Wednesday, May 28, 2014

Cold Receptors Come In From The Cold

Biology concepts – thermosensing, cool sensing, allergy, cross-reactivity, cold allergy, sperm maturation, acrosome reaction, opiate withdrawal

You can be allergic to things that touch your skin – like poison ivy,
things injected, like bee venom, things eaten – like foods, or things
inhaled – like perfumes. But now we need to add something else
to this list – cold? On the right you see a common way to test for
allergy. Anything that produces a wheal and flare reaction
(blanched and raised surrounded by red) is considered positive.
But what if they’re just allergic to the metal needle? 
Allergies can result when your immune system, specifically your mast cells, have an exaggerated response to something that should be innocuous. We have talked about the different kinds of immune hypersensitivity reactions before, but in general, allergy (or atopy, from Greek for out of place) occurs when your body produces a type of antibody (IgE) that recognizes foreign substances and causes your mast cells to release histamine.

Histamine release can lead to itching, watery eyes, runny nose, and even hives (urticaria, from Latin for nettle, see the post on nettle toxins). The IgE is good for helping you learn to avoid poisons and such, but what if your body makes and IgE to something that isn’t dangerous, like peanuts or latex?

Sometimes it isn’t even a case of building an antibody to something that is normally not deemed foreign. Sometimes a peanut molecule just looks enough like some other antigen that an IgE is tricked into binding to the peanut molecule or the banana molecule.

The fruit-latex syndrome is a good example of this. In many cases of people being allergic to latex (Hevea brasiliensis), they also have an allergy to avocados, kiwi fruit, bananas, or chestnuts. The IgE that recognizes the latex hevein protein cross reacts with a beta-glucanase enzyme protein from the fruits.

In the cases of cross-reacting antibodies, there are antibodies to innocuous antigens, your body reacts to them just like they were something dangerous. Histamine release results from IgEs grouping around an allergen and then attaching to a mast cell. If you have encountered this allergen before and have ramped up the number of IgEs that recognize this antigen, the mechanisms can lead to anaphylaxis. This life threatening condition is marked by inflammation that can cut off airways and a lowering of blood pressure that could kill the brain.

Spina bifida patients often develop latex and tropical fruit allergies.
Spina bifida is an incomplete closing of the spinal cord in the fetus
and can lead to severe difficulties in leg movement. It can range from
undetectable to very evident, like in the right image above. Lots of
treatment means lots of chances to develop latex hypersensitivity,
and almost 2/3 of spina bifida patients develop a latex allergy. A 2011
study says that they first develop allergy to latex, and then this cross-
reacts with the fruit. So patients without latex allergy don’t have
to avoid the fruits.
People allergic to nuts or bee stings are forced to carry around injectors of epinephrine just in case their allergies are triggered. The epinephrine constricts blood vessels, increases the heart rate and the amount of blood moved, so your blood pressure won’t drop too far if you take it soon enough. It also dilates the airways and stops inflammation so you can keep breathing. These are all good things.

Like we said, this is how allergies can and sometimes do work. But there are exceptions. Did you know that you can be allergic to cold weather? Yes, I hear you out there, chuckling that you’ve been allergic to shoveling snow for years. But what I’m talking about is a physical allergy – hives, breathing problems, itching, and cough – just because your skin and airways are exposed to cold air.

No – you can’t make an antibody to an environmental condition like cold – at least not as far as I know. But remember that TRPM8 is a cool sensor, stimulated by cold temperatures. What if your body skipped the antibody part and the cold temperature itself stimulated mast cell degranulation (release of histamine granules)? Maybe it does, but whether the cold acts via TRPM8 is another question.

Mast cells (in red) degranulate in response to allergens.
The allergen (1) is recognized and bound by the
appropriate IgE antibodies (2). The end of the antibody
opposite the allergen binding site has a receptor on the
mast cell surface (3). Crosslinking of more than one
surface recpeotr with Ab causes degranulation and
release of inflammatory mediators, like histamine (4)
from the granules usually stored in the cytoplasm (5).
There are only a couple of studies that have looked at TRPM8 and cold-induced urticaria. In 2010, a study using rat mast cells showed that they do express YRPM8 ion channels and that they do release histamine when exposed to cold or methanol (a TRPM8 agonist). The histamine release could be blocked, even at cold temperatures, by treating the cells with a TRPM8 antagonist. Pretty convincing, eh?

But the very next year, another study said it was unlikely that TRPM8 was responsible for cold-induced urticaria. This study used human mast cells and mice. Although they did find TRPM8 channels on the mouse mast cells, they didn’t release histamine in the presence of cold in their experimental model. And the researchers didn’t even find TRPM8 expressed on the human cells. This is a bit unusual, since mice are usually a great model for human physiology.

In mast cells from mice with no TRPM8 channels (TRPM8 knockout mice), the mast cell response to cold was normal, so this study concluded that TRPM8 is not involved in cold urticaria. Confusing, but a good opportunity to cheer the relentlessness of science. Study will continue until something is repeatable and can’t be proved wrong. Maybe it will be you – curing cold allergy might not make you rich, but cold-triggered asthma follows a similar stimulation – and solving that little problem will get you a Nobel Prize and a big fat check.

How about another exception? One important difference between TRPV1 warm/hot sensor and TRPM8 cool/cold sensor is that TRPV1 is often located on pain neurons, while TRPM8 is located on other types of neurons and other cell types. TRPM8 activation is not associated with pain sensation directly, since they don’t help depolarize pain neurons. But there is an exception – your teeth.

The left cartoon shows the dentinal pores and how they have
odontoblast processes in them. If the dentin is expose by
receding gums or by decay, the pores are then exposed. On
the right, different stimuli can cause the fluid in the pores to
move, which then puts strain or stretch on the processes, this
causes shifts in ions and that can cause the neurons to fire. These
neurons only carry one message – pain.
Inside the middle of each tooth is the pulp (in the pulp chamber), made up of a few layers of cells that can make more tooth material (odontoblasts), some blood vessels, and a set of nerves. Odontoblasts make a product called dentin, which is hard, but not as hard as enamel. The enamel on your teeth is not very thick, most of the structure is dentin. As you age, insults to the tooth (like decay), can stimulate the laying down of additional layers of dentin inside the pulp chamber.

The dentin has minute pores that travel out from the middle to the base of the enamel layer. If decay or some other stimulus reaches the pore, processes (like fingers) of the odontoblasts in the pores can react to the stimuli. These then signal the neurons in the pulp. However, the pulp has only pain sensing neurons. So every stimulus that reaches the pulp will be interpreted as pain.

The odontoblasts have TRPV1 channels, TRPM8 channels and TRPA1 channels (we will talk more about these next week). The hydrodynamic theory of tooth pain says that the changes in temperature that reach the odontoblast processes result in pressure changes and this puts mechanical stress (stretch or shear) on the membranes. These then trigger the channels and the signal is passed to the pain neuron.

A 2013 PLoS study says this is partially true. Their results seem to indicate that very cold and very hot stimuli do produce mechanical pressure on the membrane, so TRPV1 and TRPA1 are responsible for mechano-sensitive pain. But they suggest that in the case of TRPM8, cool/cold temperatures trigger the odontoblasts and neuron. The neuron only has one thing to say - pain – so when triggered by TRPM8 signals in the neighboring odontoblast, it responds the only way it knows how. Too bad, but it has spawned a million dollar industry in toothpastes for people with sensitive teeth.

This is a cartoon of the head of a sea urchin sperm, but many
of the concepts apply in humans as well. See all the red
arrows? Those represent the places where calcium flux is
important in maturation and function. And what do TRPV1
and TRPM8 move the best – calcium. The acrosome reaction
actually dissolved the membrane around the acrosome so that
it can more easily enter the ova. This has to be done at a proper
time; TRPM8 activation prevents it from happening too early.
Here’s another TRPM8 function that we will touch on only briefly. We talked about TRPV1 being important in sperm maturation and in entry into the egg. Well, it looks like TRPM8 is involved as well, only in the opposite direction. TRPM8 signaling, according to a 2011 study, TRPM8 activation prevents sperm maturation. This is also important, you need the capacitation and the acrosome reaction to occur at the proper point because they shorten the sperm survival time.

TRPM8 signaling prevents the acrosome reaction, but when the egg is near, a chemical called CRISP4 is released from the egg or parts near there. CRISP4 is a TRPM8 inhibitor. When TRPM8 is inhibited, now TRPV1 can be stimulated to trigger the acrosome reaction.

The interesting part here is that up to the point of CRISP4 release, something is constantly stimulating TRPM8 activity in the sperm cell. I really doubt that there's a cold stimulus way up inside the uterus, so just what is activating TRPM8? We know about lots of endogenous activators of TRPV1, but there has only been one study saying that TRPM8 might have a body-produced agonist, a type of lipid called lysophopholipids. But I think we are missing a bunch of other agonists – maybe you could look for those someday.

OK, here’s the last weird function for TRPM8 today. Would you believe it works in morphine action and withdrawal (when addicted)? Opiates like morphine are analgesic and cold antinociceptive. You take morphine and you don’t sense cold – of course, you won’t sense much of anything else either. For cold, we know how it acts. Opiates cause the internalization of TRPM8 channels on neurons. If there are no exposed channels, they can’t be triggered to allow ions into the neuron.

It goes even further; this isn’t some byproduct or side effect. Menthol is known to create analgesia (one of the reasons they use it in cigarettes). But according to a 2013 paper, if you give naloxone (an opiate blocker) at the same time as menthol – no analgesia. TRPM8 internalization is required for morphine to work.

The term, “cold turkey” is fairly old, first appearing in print
around 1910. It means “without preparation,” but just where
it came from is a matter of question. It might refer to the fact
that cold turkey after Thanksgiving doesn’t need preparation.
It might also be related to “talk turkey, which means to get
down to business. But the way that drug addicts feel cold,
sweat, are pale and have goose bumps – the visual aspect is
not wasted. By the way – who would smoke a cold turkey?
This is important when you are trying to kick a morphine habit. As you stop taking the opiates, TRPM8 quickly relocates to the membrane of the cell and is very easily activated. This causes a cold hypersensitivity and hyperalgesia. People going through withdrawal feel cold because their TRPM8 channels are firing. This is one explanation for calling it, “going cold turkey.” It is uncomfortable and painful, and is one of the main reasons that patients fail detox.

The naloxone that is used to treat morphine addiction binds to the opioid receptor, but doesn’t produce the analgesia. It also allows the TRPM8 to remain externalized, so they don’t have the rebound feeling of cold and pain. Pretty impressive – and now you know how it works.

Next week – TRPM8 is for cold, then there’s the cold that hurts. That is a different receptor, called TRPA1. It makes cold hurt, abut it also saves you from the cold.

Gibbs GM, Orta G, Reddy T, Koppers AJ, Martínez-López P, de la Vega-Beltràn JL, Lo JC, Veldhuis N, Jamsai D, McIntyre P, Darszon A, & O'Bryan MK (2011). Cysteine-rich secretory protein 4 is an inhibitor of transient receptor potential M8 with a role in establishing sperm function. Proceedings of the National Academy of Sciences of the United States of America, 108 (17), 7034-9 PMID: 21482758

Shapovalov G, Gkika D, Devilliers M, Kondratskyi A, Gordienko D, Busserolles J, Bokhobza A, Eschalier A, Skryma R, & Prevarskaya N (2013). Opiates modulate thermosensation by internalizing cold receptor TRPM8. Cell reports, 4 (3), 504-15 PMID: 23911290

Medic N, Desai A, Komarow H, Burch LH, Bandara G, Beaven MA, Metcalfe DD, & Gilfillan AM (2011). Examination of the role of TRPM8 in human mast cell activation and its relevance to the etiology of cold-induced urticaria. Cell calcium, 50 (5), 473-80 PMID: 21906810

Cho Y, Jang Y, Yang YD, Lee CH, Lee Y, & Oh U (2010). TRPM8 mediates cold and menthol allergies associated with mast cell activation. Cell calcium, 48 (4), 202-8 PMID: 20934218

For more information or classroom activities, see:

Cold allergy –

Hydrodynamic theory of tooth pain –

Sperm maturation –

Drug withdrawal –

Wednesday, May 21, 2014

The Cold Cure All

Biology concepts – thermoregulation, TRPM8, TRPV1, heat sensing, cool sensing, vasoconstriction, nasal resistance, viral cold

The common cold. The red nose is from irritation from
tissue and inflammation. The medicines are to treat the
symptoms. Colds are caused by viruses, and we don’t
really have treatments for viruses. What I don’t
understand is the thermometer; adults with colds very
rarely have fever. Kids usually run a fever, but not adults.
So either this is the oldest looking child or he is worried
about something other than a cold.
It sucks when you have a cold – or does it blow? Your nose is stuffed, it’s hard to breathe, you have a cough that won’t stop and seems to do you no good. You’re chilled, but don’t know if you want to feel warmer. Is there anything you can do? One popular treatment might just be a lie.

I have noticed two things that seem to help the stuffiness I feel with a cold. One is exercise – it always seems to open up my nasal passages and make it easier to breathe. The rush of endorphins doesn’t hurt either – I may not be getting better, but I don’t mind the cold as much with a good dose of endogenous opiates running through my veins.

For me, a second short-term nose opener is going outside into the cold weather to shovel snow or chop wood. Why would being cold help a stuffed nose? Ponder that question for a second while I vent on a common misconception. Why do we say we catch a cold? It’s a viral infection, does temperature have anything to do with it at all?

Sure, more people have colds in the winter – but you know from this blog that correlation does not imply causation. Having a cold in winter doesn’t mean that the winter weather had anything to do with catching cold.

Your mother always told you to wear your coat outside or you’d catch your death of cold. Your basketball coach did a hat check after practice to make sure you didn’t leave for home with wet hair on an uncovered head. Were there reasons for this?

Underneath all that winter gear is Randy Parker, Ralphie’s
little brother from A Christmas Story. His mother, in her
time-honored wisdom, didn’t want him to a catch cold by
being cold. The problem was that wrapping him up just
kept the cold from stimulating his metabolism and his
immune system over time. Plus, he found it hard to go
through doorways; he couldn’t put his arms down!
In a word, no. That isn’t to say that there is no effect from cold weather, but it's minimal. A cold is caused by a great many viruses. These viruses are transmitted from person to person via respiratory droplets and on surfaces. The closer people pack together, the more likely the virus will be transmitted from one person to another.

When are people most often closer to other people? The winter – people spend more time inside, heating systems recycle the air; it’s the season for sharing. The cold weather encourages people to stay inside, where they are more likely to receive a viral gift from someone else.

So the cold does play a role, including a slight decrease in immune function due to changes in blood flow, and the fact that cold air holds less moisture, so your mucosal membranes dry out and are a bit more susceptible to being invaded by a virus. But cold is by no means the main culprit, so I propose a letter writing campaign to rename the cold – maybe you could catch a crowd, or a doorknob, or maybe we could just call it Dennis.

O.K., now that that issue is resolved, back to the question of how cool air and exercise can help you breathe better when you have a “cold.” The key is a concept called nasal resistance.

Nasal resistance is the main way to slow air entering the
lungs. Being slower and higher pressure keeps the lungs
from overinflating or collapsing. The nasal vestibule (1)
squeezes together as you inhale, the smaller space prevents
too much air from entering. Inside is the vestibule is the
valve (3), the narrowest region. The turbinates (5-8, superior,
middle, and inferior) can expand slightly to decrease the
nasal volume. There are small muscles (alae nasi) in the
vestibule which can contract to decrease resistance as well.
Paralysis of these muscles leads to collapse of the naostrils.
Basically, the more stuff in the way of the air, the higher
the resistance.
Nasal resistance is an important way to keep our lungs from popping or collapsing. If the resistance to air entering the lungs is too high, the lungs will collapse like a balloon with a hole in it. If the resistance is too low, you could overinflate and pop them – also like a balloon. The nose, believe it or not, is responsible for about 50% of the resistance to air entering the lungs (see picture).

Having a cold increases mucous production (trying to catch viral particles before they reach your cells). A cold virus infection also puffs up the nasal tissues due to immune inflammation reactions. These responses increase nasal resistance and decrease airflow. It is much harder to get the same volume of air into your lungs through your nose. This observation won’t win me the Nobel Prize; we’ve all experienced it.

Exercise reduces nasal resistance through stimulation of the sympathetic nervous system. Hard physical work is a lot like the fight or flight response. Your body vasoconstricts vessels in the periphery so that more blood can go to the big muscles. You also need more oxygen, so the alae nasi muscles in your nose relax and the airways get bigger. Both of these actions decrease nasal resistance and increase airflow to the lungs. During a cold this is helpful since your ventilatory spaces are clogged with snot.

On the other hand, my sojourns into the brutal winter are against the literature. Cold air is supposed to increase nasal resistance. Cold air is bad for the lungs – it saps heat from the rest of the body. Therefore, the nose anatomy functions to warm the air. When cold air enters and triggers the TRPM8 cool sensors (there’s our first reference to the topic we have been discussing), the alae nasi muscles contract and the blood vessels in the nasal mucosa dilate. This swells the internal nasal tissues, increasing the surface area and thereby transferring more heat to the air before it reaches the lungs.

Rebreathing is a good way to reduce nasal resistance. Just
hold your breath for a while or breathe into a paper bag.
Increased CO2 in blood brings vasoconstriction, so tissues will
shrink and breathing will be easier. The same goes for changing
from a supine (lying) position to standing or sitting. The change
in filling of sinus vessels and nasal vessels will shrink tissues as
well and you will breathe easier for while.
This should increase the nasal resistance, which will be high anyway due to all the mucous production going on. The cold air should make it harder to breathe, but for me it clears my nose when I have a cold. Where’s the disconnect? In fact, what I have always attributed to the cold air is probably a result of what I was doing, not where I was doing it.

I go out in the cold to chop wood or shovel snow - I have a tendency to attack my work, so these activities become exercise. Upon reflection, I now realize that it was once again exercise that was reducing my nasal resistance and allowing me to breathe more normally, not the act of going out into the cold. I’m caught in my own correlation-causation trap; there were other factors that I had failed to take into account. I had too many variables in my experiment! I never considered going out into the snow and not working hard.

Now let’s consider another cold and cough treatment, Vicks VapoRub. The active ingredients in this concoction are menthol and camphor. We have talked recently about how menthol is a TRPM8 agonist (so mints make everything seem colder), and that camphor is an agonist for both TRPM8 and TRPV1, so it can induce feelings of warmth or cool, depending on the concentration and placement.

You rub the Vicks on your chest when you have a cold. The camphor stimulates TRPV1 and makes your trunk feel warm. The menthol vapors rise and your breathe some in, they make your nose less stuffy. Or so it seems.

Here is a late 1950’s ad for Vick’s VapoRub. Which child would
you rather have? A simple greasy rub on the chest and all the
problems are solved. If you can read the headline, it seems that
atom tracing shows that the vapors get into the lungs. One – is
there anything more 1950’s than talking about atoms? Two – we
now know that Vicks works in the nose, not the lungs.
The commercials for Vicks VapoRub show the menthol/camphor fumes entering the nose, and the cold sufferer then relaxing and breathing more easily - convincing stuff, visually. It does seem that it's easier to breathe. But it’s all a trick our brains are playing on us.  Remember that TRPM8 senses cool/cold temperature differences.

When you breathe in quickly and deeply, the rushing air is colder than the air that was just hanging out in your nose. This triggers the TRPM8 sensor, and your brain interprets it as a lot of air rushing up your nose and to your lungs – decoding the signal means that you are breathing well and deeply.

Now switch to the situation where you have a cold and can’t bring in air through your nose. The menthol/camphor of the Vicks VapoRub penetrates your nose and stimulates the TRPM8 channels there. Your brain interprets this data just as if cool air was rushing over the TRPM8 channels. It concludes that you are breathing well. You think you are breathing easier, but no actual change has occurred in nasal resistance! A 2008 study showed this to be the case. Bad brain! We can’t fool Mother Nature, but apparently she fools us all the time.

One thing that is true about the VapoRub is that it can calm a cough. Menthol, in particular, is excellent in its anti-tussive capability. Tussive is from the Latin tussis, meaning a cough, although I’ve never heard anyone say, “I have a very bad tussis today.” Likewise, does this mean that when you are coughing, you are really tussing? Do you need to “tuss up” that ten dollars you owe me? (Yes, I am aware of the snickering from those of you of the Cornish persuasion - look it up.)

Developed in 1865, Lofthouse was looking to help fisherman
with their seasonal colds. The lozenge contains menthol and
eucalyptus, just like Halls, but much earlier. The fisherman
liked them so much they called them their friend. Hence the
name; they’re still sold today. Menthol drops can calm a cough,
but menthol is also slightly analgesic, so some throat pain can
be reduced by them as well.
Nevertheless, recent studies have shown that menthol, through its activation of TRPM8, does have a calming effect on a cough. We knew this was so, Halls mentho-lyptus (for menthol and eucalyptus – another TRPM8 agonist) drops have been around since 1930’s, with other brands like Smith Brothers and Pines having predated Halls by some 80 years. But the 2012 study showed that menthol’s action on cough was through TRPM8 action.

A 2013 study went further. It assessed the anti-tussive action of menthol in guinea pigs and showed that the effect on TRPM8 was only effective when it was in vapor form and when it was applied to the nasal passages. Menthol on trachea or throat TRPM8 had no effect on cough. So – when you use Halls cough drops, it's the vapors from the dissolving drops that go up your nose and help stop the cough – don’t chew on them and swallow! You put them in your mouth, but they don’t act there. But don’t stuff them up your nose either – did I need to say that?

We have considered TRPM8 in thermoregulation, nasal resistance, and cough. Next week, let’s show some funky functions for cold receptors – like how they can stop cancer or how they screw up opiate addiction withdrawal.

Lindemann J, Tsakiropoulou E, Scheithauer MO, Konstantinidis I, & Wiesmiller KM (2008). Impact of menthol inhalation on nasal mucosal temperature and nasal patency. American journal of rhinology, 22 (4), 402-5 PMID: 18702906

Buday T, Brozmanova M, Biringerova Z, Gavliakova S, Poliacek I, Calkovsky V, Shetthalli MV, & Plevkova J (2012). Modulation of cough response by sensory inputs from the nose - role of trigeminal TRPA1 versus TRPM8 channels. Cough (London, England), 8 (1) PMID: 23199233

Plevkova J, Kollarik M, Poliacek I, Brozmanova M, Surdenikova L, Tatar M, Mori N, & Canning BJ (2013). The role of trigeminal nasal TRPM8-expressing afferent neurons in the antitussive effects of menthol. Journal of applied physiology (Bethesda, Md. : 1985), 115 (2), 268-74 PMID: 23640596

For more information or classroom activities, see:

Cold viruses –

nasal resistance –

menthol and cough –

Wednesday, May 14, 2014

Cold Keeps You Warm

Biology concepts – thermoregulation, TRPM8, vasoconstriction, brown adipose tissue, agonists/antagonists

Pep-O Mint was the first Lifesaver flavor, invented
in 1912. This was followed quickly by the Lifesaver
car in 1918. Built on a Dodge truck chassis, the
important word was dodge, since the car didn’t have
a windshield and the driver had to stick his head out
the side window to see ahead.
Let’s do a demonstration. Borrow a peppermint lifesaver from a friend (well, not borrow really - you’re not going to give it back). Place it between your teeth, so you can close your lips around it and suck air in through the hole (this will be the control for our experiment). 

Now put the candy in your mouth like normal and suck on it for a minute or two – don’t chew it up.  Swallow to get the saliva out of your mouth and take out the candy. Now take in a long slow breath of air. How does it feel? Did the room get colder in the last two minutes?

If you are like most people, the air feels colder in your mouth now that you've eaten menthol (peppermint). Just like capsaicin can make hot things seem hotter via TRPV1, the cold sensing channel we talked about last week, TRPM8, can make room temperature air seem colder.

The TRPM8 cold sensing ion channel is important for keeping our body temperature in a normal range. Just like TRPV1 senses when we are too warm and initiates cooling mechanisms, TRPM8 tells us we are cold and institutes procedures to make us warmer. One way is to stimulate vasoconstriction, so less heat is lost from the blood through our skin. I’m sure you have noticed that your skin is paler when you are out in the cold. This is from vasoconstriction limiting the amount of blood moving into the surface vessels.

When your core temperature varies from your skin
temperature by too much, TRPM8 will institute
heat conserving and/or generating mechanisms.
For heat conservation, more of the blood (left
cartoon) that goes the skin can be shunted through
the capillaries before it gets to the surface. This
helps reduce the amount of heat loss via radiation
of the heat in the blood. On the right is the effect of
the arrector pili muscles. When cold, they contract
and raise the hairs, trap air, and therefore trap body
heat against the skin so less heat is loss. The
contraction also mounds up the skin – goosebumps.

TRPM8 can also stimulate shivering and the burning of fat to generate warmth. Through the sensations and reactions of TRPV1 and TRPM8, animals learn to maintain a more or less constant body temperature, seeking out temperatures that are good for physiology and avoiding temperatures that would change their core temperature by too much. This was shown in a series of studies described in a 2013 paper, where mice without temperature sensing receptors TRPV1 and/or TRPM8 would not avoid hot or cold temperatures and were prone to hyperthermia and or hypothermia.

So how does the TRPM8 channel sense cold? We saw that with TRPV1 the heat induced a conformation change that caused the channel to open and calcium to flow in and start a neural action potential. Could cold induce a conformation change as well? Maybe. What was seen in a 2011 study was that TRPM8 neurons started firing when the temperature dropped to 28.4˚C (83 ˚F). As the temperature dropped, the neurons would fire more and more strongly, so it could act as a thermostat.

When the temperature dropped severely (to 10˚C) the core temperature changed little, but skin temperature dropped considerably. The TRPM8 thermostat was targeted to keeping the organs and brain warm, not the skin.  It accomplishes this by diverting heat via the blood away from the skin. A 2012 study showed that TRPM8 antagonists brought a systemic hypothermia, but repeated use of the antagonist reduced the magnitude of the temperature drop – so unlike most TRPV1antagonists (that bring bad hyperthermia), TRPM8 antagonists might be helpful in medicine.

We don’t yet know how cold activates TRPM8. In
warmth, the channel is closed. With cooler
temperature or menthol (M), the channel is open,
but we don’t know if cold achieves this by a
conformation change. If really cold, the regulatory
proteins break away and the channel can’t work.
So TRPM8 is active in a range of temperatures.
But this still doesn’t answer the questions as to how TRPM8 can detect cool temperatures. It may or may not be a conformation change, but what does occur is an alteration in the apparent temperature threshold of the neuron. The cold temperature should inhibit firing (cold slows metabolism and chemical activity), but here it increases the metabolism and biochemical activities. TRPM8 makes the neuron seem warmer so that the firing is easier, and this transfers information that the area is in fact colder! That’s an exception.

However it manages the feat, TRPM8 is important for keeping mammals warm. It might even help you lose weight. Chronic cold stimulates TRPM8 all the time, and this ramps up your heat production. A 2012 study showed that for mice, chronic cold could actually prevent them from becoming obese.

Heat production takes energy, and burning more energy helps you lose weight. But there is an important balancing act at work here. Our fat also protects us against losing too much heat in the cold. Look at whales, they have a layer of blubber all over their body to insulate them from the cold water. It has been shown that people with an even layer of fat all over their body make good cold weather swimmers, like Lynne Cox, who swam from her perfectly good boat to the shores of Antarctica and across the Bering Strait in 4˚C (40˚F) water.

On the other hand, bactrian camels keep their fat limited to two humps (one for dromedaries) in order to prevent against having too much insulation in their desert environment. Camels need to be able to dissipate lots of heat. And no, the humps aren’t for storing water! Remember we said that one of the great things about fat is that you can store lots of energy in a small space precisely because can be stored without water.

In brown adipose tissue, there are ATP synthase
proteins (for making ATP) and uncoupling proteins
(for generating heat) in the inner mitochondrial
membrane. When cold, the number of UCP proteins is
increased, as is the number of mitochondria. About
65% of the energy of the proto gradient formed by the
respiratory chain can be converted to ATP by the ATP
synthase. But using the UCP to allow protons back in
means that 100% of the energy is changed to
heat, no ATP is made.
It seems that TRPM8 can stimulate the burning of fat to produce heat. We talked about this before with capsaicin as well. Brown adipose tissue has more mitochondria and more of a protein called UCP1 (uncoupling protein). UCP separates mitochondrial energy burning from ATP production; all the energy goes to making heat.

A 2014 study showed that chronic cold makes brown fat AND white fat upregulate UCP and generate more mitochondria. It makes white fat more like brown fat and this means that more fat is burned. In mice, this chronic cold is enough to keep them from becoming obese, even on a high glucose diet. So if you want to stay skinny, turn your thermostat way down all year round.

The study that showed that chronic cold kept mice from getting fat wasn’t as cruel as it may sound. They didn’t keep the mice at cold temperature all the time, they used a chemical that could mimic the cold and make the TRPM8 channels fire all the time. What did they use? Menthol.

This is a good place to point out the similar exception for TRPV1 and TRPM8. They are both proteins that can be activated by both environmental factors and by chemicals. We saw that TRPV1 is activated by capsaicin and other chemicals. The opening of the channel and firing of the neurons in response to these chemicals was interpreted exactly the same as if the neurons were exposed to damaging heat.

There are many agonists for TRPM8, similar to TRPV1.
In fact, some things that activate TRPM8 also activate
TRPV1. An interesting one is the synthetic agonist called
icilin. It is thousands of time more active on TRPM8 than
cool temperatures. However, it binds to TRPM8 in a
completely different way as compared to menthol and
cool temperatures.
With TRPM8, menthol (in mints) and some other chemicals open the ion channels just as cool/cold temperatures would, and our brains trick us into thinking the mouth or skin is colder than it really is. That’s what is behind our lifesaver trick at the beginning of the post. The air wasn’t any colder; your brain makes you think it was.

Menthol is a terpene alkaloid contained in plants of the genus Mentha (mint, from the Greek mintha). This genus includes 25 species of aromatic herbs, such as peppermint, spearmint, and pennyroyals. Most mints can be and are used in making foods and drinks, but the pennyroyals also contain toxic compounds that will induce liver failure and kill you.

At low concentrations in the mouth or on skin, menthol produces a pleasant cooling sensation, but higher concentrations produce burning, irritation and pain (this has to do with how it activates TRPV1, TRPV3, TRPM8, and TRPA1, depending on the concentration).

In the oral cavity, a small amount of menthol actually desensitizes TRPV1 activation by heat and capsaicin, so chili peppers might not seem so spicy. Biochemical evidence shows that menthol sparks a release of glutamate from neurons. But an increase in glutamate neurotransmitter can actually stop the type C nociceptive neurons from firing (an inhibitory neurotransmitter in this case).

Pennyroyal is a member of the mentha genus (left). It, like
many plants (right image is orange mint), has been used in
medicine. It can be ground and drunk with water to settle a
sick tummy or to induce perspiration. However, pennyroyal
has to be used carefully. In addition to menthol, it contains a
chemical called pulegone. Too much pulegone and here
come the seizures, organ failure and death. Don’t confuse
the two mints.

At this same time, menthol (or other TRPM8 agonists) will sensitize TRPM8 receptors, the combination of these two results means that sucking in air after a wintergreen or peppermint candy will make the air seem colder, but might also make a hot cup of coffee seem cold as well.

I think the only way to resolve these ideas is to start a controlled experiment. What do you predict would happen if you froze a chili pepper and then took a bite? How about eating peppermint laced with capsaicin, or a strong peppermint flavored tea that has been heated to near boiling? Who will win out, TRPM8 or TRPV1?

It may not be so easy to figure out. The agonists and antagonists of the TRPs can have effects on multiple receptors and the effects can be different at different concentrations. Menthol sensitizes TRPM8, but if the temperature is above 37˚ C (98˚ F) it actually makes TRPV3, a heat sensor, more active (2006).

Camphor comes from some species of laurel trees (left), as well as
from some herbs of the mint family, like camphor basil (right).
Dried rosemary leaves are up to 20% camphor. Camphor has
been used for many things, including as a flavoring in asian
sweets, an analgesic, an insect repellent, and a rust proofing
agent. I find it hard to rectify those uses with one another.
Take oil of wintergreen in BenGay for example. We showed that it was a TRPV1 agonist, so it could induce analgesia by counter irritation. But it is also a TRPM8 agonist. In the oral cavity at lower concentrations than used in BenGay, it activates TRPM8 and desensitizes TRPV1. The lifesaver trick will work with peppermint, spearmint, and wintergreen flavors; they all activate TRPM8.  

And then there’s camphor. Like menthol, camphor is terpenoid chemical. Camphor can make things seem cool (by activating and sensitizing TRPM8), but it’s more complicated. It actually potentiates both heat and cold sensations. A 2013 study shows that it can sensitize or potentiate TRPV1 (painful hot) and TRPM8 (non-painful cool). Camphor can even activate the noxious cold sensor TRPA1 that we will talk about in a couple of posts. This means that it can be analgesic or painful, warming and cooling.

It becomes even more confusing when you realize that camphor activates TRPM8, just like menthol, but can inhibit the activation of TRPM8 by menthol. Weird, right? Well, consider this – Vick's VapoRub contains menthol and camphor as its active ingredients. Next week, we'll investigate how they can work together to open your nose and make you feel both warm and fuzzy while they cool and invigorate you at the same time.

Pogorzala LA, Mishra SK, & Hoon MA (2013). The cellular code for mammalian thermosensation. The Journal of neuroscience : the official journal of the Society for Neuroscience, 33 (13), 5533-41 PMID: 23536068

Rossato M, Granzotto M, Macchi V, Porzionato A, Petrelli L, Calcagno A, Vencato J, De Stefani D, Silvestrin V, Rizzuto R, Bassetto F, De Caro R, & Vettor R (2014). Human white adipocytes express the cold receptor TRPM8 which activation induces UCP1 expression, mitochondrial activation and heat production. Molecular and cellular endocrinology, 383 (1-2), 137-46 PMID: 24342393

Ma S, Yu H, Zhao Z, Luo Z, Chen J, Ni Y, Jin R, Ma L, Wang P, Zhu Z, Li L, Zhong J, Liu D, Nilius B, & Zhu Z (2012). Activation of the cold-sensing TRPM8 channel triggers UCP1-dependent thermogenesis and prevents obesity. Journal of molecular cell biology, 4 (2), 88-96 PMID: 22241835

Selescu T, Ciobanu AC, Dobre C, Reid G, & Babes A (2013). Camphor activates and sensitizes transient receptor potential melastatin 8 (TRPM8) to cooling and icilin. Chemical senses, 38 (7), 563-75 PMID: 23828908

For more information or classroom activities, see:

Thermoregulation –