(A) Representative confocal images (maximal projection from a focal series) of IHC synapses from 32 kHz region of Ntf3flox, Ntf3flox:Plp1/CreERT, Ntf3stop, and Ntf3stop:Plp1/CreERT cochleae immunolabeled for pre-synaptic ribbons (CtBP2-red) and post-synaptic receptor patches (GluA2-green) (scale bar = 10 µm). The dashed lines show the approximate outline of one IHC. CtBP2 antibody also weakly stains IHC nuclei. (B–D) Quantitative data shows that Ntf3 knockout reduces, and overexpression increases, the number of pre-synaptic ribbons (B), post-synaptic GluA2 receptor patches (C), and putative ribbon synapses, defined as juxtaposed CtBP2- and GluA2-positive puncta (D) at high frequency cochlear regions; n = 5–6. *p < 0.05, **p < 0.01, ***p < 0.001 by two-way ANOVA. (E) Relative synaptic counts vs relative ABR P1 amplitudes of Ntf3 knockouts (blue) or overexpressors (red) shows a linear correlation. Data points were obtained by normalizing synaptic counts and ABR P1 amplitudes of Ntf3 mutants to the values of their respective controls at each of the frequency regions analyzed. Key in D applies B–E.