Ultrastructural localization of phenylethanolamine N-methyltransferase in sensory and motor nuclei of the vagus nerve.

The ultrastructural localization of phenylethanolamine N-methyltransferase (PNMT), the enzyme used in the final step in the synthesis of adrenaline, was examined in the medial nuclei of the solitary tracts (m-NTS) and in the dorsal motor nuclei of the vagus. Adult rats were anesthetized with Nembutal (50 mg/kg intraperitoneally), and the brains were fixed by vascular perfusion with a solution containing 3.75% acrolein and 2% paraformaldehyde in 0.1 M phosphate buffer. Coronal Vibratome sections were collected through the intermediate portions of the m-NTS at the level of the area postrema. These sections were immunocytochemically labeled employing a rabbit polyclonal antiserum against PNMT and the peroxidase-antiperoxidase method. Immunoreactivity was detected in perikarya, dendrites, and axon terminals in the intermediate portion of the m-NTS. The labeled perikarya were either small (10-15 microns diameter) and oval or large 20-30 microns) with two or more proximal processes. The PNMT-containing dendrites received synaptic input from unlabeled, small (0.5-1.0 microns) and large (2-3 microns) vagal-like afferents as well as from a few terminals, which also showed PNMT immunoreactivity. Axons and axon terminals containing immunoreactive PNMT were more frequently observed than the perikarya or dendrites in the m-NTS and were the only labeled profiles in the dorsal motor nuclei. In both regions the PNMT-labeled terminals formed principally symmetric synapses with unlabeled dendrites. However, a few asymmetric axodendritic and symmetric axosomatic synapses also were detected. These findings indicate that the adrenergic neurons may have multiple, but principally inhibitory, actions on other neurons within cardiovagal portions of baroreflex pathways.