump by ouabain and strophanthidin (1-10 microM) 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.