Toxicity of a Treatment Associating Dopamine and Disulfiram for Catecholaminergic Neuroblastoma SH-SY5Y Cells: Relationships with 3,4-Dihydroxyphenylacetaldehyde Formation

3,4-Dihydroxyphenylacetaldehyde (DOPAL) is formed by the oxidative deamination of dopamine (DA) catalyzed by monoamine oxidases (MAO); then, the aldehyde is oxidized to 3,4-dihydroxyphenylacetic acid (DOPAC) by aldehyde dehydrogenases (ALDH) or reduced to 3,4-dihydroxyphenylethanol (DOPET) by aldose/aldehyde reductases. The present work aimed at evaluating the in vitro toxicity of DOPAL on catecholaminergic neuroblastoma SH-SY5Y cells which accumulate DA. DOPAL synthesis was stimulated by incubating cells with DA and blocking DOPAL oxidation by disulfiram, an irreversible inhibitor of ALDH. As evidenced by MTT reduction assays, DA and disulfiram treatments produced cell losses which increased with time. 10 ^–2 M DA reduced by 40% cell viability after a 1 h treatment, when its TC ₅₀ (concentration reducing viability by 50%) value was 7.3×10 ^‑5 M after a 24 h treatment. For the same treatment periods, TC ₅₀ values for disulfiram were 8×10 ^‑5 and 8.7×10 ^‑7 M, respectively. MTT reduction assay performed after a 24 h treatment followed by a 24 h incubation in a drug-free medium evidenced that the toxicity of 10 ^‑4 M DA or 10 ^‑6 M disulfiram was potentiated by the second drug. HPLC measurements showed that DOPAL was produced at the early stages of the treatment by DA and disulfiram. This was evidenced by the significant increase in the ( DOPAL+ DOPET)/DOPAC ratio observed after a combined 3 h treatment by 10 ^‑4 M DA and 10 ^‑6 M disulfiram. Total contents in DA and DOPAL were greatly reduced at the end of a 15 h treatment, and disulfiram did not significantly enhanced the ( DOPAL+ DOPET)/DOPAC ratio. For both treatment durations, DOPAL and DOPET were detectable only in the extracellular medium. So, these results suggest that an early production of DOPAL could produce delayed toxic effects on SH-SY5Y cells. Production of DOPET and release of DOPAL could be important means for reducing DOPAL concentrations in dopaminergic neurons.