Neurons immunocytochemically labeled with the adrenaline‐synthesizing enzyme phenylethanolamine N‐methyltransferase were mapped in the brain of rat pretreated with colchicine. In medulla, immunoreactive cells in the C1 and C2 groups were distributed in a more complex manner than described previously. C1 neurons were identified in the reticular formation of ventrolateral medulla and were organized into two populations:(1) a cell column extending throughout the ventrolateral medulla, and lying ventral to the ambiguus cell group and either dorsal to the precerebellar lateral reticular nucleus or interposed between its two subdivisions; (2) a rostral cell cluster forming medial to the column at caudal levels and enlarging close to and in parallel with the ventral surface of the rostral ventrolateral medulla. A large proportion of cells and processes of the rostral cell group were oriented medially and ventromedially. Processes of C1 neurons were traced dorsally toward the nucleus tractus solitarii, dorsal motor nucleus, and principal tegmental adrenergic bundle, ventrally toward the ventral surface, laterally toward the trigeminal complex, and medially or ventromedially toward the raphe. C2 neurons were located in the dorsomedial medulla and were subdivided into four distinct populations:(1) neurons in the rostral nucleus paragigantocellularis pars dorsalis (NGCd) and medial longitudinal fasciculus (MLF) were contiguous and similar in size and shape, with their long diameters oriented horizontally or diagonally along several axes; (2) neurons of the periventricular gray were located in a cytoarchitecturally undefined area dorsal to the MLF; these cells were ovoid, smaller, and organized more compactly than those in the NGCd‐MLF; (3) a cell group in the rostromedial nucleus tractus solitarii (NTS) and dorsal motor nucleus overflowed caudally into the intermediate thirds of both structures; and (4) a parvicellular group in the NTS was compactly organized in the dorsolateral NTS and was best developed at the level of the area postrema. Processes of C2 neurons were generally directed sagitally, medially, and laterally along the ventricular floor and ventrally or medially toward the raphe; other fibers arborized and terminated within the NTS and dorsal motor nucleus. In the medulla, local processes were traced from C1 and C2 neurons directly into respective ventral and dorsal parts of the medullary raphe and surrounding intraparenchymal blood vessels. Fibers from these neurons were also followed, respectively, onto the ventral subpial surface and the floor of the fourth ventricle. A new system of hypothalamic neurons expression catalytically active PNMT but none of the other enzymes required for catecholamine synthesis was identified in the lateral and perifornical nuclei and zona incerta; a substantial, although lower, number of cells were also seen in dorsal andmedial hypothalamus. The absence of tyrosine hydroxylase, aromatic L‐amino acid decarboxylase, and dopamine‐B‐hydroxylase in areas populated by the majority of these neurons suggests either that (1) quantities of these enzymes may normally be too low for immunocytochemical detection or that (2) noradrenaline may be taken up by and converted in situ to adrenaline or that (3) PNMT‐stained cells in the hypothalamus may methylate as‐yet‐ unrecognized amines.
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