A Matter of Taste - #59

Hey now everybody, and welcome to Back to Biota, where we dive into the weird, the wonderful, and the weight. What? Of the natural world that we experience every day. Today, let's start with some questions for you to consider about things we've all probably experienced at one time or another. Why does apple pie taste a little bit sweeter when it's warm rather than when it's cold? Why does spicy food cause us to literally break out in a sweat? And how does a little bit of salt somehow make watermelon taste just a little bit sweeter? Well, in this episode, we're going to answer those questions and get into a topic that's a little sweet and a little spicy. We're going to talk about flavor perception. To help us find answers to these questions, I've invited Dr. Chris Limon into the studio today. Dr. Limon is a neurobiologist at the University of Oklahoma who studies how the brain responds to sweet taste and how temperature of all things changes the way that neurons fire when something hits your tongue. He's going to share what his research team is learning, but how our brains develop and build the experience of flavor. So hello, Dr. Limon and welcome to Viota. Thank you very much. I'm glad to be here. Before we get into the details of all this, let's just back up a second and cover some basics. We're going to start simple, but not shallow. What exactly is a neuron and what makes it so special is compared to other cells in the body? Okay. Well, a neuron is an excitable cell. These are cells that have the ability to store an electrical potential and can communicate with other neurons through electricity and also chemical messengers that they release. And so, is a neuron the same thing as a nerve or those two completely different things? Really a nerve is a bundle of axons that are coming from a group of neurons. And so in the peripheral nervous system or in the body, usually those bundles of axons are called nerves. Once that nerve process enters the central nervous system, enters the brain, sometimes they're referred to as tracks or a nerve tract. So a nerve is really a collection of neurons, essentially. And so you mentioned this a little bit before, but I like to get a better idea of it. How does a neuron put together? What are its parts and what do they do? So the neuron has a cell body, sometimes referred to as a soma, has a nucleus, of course, in the soma. And it has two major input and output processes. And for input, it has sort of a tree like, in some cases, structure, which is referred to as a dendrite, and this dendritic tree will have receptors and ion channels that can be stimulated by various neurotransmitters that will lead to transmission of an electrical potential to the soma of the cell. And on the other side of some neurons, you have what's called the axon, which is this large cylindrical wire-like process that runs away from the cell body and will then ultimately couple to another neuron or set of neurons downstream somewhere. And so does that have the two neurons in this system that will communicate with each other? Is through these dendrites and then the parts of the other end? Yes, usually the input comes in via the dendrite and the output is the axon. It's not always the case, but that's sort of a general arrangement of the way neurons talk to each other. Okay. Yeah, I think back when I took intro biology, my instructor said, kind of look at our hand and forearm and think of your palm sort of the main body of the cell and then your fingers, the dendrites, and then your arm, your forearm would sort of be the axon coming down? Sure. Okay, all right. Well, that gives us an idea of how this works. So now that we've got our neurons figured out and they're all firing, let's talk about what happens when something hits your tongue because your tongue is full of neurons, right? Well, it is, but it's also with taste, for example, there are specialized, specialized what are called taste cells that live on the tongue that are coupled to neurons that transmit information, taste information to the brain. So taste actually arises through the interaction of chemicals and foods with what are called taste receptor cells. These are actually are specialized skin cells that they can do neuron-like things. They can sometimes generate action potentials. They can sometimes release neurotransmitter, but they're actually skin cells, okay? And what's really interesting about these cells is they have a very short lifespan. So they only live about seven to fifteen days. They're constantly turning over. So and they're housed within the structure sometimes called a taste bud, okay? And so you have within your taste bud, you have these taste cells. And some of the cells are receptor cells. Some of the cells are probably post-synaptic and talking to their job is to communicate information from the receptor cells to the nerve fibers that are then talking to the brain. We think this arrangement is due to the fact that these cells, like I said, they don't live very long. So they constantly are turning over. So imagine if these were neurons, and they had an axon that went out and it's basically or they had a dendritic process that was living on the tongue and they were dying off every seven to ten days. And so that would be a very complicated process to sort of replace these neuronal processes that were integrated to the tongue if they're dying off so rapidly. So the system has evolved as you have these independent cells that when they die off and replace with new ones, essentially they just pop back up into the taste bud and they can reestablish communication with the nerve fibers that are just there. So it seems like it's a very efficient process to deal with this process of turnover that happens. That seems like that would also make sense given that everybody's probably tasted that slice of pizza out of the oven that's a little too hot and it's going to burn your tongue. You don't want it damaging the neurons. That's right. You think that's why these cells turn over because your mouth actually is sort of a hostile environment in which to live. So you can have these wild changes in temperature. You can have wild changes in pH that can go from very, it can drop to quite acidic, depending on what it is that you're eating. And so there's, it's a very sort of difficult environment for neurons to live in. So with these specialized taste cells, we think that's why the system works this way. That they have the ability to sort of wear out, possibly die off, new ones take their place. But reestablishing communication with the brain is a lot easier with this arrangement as opposed to having nerve processes that would have to be regenerated and reintervading the tongue. Excellent. Excellent. All right. So I was looking at some of your papers and from reading them it seems like you studied how neurons in the brain will respond when mice taste sugar and that sugars at different temperatures from cold up to body temperature. And it, it looked like what you found is that temperature can change how sweet something is perceived to taste. So can you walk us through what's happening in the mouth and the brain when you taste something that's sweet at different temperatures? What's going on there? Well, there's, there's a couple of different things that are potentially happening. We don't fully understand this, but we know that inside these taste receptor cells that are involved in sensing sugars, they use sort of a complicated receptor, transduction pathway to actually generate the electrical signal that it's communicated to the brain and sending a sugar input to the brain. And so what happens is that we know that sugar molecules like sucrose or glucose or fructose, these are very large molecular weight molecules. So for them to actually stimulate a neural process, they're not going to very easily pass inside of an ion channel, for example, because they're so large. And so what happens is they interact with what's called a G protein coupled receptor, which is sitting on the surface of the taste receptor cells and there's many of these receptors on one cell. And what G protein coupled receptors do is it's almost like a lock and key type mechanism where the ligand binds to the receptor and it causes sort of a chain reaction of secondary or second messenger events that ultimately lead to the opening of ion channels, which are expressed on the taste cell surface. And when that happens when these channels open, there's of course an ionic gradient where you have more positively charged cations on the outside of the cell that flow into the cell and they can cause depolarization. But we know that there's a for sweet taste, for example, there is an ion channel that's engaged in this process that is actually sensitive to temperature. So it's not only gated by the presence of sugars activating the receptor for sugars, but it's also gated by temperature and so when you warm up this ion channel, you actually can increase current flow through channel, and where you'll increase the excitation that that channel is allowing the taste receptor cell to have. And this is an eye on channel that's known as It's called trip M5, and trip is not like trip, like you're tripping, but it stands for transient receptor potential. These are ion channels that were originally discovered in fruit flies to be involved in fruit fly vision, but they've been found to basically be involved, trip channels are involved in all types of sensory processes all throughout the body, including thermosensation, and they also have some real taste. And so when you're eating something that's warm and has sugar in it, you're potentially increasing the amount of sugar signal, so to speak, that's being sent to the brain because you are potentiating the current flow through trip M5, which will increase current flow with warming. Now, we know that this trip M5 can't explain every type of thermo taste effect that arises, that involves geochromatine coupled receptors. And in fact, we now know that temperature can also affect the operation of the sweet taste receptor itself, but it seems to have a big role in facilitating sweet taste activity. And this might be why when you eat things that have sugar when they're warmed up, they might be perceived sweeter than they are when they're cooled down. And this also had, this is also interesting to think about from an applied perspective or a consumer science perspective, because there's a lot of things that humans will consume that have tons of sugar in them that are consumed cold, for example. And we know that warming can facilitate sweetness, but cooling can suppress sweetness, and this has been shown nearly our own data have shown that in a neural setting, and it's also been shown in humans in rigorous laboratory studies where cooling can suppress sweetness. And so something like a soda, for example, which has a lot of sugar in it, the reason there's so much sugar in it is that if you drink it cold, you're suppressing the sweetness, and a lot of sugar has to be added to it potentially to make it sweeter and palatable to humans, so it's a pleasure to drink. So what would be the benefit of this? I mean, evolutionarily, or think you got survival. That's a great question, matter if it's, you know, warm or cold organisms don't get a lot of choice in the wild. We don't know. So we don't understand this fully. And so one of the things that could be happening here is this might be, this could be due to some conservation of a mechanism that is really good at helping cells depolarize. In other words, this trip in five channel, but maybe it has this side effect, where it's also sensitive to temperature, and maybe that's not part of why it's there. We don't know why this would happen. There's also another taste sensation called umami taste, which is, this is a Japanese word that loosely translate and translates into saveriness or a meatiness or something. Okay, and so umami is of course associated with the taste of monosodium glutamate. It's a really widely used flaring agent and a lot of different things. It also uses a very similar transduction process to sweet. It's where the G protein couple receptor and trip in five is involved in its transduction process as well. And so there's, there has been some discussion in the literature that maybe why umami is sensitive to temperature because it's associated with meats. Is that maybe an animal that's eating meats can, if the meat is freshly killed, it might be warmer than if it's, than if it's not freshly killed. But that hypothesis doesn't really hold water because umami for example is, it's not only found in meaty foods and things like that. It's found into a lot of vegetables like tomatoes and mushrooms and stuff like this, which are not going to have like a, a homeothermic warmth, so to speak. So we don't really fully understand why, why this, why this happens with the things that are sweet, but we, but we know what happens. Well, that gets into a question I had for a little later. Let's go to that now. I mean, we also have sour, salty, bitter, and then umami, you know, temperature has effects on all of those or not in so much. It's not, it's not uniform across tastes. The, the current data would suggest that sweet is, is probably the most wildly modulated by temperature. Okay. Whereas, uh, salt, for example, can also be modulated by temperature, but maybe not to the same extent that you see what sweetness. Um, the other tastes such as sour and bitter, they show much lower sensitivity to temperature. Um, at least at a neural level compared to something like sweet. But sweetness has this, has this unique feature about it where in like a neural setting, if you're recording from neurons that are firing to sugars, for example, so if you warm it up, you can cause these massive increases in firing. It's a very unique feature to things that are, that are sweet. Well, is that somehow related to? I mean, I remember my grandmother say, I know if the watermelon's not particularly sweet, put a little salt on it. And yeah, it does enhance at least the perception. So is it just sort of all these different factors coming together and influencing the signal going to the brain? Um, that would ultimately be what's, what's happening is the, the perception is, is of course, arising inside the brain. The brain is very good at making you think the perception is inside your mouth, but it's, it's a result of processing inside the CNS. Um, salt, of course, is a, is a very, you know, it's a, it's a very powerful taste in this. It's, it's, it's one of this, it's one of the taste categories where salt stimuli can cause a really strongly effective at activating neurons that will fire to salt. Um, and probably that's due to the fact that, you know, as, as mammals, we're essentially like walking bags of saltwater, you know, we have a lot of salt that in our bodies that we're losing and has to be replenished and the way we replenished is by eating salty. So our taste system is highly tuned into, to salts. Now, as I was looking at some of your papers and one of the things that you've done in the lab is you measure how quickly neurons respond to these, these different temperatures and sugar concentration. So what is it we learn from how fast a neuron responds to a stimulus? What does that tell us? Um, it can tell us a couple of different things about how the cells might be encoding the intensity of the, of the stimulus. So the speed of activation might be related in some cases related to the concentration of the taste stimulus. And we also have shown that not only is it concentration, but temperature can also affect the speed at which these neurons neurons will respond particularly with things that are sweet. So if we cool down a, a sucrose solution, for example, we can slow down the neural response. If we warm it up, we can systematically speed it up in some ways. But there's an interaction of, of temperature with concentration that, that it's controlling the speed of which the cells fire. We don't fully understand what what the significance is of the speed of the response per se in terms of the coating of the taste qualities. But it, but it is something that in some cases for, for certain stimuli that can be affected by both concentration and temperature. And I can see how that might be a sort of safety mechanism that if there's a very high concentration of something bad, you're going to know instantly and you want your nerves firing quickly. Yeah, yeah, some, and some, yeah, some tastes will elicit faster responses than others. And but that, that, that might, might also be related to the difference, the different types of transduction mechanisms that they would use. Whereas things like bitters and sweets and mommy will use G protein coupled receptor mechanisms, things like salts, for example, which you have sodium ions directly flowing into taste cells. Those are the sodium ions that are in the salt, salt shaker that you're putting on your, your french fries. Those are going to be the sodium ions that flow into the cells and actually cause the depotization to happen. So they're, they're passing through ion channels very quickly. And so you, so that, those speed differences can be related potentially to that. We're going to stop right there with part one of our interview with Dr. Lemon. I think he did a great job of explaining the basics of how taste works from the neuron detecting a new chemical to the brain making sense of what it might be. Based on taste. I don't know about you, but I think that the way temperature affects sweetness was pretty neat. In the conclusion of the interview, Dr. Lemon explains how they use blue lights to study the way flies detect sweetness and why the enjoyment some people get from the flavor of spicy food is a combination of detecting taste as well as detecting pain. But, for now that's the end of this biota episode. On behalf of Dr. Gibson and the rest of the biota team, we want to thank Dr. Lemon for his time. We also want to thank our listeners. If you haven't already subscribed, do it now so you don't miss part two of this interview. Biota is a production of Under the Juniper Studios. This episode was supported in part by an HHMI Bio Interactive Scholars Grant. All opinions expressed, however, are those of the speaker alone. Thanks for listening, have a great day, and take very good care of your genetic material.