Serotonergic neurons in the brainstem of the wallaby, Macropus eugenii

The organisation and cytoarchitecture of the serotonergic neurons in a dipr otodont marsupial were examined by using serial sections of the brainstem p rocessed for serotonin immunohistochemistry and routine histology. The topo graphic distribution of serotonergic neurons in the brainstem of the adult wallaby (Macropus eugenii) was similar to that of eutherian mammals, Seroto nergic neurons were divided into rostral and caudal groups, separated by an oblique boundary through the pontomedullary junction. Approximately 52% of the serotonergic neurons in the wallaby brainstem were located in the rost ral midline: nuclei (caudal linear nucleus, dorsal, median, and pontine rap he nuclei and the interpeduncular nucleus), whereas 21% were found in the c audal midline region (nuclei raphe magnus, obscurus, and pallidus). The rem aining serotonergic neurons (27%) were located in more lateral regions such as the pedunculopontine tegmental nuclei, the supralemniscal nuclei (B9 gr oup), and the ventrolateral medulla. The largest serotonergic group, the do rsal raphe, contained one-third of the brainstem serotonergic neurons and s howed five subdivisions, similar to that described in other species. In con trast, the median raphe did not show clear subdivisions. The internal compl exity of the raphe nuclei and the degree of lateralisation of serotonergic neurons suggest that the wallaby serotonergic system is similar in organisa tion to that described for the cat and rabbit. This study supports the sugg estion that the serotonergic system is evolutionally well conserved and pro vides baseline data for a quantitative study of serotonergic innervation of the developing cortex in the wallaby, (C) 1999 Wiley-Liss. Inc.