Fast vesicle replenishment allows indefatigable signalling at the first auditory synapse

Abstract

Ribbon-type synapses in inner hair cells of the mammalian cochlea encode the complexity of auditory signals by fast and tonic release through fusion of neurotransmitter-containing vesicles. At any instant, only about 100 vesicles are tethered to the synaptic ribbon, and about 14 of these are docked to the plasma membrane, constituting the readily releasable pool. Although this pool contains about the same number of vesicles as that of conventional synapses, ribbon release sites operate at rates of about two orders of magnitude higher and with submillisecond precision. How these sites replenish their vesicles so efficiently remains unclear. We show here, using two-photon imaging of single release sites in the intact cochlea, that preformed vesicles derived from cytoplasmic vesicle-generating compartments participate in fast release and replenishment. Vesicles were released at a maximal initial rate of 3 per millisecond during a depolarizing pulse, and were replenished at a rate of 1.9 per millisecond. We propose that such rapid resupply of vesicles enables temporally precise and sustained release rates. This may explain how the first auditory synapse can encode with indefatigable precision without having to rely on the slow, local endocytic vesicle cycle.

Time course and calcium dependence of transmitter release at a single ribbon synapse

“…Capacitance recordings from mammalian IHCs provide estimates for the fast component of 280 (12) and 680 (11) (26); so that during a 1-s stimulus additional vesicles must be transported to the ribbon or directly toward the release site to support sustained release. Similarly, vesicle release monitored with a fluorescent membrane probe (27) provides an average rate of release of 1.4 vesicles per ribbon per ms for a 150-ms IHC depolarization, comparable to what we find by integrating over the same time window (1.2 vesicles per ribbon per ms).…”

Mechanisms contributing to synaptic Ca 2+ signals and their heterogeneity in hair cells

“…The voltage dependence varies even among active zones of an individual IHC, which may relate to the observation of different stimulus thresholds for styryl dye destaining across putative ribbon synapses (19). Because the hair cell is isopotential (9), voltage errors due to series resistance were corrected, and those due to drifts in electrode potential were minor, we expect experimental procedures to contribute little to the observed variance of the voltage of half activation (24 mV 2 ), which was 4 times larger than that of the whole-cell current compared among the same experiments.…”

“…Differential efferent control of SGN activity (17) seems conceptually obvious. There are also indications for differences in structure (18) and function (19) among active zones of an IHC; however, little is known about the underlying mechanism. Here, we used time-resolved confocal imaging of the fluorescence of low-affinity Ca 2ϩ indicators together with pharmacological manipulations and modeling to characterize synaptic Ca 2ϩ microdomains in IHCs of hearing mice.…”

Hearing requires otoferlin-dependent efficient replenishment of synaptic vesicles in hair cells

“…In such a steady state, vesicle replenishment has to balance vesicle fusion at the IHC AZ. Accordingly, high rates of initial and sustained exocytosis have been found in hair cells [3][4][5][6][7][8] . Ribbon-type AZs of IHCs replenish readily releasable vesicles at hundreds of Hz over several seconds of stimulation, faster than ribbon synapses in the eye [9][10][11][12][13][14][15] and most non-ribbon-type AZs 16 (but see ref.…”