Ultrastructural identification of labeled neurons in the dorsal motor nucleus of the vagus nerve following injections of horseradish peroxidase into the vagus nerve and brainstem

Abstract

The efferent connections of two types neurons in the dorsal motor nucleus of the vagus nerve (DMV) were studied in the cat by light and electron microscopy following horseradish peroxidase (HRP) injections into the cervical vagus nerve or brainstem. After injections of HRP into the vagus nerve, up to 80% of medium-sized neurons averaging 26 x 20 micrometers in 1-micrometer-thick sections were retrogradely labeled while no small neurons were labeled in the DMV. Incubation with either diaminobenzidene (DAB) or p-phenylenediamine-pyrocatechol (PPD-PC) chromogens yielded electron-dense reaction products localized mainly in lysosomes. Identification of label at the ultrastructural level was facilitated by omitting lead citrate staining and by counting numbers of lysosomes, which were higher in labeled neurons. Quantitative comparisons of the dimensions of labeled and unlabeled somata demonstrated that retrograde transport and incorporation of HRP had no effect on cell size within the 2-3-day survival times used in this study. In order to determine whether neurons in the DMV project to higher levels of the brain stem, large injections of HRP (1-3 microliters) were made into the pons, mesencephalon, hypothalamus, and amygdala. After injections of HRP into the brainstem, only small neurons, measuring 17 x 10 micrometers, were retrogradely labeled. Approximately 90% of the small neurons remained unlabeled following the HRP injections. The ultrastructural features of the labeled small neurons included an invaginated nucleus, low cytoplasmic/nuclear ratio, and relatively fewer organelles than the medium-sized neurons. A quantitative analysis of labeled and unlabeled small neurons demonstrated that the labeled neurons were significantly larger than the unlabeled small neurons. Thus, two populations of small neurons may exist in the DMV. One population appears to have ascending projections to higher levels of the brainstem while the other more numerous population may be interneurons or project for only short distances.

Organization of synaptic transmission in the mammalian solitary complex, studied in vitro.

“…Morphologically, they resemble neurones described by Cajal (1909) as dorsal vagal motor neurones and shown using immunohistochemical techniques to be cholinergic (Armstrong, Clifford, Levey, Wainer & Terry, 1983) or monoaminergic (Ritchie, Westlund, Bowker, Coulter, & Leonard, 1982). Their axon trajectory follows the same course as that described by retrograde HRP labelling of the vagus nerve in rats (Kalia & Sullivan, 1982;McLean & Hopkins, 1982).…”

Ultrastructural characterization of pharyngeal and esophageal motoneurons in the nucleus ambiguus of the rat

“…We found that there were small neurons in the AmC that do not project to the alimentary t,ract. Their morphology is similar to that of the small interneurons in the dorsal motor nucleus of the vagus nerve (McLean and Hopkins, 1982). Afferent inputs of the swallowing reflex originating from the surface of the tongue, the soft palate, and the pharynx Cell size (area pm2) USDE n. MCE n. B P H n. enter the nucleus of the solitary tract and the sensory trigeminal nucleus (Miller, 1982;Altschuler et al, 1989).…”

Ultrastructural and cytochemical study of neurones in the rat dorsal motor nucleus of the vagus after axon crush

“…It seems certain that these neurones do not project down the vagus to the crush or labeling site at the mid-cervical level. They are either local interneurones or project to other brain areas; McLean and Hopkins (1982) were able to establish projections to other parts of the brain for about 10% of these smaller neurones in the cat.…”