The burning questions - Wake Forest University

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CHEMESTHESIS continued
That is, they are receptors that respond to a variety of
noxious, irritating stimuli (mechanical, thermal, and chemical)
and are especially sensitive to capsaicin (the active, ‘hot’
ingredient in chilli peppers). When capsaicin stimulates these
nociceptors it elicits a burning pain sensation.
Recently, a capsaicin receptor (VR1) has been isolated.
The receptor is a non-selective cation channel in the nerve
membrane, activated not only by capsaicin, but also by high
temperatures in the noxious range (Caterina et al., 1997).
That burning sensation
A variety of sensations are elicited by stimulation of
trigeminal chemoreceptors, including those described as
pungent, tingling, stinging, burning, cooling, warming, painful,
and irritating. In addition to providing a sensory experience,
chemical stimulation of trigeminal chemoreceptors activates
protective reflexes, such as increased secretion, decreased
breathing, sweating, and decreased nasal patency. Anyone
who has bitten into a chilli pepper or taken a whiff of ammonia
has experienced some of these reactions.
A curious aspect of chemesthesis is that, although many
chemesthetic substances are initially (and probably innately)
aversive, they eventually become preferred. Examples include
tobacco, chilli pepper, mustard, curry, horseradish, ginger, and
vinegar. Infants, children, and uninitiated adults typically reject
these substances on first exposure. Most adult humans,
however, reverse their natural aversion and develop strong
positive responses to at least one initially unpalatable substance.
Indeed, whole cuisines, such as Mexican and Thai,
are based on some of these chemesthetic stimuli. How the
reversal of the initial aversion occurs is not well understood but
any explanation has to account for the inability to produce
preferences for these compounds in most other animals.
Putting the tingle, fizz and bite into foods
A number of important questions remain to be answered
concerning chemesthesis. We do not yet know to what extent
it contributes to our perception of odour, tastes, or flavours.
Could chemesthetic stimulation enhance or diminish perception?
Research is also currently under way to determine
whether different chemesthetic stimuli elicit different sensations.
For example, can people lacking olfactory systems
discriminate between different irritants based on the quality of
those irritants? If the answer to this is yes, it suggests that
different stimuli have specific receptors. This could be an
important finding that would allow the creation of compounds
specifically targeted to bind to specific receptors and elicit
specific sensations. Perhaps some day we may be able to
purchase ‘designer’ hot sauces and seasonings to spice up
our foods in new ways.
All of this is good news for those of us who like food that
bites back.
Caterina, M.J., Schumacher, M.A., Tominaga, M., Rosen, T.A., Levine, J.D.,
Julius, D. (1997). The capsaicin receptor: a heat-activated ion
channel in the pain pathway. Nature 389:816-824.
Figure 1 Branches of the chemically sensitive trigeminal nerve
in the regions of the mouth and nose.
Figure 1
‘Pungency’:
The Trigeminal Chemosense (Chemesthesis)
The sensations arising from the chemesthetic sense are
described as ‘hot’, ‘warm’, ‘pain’, ‘burn’, ‘bite’, ‘sting’, ‘prickle’,
‘itch’, ‘tingle’, ‘fizz’, ‘numbness’, ‘freshness’, ‘chill’, ‘coolness’
and ‘coldness’.
Foods that invoke the chemesthetic sense include:
chilli, pepper, mustard, ginger, onion, garlic, horseradish,
peppermint, cumin, coriander, cinnamon, cloves, spearmint,
carbonated drinks, sherbet, salt, vinegar, fruit powders and food
acids.
Chemical compounds that act on the Trigeminal nerve
endings:
capsaicin (from chilli) piperine (from pepper)
gingerol (from ginger) zingerone (from ginger)
allyl isothiocyanate (from mustard)
cinnamaldehyde (from cinnamon)
cuminaldehyde (from cumin)
2-propenyl/2-phenylethyl isothiocyanate (from horseradish)
carbon dioxide (bubbles in carbonated liquid)
some salts, including sodium chloride
some acids, including acetic and citric acid
alcohol , ammonia, nicotine, menthol
thymol (from thyme)
eucalyptol (from eucalyptus plants)