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First published online on January 4, 2002.
 Copyright © 2002 by The Physiological Society
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2001.013171v1
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Received August 10, 2001
Accepted after revision October 30, 2001

Fast Ca2+ signals at mouse inner hair cell synapse: a role for Ca2+-induced Ca2+ release

H. Kennedy1* and Robert W. Meech2

1 Department of Physiology, University Walk, Bristol, BS8 1TD, UK
2 Department of Physiology, University of Bristol, Bristol, BS8 1TD, UK

* To whom correspondence should be addressed. E-mail: helen.kennedy{at}bristol.ac.uk.

Inner hair cells of the mammalian cochlea translate acoustic stimuli into 'phase-locked' nerve impulses with frequencies of 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 µm), 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+.




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