Immunohistochemical localization of serotonin- and substance P-containing fibers around respiratory muscle motoneurons in the nucleus ambiguus of the cat.

Retrograde tracing with a fluorescent dye (Fast Blue) combined with immunohistochemistry was used to determine if the putative neurotransmitters, serotonin and substance P, are present around posterior cricoarytenoid muscle motoneurons. Fast Blue was injected into the posterior cricoarytenoid muscle of the larynx. Following a 14-21 day survival time to allow for transport of the dye, the animals were perfusion fixed and the brainstem was removed for analysis under the fluorescence microscope. Retrogradely labeled cell bodies containing Fast Blue were found within the nucleus ambiguus from 0.5 to 3.0 mm rostral to obex. These motoneurons ranged in size from 23 to 38 micron. The same tissue sections containing labeled posterior cricoarytenoid muscle motoneurons were then used to determine the distribution of serotonin and substance P around these motoneurons using the indirect immunofluorescence technique. A dense network of serotonin-containing immunoreactive fibers was found around posterior cricoarytenoid muscle motoneurons. The fibers contained varicosities which were in close proximity, actually appearing to surround these motoneurons. Substance P immunoreactive fibers and varicosities were also found around posterior cricoarytenoid muscle motoneurons. The density and pattern of distribution of the substance P immunoreactivity was similar to that of the serotonin immunoreactivity. These results suggest that these putative neurotransmitters may be involved in influencing the activity of posterior cricoarytenoid muscle motoneurons. Serotonin and substance P are also present around other respiratory motoneurons such as phrenic motoneurons. Therefore, these two neurotransmitters may have a more general role in influencing respiratory motor outflow.