The burning questions - Wake Forest University

Chemo
Sense
SPECIAL AACSS ISSUE
What is it that makes an orange tangy, a
glass of beer refreshing and gives
Chicken Tandoori its bite? It’s pain! In
this issue Wayne Silver reviews the
trigeminal or chemesthetic sense,
revealing how the irritation of a special
bundle of nerves plays a major role in the
flavour and aroma of foods.
Also in this issue, Bets
Rasmussen investigates the biggest
nose of all — that of the majestic
elephant. Elephant olfaction is leading to
an understanding of how smell works at
a molecular level. Rasmussen has
shown how male elephant mucus
combines with female pheromones as a
bridging step to reproductive signalling.
The female elephant’s urine is lifted by
the tip of the male’s trunk and directed
into the ducts of a part of the nose called
the vomeronasal organ (VNO). The
elephant VNO tells the male that the
female is about to ovulate. Humans have
a VNO visible only through a microscope,
whereas the elephant’s is the size of your
finger. Does size matter? And if humans
have similar pheromones how do they
reach the human nose? And what
messages do they carry?
As a special feature this
16 page issue includes abstracts from
the most recent meeting of the Australian
Association for ChemoSensory Science.
From the world of business,
Kemal Avunduk, an Australian living and
working in Japan, brings us some advice
on judicious use of the tongue.
Plus, ChemoSense brings you
the latest news and upcoming events in
the chemosensory world.
ISSN 1442-9098
Vol. 2 No.1 November 1999
Chemesthesis:
The burning questions
By Wayne Silver, Wake Forest University
Animals have evolved a variety of chemoreceptive organs to sense chemicals in
their external environment. Gustation and olfaction (taste and smell), are usually
recognised as the ‘chemical senses’. However, there is another sense that we use to
detect chemical stimuli. This sense was named the ‘common chemical’ sense by G.H.
Parker in 1912 to describe the sensory system responsible for detecting chemical
irritants.
Although Parker noted that the common chemical sense was mediated by free
nerve endings resembling pain receptors, he concluded that “the common chemical
sense is a true sense with an independent set of receptors and a sensation quality
entirely its own”.
Today we know that the free nerve endings do not constitute a separate, independent
sense. Rather, they are part of the general somatic sensory system: a subset of
pain- and temperature-sensitive fibres which can be found throughout the skin and
mucosal membranes of the nose, mouth, respiratory tract, eye, and anal and genital
orifices.
The chemesthetic sense
Centre for ChemoSensory Research
The University of New South Wales
These nerve endings also respond to chemicals, including irritants, and the term
‘chemesthesis’ has recently been used to describe the sensations elicited by their
chemical stimulation. We stimulate our chemesthetic sense every time we eat chilli
peppers, sniff ammonia, or cut onions (see Box, next page). There are many important
reasons for seeking to understand our chemesthetic sense, not the least of which is to
better appreciate its role in flavours and aromas.
In humans, nerve fibres in the trigeminal (Vth cranial) nerve, innervating the mouth,
nose and eyes, best exemplify chemesthesis
(see Figure 1). While free nerve endings
originating from some other cranial nerves also
respond to chemical stimuli, much of the
research on chemesthesis, especially as it
relates to taste and smell, involves trigeminal
chemoreception.
Some nerve fibres in the trigeminal nerve
respond to touch, some to cold, some to heat,
and some to ‘painful’ stimuli. Those fibres that
respond primarily to chemical irritants are
called capsaicin-sensitive polymodal
nociceptors.
cont. pg 2
INSIDE:
Chemesthesis
Business in Japan
Elephant Olfaction
AACSS Abstracts
In the News
1

2
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)