Medial Olivocochlear Efferent Modulation of Cochlear Micromechanics Requires P2X4 Receptor in Outer Hair Cells

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 (WT) mice, was mainly concentrated in the intracellular apical region of OHCs, in the area of 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 (MOC) efferent circuit on OHCs. When P2X4KO mice were exposed to noise (white noise 95 dB SPL, 12 h), ABRs decreased and partially recovered much like WT mice, but DPOAEs showed faster recovery. We concluded that the MOC negative feedback modulation of cochlear micromechanics, in addition to involve Ca²⁺ permeable α9/α10 nicotinic receptors, also requires the activation of postsynaptic P2X4 receptors in OHCs.