Studies on the neurotoxicity of 6,7-dihydroxy-1-methyl-1,2,3,4-tetrahydroisoquinoline (salsolinol) in SH-SY5Y cells

We have studied the hypothesis that 6,7-dihydroxy-1-methyl-1,2,3,4-tetrahydroisoquinoline (salsolinol) is neurotoxic. Salsolinol induced a significant time and dose related inhibition of 3[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide; thiazoyl blue (MTT) reduction, and increased lactate dehydrogenase release (LDH) release from human SH-SY5Y neuroblastoma cells, at concentrations within the range of 1-methyl-4-phenylpyridinium (MPP⁺) cytotoxicity, in vitro. Cytotoxicity was not inhibited by the addition of antioxidants, monoamine oxidase inhibitors or imipramine. In confluent monolayers, salsolinol stimulated catecholamine uptake with EC₅₀ values of 17 μM and 11 μM, for noradrenaline and dopamine, respectively. Conversely, at concentrations above 100 μM, salsolinol inhibited the uptake of noradrenaline and dopamine, with IC₅₀ values of 411 μM and 379 μM, respectively. The inhibition of catecholamine uptake corresponded to the increased displacement of [³H]nisoxetine from the uptake₁ site by salsolinol, as the K_i (353 μM) for displacement was similar to the IC₅₀ (411 and 379 μM) for uptake. Salsolinol stimulated catecholamine uptake does not involve the uptake recognition site, or elevation of cAMP, cGMP, or inhibition of protein kinase C. Salsolinol also inhibited both carbachol (1 mM) and K⁺ (100 mM, Na⁺ adjusted) evoked release of noradrenaline from SH-SY5Y cells, with IC₅₀ values of 500 μM and 120 μM, respectively. In conclusion, salsolinol appears to be cytotoxic to SH-SY5Y cells, via a mechanism that does not require uptake₁, bioactivation by monoamine oxidase, or membrane based free radical damage. The effects of salsolinol on catecholamine uptake, and the mechanism of toxicity require further investigation.