Hair Cell Afferent Synapses: Function and Dysfunction

Abstract

To provide a meaningful representation of the auditory landscape, mammalian cochlear hair cells are optimized to detect sounds over an incredibly broad range of frequencies and intensities with unparalleled accuracy. This ability is largely conferred by specialized ribbon synapses that continuously transmit acoustic information with high fidelity and sub-millisecond precision to the afferent dendrites of the spiral ganglion neurons. To achieve this extraordinary task, ribbon synapses employ a unique combination of molecules and mechanisms that are tailored to sounds of different frequencies. Here we review the current understanding of how the hair cell's presynaptic machinery and its postsynaptic afferent connections are formed, how they mature, and how their function is adapted for an accurate perception of sound.

Figure 1.

Morphological organization of an inner hair cell (IHC). Hair cell diagram showing the USH1 protein network essential for mechanoelectrical transduction (upper inset). At the synaptic region the protein otoferlin, the glutamate transporter (VGLUT3), clarin, harmonin, CDHR23, and CDHR15 involved in hair cell synaptic transmission (bottom inset). “?”, the presence of CDHR23 and CDHR15 at the ribbon synapses is still debatable.

Figure 2.

Ribbon synaptic configuration in mature inner hair cells (IHCs). At the IHC modiolar side, presynaptic regions have larger ribbons (red) and display higher Ca²⁺ inputs (light blue). Afferent neurons have smaller diameters, higher-threshold (HT) sensitivity and lower spontaneous rate (LSR). At the pillar side, ribbons are smaller and Ca²⁺ influx appears more confined; afferents have larger diameter, low-threshold (LT) sensitivity, and higher spontaneous rate (HSR). (References for afferent and ribbon characteristics: Liberman 1980 and Liberman et al. 2011; for presynaptic Ca²⁺ influx: Meyer et al. 2009 and Wong et al. 2014.) (Image reprinted from Safieddine et al. 2012 with permission from Annual Review of Neuroscience © 2012.)

Figure 3.

Diagram depicting the innervation inner (IHCs) and outer hair cells (OHCs) by spiral ganglion neurons (SGNs) in the adult cochlea. Illustration of the mature pattern of cochlear afferent innervation. Mature IHCs are innervated by ∼95% of the type I SGN fibers. In mice, about 10–20 unbranched myelinated SGNs connect with a single IHC while each neuron receives input from only one IHC. The remaining ∼5% of SGNs are type II unmyelinated neurons that project toward the OHCs and spiral toward the base of the cochlea contacting multiple OHCs.

Figure 4.

Maturation of the mouse cochlea. Diagram showing the morphological and functional maturation of spiral ganglion neurons (SGNs) and hair cells in the mouse cochlea. References on the morphology part: hair cells terminal mitosis (for review, see Basch et al. 2015); SGNs; terminal mitosis, reaching the hair cell region and their refinement (Koundakjian et al. 2007; Coate and Kelley 2013; Delacroix and Malgrange 2015); ribbon present in inner hair cells (IHCs) (Huang et al. 2012); the maturation of ribbon synapses and the establishment of the gradients within an IHC (Wong et al. 2014; Liberman and Liberman 2016). References on the functional part: spontaneous action potentials in outer hair cells (OHCs) (Ceriani et al. 2018) and IHCs (Kros et al. 1998; Beutner and Moser 2001; Marcotti et al. 2003b; Tritsch et al. 2007; Johnson et al. 2017a), onset of function in OHCs (Marcotti and Kros 1999; Abe et al. 2007) and IHCs (Kros et al. 1998; Marcotti et al. 2003a), and onset of hearing (Ehret 1983; Romand 1983).

Figure 5.

Colocalization between presynaptic ribbons and postsynaptic GluA2 in mature inner hair cells (IHCs). A confocal z-stack projection of IHCs triple-immunostained for otoferlin (green), the ribbon protein RIBEYE (blue), and the GluA2 subunit of postsynaptic AMPA receptors for glutamate (red). At mature IHC synapses, all RIBEYE and GluA2 puncta “co-localize” (arrowheads indicate just a few of them). Dotted lines provide a rough indication of the basolateral membrane around the IHC synaptic region. Note that the RIBEYE antibody also stained the IHC nuclei (n). The insets (top-right corners) are an expanded view of the IHC synaptic region. Scale bar, 5 µm.

Figure 6.

The coupling between Ca²⁺ entry and exocytosis differs as a function of IHC position along the mature cochlea. (A–C) The kinetics of RRP vesicle pool release in mature gerbil IHCs from apical (A), middle (B), and basal (C) cochlear regions in the presence of 0.1 mm EGTA (black circles) and 10 mm EGTA (gray circles). Average ΔC_m points were obtained in response to voltage steps from 2 to 50 msec (to −11 mV) that elicit the release of the RRP. The x-axis time refers to the voltage step duration. Note that 10 mm EGTA blocks RRP release in the basal coil IHCs, reduces it significantly in the middle coil, but only slightly in the apex. (From Fig. 2 in Johnson et al. 2017b; reprinted, with permission, from the Journal of Neuroscience © 2017.)