Effects of Acoustic Trauma on Neurotransmitter Release by Inner Hair Cells

BOERO, LUIS E.; PAYNE, S; GÓMEZ CASATI, ME; RUTHERFORD, M; GOUTMAN, JD

Congreso; 43RD ANNUAL MidWinter Meeting; 2020

Noise exposure has gained relevance as one of the most important sources of hearing loss. The underlying causes of this condition have been investigated over years at histological and morphological levels, with much less progress in the physiological aspects. It has been demonstrated that acoustic trauma (AT) produces long-term structural damages to the inner ear, such as a reduction in the number synapses between inner hair cells (IHCs) and afferent neurons. Here we intend to address if the capacity of IHCs to release neurotransmitter is altered after noise exposure. Mice of either sex at ages P15-P16 (C57BL/6J VGluT3 KO or WT) were exposed to 1-12 kHz noise at 120 dB SPL, for 1 hour in a reverberant acoustic chamber. Hearing function was assessed before, one day after and two weeks after noise exposure through auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAE). Ca2+ currents and changes in membrane capacitance (∆Cm) were recorded in wholecell patch clamp configuration from IHCs one day after exposure. Post-exposure afferent synaptopathy was quantified by whole-mount immunostaining for CtBP2?the major component of the synaptic ribbon-, GluA3?a postsynaptic AMPA receptor subunit- and CaV1.3, to see the synaptic coupling to calcium entry. IHCs from either exposed (1 day after trauma) or unexposed mice were recorded, and the voltagedependence of release was investigated with short depolarizing pulses. Larger ∆Cm jumps were observed in exposed IHC compared to controls (20.2±3.2 fF and12.4±1.8 fF, respectively at -30 mV pulse). However, Ca2+ currents did not show any difference between exposed and control mice at these voltages. In addition, exposed IHC showed augmented ∆Cm specially with pulses above 100 ms (for 1s pulse: 104.5±7.1 fF for control, 167.7±22.6 fF for exposed). No differences in calcium entry between exposed and control cells were observed for any duration of depolarization. During AT protocol, IHC suffers a big Ca2+ influx and also releases large amounts of glutamate (which could act retrogradely). To determine which of these two is responsible for the potentiated release, we made use of the vesicular glutamate transporter vGluT3 knock-out (KO) mouse. In contrast to what was observed in WT mice, exposed KO showed reduced ∆Cm compared to controls. These results suggest that noise exposure potentiated vesicle release in IHC, possibly by accelerating vesicle recruitment, and this phenomenon would be dependent upon the intense glutamate release produced during AT.