Protective role of hydrogen peroxide in oxygen-deprived dopaminergic neurones of the rat substantia nigra

Abstract

Hydrogen peroxide (H₂O₂) is a reactive oxygen species, responsible for cytotoxic damage through the formation of hydroxyl radicals. Dopamine (DA) neurones of the substantia nigra pars compacta (SNc) are highly sensitive to metabolic stress, and they typically respond to energy deprivation with membrane hyperpolarization, mainly through opening of ATP-dependent K⁺ channels. Accordingly, H₂O₂ (3 mm) induced a tolbutamide-sensitive outward current in DA neurones. Conversely, in a hypoxic medium, H₂O₂ reverted membrane hyperpolarization, which is associated with oxygen deprivation in DA neurones, restored their action potential firing, and reduced the hypoxia-mediated outward current in a concentration-dependent manner, between 0.1 and 3 mm (IC₅₀ 0.6 ± 0.1 mm). Notably, H₂O₂ did not counteract membrane hyperpolarization associated with hypoglycaemia, moreover, when catalase was inhibited with 3-amino-1,2,4-triazole (3-AT; 30 mm), H₂O₂ did not reduce hypoxia-mediated outward current. The counteracting action of H₂O₂ on hypoxia-mediated effects was further confirmed by single-unit extracellular recordings of presumed DA neurones in acute midbrain slices preparations, using a planar multi-electrode array device. Whilst a prolonged period of hypoxia (40 min) caused firing suppression, which did not recover after perfusion in normoxic conditions, the presence of H₂O₂ (3 mm) during this prolonged hypoxic period rescued most of the neurones from irreversible firing inhibition. Accordingly, morphological studies showed that H₂O₂ counteracts the cytochrome c release provoked by prolonged hypoxic treatment. Taken together, our data suggest that H₂O₂ prevents the metabolic stress of DA neurones induced by hypoxia by serving as a supplementary source of molecular oxygen, through its degradation by catalase.