Reinvestigation of cochlear pathology in circling mice.

The main causes of early hearing deficit in circling mice have been reported to be early degeneration of the organ of Corti and deterioration of spiral ganglion neurons. As an exact cochlear pathology is essential to explain our previous results regarding the auditory brainstem circuits of developing circling mice, we reinvestigated the cochlear pathology in developing circling mice (14, 22, and 38 days old). It has been reported that the organ of Corti in circling mice completely degenerates as early as postnatal day (P) 21 and that circling mice are deaf by P18. Although we confirmed that circling mice were deaf at P15 and that hair bundles of outer hair cells were defective at P18, complete degeneration of the organ of Corti was not observed by P38 in circling mice. At P22, the type I cell-like spiral ganglion cell density in circling mice was reduced to 78% of that of control mice (ICR mice), but it was not significantly different from that of other control mice (heterozygous (+/cir) mice, littermates of circling mice) that could hear at P22. Our data suggest that other factors, such as absence of neurotransmitter release from inner hair cells, should be considered to explain the early hearing deficit observed at P15 in circling mice.