*4.3. Measurement of Cell Viability*

The cytotoxicity of the MPP<sup>+</sup>- and H2O2-treated SH-SY5Y cells was measured with an MTT assay, involving the reduction of formazan crystals [41]. SH-SY5Y cells (2.2 <sup>×</sup> 104 cells/mL) were pre-treated in 24-well plates with NE 52-QQ57 (100 nM) and left in serum-free cell culture media for 1 h; this was followed by stimulation with or without MPP<sup>+</sup> (1 mM) or H2O2 for 24 h. After MPP<sup>+</sup> or H2O2 stimulation, the medium was replaced with 0.5 mg/mL MTT solution, before the plates were incubated for 3 h at 37 ◦C. The supernatant was carefully removed and the formazan crystals were dissolved in dimethyl sulfoxide (DMSO) by gentle shaking for 10 min. A microplate reader (Molecular device, Sunnyvale, CA, USA) was utilised to measure the absorbance at 550 nm.
