Effects of early hearing deficits on olivocochlear efferent neuron morphology in mice

One of the fundamental features of age-related hearing loss is difficulty discriminating signals in the presence of background noise, a functional deficit that may be related to the loss of auditory efferent neurons. We hypothesize that the central degeneration of olivocochlear (OC) efferent neurons results from prolonged lack of afferent input. We investigated the time-course of central degeneration of medial (MOC) and lateral (LOC) OC efferent neurons in genetic mouse models that exhibit hearing loss during development. Tests to verify cochlear function were combined with anatomical and morphological quantification of somatic number, morphology, and location of OC neurons to investigate differences over time in mouse strains with developmental hearing abnormalities. Neuronal tract tracing methods were employed to label OC neurons in 1-, 3-, and 6-month-old CBA/CaH mice with normal hearing; DBA/2J mice with progressive, high frequency hearing loss; and homozygous Shaker2-/- mice with congenital deafness. Deaf Shaker2-/- animals exhibited a striking age-related decrease in numbers of both LOCs and MOCs, and MOC loss appeared to progress in a tonotopic manner. No OC loss was observed in DBA/2J mice, even after progressive elevation of low frequency auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) thresholds. In early life, numbers of OC efferent numbers can appear 'normal' in the complete absence of acoustic input. The retention of these neurons is not affected by late onset high-frequency hearing loss and may indicate functional plasticity of auditory brainstem feedback circuitry when afferent input is retained.