Tyrosine hydroxylase and dopamine transporter expression in residual dopaminergic neurons: potential contributors to spontaneous recovery from experimental Parkinsonism

1-Methyl-4-phenyl-1,2,3,6-tetrahyrdropyridine (MPTP)-exposed cats develop severe Parkinsonism that spontaneously resolves in 4-6 weeks. The present study examined the extent to which compensatory changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene and protein expression may underlie this behavioral recovery. In normal cats, TH and DAT protein levels were higher in the dorsal vs. ventral striatum. Expression of DAT and TH mRNA was higher in substantia nigra pars compacta (SNc) than in the ventral tegmental area (VTA). In symptomatic parkinsonian animals, DAT and TH protein levels were significantly decreased in all striatal areas studied. TH and DAT mRNA expression in residual SNc neurons were decreased a mean 32% and 38%, respectively. DAT gene expression in residual VTA neurons in symptomatic animals was decreased 30% whereas TH gene expression was unaffected. In spontaneously recovered cats, TH protein levels were significantly higher than the levels in symptomatic cats only in the ventral striatum, whereas no increase in DAT protein levels were observed in any striatal area. Residual neurons in most ventral mesencephalic regions of recovered cats had increased TH mRNA expression but not increased DAT gene expression, compared with symptomatic animals. Thus, increased TH protein and mRNA and suppression of DAT protein and mRNA expression in the striatum and ventral mesencephalon were associated with functional recovery from MPTP-induced parkinsonism.