Wednesday, April 30, 2014

Capsaicin Receptors – Matters Of Life And Death

Biology concepts – cochlear amplification, capsaicin, tinnitus, neural plasticity, long term depression, sperm capacitation, acrosome reaction, apoptosis, reactive oxygen species, cancer


On the left are the Red Hot Chili Peppers. I am most interested in the
member on the right – Flea. Acknowledged as the second best rock
bassist of all time (right behind John Entwhistle), he gathers as much
or more attention for his name choice and his seeming inability to
wear a shirt. On the right is Donnie Thornberry, of the Wild
Thornberrys. He was a feral child raised by animals in Africa. His
character is voiced by Flea – ahhh, it’s starting to make sense.
I have never been a fan of the Red Hot Chili Peppers (sometimes known as RHCP). The music is nails on a chalkboard to me, and their bassist is named for a plague-carrying arthropod. But in the interest of fair play, I looked deeper. In late 2013, lead singer Anthony Kiedis and “Flea” produced a film intended to honor the life of jazz pianist Joe Albany. Add to this that Kiedis acted in an ABC After School Special when he was a teenager, and I now see that the Chili Peppers are more than just bad music.

I use this story to introduce today’s topic - there is so much more to the capsaicin in chili peppers than just heat and pain. We learned last week how capsaicin and other TRPV1 agonists can combat obesity and damage from stroke, but these abilities were related to the TRPV1 function in thermoregulation. Today let’s look at some functions that are so far outside normal TRPV1/capsaicin function that they can be considered exceptional – like jazz piano from the RHCP.

Hearing
We have explained the mechanism of hair cell action in hearing before. The inner hair cells are bent by the sound wave in the cochlea and converted to a neural impulse to be detected as sound. The outer hair cells work to amplify the wave so that the sound is louder and can register a signal in the inner hair cells.

Scientists know that TRPV1 is expressed on the inner and outer hair cells, and works in the cochlear amplification system particularly. However, we don’t know its exact function(s) there. On the down side, we know that TRPV1 must work in a narrow range, because drugs and agonists that excite TRPV1 can lead to hearing problems.

Acoustic injury (acute or chronic loud noise), gentimicin (an antibiotic), and cisplatin (a cancer drug) can all cause hearing damage. A 2009 study of gentimicin damage to the cochlea showed that it increased TRPV1 expression in the outer hair cells. A 2008 study of cisplatin showed that its damage to the hair cells could be suppressed if you decreased TRPV1 activity.


Most often people think of tinnitus as a ringing in the
ears, but it doesn’t have to be. It can be clicking, buzzing,
hissing, or roaring sounds as well. If you hear voices –
well, that’s something completely different. Also, tinnitus
is usually characterized as a neural transduction issue,
often from damage to the hair cells, so only you hear it.
But there are problems in blood vessels, muscles, or
bones (TMJ) near the ear that produce soucnds other
can hear. This is called objective tinnitus.
A 2013 study indicates that TRPV1 is important for the uptake of the drugs into the hair cells, and then they do their damage. An older study shows that even without drugs to be taken up, increased TRPV1 expression is the cause of acoustic injury tinnitus (ringing) and hearing loss. Another paper showed that capsaicin itself blocks the action potential in the outer hair cells and dampens the cochlear amplification system. Why anyone would stick a chili pepper in his ear is beyond me.

So we see that a little TRPV1 activity is good, but too much is bad. This is not to say that eating a lot of peppers will harm your hearing; dietary capsaicin never gets to your cochlea unless you’re a horribly messy eater. We will return to this idea with TRPV1 and capsaicin; it can be an angel or a devil.

Memory and learning
Neural plasticity (the ability to change neural connections, building some, losing others) is important in learning and memory. We have discussed long-term potentiation (LTP) in terms of memory, where a reinforcement of action potentials between two neurons or in a pathway lead to a strong connection and a new item learned or remembered.

The flip side is long-term depression (LTD). This mechanism is what allows you to weaken and lose connections between neurons that aren’t being reinforced. You can’t learn something as well unless you keep reinventing the connections, including losing some. However, don’t get the idea that you have to forget some thing in order to remember something new, it is much more complicated than that.


This is a good carton because it helps me point out a
dichotomy. Many people say that your brain is a muscle,
if you don’t exercise it, you will lose it. This is like the
atrophy of your muscles if you don’t use them. But we are
reading in this post that losing some neural connections
is important for learning. Long term depression is similar
to not using your brain. Those connections that are not
reinforced are lost – but here it is a good thing,
a necessary thing.
TRPV1 acts in LTD. Without TRPV1 activity, all connections could continue to be strengthened, and a ceiling level of excitability would be reached. Then you would have trouble adding new information.

TRPV1 acts at the level of the hippocampus (important for memory) through an endocannabinoid called anandamide. We already know that anandamide is a TRPV1 agonist. A 2008 study indicated that there was no LTD activity in mice that had no TRPV1 channels. This study found that TRPV1 was “necessary and sufficient” for LTD, meaning that it was needed and only it was needed for the effect.

Another 2008 study points out how this is important for us. Stress brings too much LTD and not enough LTP. When you’re stressed it’s harder to learn new things and easier to lose established knowledge. Amazingly, TRPV1 agonists like capsaicin can regulate this in stress. Stress + TRPV1 agonists led to reduced LTD and improved LTP. It seems that capsaicin and other TRPV1 agonists regulate synaptic plasticity in both directions, keeping us on balance and always learning.

Reproduction
Put simply, without capsaicin channels none of us are here to read this great stuff. TRPV1 activity, through various agonists, is important for sperm motility and fertilization of the egg. The sperm meets the egg and here we go - is that the whole picture? Nope. It takes a lot to arrange their introduction, and some of it involves TRPV1 channels.

You ever wonder why the testicles are kept outside the trunk of the body? It’s to keep the sperm cool – heat kills sperm. TRPV1 receptors sense high heat and institutes local reactions to cool the sperm. A 2008 study showed that TRPV1 knockout mice had much higher levels of sperm death in the testicle when the temperature was raised. It didn’t make the mice sterile, but it was close.


On left is a cartoon of sperm capacitation, making it ready tofertilize travel to the egg. The loss of the cholesterol coat isimportant for increasing motility and for finishing the signals that point out whee the egg is. The acrosome is also prepared here, but there are more acrosome changes in the right image showing the
acrosome reaction. Here, the spem head becomes a weapon, dissolving the outer layers of the egg and stabbing it for entry.
So TRPV1 is important for sperm even before they start their journey. The first thing that happens after the sperm enters the female reproductive tract is that they undergo a process called capacitation. These changes make the sperm ready to bind to the egg. TRPV1 and its agonist anandamide (same as in LTD) are important late in capacitation. They prepare the sperm for transfer of the DNA to the egg.

TRPV1 and endocannabinoid receptors are also responsible for the increase in sperm motility after capacitation. The sperm swim toward the egg, but how do they know where she is? There are many signals, including osmolarity differences, hormones, temperature changes, pH changes, and liquid currents. We have seen that several of these (pH, temp., hormones, osmosis) are sensed by TRPV1 channels.

It’s been shown that TRPV1 on the sperm post-acrosomal and head regions are important for sperm motility. A 2013 study showed that inhibiting TRPV1 slowed down fish sperm. And in a 2013 study of infertile men, the motility of the sperm in men with low levels of anandamide was decreased.

Strangely, even though capacitated sperm are thermotactic (swim toward higher temperatures), this is one time where it DOESN’T involve TRPV1/capsaicin receptors. It still involves calcium, but the flux is through a different receptor system.

Once the sperm finds the egg, it undergoes the acrosome reaction. This makes it possible for the sperm - only one mind you - to enter the egg. A 2009 study indicated that anandamide and TRPV1 are crucial for the acrosome reaction as well.

From inside the male, to inside the female, to inside the egg, capsaicin receptors are crucial for making tiny humans. It doesn’t get more important than that. What I don’t know is if eating peppers affects any of this. Would a habanero make you more fertile or less?


The PSA test (prostate specific antigen) can be used
for monitoring treatment of prostate disease as well
as for diagnosis of prostate cancer in conjunction with
other tests. The tests measure s aprotein produced by
the prostate that is often elevated in those with prostate
cancer. PSA can be free or bound to a protein. You want
the free:bound ratio to be high, and low ratio is more
indicitive of cancer. However, there are noncancerous
reasons for an elevated PSA, so don’t go nuts if your
levels ar higher than normal (score of 4 ng/ml).
Cancer
In cancer we see the dual nature of TRPV1 again. In some cases it prevents or stops cancer, while in others, capsaicin might contribute to cancer.

Prostate cancer has been studied a lot with respect to TRPV1. Capsaicin, in particular, has an effect on prostatic cancer cells. It kills the cells and reduces tumor size in mice. However, in humans, little work as been done. One epidemiology study found that Nigerians that eat more peppers have a lower rate and later onset of prostate cancer.

In addition, a single study followed a man whose prostate cancer returned after several disease free years. His PSA value (prostate specific antigen, a sign of cancerous growth) started to double weekly, so he started taking 2 ml habanero sauce once or twice week. His PSA value increases slowed by half. He stopped and they sped up. He started daily habanero sauce and the PSA dropped. Sounds like a treatment to me.

The mechanisms of capsaicin action on prostate cancer cells has been determined in in vitro work. They are both TRPV1 dependent and TRPV1 independent. (studies) Capsaicin increases ROS, induces apoptosis, destabilizes membranes, throws off ion balances…basically everything capsaicin stops when used for protective hypothermia (from last week).

It isn’t just prostate cancer where capsaicin may help. In a mouse model of lung cancer, giving capsaicin prevent the lung tumors from forming by increasing apoptosis in the cancerous cells. Whether this is TRPV1 dependent or independent was not discussed.

But there is a darker side to capsaicin and TRPV1 in prostate cancer. The agonists have a small window of effectiveness. Low levels actually promote cancer growth, but higher levels cause cell cycle arrest and apoptosis. In one study, 10 ┬ÁM capsaicin actually created prostate tumors in mice.


Where do you come down on the question of whether
peppers are good for you or not? Do they cause cancer
or prevent it? Do they cause pain or prevent it? I think
that as with most things, moderation is the key. Too
much of anything is bad for you, except maybe this blog.
One thing I do know; we have sharp canines for a reason -
we were meant to eat meat. This statement is why
she has that look on her face.
The bad news continues... maybe. Several studies have linked capsaicin to stomach, oral cavity, and gall bladder cancer. One epidemiologic study showed that US counties with higher Mexican American, Cajun, and Creole populations have higher rates of these cancers, and it just so happens that these are the folks that eat more chili peppers. Some skin cancers are also associated with capsaicin. This is a bit disconcerting considering the number of topical capsaicin pain creams on the market.

In squamous cell skin cancer, capsaicin can sometimes stunt the growth of tumors via apoptosis, but the mechanism seems to be TRPV1 independent, since a TRPV1 antagonist brings the same effect. The take home message is that TRPV1/capsaicin effects may be diet, individual, cancer specific. This will make it hard to develop therapeutics.

Next week - now its time to start talking about cool receptors. Like how the cigarette companies use "cool" to sell smokes.


Lee JH, Park C, Kim SJ, Kim HJ, Oh GS, Shen A, So HS, & Park R (2013). Different uptake of gentamicin through TRPV1 and TRPV4 channels determines cochlear hair cell vulnerability. Experimental & molecular medicine, 45 PMID: 23470714

Amoako AA, Marczylo TH, Marczylo EL, Elson J, Willets JM, Taylor AH, & Konje JC (2013). Anandamide modulates human sperm motility: implications for men with asthenozoospermia and oligoasthenoteratozoospermia. Human reproduction (Oxford, England), 28 (8), 2058-66 PMID: 23697839

Gonzales CB, Kirma NB, De La Chapa JJ, Chen R, Henry MA, Luo S, & Hargreaves KM (2014). Vanilloids induce oral cancer apoptosis independent of TRPV1. Oral oncology PMID: 24434067

Anandakumar P, Kamaraj S, Jagan S, Ramakrishnan G, & Devaki T (2013). Capsaicin provokes apoptosis and restricts benzo(a)pyrene induced lung tumorigenesis in Swiss albino mice. International immunopharmacology, 17 (2), 254-9 PMID: 23747734


For more information or classroom activities, see:

Cochlear amplification –

Tinnitus –

Sperm capacitation –

Acrosome reaction –

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 hotTomorrow 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 –

Protective hypothermia -



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 17th 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 –

Gout –

Acupuncture –

Methyl salicylate triboluminescence –