Distribution of catecholaminergic neuronal systems in the canine medulla oblongata and pons

The distribution of catecholamine-containing neurons, fibers, and varicosities in the brainstem of both adult and juvenile dogs was mapped in detail with glyoxylic acid histofluorescence. Four separate groups of catecholamine-fluorescent neurons were identified within the canine medulla and pons in locations comparable to the A1, A2, A5, and A6 regions reported in other species. However, aspects of the pattern and density of the catecholaminergic neuronal systems appeared to be unique to the dog. The A1 neurons of the caudal ventrolateral medulla were much more scattered than in rats or rabbits, but relatively similar to cats. In the A2 region of the dorsomedial medulla, catecholaminergic cells and fibers were uniquely distributed compared to other species: fluorescent neurons were scattered only within the dorsal motor nucleus of the vagus, and a distinctive pattern of fibers and varicosities outlined the nucleus of the solitary tract and dorsal motor nucleus of the vagus. The A5 neurons of the rostral ventrolateral medulla appeared at the rostral limit of the A1 region. Fluorescent A5 cells were more sparse than in rats or primates, and were patterned similarly to cats and rabbits. The canine A6 region contained the most extensive and dense grouping of catecholamine neurons and was similar in pattern to the rabbits but less extensive than that seen in cats or primates. An ascending catecholaminergic fiber pathway was traced through the central tegmental field of the canine medulla and pons, with features similar to the primate. The present study provides the first description of the catecholaminergic neuronal systems of the canine medulla.