Nucleus ambiguus and bulbospinal ventral respiratory group neurons in the neonatal rat

The in vitro brainstem-spinal cord preparation of the neonatal rat is an important model system for studies of the respiratory control system, yet there have not been studies to anatomically characterize respiratory neuron populations in the neonate. Fluorescent retrograde tracers were used to identify bulbospinal neurons of the ventral respiratory group and motoneurons of nucleus ambiguus in neonatal rats. Fluoro-Gold injections into the C4 ventral horn labeled bulbospinal neurons within a densely packed column within the ventrolateral intermediate reticular nucleus from the level of the pyramidal decussation to the facial nucleus. This cell column corresponded closely to the location of the ventral respiratory group of the adult rat. In particular, neurons were labeled in regions corresponding to the rostral ventral respiratory group and the Bötzinger complex. Unlike adult rats, the preBötzinger complex also contained many bulbospinal neurons. Fluoro-Gold-labeled neurons were also located in the medial reticular nuclei, raphe pallidus, and obscurus and spinal vestibular nucleus. As in adult rats, bulbospinal ventral respiratory group neurons overlapped with cervical vagal motoneurons in the external formation, and partially with those in the loose formation, but not with those in the semicompact or compact formation of nucleus ambiguus. These results indicate that the distribution of bulbospinal ventral respiratory group neurons corresponds with that observed in physiological studies of neonatal rats.