Sensory information processing requires precisely ordered topographic neural networks. Developmental wiring of such networks is activity-dependent and involves synaptic refinement, thereby eliminating exuberant and strengthening remaining projections. In the mammalian auditory brainstem, the glycinergic projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO), which is involved in sound localization, is a well-suited model to investigate the maturation of topographic inhibitory projections. The aim of this proposal is to analyze activity-dependent refinement as well as histogenesis by a novel approach that distinguishes between the function of synaptic signaling molecules in the sensory organ (peripheral) and those in the brain (central). By this means, we will overcome intrinsic limitations of previous studies, including our owns, which employed systemic knock-out mice (Cav1.3 KOs, Vglut3 KOs) and could not pinpoint the source of the observed impairments to the periphery or to CNS sites. In our approach, peripheral and central protein loss are exemplified by Otoferlin KOs and brainstem-specific Cav1.3Krox20 CKOs, respectively. Otoferlin KOs lack transmitter release from cochlear inner hair cells and thus deprive the central auditory system of spontaneous activity generated in the periphery, whereas Cav1.3Krox20 CKOs have brainstem-specific Ca2+-signaling defects. We hypothesize that both KO types show impaired synaptic refinement and histogenesis, yet defects are more subtle in Cav1.3Krox20 CKOs because of the pivotal position of Otoferlin at the beginning of the ascending auditory pathway. Thus, we propose that loss of Otoferlin has more general and severe effects than on-site loss of Cav1.3. Experiments will comprise electrical and glutamate-uncaging stimulation of MNTB neurons in vitro to determine the strength and spatial extent of MNTB inputs onto patch-clamped LSO neurons. Quantal analysis will give mechanistic insight into the strengthening process. Volume of auditory brainstem nuclei will also be assessed. In in vivo experiments, we will analyze the effect of peripheral protein loss on spiking activity in the CNS. We will investigate mice at postnatal day P11 (hearing onset) and P30-50 (adulthood), a developmental stage hardly tackled thus far. Our studies will help to elucidate the molecular processes governing the wiring of auditory brainstem microcircuits. By separating peripheral protein loss from central protein loss, we will also shed light on mechanistic aspects of central auditory disorders.