ProjectSummary Thecochleaisinnervatedbytwomainclassesofneurons:thespiralganglionneuron(SGN)afferents,which transmitinformationfromtheeartothebrain,andtheolivocochlearneuron(OCN)efferents,whichprovide feedbackfromthebraintotheear.Housedintheauditorybrainstem,OCNscomprisetwosmallpopulations of cholinergic neurons that send axons along the eighth nerve and into the cochlea. One subset, the medial olivocochlear(MOC)efferents,extendmyelinatedaxonsthatfasciculatewithSGNafferentsinradialbundles and terminate on outer hair cells in the organ of Corti. The other subset, the lateral olivocochlear (LOC) efferents,developthinner,unmyelinatedaxonsthatalsofollowalongtheradialbundles,butterminateinstead ontheendingsofTypeISGNafferentscontactingtheinnerhaircells.Together,theLOCandMOCneurons modulatetheoutputofthecochlea,therebyimprovingbinauralhearingandprotectingthecochleafromthe 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 forrewiringthedamagedcochlea. OCNaxonsdevelopintightassociationwiththeSGNafferents,whichappeartoprovideascaffoldforgrowth 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 defininghowandwhenOCNaxonsinteractwithSGNafferents,bothatthecellularlevelandatthemolecular 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 populationtowardsdistincttargetsinthecochlea.Thesestudieswillbecomplementedwithafocusedanalysis 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 establishaframeworkforstudyingthedevelopmentandfunctionofOCNsandprovidenewinsightsintothe molecularpathwaysthatguidethedualinnervationofthecochleabyafferentsandefferents.