Mu-opioid and N-methyl-D-aspartate receptors are localized at laryngeal motoneurons of guinea pigs.

The glottic movement is important for generating efficient airflow in respiration and related reflexes such as coughing. The superior laryngeal nerve (SLN) innervates the cricothyroid muscle and regulates the glottic aperture. A total of 620 laryngeal motoneurons (LMNs) providing efferent fibers to the SLN were retrogradely identified with fluorescent wheat germ agglutinin (WGA) in 8 guinea pigs. The WGA-labeled cells were distributed in the ventrolateral part of nucleus ambiguus, extending 1.8-2.2 mm lateral to the midline, 0.8-1.2 mm from the ventral surface and 1.7-2.6 mm rostral to the obex. Double staining revealed that 28.4% of LMNs (44 out of 155 cells examined) were immunopositive to N-methyl-D-aspartate (NMDA) receptors and 29.2% of LMNs (55 out of 185 cells) to mu-opioid receptors. The present study demonstrates phenotypic variation in the synaptic transmission to the LMNs in guinea pigs. These results suggest modulation of the glottic movement by mu-opioid and NMDA mechanisms.