Spontaneous activity in the developing auditory system is required for neuronal survival as well as the refinement and maintenance of tonotopic maps in the brain. However, the mechanisms responsible for initiating auditory nerve firing in the absence of sound have not been determined. Here we show that supporting cells in the developing rat cochlea spontaneously release ATP, which causes nearby inner hair cells to depolarize and release glutamate, triggering discrete bursts of action potentials in primary auditory neurons. This endogenous, ATP-mediated signalling synchronizes the output of neighbouring inner hair cells, which may help refine tonotopic maps in the brain. Spontaneous ATP-dependent signalling rapidly subsides after the onset of hearing, thereby preventing this experience-independent activity from interfering with accurate encoding of sound. These data indicate that supporting cells in the organ of Corti initiate electrical activity in auditory nerves before hearing, pointing to an essential role for peripheral, non-sensory cells in the development of central auditory pathways.

Neurotransmitters are released continuously at ribbon synapses in the retina and cochlea. Notably, a single ribbon synapse of inner hair cells provides the entire input to each cochlear afferent fiber. We investigated hair cell transmitter release in the postnatal rat cochlea by recording excitatory postsynaptic currents (EPSCs) from afferent boutons directly abutting the ribbon synapse. EPSCs were carried by rapidly gating AMPA receptors. EPSCs were clustered in time, indicating the possibility of coordinate release. Amplitude distributions of spontaneous EPSCs were highly skewed, peaking at 0.4 nS and ranging up to 20 times larger. Hair cell depolarization increased EPSC frequency up to 150 Hz without altering the amplitude distribution. We propose that the ribbon synapse operates by multivesicular release, possibly to achieve high-frequency transmission.

Efferent feedback onto sensory organs provides a means to modulate input to the central nervous system. In the developing mammalian cochlea, inner hair cells are transiently innervated by efferent fibers, even before sensory function begins. Here, we show that neonatal inner hair cells are inhibited by cholinergic synaptic input before the onset of hearing. The synaptic currents, as well as the inner hair cell's response to acetylcholine, are mediated by a nicotinic (alpha9-containing) receptor and result in the activation of small-conductance calcium-dependent potassium channels.

The expression of unconventional vesicular glutamate transporter VGLUT3 by neurons known to release a different classical transmitter has suggested novel roles for signaling by glutamate, but this distribution has raised questions about whether the protein actually contributes to glutamate release. We now report that mice lacking VGLUT3 are profoundly deaf due to the absence of glutamate release from hair cells at the first synapse in the auditory pathway. The early degeneration of some cochlear ganglion neurons in knockout mice also indicates an important developmental role for the glutamate released by hair cells before the onset of hearing. In addition, the mice exhibit primary, generalized epilepsy that is accompanied by remarkably little change in ongoing motor behavior. The glutamate release conferred by expression of VGLUT3 thus has an essential role in both function and development of the auditory pathway, as well as in the control of cortical excitability.

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...

Inward rectifier K+ channels mediate the K+ conductance at resting potential in many types of cell. Since these K+ channels do not pass outward currents (inward rectification) when the cell membrane is depolarized beyond a trigger threshold, they play an important role in controlling excitability. Both a highly voltage-dependent block by intracellular Mg2+ and an endogenous gating process are presently assumed to underly inward rectification. It is shown that strong voltage dependence of rectification found under physiological conditions is predominantly due to the effect of intracellular spermine. Physiological concentrations of free spermine mediate strong rectification of IRK1 inward rectifier K+ channels even in the absence of free Mg2+ and in IRK1 mutant channels that have no endogenous rectification.

Cochlear inner hair cells (IHCs) convert sounds into receptor potentials and via their ribbon synapses into firing rates in auditory nerve fibers. Multivesicular release at individual IHC ribbon synapses activates AMPA-mediated EPSCs with widely ranging amplitudes. The underlying mechanisms and specific role for multivesicular release in encoding sound are not well understood. Here we characterize the waveforms of individual EPSCs recorded from afferent boutons contacting IHCs and compare their characteristics in immature rats (postnatal days 8 -11) and hearing rats (postnatal days 19 -21). Two types of EPSC waveforms were found in every recording: monophasic EPSCs, with sharp rising phases and monoexponential decays, and multiphasic EPSCs, exhibiting inflections on rising and decaying phases. Multiphasic EPSCs exhibited slower rise times and smaller amplitudes than monophasic EPSCs. Both types of EPSCs had comparable charge transfers, suggesting that they were activated by the release of similar numbers of vesicles, which for multiphasic EPSCs occurred in a less coordinated manner. On average, a higher proportion of larger, monophasic EPSCs was found in hearing compared to immature rats. In addition, EPSCs became significantly faster with age. The developmental increase in size and speed could improve auditory signaling acuity. Multiphasic EPSCs persisted in hearing animals, in some fibers constituting half of the EPSCs. The proportion of monophasic versus multiphasic EPSCs varied widely across fibers, resulting in marked heterogeneity of amplitude distributions. We propose that the relative contribution of two modes of multivesicular release, generating monophasic and multiphasic EPSCs, may underlie fundamental characteristics of auditory nerve fibers.

In the mature cochlea, inner hair cells (IHCs) transduce acoustic signals into receptor potentials, communicating to the brain by synaptic contacts with afferent fibers. Before the onset of hearing, a transient efferent innervation is found on IHCs, mediated by a nicotinic cholinergic receptor that may contain both ␣9 and ␣10 subunits. Calcium influx through that receptor activates calcium-dependent (SK2-containing) potassium channels. This inhibitory synapse is thought to disappear after the onset of hearing [after postnatal day 12 (P12)]. We documented this developmental transition using whole-cell recordings from IHCs in apical turns of the rat organ of Corti. Acetylcholine elicited ionic currents in 88 -100% of IHCs between P3 and P14, but in only 1 of 11 IHCs at P16 -P22. Potassium depolarization of efferent terminals caused IPSCs in 67% of IHCs at P3, in 100% at P7-P9, in 93% at P10 -P12, but in only 40% at P13-P14 and in none of the IHCs tested between P16 and P22. Earlier work had shown by in situ hybridization that ␣9 mRNA is expressed in adult IHCs but that ␣10 mRNA disappears after the onset of hearing. In the present study, antibodies to ␣10 and to the associated calcium-dependent (SK2) potassium channel showed a similar developmental loss. The correlated expression of these gene products with functional innervation suggests that Alpha10 and SK2, but not Alpha9, are regulated by synaptic activity. Furthermore, this developmental knock-out of ␣10, but not ␣9, supports the hypothesis that functional nicotinic acetylcholine receptors in hair cells are heteromers containing both these subunits.