Endocrine Abstracts

Although non-dopaminergic neurons expressing individual enzymes of dopamine (DA) synthesis are widely distributed in the brain, their functional significance remains uncertain. This study was aimed to evaluate the development and functional significance of the neurons expressing one of the enzymes of DA synthesis, tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC), in the arcuate nucleus of rats in vivo and in vitro by using immunocytochemistry, in situ hybridization, image analysis, confocal microscopy, high performance liquid chromatography with electrochemical detection and the radioimmunoassay. According to our data:

• The number of so-called monoenzymatic TH-expressing or AADC-expressing neurons highly exceeded that of DA-ergic neurons expressing both enzymes in fetuses and neonates, whereas there was a reverse in adult animals;

• Monoenzymatic TH-neurons and AADC-neurons synthesize DA in cooperation: synthesis of L-DOPA from L-tyrosine in TH-neurons is followed by its release and uptake by the neighbouring AADC-neurons, where L-DOPA is further converted to DA;

• The 6-hydroxydopamine(neurotoxin)-induced degeneration of DA-ergic neurons in the arcuate nucleus and the development of hyperprolactinemia were accompanied by the increase of the number of monoenzymatic neurons and cooperative synthesis of DA that is considered as a compensatory reaction.

Thus, non-dopaminergic neurons expressing individual complementary enzymes of the DA synthetic pathway produce this neurotransmitter in cooperation that is a compensatory reaction to the failure of DA-ergic neurons.