1-Methyl-4-phenylpridinium (MPP⁺)-induced functional run-down of GABA_A receptor-mediated currents in acutely dissociated dopaminergic neurons

We have evaluated GABA_A receptor function during treatment of 1-methyl-4-phenylpridinium (MPP⁺) using patch-clamp perforated whole-cell recording techniques in acutely dissociated dopaminergic (DAergic) neurons from rat substantia nigra compacta (SNc). γ-Aminobutyric acid (GABA), glutamate or glycine induced inward currents (I_GABA, I_Glu, I_Gly) at a holding potential (V_H) of -45 mV. The I_GABA was reversibly blocked by the GABA_A receptor antagonist, bicuculline, suggesting that I_GABA is mediated through the activation of GABA_A receptors. During extracellular perfusion of MPP⁺ (1-10 μM), but neither I_Glu nor I_Gly, declined (termed run-down) with repetitive agonist applications, indicating that the MPP⁺-induced I_GABA run-down occurred earlier than I_Gly or I_Glu under our experimental conditions. The MPP⁺-induced I_GABA run-down can be prevented by a DA transporter inhibitor, mazindol, and can be mimicked by a metabolic inhibitor, rotenone. Using conventional whole-cell recording with different concentrations of ATP in the pipette solution, I_GABA run-down can be induced by decreasing intracellular ATP concentrations, or prevented by supplying intracellular ATP, indicating that I_GABA run-down is dependent on intracellular ATP concentrations. A GABA_A receptor positive modulator, pentobarbital (PB), potentiated the declined I_GABA and eliminated I_GABA run-down. Corresponding to these patch-clamp data, tyrosine hydroxylase (TH) immunohistochemical staining showed that TH-positive cell loss was protected by PB during MPP⁺ perfusion. It is concluded that extracellular perfusion of MPP⁺ induces a functional run-down of GABA_A receptors, which may cause an imbalance of excitation and inhibition of DAergic neurons.