For sounds of a given frequency, spiral ganglion neurons (SGNs) with different thresholds and dynamic ranges collectively encode the wide range of audible sound pressures. Heterogeneity of synapses between inner hair cells (IHCs) and SGNs is an attractive candidate mechanism for generating complementary neural codes covering the entire dynamic range. Here, we quantified active zone (AZ) properties as a function of AZ position within mouse IHCs by combining patch clamp and imaging of presynaptic Ca 2+ influx and by immunohistochemistry. We report substantial AZ heterogeneity whereby the voltage of half-maximal activation of Ca 2+ influx ranged over ∼20 mV. Ca 2+ influx at AZs facing away from the ganglion activated at weaker depolarizations. Estimates of AZ size and Ca 2+ channel number were correlated and larger when AZs faced the ganglion. Disruption of the deafness gene GIPC3 in mice shifted the activation of presynaptic Ca 2+ influx to more hyperpolarized potentials and increased the spontaneous SGN discharge. Moreover, Gipc3 disruption enhanced Ca 2+ influx and exocytosis in IHCs, reversed the spatial gradient of maximal Ca 2+ influx in IHCs, and increased the maximal firing rate of SGNs at sound onset. We propose that IHCs diversify Ca 2+ channel properties among AZs and thereby contribute to decomposing auditory information into complementary representations in SGNs.auditory system | spiral ganglion neuron | dynamic range | synaptic strength | presynaptic heterogeneity T he auditory system enables us to perceive sound pressures that vary over six orders of magnitude. This is achieved by active amplification of cochlear vibrations at low sound pressures and compression at high sound pressures. The receptor potential of inner hair cells (IHCs) represents the full range (1), whereas each postsynaptic type I spiral ganglion neuron (hereafter termed SGN) encodes only a fraction (2-6). SGNs with comparable frequency tuning but different spontaneous spike rates and sound responses are thought to emanate from neighboring, if not the same, IHC at a given tonotopic position of the organ of Corti (2,5,7,8). Even in silence, IHC active zones (AZs) release glutamate at varying rates, evoking "spontaneous" spiking in SGNs. SGNs with greater spontaneous spike rates respond to softer sounds (highspontaneous rate, low-threshold SGNs), than those with lower spontaneous spike rates (low-spontaneous rate, high-threshold SGNs) (2, 9). This diversity likely underlies the representation of sounds across all audible sound pressure levels in the auditory nerve, to which neural adaptation also contributes (10).How SGN diversity arises is poorly understood. Candidate mechanisms include the heterogeneity of ribbon synapses that differ in pre-and/or postsynaptic properties even within individual IHCs (7,(11)(12)(13)(14). IHC AZs vary in the number (11, 15) and voltage dependence of gating (11) of their Ca 2+ channels regardless of tonotopic position (16). Lateral olivocochlear efferent projections to the SGNs regulate postsynaptic exc...

Ca2+ influx triggers the release of synaptic vesicles at the presynaptic active zone (AZ). A quantitative characterization of presynaptic Ca2+ signaling is critical for understanding synaptic transmission. However, this has remained challenging to establish at the required resolution. Here, we employ confocal and stimulated emission depletion (STED) microscopy to quantify the number (20–330) and arrangement (mostly linear 70 nm × 100–600 nm clusters) of Ca2+ channels at AZs of mouse cochlear inner hair cells (IHCs). Establishing STED Ca2+ imaging, we analyze presynaptic Ca2+ signals at the nanometer scale and find confined elongated Ca2+ domains at normal IHC AZs, whereas Ca2+ domains are spatially spread out at the AZs of bassoon-deficient IHCs. Performing 2D-STED fluorescence lifetime analysis, we arrive at estimates of the Ca2+ concentrations at stimulated IHC AZs of on average 25 µM. We propose that IHCs form bassoon-dependent presynaptic Ca2+-channel clusters of similar density but scalable length, thereby varying the number of Ca2+ channels amongst individual AZs.