Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Multiple Pathways for Signaling Glutamate Taste in Rodents
1 Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, FL 33136, USA and 2 Neuroscience Program, University of Miami School of Medicine, Miami, FL 33136, USA
Correspondence to be sent to: Nirupa Chaudhari, e-mail: nchaudhari{at}miami.edu
Key words: cAMP, functional imaging, taste receptor, umami
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Umami: a complex taste |
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 Top Umami: a complex taste Identifying taste receptorsUmami responses of taste...ConclusionAcknowledgementsReferences |
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L-glutamate, typically as its Na salt (MSG), elicits a taste termed umami. A characteristic feature of umami taste is the synergistic potentiation of glutamate taste by purine nucleotide (inosine, guanosine) monophosphates. This is manifested as an enhanced electrophysiological response from taste receptor cells, as an increase in nerve firing rate, or as increased preference in behavioral assays. Apart from this enhanced intensity, it is not clear whether the addition of nucleotides also leads to a change in the perceived quality of glutamate in animals and humans.
The magnitude of nucleotide-potentiation in nerve recordings varies considerably
between the chorda tympani (CT) and glossopharyngeal (GL) nerves (Ninomiya et al., 1993
). Single-unit recordings
further highlight the heterogeneity of umami responses in that nucleotide-potentiated
signals are seen in distinct fiber-types (sucrose-best or glutamate-best) in the CT and
GL nerves (Ninomiya and Funakoshi,
1989;
Yamamoto et al., 1991;
Formaker et al., 2004).
Gurmarin, a peptide that inhibits sweet taste in rodents, inhibits umami signals
differentially across the CT and GL nerves (Ninomiya et al., 1993;
Sako and Yamamoto, 1999).
Collectively, the nerve recording data suggest that responses to MSG differ significantly
between the anterior (CT innervation) and posterior (GL innervation) lingual taste
fields. The implication is that umami responses may originate from more than a single
type of receptor or receptor combination.
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Identifying taste receptors |
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TopUmami: a complex tasteIdentifying taste receptorsUmami responses of taste...ConclusionAcknowledgementsReferences |
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In the last few years, several strategies have been successfully used for identifying taste receptors. The genetic approach employed naturally occurring phenotypic variations in taste sensitivity. Mapping taste loci has revealed candidate receptors for bitter (e.g. T2R5) and sweet (T1R3) receptors. Molecular cloning approaches, on the other hand, begin with identifying cDNAs selectively expressed in taste tissue, confirming the presence of corresponding mRNA and/or protein in taste cells and examining the functional properties of cloned receptors when they are expressed in heterologous cells. An essential final step of this approach should be to determine how closely the functional properties of cloned receptors approximate those of native taste cells.
At least two distinct G protein-coupled receptors (GPCRs) have been proposed to
underlie the detection of glutamate in mammalian taste buds. A taste-specific variant of
a metabotropic glutamate receptor, taste-mGluR4, was cloned from rat circumvallate
papillae (Chaudhari et al.,
2000) and mGluR4 mRNA was localized to taste cells by in situ
hybridization (Chaudhari et al.,
1996;
Yang et al., 1999).
When expressed in transfected cells, taste-mGluR4 responds to MSG and L-AP4 (a
glutamate analog) at taste-effective concentrations (Chaudhari et al., 2000). To further explore the
significance of mGluR4 in taste buds, we carried out immunoblots on taste papillae using
antibodies specific for mGluR4. Extracts from circumvallate and foliate papillae from
rats and mice contained immuno-reactive bands of molecular weight predicted for both
taste- and brain-mGluR4. These bands were not apparent in extracts from non-taste
samples. In immunocytochemical experiments also, circumvallate, foliate and palatal taste
buds were immuno-reactive with anti-mGluR4 antibodies. Only a subset of spindle-shaped
cells were labeled in each taste bud (Chaudhari
et al., 2003). Using double label immunocytochemistry, we further
determined that mGluR4 expression is principally in cells that also express phospholipase
Cß2 (PLCß2), an effector implicated in taste responses.
Another candidate umami receptor, the T1R1/T1R3 heterodimer, when expressed in
heterologous cells along with promiscuous G proteins, also confers the ability to respond
to glutamate (Nelson et al.,
2002). T1R1/T1R3 dimers display nucleotide potentiation of glutamate
responses and are activated by a broad range of non-umami amino acids. When either T1R1
or T1R3 were genetically ablated, the chorda tympani (CT) response to glutamate was
eliminated and mice entirely lost taste preference for umami stimuli in brief-access
tests (Zhao et al., 2003).
The stated interpretation was that the T1R1/T1R3 dimer is uniquely necessary and
sufficient for umami taste. However, much data exists to the contrary. A knockout of
T1R3, produced by another group, showed decreased CT responses to MSG, while GL responses
to MSG were hardly changed (Damak et al.,
2003). It should be noted that T1R1 is expressed independently of T1R3 in
substantial numbers of cells, especially in the vallate taste buds (Max et al., 2001;
Kim et al., 2003). These
findings suggest that T1R1/T1R3 pairing is not obligatory in native cells, that other
partners are likely for T1R1, and that umami receptors are likely to be different in
vallate versus fungiform taste buds.
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Umami responses of taste cells |
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TopUmami: a complex tasteIdentifying taste receptorsUmami responses of taste...ConclusionAcknowledgementsReferences |
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To understand the significance of the various receptors discussed above, it is essential to compare their functional properties against the characteristics of umami responses in native taste cells. Hence, we employed two semi-intact preparations of taste tissue. First, we measured cAMP levels in intact rat and mouse taste buds stimulated with umami tastants. As we reported previously, circumvallate taste buds respond to glutamate with a concentration-dependent decrease in cAMP concentration (Abaffy et al., 2003
). This cAMP modulation is seen
in both rats and mice, and is only detected at glutamate concentrations >1 mM,
indicating that this second messenger is related to umami taste. The cAMP responses to
MSG were distinct in circumvallate taste buds relative to palatal or fungiform taste
buds. In rat (but not mouse) vallate taste buds, the addition of IMP to MSG yielded an
enhanced response. Although the significance of the cAMP signal in the overall
transduction process remains to be established, these results suggested that taste buds
exhibit more than a single type of response to glutamate.
We have also used a slice preparation of circumvallate taste papillae, loaded with
calcium green–dextran and confocal imaging to examine physiological responses
(Ca2+ transients) to umami stimuli. In this preparation, taste buds are
focally stimulated just at the taste pore, emulating in vivo stimulation
(Caicedo et al., 2000). The
basolateral membrane of taste cells is protected from exposure to the stimulus and the
measured Ca2+ signals likely represent primary taste responses. In mouse
vallate papillae, Ca2+ responses to glutamate were detected in
5% of taste cells and could be elicited by other Na and K salts. Both Na and K
salts of glutamate are known to activate taste nerves. As expected for umami responses,
low concentrations of IMP enhanced the glutamate-elicited Ca2+ responses.
We also determined that the Ca2+ responses of vallate taste cells to
monopotassium glutamate (MPG) stimulation represents release of Ca2+ from
intracellular stores. This is in keeping with the loss of umami sensitivity in mice that
are genetically deficient in PLCß2, a key mediator of Ca2+ release in
taste cells (Zhang et al.,
2003;
Dotson et al., 2004).
Some taste cells in vallate slices responded to MPG, some to a sweet tastant
(SC45647) and some to both. Interestingly, cells that responded to L-AP4 (an
umami agonist) did not respond to MPG. No cells were observed to respond to both. Taste
cells that responded to MPG also responded to varying numbers of other amino acids.
Overall, the Ca2+ responses of individual taste cells are consistent with
single-unit taste nerve recordings, but do not correspond well with the functional
properties of any single heterologously expressed receptor, neither T1R1/T1R3 (Nelson et al., 2002), nor mGluR4
(Chaudhari et al., 2000).
Our data suggest that detection of umami stimuli by native taste cells is neither as
simple nor as monotonic as the hypothesis of a unique T1R1/T1R3 umami receptor. In the
taste slice preparation, umami responses were significantly different than those reported
for mouse T1R1/T1R3 dimers, heterologously expressed (Li et al., 2002;
Nelson et al., 2002). For
instance, individual glutamate-sensitive cells in vallate papillae responded to different
combinations of amino acids, as expected given the varied behavioral responses of rodents
to different amino acids (Iwasaki et
al., 1985). In contrast, the expressed T1R1/R3 dimer responded to all of
these amino acids (Nelson et al.,
2002). Taste generalization between L-AP4 (an umami compound) and
MSG indicated that these compounds taste similar to rodents, the inference being they
activate a common receptor (Chaudhari et
al., 1996). Yet, rats also easily discriminate between the tastes of MSG
and L-AP4 (Delay et al.,
2004), suggesting that some taste receptors may exist that are activated by
one but not the other ligand. Collectively, these observations suggest that umami
responses are complex and may be generated by more than a single type of receptor
(Sako et al., 2003).
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Conclusion |
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TopUmami: a complex tasteIdentifying taste receptorsUmami responses of taste...ConclusionAcknowledgementsReferences |
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Our observations highlight possible distinctions among native glutamate-taste receptors, and suggest the presence of additional receptors for different umami stimuli or unexplored interactions among known receptors. Indeed, taste would not be unique in possessing such redundancy of receptors. Most mammalian sensory systems include more than a single receptor capable of responding to a given stimulus, whether these are multiple opsins (responding to a given wavelength of light), multiple ORs (responding to a single odorant) or multiple receptors in peripheral nociceptors (responding to metabolites arising from tissue damage).
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Acknowledgements |
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TopUmami: a complex tasteIdentifying taste receptorsUmami responses of taste...ConclusionAcknowledgementsReferences |
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Supported by grants from IGTC and NIH (5P01-DC3013, 1R01-DC6021 and 2R01-DC374).
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