Uniquantal Release through a Dynamic Fusion Pore Is a Candidate Mechanism of Hair Cell Exocytosis

The mechanisms underlying the large amplitudes and heterogeneity of excitatory postsynaptic currents (EPSCs) at inner hair cell (IHC) ribbon synapses are unknown. Based on electrophysiology, electron and superresolution light microscopy, and modeling, we propose that uniquantal exocytosis shaped by a dynamic fusion pore is a candidate neurotransmitter release mechanism in IHCs. Modeling indicated that the extended postsynaptic AMPA receptor clusters enable large uniquantal EPSCs. Recorded multiphasic EPSCs were triggered by similar glutamate amounts as monophasic ones and were consistent with progressive vesicle emptying during pore flickering. The fraction of multiphasic EPSCs decreased in absence of Ca2+ influx and upon application of the Ca2+ channel modulator BayK8644. Our experiments and modeling did not support the two most popular multiquantal release interpretations of EPSC heterogeneity: (1) Ca2+-synchronized exocytosis of multiple vesicles and (2) compound exocytosis fueled by vesicle-to-vesicle fusion. We propose that IHC synapses efficiently use uniquantal glutamate release for achieving high information transmission rates

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

International audienceInner hair cell (IHC) ribbon synapses indefatigably transmit acoustic information. The proteins mediating their fast vesicle replenishment (hundreds of vesicles/s) are unknown. Here we show that an Asp to Gly substitution in the CF domain of the synaptic vesicle protein otoferlin impairs hearing by reducing vesicle replenishment in the pachanga mouse model of human deafness DFNB9. estimates of vesicle docking, the readily releasable vesicle pool (RRP), Ca signaling and vesicle fusion were normal. Moreover, we observed postsynaptic excitatory currents of variable size and spike generation. However, mutant active zones (AZs) replenished vesicles at lower rates, and sound-evoked spiking in auditory neurons was sparse and only partially improved during longer inter-stimulus intervals. We conclude that replenishment does not keep up with release of vesicles at mutant AZs , such that a sufficient standing RRP cannot be maintained. We propose a novel role of otoferlin in replenishing synaptic vesicles

G protein-dependent presynaptic inhibition mediated by AMPA receptors at the calyx of Held

The α-amino-3-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is an ionotropic receptor mediating excitatory synaptic transmission, but it can also interact with intracellular messengers. Here we report that, at the calyx of Held in the rat auditory brainstem, activation of AMPARs induced inward currents in the nerve terminal and inhibited presynaptic Ca(2+) currents (I(pCa)), thereby attenuating glutamatergic synaptic transmission. The AMPAR-mediated I(pCa) inhibition was disinhibited by a strong depolarizing pulse and occluded by the nonhydrolyzable GTP analog GTPγS loaded into the terminal. We conclude that functional AMPARs are expressed at the calyx of Held nerve terminal and that their activation inhibits voltage-gated Ca(2+) channels by an interaction with heterotrimeric GTP-binding proteins (G proteins). Thus, at a central glutamatergic synapse, presynaptic AMPARs have a metabotropic nature and regulate transmitter release by means of G proteins

Interactions between multiple sources of short-term plasticity during evoked and spontaneous activity at the rat calyx of Held

Sustained activity at most central synapses is accompanied by a number of short-term changes in synaptic strength which act over a range of time scales. Here we examine experimental data and develop a model of synaptic depression at the calyx of Held synaptic terminal that combines many of these mechanisms (acting at differing sites and across a range of time scales). This new model incorporates vesicle recycling, facilitation, activity-dependent vesicle retrieval and multiple mechanisms affecting calcium channel activity and release probability. It can accurately reproduce the time course of experimentally measured short-term depression across different stimulus frequencies and exhibits a slow decay in EPSC amplitude during sustained stimulation. We show that the slow decay is a consequence of vesicle release inhibition by multiple mechanisms and is accompanied by a partial recovery of the releasable vesicle pool. This prediction is supported by patch-clamp data, using long duration repetitive EPSC stimulation at up to 400 Hz. The model also explains the recovery from depression in terms of interaction between these multiple processes, which together generate a stimulus-history-dependent recovery after repetitive stimulation. Given the high rates of spontaneous activity in the auditory pathway, the model also demonstrates how these multiple interactions cause chronic synaptic depression under in vivo conditions. While the magnitude of the depression converges to the same steady state for a given frequency, the time courses of onset and recovery are faster in the presence of spontaneous activity. We conclude that interactions between multiple sources of short-term plasticity can account for the complex kinetics during high frequency stimulation and cause stimulus-history-dependent recovery at this relay synapse