It’s Not Just Chili Peppers That Are Hot

The last few years have seen the rise and fall of The Cinnamon Challenge. I can’t tell you why it came, but I can explain why it went. And the reason relates to the capsaicin receptors we have been talking about.

Don’t think cinnamon candy can be hot. Your unbearably hot cinnamon bears from Jelly Belly and your Atomic Fireballs are both flavored with cinnamon oil. Fireballs have been around since the 1950’s which explains the atomic reference. It takes over two weeks to make one. Cinnamon is not ranked on the Scoville scale because the main spicy compound is cinnamaldehyde, not capsaicin. But there is some capsaicin in cinnamon oil, so I think it could be ranked without breaking some long standing policy.

The challenge goes like this: you take one tablespoon of cinnamon and try to swallow it all in 60 seconds without any water. If any one tells you they did it and came out O.K., they’re lying. If you see video of someone doing it easily, it's been faked.

Here’s how the challenge works for everyone. The compounds in cinnamon stimulate a coughing reflex (explained below). When you cough, you expel air and you have a compulsion to inhale. Here’s where the trouble starts. The inhalation carries a good portion of the cinnamon powder down your trachea and into your lungs.

Now you’ve done it. The ensuing coughing fit can be powerful enough to break ribs. The compounds in the cinnamon immediately begin to burn your lungs, make your eyes water, make your nose run, and increase your breathing rate. More inhalations carry more cinnamon into your lungs and the burn intensifies. YOU WILL blow it out, spit it out, vomit it out. The pain in your lungs will likely last for three weeks or more. Sounds like fun, doesn’t it?

Here’s the biology of the why it ends well for no one. Cinnamon contains compounds called cinnamaldehydeand eugenol, as well as capsaicin (much lower amount). The capsaicin and eugenol activate TRPV1 ion channels. Cinnamaldehyde is a different class of molecule from the capsainoids, so it does not activate TRPV1, but it does activate a powerful member of another subfamily, TRPA1. We will talk more about this receptor in later posts.

TRPV1 is involved in cough reflex, runny nose, and in the burn that the follows the challenge. The TRPA1 activation causes powerful pain in the lungs and trachea. Together, these compounds result in the involuntary cough, reflexive inhalation of cinnamon into the lungs, and all the pain that follows from activating the TRPV1 pain receptors in your lungs. Now you know WHY you should avoid the challenge.

Asthma is a trigger for airway or nasal hyper-reactivity. It is easy to see how this could get out of hand, especially when it can lead to chronic inflammation and damage of the airway tissues. And to think, it is mediated in part by the same receptor that makes your Sunday afternoon chili stew spicy.

Don’t laugh at the portion of the response that takes place in your nose. My mother-in-law got some chili pepper oil up her nose once, and she still refuses to be in the same room as a chili. Her runny nose, sneezing, and watery eyes were directly due to the capsaicin. But for some people it happens for no reason. This is called nasal hyper-reactivity.

In some cases the exaggerated nasal response is due to an allergen, but in other people the trigger is unknown. The TRPV1 ion channels in the nasal mucosa may be over-expressed (too many of them). Their activation brings mucous, bronchoconstriction, cough, and sneeze.

The evolutionary strategy here is like with the cinnamon. Your body is trying to keep toxic or harmful substances out of your lungs. A new study has linked nasal hyper-reactivity to TRPV1 action alone, without need of other receptors. What is more, the paper identifies a new TRPV1 antagonist. An antagonist is a molecule that binds to the receptor but does not activate it, and it can prevent the receptor’s stimulation by molecules that wouldnormally activate it (agonists).

This new antagonist of TRPV1 can suppress the nasal hyper-reactivity and give some relief those afflicted. You may think nasal hyper-responsiveness is trivial, but it’s snot – get it? It’s snot.

Nasal hyper-reactivity is often diagnosed by assessing an exaggerated response to capsaicin in the nose. I can’t imagine how any response to nasal capsaicin could be considered exaggerated. It’s just lucky for us that capsaicin isn’t volatile. Less of it gets into the air because it has a long hydrocarbon tail. 

Because it isn’t volatile, capsaicin doesn’t have an odor, not until you chew it and volatilize it yourself into your nose do you know you’re in trouble. The runny nose is your body recognizing there is something there that you really don’t want in your lungs. It’s bad enough with cinnamon, can you imagine getting capsaicin in your lungs?

The capsinoids are a group of compounds within the vanilloids. You can see the resemblance to vanillin, so there are all vanilloids. The different capsinoids are all found in chili peppers, but capsaicin is the most abundant and potent, but the structures of the different capsinoids are very similar.

Pure capsaicin ranks at 16 million Scoville heat units (SHU). But it isn’t the  only capsinoid in chili peppers. There is also dihydrocapsaicin (15 million SHU), nordihydrocapsaicin (9.1 million SHU), homocapsaicin (8.6 million SHU), and homodihydrocapsaicin (8.6 million SHU). Each of these can activate TRPV1 to bring the burn. But it doesn’t stop there; many other compounds can bind to TRPV1 as well. Here's some of them:

Piperine (100,000 SHU) is the spicy compound in black and white peppercorns. You already know black pepper is spicy, and it activates TRPV1 just like capsaicin. Remarkably, it's even more efficient than capsaicin at opening the TRPV1 ion channel. However, it's found in lower amounts that capsaicin in most chilies and has a greater ability to desensitize TRPV1, so it burns less. We will talk about desensitization of TRPV1 in the coming weeks.

Allicin is found in garlic and onions; they can burn too. Raw garlic is especially pungent; try it some time. Garlic is used in many folk medicines – it has been show to prevent or treat fungal infections, lowers blood pressure, is neuroprotective, and can slow the growth of some cancer cells. Some of these effects are mediated by TRPV1. Oh, and it wards off vampires too.

Eugenol is found in many foods, including cinnamon, bay leaf, clove, and allspice. It activates TRPV1, but like piperine, it can be desensitizing too. For this reason, eugenol has a numbing effect and is often used in dental preparations. If you have ever had a cavity filled with the silver amalgam, you probably smelled cloves in the process - that was the eugenol. Just recently it has been shown that eugenol also activates TRPA1 pain receptor, so maybe the dentists should be rethinking their strategy.

Radishes, horseradish, wasabi, and mustard contain allyl isothiocyanate (AITC). This compound binds to both TRPV1 and TRPA1, so they can generate a lot of pain, and the heat sensation as well. In mustard seeds, the AITC isn’t produced until the seeds are broken and an enzyme is released that converts one compound into AITC. This is why stone ground mustards with larger chunks of seeds are less spicy.

In the horse trade, a raised tail means a younger, livelier horse. When someone wanted to sell an old, worn out horse, but get more money, they might put a piece of ginger in the anus of the horse. The burn from the gingerol would make it raise it’s tail. It has also been used in horse shows, but is now illegal. Oh yes, sometimes “gingering a horse” also involved live eels.

The spicy compound is not stored in the plant as AITC because it's harmful to the plant as well. Only when an herbivorous predator comes along and starts munching on the plant is the toxic chemical produced. A new study (2013) shows that AITC actually makes TRPV1 more sensitive to heat, so using wasabi with hot food will really crank up the pain.

Ginger contains gingerol(60,000 SHU), but when you cook it gingerol is converted to the sweeter and more aromatic form called zingerone. Both can activate TRPV1.  There is also gingerol in mustard oil, so both mustard and ginger have been used in folk medicine, like plasters they use to slather on wounds. For a less appropriate use of ginger, see the photograph at the right.

Camphor is used in things like Vicks VapoRub. It activates TRPV1, so you feel warm, but it can also activate a cool receptor, so it seems to open up your nose. We will have much more to say about this in a couple of weeks. Found in certain trees, camphor is slightly analgesic (pain killing), and is antimicrobial, so it does serve a purpose in Vicks.

In addition to these plant-based agonists, TRPV1 is activated by other things as well. We already talked about how the channel is opened by acid (excess protons), but it can be activated by inflammation in tissues and some endogenous pain killers as well, like the endocannabinoids we talked about at New Year’s.

Tinyatoxin (and resiniferatoxin for that matter) are produce by the Euhorpbia poissonii plant. Native to Nigeria, its extract is used by natives as a pesticide. The tinyatoxin and resiniferatoxin are neurotoxic and can kill TRPV1- expressing neurons, so they are being looked at as a way to treat chronic pain.

There are artificially produced agonists as well. Resinferitoxin activates the heat receptor TRPV1. It rates 108.8 billion on the Scoville scale! Scientists are trying to find a use for it in chronic pain and other diseases (more in two weeks). There is also tinyatoxin from the Euphorbia plant. It is slightly less spicy, about 5.3 billion SHU. It is a neurotoxin and can kill you in large amounts.

One last agonist for TRPV1 – osmotic stress. This refers to the movement of water out of cells (so they shrink) or into cells (so they swell) when there is an imbalance of salts inside and outside of the cell. Too much salt in the extracellular fluid is called hypertonic, and water will flow out of cells and toward the more concentrated salts. Too little salt in the extracellular fluid is called hypotonic and water will move into the cells where the salt concentration is higher.  We want an isotonic environment, where the slat is the same in and out of the cell.

TRPV1 sense osmotic changes, specifically hypertonicity. A 2010 paper shows that there is a TRPV1 in the brain that does not react to heat or capsaicin, but does respond to osmotic stress. TRPV1 sense cell shrinkage and signals the hypothalamus of the brain to release a hormone called vasopressin (also called ADH). This hormone causes more water to be retained and more salt to be excreted, This lowers the salt concentration outside the cells and the cell shrinkage can be corrected.

Osmotic pressure is related to the amount of water versus the amount of salts in the water. In the cartoon, the salts are represented by the blue spheres. Water will travel to wherever salts are highest, because that means water concentration is lower. Hypertonic means water will flow out of cells, while hypotonic means water will swell the cells, even to the point of lysing them. The representative cell is a red blood cell, since they are very susceptible to osmotic changes.

Another receptor of the same subfamily, TRPV4, senses swelling during hypotonic crises. This then triggers the hypothalamus to release less vasopressin and the salt concentration will increase outside the cell; excess fluid in the cell will flow out of the swollen cells. A 2011 papershows that TRPV1 works only on shrunken cells and TRPV4 only on swollen cells.

Using TRPV1 in osmoregulation makes sense. It is closely related to thermoregulation, considering how you use sweating to get rid of excess heat. Sweating messes with osmotic pressures. Nature is smart that way.

Next week, more functions of TRPV1 – it can make pain worse and stop pain. How can that be?