J Physiol 587.8 Exocytosis at hair cell ribbon synapses 1725 in immature IHCs should be under the control of a Ca2+ microdomain. However, we found that the distance between Ca2+ channels and release sites in pre-hearing IHCs positioned at either end of the cochlea was in the range of 40 nm, which is comparable and even slightly closer to that found in adult cells (about 50 nm: Johnson et al. 2008). Therefore, exocytosis in gerbil IHCs is under a Ca2+ nanodomain control irrespective of age or position along the cochlea, suggesting that something other than Ca2+ channel localization is likely to be responsible for the modulating the Ca2+ sensitivity. The different exocytotic Ca2+ dependence observed before and after the onset of hearing (Johnson et al. 2005) and as a function of tonotopic position in adult gerbils (Johnson et al. 2008) is likely to be intrinsic to the synaptic machinery of IHCs. Therefore, corresponding differences in the expression of Ca2+ sensing molecules at IHC ribbon synapses has to be considered. So far, the proposed candidate to fulﬁl this role in auditory but not in vestibular hair cells is otoferlin (Roux et al. 2006; Beurg et al. 2008). Since otoferlin appears to be similarly distributed in immature (Figs 1E and 6D) and adult gerbil hair cells (Johnson et al. 2008; for mouse see Roux et al. 2006), it is unlikely that it alone could control both the high-order (immature IHCs N ≈ 4; low-frequency adult cells N ≈ 2–3: Fig. 4C) and linear (high-frequency adult IHCs: Fig. 4C; immature OHCs: Fig. 6C) Ca2+ dependence of exocytosis in mammalian cochlear hair cells. Therefore, additional as yet unknown Ca2+ sensing proteins or promoters of exocytosis (Jahn et al. 2003), which are differentially expressed as a function of development and cochlear position, could be involved. References Augustine GJ, Charlton MP & Smith SJ (1985). 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