Synaptic contributions to cochlear outer hair cell Ca2+ dynamics

Abstract

For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca21 levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca21 occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca21 influx have been poorly explored: voltage-gated L-type Ca21 channels (VGCCs) at synapses with Type II afferent neurons, and a9a10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca21 influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca21 ions originated in MOC synapses, but not by VGCC activation, was confined by Ca21-ATPases most likely present in nearby synaptic cisterns. Although Ca21 currents in OHCs are small, VGCC Ca21 signals were comparable in size to those elicited by a9a10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca21 signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca21 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.