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

Abstract

At the first synapse in the auditory pathway, the receptor potential of mechanosensory hair cells is converted into a firing pattern in auditory nerve fibers. For the accurate coding of timing and intensity of sound signals, transmitter release at this synapse must occur with the highest precision. To measure directly the transfer characteristics of the hair cell afferent synapse, we implemented simultaneous whole-cell recordings from mammalian inner hair cells (IHCs) and auditory nerve fiber terminals that typically receive input from a single ribbon synapse. During a 1-s IHC depolarization, the synaptic response depressed >90%, representing the main source for adaptation in the auditory nerve. Synaptic depression was slightly affected by ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor desensitization; however, it was mostly caused by reduced vesicular release. When the transfer function between transmitter release and Ca 2؉ influx was tested at constant open probability for Ca 2؉ channels (potentials >0 mV), a super linear relation was found. This relation is presumed to result from the cooperative binding of three to four Ca 2؉ ions at the Ca 2؉ sensor. However, in the physiological range for receptor potentials (؊50 to ؊30 mV), the relation between Ca 2؉ influx and afferent activity was linear, assuring minimal distortion in the coding of sound intensity. Changes in Ca 2؉ influx caused an increase in release probability, but not in the average size of multivesicular synaptic events. By varying Ca 2؉ buffering in the IHC, we further investigate how Ca 2؉ channel and Ca 2؉ sensor at this synapse might relate.hearing ͉ exocytosis ͉ hair cell ͉ synaptic transmission ͉ auditory nerve fiber M echanosensory hair cells of the inner ear release neurotransmitter continuously and with high temporal precision onto dendrites of auditory nerve fibers (AFs) (1); therefore, availability of releasable synaptic vesicles is critical. Synaptic ribbons, presynaptic structures presenting a halo of tethered vesicles, also found in retinal photoreceptors and bipolar cells (2), are thought to facilitate accumulation of vesicles presynaptically (3). Postsynaptically, ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors mediate fast synaptic transmission (4). The response of the cochlea to sound has been investigated in great detail: prolonged sound stimulation produces an increase in the firing activity of the auditory nerve that adapts on a milliseconds time scale (5). However, direct demonstration of the synaptic processes underlying adaptation and sustained release is still missing. Heretofore, hair cell exocytosis has been measured by changes in membrane capacitance that sum activity from all of the dozens of ribbon synapses in each cell (3). Capacitance measurements therefore cannot distinguish properties of individual ribbons and are limited in temporal resolution, requiring extrapolation for the earliest components. Nor does this approach reveal what role postsynaptic desensitization might play.I...

An Improved Model for the Rate–Level Functions of Auditory-Nerve Fibers

“…Our data are difficult to reconcile with the assumption of a nanodomain control of exocytosis (Neher, 1998;Augustine et al, 2003;Brandt et al, 2005;Goutman and Glowatzki, 2007;Yamashita et al, 2010), at least in its extreme form. According to this assumption, the vesicle with its Ca 2ϩ sensors is in such close proximity to a Ca 2ϩ channel that it senses the gating of essentially this single channel only, and Ca 2ϩ influx through this single channel suffices to trigger the vesicle's fusion.…”

“…These exponents are difficult to reconcile with our reasoning. However, the exponents were estimated from fits of a nonsaturating power law to normalized data based on only three cell-pair recordings, five to seven voltage steps per recording, and two to four repetitions per voltage step and recording, and associated with relatively large error bars [Goutman and Glowatzki (2007), their Fig. 3,compare A2,C,B2,D].…”

“…Fourth, changes in the whole-cell Ca 2ϩ current with membrane potential over the physiological range can also be well approximated by a linear relationship [Johnson et al (2005), their Fig. 3C; Goutman and Glowatzki (2007), their Fig. 3A1].…”

“…Fusion of synaptic vesicles with the presynaptic membrane of an IHC depends critically on the presence of Ca 2ϩ (Goutman and Glowatzki, 2007), as is the case at all other chemical synapses. Beutner et al (2001) …”

A phenomenological model of the synapse between the inner hair cell and auditory nerve: Long-term adaptation with power-law dynamics

“…The IHC-AN synapse complex is believed to be mainly responsible for this adaptation. Although the mechanism that gives rise to synaptic adaptation is not completely understood, it could be caused either by the depletion of neurotransmitter from a readily releasable presynaptic pool of neurotransmitter ͑Moser and Beutner, 2000; Schnee et al, 2005;Goutman and Glowatzki, 2007͒ or by the desensitization of post-synaptic receptors ͑Raman et al, 1994͒. Modeling the adaptation in the IHC-AN synapse has been a focus of extensive research over the last several decades. Early attempts employed a single-reservoir system with loss and replenishment of transmitter quanta ͑Schroeder and Hall, 1974;Sujaku, 1974, 1975͒, and later models added extra reservoirs ͑or sites͒ or more complex principles of transmitter flow control ͑Furukawa and Matsuura, 1978;Furukawa et al, 1982;Ross, 1982Ross, , 1996Schwid and Geisler, 1982;Smith and Brachman, 1982;Cooke, 1986;Meddis, 1986Meddis, , 1988Westerman and Smith, 1988͒.…”