Evidence for direct projections from the nucleus of the solitary tract onto medullary adrenaline cells

The anatomic relationships between projections from the intermediate portion of the nucleus of the solitary tract (NTS) and phenylethanolamine N‐methyl transferaselike‐immunoreactive (PNMT‐LI) cells in the medulla oblongata were analyzed in Sprague‐Dawley rats by using two‐color immunoperoxidase staining combined with the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA‐L). PHA‐L was iontophoretically deposited in the intermediate region of the NTS and the subjacent dorsal motor nucleus of the vagus (DMX) at the level of the middle of the area postrema. Blackstained PHA‐L‐immunoreactive (PHA‐LI) fibers and terminals were present throughout the length of the ipsilateral NTS, where many were seen in close apposition to amber‐stained PNMT‐LI cells belonging to the C2 cell group. Less profuse PHA‐LI projections were present in the contralateral NTS, especially at the level of the PHA‐L deposit. Bundles of smooth fibers emerged from the lateral edge of the DMX and could be followed to the ventrolateral surface of the medulla; these were likely vagal efferent fibers. PHA‐LI terminal arborizations were prominent in the region of the nucleus ambiguus (NA) and amongst PNMT‐LI cells in the lateral C1 cell group ventral to NA. Multiple sites of contiguity between PHA‐LI varicose fibers or boutons and PNMT‐LI cells in C1 were observed frequently and probably represent sites of functional connection. Medullary PNMT‐LI cells project to the intermediolateral cell columns of the spinal cord and to the hypothalamus and have been implicated in cardiovascular regulation. Based on the observations in this study, it is concluded that certain medullary PNMT‐LI cells in the C1 and C2 cell groups receive direct input from the intermediate NTS and thus play a role in baroreceptor‐mediated changes in cardiovascular activity.