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

:The olivocochlear projection constitutes the last stage of the descending auditory system in the mammalian brain. Its neurons reside in the superior olivary complex (SOC) and project to the inner and outer hair cell receptors in the cochlea. Olivocochlear neurons were also reported to send axon collaterals 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 ventral cochlear nucleus (VCN), we studied the distribution and number of olivocochlear neurons with and without axon collaterals into the VCN of the rat. We found that olivocochlear neurons residing in the lateral superior olive (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 collaterals. These cells may thus affect processing in the ascending auditory pathway at the level of the receptors and concurrently at the level of the secondary 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 innervate the inner hair cells, but like MOCs they do, by means of axon collaterals, project into the VCN. These findings have implications for understanding both auditory signal processing and the plasticity responses that occur following loss of cochlear function.

Horváth M ，Kraus KS ，Illing RB 《JOURNAL OF COMPARATIVE NEUROLOGY》

Superior olivary contributions to auditory system plasticity: medial but not lateral olivocochlear neurons are the source of cochleotomy-induced GAP-43 expression in the ventral cochlear nucleus.

A unilateral cochlear lesion induces expression of the growth and plasticity-associated protein 43 (GAP-43) in fibers and their varicosities on specific types of postsynaptic profiles in the ventral cochlear nucleus (VCN), suggesting the induction of synaptic remodeling. One candidate population from which GAP-43 might emerge was neurons of the lateral olivocochlear (LOC) system residing in the lateral superior olive (LSO). Upon cochleotomy, these neurons express GAP-43 mRNA and GAP-43 protein. However, retrograde axonal tracing with Fast Blue or biotinylated dextran amine from VCN revealed that the number of 6.8 +/- 1.3 neurons in the whole ipsilateral LSO labeled in normal adult rats was distinctly small and did not rise after cochleotomy. Concluding that LOC neurons cannot be the source of GAP-43 in the VCN, we reinvestigated the pattern of GAP-43 in situ hybridization and found that, after cochleotomy, shell neurons in the regions surrounding the LSO and medial olivocochlear (MOC) neurons in the ventral nucleus of the trapezoid body up-regulated GAP-43 mRNA. We then lesioned these regions by means of stereotaxic injections of kainic acid. Destruction of shell neurons preceding an ipsilateral cochleotomy did not change the emergence of GAP-43 immunoreactivity in the VCN. However, if the contralateral MOC system was lesioned, the rise of GAP-43 immunoreactivity in VCN on the side of the cochleotomy was significantly reduced. We conclude that, after cochlear dysfunction, MOC neurons are the major (if not exclusive) source of synaptic reorganization in the VCN that could possibly entail compensatory activation of the affected ascending auditory pathway.

Kraus KS ，Illing RB 《JOURNAL OF COMPARATIVE NEUROLOGY》

Descending projections from auditory brainstem nuclei to the cochlea and cochlear nucleus of the guinea pig.

Winter IM ，Robertson D ，Cole KS 《JOURNAL OF COMPARATIVE NEUROLOGY》

Cell death or survival: molecular and connectional conditions for olivocochlear neurons after axotomy.

Kraus KS ，Illing RB 《NEUROSCIENCE》

The dual origins of the olivocochlear bundle in the albino rat.

White JS ，Warr WB 《JOURNAL OF COMPARATIVE NEUROLOGY》