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.
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.
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.
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.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.
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.
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.
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
For more information or classroom activities, see:
Menthol in cigarettes –
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.