2.2. Cell Viability Assay

The 3-(3,4-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay measured cell viability by assessing the ability of mitochondria to metabolize the yellow tetrazolium salt MTT to purple insoluble crystals of formazan [38,39]. Both SH-SY5Y and rat primary cortical neurons were plated on 12-well plates, and at the end of the experimental procedure, the cells were incubated with 0.5 mL of MTT solution (0.5 mg/mL in PBS) in the dark at 37 °C and a 5% CO₂ atmosphere in a humidified incubator. After 1 h, the cells were washed with PBS, and the produced formazan crystals were dissolved in 0.5 mL of DMSO [15,40]. A decrease in mitochondrial activity resulted in a reduction in the amount of formazan produced and therefore in the absorbance value. Absorbance was read at a wavelength of 540 nm using a Victor Multilabel Counter plate reader (Perkin Elmer, Waltham, MA, USA). The results were expressed as percentages of the control value.