Efferent-mediated Enhancement of the Cochlear Amplifier

Project Summary The mammalian cochlea is endowed with a robust efferent innervation called the olivocochlear system, that begins in the ventral brainstem as two distinct neuronal clusters containing hundreds of cells. Axons from medial olivocochlear (MOC) and lateral olivocochlear (LOC) neurons travel out cranial nerve VIII where they profusely

collateralize in the cochlea to end as thousands of synaptic terminals on outer hair cells (OHC) and primary auditory afferents innervating inner hair cells, respectively. By nature of their connectivity, MOC and LOC neurons are strategically poised to modulate how the cochlea functions at some of the earliest stages of detecting

and encoding sound. As reflected in a number of audiometric measures, activation of MOC neurons can give rise to both a suppression and an enhancement of cochlear function by modulating the OHC’s contributions to sound amplification. The release of acetylcholine (ACh) and the activation of nicotinic ACh receptors on OHCs

accounts for the MOC-mediated suppression in multiple mammalian species, but the synaptic mechanisms underlying MOC-mediated enhancement have not been identified. This is further complicated by observations that indicate MOC neurons may express a dozen or more different neurotransmitters beyond ACh. As a result,

there is a clear gap in our knowledge regarding how one critical signaling arm of the MOC system operates. To facilitate a more complete understanding of MOC function in mammalian auditory physiology, two specific aims will be pursued in the peripheral auditory system of mice. The first specific aim will isolate the MOC-mediated

enhancement phenomenon after pharmacological blockade and genetic ablation of MOC-mediated suppression. This will allow for a systematic evaluation of MOC-mediated enhancement over a range of varying auditory and MOC stimulation conditions. The second specific aim will specify and characterize the MOC transmitter and

postsynaptic mechanisms required for MOC-mediated enhancement. To complete these specific aims, we will leverage recordings of distortion product otoacoustic emissions (DPOAEs) in the anesthetized mouse before, during, and after electrical stimulation of MOC neurons in the brainstem. Selective pharmacological agents will

be administered directly to the perilymphatic compartment to isolate MOC-mediated enhancement as well as identify its underlying signaling components. Immunohistochemical studies will be performed in several strains of mice to localize receptor proteins, integral to the synaptic mechanisms implicated by our pharmacological

observations. These studies are significant as they will provide much needed insights into the diverse synaptic mechanisms that the MOC neurons recruit to modulate auditory function in mammals. The data captured by this proposal is critical for probing the functional roles of the MOC system in auditory physiology as well as identifying

novel synaptic processes that can be targeted pharmacologically for combating hearing dysfunction.