Age-related auditory brainstem response (ABR) threshold changes in the dancer mouse mutant

Morphological changes of cochlea in a strain of new-mutant mice

The hearing ability and histological characteristics of the cochlea of a strain of new-mutant mice were analyzed. This new mutant arose as a spontaneous mutation in the C3H/He stock. The genetic mode is autosomal recessive and the animals show abnormal behavior such as circling, head-tossing and hyperactivity. The audiological findings exhibited no recordable auditory brain stem response (ABR) in any homozygotes at ages ranging from 11 days to 117 days. For morphological examination, we used 36 homozygote with ages ranging from 10 days to 18 months. The primary morphological abnormalities were observed in the organ of Corti. The stereocilia of the outer hair cells showed disarray throughout the whole cochlea, although outer hair cell cytoplasm became fully developed, including the nerve terminals. Age-dependent degeneration of the outer hair cells subsequently occurred from the basal to the apical part of the cochlea. The earliest change demonstrated in the outer hair cells was cuticular degeneration. Although the abnormalities of the inner hair cells occurred late, a complete loss of inner and outer hair cells was demonstrated. The stria vascularis was well preserved at a later age as were spiral ganglion cells. These histological findings confirm that this mouse is classified as a neuroepithelial-type mutant. As this animal was expected to have a single gene abnormality, molecular genetic studies on this animal can provide important information on the nature of histological changes of the hair cell from a mode of gene action.

Variability in genetically induced age-related impairment of auditory brainstem response thresholds

In genetically induced hearing impairment or deafness, the recessive mode of inheritance is by far the most common, but it has seldom been feasible to document audiological aberrations. In this study, auditory brainstem response (ABR) thresholds were analysed in groups of 10 heterozygote jerker (je/+) mice each, at ages of 3, 6 and 9 months in the frequency range 2-31.5 kHz. The animals were thereafter regularly tested up to a final age of 12 months. Forty age-related CBA/CBA mice were used as controls. The individual ABR thresholds in 3-, 6-, 9- and 12-month-old CBA/CBA mice were very similar, as also were those in 3- and 6-month-old je/+ mice (mean values within the normal range). In contrast, at 9 months--and even more so at 12 months--all je/+ animals showed large individual variations, with ABR thresholds varying randomly from near-normal to profound impairment. The wide individual spread of ABR thresholds in old je/+ animals was interpreted as reflecting the expression of a random influence of recessive jerker vis-à-vis the wild type of gene.

Aberrant frequency tuning and early stereociliary derangement in genetic inner ear disease

We have developed an auditory brainstem response (ABR) based method for frequency selective forward masking tuning curve technique. Normative data from CBA/CBA mice are compared with tuning curves (TCs) from heterozygote dancer mice which in most cases initially had a normal ABR threshold. The sharpest tuning in normal mice was obtained at 10 kHz (Q10 = 7.6) and 12 kHz (Q10 = 10.9). With increasing age the mutants showed a broadening of the TC tip before ABR threshold impairment occurred. Morphological analysis of the cochleae from dancer mutants revealed an early disarray of hair cell stereocilia which became increasingly more severely affected with advancing age. We conclude that the broadening of the TC tip is an early finding in the genetically induced cochlear susceptibility which can be correlated to stereociliary dysfunction.

The effects of auditory deprivation on morphological maturation of the ventral cochlear nucleus

The volumes of the auditory brainstem nuclei and age-related auditory brainstem response (ABR) thresholds were analyzed in homozygote (je/je) and heterozygote (je/+) jerker mutant mice. Altogether 97 mice were used in the study. Je/je mice never develop any hearing. The dorsal (DCN) and ventral (VCN) cochlear nuclei were found to have stopped their growth at 56 days after birth. In je/+ mutants, ABR thresholds remained normal or near-normal for 3-6 months, whereas VCN and DCN volumes remained unchanged at least after 56 days after birth. There is no significant difference in DCN volume in je/je and je/+ mice. However, the VCN volume and the cross-sectional area of globular cells were both significantly larger in je/+ than in je/je mice (P less than 0.01). These findings show that auditory deprivation during the maturation of hearing in je/je mutants causes an incomplete maturation of only the ventral cochlear nucleus.