BDNF and depressive symptom measurement: Noting that neuroplasticity is affected in depression [50], especially in brain regions such as the hippocampus and prefrontal cortex [50], and in other psychiatric disorders, such as schizophrenia [51] and bipolar affective disorder [52], some studies with animal models [53,54] suggest that the pathophysiology of these disorders is strongly linked to this hypothesis, and there are already studies with drugs that modulate the availability of neurotrophic factors, such as the administration of ketamine, which increased VEGF levels and BDNF in animal model neurons [55].