Expression and functional properties of TRPM2 channels in dopaminergic neurons of the substantia nigra of the rat

Abstract

Transient receptor potential melastatin 2 (TRPM2) channels are sensitive to oxidative stress, and their activation can lead to cell death. Although these channels have been extensively studied in expression systems, their role in the brain, particularly in the substantia nigra pars compacta (SNc), remains unknown. In this study, we assessed the expression and functional properties of TRPM2 channels in rat dopaminergic SNc neurons, using acute brain slices. RT-PCR analysis revealed TRPM2 mRNA expression in the SNc region. Immunohistochemistry demonstrated expression of TRPM2 protein in tyrosine hydroxylase-positive neurons. Channel function was tested with whole cell patch-clamp recordings and calcium (fura-2) imaging. Intracellular application of ADP-ribose (50–400 μM) evoked a dose-dependent, desensitizing inward current and intracellular free calcium concentration ([Ca2+]i) rise. These responses were strongly inhibited by the nonselective TRPM2 channel blockers clotrimazole and flufenamic acid. Exogenous application of H2O2(1–5 mM) evoked a rise in [Ca2+]iand an outward current mainly due to activation of ATP-sensitive potassium (KATP) channels. Inhibition of K+conductance with Cs+and tetraethylammonium unmasked an inward current. The inward current and/or [Ca2+]irise were partially blocked by clotrimazole and N-( p-amylcinnamoyl)anthranilic acid (ACA). The H2O2-induced [Ca2+]irise was abolished in “zero” extracellular Ca2+concentration and was enhanced at higher baseline [Ca2+]i, consistent with activation of TRPM2 channels in the cell membrane. These results provide evidence for the functional expression of TRPM2 channels in dopaminergic SNc neurons. Given the involvement of oxidative stress in degeneration of SNc neurons in Parkinson's disease, further studies are needed to determine the pathophysiological role of these channels in the disease process.