Projection neurons in the superior olivary complex of the rat auditory brainstem: a double retrograde tracing study

The superior olivary complex (SOC), a group of interrelated brainstem nuclei, sends efferents to a variety of neuronal structures including the cochlea and the inferior colliculus (IC). In the present study conducted in rats, we sought to investigate whether single SOC efferent neurons project to both cochlea and IC. These neurons were identified by retrograde axonal transport of the neuronal tracers fluoro-gold upon application to the cochlea and cholera toxin B subunit injected into the central nucleus of the IC. Projections to the cochlea were found to stem predominantly from the ipsilateral lateral superior olive (LSO) and medial superior olive (MSO) as well as from the bilateral superior paraolivary nucleus (SPO), dorsal periolivary groups (DPO) and the medial nucleus of the trapezoid body (MNTB). Neurons innervating the IC were seen in the bilateral LSO and DPO, in the SPO predominantly ipsilateral, as well as in the ipsilateral MSO and MNTB. We further observed that some neurons in the ipsilateral LSO, and a considerable part of those of the SPO were double-labeled. However, cells projecting to either IC or cochlea were much more numerous than double-labeled neurons.