Motoneurons of the stapedius muscle in the guinea pig middle ear: afferent and efferent transmitters.

The objective of the present study was to identify efferent and afferent transmitters of motoneurons of the stapedius muscle of the middle ear in order to gain more insight into the neuronal regulation of the muscle. To identify motoneurons, we injected the fluorescent neuronal tracer Fluorogold (FG) into the muscle after preparation of the middle ear in adult guinea pigs. Upon terminal uptake and retrograde neuronal transport, we observed FG in neurons located medial and ventral to the nucleus of the facial nerve ipsilateral to the injection site. Immunohistochemical studies of these motoneurons showed that the majority contains calcitonin gene-related peptide. Our data further demonstrate close spatial relationships of motoneurons to structures immunoreactive to either serotonin, substance P or neuronal nitric oxide and reveal that these neurons are under neuropeptidergic and nitrergic influence.