Molecular aspects of the dopaminergic neurotoxins in relation to tyrosine hydroxylase isoforms

Since the discovery of N-methyl-4-phentl-1, 2, 3, 6-tetrahydropyridine (MPTP) as a parkinsonism-producing, dopaminergic neurotoxin, efforts have been made to find MPTP-like neurotoxins in the brain of parkinsonism patients. Tyrosine hydroxylase (TH) is the key enzyme for dopamine biosynthesis, and has been known to be decreased in the activity and protein content in the nigrosniaral dopaminergic neurons of parkinsonian patients and of the MPTP-induced parkinsonian animals. Humans and monkeys are known to be highly susceptible to MPTP to produce parkinsonism. We have found that humans and monkeys have four isoforms (type -1 ~-4) and two isoforms (type-1 and -2), respectively. Using the quantitative reverse transcription-polymerase chain reaction (RT-PCR), all four types of TH mRNAs were found in the substantia nigra of the control human brains examined. We found that parkinsonian brains had very low levels of the mRNAs of all four TH isoforms and the mRNA of aromatic L-amino acid decarboxylase (AADC) in the sbstantia nigra compared with control brains, while no significant differences were found between schizophrenic brains and normal ones. Since the decrease in AADC mRNA in parkinsonian brain was comparable to that in TH mRNA, the alteration of TH in Parkinson's disease would not be a primary event, but it would reflect the degeneration of dopaminergic neurons in the substantia nigra. We also measured TH type-1 and type-2 mRNA contents in the substantia nigra, locus coeruleus, and adrenal gland of normal monkeys and in MPTP- produced parkinsonian monkeys (macaca fascicularis) by the quantitative RT- PCR method. Marked decrease in TH mRNA type-1 and 2 content were observed specifically in the substantia nigra of the monkeys with MPTP-parkinsonism compared to control monkeys. These results are similar to the data showing marked decreases in TH type -1 ~-4 mRNA content in the substantia nigra of parkinsonian patients, and suggest that MPTP-treated monkeys closely replicate changes in TH isoforms in human Parkinson's disease.