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The Journal of Neuroscience, February 20, 2008, 28(8):1798-1803; doi:10.1523/JNEUROSCI.4653-07.2008

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Brief Communications
Calcium- and Otoferlin-Dependent Exocytosis by Immature Outer Hair Cells

Maryline Beurg,¹ Saaid Safieddine,² Isabelle Roux,² Yohan Bouleau,¹ Christine Petit,² and Didier Dulon¹

¹Equipe Neurophysiologie de la Synapse Auditive, Equipe Mixte de Recherche, Inserm U587 et Université Victor Segalen, Institut des Neurosciences de Bordeaux, Centre Hospitalier Universitaire Pellegrin, 33076 Bordeaux, France, and ²Unité de Génétique et Physiologie de l'audition, Unité Mixte de Recherche, Inserm U587 et Université Pierre et Marie Curie, Collège de France, Institut Pasteur, 75015 Paris, France

Correspondence should be addressed to Christine Petit or Didier Dulon, Inserm Unité Mixte de Recherche U587, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris Cedex 15, France. Email: cpetit{at}pasteur.fr or Email: dulon{at}bordeaux.inserm.fr

Immature cochlear outer hair cells (OHCs) make transient synaptic contacts (ribbon synapses) with type I afferent nerve fibers, but direct evidence of synaptic vesicle exocytosis is still missing. We thus investigated calcium-dependent exocytosis in murine OHCs at postnatal day 2 (P2)–P3, a developmental stage when calcium current maximum amplitude was the highest. By using time-resolved patch-clamp capacitance measurements, we show that voltage step activation of L-type calcium channels triggers fast membrane capacitance increase. Capacitance increase displayed two kinetic components, which are likely to reflect two functionally distinct pools of synaptic vesicles, a readily releasable pool (RRP; = 79 ms) and a slowly releasable pool ( = 870 ms). The RRP size and maximal release rate were estimated at 1200 vesicles and 15,000 vesicles/s, respectively. In addition, we found a linear relationship between capacitance increase and calcium influx, like in mature inner hair cells (IHCs). These results give strong support to the existence of efficient calcium-dependent neurotransmitter release in immature OHCs. Moreover, we show that immature OHCs, just like immature IHCs, are able to produce regenerative calcium-dependent action potentials that could trigger synaptic exocytosis in vivo. Finally, the evoked membrane capacitance increases were abolished in P2–P3 OHCs from mutant Otof^–/– mice defective for otoferlin, despite normal calcium currents. We conclude that otoferlin, the putative major calcium sensor at IHC ribbon synapses, is essential to synaptic exocytosis in immature OHCs too.

Key words: cochlea; organ of Corti; outer hair cells; ribbon synapses; exocytosis; otoferlin

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Received July 21, 2007; revised Dec. 27, 2007; accepted Dec. 27, 2007.

Correspondence should be addressed to Christine Petit or Didier Dulon, Inserm Unité Mixte de Recherche U587, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris Cedex 15, France. Email: cpetit{at}pasteur.fr or Email: dulon{at}bordeaux.inserm.fr

This article has been cited by other articles:

				S. L. Johnson, A. Forge, M. Knipper, S. Munkner, and W. Marcotti Tonotopic Variation in the Calcium Dependence of Neurotransmitter Release and Vesicle Pool Replenishment at Mammalian Auditory Ribbon Synapses J. Neurosci., July 23, 2008; 28(30): 7670 - 7678. [Abstract] [Full Text] [PDF]

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