Strange Insect Tastes

Biology concepts – sensilla, uniporous sensilla, gustatory receptor, RNA world hypothesis

Chalcanthite (copper containing) is used in traditional

medicines, even though it is toxic. Recent study shows

it is anti-inflammatory when mixed with egg white. A

2013 study shows that after burning off the water from

chalcanthite and mixing it with egg white to eliminate

toxicity, the concoction inhibits several signaling

pathways that would otherwise stimulate

inflammation. I’m not sure you would have to taste

this mineral to know what it was.

Yukon Cornelius, you know- the prospector of Rudolph the rode-nosed reindeer fame, used to taste the end of his pick after ho tossed it into the air and it stuck in the ground. He was looking for gold, and the action implied that he could know if there was gold in the ground by how it tasted.

Don’t laugh, I’ve seen many a geologist taste a rock to get some information about it and the area from which it came. Halite is an easy one (Greek hals = salt), but other minerals have tastes as well.  

Borax is sweet and alkaline. Ulexite tastes alkaline while chalcanthite is sweet and metallic (see picture). There are others as well, just be careful that you have a clean area to taste, not one that has been exposed to overflying birds.

Believe it or not, this does have application in biology. Geologists are looking for older and older borates, like borax and ulexite, because there are hypotheses that these might have been important in stabilizing the ribose of RNA in the prebiotic RNA World. They field test samples by tasting to find borates to test for age.

But I don’t know about Mr. Cornelius using taste to find gold. Gold is famously inert, it shouldn’t have any taste at all. In fact, professional ice cream testers use gold spoons just so they won’t be influenced by anything but the product.

It’s not probable, but perhaps geologists and ice cream tasters learned from insects about taste. Arthropods are famous for finding interesting uses for taste. They taste their way to food, to find mates, and even to find appropriate places to lay eggs. But what is most exceptional is how and where they do it. Insects are our big exceptions for the day.

Insects (just one class of arthropods) are invertebrates, so they don’t have the traditional taste buds associated with mammals, fish, and birds. Instead they have taste receptors house inside gustatory sensilla. There are several types of sensilla on insect bodies, and different types can house different senses. Gustatory sensilla are usually of the trichome (or trichoid, meaning three faces) type, meaning that they look like hairs and most people call them hairs. They are not hairs because they are not made of keratin protein. They are made from chitin, the same material as the crunchy insect exoskeleton, and they are hollow.

This is a photomicrograph of an insect antenna, with

several different types of sensilla. Look closely to see

the differences. The sharper ones come in two lengths,

shorter (b) and longer (a), but they are both considered

trichome type. Others are blunter and thicker, these are

the basiconic type (c). Trichoid house touch and taste

receptors. Basiconic hold olfactory (smell) receptors.

The end of the sensilla has a single opening, a very small opening (10 nm/4 x10^-7 in). Because of the single opening, it is also called a uniporous sensilla.  The pore is so small that water molecules trapped within it can’t evaporate. If the sensilla then rubs up against a surface and some surface molecules get trapped in the water. These molecules can stimulate the gustatory receptors and the insect can taste what they have touched.

These sensilla are common among the arthropods. The arthropod phylum includes the chelicerae (scorpions, spiders, etc), myriapoda (millipedes and centipedes), hexapods (insects, beetles, ants, butterflies, etc.), and crustaceans (crabs, lobsters, shrimp, etc.). I was surprised to find that crayfish and lobsters have sensilla on their legs – antennae, O.K., but on those hard legs? Spiders, even the ones that aren’t hairy, have gustatory and olfactory sensilla, as do millipedes and similar. Since there are more than 500 arthropod species for every mammalian species on Earth, it would seem that uniporous sensilla are the common way to taste. Taste buds like ours are the exception.

The gustatory receptors of arthropods are located on neurons housed within the sensilla. The neuron depolarizes based on the strength of sense signal and transmits that information to the central nervous system. We’re talking about insects here, so “brain” isn’t really the right word to use. In arthropods, both gustatory and olfactory inputs go to an olfactory center, so one could argue if it is really a taste they are sensing. It’s perceived in the smell center, but comes from direct contact, not a gaseous molecule from a distance – let the discussions begin.

However they perceive it, gustatory receptors in uniporous sensilla of insects are used for a variety of purposes. That’s not exceptional – we do too. But insects tend to do it better and for more varied reasons. The obvious function is for finding food, but here insects take shortcuts that other animals do not. Take, for instance, one of the most important research animals ever, the fruit fly (Drosophila melanogaster).

Top left: the proboscis of the fruit fly can be seen projecting

forward, like an insect elephant trunk. Top right: the

proboscis monkey, while at the bottom is the Darwin’s hawk

moth. The term “proboscis” can be used with invertebrates

or vertebrates, but it means different things in each case.

When speaking invertebrate, proboscis means tubular

mouth parts. In the vertebrate tongue, it doesn’t mean

tongue at all, it means nose or snout.

The fruit fly shortcut is seen when it tastes something sweet or tastes water, the neural circuitry automatically causes muscular movements that extend the fly’s proboscis (pro = forward, and boskein = to feed) for feeding. Similarly, if the fly tastes something bitter, the reflex is for the proboscis to retract and feeding to either stop or not start. Muscle movements are connected directly to taste sensation. We can’t do that, unless you count the face kids make when they eat broccoli.

Also in feeding, it seems that insects are the exception to the rule of a limited number of taste receptors. Insects that feed on only one type of plant (feeding specificity) tend to evolve taste receptors for chemicals made only by that particular plant. This leads to many receptors with novel and new specificities.

Depending on the diet or other uses for receptors, insect species also vary greatly in the number of different receptor genes they have. Fruit flies have more than 70 different receptor genes, while honeybees may have as few as ten. However, they probably all have a receptor for tasting water.

The water receptor was recently identified in Drosophila. A 2010 paper in Nature describes a gene and protein called pickpocket 28 (ppk28). The scientists tracked the brain responses to water in flies when the gene was stimulated or when they removed the gene.  There was more activity when plain water was given than when it was mixed with salt or sugar (more water if no solute). When mutated to become nonfunctional, flies drank water for much less time (3 seconds versus 10 seconds) and showed much less brain activity in response to water.

Studies like this are hard, but the amount of brain space devoted to sensing things like this makes it easier. Undoubtedly, the ants (still in the arthropod phylum) are king when it comes to sensing taste and smell. For example, worker ants can distinguish between different single and double sugars, meaning that they differentiate many subtle differences in taste.

Here we witness a heartwarming act from the animal world.

C. japonicus ants have adopted the fusca caterpillar out of the

goodness of their hearts. Don’t even consider that the

caterpillar has tricked the others into think he is an ant, or

that the ants feed him just so he will secrete sugars that the

ants can snack on. No way - it’s a Hallmark special, not a

reality TV episode.

But they can also select different sugars based on immediate needs; whether they would be good for stored energy or immediate energy. This suggests nutrient sensing as well as taste differentiation. For example, Camponotus japonicus ants have a symbiotic relationship with Niphanda fusca butterfly larvae (caterpillar).

The ants protect the caterpillars because the caterpillars provide the ants with sugars, specifically, a disaccharide called trehalose + an amino acid, glycine. The ants actually adopt the caterpillars and raise them with their colony because the caterpillars secrete a pheromone that mimics that of the ants. The ants accept them because they taste like one of their own. The gift of sugar reinforces the relationship.

Other ant species will select different sugars equally, but the C. japonicus ants prefer trehalose. What is more, those ants prefer trehalose + glycine even though no ant species prefers glycine alone. So in this species, symbiosis drove evolution of a taste receptor for trehalose, and is modified by glycine.

One last example on ants, leaf cutter ants (Atta vollenweideri) produce different taste receptors based on the caste they belong to (large workers, small worker, soldiers, queens). A 2013 paper shows that the different castes and subcastes also have different numbers of taste receptors on their legs and antennae, so they respond to stimuli differently. One stimulus might be cohorts (members of the same colony) or, in other insect cases, for finding mates.

Drosophila fruit flies have been studied for this as well. A 2012 paper showed that different gustatory receptors and different pheromones are found on male and female flies. Fruit flies perform many different courtship rituals, and these take energy and time. It wouldn't pay to be courting another male – so sensing whether another fly is male or female is important.

Courtship in fruit flies is more complex than teenage dating.

The motions shown above are just a few of the actions that

take place, but the others aren’t G rated, so I decided not to

show them. The male doesn’t pick the female based on looks,

smarts, or ability to stick to a budget. New research shows

that he picks her based on the fact that she doesn’t

taste like a male.

This study mutated certain taste receptors. When male receptors are stimulated by male pheromones, courtship rituals stop. But if a female is tasted, the courtship continues. In the mutant flies, males would court males. When only mutant males were placed together in a container, they would line up in a long line, one fly courting the male ahead of it and being courted by the male behind it.

In butterflies, these different receptors give clues about evolution.  In the postman butterfly (Heliconius melpomene), researchers found 73 putative gustatory receptor genes, but the number of copies of gene and the variations of some genes varied between males and females. Fully one third of the genes shows a female bias in expression level, many being found on female legs, but not male legs. The results also showed that many of these were also the result of many recent gene duplications. 

Gene duplications allow for more genetic drift, and this would result in a greater number of possible receptors. Varied expression suggests that females are using the receptors for things that the males are not. So female behaviors seem to driving the expression and evolution of the gustatory receptors in butterflies. Once again, the women are in charge.

Next week, we should look at the weird places insects have taste receptors and how taste plays a role in egg laying. Even weirder, insects may taste plants, but it turns out that the plants are tasting them right back.

Choi EA, Park HY, Yoo HS, & Choi YH (2013). Anti-inflammatory effects of egg white combined with chalcanthite in lipopolysaccharide-stimulated BV2 microglia through the inhibition of NF-κB, MAPK and PI3K/Akt signaling pathways. International journal of molecular medicine, 31 (1), 154-62 PMID: 23128312

Briscoe AD, Macias-Muñoz A, Kozak KM, Walters JR, Yuan F, Jamie GA, Martin SH, Dasmahapatra KK, Ferguson LC, Mallet J, Jacquin-Joly E, & Jiggins CD (2013). Female behaviour drives expression and evolution of gustatory receptors in butterflies. PLoS genetics, 9 (7) PMID: 23950722

Koch SI, Groh K, Vogel H, Hansson BS, Kleineidam CJ, & Grosse-Wilde E (2013). Caste-specific expression patterns of immune response and chemosensory related genes in the leaf-cutting ant, Atta vollenweideri. PloS one, 8 (11) PMID: 24260580

Thistle R, Cameron P, Ghorayshi A, Dennison L, & Scott K (2012). Contact chemoreceptors mediate male-male repulsion and male-female attraction during Drosophila courtship. Cell, 149 (5), 1140-51 PMID: 22632976

For more information or classroom activities, see:

Mineral taste –

http://www.galleries.com/minerals/property/taste.htm

http://geology.about.com/od/mineral_ident/ss/beginminident_9.htm

https://www.google.com/search?noj=1&biw=1645&bih=926&q=taste+of+minerals&oq=taste+of+minerals&gs_l=serp.3..0j0i22i30.30242.32674.0.32848.17.16.0.1.1.1.188.1768.3j13.16.0....0...1c.1.32.serp..4.13.1201.Lyq6KNFdWjA

Uniporous sensilla –

http://www.ncbi.nlm.nih.gov/pubmed/10607382

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&ved=0CG4QFjAL&url=http%3A%2F%2Fweb.neurobio.arizona.edu%2Fgronenberg%2Fnrsc581%2Fpowerpoint%2520pdfs%2Fmechanoreception.pdf&ei=z7vlUou8JqmysASr54DADQ&usg=AFQjCNE8KfKTUBEeiGQmHoSf4JBinK5KSg&sig2=VEdmTBHsB75K5fORnIZE0w&bvm=bv.59930103,d.cWc

http://connection.ebscohost.com/c/articles/34467365/mouthparts-associated-sensilla-south-american-moth-synempora-andesae-lepidoptera-neopseustidae

Proboscis –

http://scienceprojectideasforkids.com/2010/fly-proboscis/

http://www.butterflyfunfacts.com/butterfly-proboscis.php

http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artmay98/probob.html

http://www.jove.com/video/193/proboscis-extension-response-per-assay-in-drosophila

http://www.amentsoc.org/insects/glossary/terms/proboscis

http://biologicalmarginalia.wordpress.com/tag/proboscis/

Niphanda fusca –

http://bio390parasitology.blogspot.com/2012/02/niphanda-fusca-raised-by-ants.html

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=36&ved=0CEIQFjAFOB4&url=http%3A%2F%2Fchemse.oxfordjournals.org%2Fcontent%2F26%2F8%2F983.full.pdf&ei=Rs7lUvKwBJW_sQSa1oG4Ag&usg=AFQjCNE4uh05t8QXRttG21wGQ6w4W-I6dw&sig2=FNBNnUbz8TzjeUKnadcLNg

http://books.google.com/books?id=Rl0ER01ZI8cC&pg=PA101&lpg=PA101&dq=niphanda+fusca&source=bl&ots=ReHPKi1a27&sig=634TgCSUeNGX4CLiX9hJ7_qTtrc&hl=en&sa=X&ei=dM7lUrKFLum0sQSW44DYCw&ved=0CC0Q6AEwAjgo#v=onepage&q=niphanda%20fusca&f=false

Leaf cutter ants -

http://video.nationalgeographic.com/video/kids/animals-pets-kids/bugs-kids/leafcutter-ant-kids/

http://www.arkive.org/leaf-cutter-ant/atta-cephalotes/

http://www.pbs.org/wgbh/evolution/library/01/3/l_013_01.html

http://www.rainforestanimals.net/rainforestanimal/leafcutterant.html

http://www.alexanderwild.com/Ants/Making-a-Living/The-Farming-Ants-Leafcutters

http://www.blueboard.com/leafcutters/what.htm

Who Tastes Best?

Biology concepts – taste/gustation, aposematism, carnivore, herbivore, omnivore, Jacobson’s organ, palatal organ, parasite

What animal do you think has the most taste buds? There are bunches of animals out there, so let’s make it a bit easier. What sorts of animals do you think would need more taste buds? Omnivores might be a good choice, since they eat more different kinds of foods.

Cows are big, so they have big tongues. It isn’t too surprising

that they have many taste buds. However, the density (number

per unit area) is higher than most animals too, since they eat

plants, and they have to pick the non-poisonous plants from the

poisonous. Some people eat beef tongue – Bill Cosby said he

didn’t want to taste anything that might taste him back. I would

add that I don’t want to eat anything that could have been

in a cow’s nose.

Carnivores could be a choice, since they have to worry (most of them, vultures excepted) about rotted or diseased meat. But then again, herbivores might be the answer; they eat plants, and plants are the source of most poisons. The same could be said about insectivores, the insects they eat are often toxic – formic acid in ants or stored plant toxins.

Let’s take a survey of animals and see if a pattern develops. Humans have about 10,000 taste buds on average. Young people have more than the elderly; you lose about half your taste buds by age 65. Women generally have more than men. And supertasters can have double the number of non-tasters. But 10,000 is a good number to go by.

Pigs have about 15,000 taste buds while cows have 25,000 and rabbits average 17,000. Lions and other cats come in at about 450 taste buds, but dogs fair better, averaging about 1200. Birds have very few and some fish have alot, but we will talk about them below.

Sort the numbers out and it appears that herbivores have the highest number of taste buds, carnivores and insectivores the fewest, and omnivores lie somewhere in the middle. It looks like keeping an eye (or tongue) out for poisonous plants is the most important mechanism for taste. Carnivores’ strict diet means they need fewer, as we saw when we discussed the loss of sweet taste receptors in cats.

But the winner in the taste bud count? Believe it or not – catfish! Even small catfish (6 in/15 cm) have about 250,000 taste buds. They are literally covered in taste buds. Mind you, we’re talking about taste buds, not just taste receptor cells like we discussed last week. These are full-fledged taste buds, with at least five different tastes represented (sweet, salt, bitter, sour, umami).

On this light colored fish skin, you can see the dots that

are the microscopic taste buds. I eat fish skin, but I don’t

equate it to eating cow tongue.

Most fish have the majority of their taste buds in their mouths - that makes sense. Some fish, especially bottom feeders like carp, are designed so that food coming into contact with the back of the roof the mouth will stimulate an area called the palatal organ which has thousands of taste buds. When the muscular palatal organ tastes food, it automatically clamps down on the food particle. Everything else - water, stones, inedible things - are then flushed from the mouth by blowing them out. Only the food is left and they swallow it.

That’s pretty much it for fish with scales, but catfish, sharks, and some other fish don’t have scales, they are covered with tissue that is more like skin. In these kinds of fishes, it’s like their tongues have migrated all over their bodies. They taste with everything – mouth, lips, gills, fins, body, tail, whiskers.

The catfish, as ironic as it may be, doesn’t have taste buds on its tongue! Why would it be important for catfish to have so many taste buds but none on their tongues? It’s because they live in muddy water. Their sight is impaired, so the taste buds on their body allow them to find food. In truth, most fish have very few taste buds on their tongue (called a basihyal amongst ichthyologists). 

This is fortuitous for a fish parasite called Cymothoa exigua. It may sound disgusting, but this isopod (iso = identical, and pod = foot) parasite makes its living by getting into a fish’s mouth through its gills, eating its tongue and then replacing the missing tongue with its own body. Now, everything that comes into the fish’s mouth can be nibbled on by the parasite.

C. exigua doesn’t so much eat its host’s tongue as it causes it to disappear. It grabs on to the tongue, and sucks the blood out of it. The longer this goes on, the more the tongue atrophies (a = no, and trophic = feeding) and shrinks away to nothing. Then the parasite grabs hold of the stump with its back legs and takes its place.

An amazing picture of the isopod parasite C. exigua acting

as the tongue of a captured fish. It lives in the gulf of

California and usually selects the rose snapper as a host.

Every once in a while, a fish lands in the grocery store

with a parasite still hanging on, looking for the next meal.

I’m guessing that fish gets returned and a lot of store

credit is issued. Photo credit to Dr. Nico Smit.

Talk about exceptional; this is the only known instance where a parasite functionally replaces a host’s own organ. The isopod is willing to act like a tongue, holding food up against the small teeth on the roof of the fish’s mouth, because this is how it ensures food for itself.

We know that taste is important for animals to separate toxins from those foods that are good for them, so it is probably fortunate that fish don’t use their tongue for tasting. If losing their tongue caused fish to die more often from ingested poisons or from starvation, then the parasite would be sealing its own fate – it wants its host to survive.

Moving on to something a little less disgusting. We talkedlast week about solitary chemosensory cells and their work in non-gustatory organs. An interesting 2013 paper studied the SCC and taste buds of fish and compared them to those of mammals. Their results indicate that taste buds did not evolve from SCCs or vice versa, the two developed independently. This means that taste, smell and nutrient sensing all developed on their own, yet they have similar and overlapping structures and functions. A good smell idea is a good taste idea.

Snakes show again the similarities between taste and smell. Most snakes (some sea snakes excepted) have taste buds, often lined up near their teeth. What is different is the vomeronasal organ (Jacobson’s organ) for sensing volatile chemicals. Present in amphibians, most reptiles (not crocodiles or chameleons), and some mammals, the VNO is for sensing pheromones and other scents.

On the left is a cartoon showing the snake veromonasal organ

(VNO). It sits in the roof of the mouth and neurons from it go

to the olfactory bulb. As the tongue pulls in molecules, it

presses the tongue up into the VNO. On the top right, you see

how the snake spreads its tongue out to pick up as many signals

as possible. Now you know why snakes have forked tongues.

On the bottom right, you see that the VNO doesn’t even

communicate directly with the nostrils, the moth is more

important for this smelling.

In snakes, however, the scents are not brought to the organ by breathing in. In fact, the VNO in snakes isn’t even open to the nasal compartment. Instead, snakes bring the scent molecules in with their tongue, and press them into the VNO for sensing. This makes it a quasi-direct chemosensory organ, like taste. And thus, we run up against the crossover of smell and taste again.

Humans can taste some volatile compounds as well. One nasty example is gasoline. If confronted with gasoline fumes in sufficient density, some will enter your mouth and mix with saliva. If enough molecules come into contact with your taste buds, you will taste the gas – not an especially pleasant taste.

Even more exceptional, you can also taste a gas you didn’t breathe in. There is a chemical called dimethyl sulfoxide (DMSO) that is used in many laboratories. It’s famous for being a great solvent. A solvent is a liquid in which other chemicals will dissolve. Water is a very good solvent, but there are many things that are not water-soluble.

Once inside the body, DMSO starts to be metabolized by your cells. One of the products of its breakdown is dimethyl sulfide (DMS). This travels around in your blood, but wants to a gas, so it passes from your blood to your lungs. From here you breathe it out. When you exhale DMS, you taste it. It tastes and smells like garlic, so if you taste this and haven’t had Italian for lunch, then you might have contaminated yourself with DMSO - oops.

DMSO is a by product of the wood industry and is a great

solvent. It is polar solvent, meaning that it has a partial

positive charge and a partial negative charge within its

structure. Scientists first thought that the sulfur (S) oxygen

(O) bond was a double bond, but it turns out to be a single

bond with the excess charge being shared as a huge dipole.

Being so much more polar makes it a great solvent, it can get

in between nonpolar, positive or negative structures to

help things dissolve.

The oops is because the structure of DMSO allows almost anything to go into solution – it would put your car into solution if you have enough of it. Its structure also makes it so DMSO can be absorbed directly through your skin to your bloodstream. Any molecule dissolved in DMSO will also be carried straight into your tissues.

This works out well for medicine and chemistry, as certain tests or drugs require that the chemical be dissolved. But, if you happen to be using a toxic chemical in DMSO and get some of it on your skin or mucous membranes - you’ve now been poisoned and you just hope it wasn’t enough to kill you.

Back to taste buds. At the low end of the taste bud spectrum are the birds. They have merely dozens (Japanese quail) to a couple hundred taste buds; chickens and songbirds are especially taste-poor. But that doesn’t mean they can’t be interesting. In the 1970’s, it was discovered that ducks had taste buds in a weird place – on their beaks.

Mallard ducks therefore have about 400 taste buds on their jaws, since beak parts are extensions of the maxilla (upper jaw) and mandible (lower jaw) bones. The taste buds are located in five concentrations, four on the maxilla and one on the mandible.

These positions just happen to correspond to where the duck grasps and holds food as it decides whether it is safe to swallow. Once again, life is protected by taste sense.

The leopard lacewing caterpillar on the left warns predators

with its bright colors that it is poisonous. This is

aposematism, although some animals aren’t above

faking that they are poisonous by just adopting the colors.

On the right is the crimson speckled moth, it is bright and

patterned, but it does it for a different reason. This is a

secondary sex characteristic for attracting mates.

So birds can taste their food, but they don’t rely just on that. It was back in Darwin’s day that the question was raised as to why birds would eat green and brown caterpillars but leave bright colored caterpillars alone. This led Alfred Russel Wallace (the other guy who came up with natural selection) to demonstrate the concept of aposematism (apo = away, and semantic = sign), coloration to warn of toxicity in order to ward off predation.

The bright color says, “Don’t eat me, I’m poisonous.” This is important because even a single peck with a beak could be lethal to a caterpillar. Birds learned quickly to avoid the bright caterpillars, suggesting that birds could taste. This tale is well recounted in the 2012 book of Tim Birkhead, called Bird Sense.

In a strange twist, there are a few birds that are toxic because of their diet, the pithoui and the ifrita. We have discussed these birds in terms of toxins. These birds are brightly colored, so they are using the concept of aposematism to keep themselves safe, just like many potential bird snacks do.

Speaking of caterpillars, we’ll get to arthropod gustatory sense next week. It turns out that some insects taste with their wings, while others taste different things based on their jobs.

Kirino M, Parnes J, Hansen A, Kiyohara S, & Finger TE (2013). Evolutionary origins of taste buds: phylogenetic analysis of purinergic neurotransmission in epithelial chemosensors. Open biology, 3 (3) PMID: 23466675

Tim Birkhead (2012). Bird Sense: What It Is Like To Be Bird Walker Publishing, New York

For more information, see:

Cymothoa exigua –

http://www.wired.com/wiredscience/2013/11/absurd-creature-of-the-week-the-parasite-that-eats-and-replaces-a-fishs-tongue/

http://phenomena.nationalgeographic.com/2013/02/28/tongue-eating-fish-parasites-never-cease-to-amaze/

http://www.talksciencetome.com/treasures-the-tongue-eating-sea-louse/

Tasting With Every Part Of Your Body

Biology concepts – taste sensation, non-lingual taste receptors, solitary chemoreceptor cells

Bitter melon (Momordica charantia) goes by many names,

like goya, kalera, etc. It is native to the subcontinent and Asia,

and is used in many cuisines. I have not tried it, but I have

heard it described as tasting like evil, or a cross between

uncooked collard greens and chewing on aspirin. Despite this,

there is a bittermelon soda sold in Japan. My point – even

though bitter is supposed to warn of poison, people can learn

to love it. Are they more likely to be poisoned?

In our recent discussions of the gustatory sense (here, here, and here), we have highlighted the idea that taste is basically a nutrient/poison detection system. You can avoid toxins (sour, bitter) or find nutrients (salty, sweet, umami, fat) of based just on taste. In terms of avoiding toxins; you taste it, and hopefully don’t swallow enough to be harmful to you.

A question occurs to me in this scenario – why not taste things somewhere other than the opening of your gastrointestinal (GI) system? You would be much less likely to ingest a toxin if you never put it in your mouth. Tasting something with the ends of your fingers, for example, would identify sour and bitter – things to toss toward your enemy, and could identify cupcakes and filet mignon as well. Everything would be finger food.

Yet there they are, all those taste buds sitting on our tongue, the inviting front porch swing to our GI tract. Frogs, civet cats, owls – vertebrates of all types have oral taste buds. But it gets weird if you want to start counting all of them, because they aren’t all on your tongue. There are taste buds on your palate (roof of your mouth) and in your throat, and these are just the taste buds. It gets weirder - animals have taste cells and taste receptors in some really weird places.

We have discussed, and will discuss again, how closely related are smell and taste, how they work on a molecular level and how the senses work together to form a flavor. We know now that mammals have taste receptors in their noses!

Bitter taste receptor cells are scattered throughout the nasal cavity, not grouped together in taste buds, so they are called solitary chemoreceptor cells. Maybe this is a way to detect bitter and possibly toxic stimuli before you put them in your mouth - our idea of evolving protective taste sense outside the GI tract may not be some dumb after all. However, the original paper saw that they were linked to increasing respiration rate (to get toxic substances out of the lungs faster).

Here is most of your gastrointestinal tract. The elements

underlined in red have been shown to express taste

receptors. They may acts as nutrient sensors, as hunger

modulators, or even protection against poisoning. To

understand what those mean – read the post! Those not

underlined most likely express the receptors too, they

just haven’t been studied for that yet.

The GI system happens to have many taste receptors; the stomach, small bowel, and large bowel (colon) of mammals have receptors for sweet, fat, bitter, and umami. Interesting that these are the receptors for nutrients and toxins. If you happen to swallow a bitter toxin, the gut receptors stimulate an ion release into the gut.

Water follows the ions due to osmotic pressure, and this would help to flush the toxin through the system faster (ie. diarrhea). This makes sense, but I don’t know if it plays out that way in real life. Not everybody gets to spend hours in the bathroom after eating something bitter - there are probably other issues in play.

On a sweeter note, it has been shown that many intestinal cells express functional sweet heterodimer receptors (T1R2 + T1R3). Cells call enteroendocrine cells (entero = gut, endo = within, and crine = distinguish) produce many of these receptors, and act as sensors for sugar in the gut.

When the enteroendocrine cells detect sugar or artificial sweeteners, they produce hormones that stimulate other gut cells to make more glucose transporters. This is another way that your body works hard to make sure you get all the carbohydrate you can for energy production – it never wants a carbohydrate to get through the gut without being snatched up for use.

Umami receptors are also found in the gut, including the enteroendocrine cells. It was shown in 2013 (here and here) that amino acids are sensed by these receptors, and stimulate release of a hormone called cholecystokinin (CCK), which works in part to tell your brain you that are full.

Cholecystokinin (CCK) is a multifunctional hormone

released from the small intestine after consuming proteins

or fats. It is a hunger suppressant; appetite is controlled by

the hypothalamus of the brain. It can’t cross the blood brain

barrier (BBB), but the hypothalamus isn’t protected by the

BBB. Interesting, no? It just happens that CCK is inhibited

by capsaicin, so maybe it isn’t a good idea to eat spicy and

fatty foods in the same sitting.

CCK also stimulates the release of bile from the gall bladder to aid in the digestion of fats. Fats and amino acids go hand in hand, since meat contains much of both. This may account for the presence of fat taste receptors (CD36 and GPR120) in the gut as well. Fat is harder to digest that other nutrients, so CCK stimulates a slowing of the bowel and a longer retention time in the stomach and gut.

Outside the gut, your GI system also expresses taste receptors in the pancreas. You are constantly sensing how much sugar is your blood, and how much is coming in via your gut absorption. Your pancreas has beta cells that produce insulin to increase the amount of sugar taken up by your cells; this reduces your blood sugar levels.

There are multiple mechanisms by which your pancreas knows to make or release more insulin. Chewing, blood glucose levels, stress levels, exercise, and other signals control the balance between hormone signals that reduce (insulin) or increase (glucagon) blood glucose levels. Now these mechanisms must include sweet receptors on the beta cells.

A 2009 paper showed that functional sweet taste receptors were located on the beta cells in mouse pancreas, they have also been found now in humans. Activation of these receptors by sugars or artificial sweeteners stimulate the beta cells to release insulin and lower blood sugar levels.  A more recent study indicates that fructose is also sensed by sweet receptors on beta cells and can amplify insulin signals triggered by binding glucose. This is just more evidence that postprandial (after a meal) nutrient sensing in the pancreas is mediated, at least in part, through taste receptors.

The hypothalamus works in several systems, including

appetite. Interestingly, different parts of the hypothalamus

are in charge of satiety (being full) and the want of feeding.

Water is a whole other matter. All together, the different

nuclei manage the basic control systems of the body, food

water, temperature, blood pressure, etc.

There are even taste receptors for sugars in your brain. The hypothalamus is a part of your brain – an important part - O.K. so all the parts are important. But it seems that blood glucose sensing is particularly acute in the hypothalamus. This is a little harder than it would seem at first thought. You have to sense the glucose levels without being thrown off by sensing the levels of glucose being metabolized in the cell itself. Since cells manage their internal glucose levels, any monitoring system based on this would always report the same result.

But the hypothalamus expresses sweet receptors on the outside of its neurons. Even more, the numbers of receptors is influenced by the nutrient state of the animal. More glucose sensed in other parts of the body (like the taste receptors of the gut), will result in reduced expression in the hypothalamus. This relates to the function of the hypothalamus in appetite as well. More receptor activity relative to the number of receptors, the more signals will be sent out that you are full.

There are more even brain areas that express taste receptors. A 2012 study shows that the rat brainstem has bitter receptors to sample the extracellular fluids for bitter compounds. This may be to try and act as a late protector of the brain against toxins. We have talked before about how the blood brain barrier is designed to protect the brain from toxins better than the blood vessels of the rest of the body.

Taste receptors in the brain (brainstem, hypothalamus, etc)

are attached to neurons, not epithelial cells. It’s the brain, for

gosh sakes, there’s nothing but neurons. This makes the system

more like smell than taste. No taste cells; the signal is directly

transduced to an action potential.

Perhaps sensing bitter compounds in the brain fluid results in a further clamping down on what molecules can get into the brain by manipulating the BBB. The 2012 paper only speculated that the receptors might have other, non-gustatory functions. We have seen above how taste receptors help sense nutrient levels, so it is plausible that bitter receptors in the brain could be sampling for toxins, and then induce some protective response. I bet that's being studied as we speak.

Lastly, recent evidence may show that monitoring taste receptor expression in different tissues may be moot.  It may be that every cell in your body has taste receptors! It may be that the T1R1+T1R3 amino acid taste receptor is in/on every cell sensing whether it has enough free amino acids available.

Amino acid availability is crucial for cellular function. Your cells are producing new proteins all the time, to replace old proteins and to make different proteins that would respond to changes in cell condition and environment. Without a constant source of amino acid building blocks, each cell has to seek out an amino acid supplier. Autophagy (auto = self, phagy = eat) is the answer in most cases.

In times of low amino acid stores, a 2012 study indicates that decreased signaling through the umami taste receptors on your cells will trigger a cascade of responses and the cells start to digest themselves (autophagy). They will break down organelles and proteins so that the amino acids can be recycled for proteins of immediate need. As you can guess, this isn’t the best way to run a business, robbing Peter to pay Paul, so autophagy beyond the normal (getting rid of unneeded or old structures) will have consequences. Muscle wasting in starved individuals is often a result of autophagy.

Every cell has to have a source of free amino acids. There is a

balance that must be maintained. On the bottom right, “intake”

represents the eating of protein. If that doesn’t occur, then

amino acids must be made from building blocks (bottom left).

If there is intake, then new proteins can be made (synthesis).

But if there is no intake, and little de novo synthesis, then

proteolysis (a form of autophagy) will result in free amino

acids for protein synthesis.

A 2005 paper hypothesized (now proven) that all these extraoral taste receptors on solitary chemosensory cells form a diffuse chemosensory system. The taste buds are just the most visible part of a much larger, more complex system of taste. The big picture – organisms taste themselves to monitor their nutrition and health. This isn’t really an exception, just a huge misconception.

So nature evolved extraoral taste receptors for mammals. Why didn’t it take the next step and get rid of oral taste buds? Come back in a million years and maybe we’ll be tasting with our elbows. Sounds ridiculous, doesn’t it. Well, not so fast. Next week we will see that some organisms taste things with some very peculiar body parts.

Sundaresan S, Shahid R, Riehl TE, Chandra R, Nassir F, Stenson WF, Liddle RA, & Abumrad NA (2013). CD36-dependent signaling mediates fatty acid-induced gut release of secretin and cholecystokinin. FASEB journal : official publication of the Federation of American Societies for Experimental Biology, 27 (3), 1191-202 PMID: 23233532

Kyriazis GA, Soundarapandian MM, & Tyrberg B (2012). Sweet taste receptor signaling in beta cells mediates fructose-induced potentiation of glucose-stimulated insulin secretion. Proceedings of the National Academy of Sciences of the United States of America, 109 (8) PMID: 22315413

Wauson EM, Zaganjor E, Lee AY, Guerra ML, Ghosh AB, Bookout AL, Chambers CP, Jivan A, McGlynn K, Hutchison MR, Deberardinis RJ, Cobb MH. (2012). The G protein-coupled taste receptor T1R1/T1R3 regulates mTORC1 and autophagy. Mol Cell., 47 (6), 851-862 DOI: 10.1016/j.molcel.2012.08.001

Dehkordi O, Rose JE, Fatemi M, Allard JS, Balan KV, Young JK, Fatima S, Millis RM, & Jayam-Trouth A (2012). Neuronal expression of bitter taste receptors and downstream signaling molecules in the rat brainstem. Brain research, 1475, 1-10 PMID: 22836012

For more information, see:

Extra-oral taste perception – Most of this information is recent enough that scientific journal articles are the only source of information. Follow the links in the post and those below.

http://www.sciencedirect.com/science/article/pii/S1084952112001462

http://www.sciencedaily.com/releases/2007/08/070820175426.htm

http://www.adelaide.edu.au/news/news64082.html

http://consumer.healthday.com/diabetes-information-10/diet-diabetes-news-178/pancreas-may-taste-fructose-hinting-at-links-to-diabetes-661517.html

http://www.physoc.org/proceedings/abstract/Proc%20Physiol%20Soc%2016PC35

http://www.sciencedirect.com/science/article/pii/S0006291X11001999