Localization of l-DOPA uptake and decarboxylating neuronal structures in the cat brain using dopamine immunohistochemistry

The present study examined dopamine-immunoreactive neuronal structures using immunohistochemistry in conjunction with an anti-dopamine antiserum, following injection of l-3,4-dihydroxyphenylalanine (l-DOPA) with or without an inhibitor of monoamine oxidase (Pargyline) in the cat brain. l-DOPA injection made it possible to detect dopamine immunoreactivity in presumptive serotonergic and noradrenergic cell bodies and axons. Weak to moderate dopamine immunoreactivity was observed in non-aminergic cells (possibly so-called “D” cells containing aromatic l-amino acid decarboxylase (AADC)) in several hypothalamic, midbrain, pontine and medullary nuclei. Intense dopamine immunoreactivity became visible in a large number of cells and axons (possibly containing AADC) with wide distribution in the brain following administration of l-DOPA with Pargyline. AADC is most likely active in cells and axons that take up l-DOPA, where it decarboxylates the l-DOPA to dopamine. However, newly synthesized dopamine in such cells is rapidly oxidized by monoamine oxidase.