NONDOPAMINERGIC CATECHOLAMINERGIC NEURONS OF MESENCEPHALIC AND MEDULLARY NUCLEI CONTAIN DIFFERENT LEVELS OF DOPAMINE

The present study addresses the question whether metabolic dopamine ca n be immunocytochemically detected within nondopaminergic catecholamin ergic axonal fibers. For this purpose, confocal microscopy was used to analyze sections treated for the double fluorescence immunostaining o f dopamine and either noradrenaline or phenylethanolamine-N- methyltra nsferase (the enzyme in adrenergic neurons that converts noradrenaline into adrenaline). Our data demonstrate that throughout the brain and spinal cord, the majority of the axonal fibers that reacted with the a nti-phenylethanolamine-N-methyltransfer antibodies also exhibited fain t to intense dopamine immunoreactivity. Similarly noradrenaline and do pamine immunoreactivities were frequently colocalized within axonal fi bers innervating brain and spinal cord regions that receive a dense in nervation from medullary noradrenergic neurons. On the contrary, dopam ine was rarely detected within noradrenaline-immunoreactive fibers in those regions where the noradrenergic innervation essentially arises f rom noradrenergic neurons of the locus coeruleus. A similar differenti al dopamine immunostaining was observed in the corresponding neuronal perikarya of the medulla oblongata and the locus coeruleus. These data indicate that two types of non-dopaminergic catecholaminergic neurons can be distinguished according to their content in dopamine: (i) the noradrenergic and adrenergic neurons located in the medulla oblongata, whose cell bodies and axons contain high concentrations of metabolic dopamine and (ii) the noradrenergic neurons located in the mesencephal on, which contain low levels of metabolic dopamine.