'Female sex hormone' protects hearing in males and females

3 April 2008

The development of new treatments for hearing loss may soon be possible after new research on receptors (proteins which receive chemical signals) for the female sex hormone estradiol.

One of three types of oestrogen, estradiol normally regulates the menstrual cycle and helps the development of a female body. It is known that changes in levels of the hormone can affect hearing in women throughout their menstrual cycle and after the menopause. Estradiol is also present in men, synthesised from testosterone by the enzyme aromatase. In elderly men, the hormone regulates bone formation and cardiovascular tone. In addition to its gender-specific effects, the hormone also promotes cell survival in both men and women.

Now Professor Barbara Canlon and her colleagues at Sweden’s Karolinska Institute have found that estradiol has a protective effect on hearing. The group tested mice engineered to lack various estradiol-binding proteins – known as oestrogen receptors – in order to examine the role of the receptors in recovery from ‘acoustic trauma’. These ‘knockout mice’ were exposed to 45 minutes of noise at 100 decibels of sound pressure level as this is known to cause a temporary hearing loss. Hearing thresholds were measured both before and after noise exposure in order to determine how much hearing loss had occurred.

Knockout mice without a type of oestrogen receptor called ER-beta had higher hearing thresholds after acoustic trauma than either mice lacking ER-alpha or normal mice. The same was also true for mice lacking the receptor for aromatase. In addition, the aromatase and ER-beta knockout groups had reduced levels of brain-derived neurotrophic factor (BDNF), a molecule with a nerve-protecting effect. Treatment with a protein that mimics the effect of estradiol protected both normal and aromatase knockout mice from acoustic trauma and was found to increase BDNF levels in the aromatase knockouts. All effects were found equally in males and females.

“This is the first study to show that the male auditory system has oestrogen receptors and they seem to function in the same manner as in females” says Professor Canlon, who adds that because products which activate ER-beta are already available on the market, “clinical trials should not be far off”. The results were published in the Journal of Clinical Investigation.