Introduction
In the last
decade, studies using approaches from molecular biology
have substantially advanced our understanding of the
early events in olfaction and taste. Of the four taste
qualities that humans recognize, salty and sour tastes
involve ion channels in the membrane of receptor cells
while sweet and bitter tastes result from binding to
receptor proteins. The roles that taste and smell play
in the world of the newborn are very different from
those of adults. Acceptance of sweet and rejection of
bitter appear to be hard-wired while the affect associated
with odors depends much more on experience. Genetic
variation may produce total losses or losses specific
to certain stimuli. Clinical studies, some of which
Bartoshuk described, reveal pathologies responsible
for these total or partial losses. Three cranial nerves
carry taste and two of those nerves inhibit one another
such that damage to one disinhibits the other and preserves
over-all taste function.
Genetics
of taste
Taste worlds
of humans vary because of taste blindness to phenylthiocarbamide
(PTC) and its chemical relative, 6-n-propylthiouracil
(PROP). Bartoshuk reviewed early PTC studies and applied
modern statistical analyses, showing that a higher frequency
of women tasted PTC crystals, and were tasters (threshold
classification).
In Bartoshuk’s
laboratory, sensory scaling of PROP bitterness identified
a subset of tasters ("supertasters") who rate
PROP as intensely bitter. Supertasters also perceive
stronger tastes from a variety of bitter and sweet substances,
and perceive more burn from oral irritants (alcohol
and capsaicin). The density of taste receptors on the
anterior tongue (fungiform papillae, taste buds) correlate
significantly with perceived bitterness of PROP and
support the supertaster concept. Psychophysical data
from Bartoshuk’s lab also show a sex effect: women
are supertasters more frequently than men are. The anatomical
data also support the sex difference: women have more
fungiform papillae and more taste buds.
It is well-known
that the ability to taste low concentrations of 6-n-propylthiouracil
(PROP) and related bitter compounds such as phenylthiocarbimide
(PTC) and caffeine is heritable. Bartoshuk and her colleagues
set out to determine whether the distribution of PROP
taste thresholds is consistent with an additive or a
dominant mode of Mendelian transmission. To that end,
they determined the lowest concentration of PROP detectable
by 1015 subjects and tested models of bi- or tri-modal
distributions of PROP taste thresholds. The model with
the greatest likelihood had three distributions and
followed an additive model of PROP taste sensitivity.
Studies
of taste after nerve damage
Patients
with localized damage to the taste system often experience
no subjective change in real-world taste experience.
In an effort to understand this, eight patients who
recently underwent acoustic neuroma removal were evaluated
for taste loss. Localized taste testing showed that
taste intensities decreased in the distribution of cranial
nerve VII ipsilateral to tumor removal as expected,
but asymmetries occurred for IX. Intensities were greater
on the side contralateral to the tumor removal. In addition,
palatal taste, also thought to be mediated by VII, was
not totally abolished. It is concluded that cranial
nerve IX is normally inhibited by cranial nerve VII
in the taste network. When VII is damaged, this inhibition
is abolished. This release of inhibition,
Bartoshuk suggested, serves as a compensation mechanism
that preserves normal taste experience.
Studies
of taste interactions (capsaicin adaptation)
The desensitization
effects on taste resulting from application of 100 or
10 ppm capsaicin, accompanied by daily testing of a
capsaicin series (1-1000 ppm, in log steps), or the
desensitization resulting from application of 100 ppm
capsaicin without the daily capsaicin testing, were
investigated. The taste stimuli were three concentrations
each of NaCl, sucrose, citric acid, quinine and 6-n-
propylthiouracil. Following either type of 100 ppm desensitization,
the magnitude estimates of the two bitter tastes, in
particular, and citric acid showed significant decrements.
Following 10 ppm capsaicin or an ethanol control procedure,
there were no such effects. Recovery was complete in
1-3 days. Bartoshuk suggested that the taste decrements
are due to effects on both the taste and tactile components
of taste, though there is a stronger case for effects
on the tactile component.
Studies
of pain management
Pain from
oral mucositis afflicts from 40% to 70% of patients
receiving chemotherapy or radiation therapy. Current
methods of clinical pain management (for example, topical
anesthetics, systemic analgesics) have limited success.
In a pilot study, Bartoshuk and her colleagues showed
that oral capsaicin provided temporary relief of oral
mucositis pain, which is a common side-effect of radiation
therapy. Capsaicin, the active ingredient in chili peppers,
is known to desensitize some neurons and has been shown
to offer moderate pain relief when it is applied to
the skin surface. Bartoshuk and colleagues found that
oral capsaicin in a candy (taffy) vehicle produced substantial
pain reduction in 11 patients with oral mucositis pain
from cancer therapy. Although this pain relief was not
complete for most patients and was only temporary, Bartoshuk
believes it may be potentially quite useful as a therapeutic
tool.
