Synaptic refinement via the elimination of inappropriate synapses and strengthening of appropriate ones is crucially important for the establishment of specific, topographic neural circuits. The mechanisms driving these processes are poorly understood, particularly concerning inhibitory projections. Here, we address the refinement of an inhibitory topographic projection in the auditory brainstem in functional and anatomical mapping studies involving patch-clamp recordings in combination with minimal and maximal stimulation, caged glutamate photolysis, and single axon tracing. We demonstrate a crucial dependency of the refinement on Ca V 1.3 calcium channels: Ca V 1.3 Ϫ/Ϫ mice displayed virtually no elimination of projections up to hearing onset. Furthermore, strengthening was strongly impaired, in line with a reduced number of axonal boutons. The mediolateral topography was less precise and the shift from a mixed GABA/ glycinergic to a purely glycinergic transmission before hearing onset did not occur. Together, our findings provide evidence for a Ca V 1.3-dependent mechanism through which both inhibitory circuit formation and determination of the neurotransmitter phenotype are achieved.

Within the Ca v 1 family of voltage-gated calcium channels, Ca v 1.2 and Ca v 1.3 channels are the predominant subtypes in the brain. Whereas specific functions for each subtype were described in the adult brain, their role in brain development is poorly understood. Here we assess the role of Ca v 1.3 subunits in the activity-dependent development of the auditory brainstem. We used Ca v 1.3-deficient (Ca v 1.3 Ϫ/Ϫ ) mice because these mice lack cochlea-driven activity that deprives the auditory centers from peripheral input. We found a drastically reduced volume in all auditory brainstem centers (range 25-59%, total 35%), which was manifest before hearing onset. A reduction was not obvious outside the auditory system. The lateral superior olive (LSO) was strikingly malformed in Ca v 1.3 Ϫ/Ϫ mice and had fewer neurons (1/3 less). The remaining LSO neurons displayed normal dendritic trees and received functional glutamatergic input, yet they fired action potentials predominantly with a multiple pattern upon depolarization, in contrast to the single firing pattern prevalent in controls. The latter finding appears to be due to a reduction of dendrototoxin-sensitive potassium conductances, presumably mediated through the K v 1.2 subtype. Fura2 imaging provided evidence for functional Ca v 1.3 channels in the LSO of wild-type mice. Our results imply that Ca v 1.3 channels are indispensable for the development of the central auditory system. We propose that the unique LSO phenotype in Ca v 1.3 Ϫ/Ϫ mice, which hitherto was not described in other hereditary deafness models, is caused by the synergistic contribution of two factors: on-site loss of Ca v 1.3 channels in the neurons plus lack of peripheral input.