Our hearing is one of the wonders of the human body which is all too often taken for granted. Like many things in life, we don't really appreciate our ability to hear clearly until we begin to lose it. As we age, our ability to discern sounds begins to deteriorate. Therefore, new innovative therapies to guard the hearing of patients are eagerly sought after. Sometimes these treatments can come from unexpected sources, such as the 'female sex hormone' estradiol.

A team of researchers from Sweden's prestigious Karolinska Institute studied the role of estradiol in hearing loss recovery by examining mice with deficiencies in various estrogen receptors. They found that mice of both sexes deficient in only one estrogen receptor had reduced recovery from auditory trauma and that treatment with estrogen receptor drugs protected the animals against auditory damage. Their findings have recently been published in the Journal of Clinical Investigation (JCI).

Estradiol is a sex hormone, which although labeled 'female', is found in both men and women. In women, estradiol acts as a growth hormone for tissue of the reproductive organs and plays a critical role in sexual development. In men, the hormone is generated in testosterone by aromatase, a protein which is found throughout the male reproductive system and shares in the development of sperm.

In addition to the critical role it plays in reproductive and sexual functioning, Estradiol is also known to affect other organs. Not only does it regulate bone formation and cardiovascular tone in elderly men, the hormone is known to help maintain cell survival within the central nervous system of both sexes.

Previous clinical and experimental studies have also indicated that estradiol plays a significant role in auditory physiology. In women, it was found that hearing thresholds vary with sex hormone levels during the menstrual cycle, and postmenopausal women have higher auditory brainstem response (ABR) thresholds than younger women or men. These studies indicate a protective function of estradiol in the female auditory system.

However, until now, little was known about the impact of estradiol-binding proteins known as estrogen receptors (ERs) on hearing or whether there are sex specific facets in the inner ear. In this latest investigation, the scientists from the Karolinska Institute examined the role of known estrogen receptors in response to auditory damage. This was done by testing hearing loss recovery in mice that had deficiencies in various estrogen receptors.

Researchers found that mice only lacking in the estrogen receptor ER-beta recovered less well from hearing loss. They also discovered that treatment with ER-beta-binding drugs protected mice from auditory damage. In addition, not only was ER-beta found in the ears of mice of both sexes, but levels of brain-derived neurotrophic factor (BDNF), a protein which protects nerves, were found to be lower in mice that lacked either ER-beta or aromatase.

These findings are the first experimental evidence directly linking ER expression to the protection of auditory function.The researchers involved in the study are confident that their findings will now pave the way for the development of new treatments against hearing loss.

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