PPA1 and major depressive disorder: We demonstrated that susceptibility but not resilience to stress-induced anhedonia, a core symptom of depression, is associated with significant downregulation of genes encoding mitochondrial respiratory chain complexes, such as NADH dehydrogenase, succinate dehydrogenase, cytochrome bc1, cytochrome c oxidase, F-type and V-type ATPases, and inorganic pyrophosphatase, suggesting suppression of mitochondrial energy.