JOE PALCA, host:
This is TALK OF THE NATION/SCIENCE FRIDAY from NPR News. I'm Joe Palca, in for Ira Flatow.
People say there's no accounting for taste, but in the next hour, we're going to try and prove to you that that's just not true. Now we won't explain why some people actually like William Shatner's singing. We'll be talking about how your tongue lets your brain know whether you're eating chocolate cake or salt cod. The answer starts, at least, with your taste buds. Your taste buds let you know whether what you're eating is sweet, sour, salty or bitter. Exactly how taste buds works remains a bit of an enigma, and now scientists are one step closer to figuring it out.
In this week's Proceedings of the National Academies of Sciences, scientists demonstrated that a peptide called NPY is involved in the communication between taste cells in a taste bud, and that communication appears to be key to decoding taste. We'll talk with the author of the study to find out what he did and what it means.
If you'd like to join the discussion, give us a call. Our number is 1 (800) 989-8255; that's 1 (800) 989-TALK. For more information about taste, you can go to our Web site at sciencefriday.com, where you'll find links to our topic.
Now let me introduce my guest. Scott Herness is a professor in the College of Dentistry at Ohio State University in Columbus, Ohio, and was the senior author on the PNAS paper. He joins me now by phone.
Welcome to SCIENCE FRIDAY, Dr. Herness.
Dr. M. SCOTT HERNESS (College of Dentistry, Ohio State University): Hi, Joe. It's great to be here.
PALCA: So before we get into the details of what you did, I have to ask this question that's been bugging me. I mean, you know, taste buds--I thought that the taste buds were sort of all over the tongue and that you--I mean, that--but that there were special places where you could taste salty and more likely in one spot; there was that map that you learned in physiology class.
Dr. HERNESS: Oh, yeah. That's...
PALCA: Is that...
Dr. HERNESS: ...the famous taste map.
PALCA: Yeah. What about that? Is your work undercutting that at all? 'Cause I would hate to think that the foundations of my academic training are being shattered like that.
Dr. HERNESS: Well, I think you better grab something, Joe, because...
(Soundbite of laughter)
Dr. HERNESS: ...might be a little rattling.
Dr. HERNESS: Actually, that famous taste map that we see in textbooks all the time, even in elementary school textbooks--it's a really nice little visual tool about the tongue, but there's only really one problem with it, and that's that it's completely wrong.
PALCA: Oh, I hate those little problems, you know?
Dr. HERNESS: Yeah.
PALCA: But surely there's--I mean, they didn't bring this--make this up out of whole cloth. What does that map...
Dr. HERNESS: Yeah.
PALCA: ...purport to show, and what are they--what are you disagreeing with it about?
Dr. HERNESS: Well, so the taste map--if you read it in the textbooks, it would tell you that you taste sweet in the front of your tongue and sour on the sides of your tongue, bitter in the back, and that part of it's actually not true at all. We taste all the taste qualities everywhere across the tongue, and even in places in the oral cavity that aren't on the tongue. So you can taste sweet, sour, salty and bitter just fine on the front of your tongue. What that map really means is where you're most sensitive to these taste qualities. You are more sensitive to bitter qualities in the back of your mouth and more sensitive to sweet on the front, and I don't have that many friends that are wine connoisseurs, but I've heard that wine tasters can pick up the sour taste on the sides of their tongue first, and that is where the tongue seems to be most sensitive to sour.
PALCA: So--OK. Before--I don't want to dwell on this, because it's not exactly on your topic, but why would you be more sensitive in different places? Is there a greater concentration of taste--particular kind of taste--well, you said all taste buds can taste all tastes, so is it...
Dr. HERNESS: That's true, but they don't do an equally good job at it. Recently in the field, we've finally cloned some of the receptors that are the actual molecules that interact with the taste molecules, like sucrose or quinine for sweet and bitter, and they're not distributed evenly across the tongue, so there are more bitter receptors in the taste buds in the back of the mouth, so those buds are a little more sensitive to bitter. And that helps to explain the regional differences that we see across the tongue.
PALCA: OK. So tell us a little bit about what a taste bud is. What we know now is it has a receptor, some kind of molecule on its surface, that attaches to flavorings, but what else do we know about it?
Dr. HERNESS: Well, one thing we know about taste buds is that they are unique across sensory systems, and they're highly conserved within the taste system, so that is to say, if you look at any kind of organism that has a backbone in it, whether it's a fish or a chicken or a person, they have a taste system; they're going to have taste buds. But if you look at other sensory systems, like vision or audition or even smell, you have the receptor molecules distributed across a wide array of the epithelium. So there's something unique about taste that it seems to require taste buds, 'cause that's--all systems that have taste have these taste buds. So we've known that there are these buds for hundreds of years, and we always thought there must be a good reason why you have to have taste buds in order to have a taste system that works well, and some of the research that we've been doing is beginning to address that question of why there are taste buds.
PALCA: And--but what's the story, then, with this peptide? I guess that's a string of amino acids called NPY.
Dr. HERNESS: That's right.
PALCA: What's it doing in the equation here?
Dr. HERNESS: Well, what we think it's doing--and we've looked at another peptide as well that's called cholecystokinin, or CCK for short, and these two peptides, CCK and NPY, seem to go hand in hand in the taste bud. And what we think that they're doing is they are a mechanism that these cells can use to talk to one other within the bud. It seems like the special structure that you have, a taste bud, where you pull all the receptors very close to one another into this cloistered arrangement, is really the perfect anatomical structure to allow the cells to begin to communicate with one another. And we think these peptides are agents that the buds are using for individual cells to talk to one another.
PALCA: Interesting. Well, I'd like to include my listeners--our listeners--on this. It's--our number's (800) 989-8255. And let's take a call now from--Is it Navandrea(ph)?...
NAVANDREA (Caller): Yes, that's it.
PALCA: ...in Tallahassee, Florida. Welcome to SCIENCE FRIDAY. Thanks for calling.
NAVANDREA: Thank you. I'm a longtime listener, first-time caller.
PALCA: Oh, great.
NAVANDREA: And I have a question for the speaker. This is kind of a weird question 'cause it's a strange phenomenon and maybe it's just for me, but when I eat something sweet and then I go back to drink maybe a sweet drink, the drink never really tastes quite as sweet before--as it did before I ate the sweet food. Is there a reason for that?
Dr. HERNESS: Oh, there sure is a reason for that. That's a phenomenon we call adaptation.
Dr. HERNESS: And it's interesting; all the animals that we've studied in the taste systems have adaptation, but one of the animals that has the greatest amount of adaptation is us, humans.
NAVANDREA: Oh, OK.
Dr. HERNESS: And if you taste something sweet and you continue to have it in your mouth, your taste receptor cells actually adapt to that stimulus, which is to say they don't respond as well to it as they did when you first put it in your mouth. And a good example of that is if you take a piece of hard candy and you put it in your mouth, it tastes sweet. And if you don't move it around, if you're very careful and leave it exactly in the same spot, pretty soon the sweet taste will go away.
NAVANDREA: Very, very. Well, thank you so much. I appreciate that.
PALCA: Navandrea, thanks for that question.
So I'm still a little--I need--I want to be a little clearer about exactly how the taste buds--I mean, they're not quite like the eyeball, where everything in the back of the retina connects straight to the brain. The taste buds don't all connect--or do all taste buds connect directly to the brain, or do all the cells in the taste bud connect to the brain?
Dr. HERNESS: Yeah. No, that's one of the puzzling features of taste buds that we've been really scratching our heads about, those of us who spend some time trying to figure out how taste buds operate. Actually, only a minority of the cells in the bud are actually connected to the brain by the nerve fiber, and the rest of the taste cells are not connected to the brain at all. But now we've learned that these taste cells that are not connected to the brain are actually the ones that express the bitter receptors. So it's a little puzzling, 'cause here you have a cell; it's going to interact with a bitter stimulus, it's going to get excited, but how is it going to relay that message to the brain that you're tasting something bitter when it's not connected to the brain?
Dr. HERNESS: And that's where we think these cells talking to one another within the bud is going to help us to explain how the bud operates.
PALCA: I see. So it's this communication that's essential for the taste bud to work properly.
Dr. HERNESS: It seems to be...
PALCA: Seems to be.
Dr. HERNESS: ...turning out that way, yes.
PALCA: OK. And we should emphasize here that your work was done primarily in rodents, isn't that right?
Dr. HERNESS: This is correct. Yeah.
PALCA: Right. But we're making the assumption, since this taste is so highly conserved across species, that what's true for rodents at least is going to be reasonably close to being true for humans.
Dr. HERNESS: Well, we have looked at these peptides in a number of different species, such as hamsters and rabbits. We haven't looked at them in primates yet, but we are finding them in the other species that we look at, so we're going to make an assumption that something like this could be operating in the human system as well.
PALCA: OK. Let's go to Kim in Brooklyn. I suppose that's Brooklyn, New York. Kim, welcome to the program.
KIM (Caller): Hi. Thank you.
PALCA: What's your question?
KIM: Well, my question is about a taste that my Japanese students have told me about that Americans and, I guess, all other cultures don't consider. You know how we have salty tastes and bitter and sweet? They have one other taste. I think it's called umami and...
Dr. HERNESS: That's correct.
KIM: Yeah. And I don't know what that taste is in our cuisine.
PALCA: Oh, I--well, go ahead. I've heard of examples, but maybe you can tell us, Scott Herness.
Dr. HERNESS: Sure. I can give it a shot. Umami has been studied in Japan for many, many years, and I've been in the field over 20 years myself. When I first started out in this field, we talked about four taste qualities: sweet, sour, salty and bitter. And now we talk about five, and that fifth one is umami, which is--the most common example of that would be glutamate, MSG, that's used as a flavor enhancer. But it seems to be a little more general than that, and it's the flavor of amino acids. So a good common example of that would be sort of that brothy taste that you might have in a very--in a consomme or a very bland soup--the taste of the amino acids from the proteins of the meat.
PALCA: Huh. And someone told me that eel has an umami (pronounced ooMAmee), umami (pronounced yooMAmee) taste quality to it. Is that possible?
Dr. HERNESS: Oh, I'm sure that it does.
KIM: OK. Thank you very much.
PALCA: Thanks very much.
Well, so how--you know, are we going to get the complete picture, or is there a--still ways to go here, do you think?
Dr. HERNESS: Well, I think that we still have a ways to go. I've been doing this for 20 years. I've got a little bit to go before retirement, but I don't think I'm going to run out of things to do.
PALCA: OK. Excellent. Well, unfortunately, you may have things to do, but we have no time to hear about them 'cause we've run out of time for this segment. So thanks very much for joining us today.
Dr. HERNESS: It's been my pleasure, Joe. Thanks for having me.
PALCA: Scott Herness is a professor in the College of Dentistry at Ohio State University in Columbus, Ohio.
We're going to take a short break now, and when we come back, we're going to talk about the science of bicycling. What makes one bicyclist better than another? What makes one bicycle faster than another? And a simple question: How do you keep a bicycle upright? All that when we get back. Stay with us.
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