Abstract:
The role of P2X4, one of the most abundant ionotropic purinergic receptors in the central nervous system, is explored in the context of auditory function. We observed, by using constitutive and conditional P2X4mCherryIN knock-in adult mouse models of either sex, a specific high expression of mCherry-tagged P2X4 in living cochlear outer hair cells (OHCs), from immature postnatal stages to adulthood. This P2X4-mCherry expression, confirmed by confocal immunofluorescence microscopy in wild-type mice, was mainly concentrated in the intracellular apical region of the OHCs, in the area of the Hensen's body, a lysosomal rich region, specifically labeled with the fluorescent dye lysotracker. In addition, the basal cholinergic efferent synaptic region of the OHCs was found to express P2X4 at the cell membrane. Surprisingly, the assessment of the hearing function in constitutive P2X4 knock-out (P2X4KO) mice showed improved auditory brainstem responses (ABRs) with smaller latencies and lower thresholds. These P2X4KO mice, as well as conditional Myo15-Cre:P2X4KO mice, displayed enhanced distortion product otoacoustic emissions (DPOAEs), suggesting an improved electromechanical 'amplification' activity by OHCs. These mutant animals showed reduced inhibition of DPOAEs by contralateral noise, consistent with a weaker inhibitory effect of the medial cholinergic olivocochlear efferent circuit (MOC) on OHCs. When P2X4KO mice were exposed to noise (white-noise 95dB-SPL, 12h) ABRs decreased and partially recovered much like wild-type mice, but DPOAEs showed faster recovery. We concluded that the MOC negative feedback modulation of cochlear micromechanics, in addition to involve Ca(2+) permeable alpha9/alpha10 nicotinic receptors, also requires the activation of postsynaptic P2X4 receptors in OHCs.Significance statement Our study reveals a specific strong expression of the purinergic P2X4 receptor, the main ATP-gated-cation channels in the central nervous system, in mouse cochlear outer hair cells. These cells are essential for generating the distortion products otoacoustic emissions (DPOAES) and tuning the sensitivity and frequency selectivity of the hearing organ, the cochlea. Mice lacking P2X4 showed a deficient inhibitory control of their cochlear DPOAEs when activating the medial olivocochlear efferent pathway innervating the OHCs. We propose P2X4 receptors as an important Ca(2+) regulatory component of the micromechanics of OHCs and that genetic defects in these purinergic receptors may potentially lead to hearing disorders.