The effect of GPR4 antagonist, NE52-QQ57, on the cellular morphology and on GPR4 mRNA expression in MPP+-treated SH-SY5Y cells that were stably GPR4-OE or GPR4-KO. 24 h serum-starved SH-SY5Y cells were treated for 1 h with NE52-QQ57 followed by either a 24 h incubation or MPP+ (1 mM) stimulation for 24 h in serum-free culture media. (A) The cell viability and morphology of SH-SY5Y, GPR4-OE, and GPR4-KO cells that were pre-treated for 1 h with NE52-QQ57. (B) An RT-PCR highlighting the mRNA expression of GPR4 and GAPDH in SH-SY5Y, GPR4-OE, and GPR4-KO cells pre-treated for 1 h with NE52-QQ57. (C) A semi-quantification of GPR4 mRNA expression relative to GAPDH. This semi-quantification of the respective mRNA expression levels was performed on ImageJ software; GAPDH was utilised as an internal control. Mean ± SEM (n = 3) was employed to express the data. Tukey’s multiple comparison test was performed using a one-way ANOVA. Each * p < 0.05 refers to the sample concentration compared with the same group of non-treated cells.