Chem. Senses 27: 123-131,
2002
© Oxford University Press 2002
Cross-adaptation and Bitterness Inhibition of L-Tryptophan, L-Phenylalanine and Urea
Further Support for Shared Peripheral Physiology
Monell Chemical Senses Center, 3500 Market St, Philadelphia, PA 19104, USA
Correspondence to be sent to: Paul A.S. Breslin, Monell Chemical Senses Center, 3500 Market St, Philadelphia, PA 19104, USA. e-mail: breslin{at}monell.org
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Abstract |
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A previous study investigating individuals' bitterness sensitivities found a close association among three compounds: L-tryptophan (L-trp), L-phenylalanine (L-phe) and urea (Delwiche et al., 2001
, Percept. Psychophys. 63, 761-776). In the
present experiment, psychophysical cross-adaptation and bitterness inhibition
experiments were performed on these three compounds to determine whether the
bitterness could be differentially affected by either technique. If the two
experimental approaches failed to differentiate L-trp, L-phe and urea's
bitterness, then we may infer they share peripheral physiological mechanisms
involved in bitter taste. All compounds were intensity matched in each of 13
subjects, so the judgments of adaptation or bitterness inhibition would be
based on equal initial magnitudes and, therefore, directly comparable. In the
first experiment, cross-adaptation of bitterness between the amino acids was
high (>80%) and reciprocal. Urea and quinine-HCl (control) did not
cross-adapt with the amino acids symmetrically. In a second experiment, the
sodium salts, NaCl and Na gluconate, did not differentially inhibit the
bitterness of L-trp, L-phe and urea, but the control compound,
MgSO4, was differentially affected. The bitter inhibition
experiment supports the hypothesis that L-trp, L-phe and urea share peripheral
bitter taste mechanisms, while the adaptation experiment revealed subtle
differences between urea and the amino acids indicating that urea and the
amino acids activate only partially overlapping bitter taste mechanisms. |
Introduction |
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A primary function of the peripheral gustatory system is to discriminate between nutritive and toxic chemicals among potential ingesta. Presumably, bitter taste perception evolved to detect potential toxins (Glendinning, 1994
). The
gustatory system identifies many classes of compounds as bitter: inorganic
salts (e.g. potassium chloride), amines (e.g. denatonium), amino acids (e.g.
tryptophan), peptides, alkaloids (e.g. quinine), acetylated sugars (e.g.
sucrose octaacetate), flavanols/phenols (e.g. epicatechin), carbamates (e.g.
phenyltuiocarbamide) and isohumulones, to name some. To have the ability to
taste such divergent structures, mammals have evolved multiple peripheral
mechanisms, which have an affinity for the chemical structures. Psychophysical
experiments (McBurney, 1969;
McBurney et al.,
1972; Lawless,
1979; Yokomukai et
al., 1993; Cowart et
al., 1994; Delwiche
et al., 2001) have supported multiple physiological
mechanisms involved in bitter taste, while electrophysiological and
biochemical experiments have elucidated several bitter taste transduction
systems [for review of bitter taste see Spielman et al.
(Spielman et al.
1992) and Dulac (Dulac,
2000)].
It is logical to assume that bitter compounds will share taste receptor
cells (TRCs) or transduction mechanisms, as it seems improbable that each of
the thousands of bitter compounds would have its own unique transduction
sequence. Molecular cloning and functional studies
(Adler et al., 2000;
Chandrashekar et al.,
2000; Matsunami et
al., 2000) have revealed a family of 40-80 putative bitter
receptors (Tas2Rs), many of which are co-expressed on the same cells, which
indicates bitter taste cells will respond to a number of bitter stimuli [cf.
(Caicedo and Roper, 2001)].
Further to this, Chandrashekar et al., demonstrated that the bitter
compounds, PROP and denatonium benzoate, could activate the same receptor,
thereby showing that the Tas2Rs also share ligands
(Chandrashekar et al.,
2000). Given the evidence that taste cells can express multiple
Tas2Rs and that one Tas2R can be activated by a variety of ligands, it is
probable that the bitter response activated by a group of structurally related
bitter compounds may be similar.
In addition to receptor-mediated bitter taste transduction, bitter
compounds may directly activate transduction components downstream of the
G-protein coupled receptors. Many bitter compounds are lipophilic or
amphipathic and have the ability to permeate rapidly through cell membranes,
such as the cyclic dipeptide Leu-Trp and quinine
(Peri et al., 2000).
Compounds such as quinine and certain peptides can directly activate mixtures
of G-proteins in vitro. Therefore, direct activation of G-proteins
could result in bitter taste transduction
(Naim et al., 1994;
Chahdi et al., 1998).
Certain compounds, such as caffeine, may also directly interact with bitter
taste transduction enzymes (Rosenzweig
et al., 1999).
In research testing the hypothesis that bitter tasting compounds share
transduction mechanisms, hence bitter compound sensitivities, Delwiche et
al., examined individual differences in sensitivity to 11 bitter
compounds in 26 subjects and identified several tight compound groupings
(Delwiche et al.,
2001). Among them, three bitter compounds, L-tryptophan (L-trp),
L-phenylalanine (L-phe) and urea, correlated the most tightly as a function of
individual sensitivities to bitterness from them. Those who were very
sensitive to one compound were very sensitive to the other two, independent of
their sensitivity to the other eight compounds. This correlation of compound
specific differences in sensitivity may be caused by shared TRCs or
receptor/transduction mechanisms.
To compliment the close associations revealed by individual differences
analyses, this study was designed to determine whether this cluster of
compounds, L-trp, L-phe and urea, could be differentiated perceptually by two
additional psychophysical techniques: cross-adaptation and bitterness
inhibition. Cross-adaptation studies can help determine whether compounds are
likely to share TRCs/ receptor/transduction mechanisms. In the gustatory
system, when a compound cross-adapts a taste quality of another compound, this
strongly suggests the compounds share a physiological process involved with
that taste quality, most likely at the TRCs or the receptor/transduction level
(McBurney, 1969;
McBurney et al.,
1972; Schiffman et
al., 1981; Lawless,
1982; Michel et al.,
1993; Smith and van der
Klaauw, 1995; Froloff et
al., 1998), although more central adaptation affects cannot
be ruled out. If the amino acids L-trp, L-phe and urea share bitter TRCs or
receptor/transduction mechanisms, they should symmetrically cross-adapt each
other's bitterness and affect the bitterness of unrelated compounds
comparably.
As an additional test, it might be possible to differentially affect the
stimuli with a bitter inhibitor and, thereby, infer that L-trp, L-phe and urea
act on independent peripheral physiological mechanisms. Sodium inhibits the
bitterness of different bitter compounds to widely varying degrees
(Frijters and Schifferstein,
1994; Breslin and Beauchamp,
1995,
1997). Furthermore, the
bitterness inhibiting properties of sodium are peripheral, acting in the
mouth, rather than a central cognitive effect of the perceived saltiness; mole
for mole, sodium salts with little salt taste are comparably as effective at
blocking bitterness as highly salty salts (Bartoshuk,
1979,
1980;
Bartoshuk and Seibyl, 1982;
Kroeze and Bartoshuk, 1985;
Kemp and Beauchamp, 1994).
Therefore, as sodium salts suppress the bitterness of urea and the effect is
peripheral, other compounds that may share TRCs and/or receptor/transduction
mechanisms with urea, such as L-trp and L-phe, should be suppressed to a
similar extent.
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Materials and methods |
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Experiment 1: Cross adaptation of bitterness
Subjects
Thirteen subjects (seven female, six male) aged between 20 and 51 (mean
29.9 years) were paid to participate after providing their informed consent on
an Institutional Review Board (IRB) approved form. All but one were employees
of Monell Chemical Senses Center (Philadelphia, PA). Each subject participated
in 48 sessions over 3 months. They were asked not to eat, drink or chew gum 1
h prior to each session.
Training
Subjects were initially trained in the use of the Labeled Magnitude Scale
(LMS) following standard published procedures (Green et al.,
1993,
1996), except the top of the
scale was described as the `strongest imaginable' sensation of any kind
(Bartoshuk, 2000). The LMS is a
psychophysical tool that requires subjects to rate the perceived intensity
along a vertical axis lined with adjectives: barely detectable, weak,
moderate, strong, very strong, strongest imaginable; the adjectives are spaced
semi-logarithmically, based upon experimentally determined intervals (Green
et al., 1993,
1996) to yield ratio quality
data. The scale shows adjectives not numbers to the subjects, but the
experimenter receives numerical data from the computer program.
Subjects were trained to identify each of the five taste qualities by presenting them with exemplars. Salty taste was identified as the predominant taste quality from 150 mM NaCl, bitterness as the predominant quality from 0.05 mM quinine-HCl, sweetness as the predominant quality from 300 mM sucrose, sourness as the predominant quality from 3 mM citric acid and savory the predominant quality from a mixture of 100 mM glutamic acid monosodium salt and 50 mM inosine 5'-monophosphate. To help subjects understand a stimulus could elicit multiple taste qualities, 300 mM urea (bitter and slightly sour) and 50 mM NH4Cl (salty, bitter and slightly sour) were employed as training stimuli.
Stimuli
L-trp, L-phe and urea were all purchased from Sigma (St Louis, MO) and were
Sigma-ultra grade. Quinine-HCl (QHCl) (>99%) was purchased from Fluka
(Buchs, Switzerland). Aqueous solutions were prepared every second day with
deionized (di) Millipore filtered water and stored in amber glass at
room temperature. All solutions were fully dissolved and there were no visible
signs of undissolved solids or precipitation from solutions.
Tongue adaptation method: intensity matching
An anterior tongue adaptation method was developed because whole mouth
adaptation has been shown to be inconsistent and ineffective
(Meiselman, 1968). This is
likely due to the presence of posterior lingual and pharyngeal bitter
receptors and inconsistency both within and between subjects in stimulating
the same posterior receptors with repeated stimulation.
Most individuals in a sample population will perceive a single fixed
concentration of a bitter compound differently
(Yokomukai et al.,
1993; Delwiche et
al., 2001). Therefore, the concentration of bitter compounds
was adjusted so that all subjects judged the compounds to be of equal
intensity on a large but well defined and controlled area of the tongue in
order to compare them for psychophysical effects. Consequently,
cross-adaptation was assessed with bitter compounds of equal intensities but
different molarities.
Intensity matching
Subjects were required to rate the bitter intensity of L-trp (0.06 M),
L-phe (0.17 M), urea (2 M) and QHCl (0.1 mM) in separate sessions on the LMS.
Both L-trp and L-phe were presented as saturated solutions and all subjects
rated bitter intensities as `moderate' or weaker on the LMS. Whichever of the
two amino acids, L-trp or L-phe, was rated least bitter, was then chosen as
the compound to which the other compounds were matched for intensity, as the
concentration (therefore the intensity) of the other amino acid could not be
increased. Subjects (seven of 22 subjects screened) were not included in the
study if they rated either L-trp or L-phe as less than `weak' on the LMS, as a
study on bitterness adaptation must elicit bitterness to begin.
Subjects were instructed to extend their tongue out of their mouth so a
significant portion of there anterior tongue (
2.5 cm) was exposed, then a
good seal was formed around the tongue with their lips, thereby isolating the
anterior portion from the rest the oral cavity. Subjects completely immersed
their exposed tongues into 30 ml plastic medicine cup containing 25 ml of
stimuli so that their lips were in contact with the solution. After rating the
intensity of the taste qualities (sweet, sour, bitter, savory and salty) on
the LMS, subjects removed their tongue from the solution and rinsed with
di water. There was a break of at least 60 min prior to the next test
to eliminate any possible adaptation or sensitization effects. The intensity
matching procedure continued until individual concentrations of L-trp, L-phe,
urea and QHCl were judged to be equal in bitterness magnitude for each
subject. Subjects were not included in the study (two of 15 subjects
screened), if reproducibility for a particular compound was not within 25% of
the determined LMS matched intensity over a series of at least three separate
trials.
Cross-adaptation of bitterness
Each subject was presented with eight 30 ml medicine cups filled with 25 ml
of intensity matched solutions in numbered trays. Solutions 1 and 8 were the
`test' solutions while 2-7 were the `adapting' solutions
(Figure 1). New adapting
solutions were given to subjects every 30 s in case any saliva ran into the
cup during adaptation and so the test cup (no. 8) would be experienced the
same way the adapting cups (no. 2-7) were experienced. Sample 1
(pre-adaptation) was used as the reference against which sample 8
(post-adaptation) was compared. Subjects followed the tongue immersion
methodology (described above) for sample 1; once rated, subjects rinsed their
mouth with di water four times during a 60-s interstimulus interval.
Subjects then followed the tongue immersion methodology for sample 2, but
after rating the taste intensities, their tongue remained in the solution for
30 s. After 30 s subjects removed their tongue from solution 2 and repeated
the immersion procedure with solution 3 through 7. The subjects tongue was not
retracted into the oral cavity and no water rinsing occurred between samples 2
and 8. The procedure was the same for sample 8, except once sample 8 had been
rated for taste intensity, subjects could retract their tongue into the oral
cavity and rinse with di water. The procedure took 4 min per
adaptation trial.
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A factorial matrix design ensured that every compound was the adapting solution for every other compound, including itself. Water (di) was included in the matrix design as a control.
Experiment 2: Bitterness inhibition by sodium salts
Subjects
Thirteen subjects (five male, eight female) between the ages of 20 and 35
(mean 27.9 years) were paid to participate after providing their informed
consent on an IRB-approved form. All but two were employees of Monell Chemical
Senses Center. Seven subjects who participated in the adaptation study also
participated in the bitterness inhibition study. Each subject participated in
three sessions over a period of 1 month. They were asked not to eat, drink or
chew gum for 1 h prior to each session.
Stimuli
L-Trp, L-phe, urea and magnesium sulfate (MgSO4) were purchased
from Sigma and were Sigma-ultra grade. QHCl (>99%) was purchased from
Fluka. Sodium chloride (NaCl) and sodium gluconate (NaGlu) were purchased from
Sigma and were Sigma-ultra grade. NaGlu was used in the experiment because of
the reduced salty taste caused by the larger anion
(Ossebaard and Smith, 1995);
low saltiness allows us to distinguish between the peripheral inhibition of
bitterness by sodium ions and the central cognitive inhibition of bitterness
by perceived saltiness (Breslin and
Beauchamp, 1995). Aqueous stock solutions were prepared every
second day with di Millipore filtered water and stored in amber glass
at room temperature.
Intensity matching
The bitterness inhibition experiment was a whole-mouth sip and spit
procedure, thereby activating the fungiform papillae, as in the
cross-adaptation experiment, as well as the foliate and circumvallate
papillae. The foliate and circumvallate papillae have been shown to have a
greater proportion of the putative bitter taste receptors
(Adler et al., 2000).
Therefore, given the phenomena of spatial summation
(Smith, 1971), we expected
equimolar solutions from the adaptation experiment to be more intense in the
bitterness inhibition experiment. The intensity matching procedure involved
adjusting the concentrations until the intensity of stimulus was rated as
`moderate' on the LMS by each subject. The matching methodology follows:
subjects were instructed to wear nose-clips to eliminate olfactory cues when
sampling and to rate the perceived total intensity of solution presented while
the solution remained in the subjects mouth. Subjects rated the intensity of
predetermined concentrations of bitter solutions (0.0198 M L-trp, 0.04 M
L-phe, 0.6 M urea, 0.2 mM QHCl, 0.45 M MgSO4). Taste intensity was
recorded on a computerized LMS and transferred in real time to the technician
making solutions who altered the concentration of solutions up or down
depending on the individual subject's response. The new solution was tasted
and rated by the subject, and depending on the response, new concentrations
were made until the intensity was rated as `moderate'. There was an
interstimulus interval of 60 s, during which time the subject was
required to rinse with di water at least four times. When randomly
presented with a `matched' bitter stimulus, subjects were required to rate the
intensity of the bitter compound as `moderate' on the LMS. If the LMS rating
(±25%) did not match `moderate' on subsequent evaluations of the
matched intensities, the subject was retested or excluded from the study. Five
of 18 subjects screened were excluded from the study by this criterion because
bitterness cannot be inhibited unless it is first elicited.
Methodology
Subjects, wearing nose-clips, were given trays of bitter compounds at
concentrations individually assessed in the intensity matching phase. The
solutions, which included bitter stimuli and water, were presented without
salt or with 300 mM of NaCl or NaGlu added. The testing protocol was as
follows: randomized solutions (10 ml) were presented in 30 ml plastic medicine
cups and on numerically labeled trays. Subjects rinsed with di water at least
four times over a 2-min period prior to testing. Each subject tasted and then
rated each solution for sweetness, sourness, saltiness, bitterness and
savoriness, prior to expectorating. All subjects rinsed with di water
four times during the interstimulus interval of 85 s. The LMS was used as the
rating method. Each sample was tasted only once per session and there were
three sessions in total as a test of reliability.
Statistical analysis of experiments 1 and 2
Numerical results are expressed as mean ± SE. Statistical variation was determined by one- or two-way analysis of variance (ANOVA) using Statistica 4.5 software package. P < 0.05 was considered statistically significant. Bitter intensities pre- and postadaptation were analyzed by one-way ANOVA. Mean bitter intensity data from bitter inhibition experiment were analyzed by a 5 x 3 (bitter salt) repeated measures ANOVA. All post-hoc pairwise comparisons were conducted with the Scheffé test.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Cross-adaptation
Intensity matching
Table 1 shows the average
molarity along with the range and average LMS score for each of the intensity
matched stimuli used in this experiment. At their limits of solubility, the
bitter intensity of L-trp and L-phe was rated between `weak' and `moderate' on
the LMS for all subjects tested. As a result, an individual's bitterness
rating of saturated L-trp or L-phe dictated the bitterness intensity to which
the other compounds were matched. The results revealed that eight of 13 (62%)
subjects perceived the amino acids to be isointense at their maximum
solubility. Given the variable nature of human taste sensitivities, the
concentrations of L-trp and L-phe required to elicit isointense bitterness
were remarkably similar over the majority of subjects, which of its own accord
supports the findings of Delwiche et al.
(Delwiche et al.,
2001); i.e. sensitivities to these two compounds correlate.
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Adaptation
Overall, there was a significant effect of adaptation on bitterness of the
compounds pre- and postadaptation [F(19,228) = 41.4, P <
0.001)]. Figure 2 and
Table 2 summarize the results
of self- and cross-adaptation. Self-adaptation for the compounds tested were
almost complete (96% L-trp, urea and QHCl, 94% L-phe). In all cases,
self-adaptation was greater than cross-adaptation of other compounds, which
may indicate that each compound has at least partially independent peripheral
bitter taste mechanisms. Water (di) was also used as an adapting
stimulus and results show a significant increase in bitterness post-water
adaptation for L-trp (P < 0.05), L-phe (P < 0.001) and
urea (P < 0.05).
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Cross-adaptation was not reciprocal in all cases. QHCl was chosen as a
stimulus to control for spuriously finding symmetrical cross-adaptation as
McBurney et al. (McBurney et
al., 1972) has shown that urea can cross-adapt QHCl
bitterness, but adaptation to QHCl only partially cross-adapts urea
bitterness. Results from this experiment support McBurney's finding, as urea
effectively cross-adapted 67% of QHCl's original bitterness (P <
0.001), yet adaptation to QHCl only inhibited 26% of urea's bitterness
(P = 0.87).
Cross-adaptation between the two amino acids was homogeneous and symmetrical: adaptation to L-trp decreased L-phe bitterness by 80%, while adaptation to L-phe decreased L-trp bitterness 85%. Adaptation to urea decreased L-trp bitterness 82% and L-phe bitterness 77%. Overall, urea was very effective at cross-adapting the bitterness of the three other compounds, while the other three compounds were more variable and less effective at cross-adapting urea's bitterness.
Bitterness inhibition
Intensity matching
The mean level of bitterness intensity for MgSO4 was below the
targeted `moderate' rating on the LMS. There was a significant difference in
bitterness of MgSO4 and L-trp and L-phe (P < 0.05)
(Table 3). Attempts to increase
the concentration of MgSO4 during the matching phase produced
significant irritation among the majority of subjects; therefore, we decided
that the irritation produced by higher molarities of MgSO4 would be
too distracting to subjects. Even though MgSO4 was significantly
less bitter than L-trp or L-phe, it was imperative to have a control compound
in the experimental design whose bitterness should not be inhibited by the
addition of sodium salts (Breslin and
Beauchamp, 1995). Prior research has shown that less bitter
concentrations are more easily suppressed, so the bitterness of
MgSO4 in this study should have been easier to inhibit based on its
intensity (Breslin and Beauchamp,
1995).
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Inhibition
There was a significant overall inhibition of bitterness by salt
[F(2,24) = 67.3, P < 0.001] and bitter salt interaction
[F(8,96) = 7.8, P < 0.001], which suggests some compounds
were inhibited more than others. On average, NaCl and NaGlu significantly
decreased bitterness (51% and 41% respectively, P < 0.001). There
was no statistical difference between the overall bitter inhibition ability of
the two sodium salts (P = 0.13).
Post-hoc tests revealed that NaGlu significantly reduced bitterness of L-trp (P < 0.001), L-phe (P < 0.001) and QHCl (P < 0.001) and the suppression of urea was marginal (P = 0.09). NaCl significantly decreased the bitterness of L-trp (P < 0.001), L-phe (P < 0.001), urea (P < 0.05) and QHCl (P < 0.001).
Breslin and Beauchamp had previously reported 300 mM NaCl inhibited the
bitterness of urea (60%) and QHCl (40-60%), while MgSO4 bitterness
was not affected (Breslin and Beauchamp,
1995). The present experiment showed
(Figure 3) that both NaGlu and
NaCl were more effective at reducing the bitterness intensity of QHCl (45% and
56% respectively) than urea (37% and 42% respectively), while not affecting
the bitterness of MgSO4. NaGlu and NaCl also suppressed the
bitterness of L-trp (52% and 64% respectively) and L-phe (54% and 66%
respectively). Note that both bitterness inhibitors blocked the two amino
acids symmetrically; NaCl inhibited bitterness L-trp 64%, L-phe 66% while
NaGlu inhibited bitterness of L-trp 52% and L-phe 54%.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Adaptation
Self-adaptation of a taste quality is a phenomenon that involves a
reduction of the initial taste intensity with constant or repeated application
of a taste stimulus. Cross-adaptation occurs between different stimuli. After
adaptation to one stimulus, the taste intensity of a different second stimulus
is reduced. Cross-adaptation is generally regarded as evidence that the two
stimuli share taste physiology within the transduction sequence
(McBurney et al.,
1972; Lawless,
1982; Smith and van der
Klaauw, 1995). L-trp and L-phe cross-adapted each other's
bitterness in excess of 80%, compared with 96% and 94% self-adaptation
respectively. Symmetrical cross-adaptation of bitterness between the two amino
acids supports the theory that L-trp and L-phe share bitter TRCs or
receptor/transduction mechanisms. It is worth noting that cross-adaptation
was, in all cases, less than self-adaptation, and while the difference was not
statistically significant, the trend suggests that the amino acids may have
partially independent bitter taste mechanisms, albeit a small proportion
(10-15%) of the total. Urea cross-adapted the amino acid bitterness by
80%, but cross-adaptation was not reciprocal; adaptation to L-trp
decreased urea's bitterness 58%, while adaptation to L-phe decreased urea's
bitterness 69%. One-way ANOVA of cross-adaptation between the compounds
revealed the difference between urea and L-trp was significant (P
< 0.05), but the asymmetry between urea and L-phe was not (P =
0.16). This experiment supports the theory that urea has TRCs or
receptor/transduction mechanisms in common with the amino acids; however, in
addition, urea appears to activate bitter taste mechanisms that are
independent of the amino acids.
An important feature of the adaptation results was the general symmetry between the amino acids and their interactions with the other compounds tested, whether the amino acids were adapting or test stimuli (Figure 2 and Table 2). Further analysis of the results revealed two subpopulations of subjects that were demarcated by whether QHCl cross-adapted the amino acids more than the amino acids cross-adapted QHCl or the opposite (Figure 4). Even within the two subpopulations, there was symmetry between the amino acids. For example, adaptation to QHCl suppressed bitterness of L-trp and L-phe 25 and 20%, respectively, for group A, or 87 and 84%, respectively, for group B. The consistency or symmetry observed between L-trp and L-phe in these two different groups is further evidence of shared bitter taste transduction mechanisms for the two amino acids.
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For nine of the 13 subjects (group A), QHCl had limited efficacy when cross-adapting the amino acids bitterness, but these compounds were able to cross-adapt QHCl bitterness. Conversely, the remaining four subjects (group B) reported that QHCl was able to cross-adapt the amino acids bitterness, while the amino acids were less effective in crossadapting QHCl bitterness. There is no simple explanation for the observed variation, and it may indicate variation in bitter taste transduction mechanisms among the subjects or relative proportion of amino acid/quinine receptors on the TRCs.
In addition, if urea replaces quinine at the center of Figure 4, then the differences between groups A and B disappear and the interactions appear as in Figure 2.
Results from this study replicate McBurney et al. who showed that
urea was able to cross-adapt the bitterness of QHCl, while QHCl was not as
effective at cross-adapting urea's bitterness
(McBurney et al.,
1972). Others have also inferred that urea and QHCl activate
separate bitter taste transduction sequences
(Lawless, 1979;
Yokomukai et al.,
1993), which was supported in the present study. The present
observation that urea was able to cross-adapt QHCl's bitterness suggests that
urea at least activates overlapping bitter TRCs, or receptor/transduction
mechanisms involved in transducing QHCl's bitter taste.
One hypothesis regarding urea's ability to cross-adapt many bitter
compounds relates to its ability to disrupt non-covalent interactions in
proteins and enzymes, and permeate through cellular membranes
(Lyall et al., 1999).
Urea could potentially modulate a wide variety of processes interfering with
receptor protein conformation or altering enzyme activity involved in bitter
taste transduction. Closer examination of results from McBurney et
al. appear to support this hypothesis and, although the adapting
concentration of urea (1 M) used by McBurney et al. was not as
concentrated as used in the present experiment, it appears that urea
cross-adapted at least 50% of the bitterness of the majority of compounds
(QHCl, quinine-SO4, caffeine, KNO3, MgSO4,
SOA), with the exception of PTC (McBurney
et al., 1972).
Bitterness inhibition
Sodium salts' influence on bitterness is believed to occur in the
peripheral taste system as a result of sodium's action on the gustatory
physiology, rather than more central action caused by cognitive effects of
perceived saltiness (Bartoshuk,
1979,
1980;
Bartoshuk and Seibyl, 1982;
Kroeze and Bartoshuk, 1985;
Kemp and Beauchamp, 1994;
Breslin and Beauchamp, 1995).
Keast et al. proposed four potential modes of action for sodium salts
in the peripheral taste system: (i) shielding of the receptor protein; (ii)
moderating or modulating ion channels or pumps; (iii) stabilizing the cell
membrane; and (iv) interfering with second messenger systems after entering
cells (Keast et al.,
2001). If L-trp, L-phe and urea were activating the same taste
TRCs or receptor/transduction pathways, sodium salts should not differentially
inhibit their bitterness. The findings show that sodium salts could not
differentially inhibit the bitterness of L-trp, L-phe and urea, but did
differentially affect MgSO4, although the impact of the sodium
salts on urea's bitterness was less than previously observed
(Breslin and Beauchamp, 1995).
This may be due to the current use of very high concentrations (2.33 M) of
urea compared with the previously used concentration (1 M).
When results for inhibition of urea's bitterness were analyzed according to
the concentration of urea required to elicit isointense bitterness, subjects
who required between 3.6 M and 2.5 M (n = 5) reported the inhibition
of urea's bitterness was only 12% with the addition of NaCl. Subjects who
required a urea concentration of 2.4 M or below (n = 8) reported NaCl
reduced bitterness by 58%, which was similar to bitterness reduction of the
amino acids by NaCl (Figure 5).
This analysis supports earlier research
(Breslin and Beauchamp, 1995)
that demonstrated a 60% reduction in urea's (1 M) bitterness when NaCl was
added. Perhaps concentrated urea solutions (>2.4 M) influence TRC's sensory
activity; these concentrated solutions, as well as urea's ability to permeate
cellular membranes and disrupt protein configuration, may decrease the effect
sodium has on bitter taste transduction.
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Conclusions |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Self- and cross-adaptation results showed that the two amino acids were very similar but that the amino acids and urea were not identical adapting stimuli; cross-adaptation between the L-trp and L-phe was symmetrical and nearly complete, whereas cross-adaptation of urea and the amino acids was not symmetrical, indicating that urea has a portion of independent taste mechanisms involved in its bitter taste. The bitterness inhibition experiments were unable to differentiate between urea and the amino acids, but did show they differed from MgSO4. As the adaptation experiment activated only fungiform papillae taste cells and the bitterness inhibition experiment activated whole mouth taste cells, the two experiments cannot be directly compared. Rather, each experiment should be viewed as an independent test of whether the amino acids and urea can be psychophysically distinguished.
Three independent psychophysical techniques, namely correlation of
individual sensitivities (Delwiche et
al., 2001), cross-adaptation and bitterness inhibition,
illustrate a close perceptual association between L-trp, L-phe and to a lesser
degree urea, which suggests shared peripheral physiological mechanisms
involved in bitter taste transduction.
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Acknowledgments |
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This research was supported by NIH grant DC02995 (P.A.S.B.). We thank Dr Gary Beauchamp for his valuable advice in preparation of this manuscript.
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