Chem. Senses 27: 105-115,
2002
© Oxford University Press 2002
A New Specific Ageusia
Some Humans Cannot Taste L-Glutamate
Laboratoire de Neurobiologie Sensorielle, 1 avenue des Olympiades, F-91744 Massy cedex and EA 359, Université Paris VII, France
Correspondence to be sent to: Dr Annick Faurion, Laboratoire de Neurobiologie Sensorielle, 1 avenue des Olympiades, F-91744 Massy cedex, France. e-mail: faurion{at}ccr.jussieu.fr
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A new specific ageusia was found in human subjects for monosodium L-glutamate (MSG). Four tests were successively applied to discriminate non-tasters and hypotasters from tasters. (i) NaCl and MSG thresholds, and (ii) suprathreshold sensitivity were evaluated using the up-and-down procedure. Only 73% of 109 subjects common to both tests demonstrated a sensitivity for MSG significantly higher than their sensitivity to NaCl, and hence a specific sensitivity to L-glutamate. The remaining 27% who showed no significant difference in sensitivity to MSG and NaCl solutions were considered as putative hypotasters. (iii) Perception profiles (time—intensity) for MSG and NaCl were tested in 58 subjects and appeared significantly different in 47 tasters (81%). This technique helped in identifying among putative hypotasters of tests 1 and 2 a few tasters who perceived equal intensity for isoconcentration of NaCl and MSG but who could discriminate isomolar solutions on other cues. Thus, 19% of subjects, for whom no significant differences were found between MSG and NaCl time—intensity profiles, remained in the hypotaster group. (iv) A discrimination task including 24 triangular presentations per subject of NaCl and MSG 29 mM applied to the eight most severe hypotasters showed that two subjects at least (two of 58; 3.5%) could not discriminate between both stimuli. Moreover, these subjects probably perceived identical sensations for MSG and NaCl solutions. The six other hypotasters (10.3%) could discriminate both stimuli at the limit of significance. None of these eight subjects were able to identify the typical umami taste in 29 mM MSG.
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Introduction |
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Monosodium L-glutamate (MSG) elicits a specific taste, which was named umami after Ikeda (Ikeda, 1912
). MSG has two specific properties: a preference enhancer
effect when added to foods, although MSG is not preferred in pure water
(Yamaguchi and Takahashi,
1984; Roininen et
al., 1996), and a synergistic effect in mixtures with
5'-ribonucleotides such as inosine-, adenosine-, guanosine
5'-monophosphate, etc. (Yamaguchi,
1967).
Electrophysiological studies showed that several receptors should be
involved in umami taste perception. The analysis of chorda tympani (CT) nerve
responses to various ionotropic and metabotropic agonists of L-glutamate
central nervous system (CNS) receptors
(Faurion, 1991) suggested a
similarity between taste and CNS glutamate receptors. The metabotropic
L-glutamate receptors (mGluR4), and ionotropic L-glutamate receptors were
further identified in circumvallate papillae taste cells of the rat by
Chaudhari et al. (Chaudhari
et al., 1996). Moreover, the same authors showed that
conditioned taste aversion to L-2-amino-4-phosphonobutyrate (L-AP4), an
agonist of the CNS receptor mGluR4, generalized to L-glutamate in rats.
Converging studies showed the involvement of at least two mGluRs and
ionotropic receptors (Bigiani et
al. 1997; Lin and
Kinnamon, 1999; Sako and
Yamamoto, 1999). More recently, Chaudhari et al.
(Chaudhari et al.,
2000) identified the `taste-mGluR4', a truncated form of the CNS
mGluR4 receptor, as well as the original CNS form in circumvallate taste
cells. Lin and Kinnamon (Lin and Kinnamon,
1999) showed by patch-clamp recordings that
N-methyl-D-aspartate (NMDA) ionotropic receptors were present in the
taste cells of fungiform papillae. Stapleton et al.
(Stapleton et al.,
1999) on the one hand and Nakashima et al.
(Nakashima et al.,
2001) on the other hand, studied by conditioned taste aversion the
relative role of NMDA receptors in MSG taste transduction. Finally, it has
also been suggested that L-glutamate might have a neurotransmitter function
either at the synapse between taste cells and taste fibers
(Lawton et al., 2000;
Caicedo et al., 2000)
or between taste bud cells (Caicedo et
al., 2000).
If several receptors to L-glutamate, ionotropic receptors, mGluRs receptors
and maybe sweet sensitive receptors
(Nakashima et al.,
2001), were actually involved in taste transduction, that is if
the determinism of glutamate taste sensitivity were polygenic, we might find
interindividual differences of quantitative sensitivity to MSG among subjects.
Although studies already considered the evaluation of threshold to MSG or
glutamic acid compared with NaCl (Schiffman et al.,
1981,
1991;
Yamaguchi, 1991;
Mojet et al., 2001)
or the intensity functions recorded for these chemicals
(Schiffman et al.,
1991; Yamaguchi,
1991), none considered individual sensitivities.
Population studies can reveal interindividual differences of sensitivity as
in the case of phenylthiocarbamide (PTC) or 6-n-propyl-2-thiouracil
(PROP). For these compounds, subjects can be classified as tasters or
non-tasters, on a genetic heritable trait
(Snyder, 1931;
Lugg, 1966;
Kalmus, 1971). Large
interindividual differences in taste sensitivity were also shown for other
organic compounds (Blakeslee and Salmon,
1935), which were demonstrated to be significantly higher than the
intra-individual variance (Faurion et
al., 1980; Faurion,
1993; Froloff et al.,
1996). Understanding the glutamate taste receptor mechanism might
benefit from a population study and the purpose of the present experiment was
to look for a specific ageusia to L-glutamate. Psychophysical experiments were
set up to study the distribution of the sensitivity to L-glutamate in human
subjects. Non-tasters or hypotasters were distinguished from the taster
population after a series of four successive experiments. The first two tests
(detection threshold determination, suprathreshold evaluation of taste
intensity) included a large number of subjects to screen for the possible
incidence of hypotasters and the last two (time—intensity profiles and
taste quality discrimination) verified this in a reduced part of the same
sample.
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Materials and methods |
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As MSG contains sodium, great care was taken to compare individual sensitivities to MSG relative to NaCl. Only subjects perceiving MSG at concentrations below their NaCl threshold concentration can be directly considered as tasters; however, we must also consider that a subject may perceive glutamate at concentrations equivalent to the perceived concentrations of NaCl. In this case, the evaluation of quantitative sensitivity cannot distinguish between a non-taster experiencing only the taste of Na+ in MSG and a subject perceiving both, and therefore experiencing iso-intensity at isomolar concentrations. Those subjects, probably hypotasters to a variable degree, should not be considered as non-tasters. In order to sort out the tasters from the hypotaster group, we used the four tests described further below.
Subjects
Subjects came from the local university and the local community. A population of 171 subjects (98 women and 73 men, 25 ± 9 years old) participated in the threshold experiment. A population of 119 subjects (73 women and 46 men, 27 ± 10 years old) participated in the suprathreshold experiment. A common group of 109 subjects participated in both. Subjects performed one session per day, which lasted 90 min. Each subject fulfilled four sessions. They were asked not to eat, drink or smoke at least for 30 min before the session. Fifty-eight of 109 subjects also participated in the time—intensity test. Eight subjects with a very poor sensitivity to MSG compared with NaCl were included in the last discrimination task. Finally, 64 among the same group of subjects were tested for their sensitivity to PROP.
Stimuli
NaCl (Merck, France, MW 58) and MSG (Eurolysine, France, MW 187) concentrations ranged from 0.002 mM to 10 mM in a geometric progression of ratio 2 for threshold determination. For iso-intensity evaluation, the reference was 29 mM NaCl and MSG concentrations ranged from 0.12 to 60 mM in a geometric progression of ratio 2. Solutions were daily prepared in sterile conditions, using ultraviolet sterilized water (Aqua-Stoutz, Actini, France).
The time—intensity perception profile (finger-span) and the
discrimination triangular tests were performed with 29 mM solutions of NaCl
and MSG. PROP taster and non-taster subjects were discriminated with a 0.29 mM
PROP solution (Sigma, France, MW 170.2), a concentration surely perceived by
tasters (Drewnowski et al.,
1997).
Experimental procedures
Test 1: detection thresholds
Subjects tasted each pair, comparing the stimulus (NaCl or MSG) against
water, without swallowing. They answered the question: `which is not water?',
rinsed their mouth with tap water, then rested. The concentration of the
stimulus of the following pair was increased by one log to base 2 step up
after a false response or decreased by one log to base 2 step down after a
correct response. Subjects had to fulfill one paired test within no less than
30 s for NaCl threshold, and within no less than 50 s for MSG threshold to
take account of the long-lasting perception of umami substances.
Thresholds were determined using the up-and-down procedure (U&D) of
Dixon (Dixon and Massey, 1960;
Faurion, 1993), associated
with forced choice pair comparisons. It included five pairs after the first
change of response from correct to false or vice-versa. The pattern of correct
and false responses allowed determining a k coefficient in the
Dixon's table (Dixon and Massey,
1960). The threshold concentration, T, was calculated
according to the formula T = Cf x
rk, with Cf being the final
concentration presented and, r, the ratio between successive
concentrations (geometric progression). The duration of one U&D test was
7 min. During one session, each subject performed four U&D tests for
NaCl and four for MSG threshold determinations and during the four sessions,
16 determinations of each.
Test 2: suprathreshold measurements
At suprathreshold level, the U&D technique was also used to evaluate
the individual concentration of MSG eliciting an isointense perception
compared with the 29 mM NaCl reference. Subjects compared MSG concentrations
versus 29 mM NaCl in forced choice pairs and answered the question `which is
stronger?'. They had to wait for a resting delay of 45 s after rinsing to take
account of the long-lasting MSG perception so that each paired test lasted at
least 1.15 min. The concentration of the stimulus (MSG) varied according to
the U&D technique: increasing one log to base 2 step up if the stimulus
had been evaluated as less strong as the reference (NaCl) and decreasing one
log to base 2 step down if the stimulus had been evaluated stronger than the
reference. The 29 mM NaCl isointense concentration of MSG,
Ciso MSG, was calculated according to the formula
Ciso MSG = Cf x
rk. Each subject performed four U&D suprathreshold
tests per session (16 in four sessions).
Both threshold and suprathreshold tests were computer assisted. Two laboratory-built automata based on Apple II computers and laboratory-made interfaces diluted stock solutions and delivered the right concentration to each of the eight subjects working simultaneously. Twelve U&D tests were run within the same session in the following order: NaCl thresholds, MSG thresholds, then suprathreshold isointensity evaluations, within 1.30 h. The concentrations were computer delivered according to the previous subject's response but the four U&D tests of the same nature were intermingled so that, for the subject, the concentrations looked randomized. The temperature difference between both stimuli within a pair was controlled and kept below 0.1°C.
Test 3: time—intensity profiles
A long-lasting lingering effect is usually observed for glutamate and, as a
consequence, a long rinse is often necessary between pairs, contrasting with
NaCl solutions. The time course of the perceived intensity for 29 mM solutions
of NaCl and MSG was measured in 58 subjects of the initial group thanks to the
finger-span technique (Berglund et
al., 1978), using a potentiometer in a circuit. The subject
took a sip of the solution and, with closed eyes, continuously indicated the
perceived intensity elicited by the stimulus. This intensity was evaluated as
the distance between the subject's thumb, which was fixed on the potentiometer
end and his/her forefinger tip attached to the moving cursor of the
potentiometer. The voltage was digitalized, acquired and stored for further
treatment. After a mouth rinse, the subject waited 2 min between NaCl and
MSG. The aim of this experiment was to observe a similarity or dissimilarity
between the time—intensity profiles of both stimuli.
Test 4: discrimination test
At this step, putative hypotasters were subjects who seemed not to perceive
L-glutamate anion in any of the first three tests. Triangular tests were used
to check whether putative MSG hypotasters having passed the three preceding
tests could discriminate NaCl and MSG at isomolarity (29 mM) on a qualitative
basis. Eight subjects participated in a 24-triangle test presentation. Each
triangle test was presented at a time interval of 1.30 min.
Statistical analysis
Threshold and suprathreshold evaluations
After four sessions, 16 NaCl detection thresholds, 16 MSG detection
thresholds, and 16 MSG isointense concentrations were collected for each
subject. The individual NaCl detection threshold, MSG detection threshold, and
the MSG isointense concentration were calculated as means of 16 U&Ds,
except in certain cases where the U&D data from first sessions were
discarded if statistically different from the rest of the data (Student's
t-test, 
< 0.05).
The distributions of NaCl and MSG thresholds and of MSG isointense
concentrations were plotted on a logarithmic abscissae. Each distribution was
compared with a normal theoretical distribution ( < 0.05,
2). These distributions were studied with a multigaussian
fitting analysis (Biopatch software, Biologic, Claix, France).
Statistical criteria for subject classification
Subjects exhibiting MSG sensitivity at isomolarity with NaCl were suspected
not to be sensitive to the L-glutamate anion but only to the sodium cation in
the MSG solution. The taster or hypotaster status was defined in the group of
109 subjects. In test 1, a subject was defined as a taster when his/her MSG
threshold was lower than his/her NaCl threshold at the level of P
< 0.10 (Student's t-test). This conservative criterion (P
= 0.10) was selected in order to avoid the risk that tasters might be
classified into the hypotaster category. On the contrary, borderline subjects
were kept in the taster category. At suprathreshold level, the criterion
chosen to classify a subject into the taster group was a MSG isointense
concentration lower than the NaCl reference concentration
(Ciso MSG + 2 SD < 29 mM NaCl).
The subject's classification was verified with the time—intensity test. The MSG time—intensity individual raw profile was normalized to the subject's maximal perceived intensity for NaCl (= unity). Profiles were then grouped into two categories according to the taster/hypotaster status of the subject after tests 1 and 2. A few hypotasters showing a difference between NaCl and MSG time—intensity profiles were reclassified into the taster group. Time intensity profiles for NaCl and MSG were averaged for each group of subjects, SEM were calculated. As the maximal intensity for NaCl was not localized at the same time depending on subjects, some variation remained in the NaCl group profile and the maximum of the averaged NaCl profile resulted lower than unity. Averaged profiles were finally smoothed with a sliding window on 5 s.
Student's t-test compared NaCl and MSG averaged time—intensity profiles at maximum intensity within each group of tasters and hypotasters (the degrees of freedom depending on the numbers of subjects). Similarly, tasters and hypotasters averaged time—intensity profiles were also compared at maximum intensity. Student's t-test was also used for paired comparison of total averaged profiles (frequency of data acquisition: 1 Hz; time window superior to 200 s, d.f. > 200).
Finally, subjects were declared hypotasters if they exhibited a non-significant difference between NaCl and MSG sensitivities at both levels and if time—intensity profiles for NaCl and MSG were non-significantly different. Hypotasters were declared non-tasters if they met the ß risk criterion at P = 0.10 for identity of perception for both stimuli in the triangular test (Table 1).
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Results |
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Three examples of raw results are presented in Table 2 for tests 1, 2 and 4 and, in Figure 1, for test 3.
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NaCl and MSG thresholds distributions
The group threshold for NaCl was 0.82 ± 1.05 mM (mean ± SD;
n = 171) and the group threshold for MSG was 0.32 ± 0.35 mM
(mean ± SD; n = 171). The distribution of individual values
for MSG (Figure 2b), ranging
from 0.006 to 2.89 mM, could not be fitted with a single normal distribution
(P < 10-8; 2; d.f. = 19). A
multigaussian fitting analysis (Biopatch software, Biologic) indicated at
least two distinct normal distributions with respective means at
m1 = 0.08 mM (ranging from 0.03 mM to 0.18 mM: mean
± 2 SD) and m2 = 0.39 mM (ranging from 0.14 mM to
1.07 mM: mean ± 2 SD). These subpopulations were statistically
different (P = 0.03; t-test; d.f. = 155). Two subjects
reached threshold values for MSG above 1.66 mM, i.e.: 3 SD above the higher
mean threshold; and six subjects reached values below 0.02 mM, i.e.: 3 SD
below the lower mean threshold m1. Although the
distribution of individual NaCl thresholds
(Figure 2a) could not be fitted
with a single normal function (P < 10-4;
2; d.f. = 19), the multigaussian analysis could not reveal any
clearly defined subpopulation for this stimulus.
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Distribution of MSG sensitivity at suprathreshold level
Suprathreshold sensitivity for MSG was measured as the MSG concentration
eliciting the same perceived intensity as 29 mM NaCl in 119 subjects. The
group MSG isointense concentration was 22 ± 21 mM (mean ± SD).
Although the distribution of isointense concentrations
(Figure 3) was not fitted by a
normal function (P < 10-10; 2; d.f. =
15) sub-populations could not be established using multigaussian analysis.
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Subject classification
Based on the statistical criteria established with the results of tests 1 and 2 (threshold and suprathreshold isointense evaluations), 80 of 109 subjects (73%) who perceived L-glutamate either at one or at both levels were classified as tasters. Of 80 subjects, 34 perceived L-glutamate at both levels (example `Nic' in Figure 4), 28 showed they perceived L-glutamate specifically in the suprathreshold test only and 18 in the threshold test only (examples `Mou' and `Lab' respectively).
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Table 3 exhibits the number of subjects that could be classified as putative hypotasters or non-tasters after each successive test. Figure 2c shows the ratio of individual threshold concentrations for MSG and NaCl (r = TMSG/TNaCl). Subjects with a ratio below unity perceived MSG at a lower concentration than NaCl and were easily discarded, after checking the significant difference of their average MSG and NaCl thresholds, from the group of hypotasters that was looked for in this study.
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Twenty-nine of 109 subjects (27%) were classified as putative hypotasters
considering their poor sensitivity to MSG at threshold level as well as at
suprathreshold level. Among these 29 subjects, 19 showed a threshold for MSG
not significantly different from their threshold to NaCl (P 
0.10; Student's t) and their Ciso MSG not
significantly inferior to 29 mM, the concentration of the NaCl reference
solution. Examples of data from such subjects are represented along the
isomolarity line in Figure 4
(open symbols). The 10 other subjects among the 29 putative hypotasters
exhibited a significantly lower sensitivity to MSG than to NaCl (instead of a
better sensitivity to MSG than to NaCl in tasters) either at threshold level
(n = 3; P < 0.04; Student's t) or at
suprathreshold level (n = 7; Ciso MSG -3 SD >
29 mM). These 10 subjects could not taste L-glutamate in the MSG solution;
moreover, they perceived the sodium taste less well in the L-glutamate salt
than in the chloride salt (example `Dou' in
Figure 4).
Independence of sensitivities to NaCl and MSG
The comparison of individual thresholds failed to reveal a correlation between the sensitivity to MSG and the sensitivity to NaCl (rs = 0.25; n = 171; Spearman test). On the contrary, putative hypotasters alone clearly exhibited a positive correlation between MSG and NaCl thresholds (rs = 0.75; n = 19; Spearman test).
Time—intensity profile
Two different time—intensity profiles for MSG were clearly observed within the group of 58 subjects among 109 who participated in the time—intensity test. Three examples of individual results are shown in Figure 1. The average tasters' time—intensity profile (Figure 5) was characterized by an eightfold difference between MSG and NaCl at maximum perceived intensity (P < 0.001; Student's t; d.f. = 92) and a long-lasting effect of 10 min or more for MSG without rinse. The overall paired (NaCl, MSG) profiles were significantly different (P < 0.001; paired Student's t; d.f. = 205). Conversely, the hypotaster category presented similar profiles for NaCl and MSG in intensity and duration of response (Figure 5). The time—intensity test confirmed hypotasters did not perceive MSG more intensely than the equimolar NaCl [Student's t = not significant (NS); d.f. = 20]. Their perception of MSG was significantly lower than the tasters' perception (P = 0.003; Student's t at 25 s; d.f. = 56).
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After this test on 58 subjects, 47 subjects (81%) fell into the taster category and 11 subjects (19%) actually fell into the hypotaster category. Among the 47 tasters were eight subjects who had been previously classified among the putative hypotasters (Table 3). The time—intensity test showed they were able to discriminate both solutions, although they proved to be poorly sensitive on a quantitative basis in the first two tests.
Discrimination test
A discrimination test was run to verify whether the preceding 11
hypotasters were also confusing MSG with NaCl at the same concentration as if
the stimuli elicited the same quality. Eight among the 11 hypotasters of the
preceding test performed the triangular discrimination test. Among them
(Table 3), six subjects
discriminated isomolar MSG and NaCl solution on a qualitative basis with 13-18
correct answers of 24 tests (P 
0.05, error). These
hypotasters, however, could not be considered as tasters in any of the first
three tests. They presented a low score in the discrimination test compared
with glutamate-sensitive subjects who clearly succeeded throughout this test.
The two remaining subjects were assumed to be non-tasters as they could not
discriminate MSG from NaCl (nine and 12 correct answers respectively of 24
tests; P > 0.05, error). They met the statistical
criterion of 0.10 for the ß risk, which allows considering they perceived
`identical' tastes. The three subjects who missed this test remained
undetermined either hypotasters or non-tasters and were not classified.
Subjects could be classified into the taster, hypotaster and non-taster categories after all four tests. Tasters represented 81% (47/58) of subjects. At least 10% (6/58) of subjects were considered as glutamate hypotasters and 3.5% (2/58) of subjects could be considered as glutamate non-tasters.
Comparison of MSG and PROP sensitivities.
Sixty-four subjects also tasted a 0.29 mM solution of PROP. The
2 calculated with a 2 x 2 contingency table on the
number of PROP tasters/non-tasters and MSG tasters/hypotasters showed
independence between MSG and PROP sensitivities (2 = 0.12; NS;
d.f. = 1).
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Discussion |
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The present study has shown a high interindividual difference of sensitivity for MSG as exhibited by distributions; the sensitivity to MSG is clearly continuous on the concentration axis. Comparing the individual sensitivity to NaCl and MSG allowed separating subjects into two groups: those who clearly discriminated MSG from NaCl because they perceived MSG at a significantly lower concentration than NaCl (tasters) and other subjects. The time—intensity and the triangular tests showed that among these other subjects, some could perceive MSG at isoconcentration with NaCl (hypotasters, 10%) and two subjects actually seemed to confuse MSG with NaCl (non-tasters, 3.5%). It is noteworthy that all these tests could be performed satisfactorily only after some learning (familiarization to MSG) and that no semantic description was found useful to pick out hypotasters in the population.
Working at constant intensity, with the U&D procedure, rather than at constant concentration, as with magnitude estimation, results in a very efficient method. The U&D procedure allows collecting several threshold or suprathreshold results per subject in a short time, which allows a statistical evaluation of the individual threshold, including the expression of the individual variance. The reproducibility of individual data can be checked across days and even within each session.
Distributions showed that the subjects of this study exhibited very different levels of sensitivity to MSG, thresholds ranging from 0.006 to 2.89 mM, and spanning a 500-fold range of concentration. Comparatively, the error on the individual subject's threshold was about twofold (with an average coefficient of variation of 0.9 ± 0.4, n = 171, for MSG and NaCl similarly), which is amazingly low compared with interindividual differences. Hence, a large continuous variation of the sensitivity to L-glutamate was demonstrated in this population.
The absence of a correlation between MSG and NaCl individual thresholds
shows the independence of both sensitivities in this group of subjects.
Yamaguchi (Yamaguchi, 1991),
Breslin and Tharp (Breslin and Tharp,
2001) and Mojet et al.
(Mojet et al., 2001)
also stressed the independence of NaCl and MSG taste sensitivities. In the
present study, hypotasters, on the contrary, showed a positive correlation
between MSG and NaCl thresholds (rs = 0.75). This showed
that hypotasters probably perceived mostly the sodium cation in the glutamate
salt.
It should be noted that detection threshold and suprathreshold results do
not covary. Some subjects had similar threshold concentrations for MSG and
NaCl but showed a MSG isointense concentration significantly lower than the 29
mM of the NaCl reference. Other subjects showed isointensity for isomolar
solutions of NaCl and MSG but had MSG thresholds significantly lower than
their NaCl threshold. Examples in Figure
4 showed that the sensitivity at threshold level could not predict
the quantitative sensitivity of the same subject at suprathreshold level (and
vice versa). This indicates a great difference in intensity functions among
subjects as already shown in the literature
(Faurion et al.,
1980). Schiffman et al.
(Schiffman et al.,
1991) also showed different slopes of intensity functions for
L-glutamic acid, in two groups of young and elderly subjects.
Both threshold and suprathreshold levels were necessary to determine the glutamate sensitivity status of a given subject. But qualitative tests also appeared necessary to discriminate MSG hypotasters and non-tasters from tasters, as exemplified in Table 3. Results from quantitative psychophysics lead to conclude that when subjects perceived isointensity at isomolarity, they may perceive only the sodium cation in the L-glutamate sodium salt. However, they might, on the contrary, be able actually to perceive L-glutamate at isomolarity with sodium. To overcome this uncertainty, we checked the ability of putative hypotasters to discriminate suprathreshold isomolar solutions of MSG and NaCl either by the time—intensity test or on a qualitative basis with triangular presentations. The time—intensity technique discarded at once among hypotasters, subjects who could discriminate the umami taste from the sodium taste. Though, these subjects were neither able to name it nor describe it even after the psychophysical training.
We noticed that subjects used the finger-span technique as a semiquantitative measurement: hypotasters used the full scale of the finger-span for small differences of perception as well as did tasters for large differences of perception. This technique acted as a sensitivity difference amplifier.
In the triangular discrimination test, eight hypotasters could statistically discriminate the glutamate anion from the sodium cation in the monosodium glutamate salt, on a qualitative basis. However, they were actually hypotasters regarding their quantitative data and definitely unable to comment on any difference of taste quality between both solutions. Furthermore, they could not give correct responses to all 24 triangles. Two subjects who could not discriminate MSG from NaCl might even experience an identical perception for NaCl and for MSG: they were therefore considered as `non-tasters'.
The study of a salt such as MSG presents a specific difficulty relative to the presence of a cation should it be Na+ or K+ or Ca+, etc. The sensitivity to L-glutamate is measurable up to the limit of the subject's sensitivity to this cation (here NaCl).
Distribution multimodality
The MSG threshold distribution revealed that hypotasters constitute a
continuous group between the tasters and the non-tasters. The multigaussian
analysis revealed two taster populations of average thresholds at 0.08 mM and
0.39 mM, respectively. This latter value is similar to findings by Yamaguchi
(Yamaguchi, 1991), Yamaguchi
and Ninomiya (Yamaguchi and Ninomiya,
2000) who could localize MSG threshold at 0.6 mM and also showed a
slightly lower threshold concentration for MSG than for NaCl in their group of
subjects. Comparatively, Schiffman et al.
(Schiffman et al.,
1991) showed a group threshold for MSG at 0.9 mM for young
subjects. Recently, Mojet et al.
(Mojet et al., 2001)
found MSG thresholds from young subjects to elderly men and women ranging from
1.3 to 2.5 mM. At the other end of the distribution found in this study, a few
subjects appeared extremely sensitive to glutamate. Similarly, in the
literature, the distribution of PTC sensitivity also appears more than
bimodal: Lugg (Lugg, 1966)
showed that the PTC sensitivity of a population of 500 subjects could be
divided into six modes. Moreover, a population of hypertasters was clearly
identified for PTC (Lugg,
1962), and for PROP (Bartoshuk
et al., 1996).
PROP [or PTC, the ageusia to PTC
(Kalmus, 1971) and to PROP is
one and the same] and MSG, are presently the only two representatives of taste
blindness in humans. Both show a multimodal distribution of human taste
sensitivities indicating a multireceptor mediation. However, the independence
between PROP and L-glutamate sensitivities, found in the present study,
further indicates these ageusiae to be independent and suggests the
contribution of different receptors or transduction mechanisms to recognize
these stimuli.
Besides all statistical criteria on which categories can be built, it remains unclear whether the concept of `non-taster' can be used for two reasons. The Na+ cation perception may hide a low, although existing, umami stimulation due to the L-glutamate anion with no resulting perception. Many tests are necessary in putative non-tasters to make sure that no perception is possible under all conditions. Moreover, within the literature relevant to PROP ageusia, the definition of a PROP `non-taster' itself is ambiguous, as PROP non-tasters can perceive PROP by increasing the concentration of the solution.
Most interestingly, at threshold level or suprathreshold level, some
subjects among hypotasters apparently could not perceive the Na+
cation as well in the MSG solution as in the isomolar NaCl solution. This may
be due to a difference in the degree of dissociation of both salts, MSG being
less dissociated than NaCl. But another interpretation may be kept in mind.
Although inefficient to produce an umami perception or any other measurable
perception, L-glutamate may interfere with some transduction process and
inhibit the sodium sensation. Together with the synergy effect between MSG and
ribonucleotides (Yamaguchi,
1967; Sato et al.,
1970; Torii and Cagan,
1980), this inhibitory effect might be a new cue for the
understanding of receptor events.
Mechanistic hypotheses: multiple receptors for umami taste?
Using psychophysical measurements, electrophysiological recordings and
molecular modeling on various sweet and bitter compounds, Faurion et
al. (Faurion et al.,
1980), Faurion and Vayssettes-Courchay
(Faurion and Vayssettes-Courchay,
1990), Faurion (Faurion,
1993) and Froloff et al. (Froloff et al.,
1996,
1998) suggested a
multireceptor hypothesis in which several receptor sites of low specificity
cooperatively recognize organic molecules. Shimazaki et al.
(Shimazaki et al.,
1981) and Cagan (Cagan,
1986) convergingly documented with biochemical assays the low
affinity and low specificity of molecular chemoreceptors for taste. Other
arguments converging on the multireceptor hypothesis can be found in recent
literature. Abe et al. (Abe et
al., 1993) found 60 clones coding for putative receptor
proteins in taste cells. Bernhardt et al.
(Bernhardt et al.,
1996) showed that different stimuli (sucrose and saccharin) of the
same quality could use different transduction pathways, hence different
receptors, which was further confirmed by Uchida and Sato
(Uchida and Sato, 1997) with
D-phenyl-alanine and D-tryptophan, and reviewed by Herness and Gilbertson
(Herness and Gilbertson,
1999).
For MSG particularly, a polygenic mechanism of L-glutamate chemoreception
could be responsible for the multigaussian distribution observed. Arguments
from the literature suggest the existence of several MSG receptor types in
taste cells (G protein coupled receptors) or several transduction pathways.
(i) Both, the `taste-mGluR4' receptor and the CNS form of the mGluR4 [known to
decrease intracellular cyclic adenosine monophosphate (cAMP)] are present in
taste cells (Chaudhari et al.,
1996,
2000). As several other forms
of L-glutamate receptors are expressed in the CNS, the same might be true for
taste cells. (ii) An NMDA-like receptor was confirmed by patch-clamp studies
(Lin and Kinnamon, 1999). (3)
From an another point of view, an increase of the cAMP and of IP3
(Ninomiya et al.,
2000) were both found involved after MSG stimulation. On the
contrary, Bigiani et al. (Bigiani
et al., 1997), Lin and Kinnamon
(Lin and Kinnamon, 1998) and
Brand (Brand, 2000) suggested
that `taste-mGluR4' couples negatively to a cAMP cascade. As Varkevisser and
Kinnamon (Varkevisser and Kinnamon,
2000) suggested, different transduction pathways may interact
within a single taste cell either at the level of second messengers or at the
target channels of the basal membrane. (iv) Furthermore, converging arguments
suggest the implication of gurmarin sensitive receptors in addition to
ionotropic and metabotropic CNS-like receptors for MSG taste sensitivity
(Sako and Yamamoto, 1999;
Ninomiya et al.,
2000; Nakashima et
al., 2001). These authors conclude to the involvement of
sweet sensitive receptors. However, the relationship between MSG and sucrose
chemoreception is not yet clearly understood. If MSG increases cAMP in taste
cells as sucrose does (Striem et
al., 1989), this would be sufficient to explain some partial
similarities between electrophysiological responses to L-glutamate and some
sweet stimuli (Yamamoto et al.,
1991; Nakashima et
al., 2001).
The present study, showing more than one mode in the distribution of taster
MSG thresholds, suggests that L-glutamate may interact with a series of
receptors at the surface of taste cells. The coding of the L-glutamate taste
signal could result from the cooperation of these various receptors and,
therefore, depending on genetic peculiarities, human subjects might perceive
or not the proper umami taste. Glutamate is probably a good example of a
`tastant using more than one mechanism'
(Herness and Gilbertson,
1999). Finally, the existence of non-tasters together with the
already accumulated knowledge on L-glutamate CNS receptors could be a very
efficient entry into the study of taste receptors.
|
Acknowledgments |
|---|
Part of this work was presented at the AchemS XXXIIIrd Annual meeting, April 25-29, 2001, Sarasota, Florida, USA. Supported by the International Glutamate Technical Committee (IGTC) and le Comité des Fabricants d'Acide Glutamique (COFAG).
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Accepted October 1, 2001
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