Department of Physiology, Virginia Commonwealth University, Richmond, Virginia 23298-0551
THE TASTE QUALITIES of sweet and bitter have powerful
influences on food selection for humans and other species. Generally speaking, sweet-tasting substances are innately appetitive, whereas those that are bitter tasting are aversive (2, 10). It is, therefore,
not surprising that sweet-tasting (and calorie-rich) carbohydrates, in
their various forms, are highly preferred meals. The association of the
appetitive sweet with high-caloric intake has obvious survival value.
However, modern society affords us an abundance (perhaps an
overabundance) of sweet-tasting foods, and, as every dieter can attest,
the persistent present-day challenge is limiting their ingestion to
avoid excessive caloric intake. The chance discovery that some
compounds (with negligible caloric value) can stimulate a sweet taste
response at concentrations far lower than sugars, has, not
surprisingly, spawned an entire industry. Similarly, the survival value
in an aversive reaction to most naturally occurring bitter-tasting
compounds is clear, considering that among these are many potentially
toxic substances (e.g., plant alkaloids). Interestingly, the pleasant
systemic effects that humans have discovered in less than acutely
lethal encounters with some alkaloids (e.g., nicotine, caffeine, and others) results in their acceptance despite the bitterness. The structural diversity in bitter-tasting compounds has long been noted
and includes mineral salts, amines, and ammonium salts (including those
used pharmacologically as ion channel blockers), some
L-amino acids and peptides (including the peptide toxin
mastoparan), saponins, local anesthetics, and others (13, 20, 32, 35).
The same sort of diversity is observed among sweet-tasting compounds.
Sucrose and other naturally occurring saccharides form a unique class of compounds that are intensely sweet only at concentrations high enough to be nutritionally beneficial (20, 35). Nonsaccharide sweet-tasting substances include inorganic salts (including some lead
salts, in this case an undesirable property of a potentially harmful
substance), sulfimides (e.g., saccharin), amino acids (e.g., glycine,
L-alanine, and some D-amino acids), peptides
(e.g., aspartame), proteins (e.g., thaumatin and monellin), and other synthetic compounds (20, 35). In short, a large variety of chemical
structures end up producing mainly one of only two chemical sensations.
From a physiological standpoint this suggests multiple input routes to
a common sensory transduction process within a submodality, or multiple
transduction cascades within a submodality, or both multiple inputs and
multiple transduction cascades.
Cross adaptation psychophysical studies on humans (30) and cross
adaptation electrophysiological studies in other species (15) within
either the sweet- or bitter-tasting stimulus group have produced
evidence generally favoring the existence of multiple parallel
transduction pathways within a submodality. Interestingly, data
indicating the existence of more than one peripheral sensory pathway
were often interpreted as evidence in favor of multiple taste cell
apical membrane receptor sites, although the existence of the latter
had not been unequivocally demonstrated. However, the concept of taste
receptor site took a quantum leap toward demonstrated fact with the
discovery of a taste receptor cell-specific G protein The recent cloning of two seven-transmembrane domain proteins expressed
in the apical poles of taste receptor cells by Hoon et al. (14) and a
probable glutamate taste receptor protein by Chaudhari et al. (7) have
provided the first candidate taste receptor molecules. Although
functionality remains to be determined, they would appear to fulfill
many of the expected properties of membrane taste receptors, and it
seems only a matter of time before functionality is demonstrated.
Although it is tempting to say at this point "end of story,"
there remain a number of outstanding observations that strongly suggest
that many sweet and bitter tastants can initiate the sensory
transduction chain at various points, i.e., transduction may not always
require the binding of these tastants to a specific membrane receptor
protein. For bitter tastants the evidence for this was extensively
reviewed and discussed by Spielman et al. (32). Many intensely
bitter-tasting stimuli, e.g., quinine, tetraethylammonium,
BaCl2, CsCl, and 4-aminopyridine are part of the cell
physiologist's arsenal of K+ channel blockers. Each has
been shown to depolarize taste receptor cells by blocking one or more
K+ channel conductance (3, 17, 33), a mode of action that might function in situ. However, most of the K+
conductances in mammalian taste receptor cells are located on the
basolateral side of the cells and, therefore, are not immediately in
contact with taste stimuli (11, 41).
For receptor cell depolarization through K+ conductance
blockage to be a factor in taste transduction, the conductances would have to be blocked from the cell cytoplasmic side or from the cell
interstitial fluid compartment below the tight junctions. They are
blocked from the cytoplasmic side when, for example, saccharides effect
a G protein-mediated increase in cell cAMP, ultimately resulting in
protein kinase A-mediated closure of K+ channels (8).
Isolated taste receptor cells undergo sustained volume decrease when
subjected to hyperosmotic concentrations of mannitol, suggesting
relatively low membrane permeability for this and perhaps other
saccharides (22). Sweet taste transduction involving
saccharides, therefore, probably begins with specific cell surface
receptors that are selective among saccharides, but specific membrane
saccharide transporters, which could perform the same selectivity
function, should not be completely ruled out. On the other hand, in
taste receptor cells some hydrophilic tastants, such as urea, have the
osmotic characteristics of solutes that are very membrane permeable
(22). Moreover, many artificial sweeteners and bitter tastants are
either amphipathic or can exist in both hydrophilic and lipophilic
forms (usually depending on pH), so the possibility that they too enter
taste receptor cells must be considered. Indeed, taste receptor cells
readily absorb the nonionic forms of various fluorescent dyes (4, 21).
Moreover, taste cells contain the multidrug resistance P-glycoprotein
(MDR1) membrane protein, suggesting that some substances regularly
enter taste cells in amounts that warrant a robust mechanism for their active removal (16).
Naim et al. (25) have already shown that saccharin, quinine, and other
amphipathic tastants can directly activate transducin and a mixture of
Gi/Go proteins in vitro. On this basis they
proposed that direct activation of taste transduction could occur in
some cases at the G protein level. This conclusion was supported by the
fact that some amphipathic peptides do activate G proteins directly in
other cell types. Perhaps the best-studied example is mastoparan, an
amphipathic peptide in wasp venom, which activates G proteins directly
in mast cells and must cross the cell membrane to reach its
intracellular target (6, 24). Other agonists that can stimulate
histamine secretion from mast cells by direct G protein activation are
the neuropeptide substance P, bradykinin, and compound 48/80, a
synthetic polyamide. It is interesting that bradykinin, substance P,
and mastoparan all contain basic amino acids flanked by hydrophobic
amino acids. Bradykinin is highly bitter tasting, as are synthetic
short peptides containing similar arrangements of amino acids (32). The
bitter-tasting local anesthetics procaine and tetracaine are also
direct activators of G proteins (12). Further evidence supporting
direct activation, from Naim and colleagues and others, is that
artificial sweeteners stimulate G protein-dependent adenylyl cyclase
activity in non-taste tissue (9, 38) and insulin secretion from
isolated pancreatic islets (19). It is unlikely that muscle, liver, and
pancreas contain taste receptor molecules for saccharin and other
artificial sweeteners.
Key unanswered questions that remained are: Do tastants such as
saccharin and quinine enter taste receptor cells and, if so, are the
amount and rate of uptake in the time frame of taste responses? In the
current article in focus by Peri et al. (Ref. 26, see page
C17 in this issue), Naim and colleagues investigate the
time course of uptake of saccharine, quinine, and the cyclic dipeptide (cyclo)Leu-Trp into rat circumvallate taste bud cells. The authors used
the confocal microscope to monitor increasing intracellular concentrations of the tastants using their autofluorescence properties. The uptake was rapid and confined to the cytosol. The kinetics of
uptake were determined by permeabilizing the cells following exposure
to a tastant and then analyzing the cytosol using HPLC methods. The
initial rates of uptake are clearly sufficiently fast to support the
possibility of direct intracellular activation of transduction. In a
parallel set of experiments Naim and colleagues studied the uptake of
the tastants by liposomes. In each case the uptake was rapid and
mechanistically consistent with a partitioning of the tastant into the
lipid followed by diffusion into the vesicle interior. Cyclo(Leu-Trp)
is uncharged at the pH values of the experiments and quinine is
partially so, suggesting that these lipid-permeable forms are
responsible for the membrane transit into taste receptor cells.
However, saccharin is strongly anionic yet still transported, an
observation meriting further investigation, especially in view of its
very different taste quality. These studies, in so far as they suggest
that the causal chemosensory stimulus may be intracellular
concentration, complement recent work on cellular pH sensors, where it
appears that intracellular receptor pH is the excitatory stimulus for
acid taste receptors (21, 36), carotid body pH-sensing cells (5), and
brain stem chemoreceptors (28). This study by Peri et al. (26) serves as a timely reminder that the multiple transduction pathways mediating taste sensation may themselves contain an additional level of diversity, i.e., different points of initiation. These may be 1) at a G protein-coupled receptor molecule, 2) at any
of several intracellular loci, and 3) in some cases, no doubt,
both of the above.
ARTICLE
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ARTICLE
REFERENCES
-subunit
(
-gustducin) by McLaughlin et al. (23). This G protein
-subunit is >80% homologous to
-transducin, and, like the
latter, activates a phosphodiesterase. Interestingly, rod transducin is
also present in taste receptor cells, where it has been shown to be
capable of activating taste-cell-specific phosphodiesterases when
stimulated with the bitter-tasting compound denatonium (29). That
bitter taste transduction might depend on phosphodiesterase activation
was itself confirmation of Price's earlier observations (27). Knockout
mice lacking the
-gustducin gene showed significantly reduced
behavioral and neural responses to denatonium, reinforcing the idea
that
-gustducin mediates a transduction cascade associated with the
bitter submodality and that a reduction in cell cAMP levels is part of
the process (18, 40). This transduction pathway, however, does not
appear to be unique for this bitter-tasting stimulus. Earlier work had already shown that denatonium activates taste cell inositol
trisphosphate (IP3) through a G
protein-dependent mechanism sensitive to pertussis toxin (31, 34),
resulting in the mobilization of intracellular Ca2+ (1). It
appears, therefore, virtually certain that taste cell membrane receptor
molecules exist and that they can initiate multiple G protein-mediated
transduction cascades. A similar conclusion holds for the sweet
submodality. Striem et al. (37, 39) showed that saccharides stimulate a
G protein-dependent increase in adenylyl cyclase activity and cAMP
production. Moreover, artificial sweeteners did not increase cAMP
levels in isolated rat taste receptor cells but caused IP3
levels to increase and Ca2+ to be released from
intracellular stores (4).
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FOOTNOTES |
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Address for reprint requests and other correspondence: J. A. DeSimone, Dept. of Physiology, Virginia Commonwealth Univ., Sanger Hall, Rm 3-002, 1101 E. Marshall St., Richmond, VA 23298-0551 (E-mail: jdesimon{at}hsc.vcu.edu).
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