Identification of catecholaminergic cell groups in the brainstem of the canary, zebra finch, white-throated sparrow and budgerigar by tyrosine hydroxylase immunocytochemistry

In Passeriformes and Psittaciformes, the learning and production of complex learned vocalizations is controlled by a network of telencephalic, diencephalic and mesencephalic nuclei, the so-called song-control system. Specialized telencephalic song control nuclei such as the high vocal center, nucleus robustus archistriatalis and area X in the basal ganglia receive dense and discrete catecholaminergic inputs. Catecholaminergic fibers also innervate the telencephalon of other birds and other vertebrate species but there appear to be unique specializations of the catecholaminergic inputs in songbirds. In this paper the distribution of tyrosine hydroxylase-immunoreactive cell groups in the brainstem is described in 4 avian species that are known to be vocal learners. The goal of this work is to identify whether novel cell groups are present in the brainstem that may give rise to the specialized catecholaminergic projections in the telencephalon of these vocal learners. These brainstem cell groups are known to be the source of most telencephalic catecholaminergic input in birds and other vertebrates. Three songbird species, the zebra finch, the canary and the white-throated sparrow and one psittaciform, the budgerigar were analyzed. Immunocytochemical analysis identified in the brainstem of the 4 species the same groups of tyrosine hydroxylase-immunoreactive cells that were previously described in the brain of other non-song birds. These were located in the area ventralis of Tsai (A10), around the substantia nigra (A9) and its caudal extension (retroruberal field, A8), and in the nucleus ceruleus and subceruleus (A6). No additional cell group could be detected in these species by comparison with the other species of non oscines studied previously. This suggests that both in song birds and in budgerigars specialized innervations by catecholaminergic neurones of the telencephalic song control nuclei are not associated with the evolution of novel catecholaminergic cell groups in the brainstem as compared to those that are present in species that do not display these specializations.