In the tongue, distinct classes of taste receptor
cells detect the five basic tastes; sweet, sour,
bitter, sodium salt and umami. Among these qualities,
bitter and sour stimuli are innately aversive,
whereas sweet and umami are appetitive and generally
attractive to animals. By contrast, salty taste is
unique in that increasing salt concentration
fundamentally transforms an innately appetitive
stimulus into a powerfully aversive one.
But Charles Zuker, PhD, and colleagues at Columbia
University Medical Center have discovered how the
tongue detects high concentrations of salt (think
seawater levels, not potato chips), the first step
in a salt-avoiding behavior common to most mammals.
The findings, which were published today online in
the journal Nature, could serve as a springboard for
the development of taste modulators to help control
the appetite for a high-salt diet and reduce the ill
effects of too much sodium.
“Salt taste in mammals can trigger two opposing
behaviors,” said Dr. Zuker, professor in the
Departments of Biochemistry & Molecular Biophysics
and of Neuroscience at Columbia University College
of Physicians & Surgeons. “Mammals are attracted to
low concentrations of salt; they will choose a salty
solution over a salt-free one. But they will reject
highly concentrated salt solutions, even when
salt-deprived.”
Over the past 15 years, the receptors and other
cells on the tongue responsible for detecting sweet,
sour, bitter, and umami tastes—as well as low
concentrations of salt—have been uncovered largely
through the efforts of Dr. Zuker and his
collaborator Nicholas Ryba from the National
Institute of Dental and Craniofacial Research.
“But we didn’t understand what was behind the
aversion to high concentrations of salt,” said Yuki
Oka, a postdoctoral fellow in Dr. Zuker’s laboratory
and the lead author of the study.
The researchers expected high-salt receptors to
reside in cells committed only to detecting high
salt. “Over the years our studies have shown that
each taste quality—sweet, bitter, sour, umami, and
low-salt—is mediated by different cells,” Dr. Ryba
said. “So we thought there must be different taste
receptor cells for high-salt. But unexpectedly, Dr.
Oka found high salt is mediated by cells we already
knew.”
In experiments conducted by Dr. Oka, the researchers
found that high salt concentrations activate
previously discovered bitter- and sour-sensing cells.
When one of these cell types was silenced and made
incapable of sending messages to the brain, aversion
to high-salt solutions was reduced, but not
eliminated. When both cell types were silenced, the
mammals completely lost their aversion to high-salt
solutions, even showing unrestrained attraction to
exceedingly salty solutions equivalent to those of
seawater.
For mammals, ingesting high concentrations of
seawater can lead to extreme dehydration, kidney
failure, and death. With two aversion pathways, Dr.
Oka said, animals have a safeguard to make sure that
high salt is always aversive.
Now that all the salt pathways have been identified,
Dr. Oka said, it may be possible to use that
knowledge to make low concentrations of salt taste
saltier, to reduce NaCl intake. It also may be
possible to make the taste of KCl (potassium
chloride), which has fewer long-term health effects
than sodium chloride, more appealing to encourage
its use as a salt substitute.
Though the commercial implications of the work are
clear, the researchers’ objective is not to find
ways to alter our tastes, but to understand how we
perceive the sensory world. How does the detection
of high salt concentrations on the tongue lead to a
decision to turn away from a source of water? How
can we tell the difference between chocolate cake
and pumpkin pie? How do our taste sensations change
over time? The answers are in the firing of neurons
in the brain.
With the taste receptor cells in hand, the
researchers have recently turned to brain imaging,
mapping the neurons that receive information from
the tongue’s taste buds [http://www.hhmi.org/news/zuker20110902.html].
The map was a surprise. Instead of finding the
neurons scattered, as the taste receptor cells on
the tongue are, they found discrete hotspots of the
brain for each of four tastes: sweet, bitter, umami,
and salty (sour has not yet been located).
Ultimately, they hope to understand how the firing
of these neurons produces the sensations we call
tastes.
For more information
The title of the paper is “High salt recruits
aversive taste pathways.” Contributors areYuki Oka (CUMC/HHMI),
Matthew Butnaru (CUMC/HHMI), Lars von Buchholtz
(National Institute of Dental and Craniofacial
Research, NIH), Nicholas J. P. Ryba (National
Institute of Dental and Craniofacial Research, NIH),
and Charles S. Zuker (CUMC/HHMI).
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11905.html
http://www.cumc.columbia.edu
(MDN)
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