Figure 3.

Photomicrographs illustrating retrograde labeling of parasympathetic preganglionic cardiac motor neurons in the medulla of the rat after pericardial injections of cholera toxin B (CTB) and their activation by diving. Photomontage of a medullary section caudal to the obex (A) showing the distribution of caudal preganglionic cardiac motoneurons (arrows) along an oblique line from the ventrolateral part of the dorsal motor nucleus of the vagus nerve toward the nucleus ambiguus. Such distribution is similar to that described in other mammals. Most neurons rostral to the obex (B,C,E) were retrogradely-labeled adjacent or ventral to the compact formation of nucleus ambiguus (Amb-c). Note the long dendrites of neurons ventral to Amb-c (C, red arrows) extending into the reticular formation, many surrounding Amb-c or approaching the spinal trigeminal nucleus, pars interpolaris (Sp5I). Double labeling of preganglionic cardiac motor neurons after underwater submersion with both cFos (black immunolabeled nuclei) and retrograde transport of CTB (brown cytoplasmic labeling) is shown in (D,E). Two retrogradely-labeled neurons in the caudal medulla are seen in (D), one of which was activated by underwater submersion (arrow); note the long obliquely orientation of the dendrites. While some Amb-c neurons were retrogradely-labeled (E) after pericardial injections, neurons labeled with cFos were found on its periphery, some of which were double-labeled (black arrows). A single retrogradely labeled neuron also is shown more ventrally (red arrow).