BACKGROUND: Activity-dependent neuroprotective protein (ADNP) is essential for brain formation and neuronal survival. It is possible that intracellular alpha-synuclein (?-syn) inclusions may be due to, or may cause, down-regulation of ADNP expression. OBJECTIVE: This study were to determine whether ADNP protein levels are altered in nigral dopaminergic neurons, establish whether ADNP alterations are associated with ?-syn accumulation, and evaluate potential correlations between levels of ADNP expression and axonal transport motor proteins in sporadic and experimental Parkinson's disease (PD). METHODS: Twenty human brains from PD (n?=?12) and age-matched controls (n?=?8) and sixteen rat brains received ?-synuclein gene (n?=?8) or empty vector (n?=?8) were analyzed using immunohistochemistry. The number of ADNP labeled nigral neurons were estimated with stereology and the levels of ADNP were determined using densitometry. RESULTS: Compared to age-matched controls, a marked reduction in ADNP protein levels was observed in neuromelanin-containing nigral neurons of PD. Reduced ADNP levels did no relate to the progression of PD symptoms, but instead occurred at early PD stages, before reductions in tyrosine hydroxylase could be detected. Reductions in ADNP were also positively correlated with alterations in axonal transport motor protein. Reductions in ADNP levels were recapitulated in a rat model of PD based on viral over-expression of human wild-type ?-synuclein, suggesting that ADNP reductions in PD are a direct result of ?-synuclein overexpression. CONCLUSION: These findings demonstrate that the down-regulation of protein ADNP is an early pathological alteration and may contribute to dopaminergic neurodegeneration in PD.