catecholamines (儿茶酚胺) is the collection name for three neurotransmitters: epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine.
L-DOPA is its precursor. Its corresponding drug is called levodopa.
L-DOPA is produced from the amino acid L-tyrosine by the enzyme tyrosine hydroxylase (TH).
Tyrosine cannot pass the BBB (blood brain barrier), but L-DOPA can.
L-DOPA can be converted to dopamine, which can be further converted to epinephrine (adrenaline), norepinephrine (noradrenaline).
Dopaminergic neurons can be divided into 11 cell groups according to where they located, using histochemical fluorescence method. It includes A8-A16, Aaq and Telencephalic group. Among that, A9 is where the substantia nigra pars compacta (SNpc) refers.
Number of TH-positive neurons in SN declines with age, while a-synuclein level increases with age. This inverse relationship is also seen in the surviving DA neurons of PD patients.
And it shows that the down-regulation of TH is linked to a reduction of expression of transcription factor called Nurr1 (orphan nuclear receptor, encoded by gene NR4A2).
Nurr1 is shown to regulate a category of nuclear-encoded mitochondrial genes. (http://www.ncbi.nlm.nih.gov/pubmed/23341612)