Moreover, the connection between peripheral and central BDNF is still currently unknown, and reduced blood BDNF levels have been reported in a multitude of neuropsychiatric and neurodegenerative diseases, as well as in medical diseases, making circulating BDNF a non-specific biomarker [191,192,193,194,195,196,197,198,199,200]. This evidence concerns the gene BDNF and neurodegenerative disease.