Distribution and dendritic features of three groups of rat olivocochlear neurons. A study with two retrograde cholera toxin tracers.

Cholera toxin B subunit conjugated to horseradish peroxidase, and unconjugated cholera toxin B subunit are useful tools for retrograde tract tracing. Unilateral injection of either cholera toxin preparation into the cochlea results in excellent labeling of olivocochlear neurons, as judged by the Golgi-like filling of cell bodies, dendrites, and even axons. By this approach, we have studied the light microscopic cytology and topographic distribution of olivocochlear neurons and counted their numbers in Sprague-Dawley rats. The olivocochlear system of rats can be divided into three subgroups. The lateral olivocochlear system, composed of small cells located exclusively within the ipsilateral lateral superior olive (relative to the test cochlea), and a medial olivocochlear system, composed of large cells bilaterally dispersed within the ventral nucleus of the trapezoid body, conformed to previous topographic descriptions. A third subgroup of approximately 110 large cells, herein termed shell neurons, was labeled by both tracers, but was not well recognized in previous studies. Shell neurons and their dendrites surround the ipsilateral, and to a much lesser extent the contralateral, lateral superior olive. Lateral olivocochlear neurons do not project their dendrites outside the gray matter of the lateral superior olive, while dendrites belonging to shell neurons penetrate into that nucleus as well as into other auditory brain stem nuclei and the surrounding reticular formation. Medial olivocochlear neurons usually project dendrites ventrally into the trapezoid body and are always excluded from the lateral superior olive.