BDNF and Parkinson disease: Previous research has shown that a decrease in overall BDNF levels has been associated with neurodegenerative disorders such as Alzheimer's and Parkinson's disease (Phillips et al., 1991; Levivier et al., 1995; Connor et al., 1997; Parain et al., 1999; Howells et al., 2000; Michalski and Fahnestock, 2003).