Neuronal organization of the stapedius reflex pathways in the rat: a retrograde HRP and viral transneuronal tracing study.

The location of stapedius motoneurons in the rat was determined with horseradish peroxidase (HRP) retrograde tracing techniques. After injection of free HRP or wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in the stapedius muscle on one side, labeled neurons were seen ipsilaterally in a region ventromedial to the rostral half of the facial motor nucleus (VII), extending rostrally to the caudal part of the superior olivary complex (SOC). These labeled neurons, located outside the SOC and facial motor nuclei themselves, constitute the pool of stapedius motoneurons, in agreement with previous descriptions for other species. In order to identify the origin of some inputs to the stapedius motoneurons, injections of herpes virus suis were performed in the stapedius muscle. After replication in the motoneurons, the viruses are transported transneuronally to some premotor neurons, as previously reported in other systems. The presence of the virus was detected by immunofluorescence in neurons corresponding to the stapedius motoneurons labeled with HRP or WGA-HRP. In addition, infected neurons were seen bilaterally at the level of the SOC, in the mediotrapezoid region, where no labeled cells were observed following HRP or WGA-HRP injections in the stapedius muscle. These neurons were considered as infected transneuronally and therefore providing inputs to the pool of stapedius motoneurons. No virus could be detected in cochlear nucleus neurons. These data are consistent with previous observations in the rabbit based on lesion experiments, suggesting that neurons at the level of the SOC are involved in the reflex arc of middle ear muscles.