The location of chronotropic cardioinhibitory vagal motoneurons in the medulla of the rabbit

Vagal preganglionic motoneurons originating in nucleus ambiguus (NA) and dorsal vagal nucleus (DVN) were identified via retrograde labeling with horseradish peroxidase (HRP). DVN and NA were then explored for cardiovascular responsive sites using microstimulation. Stimulation within DVN from slightly caudal to obex to 3.00 mm rostral to obex produced a primary bradycardia (n = 15, X = -123 bpm). Stimulation within NA from slightly rostral to obex to 1.5 mm caudal to obex produced a similar primary bradycardia (n = 15, X = -127 bpm). Extracellular recordings were made from 7 cells in DVN and 10 cells in NA in regions producing maximal bradycardia to electrical stimulation. These cells were antidromically activated by cervical vagus nerve (VN) stimulation, increased their firing rates to systemic injection of phenylephrine (PE), revealed an expiratory rhythm, showed an increase in firing rate coinciding with spontaneous and elicited decreases in heart rate, had conduction velocities in the A-delta and B-fiber range, and produced bradycardia upon stimulation through the recording electrode with thresholds as low as 4 μA. The data indicate that in rabbits, chronotropic cardioinhibitory vagal motoneurons are discretely localized on the lateral, caudal portions of DVN and NA between 0.5 mm caudal and 1.5 mm rostral to obex.