BACE1 is expressed in spiral ganglion neurons and olivocochlear terminals. Expression of BACE1 was analyzed with immunohistochemistry on (A,B,E) cochlear cryosections and (C,D) whole-mount preparations of the apical cochlear turn of the organ of Corti. All images are representative maximum intensity projections of confocal z-stacks (see material and methods for details). A, We detected strong anti-BACE1 immunosignals in cell bodies of SGN in BACE1^+/+ mice that (B) were strongly reduced in BACE1^−/− mice. The panels (A+B) show representative transmitted light (left) and corresponding confocal images (right) of cryo-sections through the spiral ganglion stained with primary antibodies directed against BACE1 (green). C, In the organ of Corti of wild-type mice, we detected strong anti-BACE1 signals in synapsin1,2-positive structures that represent efferent olivocochlear terminals. D, These anti-BACE1 signals in efferent boutons were completely absent in the organ of Corti of BACE1^−/− mice. C and D show confocal images of the organ of Corti in the IHC (top) or the OHC (bottom) region stained with primary antibodies against BACE1 (green) and synapsin1,2 (red). The right images in C and D show merged images of anti-BACE1 and the respective anti-synapsin1,2 staining. E, BACE1 expression was not detectable in the stria vascularis. We found comparable diffuse anti-BACE1-immunosignals in the stria vascularis of BACE1^+/+ (left) and BACE1^−/− (right) mice. We thus consider these signals as caused by unspecific binding of the primary antibody. F, As demonstrated with antibodies directed against KCNQ1 channels (K_V7.1), localization and expression of KCNQ1 channels (green) in the stria vascularis was indistinguishable between BACE1^+/+ (left) and BACE1^−/− (right) mice. E and F show merged transmitted light and confocal images of cryo-sections through the stria vascularis. Scale bars, 20 μm.