Increased dopamine synthesis in aging substantia nigra neurons.

Striatal dopamine (DA) and metabolite (DOPAC) levels in 8-, 21-, 52- and 104-week-old C57BL mice were compared with those in 11-week-old mice, 20 days after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment. DA and DOPAC concentrations expressed relative to striatal wet weight did not change with age. In contrast, DA and DOPAC levels increased almost linearly when values were expressed relative to the proportion of remaining tyrosine hydroxylase-positive (TH+) SNc neurons, reaching a 5-7-fold increase per average remaining TH+ neuron by 104 weeks of age (corresponding to neuronal loss of 70%) relative to that found per average neuron in 8-week-old mice. DA and DOPAC levels per average remaining TH+ SNc neuron following MPTP increased for low doses (neuronal losses less than 42%) but decreased for higher doses (55 and 70% losses) but the DOPAC/DA ratio per SNc neuron increased and was 9-fold higher in the 300 mg/kg MPTP-treated animals in comparison to saline controls. Cytoplasmic TH protein (estimated by somal TH immunodensity) was increased by 45% in SNc somata from mice treated with 150 mg/kg MPTP in comparison to saline controls, and by 63% in 104-week-old mice in comparison to 8-week-old animals. This study provides evidence that an average surviving TH+ SNc neuron compensates for the age-related loss of other SNc neurons by increasing dopamine synthesis similar to younger SNc neurons surviving low levels of toxically induced damage and that the compensation may be in part mediated by increased synthesis of TH.