Outer hair cells (OHC) are innervated by type II afferent fibres of unknown function. If OHCs had an afferent function presynaptic Ca2+ channels would be required. We recorded Ba2+ currents (IBa) in mouse and rat hair cells aged P1-P32. OHC IBa peaked at P2 and declined upon further maturation, in parallel to the acquisition of a mature OHC phenotype. Average IBa in rat OHCs amounted to 66 pA ± 18 (n=13; P28). Properties of rat IHC and OHC currents were similar to those of neonatal mouse IHC/OHC Ba2+ currents carried by the Cav1.3 subunit (Platzer et al. Cell 2000; Michna et al. J. Physiol. 2003). In situ hybridisation, RT-PCR and immunostaining confirmed the presence of Cav1.3 in mature IHCs and OHCs. Mature rat OHCs showed 3.3fold smaller Ba2+ currents compared to IHCs (219 ±26 pA, n=6, P30) but the ratio of IBa to the number of afferent fibres was identical in OHCs and IHCs (20 pA/fibre), thus strongly suggesting OHC afferent signalling. The presence of the synaptic vesicle protein otoferlin and the presynaptic protein CtBP2 add further pieces of evidence for functional exocytosis in OHCs.

Supported by DFG En 294/2-4, DFG Kni-316/3-1; 4-1, and Forschungsschwerpunkt Baden-Wuerttemberg