Posts tagged taste
Articles and news from the latest research reports.
Taste memory
Have you ever eaten something totally new and it made you sick? Don’t give up; if you try the same food in a different place, your brain will be more “forgiving” of the new attempt. In a new study conducted by the Sagol Department of Neurobiology at the University of Haifa, researchers found for the first time that there is a link between the areas of the brain responsible for taste memory in a negative context and those areas in the brain responsible for processing the memory of the time and location of the sensory experience. When we experience a new taste without a negative context, this link doesn’t exist.
The area of the brain responsible for storing memories of new tastes is the taste cortex, found in a relatively insulated area of the human brain known as the insular cortex. The area responsible for formulating a memory of the place and time of the experience (the episode) is the hippocampus. Until now, researchers assumed that there was no direct connection between these areas – i.e., the processing of information about a taste is not related to the time or the place one experiences the taste. The accepted thinking was that a negative experience – for example, being exposed to a bad taste – would be negative in the same way anywhere, and the brain would create a memory of the taste itself, divorced from the time or place.
But in this new study, conducted by doctoral student Adaikkan Chinnakkaruppan in the laboratory of Prof. Kobi Rosenblum of the Sagol Department of Neurobiology at the University of Haifa, in cooperation with the Riken Institute, the leading brain research institute in Tokyo, the researchers demonstrate for the first time that there is a functional link between the two brain regions.
In the study the researchers sought to examine the relationship between the taste cortex (which is responsible for taste memory), and three different areas in the hippocampus: CA1, which is responsible for encoding the concept of space (where we are located); DG, the area responsible for encoding the time relationship between events; and CA3, responsible for filling in missing information. To do this the researchers took ordinary mice and mice that were genetically engineered by their Japanese colleagues such that these three areas of the brain functioned normally but were lacking plasticity, which did not allow new memories reliant on them to be created.
“In brain research, the manipulation we do must be very delicate and precise, otherwise the changes can make the entire experiment irrelevant to proving or refuting the research hypothesis,” said Prof. Rosenblum.
The mice were exposed to two new tastes, one that caused stomach pains (to mimic exposure to toxic food) and another that didn’t cause that feeling. By comparing the two groups it emerged that when the new taste was not accompanied by an association with toxic food, there was no difference between the normal mice and those whose various functional areas in the hippocampus didn’t allow plasticity. But when the taste caused a negative feeling, there was clear involvement of the CA1 area, which is responsible for encoding the space.
“The significance of this is that the moment we go back to the same place at which we experienced the taste associated with a bad feeling, subconsciously the negative memory will be much stronger than if we come to taste the same taste in a totally different place,” explained Prof. Rosenblum. Similarly, the DG area, which is responsible for encoding the time between incidents, was involved the more time that passed between the new taste and the stomach discomfort. “This means that even during a simple associative taste, the brain operates the hippocampus to produce an integrated experience that includes general information about the time between events and their location,” he said.
The findings, which were recently published in the Journal of Neuroscience, expose the complexity and richness of the simple sensory experiences that are engraved in our brains and that in most cases we aren’t even aware of. Moreover, the study can help explain behavioral results and the difficulty in producing memories when certain areas of the brain become dysfunctional following and illness or accident. The better we understand the encoding of simple sensory experiences in the brain and the link between the feeling, time and place of the experiences; we will better understand the complex process of creating memories and storing them in our brains.
(Source: newmedia-eng.haifa.ac.il)
Neural sweet talk: Taste metaphors emotionally engage the brain
So accustomed are we to metaphors related to taste that when we hear a kind smile described as “sweet,” or a resentful comment as “bitter,” we most likely don’t even think of those words as metaphors. But while it may seem to our ears that “sweet” by any other name means the same thing, new research shows that taste-related words actually engage the emotional centers of the brain more than literal words with the same meaning.
Researchers from Princeton University and the Free University of Berlin report in the Journal of Cognitive Neuroscience the first study to experimentally show that the brain processes these everyday metaphors differently than literal language. In the study, participants read 37 sentences that included common metaphors based on taste while the researchers recorded their brain activity. Each taste-related word was then swapped with a literal counterpart so that, for instance, “She looked at him sweetly” became “She looked at him kindly.”
The researchers found that the sentences containing words that invoked taste activated areas known to be associated with emotional processing, such as the amygdala, as well as the areas known as the gustatory cortices that allow for the physical act of tasting. Interestingly, the metaphorical and literal words only resulted in brain activity related to emotion when part of a sentence, but stimulated the gustatory cortices both in sentences and as stand-alone words.
Metaphorical sentences may spark increased brain activity in emotion-related regions because they allude to physical experiences, said co-author Adele Goldberg, a Princeton professor of linguistics in the Council of the Humanities. Human language frequently uses physical sensations or objects to refer to abstract domains such as time, understanding or emotion, Goldberg said. For instance, people liken love to a number of afflictions including being “sick” or shot through the heart with an arrow. Similarly, “sweet” has a much clearer physical component than “kind.” The new research suggests that these associations go beyond just being descriptive to engage our brains on an emotional level and potentially amplify the impact of the sentence, Goldberg said.
"You begin to realize when you look at metaphors how common they are in helping us understand abstract domains," Goldberg said. "It could be that we are more engaged with abstract concepts when we use metaphorical language that ties into physical experiences."
If metaphors in general elicit an emotional response from the brain that is similar to that caused by taste-related metaphors, then that could mean that figurative language presents a “rhetorical advantage” when communicating with others, explained co-author Francesca Citron, a postdoctoral researcher of psycholinguistics at the Free University’s Languages of Emotion research center.
"Figurative language may be more effective in communication and may facilitate processes such as affiliation, persuasion and support," Citron said. "Further, as a reader or listener, one should be wary of being overly influenced by metaphorical language."
Colloquially, metaphors seem to be employed precisely to evoke an emotional reaction, yet the actual emotional effect of figurative phrases on the person hearing them has not before been deeply explored, said Benjamin Bergen, an associate professor of cognitive science at the University of California-San Diego who studies language comprehension, and metaphorical language and thought.
"There’s a lot of research on the conceptual effects of metaphors, such as how they allow people to think about new or abstract concepts in terms of concrete things they’re familiar with. But there’s very little work on the emotional impact of metaphor," said Bergen, who had no role in the research but is familiar with it.
"Emotional impact seems to be one of the main reasons people use metaphors to begin with. For instance, a senator might describe a bill as ‘job-killing’ to evoke an emotional reaction," he said. "These results suggest that using certain metaphorical expressions induces more of an emotional reaction than saying the same thing literally. Those expressions that have this property are likely to have the effects on reasoning, inference, judgment and decision-making that emotion is known to have."
The brain areas that taste-related words did not stimulate are also an important outcome of the study, Citron said. Existing research on metaphors and neural processing has shown that figurative language generally requires more brainpower than literal language, Citron and Goldberg wrote. But these bursts of neural activity have been related to higher-order processing from thinking through an unfamiliar metaphor.
The brain activity Citron and Goldberg observed did not correlate with this process. In order to create the metaphorical- and literal-sentence stimuli, they had a group of people separate from the study participants rate sentences for familiarity, apparent arousal, imageability — which is how easily a phrase can be imagined in the reader’s mind — and how positive or negative each sentence was interpreted as being. The metaphorical and literal sentences were equal on all of these factors. In addition, each metaphorical phrase and its literal counterpart were rated as being highly similar in meaning.
These steps helped to ensure that the metaphorical and literal sentences were equally as easy to comprehend. Thus, the brain activity the researchers recorded was not likely to be in response to any additional difficulty study participants had in understanding the metaphors.
"It is important to rule out possible effects of familiarity, since less familiar items may require more processing resources to be understood and elicit enhanced brain responses in several brain regions," Citron said.
Citron and Goldberg plan to follow up on their results by examining if figurative language is remembered more accurately than literal language, if metaphors are more physically stimulating, and if metaphors related to other senses also provoke an emotional response from the brain.
Stress hormone receptors localized in sweet taste cells
According to new research from the Monell Center, receptors for stress-activated hormones have been localized in oral taste cells responsible for detection of sweet, umami, and bitter. The findings suggest that these hormones, known as glucocorticoids, may act directly on taste receptor cells under conditions of stress to affect how these cells respond to sugars and certain other taste stimuli.
"Sweet taste may be particularly affected by stress," said lead author M. Rockwell Parker, PhD, a chemical ecologist at Monell. "Our results may provide a molecular mechanism to help explain why some people eat more sugary foods when they are experiencing intense stress."
Glucocorticoid (GC) hormones affect the body by activating specialized GC receptors located inside of cells. Knowing that stress can have major effects on metabolism and food choice, the researchers used a mouse model to ask whether taste receptor cells contain these GC receptors.
The findings, published online ahead of print in the journal Neuroscience Letters, revealed that GC receptors are present on the tongue, where they are specifically localized to the cells that contain receptors for sweet, umami and bitter taste. The highest concentrations of GC receptors were found in Tas1r3 taste cells, which are sensitive to sweet and umami taste.
GC hormones act on cells via a multi-step process. After GCs bind to their receptors within target cells, the activated receptor complex moves, or translocates, to the cell nucleus, where it then influences gene expression and protein assembly.
To explore whether GC receptors in taste tissue are activated by stress, the researchers compared the proportion of taste cells with translocated receptors in stressed and non-stressed mice. Compared to controls, the stressed mice had a 77 percent increase of GC receptors within taste cell nuclei.
Together, the results suggest that sweet taste perception and intake, which are known to be altered by stress, may be specifically affected via secretion of GCs and subsequent activation of GC receptors in taste cells.
"Taste provides one of our initial evaluations of potential foods. If this sense can be directly affected by stress-related hormonal changes, our food interaction will likewise be altered," said Parker.
Parker noted that although stress is known to affect intake of salty foods, GC receptors were not found in cells thought to be responsible for detecting sally and sour taste. One explanation, he said, is that stress may influence salt taste processing in the brain.
Implications of the findings extend beyond the oral taste system. Noting that taste receptors are found throughout the body, senior author and Monell molecular neurobiologist Robert Margolskee, MD, PhD, said, “Taste receptors in the gut and pancreas might also be influenced by stress, potentially impacting metabolism of sugars and other nutrients and affecting appetite.”
Future studies will continue to explore how stress hormones act to affect the taste system.
(Source: eurekalert.org)
Citizens help researchers to challenge scientific theory
Science crowdsourcing was used to disprove a widely held theory that “supertasters” owe their special sensitivity to bitter tastes to an usually high density of taste buds on their tongue, according to a study published in the open-access journal Frontiers in Integrative Neuroscience.
Supertasters are people who can detect and are extremely sensitive to phenylthiocarbamide and propylthiouracil, two compounds related to the bitter molecules in certain foods such as broccoli and kale. Supertasting has been used to explain why some people don’t like spicy foods or “hoppy” beers, or why some kids are picky eaters.
The sensitivity to these bitter tastants is partly due to a variation in the taste receptor gene TAS2R38. But some scientists believe that the ability to supertaste is also boosted by a greater-than-average number of “papillae”, bumps on the tongue that contain taste buds. Nicole Garneau, Curator and Chair of the Department of Health Sciences, Denver Museum of Nature & Science, and colleagues tested if this is true.
"There is a long-held belief that if you stick out your tongue and look at the bumps on it, then you can predict how sensitive you are to strong tastes like bitterness in vegetables and strong sensations like spiciness," says Garneau. "The commonly accepted theory has been that the more bumps you have, the more taste buds you have and therefore the more sensitive you are."
Over 3000 visitors to the museum’s Genetics of Taste Lab volunteered to stick their tongue out so that their papillae could be counted and their sensitivity to phenylthiocarbamide and propylthiouracil measured. In total, 394 study subjects were included in the analysis. Cell swabs from volunteers were taken to determine their DNA sequence at TAS2R38. Results confirmed that certain variations in TAS2R38 make it more likely that somebody is sensitive to bitter, but also proved that the number of papillae on the tongue does not affect increased taste sensitivity.
"No matter how we looked at the data, we couldn’t replicate this long held assumption that a high number of papillae equals supertasting," says Garneau.
The authors argue against the continued misuse of the term supertaster, and for the use of the more objective term hypergeusia – abnormally sensitized taste – to describe people who are sensitive to all tastes and sensations from food.
"What we know and understand about how our bodies work improves greatly when we challenge central dogmas of our knowledge. This is the nature of science itself," adds Garneau. "As techniques improve, so too does our ability to do science, and we find that what we accepted as truth 20, 30, or 100 years ago gets replaced with better theories as we gather new data, which advances science. In this case, we’ve proven that with the ‘Denver Papillae Protocol’, our new method for objective analysis for papillae density, we were unable to replicate well-known studies about supertasting."
What make this study unique is that most of the results were collected by citizen scientists including over 130 volunteers who had been specially trained by Garneau and her colleagues. The Genetics of Taste Lab is located in the heart of the museum, uniquely situated to attract volunteers and dedicated citizen scientists who conduct population-based research about human genetics, taste, and health.
Taste Test: Could sense of taste affect length of life?
Perhaps one of the keys to good health isn’t just what you eat but how you taste it.
Taste buds – yes, the same ones you may blame for that sweet tooth or French fry craving – may in fact have a powerful role in a long and healthy life – at least for fruit flies, say two new studies that appear in the Proceedings of the National Academy of Sciences of the United States of America.
Researchers from the University of Michigan, Wayne State University and Friedrich Miescher Institute for Biomedical Research in Switzerland found that suppressing the animal’s ability to taste its food –regardless of how much it actually eats – can significantly increase or decrease its length of life and potentially promote healthy aging.
Bitter tastes could have negative effects on lifespan, sweet tastes had positive effects, and the ability to taste water had the most significant impact – flies that could not taste water lived up to 43% longer than other flies. The findings suggest that in fruit flies, the loss of taste may cause physiological changes to help the body adapt to the perception that it’s not getting adequate nutrients.
In the case of flies whose loss of water taste led to a longer life, authors say the animals may attempt to compensate for a perceived water shortage by storing greater amounts of fat and subsequently using these fat stores to produce water internally. Further studies are planned to better explore how and why bitter and sweet tastes affect aging.
“This brings us further understanding about how sensory perception affects health. It turns out that taste buds are doing more than we think,” says senior author of the University of Michigan-led study Scott Pletcher, Ph.D., associate professor in the Department of Molecular and Integrative Physiology and research associate professor at the Institute of Gerontology.
“We know they’re able to help us avoid or be attracted to certain foods but in fruit flies, it appears that taste may also have a very profound effect on the physiological state and healthy aging.”
Pletcher conducted the study with lead author Michael Waterson, a Ph.D graduate student in U-M’s Cellular and Molecular Biology Program.
“Our world is shaped by our sensory abilities that help us navigate our surroundings and by dissecting how this affects aging, we can lay the groundwork for new ideas to improve our health,” says senior author of the other study, Joy Alcedo, Ph.D, assistant professor in the Department of Biological Sciences at Wayne State University, formerly of the Friedrich Miescher Institute for Biomedical Research in Switzerland. Alcedo conducted the research with lead author Ivan Ostojic, Ph.D., of the Friedrich Miescher Institute for Biomedical Research in Switzerland.
Recent studies suggest that sensory perception may influence health-related characteristics such as athletic performance, type II diabetes, and aging. The two new studies, however, provide the first detailed look into the role of taste perception.
“These findings help us better understand the influence of sensory signals, which we now know not only tune an organism into its environment but also cause substantial changes in physiology that affect overall health and longevity,” Waterson says. “We need further studies to help us apply this knowledge to health in humans potentially through tailored diets favoring certain tastes or even pharmaceutical compounds that target taste inputs without diet alterations.”
(Source: uofmhealth.org)
Does obesity reshape our sense of taste?
Obesity may alter the way we taste at the most fundamental level: by changing how our tongues react to different foods.
In a Nov. 13 study in the journal PLOS ONE, University at Buffalo biologists report that being severely overweight impaired the ability of mice to detect sweets.
Compared with slimmer counterparts, the plump mice had fewer taste cells that responded to sweet stimuli. What’s more, the cells that did respond to sweetness reacted relatively weakly.
The findings peel back a new layer of the mystery of how obesity alters our relationship to food.
“Studies have shown that obesity can lead to alterations in the brain, as well as the nerves that control the peripheral taste system, but no one had ever looked at the cells on the tongue that make contact with food,” said lead scientist Kathryn Medler, PhD, UB associate professor of biological sciences.
“What we see is that even at this level — at the first step in the taste pathway — the taste receptor cells themselves are affected by obesity,” Medler said. “The obese mice have fewer taste cells that respond to sweet stimuli, and they don’t respond as well.”
The research matters because taste plays an important role in regulating appetite: what we eat, and how much we consume.
How an inability to detect sweetness might encourage weight gain is unclear, but past research has shown that obese people yearn for sweet and savory foods though they may not taste these flavors as well as thinner people.
Medler said it’s possible that trouble detecting sweetness may lead obese mice to eat more than their leaner counterparts to get the same payoff.
Learning more about the connection between taste, appetite and obesity is important, she said, because it could lead to new methods for encouraging healthy eating.
“If we understand how these taste cells are affected and how we can get these cells back to normal, it could lead to new treatments,” Medler said. “These cells are out on your tongue and are more accessible than cells in other parts of your body, like your brain.”
The new PLOS ONE study compared 25 normal mice to 25 of their littermates who were fed a high-fat diet and became obese.
To measure the animals’ response to different tastes, the research team looked at a process called calcium signaling. When cells “recognize” a certain taste, there is a temporary increase in the calcium levels inside the cells, and the scientists measured this change.
The results: Taste cells from the obese mice responded more weakly not only to sweetness but, surprisingly, to bitterness as well. Taste cells from both groups of animals reacted similarly to umami, a flavor associated with savory and meaty foods.
The Bitter and the Sweet: Fruit Flies Reveal a New Interaction Between the Two
Fruit flies have a lot to teach us about the complexity of food. Like these tiny little creatures, most animals are attracted to sugar but are deterred from eating it when bitter compounds are added.
A new study conducted by UC Santa Barbara’s Craig Montell, Duggan Professor of Neuroscience in the Department of Molecular, Cellular and Developmental Biology, explains a breakthrough in understanding how sensory input impacts fruit flies’ decisions about sweet taste. The findings were published today in the journal Neuron.
It is generally well known that the addition of bitter compounds inhibits attraction to sugars. However, until now the cellular and molecular mechanisms underlying an important aspect of this ubiquitous animal behavior were poorly understood.
When animals encounter bitterness in foods, two factors cause them to stop eating. First, bitter compounds bind to proteins called bitter gustatory receptors (GRs), which inhibits feeding. The second –– and more elusive –– factor involves inhibition of the sugar response. This is the focus of Montell’s research.
At the center of the team’s discovery is the function of an odorant-binding protein (OPB) in the gustatory system. These proteins are usually but not exclusively resident in the olfactory system. Montell’s team found definitive evidence that an OBP, synthesized and released from non-neuronal cells, not only binds bitter tastants, but also moves and binds to the surface of nearby gustatory receptor neurons (GRNs) that contain sugar-activated GRs.
This unanticipated process inhibits the activity of these GRNs and reduces the fruit flies’ attraction to sugars. These results not only reveal an unexpected role for an OBP in taste, but also identify the first molecular player (OBP49a) involved in the integration of opposing attractive and aversive gustatory stimuli in fruit flies.
The researchers used two different fruit flies, wild-type and mutants missing the OBP49a protein, to demonstrate that bitter compounds suppress feeding behavior by binding to the OBP49a protein. As expected, wild-type flies find bitter aversive and prefer the lower concentration of sucrose when the higher concentration of sucrose is laced with bitter tastants such as quinine.
The same was not true of the mutant flies, which do not express OBP49a. Their avoidance behavior was impaired because the bitter compounds did not inhibit the sweet response by binding to OPB49. However, loss of OBP49a did not affect gustatory behavior or action potentials in sugar- or bitter-activated GRNs when the GRNs were presented with just one type of tastant.
"We showed that the OBP49a protein was in very close proximity or even touching the sugar GRs," said Montell. "If the bitter compound weren’t present, there would be normal sugar activation. We found that decreased behavioral avoidance to a sucrose/aversive mixture in the mutant flies was due to a deficit in the sugar-activated GRNs and not due to effects on GRNs activated by bitter compounds."
OBP49a is the first molecule shown to promote the inhibition of the sucrose-activated GRNs by aversive chemicals in fruit flies. The findings demonstrate at least one important cellular mechanism through which bitter and sweet taste integration occurs in the taste receptor neurons. However, the findings do not exclude the possibility that suppression of sweet by bitter compounds could also take place through the integration of separate bitter and sweet inputs in the brain.
"As we get a better understanding of aversive and attractive chemosensory behaviors in flies, it helps us understand how insect pests can be controlled," said Montell. "This is a step toward understanding the behaviors of related insects that spread disease. Molecules related to the OBPs and GRs in fruit flies are also in ticks and mosquitos that spread parasites and viruses."
‘Seeing’ the flavor of foods
The eyes sometimes have it, beating out the tongue, nose and brain in the emotional and biochemical balloting that determines the taste and allure of food, a scientist said here today. Speaking at the 245th National Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society, he described how people sometimes “see” flavors in foods and beverages before actually tasting them.
“There have been important new insights into how people perceive food flavors,” said Terry E. Acree, Ph.D. “Years ago, taste was a table with two legs — taste and odor. Now we are beginning to understand that flavor depends on parts of the brain that involve taste, odor, touch and vision. The sum total of these signals, plus our emotions and past experiences, result in perception of flavors, and determine whether we like or dislike specific foods.”
Acree said that people actually can see the flavor of foods, and the eyes have such a powerful role that they can trump the tongue and the nose. The popular Sauvignon Blanc white wine, for instance, gets its flavor from scores of natural chemicals, including chemicals with the flavor of banana, passion fruit, bell pepper and boxwood. But when served a glass of Sauvignon Blanc tinted to the deep red of merlot or cabernet, people taste the natural chemicals that give rise to the flavors of those wines.
The sense of smell likewise can trump the taste buds in determining how things taste, said Acree, who is with Cornell University. In a test that people can do at home, psychologists have asked volunteers to smell caramel, strawberry or other sweet foods and then take a sip of plain water; the water will taste sweet. But smell bread, meat, fish or other non-sweet foods, and water will not taste sweet.
While the appearance of foods probably is important, other factors can override it. Acree pointed out that hashes, chilies, stews and cooked sausages have an unpleasant look, like vomit or feces. However, people savor these dishes based on the memory of eating and enjoying them in the past. The human desire for novelty and new experiences also is a factor in the human tendency to ignore what the eyes may be tasting and listening to the tongue and nose, he added.
Acree said understanding the effects of interactions between smell and vision and taste, as well as other odorants, will open the door to developing healthful foods that look and smell more appealing to finicky kids or adults.
(Source: portal.acs.org)
How the Body’s Energy Molecule Transmits Three Types of Taste to the Brain
Saying that the sense of taste is complicated is an understatement, that it is little understood, even more so. Exactly how cells transmit taste information to the brain for three out of the five primary taste types was pretty much a mystery, until now.
A team of investigators from nine institutions discovered how ATP – the body’s main fuel source – is released as the neurotransmitter from sweet, bitter, and umami, or savory, taste bud cells. The CALHM1 channel protein, which spans a taste bud cell’s outer membrane to allow ions and molecules in and out, releases ATP to make a neural taste connection. The other two taste types, sour and salt, use different mechanisms to send taste information to the brain.
Kevin Foskett, PhD, professor of Physiology at the Perelman School of Medicine, University of Pennsylvania, and colleagues from the Monell Chemical Senses Center, the Feinstein Institute for Medical Research, and others, describe in Nature how ATP release is key to this sensory information path. They found that the calcium homeostasis modulator 1 (CALHM1) protein, recently identified by the Foskett lab as a novel ion channel, is indispensable for taste via release of ATP.
“This is an example of a bona fide ATP ion channel with a clear physiological function,” says Foskett. “Now we can connect the molecular dots of sweet and other tastes to the brain.”
Taste buds have specialized cells that express G-protein coupled receptors (GPCRs) that bind to taste molecules and initiate a complex chain of molecular events, the final step of which Foskett and collaborators show is the opening of a pore in the cell membrane formed by CALHM1. ATP molecules leave the cell through this pore to alert nearby neurons to continue the signal to the taste centers of the brain. CALHM1 is expressed specifically in sweet, bitter, and umami taste bud cells.
Mice in which CALHM1 proteins are absent, developed by Feinstein’s Philippe Marambaud, PhD, have severely impaired perceptions of sweet, bitter and umami compounds; whereas, their recognition of sour and salty tastes remains mostly normal. The CALHM1 deficiency affects taste perception without interfering with taste cell development or overall function.
Using the CALHM1 knockout mice, team members from Monell and Feinstein tested how their taste was affected. “The mice are very unusual,” says Monell’s Michael Tordoff, PhD. “Control mice, like humans, lick avidly for sucrose and other sweeteners, and avoid bitter compounds. However, the mice without CALHM1 treat sweeteners and bitter compounds as if they were water. They can’t taste them at all.”
From all lines of evidence, the team concluded that CALHM1 is an ATP-release channel required for sweet, bitter, and umami taste perception. In addition, they found that CALHM1 was also required for “nontraditional” Polycose, calcium, and aversive high-salt tastes, implying that the deficit displayed in the knockout animals might best be considered as a loss of all GPCR-mediated taste signals rather than simply sweet, bitter and umami taste.
Interestingly, CALHM1 was originally implicated in Alzheimer’s disease, although the link is now less clear. In 2008, co-author Marambaud identified CALHM1 as a risk gene for Alzheimer’s. They discovered that a CALHM1 genetic variant was more common among people with Alzheimer’s and they went on to show that it leads to a partial loss of function. They also found that this novel ion channel is strongly expressed in the hippocampus, a brain region necessary for learning and memory. So far, there is no connection between taste perception and Alzheimer’s risk, but Marambaud suspects that scientists will start testing this hypothesis.
Scientists Discover How Animals Taste, and Avoid, High Salt Concentrations
For consumers of the typical Western diet—laden with levels of salt detrimental to long-term health—it may be hard to believe that there is such a thing as an innate aversion to very high concentrations of salt.
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 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.
The sensation of saltiness is unique among the five basic tastes. Whereas mammals are always attracted to the tastes of sweet and umami, and repelled by sour and bitter, their behavioral response to salt dramatically changes with concentration.
“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.”