Synaptic Ca2+ Signals in Cochlear Outer Hair Cells

MARCELO J. MOGLIE; JUAN D. GOUTMAN; A. BELEN ELGOYHEN

San Jose, California

Congreso; 43rd Annual MidWinter meeting of the Association for Research in Otolaryngology; 2020

Association for Research in Otolaryngology

Ca2+ regulation seems as a fundamental prerequisite for the normal operation of cochlear outer hair cells (OHCs). The existence of multiple cellular specializations, such as large amounts of cytoplasmic Ca2+ binding proteins, cisterns and pumps involved in Ca2+ extrusion mechanisms, points in that way. The main Ca2+ source is through mechanotransducer channels, but influx through voltage-gated Ca2+ channels (VGCCs) and the nicotinic α9α10 receptors has been also found. While VGCCs may be involved in synaptic transmission with type II afferent fibers, the highly Ca2+ permeable α9α10 receptors mediate the cholinergic synapse with efferent fibers from the medial olivocochlear complex in the brainstem. Ca2+ influx through nicotinic receptors is coupled to the activation of Ca2+-dependent K+ (SK) channels within a very narrow domain limited by a near-membrane postsynaptic cistern. Thus, efferent cholinergic synapse exerts inhibition over OHCs? functions: it hyperpolarizes the OHCs and reduces cochlear amplification. We aimed to determine the magnitude and spread of synaptic Ca2+ signals and the specific role of the efferent postsynaptic cisterns during synaptic activity using simultaneous electrophysiological recordings and Ca2+ imaging techniques.Electrical stimulation of cholinergic axons revealed single efferent Ca2+ entry sites. The signal was spatially restricted and its amplitude (2.7 ± 0.3 %∆F/F0) correlated with the size of evoked inhibitory postsynaptic currents. Trains of stimuli gave rise to larger Ca2+ signals and their amplitude was dependent on stimulation frequency (20 Hz: 8.1 ± 2.6 %∆F/F0; 40 Hz: 13.0 ± 3.1 %∆F/F0; 80 Hz: 21.2 ± 2.7 %∆F/F0). Additionally, pharmacological blockade of Ca2+ uptake during efferent stimulation suggests a role in efferent Ca2+ signal decay. Finally, depolarization protocols used to activate VGCCs, allowed detection of afferent Ca2+ entry sites. These were closely positioned to efferent ones. Interestingly, the maximal amplitude reached during depolarizations (7.8 ± 1.3 %∆F/F0) was smaller than that attained by the cholinergic input. This result is consistent with previous reports showing limited voltage-gated Ca2+ currents in mature OHC. The larger size of efferent Ca2+ signals compared to afferent influx and their close proximity, suggests that afferent activation evoked by spilled-over efferent Ca2+ is a possibility.