BDNF and depressive symptom measurement: The specific tyrosine kinase receptor TRKB is the main receptor of brain-derived neurotrophic factor (BDNF) and plays an important role both in neuron survival and the growth and differentiation of new neurons and synapses; it can thus be seen, on the scale of overall brain function, as an agent that promotes neuron plasticity, the increase of which, through a combination of drug and talk therapies, is currently seen as one of the most promising routes, following a mechanistic biophysical paradigm, for the treatment of clinical depression.