Superior olivary and lateral lemniscal neurons projecting to the cochlea in the guinea pig

Efferent neurons to the cochlea in the guinea pig were observed in the superior olivary complex and ventral lateral lemniscal nuclei after injection of WGA-HRP into the cochlea via the round window. Of HRP-labeled nerve cells found in the brainstem, about 60% and 10% were located in the ipsilateral lateral superior olivary nucleus (LSO) and the contralateral ventral nucleus of the trapezoid body (VTB), respectively. A few labeled neurons were found in the ipsilateral VTB, contralateral LSO and the bilateral superior paraolivary nucleus (SPO). Seven percent and 5% of the labeled neurons were definitely observed in the ventrocaudal part of the contra- and ipsilateral ventral nucleus of the lateral lemniscus (VLL), respectively. A combination of acetylcholine esterase (AChE) histochemistry with the tracing of retrograde axonal transport of a fluorescent dye (DAPI) revealed that VLL neurons projecting to the cochlea, as well as about 85% of LSO neurons projecting to the ipsilateral cochlea, were AChE-positive.