N-Methyl-D-Aspartate Receptors Involvement in the Gentamicin-Induced Hearing Loss and Pathological Changes of Ribbon Synapse in the Mouse Cochlear Inner Hair Cells

Figure 7

Cochlear ribbon synapses were maintained when MK801 was coinjected with gentamicin. (a-b) The elevation of ABR thresholds and the decline of ABR wave I amplitudes induced by gentamicin injury were successfully rescued by MK801 treatment in the 4-day treatment group (, versus gentamicin-only group). (c-d) The elevation of ABR thresholds and the decline of ABR wave I amplitudes induced by gentamicin injury were partly blocked by MK801 treatment in the 7-day treatment group (, versus gentamicin-only group). (e–i) Afferent ribbon synaptic pairs were observed by immunostaining presynaptic ribbons with anti-CtBP2 (green) and postsynaptic AMPARs with anti-GluA2 (red). Nuclei were labeled by DAPI (blue). (e) Ribbons and AMPARs were paired at the basal poles of the IHCs in the undamaged cochlea. (f) After gentamicin treatment for 4 days, AMPARs and ribbons moved upwards towards the bundle poles of the IHCs. The translocation of AMPARs occurred earlier than that of the presynaptic ribbons. (g) After the combined treatment with gentamicin and MK801 for 4 days, the ribbon synaptic pairs had similar location, morphology, and distribution as undamaged controls. (h) After gentamicin treatment for 7 days, AMPARs and ribbons were further relocated towards the bundle poles of the IHCs. Some large ribbons and/or AMPARs were also observed at the basal poles of the IHCs. (i) After the combined treatment with gentamicin and MK801 for 7 days, the quantity of the ribbon synaptic pairs was significantly increased in the IHC basal pole region compared with that in gentamicin-only treatment. GM 4 d, 7 d: gentamicin treatment for 4 days or 7 days; GM + MK 4d, 7d: combined treatment with gentamicin and MK801 for 4 days or 7 days; IHC: inner hair cell. Scale bar = 10 μm. ^∗

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