Tasting With Your Brain

May 15, 2018

Written by: Sarah Reitz

 

Imagine biting into a warm chocolate chip cookie, or eating a salty potato chip. Now imagine drinking milk that has been sitting in the refrigerator for quite some time past its expiration date. Probably doesn’t conjure the same taste as the cookie or the chip, right? The sourness of the spoiled milk, the sweetness of the cookie, and the saltiness of the pretzel are three of the major tastes we as humans have evolved to detect, with the others being bitterness and umami (often referred to as “savory”). But why did we develop the ability to taste these distinct tastes? And what role do our nervous system and brain play in detecting and deciphering them?

Our sense of taste likely developed as a way to keep us fit and healthy. In a world before grocery stores, or even agriculture, our ancestors had to be able to determine which foods were not only safe to eat, but also would give them the most energy. For example, foods that contained a lot of sugar also contained a lot of beneficial energy, so we likely evolved to recognize these foods as sweet and pleasant. On the other hand, a plant that was bitter was very often poisonous, so this taste may have evolved as a way to warn us against eating something with the potential to be harmful. Though we are no longer hunters and gatherers like our ancestors, our sense of taste has not quite caught up to our modern day eating habits. Think about the first time you drank coffee, or had a beer. Your first reaction was likely to grimace and maybe even spit it out! That’s our sense of taste detecting the bitterness in these beverages and telling us that they may be poisonous. It’s only after we learn to associate these drinks with the pleasant effects they bring that we can override this initial aversion implemented by our tastes.

The taste of a particular food or drink is initially detected on taste cells that are clustered into the taste buds covering the entire top surface of our tongue. Contrary to popular belief, the five tastes are not clustered into distinct regions of the tongue. Instead, taste buds that detect one of the five tastes, like sweetness, are scattered across the tongue along with taste buds that detect the other tastes. But how exactly does a sweet taste cell know when we’ve eaten something sweet, or a bitter taste cell recognize a food as bitter?

 

tastepathway
Figure 1: Overview of the gustatory pathway.
Molecules from our food bind to receptors on taste cells on the tongue. This enables the taste cell to produce an action potential, or electrical signal, that allows it to communicate with neighboring neurons. This electrical signal travels from the taste cells in the tongue through neurons in three cranial nerves to the nucleus of the solitary tract (NST), a region in the brainstem. The NST then passes this taste information to the thalamus, which in turn relays the information to two adjacent regions in the cortex: the anterior insula and frontal operculum. These two cortical regions are oftentimes referred to together as the gustatory cortex, due to their role in taste processing.

 

It turns out that there are specialized receptors on each of these cells that are designed to respond only to the chemicals found in specific tasting foods. Receptors are specialized proteins that allow a cell to sense and react to specific signals. When we eat, molecules from the food interact with our saliva and end up binding to the taste receptors on our tongue. So for example, sweet taste cells have sweet receptors that respond only when a sugar molecule binds to it, but do not respond to salt or any other molecule. Interestingly, scientists have recently identified receptors that respond to lipids — molecules found in fats — on the tongue, suggesting that fat could be considered a sixth taste1,2. When these receptors are activated by the appropriate molecule, they set off a chain of signals inside the taste cell that tell it to fire an action potential, allowing the taste cell to communicate with the neurons involved in perceiving tastes. This is unique because taste cells are not actually neurons, yet they are capable of electrically communicating just like neurons do3. This electrical signal travels from the taste cells in the tongue to the brain’s sensory relay station, which passes the information to the gustatory cortex, the region of the brain that processes tastes (Figure 1).

While it is well understood that the tongue plays a major role in detecting different tastes, recently scientists have discovered that it’s actually possible to perceive tastes without activating taste receptors on the tongue at all. We know from previous research that sweet tastes and bitter tastes are received and processed in separate regions of the gustatory cortex4. Drs. Yueqing Peng and Charles Zuker at Columbia University wanted to determine whether activating these specific regions could produce and alter taste perception in mice5. To do this they used a technique called optogenetics, which uses light to control neuronal firing in living organisms. By expressing alight-sensitive protein in the neurons of either the sweet-processing or the bitter-processing cortical areas, they could shine blue light in the mouse’s brain and activate only the “sweet” or only the “bitter” regions.

The researchers wanted to find out if they could activate either of these regions and make a mouse perceive regular water as either sweet or bitter. Similar to humans, mice will drink more of a sugary drink and also drink it faster, while they will try to avoid drinking a bitter liquid. When the “sweet” cortical region was activated during drinking, the mice began to drink the plain water more often and faster than before, as if the water tasted sweet. Conversely, when the “bitter” region was stimulated, the mice significantly slowed down their drinking of the plain water. Some mice perceived the water as so bitter that they even began trying to scrape their tongue or gag after drinking, even though the water itself hadn’t changed! Even more interesting is that when the sweet or bitter brain regions were activated in mice that had never been exposed to sweet or bitter tastes before, they reacted the same way, suggesting that behavioral responses to certain tastes are so important that we are actually born with them and don’t have to learn by trial and error. Knowing how aversive bitter liquids are, the researchers then wanted to see if they could make a mouse drink a bitter liquid by stimulating the sweet region of their brain. Sure enough, when the “sweet neurons” were optogenetically activated, the mice began to drink more of the bitter liquid and no longer exhibited aversive .

Together, these experiments suggest that the true perception of different tastes takes place in the brain, not on the tongue. According to Dr. Zuker, “taste, the way you and I think of it, is ultimately in the brain. Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it’s the brain that affords meaning to these chemicals” 6. These results also suggest that our initial perception of tastes, at least for sweetness and bitterness, is hardwired into our brain and doesn’t necessarily depend on prior exposure to the taste molecules. Future studies will hopefully be able to explore whether this is the same for our other tastes.

While showing that we can alter taste perception by stimulating the brain is incredibly cool, it also has important implications for people who have a limited or absent sense of taste. One common example of this occurs in people undergoing chemotherapy. Many chemotherapy drugs are designed to kill any rapidly reproducing cells in the body, in order to eliminate cancerous cells. While most cells in our body do not reproduce as quickly as cancer cells, it turns out that our taste receptor cells on our tongue regenerate every 5-20 days7,8. Therefore, the chemotherapy also unintentionally kills off healthy taste cells, leaving many patients with a blunted sense of taste9. The results from Drs. Peng and Zuker suggest that perhaps we can engineer the perception of different tastes by bypassing the affected taste cells on the tongue and instead directly stimulating the brain. While there is much work to be done before this occurs, it is an exciting avenue for future studies to consider.

 

Images:

Cover Image: Image by Lottie, CC0 1.0 Universal, via Flickr

 

References:

  1. Pepino MY, Love-Gregory L, Klein S, Abumrad NA. The fatty acid translocase gene CD36 and lingual lipase influence oral sensitivity to fat in obese subjects. (2012). J Lipid Res 53(3):561-6.
  2. Running CA, Craig BA, Mattes RD. Oleogustus: the unique taste of fat. (2015). Chem Senses 40(7):507-16
  3. Vandenbeuch A, Kinnamon SC. Why do taste cells generate action potentials? (2009) J Biol 8(4):42.
  4. Chen X, Gabitto M, Peng Y, Ryba NJ, Zuker CS. A gustotopic map of taste qualities in the mammalian brain. (2011) Science 333(6047):1262-6
  5. Peng Y, Gillis-Smith S, Jin H, Trankner D, Ryba NJ, Zuker CS. Sweet and bitter taste in the brain of awake behaving animals. (2015). Nature 527(7579):512-5
  6. Wein, H. How taste is perceived in the brain. (2015). At https://www.nih.gov/news-events/nih-research-matters/how-taste-perceived-brain
  7. Beidler LM, Smallman RL. Renewal of cells within taste buds. (1965). J Cell Biol 27(2):263-72
  8. Farbman AI. Renewal of taste bud cells in rat circumvallate papillae. (1980). Cell Tissue Kinet 13(4):349-57
  9. Murtaza B, Hichami A, Khan AS, Ghiringhelli F, Khan NA. Alteration in taste perception in cancer: Causes and strategies of treatment. (2017). Front Physiol 8:134

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