BDNF and major depressive disorder: However, in our experimental conditions, in both individual models and in the model of the co-occurrence of hypothyroidism and endogenous depression, this pathway does not seem to be directly or indirectly affected because we did not observe any changes in markers of oxidative stress (MDA and 4-HNE) or hippocampal BDNF and NGF expression, which suggests that learning and memory impairment does not occur through oxidative stress induction.