Blocking the glucocorticoid receptor with RU-486 does not prevent glucocorticoid control of autoimmune mouse hearing loss

Abstract

Background/aims: Glucocorticoids effectively manage autoimmune hearing loss, although the cochlear mechanisms involved are unknown. Previous studies of steroid-responsive hearing loss in autoimmune (lupus) mice showed glucocorticoids and mineralocorticoids were equally effective, suggesting the ion homeostasis functions of glucocorticoids may be as relevant as immunosuppression for control of autoimmune-induced inner ear disease. Therefore, to better characterize the role of the glucocorticoid receptor in autoimmune hearing loss therapy, its function was blocked with the antagonist RU-486 (mifepristone) during glucocorticoid (prednisolone) treatments.

Methods: Following baseline auditory brainstem response (ABR) thresholds, MRL/MpJ-Fas(lpr) autoimmune mice were implanted with pellets providing combinations of 1.25 mg/kg of RU-486, 4 mg/kg of prednisolone, or their respective placebos. After 1 month, animals were retested with ABR and blood was collected for immune complex analyses.

Results: Mice receiving no prednisolone (placebo + placebo and placebo + RU-486) showed continued declines in hearing. On the other hand, mice receiving prednisolone (prednisolone + placebo and prednisolone + RU-486) had significantly better hearing (p < 0.05) than the non-prednisolone groups. Immune complexes were significantly elevated in the placebo + RU-486 group, suggesting RU-486 effectively blocked glucocorticoid receptor-mediated immune suppression. These results showed that blockage of the glucocorticoid receptor with RU-486 did not prevent prednisolone's effects in the ear, suggesting its ion homeostasis actions via the mineralocorticoid receptor were more relevant in hearing control.

Conclusion: The mineralocorticoid receptor-mediated actions of glucocorticoids are potentially relevant in steroid-responsive hearing disorders, implying disrupted cochlear ion transport functions may underlie the vascular problems proposed in some forms of immune-mediated hearing loss.

Fig. 1.

a Threshold elevations in placebo + placebo mice (8 mice, 15 ears) after 1 month are typical for untreated autoimmune-diseased mice. The major shift in thresholds occurs at 32 kHz. Vertical bars represent 8 1 SD of the baseline measures. b The prednisolone + placebo group (10 mice, 20 ears) did not show the typical rise in thresholds at any frequency. In fact, thresholds at 8 and 16 kHz trended lower. c The prednisolone + RU-486 group (10 mice, 20 ears) also did not show the typical rise in thresholds at any frequency and trended lower at 8 and 16 kHz.

Fig. 2.

Changes in hearing thresholds after 1 month of treatment. At 4 kHz, the 2 prednisolone groups were unchanged from baseline, but the small elevation in the placebo + placebo group lead to no statistical difference among the 3 groups (p = 0.37). In contrast, the improvement in thresholds of the 2 prednisolone groups at 8 and 16 kHz led to their being significantly better than the placebo + placebo group. Although no statistical group differences were seen at 32 kHz, the placebo + placebo mice showed a trend towards higher thresholds than the 2 prednisolone groups. However, the slight rise in thresholds in the 2 latter groups was sufficient to make the overall group difference significant at only the p = 0.09 level. However, across all frequencies, the prednisolone + RU-486 mice did not differ from the prednisolone + placebo mice, suggesting that blockage of the glucocorticoid receptor did not prevent prednisolone from preserving hearing levels.

Fig. 3.

An ANOVA of the level of immune complexes revealed an overall group difference. The biggest increase in immune complexes was seen in the mice receiving RU-486, presumably due to its blockage of the glucocorticoid receptor and prevention of immune suppressive functions.

Fig. 4.

The prednisolone + placebo mice showed virtually no correlation (r = 0.04; p = 0.87) between hearing change and immune complex levels, suggesting the latter was not a major factor in the level of hearing following steroid treatment.