Exocytosis in the Frog Amphibian Papilla

Using whole-cell patch-clamp recordings, we measured changes in membrane capacitance (ΔC _m) in two subsets of hair cells from the leopard frog amphibian papilla (AP): the low-frequency (100–500 Hz), rostral hair cells and the high-frequency (500–1200 Hz), caudal hair cells, in order to investigate tonotopic differences in exocytosis. Depolarizations of both rostral and caudal hair cells evoked robust ΔC _m responses of similar amplitude. However, the calcium dependence of release, i.e., the relationship between ΔC _m relative to the amount of calcium influx (Q _Ca ²⁺ ), was found to be linear in rostral hair cells but supra-linear in caudal hair cells. In addition, the higher numbers of vesicles released at caudal hair cell active zones suggests increased temporal precision of caudal hair cell exocytosis. ΔC _m responses were also obtained in response to sinusoidal stimuli of varying frequency, but neither rostral nor caudal hair cell ΔC _m revealed any frequency selectivity. While all AP hair cells express both otoferlin and synaptotagmin IV (SytIV), we obtained evidence of a tonotopic distribution of the calcium buffer calretinin which may further increase temporal resolution at the level of the hair cell synapse. Our findings suggest that the low (rostral) and high (caudal) frequency hair cells apply different mechanisms for fine-tuning exocytosis.