Figure 1. DOR activation attenuated hypoxic and/or MPP+ insults induced mitochondrial membrane potential depolarization and mitochondrial dysfunction. (A) PC12 cells were exposed to hypoxia at 1% O2 for 48 hrs, the mitochondrial membrane potential was measured using TMRM reagent. C: normoxic control. H: hypoxia. H+U: DOR was activated using UFP-512 in hypoxic conditions. H+U+N: PC12 cells were treated with UFP-512 plus naltrindole at the same time in hypoxic conditions. H+N: PC12 cells were treated with DOR antagonist naltrindole alone in hypoxic condition. N=3 in each group. **p<0.01 vs. C; Δp<0.05 vs. H; Note that hypoxia significantly decreased the red fluorescent intensity compared to the control, suggesting the collapse of mitochondrial membrane potential. The application of DOR agonist UFP-512 attenuated these changes and the flow cytometer results were consistent with the fluorescence microscope observation. (B) PC12 cells were exposed to 1.0 mM MPP+ for 24 hrs. C: control. M: MPP+. M+U: DOR was activated using UFP-512 and exposed to MPP+. M+U+N: PC12 cells were treated with UFP-512 plus naltrindole and exposed to MPP+. M+N: PC12 cells were exposed to naltrindole along with MPP+. N=3 in each group. **p<0.01 vs. C; Δp<0.05, ΔΔp<0.01 vs. M. Note that MPP+ insults also caused a depolarization of mitochondrial membrane potential, while activating DOR using UFP-512 significantly reversed these destructive changes induced by MPP+ insults. In contrast, applying DOR antagonist naltrindole alone further aggravated the collapse of mitochondrial membrane potential under MPP+ insults. The results measured by flow cytometer were consistent with the florescence observation. (C) PC12 cells were exposed to 1% O2 for 48 hrs or 1.0mM MPP+ for 24 hrs. N=3 in each group. **p<0.01 vs. C; Δp<0.05, ΔΔp<0.01 vs. H or M. Note that hypoxia and/or MPP+ caused a significant decrease in ATP generation, while DOR activation restored the capacity of mitochondria in ATP production.