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The Journal of Neuroscience, June 13, 2007, 27(24):6442-6451; doi:10.1523/JNEUROSCI.5364-06.2007

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Cellular/Molecular
Persistence of Ca_v1.3 Ca²⁺ Channels in Mature Outer Hair Cells Supports Outer Hair Cell Afferent Signaling

Martina Knirsch,^1 * Niels Brandt,^1 * Claudia Braig,² Stephanie Kuhn,¹ Bernhard Hirt,³ Stefan Münkner,¹ Marlies Knipper,² and Jutta Engel¹

¹Institute of Physiology II, ²Department of Otorhinolaryngology, Molecular Neurobiology, and ³Institute of Anatomy, Tübingen Hearing Research Centre, University of Tübingen, D-72076 Tübingen, Germany

Correspondence should be addressed to Dr. Jutta Engel, University of Tübingen, Institute of Physiology II and Department of Otolaryngology, Tübingen Hearing Research Centre, Gmelinstrasse 5, D-72076 Tübingen, Germany. Email: jutta.engel{at}uni-tuebingen.de

Outer hair cells (OHCs) are innervated by type II afferent fibers of as yet unknown function. It is still a matter of debate whether OHCs perform exocytosis. If so, they would require presynaptic Ca²⁺ channels at their basal poles where the type II fibers make contacts. Here we show that L-type Ca²⁺ channel currents (charge carrier, 10 mM Ba²⁺) present in neonatal OHCs [postnatal day 1 (P1) to P7] decreased from 170 to 50 pA at approximately the onset of hearing. Ba²⁺ currents could hardly be measured in mature mouse OHCs because of their high fragility, whereas in the rat, the average Ba²⁺ current amplitude of apical OHCs was 58 ± 9 pA (n = 20, P19–P30) compared with that of the inner hair cells (IHCs) of 181 ± 50 pA (n = 24, P17–P30). Properties of Ba²⁺ currents of mature OHCs resembled those of neonatal OHCs. One exception was the voltage dependence of activation that shifted between birth and P12 by +9 mV toward positive voltages in OHCs, whereas it remained constant in the IHCs. Ca_v1.3-specific mRNA was detected in mature OHCs using cell-specific reverse transcription (RT)-PCR and in situ hybridization. Ca_v1.3 protein was stained exclusively at the base of mature OHCs, in colocalization with the ribbon synapse protein CtBP2 (C-terminal binding protein 2)/RIBEYE. When current sizes were normalized to the estimated number of afferent fibers or presynaptic ribbons, comparable values for IHCs and OHCs were obtained, a finding that together with the colocalization of Ca_v1.3 and CtBP2/RIBEYE protein strongly suggests a role for Ca_v1.3 channels in exocytosis of mature OHCs.

Key words: mouse; rat; hair cell; ribbon synapse; spiral ganglion; development

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Received Dec. 12, 2006; revised May 9, 2007; accepted May 11, 2007.

Correspondence should be addressed to Dr. Jutta Engel, University of Tübingen, Institute of Physiology II and Department of Otolaryngology, Tübingen Hearing Research Centre, Gmelinstrasse 5, D-72076 Tübingen, Germany. Email: jutta.engel{at}uni-tuebingen.de

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