Afferent-Efferent Interactions in the Developing Cochlea
The cochlea is innervated by two main classes of neurons: the spiral ganglion neuron (SGN) afferents, which transmit information from the ear to the brain, and the olivocochlear neuron (OCN) efferents, which provide feedback from the brain to the ear. Housed in the auditory brainstem, OCNs comprise two small populations of cholinergic neurons that send axons along the eighth nerve and into the cochlea. One subset, the medial olivocochlear (MOC) efferents, extend myelinated axons that fasciculate with SGN afferents in radial bundles and terminate on outer hair cells in the organ of Corti. The other subset, the lateral olivocochlear (LOC) efferents, develop thinner, unmyelinated axons that also follow along the radial bundles, but terminate instead on the endings of Type I SGN afferents contacting the inner hair cells. Together, the LOC and MOC neurons modulate the output of the cochlea, thereby improving binaural hearing and protecting the cochlea from the effects of excess noise and aging. By investigating how LOC and MOC neurons develop and establish connections, we can gain valuable insights into how the cochlea is wired and maintained for a lifetime of hearing. This knowledge will improve cochlear implant technology and identify new molecular entry points for rewiring the damaged cochlea. OCN axons develop in tight association with the SGN afferents, which appear to provide a scaffold for growth within the cochlea. In turn, OCN efferents influence SGN activity both indirectly, by forming transient synapses with the IHCs during development, and directly, by forming synapses on Type I peripheral processes that can regulate mature SGN firing properties. Based on the intimate relationship between these two populations, we hypothesize that reciprocal interactions between efferents and afferents sculpt the final wiring pattern of the cochlea. To investigate this idea, we propose to launch a new research project aimed at defining how and when OCN axons interact with SGN afferents, both at the cellular level and at the molecular level. We will start by using genetic approaches to document afferent-efferent interactions with high spatial and temporal resolution. In parallel, we will use newly available molecular biology techniques to identify genes that are differentially expressed in LOC and MOC neurons, including those that might direct each population towards distinct targets in the cochlea. These studies will be complemented with a focused analysis of the transcription factor Gata3, which we found is required in OCNs for proper innervation of the cochlea, with secondary effects on SGN afferent growth and targeting. Results from the proposed experiments will establish a framework for studying the development and function of OCNs and provide new insights into the molecular pathways that guide the dual innervation of the cochlea by afferents and efferents.