BDNF and early-onset autosomal dominant Alzheimer disease: BDNF levels have been hypothesized to be reduced in Alzheimer's disease, Parkinson's disease, and Huntington's disease.21–23 BDNF depletion has also been suggested to play a role in glaucoma,24 with evidence for impaired retrograde BDNF transport from the brain to the retina demonstrated in experimental models of ocular dysfunction.25–27