Programme Contents

Noise-induced hearing adaptation kinetics of the ‘cochlear amplifier’ maps to P2X₂ receptor-dependent auditory brainstem response temporary threshold shift

ATP-gated ion channels assembled from P2X₂ receptor (P2X₂R) subunits are expressed in the cochlea and activated by elevated sound levels. P2X₂R contribute significantly to the development of auditory brainstem response (ABR) temporary threshold shift in wildtype mice exposed to sustained sound around safe workplace limits (85 dB SPL). In contrast, mice null for the P2rx2 gene (P2rx2^(-/-)) that encodes the P2X₂R failed to develop this reversible noise-induced hearing loss. Moreover, when these knockout mice were exposed to louder noise levels, they exhibited disproportionately greater permanent hearing loss than the wildtype controls; hence purinergic hearing adaptation is otoprotective (Housley, et al., 2013). Loss of function mutations in the human P2X₂R gene cause vulnerability to noise-induced and age-related hearing loss (Yan et al., 2013; Faletra et al., 2014; Moteki et al., 2015), supporting a protective role for P2X₂R. The time constant for development of the reversible purinergic hearing adaptation in wildtype mice was previously estimated at ~ 20 minutes based on shifts in ABR thresholds with cumulative noise exposure from 10 minutes to two hours (Housley, et al., 2013).

Here we sought to improve resolution of the kinetics of this purinergic hearing adaptation and to investigate the contribution of the outer hair cell ‘cochlear amplifier’ to this mechanism. To achieve this, rates of adaptation of hearing sensitivity with noise exposure were compared between wildtype and P2rx2^(-/-) mice (C57Bl/6J background; anaesthetized (i.p.) with a cocktail of ketamine (40 mg/kg), xylazine (8 mg/kg), acepromazine (0.5 mg/kg) or isoflurane (4% induction, 1-1.5% maintenance with O₂)) using ABR and cubic (2f₁-f₂) distortion product otoacoustic emission (DPOAE) measurements. DPOAEs report changes in the gain of the cochlear amplifier that stem from alterations in cochlear outer hair cell electromechanical transduction and associated organ of Corti micromechanics. We found that both the ABR and DPOAE threshold shifts were largely complete within the first 7.5 minutes of moderate noise exposure (85 dB SPL; 8 – 32 kHz) of wild-type mice. ABR threshold shift after 7.5 minutes noise was 0.77 ± 0.07 of the 7.92 ± 1.06 dB threshold shift measured at 17.5 minutes (means ± s.e.m.; n = 12). Similarly, for the DPOAE, threshold shift at 7.5 minutes noise was 0.76 ± 0.08 of the 14.4 ± 1.6 dB shift at 17.5 minutes (n = 8). As previously noted, the noise exposure failed to produce significant changes in either ABR or DPOAE thresholds in the P2rx2^(-/-) mice.

These findings document a considerably faster purinergic hearing adaptation to noise than previously reported. Moreover, the similarity in kinetics of ABR and DPOAE measurements implicate the ‘cochlear amplifier’ as the site of action of adaptation, as ABR reflects downstream neural activity.

Faletra F, Girotto G, D'Adamo AP, Vozzi D, Morgan A, Gasparini P. (2014) Gene 534(2):236-239.

Housley GD, Morton-Jones R, Vlajkovic SM, Telang RS, Paramananthasivam V, Tadros SF, Wong AC, Froud KE, Cederholm JM, Sivakumaran Y, Snguanwongchai P, Khakh BS, Cockayne DA, Thorne PR, Ryan AF. (2013) Proc Natl Acad Sci USA 110(18):7494-7499.

Moteki H, Azaiez H, Booth KT, Hattori M, Sato A, Sato Y, Motobayashi M, Sloan CM, Kolbe DL, Shearer AE, Smith RJ, Usami S. (2015) Ann Otol Rhinol Laryngol 124 Suppl 1:177S-183S.

Yan D, Zhu Y, Walsh T, Xie D, Yuan H, Sirmaci A, Fujikawa T, Wong AC, Loh TL, Du L, Grati M, Vlajkovic SM, Blanton S, Ryan AF, Chen ZY, Thorne PR, Kachar B, Tekin M, Zhao HB, Housley GD, King MC, Liu XZ. (2013) Proc Natl Acad Sci USA 110(6):2228-2233.