INS and major depressive disorder: Many neurological diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), epilepsy, bipolar disorder (BD), schizophrenia (SZ), and major depressive disorder (MDD) are characterized by cerebral glucose hypometabolism, insulin resistance, neurotransmitter imbalances, mitochondrial dysfunction, oxidative stress, and inflammation as potential causative factors [5,6].