Nuclear organization of cholinergic, catecholaminergic, serotonergic and orexinergic systems in the brain of the Tasmanian devil (Sarcophilus harrisii)

• Several neural systems of the Tasmanian devil are described for the first time.
• While the organization of the nuclei of these systems is generally similar to that seen in other mammals, there were some interesting variants, mostly in the cholinergic system not seen in other mammals.
• A specific dorsal subdivision of the cholinergic laterodorsal tegmental nucleus, not seen in other mammals studied to date, was observed in the Tasmanian devil.
• Cholinergic interneurons were observed in the gelatinous layer of the caudal spinal trigeminal nucleus, a feature not previously observed in other mammals.
• The cholinergic neurons of the hypothalamus only possessed a single nucleus, instead of the three typically found in placental mammals and the complete absence of these in monotremes.

This study investigated the nuclear organization of four immunohistochemically identifiable neural systems (cholinergic, catecholaminergic, serotonergic and orexinergic) within the brains of three male Tasmanian devils (Sarcophilus harrisii), which had a mean brain mass of 11.6 g. We found that the nuclei generally observed for these systems in other mammalian brains were present in the brain of the Tasmanian devil. Despite this, specific differences in the nuclear organization of the cholinergic, catecholaminergic and serotonergic systems appear to carry a phylogenetic signal. In the cholinergic system, only the dorsal hypothalamic cholinergic nucleus could be observed, while an extra dorsal subdivision of the laterodorsal tegmental nucleus and cholinergic neurons within the gelatinous layer of the caudal spinal trigeminal nucleus were observed. Within the catecholaminergic system the A4 nucleus of the locus coeruleus complex was absent, as was the caudal ventrolateral serotonergic group of the serotonergic system. The organization of the orexinergic system was similar to that seen in many mammals previously studied. Overall, while showing strong similarities to the organization of these systems in other mammals, the specific differences observed in the Tasmanian devil reveal either order specific, or class specific, features of these systems. Further studies will reveal the extent of change in the nuclear organization of these systems in marsupials and how these potential changes may affect functionality.