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Departments of
*
Oral Physiology and
Chemistry, Asahi University School of Dentistry, Hozumi, Motosu, Gifu 501-0223 Japan;
**
Department of Physiology, Hamamatsu University School of Medicine, Hamamatsu 431-3192 Japan;
Department of Physiology, Nagoya City University School of Medicine, Nagoya 467-0001 Japan;
Molecular Engineering Laboratory, National Food Research Institute, MAFF, Tukuba 305-8642 Japan; and
University of Wisconsin and Wisconsin Regional Primate Center, Madison, WI 53706
3To whom correspondence should be addressed.
 |
ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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KEY WORDS: • umami • glutamate receptors • taste cells • chorda tympani nerve • glossopharyngeal nerve
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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, Harada and Smith 1992
, Ninomiya and Funakoshi 1987
and 1989a
,
Ninomiya et al. 1991
and 1993
, Shingai and Beidler 1985
). The most clear-cut difference may be
sensitivity to amiloride (a Na+ channel blocker
in various epithelial cells, and an inhibitor of NaCl responses in
taste cells) in some strains of mice and rats; this response is
robust in taste cells of the fungiform and a portion of the folliate
papillae located on the anterior two thirds of the tongue [innervated
by the chorda tympani
(CT)4
nerve], but weak in taste cells of the vallate and foliate papillae
located on the posterior third of the tongue [innervated by the
gossopharyngeal (GL) nerve] (Doolin and Gilbertson 1996
, Formaker and Hill 1991
,
Kitada et al. 1998
, Ninomiya et al. 1991
,
Ninomiya 1998
). Similarly, a prominent difference in
inhibition of sweetener responses by gurmarin (a 4209-Da peptide
isolated from the plant, Gymnema sylvestre) of the
CT and GL nerves has been found in mice (Ninomiya et al. 1997
). Gurmarin inhibited sweetener responses of the CT nerve
by ~50% of control response, whereas it had a minimal effect on
responses of the GL nerve, suggesting a lack of
gurmarin-sensitivity in taste cells innervated by the GL nerve.
Our previous studies in mice (Ninomiya and Funakoshi 1987
and
1989a
) and rhesus monkeys (Hellekant et al. 1997a
) have also demonstrated a difference in the two
taste nerves regarding their response to umami substances (US) such as
monosodium glutamate (MSG), disodium 5'-inosinate (IMP) and disodium
5'-guanylate (GMP). In both species, single fibers having highly
selective sensitivity to umami substances appeared in the GL nerve,
whereas no such MSG-selective fibers were found in the CT nerve.
Consistently, mouse behavioral studies using a conditioned taste
aversion paradigm suggested that the GL nerve plays a more important
role than the CT nerve in the discrimination of umami substances from
other basic taste substances (Ninomiya and Funakoshi 1987
and
1989b
). In this paper, we address possible properties of umami
taste receptors in taste buds innervated by the CT and GL nerves by
discussing data from previous electrophysiologic studies of the
responses of taste nerves to umami substances as well as recent
findings from our laboratory regarding changes in the intracellular
concentrations of Ca2+ ([Ca2+]i)
and second messengers [e.g., cAMP or inositol 1,4,5-trisphosphate
(IP3)] of taste cells in response to umami substances.
|
Differential responsiveness of the CT and GL nerves to umami substances |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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). They also found that most fibers showing this synergism
were sensitive predominantly to sucrose. This relatively low taste
selectivity of MSG-sensitive CT fibers was also observed in
subsequent studies in hamsters (Yamamoto et al. 1988
)
and rhesus monkeys (Hellekant et al. 1997a
).
Consistently, behavioral studies using a conditioned taste aversion
paradigm in rats (Yamamoto et al. 1985
) and hamsters
(Yamamoto et al. 1988
) have shown that these animals had
a generalized aversion to 100 mmol/L MSG + IMP from 100 mmol/L
NaCl. This result might be caused by the Na+
contained in these three taste substances, which strongly stimulates
taste bud cells innervated by the CT nerve in these species. But
Yamamoto et al. (1991)
reported that rats could discriminate 100 mmol/L
MSG from 100 mmol/L NaCl when they were dissolved in 0.01–0.03 mmol/L
amiloride, though they did not discriminate MSG from 500 mmol/L
sucrose.
About 10 y ago, we first identified taste fibers in the
mouse GL nerve that responded strongly to 100 mmol/L MSG, but very
weakly (if at all) to 100 mmol/L NaCl, 500 mmol/L sucrose, 10 mmol/L
HCl and 20 mmol/L quinine (Ninomiya et al. 1987
and
1989a
). These MSG-sensitive fibers exhibited the
synergistic effect of 0.5 mmol/L GMP on the MSG response. Consistently,
behavioral studies using a conditioned taste aversion demonstrated that
normal mice could discriminate MSG from the original group of four
basic taste stimuli. However, mice that had undergone bilateral
sectioning of the GL nerve could not discriminate MSG from NaCl
(Ninomiya and Funakoshi 1987
and 1989b
). These results
suggest that the GL nerve conveys taste information for umami
substances, which can be discriminated from that of the other basic
tastes. The greater sensitivity to umami substances of the posterior
than the anterior parts of the tongue was subsequently identified in a
psychophysical study in humans (Yamaguchi 1998
) and an
electrophysiologic study in rhesus monkeys (Hellekant et al.
1997
). In rhesus monkeys, a hierarchical cluster analysis on
the taste responses of 33 GL fibers resulted in three major clusters
(M, Q and S). Five fibers were grouped as the M-cluster in which
MSG elicited the largest responses among 20 stimuli.
Although the posterior part of the tongue may be the key site for
glutamate taste perception in many mammals, chimpanzees
(Hellekant and Ninomiya, 1991
, Hellekant et al. 1997b
) and pigs (Danilova et al. 1999
),
in which the response to sodium is very weakly (if at all) inhibited by
amiloride, reportedly have CT fibers that are specifically sensitive to
US. In pigs, such fibers, grouped as an M-cluster, constituted
~15% of the CT and 10% of the GL fibers. Interestingly, 70 mmol/L
MSG was one of best stimuli for the CT nerve among the various taste
stimuli tested, including 100 mmol/L NaCl, 300 mmol/L sucrose, 20
mmol/L citric acid and 5 mmol/L quinine HCl (Danilova et al. 1999
). Therefore, it is possible in rodents that some
populations of specific receptors for glutamate may also exist in the
anterior part of the tongue, although the much higher sensitivity to
sodium on the anterior part of the tongue may mask the taste response
occurring through such specific glutamate receptors.
|
[Ca2+]i response to US in isolated taste cells from the mouse fungiform papillae |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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Importantly, in three of four MSG-sensitive cells tested, lowering the Ca2+ concentration in the stimulus solution did not reduce substantially the increases in [Ca2+]i that resulted from exposure to a mixture of MSG and IMP (or GMP). Therefore, the increase in [Ca2+]i in these cells did not require the presence of extracellular Ca2+, suggesting that the observed Ca2+ release occurred mainly from intracellular stores. This result strongly suggests that the IP3 pathway may be involved in the transduction mechanisms for US.
|
Changes in the concentrations of IP3 and cAMP in fungiform papillae in response to US |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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and 1998
). This procedure
was repeated, and 60 fungiform papillae from two mouse tongues were
pooled. The levels of both IP3 and cAMP in each
tissue pool were measured by radiobinding assay kits. As expected,
stimulation with US (100 mmol/L MSG, 0.5 mmol/L IMP and a mixture of 30
mmol/L MSG and 0.5 mmol/L IMP for cAMP levels, and 30 and 100 mmol/L
MSG, 0.5 mmol/L IMP and a mixture of 30 mmol/L MSG and 0.5 mmol/L IMP
for IP3 levels) significantly increased both cAMP
and IP3 levels in the fungiform papilla compared
with the control levels of cAMP and IP3 achieved
after stimulation with distilled water (t test, P
< 0.05–0.01). Stimulation with 500 mmol/L sucrose also increased
the cAMP level (t test, P < 0.01) but not
the IP3 level (t test, P
> 0.05), whereas 100 mmol/L NaCl produced no significant changes
in either cAMP or IP3 levels (t test,
P > 0.05). These results suggest that both cAMP and
IP3 pathways may be involved in taste
transduction for US in the mouse fungiform papilla. Interestingly,
increases in both cAMP and IP3 levels elicited by
a mixture of 30 mmol/L MSG and 0.5 mmol/L IMP were larger than the sum
of the increases produced separately by each component stimulus,
suggesting that the synergism between MSG and IMP occurred separately
in the two different transduction pathways. Therefore, it is probable
that the site of occurrence of the synergism of US may be at the
receptor level, as previously proposed by Kurihara and colleagues
(Kumazawa et al. 1991
). |
Gurmarin-sensitive and -insensitive receptor mechanisms for US |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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, Ninomiya and Imoto 1995
,
Ninomiya et al. 1997
). In mice, the existence of two
different sweet receptors, gurmarin-sensitive and -insensitive
types, has been postulated (Ninomiya and Imoto 1995
,
Ninomiya et al. 1997
). Gurmarin has been reported to
suppress completely the response of the CT nerve in rats to a mixture
of 100 mmol/L MSG and 10 mmol/L IMP dissolved in amiloride solution
(Yamamoto et al. 1991
). This suggests that in rats, the
amiloride-insensitive response component to the mixture of US might
occur through the gurmarin-sensitive sweet receptor. However, our
own data involving measurements of [Ca2+]i
and intracellular concentrations of cAMP and IP3 in mouse
taste cells (discussed above) suggest that receptor mechanisms for US
may not necessarily be common to that for sweeteners. To examine this
issue, we explored possible effects of gurmarin on the responses of the
CT and GL nerves to US in C57BL mice. We used gurmarin at a level of
100 µg/mL, which is higher than the concentrations
(30–50 µg/mL) that produce maximal inhibition of the
sweet responses of the CT nerve of C57BL mice. As previously reported
(Ninomiya et al. 1997
), gurmarin did not affect the
responses of the GL nerve to any of the taste stimuli tested, including
US. In the CT nerve, however, gurmarin suppressed the responses to
sweeteners and to a mixture of MSG and IMP (to ~65% of control).
Unlike in rats, however, the enhancement of MSG responsiveness by the
addition of IMP was clearly observed after gurmarin treatment in mice.
This suggests that the synergism of US occurs not only through
gurmarin-sensitive, but also gurmarin-insensitive receptor
mechanisms. Further treatment of the tongue with pronase E, a
proteolytic enzyme that inhibits sweet responses in rats and mice
(Hiji 1975
, Ninomiya et al. 1992
),
abolished the responses to sweeteners almost completely, but minimally
suppressed the residual responses to the US mixture after gurmarin. In
the CT nerve, 2-amino-4-phosphonobutyrate (L-AP4, at 1.0
mmol/L or more of concentrations), a metabotropic glutamate receptor
(mGluR4) agonist, and N-methyl d-aspartic
acid (NMDA, at 3.0 mmol/L or more of concentrations), an ionotropic
glutamate receptor agonist, were also effective stimuli; responses to
10 mmol/L L-AP4 and NMDA were potentiated by 0.5 mmol/L
IMP. Neither gurmarin nor pronase significantly suppressed responses to
L-AP4 and NMDA, or their mixtures with IMP. Therefore, it
is possible that a gurmarin-insensitive component of responses to
the MSG-IMP mixture may occur through glutamate receptors that
could be activated by L-AP4 and/or NMDA. |
Possible receptor mechanisms for US in taste cells innervated by the CT and GL nerves |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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Hayashi et al. (1996)
measured membrane potentials and
[Ca2+]i in isolated taste
buds from vallate and foliate papillae of C3H mice in response to 1
mmol/L glutamate, L-AP4 and NMDA by using both the
calcium-sensitive dye Fura-2 and the fast voltage-sensitive dye
di-8-ANEPPS. They found that L-AP4 elicited primarily
decreases in [Ca2+]i,
whereas NMDA elicited increases in
[Ca2+]i.
L-Glutamate elicited both increases and decreases in
[Ca2+]i in different
cells. Measurements in taste cells doubly labeled with Fura-2 and
di-8-ANEPPS indicated that the increases in
[Ca2+]i occurred
simultaneously with cell depolarization, whereas the decreases in
[Ca2+]i were not
accompanied by measurable changes in membrane potential. These data
suggest that in mouse taste cells, there are multiple receptors for
glutamate including metabotropic (L-AP4) and ionotropic
(NMDA) types, but that the metabotropic receptors responding to
L-AP4 do not mediate excitatory glutamate taste
responses. In their subsequent studies using C57BL mice, Hayashi et al. (1997)
found that unlike C3H mice, C57BL mice showed
increases in [Ca2+]i in
~40% of taste cells in response to L-AP4. The
concentration of L-AP4 employed was 1.0 mmol/L, close to
the threshold for excitatory responses by the CT nerve in C57BL mice,
as shown above. Therefore, in this concentration range, the
responsiveness to L-AP4 may vary widely even within
L-AP4–sensitive taste cells in mice.
By using reverse transcriptase-polymerase chain reaction,
Chaudhari et al. (1996
) found that several ionotropic
and metabolic glutamate receptors are expressed in the rat lingual
tissue, including vallate and foliate papillae. Among these glutamate
receptors, ionotropic receptors were detected in lingual tissue devoid
of taste buds, whereas the metabolic glutamate receptor, mGluR4, was
expressed selectively in taste buds. They also found that expression of
mGluR4 in taste buds is higher in preweanling rats compared with adult
rats, corresponding to greater responses of the GL nerve to MSG in
juvenile mice (Ninomiya et al. 1991
). Their behavioral
studies using a conditioned taste aversion paradigm have indicated that
5 and 10 mmol/L L-AP4 and 15 and 50 mmol/L MSG dissolved in
an amiloride solution elicited similar tastes in rats. From these
results, they concluded that mGluR4 may be a chemosensory receptor
responsible in part for the taste of MSG.
Their subsequent studies (Bigiani et al. 1997
), using
whole-cell patch-clamp techniques in isolated rat taste cells,
demonstrated that MSG (10–20 mmol/L) elicited inward currents with an
increase in membrane conductance (transient responses) and outward
currents with a decrease in membrane conductance (sustained responses).
These two different membrane currents may be generated by two different
sets of nonselective cation channels, one closed (sustained response)
and the other opened (transient response) by glutamate.
L-AP4 (0.83 mmol/L) mimicked only the sustained
response. Similar two-type responses to MSG (10 mmol/L) and the
sustained response to L-AP4 (1.0 mmol/L) were also observed
in taste cells isolated from vallate and foliate papillae in C3H and
C57BL mice (Oh et al. 1997
, Sugimoto 1996
).
From these results, it is likely that taste cells of the vallate and foliate papillae in rodents may possess multiple receptors including metabotropic (mGluR4) and ionotropic (NMDA-type) glutamate receptors for transduction of umami taste.
Receptors responsible for MSG responses in taste cells innervated by the CT nerve.
Data from previous electrophysiologic studies on single CT
responses in rats and mice (Ninomiya and Funakoshi 1987
and 1989a
, Sato et al. 1970
, Yamamoto et al. 1991
) suggested that the CT responses to MSG may occur at least
through amiloride-sensitive sodium and gurmarin-sensitive sweet
receptors. In addition to these two receptors, our data (discussed
above) suggest the participation of gurmarin-insensitive components
of receptors in the mouse CT responses to MSG, which may include the
metabotropic (mGluR4) and ionotropic (NMDA-type) glutamate receptors.
MSG, like sucrose and saccharin, elicited increases in cAMP in the
fungiform papilla. This increase in cAMP in response to MSG might occur
through sweet receptors, because increases in cAMP in response to
sucrose and saccharin disappear after lingual treatment with pronase
(Nakashima and Ninomiya 1997
and 1998
). In addition, MSG
elicited increases in IP3 in the fungiform
papilla. Data from calcium imaging studies demonstrated that the
increase in [Ca2+]i in
taste cells in response to a mixture of MSG and IMP or GMP did not
require the presence of extracellular Ca2+. This
result also suggests an involvement of the IP3
pathway in the transduction for MSG responses, in which
Ca2+ release could be elicited from intracellular
stores. Although there are no available data suggesting directly which
receptor could be responsible for this increase in
IP3, it is possible that
L-AP4-sensitive receptors (mGluR4) are involved. That is, a
recent study in salamander retinal ganglion cells (Shen and Slaughter 1998
) showed that L-AP4 stimulates
Ca2+ release from internal stores, and
antagonists of phospholipase C, IP3 and ryanodine
receptors inhibited the action of L-AP4. This indicates
that L-AP4–sensitive metabotropic glutamate receptor
transduction is linked to the IP3 pathway.
Generally, it is accepted that mGluR4 (group III of metabotropic
glutamate receptors) is linked to inhibition of adenyl cyclase
(Pin and Duvoisin 1995
). Comparably, it has been shown
that MSG (10–20 mmol/L) elicits a decrease in cAMP concentrations in
taste buds from rat vallate and foliate papillae (Zhou and Chaudhari 1997
). Taking these facts together with our findings,
it is possible that MSG activates different transduction pathways in
different taste cells in the mouse fungiform papilla, eliciting
increases in cAMP in sweet-sensitive taste cells, and both
decreases in cAMP and increases in IP3 in
L-AP4-sensitive taste cells. However, future extensive
studies are required to evaluate these possibilities.
In conclusion, our data together with those from previous studies suggest that gurmarin-insensitive components of receptors for US, including metabotropic and ionotropic glutamate receptors, may commonly be involved in the transduction of the umami taste in taste bud cells on both anterior and posterior parts of the tongue in mice.
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FOOTNOTES |
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2 Supported in part by Grant-in-Aid Nos.
09470407 and 09557147 for Scientific Research from the Ministry of
Education, Science, Sports and Culture of Japan.
4 Abbreviations used: L-AP4,
2-amino-4-phosphonobutyrate; CT, chorda tympani; GL, glossopharyngeal;
GMP, disodium 5'-guanylate; IMP, disodium 5'-inosinate;
IP3, inositol 1,4,5-triphosphate; MSG, monosodium
glutamate; NMDA, N-methyl-D-aspartic acid;
US, umami substances.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONDifferential responsiveness of...[Ca2+]i response to US...Changes in the concentrations...Gurmarin-sensitive and ...Possible receptor mechanisms for...REFERENCES |
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