A novel method for selectively labelling olivocochlear collaterals in the rat

Axons of olivocochlear neurons originate from the brainstem and project to the cochlea. A subpopulation, medial olivocochlear (MOC) neurons, also projects collateral branches to the cochlear nucleus. The precise targets of these collaterals are as yet unknown. Previous methods for labelling these collaterals include firstly, cochlear injections of retrograde tracers, but this is technically demanding and can also label afferent projections or secondly, labelling by injecting tracers into the nuclei of origin of MOC neurons. However, this latter method is non-specific because it also labels non-MOC projections. A technique was used to specifically label MOC collaterals, which involved injections of the tracer biocytin at the floor of the fourth ventricle and fixation 3 hours later. Biocytin injections resulted in labelled neurons in the ventral nucleus of the trapezoid body and rostral periolivary nucleus, confirming MOC axonal labelling. Labelled neurons in dorsal cochlear nucleus indicated labelling of the dorsal acoustic stria and these injections were discarded. After selective MOC labelling, collateral branches were found to innervate granule cell regions, medial edge and core of the ventral cochlear nucleus, as well as the dorsal cochlear nucleus, in agreement with previous data. Therefore we conclude that injections at the floor of the fourth ventricle provide a simple, rapid and specific technique for labelling the majority of MOC axons and their collaterals and this technique may assist in defining the precise neuronal targets of olivocochlear collaterals in cochlear nucleus.