The findings, published in today's issue of The Journal of Neuroscience, show for the first time that a sex hormone can directly affect auditory function, and point toward the possibility that oestrogen controls other types of sensory processing as well. Understanding how oestrogen changes the brain's response to sound, say the authors, might open the door to new ways of treating hearing deficiencies.
"We've discovered oestrogen doing something totally unexpected," says Raphael Pinaud, assistant professor of brain and cognitive sciences at the University of Rochester and lead author of the study. "We show that oestrogen plays a central role in how the brain extracts and interprets auditory information. It does this on a scale of milliseconds in neurons, as opposed to days, months or even years in which oestrogen is more commonly known to affect an organism."
Previous studies have hinted at a connection between oestrogen and hearing in women who have low oestrogen, such as often occurs after menopause, says Pinaud. No one understood, however, that oestrogen was playing such a direct role in determining auditory functions in the brain, he says. "Now it is clear that oestrogen is a key molecule carrying brain signals, and that the right balance of hormone levels in men and women is important for reasons beyond its role as a sex hormone," says Pinaud.
Pinaud, along with Liisa Tremere, a research assistant professor of brain and cognitive sciences, and Jin Jeong, a postdoctoral fellow in Pinaud's laboratory, demonstrated that increasing oestrogen levels in brain regions that process auditory information caused heightened sensitivity of sound-processing neurons, which encoded more complex and subtle features of the sound stimulus. Perhaps more surprising, says Pinaud, is that by blocking either the actions of oestrogen directly, or preventing brain cells from producing oestrogen within auditory centres, the signaling that is necessary for the brain to process sounds essentially shuts down. Pinaud's team also shows that oestrogen is required to activate genes that instruct the brain to lay down memories of those sounds.
"It turns out that oestrogen plays a dual role," says Pinaud. "It modulates the gain of auditory neurons instantaneously, and it initiates cellular processes that activate genes that are involved in learning and memory formation."
Pinaud and his group stumbled upon these findings while investigating how oestrogen may help change neuronal circuits to form memories of familiar songs in a type of bird typically used to understand the biology of vocal communication. "Based on our findings we must now see oestrogen as a central regulator of hearing," he says. "It both determines how carefully a sound must be processed, and activates intracellular processes that occur deep within the cell to form memories of sound experiences."
Pinaud and his team will continue their work investigating how neurons adapt their functionality when encountering new sensory information and how these changes may ultimately enable the formation of memories. They also will continue exploring the specific mechanisms by which oestrogen might impact these processes.
"While we are currently conducting further experiments to confirm it, we believe that our findings extrapolate to other sensory systems and vertebrate species," says Pinaud. "If this is the case, we are on the way to showing that oestrogen is a key molecule for processing information from all the senses."
