As Many Exceptions As Rules: taste

Showing posts with label taste. Show all posts

Wednesday, July 9, 2014

What’s So Repelling About Repellents?

Biology concepts – thermosensing, repellent, odor receptors, gustatory receptors, semiochemcials

Science explains our world, and then technology and engineering

build a model of that for our use. The better we know how our

universe works, the better we can make use of it. In the 1985

film Real Genius, this difference is stated when the scientist

students ask what a 6 megawatt laser might be for, one student

says, “Let the engineers figure out a use for it.” In this case, they

used it to fill a house with popcorn.

Science exists to describe our universe in terms of rules and mechanisms; what is and how it comes to be. Knowing that something exists is only half the equation. Science seeks to explain how something exists in terms of the rules of the universe. Observation is good, but it only shows us the question – mechanisms of action and interactions show us the answers.

As an example – we know that certain naturally occurring oils and well as some man made chemicals keep mosquitoes from feeding on us. This is the observation. But the question is – how do mosquito repellents work? The answer is more interesting and more complicated than you would initially think. Repellents rarely repel.

Investigating how chemicals keep us from getting bitten will teach us about how the living systems work, will give us a better understanding of our universe, and then give us better insect repellents. Don’t think that’s important? Consider the hundreds of millions of people who are infected every year (several million die) with mosquito-borne diseases (malaria, encephalitis, dengue fever, yellow fever, filiariasis). So yes, we need more repellents.

Mosquito borne diseases can be unpleasant at best. Top left is

filariasis, a worm is transmitted via mosquito and it clogs up

your lymphatic vessels, so that body parts swell from excess

fluid. Top right – malaria can result in so much red blood cell

lysis that your spleen (the guy who cleans them up) can

rupture. Bottom left – Dengue fever is often called breakbone

fever, the pain is not something an image can express. But the

hemorrhagic form of the disease can produce some bleeding

in weird places. Oh, and it can kill you too. Bottom right –

yellow fever is caused by a virus transmitted by mosquito. Your

liver breaks down and causes your whole body to turn yellow

and you bleed into your skin.

We should start with the repellents for which we have good ideas of their mechanism of action. But there aren’t any. We have some hypotheses and working ideas of the modes of action of mosquito repellents, but nothing is definitive yet. Let’s look at two of them and see if we can find some common pathways.

Citronella oil

Citronella is a combination of many different natural oils produced in lemongrass plants (Cymbopogon nardus and Cymbopogon winteratu). As a natural oil and a flavoring in Asian cooking, one would think that citronella oil would be considered just about the safest insect repellent this side of a slap with an open palm.

But no, Canada says that one small component of citronella oil called methyleugenol, can increase the likelihood of tumor formation in rats. Of course this was when methyleugenol was distilled from the oil, given by itself in large doses, and introduced directly into the stomach. But Canada is still in the process of banning citronella oil as an insect repellent. Of course, you can still eat thai food in Canada, which is often flavored with lemongrass.

The EU, on the other hand, said that the repelling function of citronella oil hadn’t been proven and it was deemed illegal to use in the EU in 2006. Oh, you could eat it, and use it soap or perfumes, you just couldn’t use it to keep mosquitoes away. They reconsidered in 2014 and some restricted uses of citronella oil as a repellent are now allowed.

Citronella oil comes from the lemongrass plant (Cymbopogon

nardus or Cymbopogon winteratu). There are two major species

for acquiring the oil, and the oil from each is a little different in

the percentage of each chemical. Lemon grass is also used in

cooking, the woody stalks are used with extra long cook times.

The torches that burn citronella oil work pretty well, but you

have to stay in the volatilized cloud of oil for them to be efficient.

Despite these issues, the U.S. Environmental Protection Agency (EPA) says citronella is safe and effective as an insect repellent. One weird side issue – you can take all the lemongrass you want from the US to Canada, where its oil is under attack, but you can’t bring any lemongrass from Canada to the US, where it is considered safe. Hmmmm.

Citronella oil probably works in a couple of ways. It's strong and sweet smelling, so it covers up and dilutes the odors that mosquitoes use to find you. If they’re detecting all the citronella in the air, then they aren’t smelling you. But research also shows that citronella oil activates TRPA1 ion channels. In us, they detect cold and noxious chemicals and are interpreted as pain. It is very possible that the detected signals in mosquitoes just come through as something unpleasant and to be avoided.

In this way, citronella would be an actual repellent. It repels on contact as well, as the taste is thought to activate bitter taste receptors and contact greatly reduces feeding time.

But citronella only seems to work when you are in the cloud produced by burning the candles or torches, or within the area of the spray. And if you’re using an oil or cream with citronella, it should really be reapplied every 30-45 minutes - not the most user-friendly method for discouraging pests.

DEET

World War II in the Pacific was an insect nightmare for the US Army. In response to the plethora of insect-borne disease that ran through the allied forces, defense scientists starting looking for better insect repellents. In 1946, their efforts produced N,N-Diethyl-meta-toluamide, or DEET.

Just how they came up with DEET is a mystery to me, it must have been a massive exercise in trial and error. Why? Because we know less about how DEET works than we do about citronella oil. And that’s with the benefit of 40 years of research. They didn’t have a clue how it worked or even what systems it was targeting when developed in the 40’s.

Guess which hand has been treated with DEET. The

mosquitoes come very close to the hand that was treated,

but don’t land on it. This argues that DEET is less repelling,

than it is disguising. On the right, the structure of DEET is

similar to several human semiochemicals, it fits into the lock

and key system of several odor receptors and activates or

inhibits them.

Originally it was believed that DEET disrupted the mosquito’s ability to detect semiochemicals (octenol) produced by mammals, especially humans, so mosquitoes couldn’t find a mammalian host to feed on. Then they played around with the idea that it blocked detection of CO₂.

More recent studies have been more rigorous, but haven’t helped solve the puzzle. A 2008 study suggested that DEET was actually repellent; the mosquitoes didn’t like the smell and would avoid it. But other studies have shown different mechanisms of action.

A study in the journal Nature in 2011 found that mosquito odor receptors could be confused by DEET. The receptors for octenol were less responsive in the presence of DEET, but other receptors more more responsive. The conclusion of the study was that odorants from humans could be detected, but their pattern was confused, so the mosquito didn’t recognize the target as a target. It’s as if we disappear from the mosquitoes radar when we wear DEET.

A 2010 study showed similar results. DEET activated certain odor receptors but not others when given alone, but the opposite effects were seen when DEET was given in the presence of things from human sweat that would normally attract a mosquito. Once again, the signals were confused. This is really more of a chemical disguise for us, not a repellent. Next time your kids go outside, you should insist that they apply their mosquito confusant.

However, a 2013 study in the Journal of Vector Ecology found that heat and moisture were critical elements for recognition of targets by female mosquitoes, and that DEET messed not with odor, but with detection of heat and/or moisture. Different from the other studies, but still more of a masking than a repellent.

Something a little disturbing. Mosquitoes can learn to ignore

DEET. Most mosquitoes will be confused by DEET and never

find you. But if they do and then are repelled by the taste, they

learn from that and the second taste is not repellent. Hopefully

they just don’t find you a second time.

There was an interesting study from 2013 that showed that if you mutate or knock out Orco, one of the co-receptors (a protein that works with many different odor receptors so that they can function properly), then two things happened. One, DEET didn’t have any effect on the mosquitoes, and two, mosquitoes that normally preferred humans greatly would then settle for any mammal.

Weird - Orco is needed for both DEET to work and for mosquitoes to find humans more attractive. I haven’t figured that one out yet. The researchers showed that DEET only maintained an effect on the Orco mutant mosquitoes when they landed on a DEET covered surface, and then they didn’t like it at all.

This suggested that DEET might have more than one mechanism, confusion in the air and repellent taste on contact. Older studies supported this idea, as a couple of studies in 2005 and 2006 showed that contact with DEET would reduce feeding behavior in mosquitoes and one in 2010 showed that fruit fly bitter taste receptors are activated by DEET.

So, we have studies that say DEET is a confusant rather than a repellent, others that say it is a true obnoxious smell that they can’t stand, and yet others that say DEET is confusing to the smell and repellent to the taste. But there are more. Other studies suggest that DEET actually inhibits the smelling of anything, while others say that it inhibits an important protein called cytochrome p450.

Used commercially since the 1950’s, DEET has been the gold standard for efficiency for many years. Although it has to be used at fairly high concentrations, it can keep mosquitoes away for 4-6 hours at concentrations where citronella oil might work for less than an hour. At 100% concentration, DEET is active for more than 12 hours. What’s more, if you combine DEET with 5% vanillin, it works two hours longer!

A lime with cloves stuck in it as a mosquito repellent – really?

Well, lime is kind of like citronella oil, and clove has eugenol,

which acts on TRPV1 ion channels. But how many would you

have to have, or do you wear them like earrings? Penny royal

contains menthol and mosquitoes stay away from it. But it

also has toxins that will kill you.

As good as DEET is, people still question whether it’s safe. The EPA in a 2014 review said that DEET is safe for human use and poses no identifiable risks for human health, even in children. But this doesn’t keep people from suspecting chemical usage of carrying negative effects.

On the other hand, DEET dissolves plastic, foam rubber, spandex, gore-tex, and nylon. I can see where this might make people leery about slathering it on their skin for hours at a time. And a few people are allergic to DEET, so the best current repellent isn’t without some negatives.

One last point – a newer repellent called picaridin is almost as effective as DEET and doesn’t eat your back packing equipment and clothes. The interesting point is that picaridin is a synthetic version of piperine, the spicy chemical in black peppercorns. Add to this that menthol is also a fairly decent mosquito repellent, and we have some good arguments that TRP receptors might be involved in repelling activity – as with citronella oil. Piperine is a TRPV1 agonist, and menthol activates TRPM8 and TRPV1. All our talk about spicy food and heat/cold receptors has an impact even in the spread of malaria and other deadly diseases!

Next week, another question to answer - do sunflowers really turn with the sun?

DeGennaro M, McBride CS, Seeholzer L, Nakagawa T, Dennis EJ, Goldman C, Jasinskiene N, James AA, & Vosshall LB (2013). orco mutant mosquitoes lose strong preference for humans and are not repelled by volatile DEET. Nature, 498 (7455), 487-91 PMID: 23719379

Stanczyk NM, Brookfield JF, Field LM, & Logan JG (2013). Aedes aegypti mosquitoes exhibit decreased repellency by DEET following previous exposure. PloS one, 8 (2) PMID: 23437043

Klun JA, Kramer M, & Debboun M (2013). Four simple stimuli that induce host-seeking and blood-feeding behaviors in two mosquito species, with a clue to DEET's mode of action. Journal of vector ecology : journal of the Society for Vector Ecology, 38 (1), 143-53 PMID: 23701619

Wednesday, April 23, 2014

Chili Peppers Run Hot And Cold

Biology concepts – obesity, brown adipose tissue, agonist/antagonist, protective hypothermia, hyperthermia, reactive oxygen species, ischemia, hypoxia

When The Wizard of Oz was released in 1939, it just barely turned a profit. The '39 version was the third attempt at filming the children’s classic, and the first two efforts had not fared much better.

I don’t see how people didn’t take to the Wizard of Oz right away.

It had new technology for the movies, a good villain, and all those

little people. The tin man on the left was played by Jack Haley, but

originally it was supposed to Buddy Ebsen (Jed from the Beverly

Hillbillies). Unfortunately, the lead metal in the makeup almost

killed him during the makeup/costume tests. Glenda the good witch

(Billie Burke) had that squeaky voice. She only began acting

after her husband, Flo Ziegfeld, Jr. (son of the Ziegfeld Follies

creator), went belly up on Black Monday in 1929.

Over time, what was first considered bad has become a classic. In what many people consider the best year ever in film, The Wizard of Oz is now a favorite among favorites, more than Goodbye Mr. Chips, Mr. Smith Goes to Washington, Stagecoach, or even Gone With the Wind – all produced in 1939.

It’s smart to hang on to useless things and knowledge, something might change. For Oz – it was television. For some reason, this film translated better to TV than it did the big screen. The Library of Congress now rates it as the most viewed film ever. And it wasn’t even shown on TV until 1956. The weird part – very few people in 1956 owned a color television, so Dorothy’s entrance into the land of Oz was no big deal for most folks until the late 1960’s.

Why am I telling you this story? Because the same thing happens in biology and medicine. Problems can become assets if the right environment is created or the proper setting is found. We've been discussing the capsaicin receptor, TRPV1, for some weeks, and this is where I find a negative being turned into a positive.

As you know, the TRPV1 capsaicin receptor is primarily a heat sensing receptor for thermoregulation of the body. If activated by noxious (painful) high temperatures, it generates a pain signal and initiates a cooling program for the body, including sweating.

In an effort to block TRPV1 to create analgesia (no pain), the problem has been that blockers also stop thermoregulation and the patient overheats. This prevents most TRPV1 antagonists (substances that bind the receptor but don’t allow function) from being used as analgesics. But what about in other situations?

I was wondering if TRPV1 antagonists might be helpful in obesity, by helping burn off some fat through increased cooling activity. If they are indeed helpful, nobody knows about it yet. I couldn’t find even one paper that studied TRPV1 antagonists as a way to induce increased energy expenditure and weight loss. In fact, I learned just the opposite. Capsaicin and other TRPV1 agonists might help with weight loss.

On the left is brown fat and white fat. You can see that brown fat

actually looks browner because of all the mitochondria that it

contains. White fat contains a lot more lipid. The right cartoon

shows that a cold challenge initiates uncoupled fat metabolism in

brown fat, creating heat. But the cold also releases more fatty acids

from white fat, which can then be burned by the brown fat. The

involvement of bone comes from bone breakdown. Breakdown

releases a protein that stimulate white fat to release fatty acids,

this would provide energy for the brown fat.

We have discussed how TRPV1 activation by noxious heat helps to cool the body, but it turns out that noxious cold leads to TRPV1 activation as well, but in these cases, it brings an increase in heat production. So TRPV1 can cool you down or warm you up as needed. Pretty cool. You'll have to wait a few weeks to find out how a heat receptor senses noxious cold.

The heat induced by cold comes from increased activity of brown adipse tissue (BAT) – brown fat. We have talked about BAT before, how it is especially important for infants because they lose heat so easily. Brown fat has lots of mitochondria, but they don’t make ATP. They convert all the energy they burn into heat.

New research is showing that BAT can be important to adults as well. Those people that have more BAT tend to have less white fat, the kind that makes you bigger. What is more, a 2013 paper shows that cold temperature exposure can help create more BAT, and this effect is mimicked by capsaicin and other TRPV1 agonists.

If you expose adults to mildly cold temperatures for six hours a day, they start to make more BAT and this means they burn more energy for heat; therefore less energy is left to be stored as white fat. But the study also showed that giving the people capsaicin for weeks in a row generated the same increase in BAT and stopped white fat accumulation.

One mechanism involved is that TRPV1 agonists stimulate an increase in uncoupling protein (UCP) expression in BAT. This is the protein that permits the BAT mitochondria to produce lots of heat instead of lots of ATP and a little heat. The uncoupling protein activity in BAT uses excess calories to produce heat, so those calories are not available to make fat.

Here is how a stem cell becomes a fat cell (adipocyte).

The mesenchymal cell can go two directions, one

toward fat and one toward muscle. But notice you can

get to a brown fat cell through the pathway meant for

muscle cells. PG stands for prostaglandins; different

profiles of prostaglandins lead to a decision to become

a brown fat cell or a white fat cell. We know this picture

is incomplete now, because we have evidence that

TRPV1 agonists can drive the decision between

brown fat and white fat.

But there may also be another mechanism at work. A 2014 study in laboratory petri dishes shows that cells destined to become white fat cells can be stopped from changing by capsaicin. In cells called preadipocytes, capsaicin stopped their proliferation (dividing to become more cells) and their differentiation (changing) to become full-fledged adipocytes (fat cells). Another study (2012) showed that in liver, capsaicin could prevent the accumulation of white fat build up (called fatty liver) and could actually induce UCP protein expression in some fat cells, turning them into liver BAT. Amazing.

This all sounds fine, but the proof is in the pudding, so to speak. Capsaicin and other TRPV1 agonists have been shown to reduce white fat and total body mass in rabbits fed a high-fat/1% capsaicin diet, in mice fed a high sucrose diet, and in human patients kept cold or fed hot. Tomorrow I’m going to start eating hot peppers in a cold house – I’ll shrink away before your eyes.

What about on the other end of the thermometer? People freeze to death when they get too cold, and TRPV1 agonists will cool you off when too warm. No TRPV1 activity causes a reactive hyperthermia, and too much TRPV1 activity will induce a reactive hypothermia. But is there a time when inducing cold in a body with capsaicin would be a good thing?

Would we be talking about it if there weren’t an exception? It's called protective hypothermia, and it has become a very important treatment adjunct during stroke and some over conditions.

Ischemia (left) is often associated with coronary (heart) arteries.

Ischemia means a reduction in blood flow to a tissue or the whole

body. With less blood flow comes less oxygen, so tissue cells suffer.

Several mechanisms can lead to a lessening of blood flow. On the

right is hypoxia, which is often used when referring to the brain or

specific organs. Hypoxia is a reduction in oxygen to the tissues,

whether it comes from a reduction in blood flow or some other

reason, like fewer red blood cells, lower oxygen in the air, etc.

Protective hypothermia is an induced cold that is used to protect tissues from post-ischemic injury. When there is a reduction in blood (ischemia) or oxygen (hypoxia) to a tissue or organ, the cells are starved for oxygen and then become starved for ATP (you need oxygen to make ATP). With lower oxygen over time, either from low oxygen or reduced blood flow, the tissues get used to having lower oxygen levels.

Getting used to it would include down-regulating the systems that would normally combat the damage that could be caused by reactive oxygen species (ROS). Whenever oxygen is being used in tissues, ROS are an unfortunate by-product. Their name tells you that they’re reactive, which means they can react with many molecules in the cell and they will do significant damage.

When normal blood flow or oxygen perfusion is re-established, the sudden increase in O₂ causes a spike in ROS (reperfusion injury) – until the cell can ramp up its antioxidant capabilities again. What medicine needs to do is find a way to increase the O₂ without increasing the ROS damage.

Cold seems to do the trick. Reducing the temperature of the body reduces cell death and ROS after cardiac arrest, stroke, neonatal encephalopathy, or traumatic spinal/brain injury. Why? There have been a few ideas why.

The old hypothesis was that the lower temperature would reduce cellular metabolism, so that there is less need for O₂. This would imply that the lower the temperature, the better. But very low temperatures might lead to injury or damage on their own. Also, extended cold could bring pneumonia or promote sepsis. Maybe colder isn’t always better.

There are many ways to get a perfusion injury when

oxygenation of the tissues is reestablished after hypoxia.

We talked about the free radicals (ROS) in the post. The

other injuries are a bit less obvious. We mentioned the

problems with membranes and the increase in apoptosis.

The other two are related to spasm of the muscle cells in

the vessels which would again reduce oxygen levels, and

a nonspecific activation of coagulation and cell killing that

would lead to damage as well.

Now scientists think protective hypothermia works in a couple of different ways. Colder temperatures bring a neuroprotective effect by preventing apoptosis (programmed cell death). Less O₂ means less ATP being made, and a decrease in ATP usually means that the mechanisms for maintaining proper ion movements in and out of the cell are hampered. Increased ion flux triggers apoptosis. So lower temperature brings less ion flux, less damage, and less cellular suicide.

Even a small decrease in temperature can stabilize the cell membrane independent of ATP levels. This makes sense; membranes are mostly lipid, and lower temperatures make fats stiffer – like cold butter. This will decrease ion movement across the membrane and reduce cell damage.

Lastly, decreased body temperature brings less reperfusion injury. In this case, maybe the old hypothesis was correct. Colder tissues metabolize less, so less oxygen will be needed and less ROS will be produced.

So cold is helpful, but how do you do it? You can lower the body temperature by using cooled IV fluid, cold mist in the nose, or even wrapping specific body parts in cooled blankets. But perhaps TRPV1 agonists could help cool the body from the inside.

As of early 2014, the evidence for TRPV1 agonists is only in mouse models, but it’s looking good. A study in 2011 showed the an injection of capsaicin into the abdominal cavity three hours before inducing hypoxia reduced the volume of dead tissue and the amount of apoptosis in the brains of the mice.

This is the fruit of the Evodia rutaecarpa Bentham plant. It has been

used in Chinese herbal medicine for hundreds of years. We are

starting to learn why it does what it does. It has been shown to be

an anti-cancer, anti-obesity, anti-vomiting, anti-hypertension

anti-ulcer, anti-pain drug. Five thousand years of

culture leads to good drugs like this.

Two 2013 studies added strength to the 2012 study. One experiment used a Chinese herbal medicine that contained a chemical called evodiamine. It had been known that evodiamine helped in stroke victims, but we didn’t know why. Evodiamine was shown to be a TRPV1 agonist in 2012, and the 2013 study showed that after a stroke, the agonist increased cell survival mechanisms and reduced apoptosis.

The other study from 2013 showed that capsaicin also helps in reperfusion injury. Mice were given strokes by blocking an artery in the brain and then unblocking it to replenish the blood and oxygen. Injecting capsaicin within 90 minutes of the re-establishment of blood flow produced a mild hypothermia, reduced the volume of dead tissue in the brain, and increased neural function. This didn’t occur in mice without TRPV1, so we know the capsaicin receptor was responsible. Sounds like emergency rooms are going to start stocking hot peppers.

Today we discussed interesting uses for capsaicin and its receptor in temperature-related functions. Next week, some weird functions for TRPV1 that have little or nothing to do with temperature.

Yoneshiro T, Aita S, Matsushita M, Kayahara T, Kameya T, Kawai Y, Iwanaga T, & Saito M (2013). Recruited brown adipose tissue as an antiobesity agent in humans. The Journal of clinical investigation, 123 (8), 3404-8 PMID: 23867622

Feng Z, Hai-Ning Y, Xiao-Man C, Zun-Chen W, Sheng-Rong S, & Das UN (2014). Effect of yellow capsicum extract on proliferation and differentiation of 3T3-L1 preadipocytes. Nutrition (Burbank, Los Angeles County, Calif.), 30 (3), 319-25 PMID: 24296036

Yoneshiro T, & Saito M (2013). Transient receptor potential activated brown fat thermogenesis as a target of food ingredients for obesity management. Current opinion in clinical nutrition and metabolic care, 16 (6), 625-31 PMID: 24100669

Muzzi M, Felici R, Cavone L, Gerace E, Minassi A, Appendino G, Moroni F, & Chiarugi A (2012). Ischemic neuroprotection by TRPV1 receptor-induced hypothermia. Journal of cerebral blood flow and metabolism : official journal of the International Society of Cerebral Blood Flow and Metabolism, 32 (6), 978-82 PMID: 22434066

Cao Z, Balasubramanian A, & Marrelli SP (2014). Pharmacologically induced hypothermia via TRPV1 channel agonism provides neuroprotection following ischemic stroke when initiated 90 min after reperfusion. American journal of physiology. Regulatory, integrative and comparative physiology, 306 (2) PMID: 24305062

For more information or classroom activities, see:

Brown adipose tissue –

https://exploreb2b.com/articles/how-does-brown-fat-help-to-reduce-obesity

http://www.medicalnewstoday.com/articles/240989.php

http://www.webmd.com/diet/features/the-truth-about-fat

http://www.landesbioscience.com/curie/chapter/623/

http://diabetes.diabetesjournals.org/content/58/7/1482.full

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/misc_topics/brownfat.html

http://www.marksdailyapple.com/a-primal-primer-brown-adipose-tissue/

http://wholehealthsource.blogspot.com/2013/07/brown-fat-its-big-deal.html

Protective hypothermia -

https://www.google.com/#q=protective+hypothermia

http://www.qscience.com/doi/full/10.5339/qmj.2012.2.19

http://ccforum.com/content/16/S2/A2

http://www.ctnow.com/health/ctn-woman-dead-revived-with-cpr-hypothermia-web,0,3890140.htmlstory

https://www.med.upenn.edu/resuscitation/hypothermia/

http://emedicine.medscape.com/article/812407-overview

Wednesday, April 16, 2014

Using Pain To Stop Pain

Biology concepts – desensitization, habituation, counter irritation, cautery, heat sensing, pain, chronic, acute, analgesia

Gout usually attacks middle-aged men and the big toe

joint is a favorite spot. But it can occur anywhere and

in anyone. The accretions or urates build up and clog

the joint, causng poor function and intense pain,

painful enough that even the weight of a sheet on it at

night is too much. Usually the acute attacks are far

worse, and become less painful gouty arthritis

as they become chronic.

Sometimes people use pain to combat pain, as silly as it may sound. Gout is an arthritis-like disease where uric acid crystals (a waste product from many different pathways, especially purine nucleotide metabolism) buildup in the joints and can cause life-altering pain.

Before adequate drugs and diet suggestions came along to help to rid the body of excess urates, people were sometimes left to to their own devices in trying to relieve the pain of gout. One home remedy was described in a history text from the 500’s CE called Historium Libri Decem.

In the book, the Bishop of Cahors had gout so bad that he would stick a fireplace poker in the embers and then apply it to his foot and shin. Man oh man, it must be some major discomfort if cauterizing your toes and foot becomes a good idea.

The practice of cautery (from Greek for branding iron) for gout lasted for hundreds of years, with a 17^th century century surgeon from France pronouncing that he didn’t really believe in external remedies for gout, except of course, for cautery with a red hot poker.

The above example falls under the heading of analgesia (an = not, and gesia = pain) by counter irritation. Counter irritants basically substitute one pain for another. You have two competing stimuli, both of which create pain. One is probably chronic (long-term, comes from chronus = time); this is pain from which the person wants relief.

The second pain is acute (from Greek for sharp). The second pain is geared toward relieving the first pain. Who hasn’t bitten a knuckle or lip while getting an injection or experiencing some other pain? Is it just a distraction, a placebo, or is there biology at work?

The concept of counter-irritation is old. Before we had any idea how it worked or even if it worked, counter irritant uses approached the bizarre. Heart attack and angina pectoris (pain from partially blocked cardiac vessels) often shoot a pain down the left arm. So early physicians decided that if counter irritants were placed on the left arm, they could short circuit the pain as it was sent out but before it could come back to the heart.

Cupping is one example of a counter irritant to relieve pain. The

little pins on the tops of the cups are for drawing a vacuum. Dry

cupping is shown on the left, no blood is drawn. Wet cuppin is

on the right, where more vacuum is used and blood is suck out

of the skin through the pores. Believe it or not,

wet cupping is more common.

Counter irritants these days are sometimes just as odd. For example, blistering horse legs to help their knees goes against common sense. There is also something called cupping, which is Chinese in origin. Suction is created in cups that then are placed on the skin. The skin is drawn up into the cup, and blood is drawn to the surface.

Originally used to promote healing, cups are used to quell pain too. As far as truly scientific studies, the only thing we know about cupping and pain relief is that the studies have been poorly conducted – better studies are needed. But never underestimate the power of the placebo. If a patient thinks it works, then it works.

Other types of counter irritants include scarification – scratching until the epidermis is removed. Many people with chronic itch practice this everyday of their lives. Chemical irritants that act on the skin are called rubefacients because they turn the skin red (through dilation of blood vessels underneath). Capsaicin, menthol, camphor, methylsalicylate (more on this below), eucalyptus oil, all have been tried as rubefacient counter-irritants.

This youngster suffers from Alagille syndrome.

It causes bile to stockpile in his liver which

makes him itch all over, all the time. He wears a

special suit to prevent him from scratching until he

bleeds. The scratching is a type of counter irritant

and will will talk more about itch and capsaicin in a

couple of weeks. The young boy is on the

liver transplant list.

Often, the counter-irritant is applied at the same place where the deep pain is, because the users believe it must act on the set of nerves that sense the pain in both areas. Capsaicin rubs, or things like BenGay (methyl salicylate) are rubbed on the skin where the muscles or joints ache. But this may not be the way that they act.

A study from 2009 showed you could counter a long-standing pain in the right leg by immersing your left foot in cold water. Painfully cold water activates TRPM8 and TRPA1. Doing this only to the left foot reduced shock pain in the right leg by 50%. Apparently it acts to confuse pain signals at the level of the spinal column.

So just how does counter irritation relieve pain? There are competing ideas. Perhaps the acute pain of the irritant sparks a release of endorphins (opiate pain killer made by our own body). This seems plausible, but chronic pain patients have very high levels of endorphins in their blood; they just seem to not respond to them.

The hypothesis of a nerve overload is less precise, and may actually reflect other mechanisms at work. Overload in general would mean that a huge amount of neural input at the same time overloads the spinal nerves at that point and results in no signals getting through. This may be how the left foot right leg example works.

Even if some of these mechanisms contribute to counterirritant action, additional processes are probably at work as well, namely desensitization and habituation. You’ll be surprised how much TRPV1 heat/capsaicin sensors are involved. Capsaicin causes pain because your body interprets it as noxious heat (TRPV1), but somehow, you can also use capsaicin to take away pain.

I have used the joke before, but it keeps working. Hitting

yourself with a hammer over and over could desensitize

pain receptors, induce endorphin release, distract from

other pain, confuse spinal signaling – all of which are

plausible mechanisms for counter irritation.

The truth is what makes it funny.

In simple terms, desensitization of the TRPV1 channels means that while some activation causes pain, continued activation depletes the neuron of the molecules need to create or transmit the signal, and the pain neuron can’t fire any longer. If it can’t fire, there’s no pain – analgesia.

There are two kinds of desensitization, homologous means that continued use of one agonist on a receptor makes that receptor less able to respond to that agonist. Heterologous desensitization is when other agonists work on receptors in another part of the tissue.

The heterologous type of desensitization sounds an awful lot like the example above where really cold water on one foot reduces pain in the other foot. So this could be one of the mechanisms of counter irritants.

On the other hand, consider the old example of placing a frog in hot water (video here) – it jumps out due to TRPV1 heat/pain signaling. But if you place a frog in warm water and then heat it slowly, the frog won’t jump out. It will sit there until it’s a frog leg dinner. This is habituation (tolerance) and perhaps homologous desensitization as well. Perhaps capsaicin pain creams act by habituation and counter irritation, nothing says they can’t do both.

Another desensitization/habituation model uses resiniferatoxin injected into the covering of the brain (epidura) to stop neuropathic pain. Remember that resinferitoxin is a strong capsaicin-like molecule, with a score of 108.8 billion Scoville units. It is such a strong agonist that it alone can desensitize TRPV1 in short order - so much that it is termed an anti-nociceptive agonist.

Even more amazing, a Sept. 2012 study used two agonists of TRPV1, capsaicin and MRS1477. Use one or the other and you get hyperalgesia. But administer both at the same time and you get analgesia. In this case, they are hoping that this will be an alternative to opiates in cancer-mediated pain.

This all sounds great – TRPV1 is on small (C type) and some larger (Adelta) pain fibers, and wearing them out can reduce pain. But wouldn’t it just be easier to block them with an antagonist (something that binds but doesn’t activate)? Well, there’s a problem with that – TRPV1 does more than just signal pain.

If you block TRPV1 activity to produce analgesia, you also block its heat-sensing role. Now your body won’t know when it is getting hotter and won’t cool itself down. You end up with hyperthermia and that can kill you. This has been a consistent problem with TRPV1 agonists as analgesics, including if resinferitoxin is given orally – but who would want 108.8 billion SHU in their mouth?

It is the oral TRPV1 antagonists that seem to bring the hyperthermia when trying to treat osteoarthritis. The hyperthermia has been attributed to the action of TRPV1 antagonists in the GI tract. New work shows that activity of TRPV1 is increased (and the expression) in the joint during osteoarthritis, but no increase in expression or activity in the spinal column. Injection of a TRPV1 antagonist into the joint to stop the pain signals from the joint without the hyperthermia.

Wint-O-Green Life Savers are famous for emiting light

when chewed. This is called triboluminescence and also

occurs when you rub quartz together or pull tape from

a roll. The mechanism is through energy release by

mechanical breaking of crystals or bonds. In the case

of life savers, sugar dried to crystal will undergo

triboluminescence, but the light is often in the UV

range. But the oil of wintergreen chemistry converts

the UV to visible light. And now you know.

But there's hope for oral TRPV1 antagonists. New polypeptides called APHC1 and APHC3 show analgesic activity in vivo at reasonable doses (0.01-0.1 mg/kg) by blocking capsaicin, heat, and acid activation and did not cause hyperthermia. They can be used IV or perhaps orally, they don’t need to be injected into a specific joint or into the spinal column.

Some chemicals may be both agonistic and antagonistic for TRPV1. A new paper states that methyl salicylate (oil of wintergreen) activates TRPV1, so it induces a warm feeling, but it also blocks TRPV1 activation by capsaicin, acid, anandamide, and perhaps inflammatory mediators. This means that it can be analgesic, which is why it's the main ingredient in BenGay.

However, the same paper indicates that analgesic activity of methyl salicylate might be due to its TRPV1-independent activity on a different system, the same pain generating system blocked by aspirin (cyclooxygenase). Arguing against this - BenGay is amazingly painful when loaded into a teammate’s underwear or jock. Take my word for it.

Acupuncture is a source of constant argument in science.

Does it really do something or is it all placebo. Recent

(2012-2014) papers are starting to show that it does have

specific physiologic actions. Here is shown the

electroacupuncture. In medical terms, this is equivalent to

TENS (transcutaneous electrical nerve stimulation). In TENS,

the current is passed through the area that is being

affected, but in acupuncture, a remote area may be used,

according to acupuncture charts.

On a completely different front, a 2012 study of electroacupuncture showed that it could reduce the size and frequency of the action potentials from TRPV1 nociceptive neurons. To determine how this might work, the same acupuncture point (st36) was shown in a 2014 paper to block pain by stimulating a pain pathway. If you block the anti-nociceptive TRPV1 channels with 1% capsaicin, then the acupuncture won’t stop the pain. Once again, TRPV1 works in both pain and anti-pain. That’s a confusing exception.

Next week, capsaicin receptors are also used in some other systems, not just heat and pain. Who would have guessed that eating chili peppers could stop but also cause cancer?

Andreev YA, Kozlov SA, Korolkova YV, Dyachenko IA, Bondarenko DA, Skobtsov DI, Murashev AN, Kotova PD, Rogachevskaja OA, Kabanova NV, Kolesnikov SS, & Grishin EV (2013). Polypeptide modulators of TRPV1 produce analgesia without hyperthermia. Marine drugs, 11 (12), 5100-15 PMID: 24351908

Tobaldini G, de Siqueira BA, Lima MM, Tambeli CH, & Fischer L (2014). Ascending nociceptive control contributes to the anti-nociceptive effect of acupuncture in a rat model of acute pain. The journal of pain : official journal of the American Pain Society PMID: 24412800

Lee MG, Huh BK, Choi SS, Lee DK, Lim BG, & Lee M (2012). The effect of epidural resiniferatoxin in the neuropathic pain rat model. Pain physician, 15 (4), 287-96 PMID: 22828682

Kelly S, Chapman RJ, Woodhams S, Sagar DR, Turner J, Burston JJ, Bullock C, Paton K, Huang J, Wong A, McWilliams DF, Okine BN, Barrett DA, Hathway GJ, Walsh DA, & Chapman V (2013). Increased function of pronociceptive TRPV1 at the level of the joint in a rat model of osteoarthritis pain. Annals of the rheumatic diseases PMID: 24152419

Because pain is involved, I am including demonstration links only for triboluminescence.

For more information, see:

Counter irritants –

http://www.racehorseherbal.com/Herbal_Therapies/Liniments/Counterirritation/counterirritation.html

http://www.medicine.ox.ac.uk/bandolier/booth/painpag/topical/topintro.html

http://relieffrompain.net/counterirritants/

http://arthritis.about.com/od/topicalcounterirritants/

http://www.pharmacytimes.com/publications/issue/2010/September2010/TopicalAnalgesics-0910

http://abcnews.go.com/Health/PainArthritis/story?id=5185473

Gout –

http://www.webmd.com/arthritis/tc/gout-topic-overview

http://www.niams.nih.gov/Health_Info/Gout/