Transient receptor potential vanilloid subtype 1 mediates cell death of mesencephalic dopaminergic neurons in vivo and in vitro

Intranigral injection of the transient receptor potential vanilloid subtype 1 (TRPV1; also known as VR1) agonist capsaicin ( CAP) into the rat brain, or treatment of rat mesencephalic cultures with CAP, resulted in cell death of dopaminergic (DA) neurons, as visualized by immunocytochemistry. This in vivo and in vitro effect was ameliorated by the TRPV1 antagonist capsazepine (CZP) or iodoresiniferatoxin, suggesting the direct involvement of TRPV1 in neurotoxicity. In cultures, both CAP and anandamide ( AEA), an endogenous ligand for both TRPV1 and cannabinoid type 1 (CB1) receptors, induced degeneration of DA neurons, increases in intracellular Ca2+ ([Ca2+](i)), and mitochondrial damage, which were inhibited by CZP, the CB1 antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1( 2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) or the intracellular Ca2+ chelator BAPTA/AM. We also found that CAP or AEA increased mitochondrial cytochrome c release as well as immunoreactivity to cleaved caspase-3 and that the caspase-3 inhibitor z-Asp-Glu-Val-Asp-fmk protected DA neurons from CAP- or AEA-induced neurotoxicity. Additional studies demonstrated that treatment of mesencephalic cultures with CB1 receptor agonist (6aR)- trans 3-(1,1-dimethylheptyl)-6a,7,10,10a-tetrahydro-1-hydroxy-6,6- dimethyl-6H-dibenzo[b, d] pyran-9-methanol (HU210) also produced degeneration of DA neurons and increases in [Ca2+](i), which were inhibited by AM251 and BAPTA/AM. The CAP-, AEA-, or HU210-induced increases in [Ca2+](i) were dependent on extracellular Ca2+, with significantly different patterns of Ca2+ influx. Surprisingly, CZP and AM251 reversed HU210- or CAP- induced neurotoxicity by inhibiting Ca2+ influx, respectively, suggesting the existence of functional cross talk between TRPV1 and CB1 receptors. To our knowledge, this study is the first to demonstrate that the activation of TRPV1 and/or CB1 receptors mediates cell death of DA neurons. Our findings suggest that these two types of receptors, TRPV1 and CB1, may contribute to neurodegeneration in response to endogenous ligands such as AEA.