Tonotopic Variation in the Calcium Dependence of Neurotransmitter Release and Vesicle Pool Replenishment at Mammalian Auditory Ribbon Synapses

Abstract

The mammalian cochlea is specialized to recognize and process complex auditory signals with remarkable acuity and temporal precision over a wide frequency range. The quality of the information relayed to the auditory afferent fibers mainly depends on the transfer characteristics of inner hair cell (IHC) ribbon synapses. To investigate the biophysical properties of the synaptic machinery, we measured changes in membrane capacitance (ΔC_m) in low-frequency (apical region, ∼300 Hz) and high-frequency (basal, ∼30 kHz) gerbil IHCs maintained in near physiological conditions (1.3 mmextracellular Ca²⁺and body temperature). With maturation, the Ca²⁺efficiency of exocytosis improved in both apical and basal IHCs and was more pronounced in the latter. Prehearing IHCs showed a similar Ca²⁺cooperativity of exocytosis despite the smaller ΔC_min apical cells. After maturation, ΔC_min high-frequency IHCs increased linearly with the Ca²⁺current, whereas, somewhat surprisingly, the relationship was significantly more nonlinear in low-frequency cells. This tonotopic difference seemed to be correlated with ribbon synapse morphology (spherical in apical and ellipsoid in basal IHCs) but not with the expression level of the proposed Ca²⁺sensor otoferlin or the spatial coupling between Ca²⁺channels and active zones. Repetitive stimulation of adult IHCs showed that vesicle pool refilling could become rate limiting for vesicle release, with high-frequency IHCs able to sustain greater release rates. Together, our findings provide the first evidence for a tonotopic difference in the properties of the synaptic machinery in mammalian IHCs, which could be essential for fine-tuning their receptor characteristics during sound stimulation.