Minty Freshness: Why Does It Feel Cold?

[Andrew Buren]

Why does the mint flavore make your mouth feel cold?
I thought it might be chemistry.

 

[Dadface]

It is chemistry namely an endothermic reaction.The mint dissolving in the water in your mouth takes in energy giving a slight cooling effect.

 

[Monique]

That is not entirely correct. It is in fact a biological process.

The menthol in mint attaches to and sensitizes the TRP-M8 neuron endings in your mouth. TRP-M8 is required for cold sensation, by sensitizing the receptor it more easily sends a signal to the brain. The brain then perceives that as cold sensation.

In fact, capsaicin works in a similar manner. Instead it sensitizes the TRP-V1 nerve endings, which are required for heat sensation. The neuron is more easily triggered, sends a signal to the brain, which then thinks there is something hot in the mouth.

 

[Dadface]

Thank you Monique,we live and learn .Is there no temperature change at all with mints or is it a sort of taste illusion?

 

[Monique]

Basically the effect is an illusion.

From ScienceDaily, http://www.sciencedaily.com/releases/2007/05/070530132405.htm":

"It's been known for years that menthol and related cooling agents evoke the psychophysical sensation of cold -- somehow by interacting with the aspect of the sensory nervous system that's related to cold detection," says Julius.

The current study, he says -- led by Diana M. Bautista, PhD, and Jan Siemens, PhD, of the Julius lab and Joshua M. Glazer, PhD, of the lab of co-senior author Cheryl Stucky, PhD, of the Medical College of Wisconsin -- puts that question to rest.

Taken from a review in Annual Review of Neuroscience, published in 2006 (doi:10.1146/annurev.neuro.29.051605.112958) http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.neuro.29.051605.112958"[/URL][quote]Menthol derived from mint elicits a sensation of cold when applied to the skin or mucous membranes. Remarkably, menthol modulates the activity of cool-induced currents in individual free nerve endings. In recordings of cold-sensitive afferents, menthol stimulates these fibers at subthreshold activation temperatures, which suggests that menthol acts directly on the molecule(s) responsible for cold transduction (Hensel & Zotterman 1951a, Schafer et al. 1986). In analogous studies using cultured rodent sensory neurons, electrophysiological and calcium-imaging experiments identified a population of neurons that responded to both innocuous cool and menthol, corresponding to 10% of total neurons (Reid & Flonta 2001b, Viana et al. 2002). An important step forward came when Reid & Flonta (2001b) reported that the stimulation of cultured DRGs with cool and/or menthol leads to the activation of a nonselective cation channel.

An intense search to identify a cold-activated ion channel led to the identification of a cool/menthol receptor TRPM8 (CMR1) by two independent groups (McKemy et al. 2002, Peier et al. 2002). One group used a genomics-based approach, reasoning that TRP channels, which encode a family of nonselective cation channels that are involved in thermosensation, may encode additional thermoreceptors. TRPM8 was identified by its expression in sensory neurons and its ability to be activated by cold and menthol (Peier et al. 2002). Using menthol as a stimulus, the same group that first identified TRPV1 used expression cloning to isolate TRPM8 from a rat trigeminal neuron cDNA library (McKemy et al. 2002). In heterologous expression systems, TRPM8 was activated with a threshold of 25°C–28°C similar to the threshold temperature observed in cold/menthol-sensitive sensory neurons (27°C–33°C). Additionally, TRPM8 has many of the same characteristics of the native cool/menthol channel, including outward rectification, ion selectivity, adaptation, and the ability of subthreshold levels of menthol to shift the activation temperature (McKemy et al. 2002, Peier et al. 2002).[/quote]
Here is a link to the original research published in 2007 in Nature that really established that menthol is able to activate the neurons: http://www.nature.com/nature/journal/v448/n7150/pdf/nature05910.pdf" [quote="Abstract"]Sensory nerve fibres can detect changes in temperature over a remarkably wide range, a process that has been proposed to involve direct activation of thermosensitive excitatory transient receptor potential (TRP) ion channels. One such channel--TRP melastatin 8 (TRPM8) or cold and menthol receptor 1 (CMR1)--is activated by chemical cooling agents (such as menthol) or when ambient temperatures drop below approximately 26 degrees C, suggesting that it mediates the detection of cold thermal stimuli by primary afferent sensory neurons. However, some studies have questioned the contribution of TRPM8 to cold detection or proposed that other excitatory or inhibitory channels are more critical to this sensory modality in vivo. [b]Here we show that cultured sensory neurons and intact sensory nerve fibres from TRPM8-deficient mice exhibit profoundly diminished responses to cold. These animals also show clear behavioural deficits in their ability to discriminate between cold and warm surfaces, or to respond to evaporative cooling. At the same time, TRPM8 mutant mice are not completely insensitive to cold as they avoid contact with surfaces below 10 degrees C, albeit with reduced efficiency. Thus, our findings demonstrate an essential and predominant role for TRPM8 in thermosensation over a wide range of cold temperatures, validating the hypothesis that TRP channels are the principal sensors of thermal stimuli in the peripheral nervous system.[/quote]

 

[turbo]

Interestingly, oil of Wintergreen (Methyl Salicylate) and Capsaicin (plus camphor) are the basic ingredients of "Heet" brand pain-reliever, stimulating nerves responsible for sensation of both cold and heat. When the weather turns cranky and my arthritic knees start hurting, this old remedy helps.

 

[Monique]

That old remedy probably drowns-out the noxious signal by stimulating multiple neurons, thereby desensitizing you to the pain? (like how menthol relieves an itch)

An interesting statement in the ScienceDaily link:

As such, the receptor -- known as menthol receptor TRPM8 -- provides a target for studying acute and chronic pain, as can result from inflammatory or nerve injury, the researchers say, and a potential new target for treating pain.

"By understanding how sensory receptors work, how thresholds for temperature are determined, we gain insight into how these thresholds change in the setting of injury, such as inflammatory and nerve injury, and how these changes may contribute to chronic pain," says senior author David Julius, PhD, chairman and professor of physiology at UCSF.

The menthol receptor, and other temperature receptors discovered in recent years by the Julius lab, offer potential targets for developing analgesic drugs that act in the peripheral, nervous system, rather than centrally, where opiate receptors act, he says.

 

[turbo]

That is probably the case. There are other formulations on the market that are similar, such as "Icy-Hot". They do offer some relief, not by addressing the cause of the pain, nor by providing physical heating or cooling, but by chemically stimulating nerves in the skin to perceive both heating and cooling, which seems to drown out the perception of pain to some extent.

 

[Dadface]

Interesting stuff and I wonder what advantage it is to the plants in synthesising these compounds.I have just checked on the net and apparently oils extracted from mints,chillis and certain other plants make good insecticides.Perhaps there is the answer.

 

[turbo]

Interesting stuff and I wonder what advantage it is to the plants in synthesising these compounds.I have just checked on the net and apparently oils extracted from mints,chillis and certain other plants make good insecticides.Perhaps there is the answer.

Tell that to the Tomato Horntails! Those creeps ate holes in so many of my chili peppers last year. This one was bigger than my middle finger (I know, because I showed it to him before he accidentally got under my boot.)

 

[Dadface]

Your Tomato Horntail looks like a green cutie and I am sad to hear of his accidental demise.Perhaps you should try planting mints amongst your chillis.

 

[turbo]

Your Tomato Horntail looks like a green cutie and I am sad to hear of his accidental demise.Perhaps you should try planting mints amongst your chillis.

Mints propagate through rhizomes, and they will never be welcome in the vegetable garden for that reason. Herb gardens, OK.

At least, I bothered to take an obituary photo before his "accidental" demise.

 

[Dadface]

Its because they propogate through rhizomes that I keep my mints in pots and it works pretty well provided that you check them every so often.I have never tried chillis but I might have a crack at them this summer.Dont send any of little green friends over-they are not that cute.

 

[hypatia]

I use to sink pots of mint in my up north garden, until the deer found them. Then we had a year of deer with minty fresh breath.