A peek behind the science curtain

Bethany Brookshire



There’s a sour side to serotonin

Chemical often associated with mood also senses sour in the mouth

reuben sandwich

A Reuben sandwich is no good without the sour tang of sauerkraut. A new study shows we can thank serotonin for communicating the taste.

Sponsor Message

Unless you’re in the middle of biting into a delicious Reuben sandwich, you might forget that taste is one of the fundamental senses. “It’s required for our enjoyment of food,” explains Emily Liman, a taste researcher at the University of Southern California in Los Angeles. “Without taste … people stop eating. They don’t enjoy their food.” A life without the sweet jolt of sugar or the savory delights of umami seems, well, tasteless.

When you put that mouthwatering combination of corned beef, Swiss cheese, Thousand Island dressing, sauerkraut and rye in your mouth, the chemicals in the sandwich stimulate taste buds on your tongue and soft palate. Those taste buds connect to the ends of nerve fibers extending delicately into the mouth. Those nerve fibers are the ends of cells located in the geniculate ganglion, a ball of cells nestled up against the ear canal on the side of your head. From there, taste sensations head toward the brain.

Chemical messengers bridge the gap between the taste bud and the end of the nerve fiber. But what chemical is involved depends on the type of cell within the bud. There are three types of taste cells (imaginatively titled I, II and III). Type I is not well-understood, but it may be a kind of support cell for other taste cells. Type II, in contrast, is better known. These taste cells sense the slight bitterness of the rye seeds, the sweet edge of the Thousand Island dressing and the savory umami of the beef. They pass that delightful message on using the chemical ATP.

The lovely sour tang of the sauerkraut is left to type III. Scientists knew that these cells sensed sour by detecting acidity in foods. But the chemical connecting the taste cell to the nerve remained unknown.

The mystery chemical messenger, a new study in mice shows, is one usually associated with mood — serotonin. The findings help us understand one of our more mysterious senses. But they also suggest that some of our taste knowledge may have been hidden, not by lack of interest, but by use of the wrong techniques.

Eric Larson, a cell biologist at the University of Colorado School of Medicine in Aurora, says scientists had long suspected that serotonin was a key player in taste. “It’s been known for a while that type III cells … synthesize and store serotonin,” he says. “You can stain [the taste buds] for serotonin and see it accumulating.”

And the serotonin doesn’t just sit there. In a study published in 2005 in the Journal of Neuroscience, Stephen Roper and colleagues at the University of Miami School of Medicine showed that mouse taste buds release their serotonin, which is presumably acting as a messenger.

A chemical messenger can’t just bump into a cell, though. It has to contact a receptor, fitting into it as a key into a lock, to send the message on. In a new study, Larson and his colleagues set out to find the corresponding lock for the serotonin key. They hooked a potential receptor, serotonin 3, to green fluorescent protein and showed that the nerve terminals in the mouse tongue have serotonin 3 receptors, and these receptors are made by the nerve cells located in the geniculate ganglion. In the presence of sour tastes, the type III taste buds release serotonin, and the nerve cells with serotonin 3 receptors take it up. Larson and his colleague published their results December 2 in the Journal of Neuroscience.

But the finding only emerged after several months of failures. “To report the output of the tongue, you anesthetize the mouse and you record from the nerve,” Larson explains. Usually, he says, taste researchers anesthetize mice with pentobarbital. Pentobarbital is a barbiturate, which stimulates the inhibitory chemical messenger GABA, putting animals (and humans) to sleep. But Larson and his group found an unexpected side effect: “It blocked the serotonin signal,” he says. When mice were anesthetized with the barbiturate, the serotonin effect was barely present. When Larson used another anesthetic, urethane, the signal shot upward.

“When we switched [anesthetics] it was quite surprising,” Larson says. “We were super excited, now our mouse had a phenotype!”

The findings may call into question other taste studies that used pentobarbital, says Liman. “It’s surprising that pentobarbital affected the response,” she says. “The anesthetic may have masked interactions that need to be re-investigated.”

The idea that serotonin was mediating the taste of sour had been proposed before, but “there was no direct evidence,” she says. Now, “all the different pieces of evidence lead to the conclusion that serotonin acts in the taste bud.”

So the next time you face the mouthwatering prospect of a Reuben, think of serotonin. And don’t forget that even seemingly unimportant choices in science — such as the choice of one anesthetic technique over another — can sometimes make all the difference.

Editor's note: This post was updated on January 25, 2016, to clarify that taste buds contain multiple types of cells.

Biomedicine,, Science & Society

Building standards aren’t to blame for chilly offices

By Bethany Brookshire 4:26pm, August 11, 2015
A recent study made headlines for finding differences between men and women in comfort level for heating and cooling. But that’s not why women are cold in the office.
Nutrition,, Science & Society

How trans fats oozed into our diet and out again

By Bethany Brookshire 2:24pm, July 29, 2015
Trans fats are no longer “generally recognized as safe” by the FDA. In a world where we want to have our doughnuts and eat them, too, it’s back to the drawing board, and back to butter.

The weekly grind of social jetlag could be a weighty issue

By Bethany Brookshire 7:38am, July 20, 2015
Even those of us with nine-to-five jobs don’t always respect our body’s clocks. Research shows that even slight disruptions might be associated with obesity.
Physiology,, Health

Shifted waking hours may pave the way to shifting metabolism

By Bethany Brookshire 2:30pm, July 15, 2015
Shift workers are at higher risk for obesity and metabolic problems. Scientists are working hard to understand why the night shift makes our hormones go awry.
Physiology,, Health

Women blush when ovulating, and it doesn’t matter a bit

By Bethany Brookshire 4:59pm, July 9, 2015
Women don’t signal their fertility in obvious ways like nonhuman primates. A new study shows that even skin flushes are too subtle to detect.
Science & Society,, Health

No matter the language, disease risk is hard to communicate

By Bethany Brookshire 4:56pm, June 29, 2015
Reassuring messages about MERS might seem designed to stop panic. But in reality, people need to hear the truth, even if it’s uncertain.
Psychology,, Science & Society

The guilty pleasure of funny cat videos

By Bethany Brookshire 7:30am, June 25, 2015
Many people love posting and looking at cute kitty content online. A new survey shows that this could be because it helps us manage our emotions.
Science & Society

Home-brewed heroin: Hold the hype

By Bethany Brookshire 3:00pm, June 17, 2015
Now is the time to think about policy for synthetically produced morphine, but the process, if it bears out, is years away from working.

Diet and nutrition is more complex than a simple sugar

By Bethany Brookshire 4:04pm, May 26, 2015
A new study shows that fructose may leave you wanting more when compared to the same dose of glucose. But in studies of single nutrients, it’s important to be cautious.

A vivid emotional experience requires the right genetics

By Bethany Brookshire 11:43am, May 8, 2015
A single gene deletion gives some people an extra vivid jolt to their emotional experience, a new study shows.
Subscribe to RSS - Scicurious