Olivocochlear neurons sending axon collaterals into the ventral cochlear nucleus of the rat

The olivocochlear projection constitutes the last stage of the descending a uditory system in the mammalian brain. Its neurons reside in the superior o livary complex (SOC) and project to the inner and outer hair cell receptors in the cochlea. Olivocochlear neurons were also reported to send axon coll aterals into the cochlear nucleus, but controversies about their number and about species differences persist. By injecting the fluorescent retrograde axonal tracers diamidino yellow and fast blue into the cochlea and the ven tral cochlear nucleus (VCN), we studied the distribution and number of oliv ocochlear neurons with and without axon collaterals into the VCN of the rat . We found that olivocochlear neurons residing in the lateral superior oliv e (LSO), the intrinsic lateral olivocochlear cells (intrinsic LOCs), do not send axon collaterals into the VCN. By contrast, a majority, and possibly all, olivocochlear neurons residing in the ventral nucleus of the trapezoid body (VNTB), the medial olivocochlear cells (MOCs), do have such axon coll aterals. These cells may thus affect processing in the ascending auditory p athway at the level of the receptors and concurrently at the level of the s econdary sensory neurons in the cochlear nucleus. Belonging to the lateral olivocochlear system, shell neurons reside around the LSO and form a third group of olivocochlear cells (shell LOCs). Like intrinsic LOCs, they innerv ate the inner hair cells, but like MOCs they do, by means of axon collatera ls, project into the VCN. These findings have implications for understandin g both auditory signal processing and the plasticity responses that occur f ollowing loss of cochlear function. (C) 2000 Wiley-Liss, Inc.