Increases in TH immunoreactivity, neuromelanin and degeneration in the substantia nigra of middle aged mice

The dopaminergic (DArgic) neurons in the substantia nigra (SN) are particularly vulnerable to oxidative stress and during aging. The present study was undertaken in order to determine whether aging is associated with changes in the DA synthesizing enzyme tyrosine hydroxylase (TH) as early as middle age by comparing 7- and 50-week-old mice. Quantitative analysis, performed by measuring the density of TH-immunopositive neurons, revealed that in the older animals, the number of DArgic neurons was decreased by 10% while TH immunodensity was 24 ± 3% higher compared to the younger animals. Based on Masson-Fontana staining for neuromelanin (NM), the number of NM-containing neurons in the SN and the volume of NM per NM-positive neurons in the older animals were 5- and 11.6 ± 0.1-fold higher, respectively. The silver stain-positive fibers, indicative of degeneration, were higher in the SN and striatum of the older animals, with the optical density 3.3 ± 0.1- and 5.4 ± 0.2-fold of the younger animals. The present study demonstrates that aging is associated with changes in the DA synthesizing enzyme TH as early as middle age and that this is associated with dramatic increases in the number of NM-containing neurons, volume of NM per cell, and degeneration.