title: Fast Ca2+ signals at mouse inner hair cell synapse: a role for Ca2+-induced Ca2+ release.
abstract: Inner hair cells of the mammalian cochlea translate acoustic stimuli into 'phase-locked' nerve impulses with frequencies of up to at least 1 kHz. Little is known about the intracellular Ca2+ signal that links transduction to the release of neurotransmitter at the afferent synapse. Here, we use confocal microscopy to provide evidence that Ca2+-induced Ca2+ release (CICR) may contribute to the mechanism. Line scan images (2 ms repetition rate) of neonatal mouse inner hair cells filled with the fluorescent indicator FLUO-3, revealed a transient increase in intracellular Ca2+ concentration ([Ca2+]i) during brief (5-50 ms) depolarizing commands under voltage clamp. The amplitude of the [Ca2+]i transient depended upon the Ca2+ concentration in the bathing medium in the range 0-1.3 mM. [Ca2+]i transients were confined to a region near the plasma membrane at the base of the cell in the vicinity of the afferent synapses. The change in [Ca2+]i appeared uniform throughout the entire basal sub-membrane space and we were unable to observe hotspots of activity. Both the amplitude and the rate of rise of the [Ca2+]i transient was reduced by external ryanodine (20 microM), an agent that blocks Ca2+ release from the endoplasmic reticulum. Intracellular Cs+, commonly used to record at presynaptic sites, produced a similar effect. We conclude that both ryanodine and intracellular Cs+ block CICR in inner hair cells. We discuss the contribution of CICR to the measured [Ca2+]i transient, the implications for synaptic transmission at the afferent synapse and the significance of its sensitivity to intracellular Cs+.
authors.affiliation: Department of Physiology, University of Bristol, Bristol, BS8 1TD, UK. helen.kennedy@bristol.ac.uk
authors.forename: Helen J
authors.initials: HJ
authors.lastname: Kennedy
authors.forename: Robert W
authors.initials: RW
authors.lastname: Meech
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D020013:Calcium Signaling; D002586:Cesium / Q000494:pharmacology; D004553:Electric Conductivity; D006199:Hair Cells, Auditory, Inner / Q000502:physiology*; D051379:Mice; D008815:Mice, Inbred Strains; D012433:Ryanodine / Q000494:pharmacology; D013569:Synapses / Q000502:physiology*; D013997:Time Factors
chemical_list: D012433:Ryanodine; D002586:Cesium; D002118:Calcium