Dopamine transporter-immunoreactive neurons decrease with age in the humansubstantia nigra

Unbiased disector stereologic cell counting was applied to sections from th e human substantia nigra that were immunostained by using a monoclonal anti body against the dopamine transporter (DAT). This antibody was found to pen etrate the full thickness of the stained section. Quantification of the num ber of DAT immunostained neurons was performed in human cases stratified in to three age groups, young (ages 0-49 years), middle aged (ages 50-69 years ), and aged (ages 70-85 years). The number of DAT-immunoreactive nigral neu rons was normalized for each case by constructing a ratio of the number of DAT-containing neurons to total number of neuromelanin-containing cells in each subject's sample. Three types of DAT nigral neurons were seen: type 1, intensely stained; type 2, lightly stained; and type 3, DAT-immunonegative neuromelanin-containing perikarya. By 50 years of age, the number of type 1 neurons decreased significantly (P < 0.0001), whereas the number of type 2 neurons increased with age (P < 0.0001). Type 3 neurons also increased wi th age (P < 0.01), although less robustly than type 2 neurons. Type 1 neuro ns decreased by 11.2% per decade, and the total number of nigral neurons (t ypes 1-3) decreased by 6.7% per decade. Relative to the young group, there were 75% and 88% reductions in type 1 neurons in the middle-aged and aged g roups, respectively. This contrasts with the 35% and 41% reductions in tota l number of neuromelanin-containing neurons seen in middle-aged and aged gr oups, respectively. The young group had significantly more type I neurons a nd fewer type 2 neurons compared with middle-aged and aged participants. Pa st-hoc analyses indicated that the young group had significantly fewer type 3 neurons compared with middle-aged and aged participants. These findings demonstrate an age-related reduction in the number of substantia nigra DAT- immunoreactive neurons. Therefore, insight into the mechanisms regulating t he rate of DAT synthesis may aid in our understanding of the decline of DAT s with aging and its functional significance. (C) 1999 Wiley-Liss, Inc.