Age-related and regional differences in dopamine transporter mRNA expression in human midbrain.

We assessed the abundance of dopamine transporter messenger RNA (DAT mRNA) in various human midbrain dopamine cell groups using in situ hybridization. The youngest individuals studied (17-23 years of age) exhibited significant regional heterogeneity of DAT expression, both in terms of the number of dopamine neurons expressing DAT mRNA and the abundance of DAT mRNA per cell, with the highest levels of expression evident within the ventral tier of the substantia nigra and lowest expression within the retrorubral field. In the older subjects (65-72 years old) analyzed, DAT mRNA in all regions was reduced to the level seen in the retrorubal field, indicating a positive correlation between initial levels of DAT expression and subsequent age-related reductions, with some regions exhibiting up to a 75% loss of DAT mRNA with age. The age-related decline in DAT mRNA was due to both a decrease in the abundance of DAT mRNA per dopamine cell as well as a decrease in the total number of dopamine cells expressing DAT mRNA, although tyrosine hydroxylase expression was less affected. These results indicate that changes in dopamine neurotransmission seen in normal aging may be related to altered DAT gene expression.