A small fish with a remarkable hearing system that enables females to zero in on the love hums broadcast by males during the breeding season is providing scientists with clues that someday might provide a treatment for people with high-frequency hearing loss.
Researchers from the 糖心少女and Cornell University have duplicated a natural physiological change that occurs in the female plainfin midshipman fish (Porichthys notatus) during the breeding season. Working with non-reproductive females, the researchers boosted hormone levels that alter the fish鈥檚 inner-ear hearing sensitivity for a short period so they can better hear the males鈥� humming calls.
鈥淭his is the first time anyone has been able to hormonally induce a change in hearing sensitivity in a vertebrate,鈥� said Joseph Sisneros, a 糖心少女assistant psychology professor who studies the neural basis of behavior. He is the lead author of a paper published in the July 16 edition of the journal Science.
Sisneros and a Cornell research team headed by Andrew Bass found that a spike in levels of the hormones testosterone and estradiol (an estrogen) triggers changes in the females鈥� inner ear so they are capable of detecting higher frequencies in the males鈥� multi-harmonic humming. This process couples the transmission of sound by the males and reception by the females.
鈥淲e suspected that enhancing the sensitivity of the females鈥� ear to the upper harmonics of the males鈥� hums should improve detection of their vocalization,鈥� said Bass, who is a professor of neurobiology and behavior. 鈥淯pper harmonics propagate farther in shallow-water environments like those where midshipman males build nests and sing their love songs to attract females. But the females can鈥檛 process this vital information and respond appropriately if they can鈥檛 hear it. Steroid hormones appear to provide a key molecule that leads to shifts in the hearing sensitivity of females.鈥�
The midshipman fish live in deep water off the Pacific Coast of the United States three-quarters of the year. They move into shallow intertidal zones to reproduce. Males build nests under rocks and begin broadcasting their loud humming love calls to attract females.
The researchers collected non-reproductive females, removed their ovaries and replaced them with implants to induce hormonal levels in the fish that mimic the natural spike in testosterone and estradiol levels in reproductive females. A month later the hormone implants caused changes in the non-reproductive females鈥� hearing sensitivity.
鈥淭he importance of this study is that the neural plasticity we鈥檝e uncovered may affect all vertebrates,鈥� said Sisneros. 鈥淭his fish relies heavily on its auditory system for reproduction and our work gives us insight into how this plasticity operates. The next step is to locate where this plasticity occurs. There are four possibilities 鈥� at the sensory hair cells in the ear, in the auditory nerve, in the brain or in all three at the same time.鈥�
The jump is a big one, but Sisneros said the research eventually might have human applications. For instance, he said, 鈥減revious experiments in other laboratories suggest that steroid hormones may also play a role in causing some of the reported changes in hearing sensitivity of human females at differing stages of the menstrual cycle. Also, humans have a natural loss of hearing in relationship to age. The higher frequencies tend to drop out first along with a natural drop in testosterone and estrogen. So a question arises: Are natural decreases in hormone levels related to this loss of high-frequency hearing?鈥�
Co-authors of the paper were Paul Forlano, a Cornell graduate student, and David Deitcher, a Cornell associate professor of neurobiology and behavior. They also identified the estrogen receptor in the female inner ear where the enhancement begins.
鈥淭his was especially important since estrogen receptors are also present in the human inner ear but no one know why they might be there. Our study now suggests a possible function,鈥� said Deitcher.
The National Institutes of Health funded the research.