Neurodegenerative diseases comprehend a series of diseases characterized by progressive and apparently irreversible loss of neurons, or loss of neuronal functionality. Although each of these diseases has certain characteristics that affect different areas of the brain with different outcomes, the common denominator is that the brain neurons undergo degeneration and subsequent death. The study of the causes of these diseases is still ongoing, and unfortunately it is not easy to find the correct path to follow. As an example, it is worth to mention the case of Alzheimer’s disease (AD) where for decades the focus was on the “amyloid cascade hypothesis” without reaching any definitive conclusions. Naturally this hypothesis remains valid today but sometimes it is necessary to undertake other directions. How, for example, to explain the fact that post-mortem studies have clearly demonstrated the presence of amyloid plaques and neurofibrillary tangles in subjects without AD symptoms? It looks like something is missing to the general opinion on the mechanisms of these diseases. Besides these general considerations and going into further detail, among many therapeutic approaches proposed and/or adopted, a common denominator is the attempt to restore, or at least limit, the damage of neuronal function. Indeed, this is currently the most logical approach to follow, because as we said before, the causes of these diseases are still unknown. In this regard, we have recently proposed a cognitive rehabilitation protocol in patients with Parkinson's disease (PD), in which we used as a possible biomarker or readout of the effects of cognitive training the serum levels of a member of a class of proteins acting on neuronal function: the neurotrophin brain-derived neurotrophic factor (BDNF).