Synthesis of Dopamine by Non-Dopaminergic Neurons Containing Aromatic Amino Acid Decarboxylase in the Suprachiasmatic Nucleus of Rats in Ontogeny

—The suprachiasmatic nucleus (SCN) regulates functions in accordance with circadian rhythms thanks to the pacemaker neurons present in it and afferents modulating their activity. One type of afferent is nerve fibers containing only tyrosine hydroxylase (TH) and secreting (L-dihydroxyphenylalanine) L-DOPA. The aim of this work was to test the hypothesis that L-DOPA comes from TH-containing fibers into neurons containing only aromatic L-amino acid decarboxylase (AAAD), where dopamine is synthesized. The synthesis of dopamine was evaluated by its content in SCN in intact rats on the 10th postnatal day (P10) and on P60, as well as by the total dopamine content in SCN and in the incubation medium after incubation of this nucleus in the medium with or without L-DOPA. It was shown that in intact rats SCN contains dopamine at the same concentration at P10 and P60. Some of the experiments with incubation of SCN slices were performed with rats injected with neurotoxins of monoaminergic neurons to exclude the synthesis of dopamine from L-DOPA in monoaminergic structures. At P10, during incubation of SCN in L-DOPA-containing medium, an increase in dopamine synthesis was observed equally in animals in control (normal) and after exposure to neurotoxins, which indicates synthesis of dopamine only in monoenzymatic AAAD-containing neurons. In a similar experiment at P60, the addition of L-DOPA into the incubation medium also led to an increase in the synthesis of dopamine, however, in animals exposed to neurotoxins to a lesser extent than in the control. This means that in adult animals, dopamine is synthesized in SCN mainly in monoenzymatic AAAD-containing neurons and to a lesser extent in monoaminergic structures. Thus, it was shown that dopamine in SCN is synthesized in monoenzymatic AAAD-containing neurons from L-DOPA secreted by monoenzymatic TH-containing fibers.