Postnatal development of the efferent innervation of the rat cochlea

The postnatal development of the efferent innervation of the rat cochlea was studied by intracochlear injection of the fluorescent retrograde neuronal traces Diamidino yellow and Fast blue. Injections were performed on adult rats and on neonatal rats ranging from 0 to 8 postnatal days. It was found that the total number of neurones labelled in the brainstem after intracochlear injection was not significantly different in the newborn rat, compared to the adult. On the basis of cell body location and laterality of projections, there was a clear separation into lateral and medial efferent systems at the earliest postnatal age studied (PO). Evidence was also found in the newborn for a tonotopicity in the lateral system projection similar to that in the adult. Differences between the newborn and adult were a slight but significantly greater number of bilaterally-projecting cells in the newborn, and the presence in the newborn of a small number of cells located in the lateral superior olivary nucleus contralateral to their target cochlea. These were extremely rare in the adult brainstem. Evidence was found for the occurrence of postnatal neuronal death in nuclei of origin of both efferent systems. It is suggested that although the overall extent and general organization of the efferent projection to the cochlea in the rat appears to be established at birth, regressive changes are occurring during the postnatal shaping and maturation of this brainstem-to-cochlea pathway.