The immunocytochemical localization of the biosynthetic enzymes‐tyrosine hydroxylase (TH), dopamine‐beta‐hydroxylase (DBH), and phen‐ylethanolamine‐N‐methyltransferase (PNMT)‐was used to determine the cytological features and precise neuroanatomical location of catecholami‐nergic neurons in the medulla oblongata of rat. Perikarya labeled with TH were detected in two bilaterally symmetrical columns located in the ventrolateral and dorsomedial medulla. The distribution and the number of neuronal perikarya containing TH were the same as those containing DBH, except in the dorsal motor nucleus of the vagus at the level of the area postrema where the number of neurons immunocytochemically labeled for TH was considerably greater than those labeled for DBH. The detection of perikarya which show immunoreactivity for TH, used in the biosynthesis of dopamine, noradrenaline, and adrenaline, but not DBH, which converts dopamine to noradrenaline, suggests the existence of dopamine‐synthesiz‐ing neurons in the medulla. Perikarya labeled with PNMT, used in the biosynthesis of adrenaline, were localized in more restricted regions corresponding to rostral subsets of the dorsal and ventral groups labeled for TH and DBH. Counts of neurons immunpcytochemically labeled for TH or PNMT were obtained in order to determine the relative ratio of neurons which contain the enzymes necessary for the synthesis of dopamine, noredrenaline, or adrenaline at various levels of the medulla. At the most caudal levels no PNMT labeled neurons were detected. Further rostral, PNMT‐labeled neurons were first detected in the ventrolateral medulla. At the level of the area postrema, the number of PNMT‐labeled neurons in the ventrolateral medulla was approximately half of the number of cells showing immunoreactivity for TH. In contrast, few PNMT‐labeled cells were detected in the dorsomedial medulla at the level of the area postrema compared to many neurons labeled for TH. At rostral medullary levels, in both the ventrolateral and the dorsomedial regions, the number of neurons labeled for TH and PNMT was essentially the same. Thus most, if not all, of the catecholaminergic neurons in the rostral medulla have PNMT, necessary for the synthesis of adrenaline.
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