Medial olivocochlear efferent modulation of cochlear micromechanics requires P2X4 expression in outer hair cells

The role of P2X4, one of the most abundant ionotropic purinergic receptors in the central nervous system, is explored here in the context of auditory function. We observed, by using constitutive and conditional P2X4mCherryIN knock-in adult mouse models, 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 immuno-confocal fluorescence 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 with smaller latencies, larger amplitudes and smaller thresholds. These P2X4KO mice, as well as conditional P2X4KO-Myo15-Cre 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. We concluded that the MOC negative feedback modulation of cochlear micromechanics, in addition to involve Ca ²⁺ permeable α9/α10 nicotinic receptors, also requires the activation of postsynaptic P2X4 receptors in OHCs.