NGF and neurodegenerative disease: Many studies have shown that NGF is downregulated in the brain tissue and cerebrospinal fluid (CSF) of patients with AD, which is also observed in patients with Parkinson’s disease, another neurodegenerative disease, indicating that decreased NGF levels might be associated with nerve tissue hypofunction and degradation (Santaella et al. 2020; Williams et al. 2006).