Chem. Senses 28: 261-266,
2003
© Oxford University Press 2003
Synergistic Responses of the Chorda Tympani to Mixtures of Umami and Sweet Substances in Rats
Department of Oral Physiology, Asahi University School of Dentistry, 1851 Hozumi, Hozumi, Motosu, Gifu 501-0296 1 Department of Behavioral Physiology, Graduate School of Human Sciences, Osaka University, 1-2 Yamadaoka, Suita, Osaka 565-0871, Japan
Correspondence to be sent to: Noritaka Sako, Department of Oral Physiology, Asahi University School of Dentistry, 1851 Hozumi, Hozumi, Motosu, Gifu 501-0296 Japan. e-mail: sako{at}dent.asahi-u.ac.jp
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Abstract |
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It has been known that umami substances such as monosodium L-glutamate (MSG) and 5'-inosine monophosphate (IMP) elicit a unique taste called ‘umami’ in humans. One of the characteristics of the umami taste is synergism: the synergistic enhancement of the magnitude of response produced by the addition of 5'-ribonucleotides to MSG. In addition to this well-documented synergism, we report here for the first time on another type of synergism between a glutamate receptor agonist, L-AP4, and sweet substances, by analyzing the chorda tympani responses in rats. The results are as follows: (i) when L-AP4 was mixed with one of the sweet substances, such as sucrose, glucose, fructose and maltose, large synergistic responses were observed. (ii) These synergistic responses, except to L-AP4 + sucrose, were not suppressed by sweet taste suppressants, gurmarin and pronase E. (iii) These synergistic responses were not suppressed by either metabotropic or ionotropic glutamate receptor antagonists. (iv) Fibers that responded well to the binary mixtures of L-AP4 and sweet substances also responded well to NaCl and HCl, but very weakly to sucrose. These findings are different from the characteristics of synergism between glutamate and IMP. The multiple transduction mechanisms for the umami taste in rat taste cells are discussed.
Key words: chorda tympani, rats, sweet, synergism, umami
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Introduction |
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The umami taste has been recognized as a unique taste which does not belong to any of the four basic taste qualities, sweet, salty, sour or bitter. Many researchers have reported that a group of L-amino acids represented by monosodium L-glutamate (MSG) and the derivatives of 5'-ribonucleotides represented by inosine 5'-monophosphate (IMP) induce the umami taste (Kawamura and Kare, 1987
).
Chaudhari et al. (Chaudhari et al.,
1996,
2000;
Bigiani et al., 1997;
Chaudhari and Roper, 1998)
found by using the reverse transcriptase (RT)-PCR method in rats that the
metabotropic glutamate receptor, taste-mGluR4, a truncated form of
brain-mGluR4, was expressed in only lingual tissues containing taste buds.
They also showed that MSG and an agonist for mGluR4,
L-amino-4-phosphonobutyrate (L-AP4), elicited a similar taste in a
behavioral experiment in which the conditioned taste aversion technique was
used, and suggested that the taste-mGluR4 was a chemosensory receptor
responsible for the taste of glutamate. Despite findings, our recent
neuropharmacological study (Sako and
Yamamoto, 1999) showed that S-2-amino-2-methyl-4
phosphonobutylic acid (MAP4), an antagonist of mGluR4, did not suppress taste
nerve responses to glutamate and L-AP4, but enhanced them in chorda tympani
nerve recordings, and that gurmarin (Imoto
et al., 1991;
Miyasaka and Imoto, 1995), an
anti-sweet peptide, suppressed the synergistic response to the mixture of
glutamate + IMP or the mixture of L-AP4 + IMP in rats. Recently, Nelson et
al. (Nelson et al.,
2002) reported that T1R1 and T1R3, which are taste specific
G-protein coupled receptors (CGCRs), combine to function as a broadly tuned
L-amino acid receptor responding to the binary mixture of MSG +
IMP.
In cytophysiological studies, the activation of sweet receptors is known to
increase the cAMP level [for reviews see
(Kinnamon and Margolskee,
1996; Lindeman, 2001)], but the mGluR4 binding of MSG or L-AP4 in
the brain decreases the cAMP level (Flor
et al., 1995; Thomsen
et al., 1997). Yamamoto et al.
(Yamamoto et al.,
1991) found, by using conditioned taste aversion techniques in
their behavioral study, that the taste of umami substances was not unique, but
was similar to the taste of sucrose for rats.
Synergism is known as one of the unique characteristics of the umami taste
(Yamaguchi, 1991). Sato and
his colleagues (Sato and Akaike,
1965) found that the synergistic enhancement of the magnitude of
response occurred when MSG was mixed with IMP in rats. However, this
enhancement was observed in fibers that responded well to sucrose, but not to
NaCl. Thus, the umami and sweet receptors are confused with each other. A more
definitive explanation for the glutamate (umami) transduction mechanism is
needed.
Recently, we have found the occurrence of synergistic responses to mixtures
of L-AP4 and sweet substances (Sako et
al., 2001). This phenomenon seems to be interesting, since no
synergism has been reported previously for the mixture of MSG and sweet
substances. In the present study, therefore, we conducted an
electrophysiological study to further investigate the receptor synergistic
mechanisms for the umami taste.
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Materials and methods |
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General procedure
In total, 57 male Wistar rats (250–300 g) were used. The animals were deeply anesthetized by an i.p. injection of sodium pentobarbital (60 mg/kg). Each animal was tracheotomized and secured with a head holder. The left chorda tympani nerve was exposed, freed from its surrounding tissues, and cut at the point of its entry to the bulla. The whole bundle or single fiber of the nerve was dissected and lifted on a platinum recording-wire electrode (0.1 mm diameter). An indifferent electrode was attached to nearby tissues. The nerve activities were amplified, displayed on an oscilloscope, and monitored with an audio amplifier. In the whole nerve experiment, the amplified signal was passed through an integrator with a time constant of 0.3 s, and was displayed on a slip chart recorder. In the single nerve experiment, the amplified signal was recorded using a DAT recorder (RD-135T; TEAC Co., Tokyo, Japan) for future analyses.
Data analysis
In the whole nerve experiment, the magnitude of the nerve response was measured as the height of the integrated response from the baseline at 10 s after the onset of stimulation. Responses to the taste stimuli were expressed as the relative magnitudes of responses when the magnitude of response to 0.1 M NH4Cl was taken as the standard. In the single nerve experiment, the number of impulses occurring before and after each stimulation were counted by WorkBench and the Discovery system (DataWave Technologies Co., Longmont, CO). The mean background activity per 5 s during rinsing of the tongue with distilled water was calculated. A fiber was considered to be responsive to a stimulus if the nerve impulse rate during the first 5 s of taste stimulation was larger than the mean ± SD of the background rate. The response magnitude of each fiber to a particular stimulus was the net number of impulses produced for the first 5 s after the stimulus onset, which was obtained by subtracting the mean background impulse discharge from the total number of impulses.
The synergistic effect was expressed as a potentiation ratio (= magnitude of response to mixture/sum of magnitudes of responses to individual components in the mixture). The suppressive effect by each suppressant was expressed as a suppression ratio (= magnitude of response after treatment with suppressant/magnitude of response before treatment with suppressant).
Stimuli and chemical substances
The taste solutions were (in M): 0.1 NH4Cl (Wako Pure Chemical
Industries, Osaka, Japan), 0.1 NaCl (Na; Wako), 0.01 HCl (H; Wako), 0.02
quinine hydrochloride (Q; Wako), 0.005 L-AP4 (Sigma-Aldrich Fine Chemicals, St
Louis, MO), 0.1 monopotassium L-glutamate (MPG; Ajinomoto Co.,
Tokyo, Japan), 0.01 IMP (Ajinomoto), 0.1 sucrose (Suc; Wako), 0.1 glucose
(Glu; Wako), 0.1 maltose (Mal; Wako), 0.1 fructose (Fru; Wako), 0.001 sodium
saccharin (Sac; Sigma) and their binary mixtures. A higher concentration (0.5
M) of sucrose was used when necessary. MPG was used instead of MSG because the
latter elicits a huge sodium response in rats
(Yamamoto et al.,
1991).
As the sweet taste suppressants, 2% pronase E
(Hiji, 1975), a proteolytic
enzyme, was dissolved in a 5 mM phosphate buffer of pH 6.8, and 50 µM
gurmarin, an antisweet peptide, was dissolved in a 5 mM acetate buffer, pH
4.5. As antagonists for mGluR4, NMDA and the AMPA/kinate glutamate receptors,
40 mM MAP4 (Tocris Cookson Inc., Ballwin, MO)
(Jane et al., 1994),
40 mM 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphanic acid (CPP;
Tocris) (Davies et al.,
1986) and 3 mM
2,3-dihydroxy-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide
disodium (NBQX; Tocris) (Gill et
al., 1992) were used, respectively. Each solution and the
rinsing water were applied to the anterior part of the tongue at room
temperature (25 ± 2°C). The tongue was rinsed for at least 45 s
between successive stimulations.
Experiment 1
A total of seven male Wistar rats were used. In this experiment, we examined the taste effectiveness of MPG and L-AP4, as well as the synergistic effects when each of these substances was mixed with one of the following sweet substances, Suc, Glu, Mal, Fru or Sac.
Experiment 2
A total of 14 male Wistar rats were used. In this experiment, the effects of 50 µM gurmarin or 2% pronase E on the chorda tympani responses to the mixtures of L-AP4 and one of the sweet substances were examined.
Experiment 3
A total of 24 male Wistar rats were used. The effects of 40 mM MAP4, 40 mM CPP or 3 mM NBQX on the chorda tympani responses to the mixtures of L-AP4 and one of the sweet substances were examined.
Experiment 4
A total of 12 male Wistar rats were used. The single fiber responses to the conventional four basic taste stimuli and the mixtures of L-AP4 + Suc and MPG + IMP were recorded.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Experiment 1
Figure 1 shows sample records of the integrated chorda tympani responses (A) and the representative integrated responses (B) to the mixture stimuli. The mixtures of L-AP4 and any of the sweet substances, including Glu, Fru, Suc, Mal and Sac, showed synergistic responses with potentiation ratios ranging from 3.1 to 5.9.
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These values tended to be larger than those shown by the mixture of 5 mM L-AP4 and 0.01 M IMP or the mixture of 0.1 M MPG and 0.01 M IMP. However, the mixture of 0.1 M MPG and 0.1 M Suc showed essentially no synergism, the value being 1.2 ± 0.3 (mean ± SEM, n = 7).
Experiment 2
Figure 2 shows a graphical presentation of the suppression ratios by gurmarin (A) or pronase E (B). After treatment with one of the sweet suppressants, the responses to the mixture of 0.1 M MPG + 0.01 M IMP and 5 mM L-AP4 + 0.01 M IMP were suppressed to 20–29% of the pretreatment level. However, the mixtures of L-AP4 and all of sweet substances, with the exception of Suc, were not suppressed by either grumarin or pronase E. There were significant differences in the suppression ratios between the mixture of 0.1 M MPG + 0.01 M IMP and those of 5 mM L-AP4 with one of the sweet substances except for Suc. No statistically significant difference was detected in the suppression ratios between the mixture of 0.1 M MPG + 0.01 M IMP and that of 0.1 M MPG + 0.1 M Suc.
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Experiment 3
Figure 3 shows the integrated responses to the mixtures of L-AP4 and the sweet substances before and after treatment of the tongue with one of the glutamate antagonists for 10 min. No statistically significant differences were detected for the mixtures of L-AP4 and the sweet substances between before and after treatment with either glutamate antagonist (t-test; P > 0.05).
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Experiment 4
Figure 4 shows a graphical presentation of the number of impulses per 5 s obtained from a single fiber in the chorda tympani nerve. This fiber responded best to H, followed by Na and Q. Suc elicited the least discharges among the conventional four basic taste stimuli. Although essentially no responses were elicited by 5 mM L-AP4 or 0.1 M Suc, the mixtures of these substances elicited responses. Small numbers of impulses were elicited by 0.1 M MPG, 0.01 M IMP, and the mixture of these stimuli, but no potentiated response was detected for this mixture. Figure 5 shows the responses profiles of six representative fibers to the conventional four basic taste stimuli, the mixture of L-AP4 + Suc and the mixture of MPG + IMP analyzed with 30 chorda tympani fibers. Shown in Figure 5A-C, are fibers that had good responses to Na, H and Q, but very weak responses to Suc. These fibers showed synergistic responses to the mixture of L-AP4 + Suc, but no such potentiated responses were detected for the mixtures of MSG + IMP. However, the fibers shown in Figure 5D-F, responded well to Suc, then Na, H, and very weakly to Q. These fibers showed synergism to the mixture of MPG + IMP, but no such potentiated responses were detected for the mixture of L-AP4 + Suc.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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One of the characteristics of the umami taste is the occurrence of synergism. When the two groups of umami substances are mixed, the umami responses of the mixture become stronger than the sum of umami responses of the individual components in the mixture (Sato and Akaike, 1965
;
Kawamura and Kare, 1987;
Yamamoto et al.,
1991).
Recent molecular investigations have led to the identification of several
taste receptor candidates. Ugawa et al.
(Ugawa et al., 1998)
demonstrated that MDEG1 (mammalian degenerin-1) was a candidate receptor for
the sour taste. Since the first study by Hoon et al.
(Hoon et al., 1999),
in which they cloned and characterized the new taste receptors T1R1 and T1R2
from rat taste cells, the possible identifications of taste receptor
candidates with GPCRs have been reported
(Kitagawa et al.,
2001; Max et al.,
2001; Montmayeur et
al., 2001; Sainz et al., 2001). Nelson et
al. (Nelson et al.,
2001) suggested that these receptors (T1Rs) were candidates for
sweet taste receptors. However, identification of a family of candidates for
bitter receptors (T2R/TRB) that are members of the GPCRs superfamily has been
reported (Adler et al.,
2000; Chandrashekar et
al., 2000; Matsunami
et al., 2000).
For the umami taste, taste-mGluR4, T1R1 and T1R3 are the likeliest
receptors. Chaudhari et al. (Chaudhari et al.,
1996,
2000) employed molecular
approaches using reverse transcriptase and in situ hybridization to
find that a specific metabtropic glutamate receptor (taste-mGluR4), a subtype
of mGluR4, was expressed exclusively in taste buds from the foliate and
circumvallate papillae in the rat. Toyono et al.
(Toyono et al., 2002)
also demonstrated that both brain-mGluR4 (brain-expressed mGluR) and
taste-mGluR4 were expressed in the taste tissues by using immunoblot analysis.
They also showed that the antibody against taste-mGluR4 exhibited intense
labeling of the taste pores and taste hairs in all the taste buds of the
gustatory papillae which they examined. However, Nelson et al.
(Nelson et al., 2002)
also found that T1R1 and T1R3, which are taste-specific GPCRs, combine to
function as a broadly tuned L-amino acid receptor responding to the
binary mixture of MSG + IMP. Despite these findings, the mechanisms of
synergism by the umami substances have not been clarified.
In the present study, we showed the possibility that two different synergistic mechanisms associated with different taste modalities exist in rats. One is the MPG + IMP type, which is suppressed by antisweet substances. This type of synergism is observed in fibers which respond well to sucrose. The other is the L-AP4 + sweet substance type synergism, which is not suppressed by antisweet substances or by any antagonists for the glutamate receptors. This type of synergism is observed in fibers which respond to NaCl, HCl and quinine.
The synergism of umami substances has been reported for many species, such
as rats (Sato and Akaike,
1965; Yoshii,
1987; Yamamoto et
al., 1991; Sako and
Yamamoto, 1999; Sako et
al., 2000), mice
(Ninomiya and Funakoshi,
1987), dogs (Kumazawa and
Kurihara, 1990), bovines
(Torii and Cagan, 1980) and
humans (Yamaguchi, 1991).
Yoshii (Yoshii, 1987) showed
that the purine-based 5'-phosphate ribonucleotide leads to an increase
in the affinity of amino acids for the respective receptor sites, and has a
synergistic effect on the response to amino acids. This finding suggests that
there are specific binding sites for these nucleotides on the taste receptor
membrane. However, Torii and Cagan (Torii
and Cagan, 1980) showed that the number of exposed sites
increased, but the affinity was not changed.
In the present study, we would like to hypothesize the existence of at
least two types of synergistic mechanisms concerning umami substances. One
involves taste-mGluR4 and sweet-taste receptors co-localized in single taste
cells, which are responsible for the potentiation between glutamate (or L-AP4)
and IMP. MSG and IMP may combine with sweet taste receptors at different
sites, and synergism may occur within these receptors. This assumption
explains (i) why the synergistic enhancement is exclusively seen in the subset
of taste fibers highly responsive to sweet substances
(Figure 5) (Sato and Akaike, 1965); (ii)
why synergistic taste responses to mixtures of glutamate + IMP are greatly
suppressed by gurmarin (Yamamoto et
al., 1991; Sako and
Yamamoto, 1999); and (iii) why the mixture is very highly
preferred (Yamamoto et al.,
1991), and the taste is similar to the taste of sucrose as shown
by the conditioned taste aversion paradigm
(Yamamoto et al.,
1991). The other type of synergism, which we found in the present
study, is for the potentiation between L-AP4 and sweet substances. These
substances may combine with unidentified receptors which may be colocalized
with Na+, H+ and bitter-taste receptors.
In conclusion, two types of synergy with different mechanisms exist for umami and sweet substances in the taste cells of rats. One is the MSG (MPG) + IMP type, and the other is the L-AP4 + sweetener type.
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Acknowledgments |
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Gurmarin was provided by Dr T. Imoto, Tottori University, and MPG and IMP were provided by Ajinomoto Co. This work was supported by Grant-in-Aid for scientific research (no. 14370593 to T.Y.) from the Japan Society for the Promotion of Science, The Salt Science Research Foundation (no. 0244) and The Mishima Kaiun Research Foundation.
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Accepted March 7, 2003
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B.K. Formaker, J.R. Stapleton, S.D. Roper, and M.E. Frank Responses of the Rat Chorda Tympani Nerve to Glutamate-Sucrose Mixtures Chem Senses, July 1, 2004; 29(6): 473 - 482. [Abstract] [Full Text] [PDF]
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E.R. Delay, G.M. Sewczak, J.R. Stapleton, and S.D. Roper Glutamate Taste: Discrimination between the Tastes of Glutamate Agonists and Monosodium Glutamate in Rats Chem Senses, May 1, 2004; 29(4): 291 - 299. [Abstract] [Full Text] [PDF]
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