Inner hair cells (IHCs) of the cochlea use ribbon synapses to transmit auditory information faithfully to spiral ganglion neurons (SGNs). In the present study, we used genetic disruption of the presynaptic scaffold protein bassoon in mice to manipulate the morphology and function of the IHC synapse. Although partial-deletion mutants lacking functional bassoon (Bsn^ΔEx4/5) had a near-complete loss of ribbons from the synapses (up to 88% ribbonless synapses), gene-trap mutants (Bsn^gt) showed weak residual expression of bassoon and 56% ribbonless synapses, whereas the remaining 44% had a loosely anchored ribbon. Patch-clamp recordings and synaptic Ca_V1.3 immunolabeling indicated a larger number of Ca²⁺channels for Bsn^gtIHCs compared with Bsn^ΔEx4/5IHCs and for Bsn^gtribbon-occupied versus Bsn^gtribbonless synapses. An intermediate phenotype of Bsn^gtIHCs was also found by membrane capacitance measurements for sustained exocytosis, but not for the size of the readily releasable vesicle pool. The frequency and amplitude of EPSCs were reduced in Bsn^ΔEx4/5mouse SGNs, whereas their postsynaptic AMPA receptor clusters were largely unaltered. Sound coding in SGN, assessed by recordings of single auditory nerve fibers and their population responsesin vivo, was similarly affected in Bsn^gtand Bsn^ΔEx4/5mice. Both genotypes showed impaired sound onset coding and reduced evoked and spontaneous spike rates. In summary, reduced bassoon expression or complete lack of full-length bassoon impaired sound encoding to a similar extent, which is consistent with the comparable reduction of the readily releasable vesicle pool. This suggests that the remaining loosely anchored ribbons in Bsn^gtIHCs were functionally inadequate or that ribbon independent mechanisms dominated the coding deficit.

We studied the role of the synaptic ribbon for sound encoding at the synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) in mice lacking RIBEYE (RBE^KO/KO). Electron and immunofluorescence microscopy revealed a lack of synaptic ribbons and an assembly of several small active zones (AZs) at each synaptic contact. Spontaneous and sound-evoked firing rates of SGNs and their compound action potential were reduced, indicating impaired transmission at ribbonless IHC-SGN synapses. The temporal precision of sound encoding was impaired and the recovery of SGN-firing from adaptation indicated slowed synaptic vesicle (SV) replenishment. Activation of Ca²⁺-channels was shifted to more depolarized potentials and exocytosis was reduced for weak depolarizations. Presynaptic Ca²⁺-signals showed a broader spread, compatible with the altered Ca²⁺-channel clustering observed by super-resolution immunofluorescence microscopy. We postulate that RIBEYE disruption is partially compensated by multi-AZ organization. The remaining synaptic deficit indicates ribbon function in SV-replenishment and Ca²⁺-channel regulation.

Significance: Ca ²⁺ ions serve as a key cellular signal and are tightly controlled. One mechanism to limit free Ca ²⁺ ions is buffering by Ca ²⁺ -binding proteins, which are strongly expressed in sensory hair cells of the ear. Here we studied how genetic disruption of the Ca ²⁺ -binding proteins parvalbumin-α, calbindin-D28k, and calretinin affects exocytosis and sound encoding at the synapses of mouse inner hair cells (IHCs) and spiral ganglion neurons (SGNs). Mutant IHCs showed increased exocytosis, but the sound-evoked spiking activity in SGNs was unaltered. Together with mathematical modeling, this finding indicates that a large fraction of exocytosis in mutant IHCs occurred outside synapses. We conclude that Ca ²⁺ -binding proteins shape presynaptic Ca ²⁺ signals to restrict exocytosis to active zones, thus enabling metabolically efficient sound encoding.

Myelinating oligodendrocytes enable fast propagation of action potentials along the ensheathed axons. In addition, oligodendrocytes play diverse non-canonical roles including axonal metabolic support and activity-dependent myelination. An open question remains whether myelination also contributes to information processing in addition to speeding up conduction velocity. Here, we analyze the role of myelin in auditory information processing using paradigms that are also good predictors of speech understanding in humans. We compare mice with different degrees of dysmyelination using acute multiunit recordings in the auditory cortex, in combination with behavioral readouts. We find complex alterations of neuronal responses that reflect fatigue and temporal acuity deficits. We observe partially discriminable but similar deficits in well myelinated mice in which glial cells cannot fully support axons metabolically. We suggest a model in which myelination contributes to sustained stimulus perception in temporally complex paradigms, with a role of metabolically active oligodendrocytes in cortical information processing.

Transgenic mice have become an important tool to elucidate the genetic foundation of the human language faculty. While learning is an essential prerequisite for the acquisition of human speech, it is still a matter of debate whether auditory learning plays any role in the development of species-specific vocalizations in mice. To study the influence of auditory input on call development, we compared the occurrence and structure of ultrasonic vocalizations from deaf otoferlin-knockout mice, a model for human deafness DFNB9, to those of hearing wild-type and heterozygous littermates.