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 5

AMPARs and NMDARs were relocated on SGN dendrites after gentamicin exposure, and this was blocked by coinjection of the NMDAR antagonist MK801. Postsynaptic AMPARs, NMDARs, and nerve fibers were identified by immunostaining for GluA2 (red), GluN1 (green), and neurofilament (NF) (grey), respectively. Nuclei were labeled with DAPI (blue). (a) The different locations of NMDARs and AMPARs at afferent dendritic terminals in the normal mature cochlea. AMPAR patches were mostly observed on the ISBs around the basal poles of the IHCs, while NMDAR patches were almost all distributed on nerve terminals between the adjoining IHCs and closer to the IHC nuclei region. (b) After gentamicin treatment for 4 days, the AMPARs migrated upwards towards the bundle poles of the IHCs, while NMDARs migrated downwards towards the basal poles of the IHCs. (c) After the combined treatment with MK801 and gentamicin for 4 days, AMPARs and NMDARs were prevented from delocalizing to the dendritic terminals around the IHCs. (d) After gentamicin treatment for 7 days, the AMPARs and NMDARs had essentially switched locations along the dendritic terminals. Many colocalized AMPARs and NMDARs were observed laterally between the adjoining IHCs. (e) After the combined treatment with gentamicin and MK801 for 7 days, the gentamicin-induced rearrangements of AMPARs and NMDARs at the afferent dendritic terminals were partly blocked. 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; ISBs: inner spiral bundles. Scale bar = 10 μm.