Effects of exposing gonadectomized and intact C57BL/6J mice to a high-frequency augmented acoustic environment: Auditory brainstem response thresholds and cytocochleograms

Abstract

Gonadectomized and surgically intact adult C57BL/6J (B6) mice of both sexes were exposed for 12 hours nightly to a high-frequency augmented acoustic environment (AAE): repetitive bursts of a half-octave noise band centered at 20 kHz, 70 dB SPL. The effects of sex, gonadectomy, and AAE treatment on genetic progressive hearing loss (exhibited by B6 mice) were evaluated by obtaining auditory brainstem response thresholds at ages 3-, 6-, and 9-months; hair cell counts (cytocochleograms) were obtained at 9 months. A sex difference in the rate of genetic progressive hearing loss in B6 mice (observed by earlier studies) was confirmed, with females exhibiting a faster rate of threshold elevations and more severe loss of hair cells at age 9 months. Gonadectomy had no consistent effects on the rate or severity of hearing loss in nonexposed mice of either sex. An unexpected finding was that the high-frequency AAE treatment caused additional ABR threshold elevations and hair cell loss. In an earlier study, the same high-frequency AAE treatment on DBA/ 2J mice ameliorated hearing loss. The most severe AAE-induced losses occurred in surgically intact females, suggesting a potentiating effect of ovarian hormone(s).

Effects of exposing C57BL/6J mice to high- and low-frequency augmented acoustic environments: Auditory brainstem response thresholds, cytocochleograms, anterior cochlear nucleus morphology and the role of gonadal hormones

“…Sex effect on hearing loss-The sex effect found in earlier studies (Henry, 2002;Willott and Bross, 2004;Willott et al, 2006b) was again replicated. Intact female B6 mice that were not treated with an AAE exhibited higher thresholds than males at age 9 months (Fig.…”

“…The procedures for preparing and analyzing mouse inner ears have been described in detail in previous reports (McFadden et al, 1999a;McFadden et al, 1999b: Willott et al, 2006b. Briefly, the organ of Corti was carefully microdissected out from base to apex, and specimens stained with Ehrlich's hematoxylin solution (Ding et al, 2001).…”

“…The effects of AAE treatment on AVCN morphology were evaluated for mice used in the LAAE experiments together with those used in the HAAE experiments (whose ABR thresholds and cytocochleograms were first reported in the Willott et al, 2006b). The overall size of AVCNs (summing the serial sections from which the standard posterior and anterior sections were chosen) did not differ significantly among the groups.…”

“…Treatment of D2 mice with a high-frequency AAE (HAAE, centered at 20 kHz) increased the loss of inner hair cells (IHCs) in the extreme basal (high-frequency) cochlear region, while having otherwise beneficial effects on the survival of hair cells and AVCN neurons (Willott et al, 2006a). The same HAAE worsened high-frequency hearing loss and hair cell loss in 9-month-old B6 mice (Willott et al, 2006b). The present study evaluates the central correlates of the negative HAAE effects in B6 mice, as well as the peripheral and central effects of treatment with a low-frequency AAE (LAAE) that has not been employed previously.…”

“…Our strategy is to employ gonadectomized mice (gonadal hormone levels are greatly reduced) and intact mice. Several studies have shown that high-frequency hearing loss becomes more severe in female B6 mice by 6-9 months of age (Henry, 2002(Henry, , 2004Willott and Bross, 2004;Willott et al, 2006b). Moreover ovarian hormones may contribute to this deficit in females (Willott et al, 2006b).…”