Functional Postnatal Maturation of the Medial Olivocochlear Efferent–Outer Hair Cell Synapse

Abstract

The organ of Corti, the auditory mammalian sensory epithelium, contains two types of mechanotransducer cells, inner hair cells (IHCs) and outer hair cells (OHCs). IHCs are involved in conveying acoustic stimuli to the CNS, while OHCs are implicated in the fine tuning and amplification of sounds. OHCs are innervated by medial olivocochlear (MOC) cholinergic efferent fibers. The functional characteristics of the MOC-OHC synapse during maturation were assessed by electrophysiological and pharmacological methods in mouse organs of Corti at postnatal day 11 (P11)-P13, hearing onset in altricial rodents, and at P20-P22 when the OHCs are morphologically and functionally mature. Synaptic currents were recorded in whole-cell voltageclamped OHCs while electrically stimulating the MOC fibers. A progressive increase in the number of functional MOC-OHC synapses, as well as in their strength and efficacy, was observed between P11-13 and P20-22. At hearing onset, the MOC-OHC synapse presented facilitation during MOC fibers high-frequency stimulation that disappeared at mature stages. In addition, important changes were found in the VGCC that are coupled to transmitter release. Ca 21 flowing in through L-type VGCCs contribute to trigger ACh release together with P/Q-and R-type VGCCs at P11-P13, but not at P20-P22. Interestingly, N-type VGCCs were found to be involved in this process at P20-P22, but not at hearing onset. Moreover, the degree of compartmentalization of calcium channels with respect to BK channels and presynaptic release components significantly increased from P11-P13 to P20-P22. These results suggest that the MOC-OHC synapse is immature at the onset of hearing.

Distribution of Kv3 Subunits in Cochlear Afferent and Efferent Nerve Fibers Implies Distinct Role in Auditory Processing

“…Not surprisingly, neurotransmitter release from cochlear efferent fiber axons are modulated by multiple mechanisms [55,56]. Metabotropic glutamate receptor, voltage gated Ca 2+ channels and Ca 2+ -activated BK K + channel at the presynaptic efferent fiber axons had been reported to contribute to the efferent synaptic transmission [56][57][58]. Here, based on our finding of K v 3.4 at the MOC and LOC efferent fiber axons, we propose to add K v 3.4 to the list of proteins modulating cochlear efferent neurotransmitter release.…”

Efferent feedback enforces bilateral coupling of spontaneous activity in the developing auditory system

“…P10-P11 is considered a transition age where MOC-OHC synapses form and MOC-IHC synapses start to decompose (Simmons, 2002, Elgoyhen andKatz, 2012). Indeed, MOC mediated IPSCs can be recorded in both IHCs and OHCs at P11 (Ballestero et al, 2011, Kearney et al, 2019, Vattino et al, 2020. Consistently, bilateral coupling can be rescued with chemogenetic excitation at P11 (Figure 4 J-L), suggesting the existence of MOC-IHC synapses in vivo.…”

“…Here we reported elevated auditory thresholds in the α9/α10 knockout at hearing onset across the frequency spectrum ( Figure 5). Although MOC-OHC synapses are already functional at this age (Vattino et al, 2020), the deficit is probably not caused by loss of acute MOC inhibition on the cochlear amplifier. In fact, eliminating the efferent modulation does not change auditory sensitivity as knocking out the α9 nAChR or the coupled SK2 channel does not alter ABR thresholds in mature animals (Ryugo et al, 2011, Morley et al, 2017, while strengthening efferent inhibitions leads to threshold elevation (Taranda et al, 2009).…”

Synaptic contributions to cochlear outer hair cell Ca2+homeostasis

“…See Methods) such that inward currents were due to the activation of both α9α10 receptors and SK2 channels (K + equilibrium potential: ~-82 mV). Paired pulses were used instead of single stimuli to increase the otherwise very low release probability (Ballestero et al, 2011; Vattino et al, 2020). Due to frame rate limitations, the imaging signal appears as the ensemble activation in response to both stimuli in a pair.…”

“…Representative traces of fluorescence changes at the brightest ROI are shown in the bottom panel of Figure 2B (red traces), whereas the corresponding synaptic currents recorded in the same trials are included in the top panel (black traces). Paired pulses were used instead of single stimuli to increase the otherwise very low release probability (Ballestero et al, 2011;Vattino et al, 2020). Due to frame rate limitations, the imaging signal appears as the ensemble activation in response to both stimuli in a pair.…”