BDNF and major depressive disorder: Alteration in monoamines (serotonin and noradrenaline), increasing cortisol levels and decreasing levels of brain-derived neurotrophic factor (BDNF), could explain the association between depression and diabetes in some of the abnormalities documented in diabetic patients and animal models (Oladeji and Gureje, 2013; Lenart et al., 2016).