J Physiol 587.8 Exocytosis at hair cell ribbon synapses 1719 Table 1. Synaptic ribbon properties in gerbil hair cells Ca2+ dependence of exocytosis (power N) Ribbon no. Vesicles in the total RRP RRP vesicles per ribbon RRP release rate (vesicles s−1) RRP release rate per ribbon (vesicles s−1) SRP release rate (vesicles s−1) SRP release rate per ribbon (vesicles s−1) SRP depletion time constant (s) Immature OHCs (3rd row) Apical Basal 0.7 18 135 8 — — — — — 1.1 22 143 7 — — — — — Immature IHCs Adult IHCs Apical 3.6 36 400 11 12 911 355 8208 216 8.6 Basal 3.9 37 510 14 13 425 360 7933 214 4.8 Apical 2.2 21 534 26 10 449 507 2335 111 12.0 Basal 1.0 22 619 28 14 883 664 1695 77 21.3 All numbers reported are mean values. S.E.M. and cell numbers for immature data are given in Results. Adult values are obtained or calculated from Johnson et al. 2008. For immature OHCs, recording were only performed from the outer row (3rd row) of cells. The ribbon number and RRP vesicles per ribbon in immature OHCs are based on the estimation of two ribbons per active zone (see Results). OHC exocytotic kinetics were not measured. We did not observe any RRP depletion in both immature and adult IHCs using 50 ms voltage steps (at about 7 Hz). Figure 2. Depletion of synaptic vesicle pools in immature IHCs A and B, C m responses from basal-coil IHCs (P7) elicited using repetitive voltage steps to near −11 mV. The duration of the voltage step was selected in order to elicit the RRP (A: 50 ms) or the SRP (B: 1 s), with an interstep-interval of 100 ms or 200 ms, respectively. For clarity, only the ﬁrst few steps are shown. The voltage protocol used is shown above the traces. C and D, average cumulative C m values elicited using 50 (RRP) and 35 (SRP) steps from P7–P10 apical (RRP: n = 6; SRP: n = 7) and basal (RRP: n = 11; SRP: n = 10) IHCs, respectively. E and F, individual C m values measured following each voltage step from panels C and D, respectively, in both apical and basal IHCs. Fits in F are according to a ﬁrst-order exponential equation. C 2009 The Authors. Journal compilation C 2009 The Physiological Society