Medullary projections to the vagus nerve and posterolateral hypothalamus

Background: Vagal visceromotor reflexes are dependent upon reciprocal neural connections existing between the medulla and the hypothalamus. Medullohypothalamic neurons may provide feedback cues to the hypothalamus regarding the activity of vagal motor neurons. As yet, however, studies investigating the spatial relationships between medullohypothalamic neurons and vagal motor neurons have not been performed. Methods: A variety of retrogradely transported tracers were used for the purpose of mapping the relative locations of medullovagal and medullohypothalamic neurons. Tracers were injected into the cervical vagus nerve and/or the posterolateral hypothalamus, and subsequently the retrogradely labeled medullary neurons were plotted. Results: Labeling of the two neuronal populations was primarily observed within the ventrolateral and dorsomedial medulla. Within the ventrolateral medulla, medullovagal neurons were found within the retrofacial nucleus and nucleus retroambiguus, whereas medullohypothalamic neurons were located subjacent to these nuclei. Within the dorsomedial medulla, labeling of the two neuronal populations was primarily limited to the vagal‐solitary complex. At this location medullovagal neurons were found within the dorsal vagal nucleus, whereas medullohypothalamic neurons were largely confined to the caudal aspect of the solitary nucleus. Conclusions: Because of the spatial proximity existing between medullovagal and medullohypothalamic neurons, it is suggested that functional interrelationships may exist between these two neuronal populations. Specifically, it is suggested that the medullohypothalamic neurons identified in this study may support vagal‐related functions by providing feedback cues to the posterolateral hypothalamus. © 1995 Wiley‐Liss, Inc.