Identification of the origin of catecholaminergic inputs to HVc in canaries by retrograde tract tracing combined with tyrosine hydroxylase immunocytochemistry

The telencephalic nucleus HVc (sometimes referred to as the high vocal cent er) plays a key role in the production and perception of birdsong. Although many afferent and efferent connections to this nucleus have been described , it has been clear for many years, based on chemical neuroanatomical crite ria, that there are projections to this nucleus that remain undescribed. A variety of methods including high performance liquid chromatography, immuno histochemistry and receptor autoradiography have identified high levels of catecholamine transmitters, the presence of enzymes involved in the synthes is of catecholamines such as tyrosine hydroxylase and a variety of catechol amine receptor sub-types in the HVc of several songbird species. However, n o definitive projections to HVc have been described from cells groups known to synthesize catecholamines. These projections were analyzed in the prese nt study by retrograde tract tracing combined with immunocytochemistry for tyrosine hydroxylase. The origin of the catecholaminergic inputs to HVc wer e determined based exclusively on birds in which injections of the retrogra de tracer (latex fluospheres) were confined within the cytoarchitectonic bo undaries of the nucleus. Retrogradely transported latex fluospheres were fo und mainly in cells of two dopaminergic nuclei, the mesencephalic central g ray (All) and, to a lesser extend, the area ventralis of Tsai (A10; homolog ous to the ventral tegmental area of mammals). A few retrogradely-labelled cells were also found in the noradrenergic nucleus subceruleus (A6). Most o f these retrogradely-labelled cells were also tyrosine hydroxylase-positive . Other catecholaminergic nuclei were devoid of retrograde label. These dat a converge with others studies to indicate that HVc receives discrete dopam inergic and noradrenergic inputs. These inputs may influence the steroid re gulation of HVc, attentional processes related to song and modulate sensory inputs to the song system. (C) 2000 Elsevier Science B.V. All rights reser ved.