THE BRAIN-STEM NETWORK INVOLVED IN COORDINATION OF INSPIRATORY ACTIVITY AND CHOLINERGIC OUTFLOW TO THE AIRWAYS

The respiratory rhythm modulates cholinergic outflow to the tracheal s mooth muscle through the parasympathetic nerves. To determine the basi s of this modulation, we combined the retrograde tracer technique to i dentify bulbospinal cells projecting to phrenic motoneurons, and the t ransneuronal labeling method to visualize medullary neurons that inner vate airway-related vagal preganglionic cells. Following injections of fluorogold into the ventral horns of the cervical spinal cord and inj ections of pseudorabies virus (PRV) into the wall of the extrathoracic trachea of superior cervical ganglioctomized Sprague-Dawley rats. A l arge number of the double-labeled cells were identified along the vent ral aspect of the medulla oblongata. Most frequently, double-labeled n eurons were seen in the medial tegmental field, particularly in the pa rapyramidal region, within the gigantocellular nuclei, and the caudal raphe nuclei. Less frequently, double-labeled neurons were found in th e ventrolateral medulla. No double-labeled cell was observed in the do rsal aspect of the medulla oblongata. This study indicates that a subs et of medullary neurons that project to phrenic motoneurons also inner vate the airway-related vagal preganglionic cells, allowing the coupli ng of inspiratory activity and parasympathetic outflow to the airways.