The Unitary Event Underlying Multiquantal EPSCs at a Hair Cell's Ribbon Synapse

Abstract

EPSCs at the synapses of sensory receptors and of some CNS neurons include large events thought to represent the synchronous release of the neurotransmitter contained in several synaptic vesicles by a process known as multiquantal release. However, determination of the unitary, quantal size underlying such putatively multiquantal events has proven difficult at hair cell synapses, hindering confirmation that large EPSCs are in fact multiquantal. Here, we address this issue by performing presynaptic membrane capacitance measurements together with paired recordings at the ribbon synapses of adult hair cells. These simultaneous presynaptic and postsynaptic assays of exocytosis, together with electron microscopic estimates of single vesicle capacitance, allow us to estimate a single vesicle EPSC charge of approximately Ϫ45 fC, a value in close agreement with the mean postsynaptic charge transfer of uniformly small EPSCs recorded during periods of presynaptic hyperpolarization. By thus establishing the magnitude of the fundamental quantal event at this peripheral sensory synapse, we provide evidence that the majority of spontaneous and evoked EPSCs are multiquantal. Furthermore, we show that the prevalence of uniquantal versus multiquantal events is Ca 2ϩ dependent. Paired recordings also reveal a tight correlation between membrane capacitance increase and evoked EPSC charge, indicating that glutamate release during prolonged hair cell depolarization does not significantly saturate or desensitize postsynaptic AMPA receptors. We propose that the large EPSCs reflect the highly synchronized release of multiple vesicles at single presynaptic ribbon-type active zones through a compound or coordinated vesicle fusion mechanism.

Evidence that protons act as neurotransmitters at vestibular hair cell–calyx afferent synapses

“…(B) Evoked EPSCs recorded from (i and ii) two calyces showing adaptation of quantal release rates in response to deflection of a single hair bundle. The total current in these cells could be accounted for by a superposition of quantal events (32). (C) Example recordings from (i and ii) two calyces, where deflection of a single hair bundle simultaneously evoked nqEPSCs and quantal EPSCs.…”

“…This energy efficiency may explain why nqEPSCs and calyx terminals are prolific in modern vestibular hair cell organs responsible for sensing tonic signals such as gravity, but absent from auditory hair cell organs responsible for sensing fast transient signals (12,32).…”

Modeling the resonant release of synaptic transmitter by hair cells as an example of biological oscillators with cooperative steps

“…For example, negative cooperativity has been suggested to render release in hippocampal synapses more reliable by decreasing the standard deviation of the number of vesicles released by an action potential (21). The positive cooperativity examined here readily explains the multiquantal release characteristic of hair cells (4)(5)(6). More specifically, the large excitatory postsynaptic currents observed electrophysiologically could result from the essentially synchronous fusion of two to perhaps ten synaptic vesicles at adjacent release sites (Fig.…”

“…First, the neurotransmitter content of multiple vesicles can be released synchronously, yielding large, multiquantal postsynaptic responses (4)(5)(6). Although this behavior might originate from prefusion of several vesicles, it could alternatively stem from the tight coordination of vesicle release at the synaptic ribbon.…”

Postsynaptic Recordings at Afferent Dendrites Contacting Cochlear Inner Hair Cells: Monitoring Multivesicular Release at a Ribbon Synapse

“…Ribbon synapses of sensory cells release neurotransmitter continuously, the rate of which is modulated in response to graded changes in IHC membrane potential 5 . Ribbon synapses have been shown to operate by multivesicular release, where multiple vesicles can be released simultaneously to evoke excitatory postsynaptic currents (EPSCs) of varying amplitudes 1, 4,[6][7][8][9][10][11] . Neither the role of the presynaptic ribbon, nor the mechanism underlying multivesicular release is currently well understood.…”