Segregation of opposing Ca2+ signals in cochlear hair cells during development

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

Congreso; 2nd FALAN Congres - SAN; 2016

FALAN - Sociedad Argentina de Investigación en Neurociencia

Before the onset of hearing (P14), cochlear inner hair cells (IHCs) of rodents receive efferent and afferent innervation. The entry of Ca2+ following the firing of spontaneous action potentials evokes the release of glutamate to afferent dendrites of the auditory nerve. In the efferent synapse, the entry of Ca2+ through α9α10 cholinergic receptors activates SK2, Ca2+-dependent potassium channels, producing hyperpolarization. We aim to investigate the segregation of these opposing Ca2+ signals within a small diffusional space.Electron-micrographs of IHC exhibited thin near-membrane cisterns juxtaposed to efferent contacts and intertwined afferent ribbon synapses. Electrophysiological and Ca2+ imaging measurements in IHCs from P9-P11 mice revealed multiple and independent Ca2+ entry hotspots following single pulse efferent electrical stimulation. Trains of stimuli and ACh applications evoked global Ca2+ signal increases. In afferent boutons recordings, exogenous ACh applications produced a strong increase in the frequency of EPSCs, presumably due to release triggered by Ca2+ influx through α9α10. This was reproduced by electrical stimulation of efferent fibers with 80 Hz trains. Both morphological and functional data provide evidence for a close proximity between afferent and efferent synapses in developing IHC. Physical barriers, imposed by synaptic cisterns and/or strong Ca2+ buffering in IHC prevent efferent to afferent synaptic crosstalk only at low frequency stimulation.