DESCENDING PROJECTIONS OF THE SONGBIRD NUCLEUS ROBUSTUS ARCHISTRIATALIS

The descending, efferent projections of nucleus robustus archistriatal is were investigated in male zebra finches and greenfinches with injec tions of either biotinylated dextran amine or cholera toxin B-chain co njugated to horseradish peroxidase. The results show that in addition to the well-known projections to the tracheosyringeal motor nucleus an d the dorsomedial nucleus of the intercollicular complex, there are ot her projections of comparable density to the ipsilateral nucleus ambig uus and nucleus retroambigualis. Within nucleus ambiguus, robustus axo ns terminate in close proximity to laryngeal motoneurons which were re trogradely labelled in the same bird by injections of cholera B-chain into the laryngeal muscles; and within nucleus retroambigualis robustu s axons terminate in relation to bulbospinal neurons previously shown to project to regions of spinal cord containing motoneurons innervatin g abdominal expiratory muscles (J.M. Wild, Brain Res. 606:119-124, 199 3). These projections of nucleus robustus thus seem well placed to coo rdinate syringeal, laryngeal, and expiratory muscle activity during vo calization. Other relatively sparse, but distinct, projections of nucl eus robustus were found to nucleus dorsolateralis anterior thalami, pa rs medialis, to a narrow region between the superior olivary nucleus a nd the spinal lemniscus, and to the rostral ventrolateral medulla. Neu rons in these last two locations were retrogradely labelled bilaterall y following injections of cholera B-chain into nucleus retroambigualis of one side. Together with sparse contralateral projections of nucleu s robustus to all brainstem targets receiving ipsilateral projections, potential pathways are thus identified by which the respiratory-vocal activity controlled by one side of the lower medulla can be influence d by the nucleus robustus of either side, thereby possibly bringing ab out bilateral coordination of respiratory-vocal output. (C) 1993 Wiley -Liss, Inc.