Download files
Abstract
Age related hearing loss (ARHL) affects one in three humans aged over 65 years of age. The most common complaint in ARHL is difficulty understanding speech in noise. Olivocochlear (OC) efferents project from the brain to the cochlea and represent the final stage of the descending auditory pathway. OC efferents are involved in listening in loud environments and protect the cochlea from noise damage. We used anatomical methods to investigate OC efferent cell morphology and top-down patterns of descending input from the inferior colliculus (IC) to determine if deterioration of the descending auditory pathway accompanies peripheral auditory pathology in the mouse.
Brightfield, fluorescent, and electron microscopy were used to identify OCs and their synaptic inputs in 1-,3-, and 6-month old mice with normal hearing (CBA/CaH), progressive high frequency hearing loss (DBA/2), and congenital deafness (homozygous Shaker2). Electrophysiologically directed injections in the IC were used to elucidate the tonotopic nature of descending input to the medial OC (MOC) efferent population.
In normal hearing mice, the IC provides excitatory synaptic input to MOCs in a tonotopic manner. Higher frequencies project to medially located cells, whereas lower frequencies project to laterally located cells. Synaptic input was retained in hearing loss and congenital deafness, however, the spatial distribution of terminals and MOC somata were significantly altered. Together, these data indicate that the nature and time course of peripheral hearing pathology is correlated with loss of MOC efferent cells and a systematic deviation of descending input from the IC in a frequency dependent manner. Therefore, changes to the descending central auditory pathway may help to explain the difficulty extracting signals from background noise characteristic of ARHL.
In the course of investigating OC efferents, we consistently identified a population of cells in the ventral nucleus of the lateral lemniscus that appeared to project to the cochlea. Using immunohistochemical, neuronal tract tracing, and electron microscopic methods, we characterized these ‘dorsal efferents’ with respect to the existing OC populations and provide evidence for their inclusion as a new class of auditory efferents in mouse.