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 –

http://beyondcoldwaterbootcamp.com/en/thermo-regulation

http://www.unm.edu/~lkravitz/Article%20folder/thermoregulation.html

http://dwb.unl.edu/teacher/nsf/c01/c01links/www.science.mcmaster.ca/biology/4s03/thermoregulation.html

https://worldofbiology.wikispaces.com/biol+2+classroom+presentations

http://www.pbslearningmedia.org/resource/lsps07.sci.life.reg.heatexchange/blood-flow-and-thermoregulation/

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CDwQFjAE&url=http%3A%2F%2Fbio.classes.ucsc.edu%2Fbio131%2FThometz%2520Website%2F9%2520-%2520Thermoregulation%2520Lab%25202012.pdf&ei=zSIzU6SuFqPEyQGZ4oD4CA&usg=AFQjCNERS9ye-sPF7qZM-dJHUsFPQ031Tw&sig2=xLwl_fcQ3rIxKcZiLictmw

http://www.life.umd.edu/classroom/bsci338m/Lectures/Temperature.html

https://www.khanacademy.org/science/mcat/organ-systems/muscular-system/v/thermoregulation-by-muscles

https://www.youtube.com/watch?v=kg7QdpOug2Y

http://www.nsta.org/publications/news/story.aspx?id=53687

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=16&ved=0CEgQFjAFOAo&url=http%3A%2F%2Fwww.tarleton.edu%2F~jblevins%2Fexercise%2520physiology%2FSpring%252009%2Fpowers7_ppt_Chap12%2520%255BRead-Only%255D%2520%255BCompatibility%2520Mode%255D.pdf&ei=HSMzU-ToHaSHygGerwE&usg=AFQjCNHxnlMTdNoOKd1p9lJrSyv6yew6Jg&sig2=fsvfEpvq9j0LTxOVaSYQwQ

http://www.studyblue.com/notes/note/n/ch-40-hwpdf/file/9213703

http://vce-unit1and2biology.wikispaces.com/thermoregulation

http://en.wikioffuture.org/Dissertation_Research:_Olfactory_Modulation_of_Thermoregulation_and_Flight_in_Moths

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatreg.html

https://www.youtube.com/watch?v=TSUCdLkI474

http://people.seas.harvard.edu/~jones/cscie129/pages/health/thermreg.htm

https://www.stanford.edu/group/stanfordbirds/text/essays/Temperature_Regulation.html

https://www.mdc-berlin.de/26954638/de/research/research_teams/temperature_detection_and_thermoregulation/Research?cntx=40550329

Everybody Wants To Be Cool

Biology concepts – TRPM8 cold sensor, menthol, evolution, cold pleasure

The old ads for menthol cigarettes are fascinating, from a biology
point of view. The “cool” and the “refreshing” aspects were reflected
by using spring and summer outdoor pictures, most often with lots of
cool water. At the end of today’s post, we see why this was so. We
also see an African American, since menthol cigarettes were targeted
much more strongly urban African Americans. They were newer
smokers and in typically hotter environments, so the coldness and
soothing abilities of the menthol were great selling points.

In 1924, Lloyd “Spud” Hughes patented the menthol cigarette. Not a big deal in the beginning, Hughes sold his patent to a cigarette manufacturer who marketed them as Spud cigarettes in 1927. They became the fifth largest seller, although there still wasn’t much in the way of profit. Kool cigarettes came along in 1933 and advertised the menthol casket nails as “soothing to the throat” and claimed they were actually medicinal.

The menthol cooled the feel of the smoke in the mouth and throat (much more next week on hos menthol feels cool). Menthol made it feel as though you weren’t sucking hot smoke into your lungs. And menthol deadened the discomfort that cigarettes could generate by irritating the lining of the throat and lungs.

These days, almost 90% of cigarettes contain some menthol, even if they don’t advertise themselves as menthol cigarettes. Why? The “cool” factor lends itself to novice smokers, while the throat analgesia appeals to the seasoned addict. But that may not be the main reason. A study from 2004 showed that menthol slows the metabolism of nicotine.

Slowing the metabolism of nicotine, menthol results in nicotine staying in the system longer and at greater concentrations - just perfect for developing a physical addiction. This, combined with the ability to comfortably smoke more cigarettes because of the slight throat numbing and apparent cooling of hot smoke would encourage more consumption, more addiction, and therefore more profit.

There is now (2013-2014) a push by the US Food and Drug Administration to ban or regulate menthol cigarettes. Did you know that menthol addition to shampoo is federally regulated but its addition to cigarettes is not? Let’s look at some of the reasons a change is being considered.

The tobacco plant has supplied cells that are used to show the
danger of menthol cigarettes. I just love that. But tobacco has been
involved in science in other ways as well. New efforts are
underway to have genetically modified tobacco produce medicines
or biodisel. And the tobacco mosaic disease actually led to the
discovery of viruses and the coining of the word “virus.” This same
virus was instrumental in establishing the fields of
virology, plant virology and all of molecular biology.

A 2013 series of experiments showed that menthol-containing cigarette smoke is more toxic to cells than non-mentholated cigarette smoke. Menthol alone had no toxic effect on the cells, so it is the combination of menthol and cigarette smoke that kills cells at a higher rate. In the most delicious irony imaginable, the two cell types that the researchers used to monitor cell death after smoke exposure were human lung cells and tobacco plant cells!!!

Additional recent evidence suggests that menthol interacts with the nicotine receptor in the brain. Brody and his co-workers showed that menthol cigarette smoke up regulates the number of nicotine receptors in the brain more than regular cigarette smoke. This might explain why it is harder for menthol cigarette smokers to quit smoking and why more of them fail in their efforts to quit.

Another 2013 study showed that menthol decreased the activity of the nicotine receptor, so that more nicotine was necessary to reach the same level of activation. Once again, this would contribute to a physical addiction.  Just a bit of information if you are considering taking up the habit - the “cool” factor and refreshing cold of mentholated smoke just may contribute to your death.

So sensing cool or cold has its place in biology and in society. Chili peppers are sensed as burning hot because they just happen to bind to and activate the TRPV1 heat sensing ion channel – it’s the biological joke being played on us that we have talked about before. TRPV1 a receptor that reacts to both environmental (temperature, pH) conditions and food substances.

On the other end of the scale is the sense of cold. Do organisms sense cold like they sense heat? Isn’t cold just a lack of heat, so that a feeling of cold is just a lack of activation of TRPV receptors? Nope. There are receptors specifically designed to sense cool or cold. Are there exceptions in cold sensing like there were for heat? You should know that answer by now.

Melastatin was the first TRPM protein discovered. The name
comes from melanin (the pigment in skin and hair cells) and
statin, which means to stop. It was important because it could stop
the invasion of tumors of melanin producing cells. We call this
maliganant melanoma, one of the deadliest cancers. Tumors with
more melastatin were less aggressive and invasive, while those with
little melastatin killed patients much sooner.

We learned recently that there are six different TRPV cation channels, and at least four of them are important for sensing different ranges of temperatures. In some cases, like with TRPV1, noxious heat (or capsaicin) results in a sensation of pain and burning, and the body’s mechanisms for cooling are turned on.

Another TRP family member, TRPM8, turns out to be the receptor channel that senses cool temperatures, from about 28˚C (82 ˚F) down to about 10˚C (50 ˚F) or even lower. The M stands for melastatin, a name for the first TRPM, before they knew it was a family of proteins. Now there are eight known members of the TRPM subfamily of TRP ion channels.

TRPM5 is particularly interesting for our recent discussion of taste, since it works to change the mechanical energy of taste particles + taste receptors into an electrical signal that is sent to the brain. Once again, we see the close relationship between the ion channels, like TRPV1 for capsaicin, and the taste sense. Maybe cold and TRPM8 also influence taste. We shall see.

Less is known about TRPM8 as compared to TRPV1 although they were discovered about the same time (early 2000’s). The pain associated with capsaicin and noxious heat aspects of TRPV1 made it sexier to study. I think we will see that TRPM8 and TRPA1 can be quite interesting in their own right.

Here’s a quick overview of the thermosensing by TRPs.
We will talk about it more next week. The TRPVs are
generally for warm temperatures, while TRPM8 is for
cool temp.s. TRPA1 will be our focus in a few weeks; it
senses painful cold. But notice, the garlic and wasabi
pictured with TYRPA1 also activate TRPV1, and the camphor
shown for TRPV1 also activates TRPM8 (next week). These are
related and complex systems.

First of all, TRPM8 is involved in thermoregulation, just as is TRPV1. In humans and other mammals (the naked mole rat excepted), when TRPV1 is activated, the body automatically thinks it is too hot and initiates cooling mechanisms. With TRPM8, the effect is the opposite. Stimulation of this ion channel tells the body that it is too cold, and mechanisms are initiated to increase the core temperature. We will talk about how TRPM8 helps to regulate body temperature next week.

The big question is why it’s important to sense cold as well as heat. For some reason, we sense cool/cold with some distinct proteins and heat with different proteins. Remember, evolution doesn’t follow a plan to make things complex, functional and efficient. Sometimes the functions occur at separate times and come from different pathways; there is no evolutionary goal or roadmap to a destination. It’s all chance.

A 2013 review has an interesting hypothesis as to why sensing cold/cold is so important, aside from just alerting us to the chance we might freeze to death. Based on mouse study results from as early as the 1970’s, and on the answers that human subjects give, it seems that coolness is an evolutionary plus. No- I don’t mean that The Fonz from Happy Days was an evolutionary leap into the future, I mean that cool sensations somehow help us survive and propagate.

We typically heat food because it increases aroma, increases taste, and reduces the work in digestion. These are all important for getting us the nutrients and the calories we need. Taste, as we said several weeks ago, is nature’s way of getting us to eat those things we need and avoid those foods that might harm us.

So why would cool foods or sensations be helpful? Cooling would decrease aroma and taste, so it must be something other than taste. The obvious reason for drinking something cold would be that it cools off our body – but it doesn’t work that way. As soon as you drink a cold drink, your body reacts to the cold by constricting the blood vessels near the cold surface so that heat is not lost. TRPM8 also invokes heating mechanisms after it is activated by the cold water or soda. So in truth, cold drinks don’t cool you off.

On the left is a mint julep, famous in Kentucky and the Deep
South during the hot summers. It contains Kentucky bourbon,
which is why it is brownish. On the right is the mojito, also
good on hot days, but uses rum, so it is popular in the Caribbean
and Florida, where the rum is. The connection? They both use
mint (menthol) to increase the coolness and refreshing
characteristics of the drinks. TRPM8 hard at work to make
your Saturday evening a success. 

Yet they still feel refreshing on a hot day – what gives? Refreshing may be the key word here. People use many words that together make up “refreshing.” They say that cold drinks revive them, restore their energy, arouse them, reduce stress. All these feelings would promote survival behaviors in a hot environment. But we might also drink a cold drink on a cold day and deem it pleasant. In this case, pleasant can be equated to useful – and useful means promoting survival.

The 1970’s experiments showed that mice would lick a cold piece of metal when they were thirsty, showing that cold helps satisfy thirst. The more amazing thing was that the mice would lick the cold metal even if they could drink all they wanted. This meant that cold drinks were a reward; they activate a pleasure center in the brain. Many studies and experiments have shown these results to be true for humans as well.

So a cold drink on a cold day might be seen as unpleasant, while a cold drink on a hot day is very pleasant (useful). But more important, a cold drink on a cold day when you are thirsty is seen as pleasant and satisfying. It’s our brain helping us to garner the things we need; if cold water is all that’s available to a cold caveman, he better want to drink it. It works the same on skin, cold applied to the skin on a hot day – such as jumping into the pool on a warm day is seen as pleasant, even if it doesn’t cool the body all that much (see above). But the same cannonball on New Years day with the Polar Bear Club, is completely unpleasant.

Comedian and late night talk show host Jimmy Fallon took
the Polar Bear Plunge in Chicago this past New Years. Basically,
3000 people jump into a 34˚F (1˚C) Lake Michigan to support
Special Olympics. Some do it for the charity, some for the thrill,
some because they are unbalanced. For those with a heart
condition, it can kill you.

The brain is an amazing organ, it works with our body to get us what we need, and tricks us into doing it – that’s basically what pleasurable things are, evolutionary tricks. But remember – too much of a good thing can be bad in an environment where we can manipulate nature.

Unfortunately, evolution doesn’t look into the future, it only worries about what keeps us alive at this moment. This explains the danger of menthol in cigarettes – we find it pleasant even if it is bad for us in the long run.

We will talk more about the TRPM8 next week, about how menthol seems to cool you down, how TRPM8 is a lot like TRPV1, and how it may save your life.

Eccles R, Du-Plessis L, Dommels Y, & Wilkinson JE (2013). Cold pleasure. Why we like ice drinks, ice-lollies and ice cream. Appetite, 71, 357-60 PMID: 24060271

Noriyasu A, Konishi T, Mochizuki S, Sakurai K, Tanaike Y, Matsuyama K, Uezu K, & Kawano T (2013). Menthol-enhanced cytotoxicity of cigarette smoke demonstrated in two bioassay models. Tobacco induced diseases, 11 (1) PMID: 24001273

Brody AL, Mukhin AG, La Charite J, Ta K, Farahi J, Sugar CA, Mamoun MS, Vellios E, Archie M, Kozman M, Phuong J, Arlorio F, & Mandelkern MA (2013). Up-regulation of nicotinic acetylcholine receptors in menthol cigarette smokers. The international journal of neuropsychopharmacology / official scientific journal of the Collegium Internationale Neuropsychopharmacologicum (CINP), 16 (5), 957-66 PMID: 23171716

Ashoor A, Nordman JC, Veltri D, Yang KH, Al Kury L, Shuba Y, Mahgoub M, Howarth FC, Sadek B, Shehu A, Kabbani N, & Oz M (2013). Menthol binding and inhibition of α7-nicotinic acetylcholine receptors. PloS one, 8 (7) PMID: 23935840

For more information or classroom activities, see: 
Menthol in cigarettes – 
http://smokefree.gov/menthol-cigarettes 
http://www.cancer.org/cancer/news/expertvoices/post/2013/08/28/menthol-cigarettes-whats-the-big-deal.aspx 
http://www.npr.org/2014/01/01/258671720/fda-weighs-restrictions-possible-ban-on-menthol-cigarettes 
http://www.huffingtonpost.com/2013/11/22/latino-smokers-menthol-cigarettes_n_4326094.html 
http://www.forbes.com/sites/robwaters/2013/11/20/menthol-an-unequal-opportunity-killer-why-hasnt-the-fda-banned-it/  
http://www.webmd.com/smoking-cessation/news/20110323/are-menthol-cigarettes-riskier-than-non-menthol http://www.theguardian.com/society/2013/oct/08/menthol-cigarettes-to-be-banned-eu
http://www.fool.com/investing/general/2013/12/14/judgement-day-for-menthol-cigarettes-is-getting-cl.aspx

http://www.dw.de/european-parliament-approves-stricter-tobacco-regulations/a-17458107     

In lieu of additional web sources, I suggest investigating the National Center for Biotechnology Information site (http://www.ncbi.nlm.nih.gov/) from the National library of Medicine. This site has many resources, from looking at the amino acid or nucleotide sequences from any protein or gene you can imagine (GenBank, http://www.ncbi.nlm.nih.gov/genbank/) to scientific journal articles that may or may not be available to you. Look at PubMed Central (PMC,  http://www.ncbi.nlm.nih.gov/pmc/) where all articles are available free to the public.