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Hearing Ability as We Age is Determined by Hormone
Gene therapy may be way to correct the problem
of potassium imbalance
 Feb. 10, 2006 Why do senior citizens lose their
hearing? It is their hormones, well, actually one hormone aldosterone
says a new study. Researchers have linked this hormone, known to
adjust levels of key brain chemicals, to the quality of our hearing as
we age. The more of the hormone that older people have in their
bloodstream, the better is their hearing. And, the less of the hormone
they have, the worse is their hearing.
Aldosterone, is known to regulate kidney function
and also plays a role in controlling levels of two crucial signaling
chemicals in the nervous system, potassium and sodium.
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For nerves to send signals crisply and work
properly, potassium and sodium must be in precise proportion, without
any disruption in the molecular channels or gates through which they
move.
Levels of potassium are particularly crucial in the
sensitive inner ear, where fluid rich in potassium plays a central role
in converting sounds into signals that the nervous system recognizes.
The team of scientists in Rochester, N.Y., put 47
healthy men and women between the ages of 58 and 84 through a battery of
sophisticated hearing tests.
Scientists also measured their blood levels of
aldosterone, which is known to drop as people age. They found that
people with severe hearing loss had on average about half as much
aldosterone in their bloodstream as their counterparts with normal
hearing.
The researchers noted, however, that the levels of
aldosterone found in all the participants is considered normal, and that
no patients or physicians should consider altering aldosterone levels
without more research.
The findings come from researchers at the
International Center for Hearing and Speech Research (ICHSR), a group
funded by the National Institute on Aging that is recognized as a leader
in research on age-related hearing loss. The center includes scientists
from the National Technical Institute for the Deaf at Rochester
Institute of Technology and neuroscientists from the University of
Rochester.
"The inner ear is especially sensitive to any
disruption in potassium levels," said Robert D. Frisina, Ph.D.,
professor of Otolaryngology at the University of Rochester Medical
Center and an adjunct professor at Rochester Institute of Technology.
"We know that potassium levels in the inner ear
seem to decrease as we age and that these falling levels play a role in
age-related hearing loss, and we also know that blood levels of
aldosterone generally decrease with age.
"We found a direct link between blood levels of
aldosterone and the ability of people to hear normally as they age.
Depressed hormone levels may hurt hearing both in the inner ear and the
part of the brain used for hearing. More research is needed, however, to
understand the precise role that aldosterone plays for instance,
whether it's a cause of failed hearing, or whether it's symptomatic.
Before we understand the issue more fully, people should not worry about
their aldosterone levels or look to boost the amount in their
bloodstream."
The team led by Frisina published its results in
the November issue of the journal Hearing Research. This week at the
annual international meeting of the Association for Research in
Otolaryngology in Baltimore, the team presented its latest results
showing just how important potassium regulation is to age-related
hearing loss.
In Baltimore, Otolaryngology medical resident Jared
Spencer, M.D., presented results from "knockout" mice whose genes
controlling the potassium channels in the inner ear don't function
properly, and confirmed that malfunctioning potassium channels are
central to age-related hearing loss, or presbycusis.
The channels are highly concentrated in a part of
the brain that plays an important role providing feedback from the brain
to the ears. Frisina's team previously discovered that the feedback
system is one of the first things to go wrong in age-related hearing
loss, often declining in people who are in their 40s and 50s, usually
before they even realize their hearing is declining.
"We are now working out some of the underlying
biology about how the decline occurs," said Frisina. "We have evidence
that these potassium channels may play an important role in the failure
of the feedback system, which is a big part of age-related hearing
loss."
Nearly everyone wrestles with failing hearing at
some point. While some people suffer from hearing damage as a result of
exposure to loud noise, or from other causes such as the side effects of
some medications, for many people hearing problems occur with no known
cause. Some people notice problems in their 40s and 50s, but the process
becomes very noticeable for most people in their 60s and older.
Frisina said that until the biology of the problem
is better understood, the best advice for people concerned about hearing
loss is to limit exposure to loud, damaging noise and to medications
that are toxic to the ears. He also counsels people to eat healthy and
to exercise "all those things you know you should be doing to stay
healthy with age," he said.
Meanwhile, his team is looking at the possibility
of using gene therapy to try to correct the problem. It may be possible
some day to modify a person's inner ear to correct the potassium
imbalance that is central to hearing loss. Such an approach might also
address the biggest cause of congenital deafness, which involves a
genetic mutation that mucks up the potassium balance in the inner ear.
The new findings come from a research group founded
by Robert Frisina's father, D. Robert Frisina, Ph.D., founding director
of NTID, who heads one of the largest research groups in the world
studying age-related hearing loss. The group has attracted top
researchers from around the world to come together to study the problem.
Members of the group, which numbers more than two dozen, hail from
Egypt, Brazil, Russia, China, Korea, India, and the United States.
In addition to Frisina, Frisina and Spencer, the
team includes post-doctoral research associate Sherif Tadros, M.D., of
both the University and NTID, who is first author of the Hearing
Research paper; research nurse Susan Frisina of both NTID and the
University; audiologist Frances Mapes of NTID; and otolaryngologist
Xiaozia Zhu, M.D., of the University.
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