Synaptic Contributions to Cochlear Outer Hair Cell Ca2+ Dynamics

For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca2+ levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca2+ occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca2+ influx have been poorly explored: voltage-gated L-type Ca2+ channels (VGCCs) at synapses with Type II afferent neurons, and alpha 9 alpha 10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca2+ influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca2+ ions originated in MOC synapses, but not by VGCC activation, was confined by Ca2+-ATPases most likely present in nearby synaptic cisterns. Although Ca2+ currents in OHCs are small, VGCC Ca2+ signals were comparable in size to those elicited by alpha 9 alpha 10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca2+ signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca2+ entry sites are closely positioned, they differ in their regulation by intracellular cisterns and/or organelles, suggesting the existence of well-tuned mechanisms to separate the two different OHC synaptic functions.

Automated summary

Normal cochlear function requires precise control of intracellular Ca2+ levels, with Ca2+ influx occurring at the stereocilia tips and basolateral membrane. Two different sources of Ca2+ influx in the basolateral membrane, VGCCs and alpha 9 alpha 10 receptors, are regulated differently, suggesting well-tuned mechanisms to separate the two OHC synaptic functions.