Neuroscience of taste: unlocking the human taste code

Since antiquity human taste has been divided into 4-5 taste qualities. We realized in the early 1970s that taste qualities vary between species and that the sense of taste in species closer to humans such as primates should show a higher fidelity to human taste qualities than non-primates (Brouwer et al. in J Physiol 337:240, 1983). Here we present summary results of behavioral and single taste fiber recordings from the distant South American marmoset, through the Old World rhesus monkey to chimpanzee, the phylogenetically closest species to humans. Our data show that in these species taste is transmitted in labelled-lines to the CNS, so that when receptors on taste bud cells are stimulated, the cell sends action potentials through single taste nerve fibers to the CNS where they create taste, whose quality depends on the cortical area stimulated. In human, the taste qualites include, but are perhaps not limited to sweet, sour, salty, bitter and umami. Stimulation of cortical taste areas combined with inputs from internal organs, olfaction, vision, memory etc. leads to a choice to accept or reject intake of a compound. The labelled-line organization of taste is another example of Müller's law of specific nerve energy, joining other somatic senses such as vision (Sperry in J Neurophysiol 8:15-28, 1945), olfaction (Ngai et al. in Cell 72:657-666, 1993), touch, temperature and pain to mention a few.

The blood circulation of the tongue

This article describes the gross anatomy of the vessels which supply the mammalian tongue. It shows that there is a rich vascular supply. Available data indicate that the lingual papillae are supplied with a true capillary circulation, which is more abundant in the papillae containing taste buds. The vessels of the tongue are innervated by adrenergic sympathetic vasconstrictor fibers and it is also very likely that a cholinergic parasympathetic vasodilator influence exists.

CALHM1 Deletion in Mice Affects Glossopharyngeal Taste Responses, Food Intake, Body Weight, and Life Span

Stimulation of Type II taste receptor cells (TRCs) with T1R taste receptors causes sweet or umami taste, whereas T2Rs elicit bitter taste. Type II TRCs contain the calcium channel, calcium homeostasis modulator protein 1 (CALHM1), which releases adenosine triphosphate (ATP) transmitter to taste fibers. We have previously demonstrated with chorda tympani nerve recordings and two-bottle preference (TBP) tests that mice with genetically deleted Calhm1 (knockout [KO]) have severely impaired perception of sweet, bitter, and umami compounds, whereas their sour and salty tasting ability is unaltered. Here, we present data from KO mice of effects on glossopharyngeal (NG) nerve responses, TBP, food intake, body weight, and life span. KO mice have no NG response to sweet and a suppressed response to bitter compared with control (wild-type [WT]) mice. KO mice showed some NG response to umami, suggesting that umami taste involves both CALHM1- and non-CALHM1-modulated signals. NG responses to sour and salty were not significantly different between KO and WT mice. Behavioral data conformed in general with the NG data. Adult KO mice consumed less food, weighed significantly less, and lived almost a year longer than WT mice. Taken together, these data demonstrate that sweet taste majorly influences food intake, body weight, and life span.

CALHM1 ion channel mediates purinergic neurotransmission of sweet, bitter and umami tastes

Recognition of sweet, bitter and umami tastes requires the non-vesicular release from taste bud cells of ATP, which acts as a neurotransmitter to activate afferent neural gustatory pathways. However, how ATP is released to fulfil this function is not fully understood. Here we show that calcium homeostasis modulator 1 (CALHM1), a voltage-gated ion channel, is indispensable for taste-stimuli-evoked ATP release from sweet-, bitter- and umami-sensing taste bud cells. Calhm1 knockout mice have severely impaired perceptions of sweet, bitter and umami compounds, whereas their recognition of sour and salty tastes remains mostly normal. Calhm1 deficiency affects taste perception without interfering with taste cell development or integrity. CALHM1 is expressed specifically in sweet/bitter/umami-sensing type II taste bud cells. Its heterologous expression induces a novel ATP permeability that releases ATP from cells in response to manipulations that activate the CALHM1 ion channel. Knockout of Calhm1 strongly reduces voltage-gated currents in type II cells and taste-evoked ATP release from taste buds without affecting the excitability of taste cells by taste stimuli. Thus, CALHM1 is a voltage-gated ATP-release channel required for sweet, bitter and umami taste perception.

Responses of single chorda tympani taste fibers of the calf (Bos taurus)

In spite of a wealth of information on feed and nutrition in cattle, there little is published of what they actually can taste. Here, we attempt to remedy some of this deficiency by presenting recordings of the chorda tympani proper nerve of young Holstein calves during stimulation of approximately 30 compounds. Hierarchical cluster analysis of 46 single taste fibers separated 4 fiber clusters: N (salt best), H (sour best), and 2 clusters, which could not be related to any human taste quality. The N fibers responded best to LiCl, NaCl, urea, monosodium glutamate, and KCl, whereas the H fibers responded strongly to citric and ascorbic acid. Interestingly, propionic and butyric acid stimulated best the 3rd cluster, whereas the 4th cluster responded best to denatonium benzoate and only to a small extent to quinine hydrochloride. Sweeteners stimulated moderately all clusters. Beginning with the largest response to sweet, the order between the responses was: acesulfame-K, saccharin, D-phenylalanine, glycine, sucrose, fructose, erythritol, cyclamate, and lactose. Alitame, aspartame, and super-aspartame evoked no or little responses. Three and 5 M ethanol stimulated all clusters. Comparison with taste fibers in other species suggests that the taste world of cattle is quite different from other species'.

Self-reported severity of taste disturbances correlates with dysfunctional grade of TMD pain

Altered central neural processing of sensory information may be associated with temporomandibular disorders (TMD) pain. The objectives of this study were to compare the prevalence of self-reported taste disturbances in TMD pain patients and in a control population, and to determine whether frequency of taste disturbances was correlated with dysfunctional grade of TMD pain. Subjects were 2026 people within a German population sample and 301 consecutive TMD patients diagnosed using the Research Diagnostic Criteria. Taste disturbances were measured using two questions from the Oral Health Impact Profile. Dysfunctional grade of TMD pain was measured with the Graded Chronic Pain Scale. A two-sample test of proportions revealed that TMD patients reported a greater frequency of taste disturbances, 6%, than did the general population subjects, 2% (P < 0.001). Moreover, the frequency of taste disturbances correlated with the dysfunctional grade of TMD pain. For each 1 unit increase in taste disturbance, the odds of observing a higher grade of TMD pain increased by 29% (95% CI: 3-63%, P = 0.03). Analysis by individual taste question and adjustment for age and gender did not substantially affect the results. These findings are consistent with a central neural dysfunction in TMD pain and suggest that a common neural substrate may underlie sensory disturbances of multiple modalities in chronic pain patients. Further research regarding taste disturbances and trigeminally mediated pains such as in TMD is warranted.

Design and evaluation of new analogs of the sweet protein brazzein

We have previously modeled the interaction of the sweet protein brazzein with the extracellular domains of the sweet taste receptor. Here, we describe the application of that model to the design of 12 new highly potent analogs of brazzein. Eight of the 12 analogs have higher sweetness potency than wild-type brazzein. Results are consistent with our brazzein-receptor interaction model. The model predicts binding of brazzein to the open form of T1R2 in the T1R2-T1R3 heterodimer.

The sweet taste quality is linked to a cluster of taste fibers in primates: lactisole diminishes preference and responses to sweet in S fibers (sweet best) chorda tympani fibers of M. fascicularis monkey

Background

Psychophysically, sweet and bitter have long been considered separate taste qualities, evident already to the newborn human. The identification of different receptors for sweet and bitter located on separate cells of the taste buds substantiated this separation. However, this finding leads to the next question: is bitter and sweet also kept separated in the next link from the taste buds, the fibers of the taste nerves? Previous studies in non-human primates, P. troglodytes, C. aethiops, M. mulatta, M. fascicularis and C. jacchus, suggest that the sweet and bitter taste qualities are linked to specific groups of fibers called S and Q fibers. In this study we apply a new sweet taste modifier, lactisole, commercially available as a suppressor of the sweetness of sugars on the human tongue, to test our hypothesis that sweet taste is conveyed in S fibers.

Results

We first ascertained that lactisole exerted similar suppression of sweetness in M. fascicularis, as reported in humans, by recording their preference of sweeteners and non- sweeteners with and without lactisole in two-bottle tests. The addition of lactisole significantly diminished the preference for all sweeteners but had no effect on the intake of non-sweet compounds or the intake of water. We then recorded the response to the same taste stimuli in 40 single chorda tympani nerve fibers. Comparison between single fiber nerve responses to stimuli with and without lactisole showed that lactisole only suppressed the responses to sweeteners in S fibers. It had no effect on the responses to any other stimuli in all other taste fibers.

Conclusion

In M. fascicularis, lactisole diminishes the attractiveness of compounds, which taste sweet to humans. This behavior is linked to activity of fibers in the S-cluster. Assuming that lactisole blocks the T1R3 monomer of the sweet taste receptor T1R2/R3, these results present further support for the hypothesis that S fibers convey taste from T1R2/R3 receptors, while the impulse activity in non-S fibers originates from other kinds of receptors. The absence of the effect of lactisole on the faint responses in some S fibers to other stimuli as well as the responses to sweet and non-sweet stimuli in non-S fibers suggest that these responses originate from other taste receptors.

Interactions of the sweet protein brazzein with the sweet taste receptor

Brazzein is a small, potently sweet protein. Homology modeling has been used to construct a model of the ligand-binding domain of the sweet taste receptor, and low-resolution docking has been used to identify potential modes of brazzein-receptor binding. Published brazzein mutation-taste data were then used to select one of these as the most likely brazzein-receptor binding orientation. This orientation places brazzein in contact primarily with the T1R2 subunit of the receptor, and it accounts for 21 of the 23 mutation results examined.

Taste Responses to Sweet Stimuli in -Gustducin Knockout and Wild-Type Mice

The importance of alpha-gustducin in sweet taste transduction is based on data obtained with sucrose and the artificial sweetener SC45647. Here we studied the role of alpha-gustducin in sweet taste. We compared the behavioral and electrophysiological responses of alpha-gustducin knockout (KO) and wild-type (WT) mice to 11 different sweeteners, representing carbohydrates, artificial sweeteners, and sweet amino acids. In behavioral experiments, over 48-h preference ratios were measured in two-bottle preference tests. In electrophysiological experiments, integrated responses of chorda tympani (CT) and glossopharyngeal (NG) nerves were recorded. We found that preference ratios of the KO mice were significantly lower than those of WT for acesulfame-K, dulcin, fructose, NC00174, D-phenylalanine, L-proline, D-tryptophan, saccharin, SC45647, sucrose, but not neotame. The nerve responses to all sweeteners, except neotame, were smaller in the KO mice than in the WT mice. The differences between the responses in WT and KO mice were more pronounced in the CT than in the NG. These data indicate that alpha-gustducin participates in the transduction of the sweet taste in general.

Elucidating coding of taste qualities with the taste modifier miraculin in the common marmoset

To investigate the relationships between the activity in different types of taste fibers and the gustatory behavior in marmosets, we used the taste modifier miraculin, which in humans adds a sweet taste quality to sour stimuli. In behavioral experiments, we measured marmosets' consumption of acids before and after tongue application of miraculin. In electrophysiological experiments responses of single taste fibers in chorda tympani and glossopharyngeal nerves were recorded before and after tongue application of miraculin. We found that after miraculin marmosets consumed acids more readily. Taste nerve recordings showed that after miraculin taste fibers which usually respond only to sweeteners, S fibers, became responsive to acids. These results further support our hypothesis that the activity in S fibers is translated into a hedonically positive behavioral response.

Probing the sweet determinants of brazzein: Wild-type brazzein and a tasteless variant, brazzein-ins(R18a-I18b), exhibit different pH-dependent NMR chemical shifts

Brazzein is a small, intensely sweet protein. As a probe of the functional properties of its solvent-exposed loop, two residues (Arg-Ile) were inserted between Leu18 and Ala19 of brazzein. Psychophysical testing demonstrated that this mutant is totally tasteless. NMR chemical shift mapping of differences between this mutant and brazzein indicated that residues affected by the insertion are localized to the mutated loop, the region of the single alpha-helix, and around the Cys16-Cys37 disulfide bond. Residues unaffected by this mutation included those near the C-terminus and in the loop connecting the alpha-helix and the second beta-strand. In particular, several residues of brazzein previously shown to be essential for its sweetness (His31, Arg33, Glu41, Arg43, Asp50, and Tyr54) exhibited negligible chemical shift changes. Moreover, the pH dependence of the chemical shifts of His31, Glu41, Asp50, and Tyr54 were unaltered by the insertion. The insertion led to large chemical shift and pKa perturbation of Glu36, a residue shown previously to be important for brazzein's sweetness. These results serve to refine the known sweetness determinants of brazzein and lend further support to the idea that the protein interacts with a sweet-taste receptor through a multi-site interaction mechanism, as has been postulated for brazzein and other sweet proteins (monellin and thaumatin).

Sense of Taste in a New World Monkey, the Common Marmoset. II. Link Between Behavior and Nerve Activity

In a previous study, we characterized the gustatory system of a New World monkey the common marmoset, Callithrix jacchus jacchus, with electrophysiological techniques by recording from taste fibers of the chorda tympani proper (CT) and glossopharyngeal (NG) nerves. Hierarchical cluster analysis identified three clusters of taste fibers: S fibers, responding predominantly to sweeteners, Q fibers, responding predominantly to bitter stimuli, and H fibers, responding predominantly to acids. In this study, we employed two behavioral techniques, the two-bottle preference (TBP) and conditioned taste aversion (CTA), to study the taste of the compounds used in the previous electrophysiological study. The results showed that compounds that did not stimulate any taste fibers were neither preferred nor rejected. Compounds that activated only S fibers were always preferred over water. When aversion to sucrose was created by the CTA method, these compounds were rejected. Compounds that activated Q fibers were rejected and consumed less than water. We studied the relationship between intake and net response from S and Q fibers in the CT and NG nerves. Intake was measured as a preference ratio in TBP test. The net response was defined as: (SCT + SNG) − (QCT + QNG), where SCT + SNG denotes the sum of the responses in S fibers of the CT and NG nerves. Similarly, QCT + QNG represents the sum of the responses in Q fibers of the CT and NG nerves. The relationship between intake and the Net response was linear with a Pearson correlation coefficient 0.85. This study supports our hypothesis that intake is influenced by S and Q fibers, where S fibers serve as a hedonically positive input and Q fibers as a hedonically negative input.

Monkey electrophysiological and human psychophysical responses to mutants of the sweet protein brazzein: delineating brazzein sweetness

Responses to brazzein, 25 brazzein mutants and two forms of monellin were studied in two types of experiments: electrophysiological recordings from chorda tympani S fibers of the rhesus monkey, Macaca mulatta, and psychophysical experiments. We found that different mutations at position 29 (changing Asp29 to Ala, Lys or Asn) made the molecule significantly sweeter than brazzein, while mutations at positions 30 or 33 (Lys30Asp or Arg33Ala) removed all sweetness. The same pattern occurred again at the beta-turn region, where Glu41Lys gave the highest sweetness score among the mutants tested, whereas a mutation two residues distant (Arg43Ala) abolished the sweetness. The effects of charge and side chain size were examined at two locations, namely positions 29 and 36. The findings indicate that charge is important for eliciting sweetness, whereas the length of the side-chain plays a lesser role. We also found that the N- and C-termini are important for the sweetness of brazzein. The close correlation (r = 0.78) between the results of the above two methods corroborates our hypothesis that S fibers convey sweet taste in primates.

Critical regions for the sweetness of brazzein

Brazzein is a small, heat-stable, intensely sweet protein consisting of 54 amino acid residues. Based on the wild-type brazzein, 25 brazzein mutants have been produced to identify critical regions important for sweetness. To assess their sweetness, psychophysical experiments were carried out with 14 human subjects. First, the results suggest that residues 29-33 and 39-43, plus residue 36 between these stretches, as well as the C-terminus are involved in the sweetness of brazzein. Second, charge plays an important role in the interaction between brazzein and the sweet taste receptor.

Comparison of the responses of the chorda tympani and glossopharyngeal nerves to taste stimuli in C57BL/6J mice

BACKGROUND

Recent progress in discernment of molecular pathways of taste transduction underscores the need for comprehensive phenotypic information for the understanding of the influence of genetic factors in taste. To obtain information that can be used as a base line for assessment of effects of genetic manipulations in mice taste, we have recorded the whole-nerve integrated responses to a wide array of taste stimuli in the chorda tympani (CT) and glossopharyngeal (NG) nerves, the two major taste nerves from the tongue.

RESULTS

In C57BL/6J mice the responses in the two nerves were not the same. In general sweeteners gave larger responses in the CT than in the NG, while responses to bitter taste in the NG were larger. Thus the CT responses to cyanosuosan, fructose, NC00174, D-phenylalanline and sucrose at all concentrations were significantly larger than in the NG, whereas for acesulfame-K, L-proline, saccharin and SC45647 the differences were not significant. Among bitter compounds amiloride, atropine, cycloheximide, denatonium benzoate, L-phenylalanine, 6-n-propyl-2-thiouracil (PROP) and tetraethyl ammonium chloride (TEA) gave larger responses in the NG, while the responses to brucine, chloroquine, quinacrine, quinine hydrochloride (QHCl), sparteine and strychnine, known to be very bitter to humans, were not significantly larger in the NG than in the CT.

CONCLUSION

These data provide a comprehensive survey and comparison of the taste sensitivity of the normal C57BL/6J mouse against which the effects of manipulations of its gustatory system can be better assessed.

Partial rescue of taste responses of alpha-gustducin null mice by transgenic expression of alpha-transducin

The transduction of responses to bitter and sweet compounds utilizes guanine nucleotide binding proteins (G proteins) and their coupled receptors. Alpha-gustducin, a transducin-like G protein alpha-subunit, and rod alpha-transducin are expressed in taste receptor cells. Alpha-gustducin knockout mice have profoundly diminished behavioral and electrophysiological responses to many bitter and sweet compounds, although these mice retain residual responses to these compounds. Alpha-gustducin and rod alpha-transducin are biochemically indistinguishable in their in vitro interactions with retinal phosphodiesterase, rhodopsin and G protein betagamma-subunits. To determine if alpha-transducin can function in taste receptor cells and to compare the function of alpha-gustducin versus alpha-transducin in taste transduction in vivo, we generated transgenic mice that express alpha-transducin under the control of the alpha-gustducin promoter in the alpha-gustducin null background. Immunohistochemistry showed that the alpha-transducin transgene was expressed in about two-thirds of the alpha-gustducin lineage of taste receptor cells. Two-bottle preference tests showed that transgenic expression of rod alpha-transducin partly rescued responses to denatonium benzoate, sucrose and the artificial sweetener SC45647, but not to quinine sulfate. Gustatory nerve recordings showed a partial rescue by the transgene of the response to sucrose, SC45647 and quinine, but not to denatonium. These results demonstrate that alpha-transducin can function in taste receptor cells and transduce some taste cell responses. Our results also suggest that alpha-transducin and alpha-gustducin may differ, at least in part, in their function in these cells, although this conclusion must be qualified because of the limited fidelity of the transgene expression.

Sense of taste in a new world monkey, the common marmoset: recordings from the chorda tympani and glossopharyngeal nerves

Whole nerve, as well as single fiber, responses in the chorda tympani proper (CT) and glossopharyngeal (NG) nerves of common marmosets were recorded during taste stimulation with three salts, four acids, six bitter compounds and more than 30 sweeteners. We recorded responses of 49 CT and 41 NG taste fibers. The hierarchical cluster analysis distinguished three major clusters in both CT and NG: S, Q, and H. The S(CT) fibers, 38% of all CT fibers, responded only to sweeteners. The S(CT) fibers did not respond during stimulation with salts, acids, and bitter compounds but exhibited OFF responses after citric and ascorbic acids, quinine hydrochloride (QHCl), and salts (in 80% of S(CT) fibers). S(NG) fibers, 50% of all NG fibers, also responded to sweeteners but not to stimuli of other taste qualities (except for citric acid, which stimulated 70% of the S(NG) fibers). Some sweeteners, including natural (the sweet proteins brazzein, monellin) and artificial [cyclamate, neohesperidin dihydrochalcone (NHDHC), N-3,5-dichlorophenyl-N'-(S)-alpha-methylbenzylguanidineacetate (DMGA), N-4-cyanophenylcarbamoyl-(R,S)-3-amino-3-(3,4-methylenedioxyphenyl) propionic acid (CAMPA)] did not elicit responses in the S fibers. In general, the response profiles of the S(CT) and S(NG) clusters were very similar, the correlation coefficient between the responses to sweeteners in these clusters was 0.94. Both the Q(CT) and the Q(NG) fibers (40 and 46% of all fibers) were predominantly responsive to bitter compounds, although their responses to the same set of bitter compounds were quite different. Sweeteners with sweet/bitter taste for humans also stimulated the Q clusters. The H clusters (22 and 3% of all fibers) were predominantly responsive to acids and did not respond to stimuli of other taste qualities. However, bitter stimuli, mainly QHCl, inhibited activity in 70% of H(CT) fibers. Among a total of 90 fibers from both nerves there was only 1 NaCl-best fiber in CT. We found, however, that 35% of the CT fibers reacted to salts with inhibition of activity during stimulation, followed by an OFF response. This OFF response was diminished or eliminated by amiloride. These characteristics indicate that amiloride-sensitive sodium channels are involved in salt transduction in marmosets. In the two NG fibers responding to NaCl, we recorded neither suppression by amiloride nor OFF responses. Comparison of marmoset data with those of other nonhuman primates studied, rhesus and chimpanzee, demonstrates phylogenetic trends in the organization of taste system. This can help to uncover pathways of primate evolution.

Oral sensation of ethanol in a primate model III: responses in the lingual branch of the trigeminal nerve of Macaca mulatta

Ethanol administered orally has been shown to elicit a powerful response in rhesus monkey taste nerves. In this study we focused on the effects of ethanol on lingual non-gustatory receptors by recording from 70 single lingual nerve fibers. Of these 70 fibers, 54 (78%) responded to one or more concentrations of 0.7-12 M ethanol; 16 fibers (22%) were not affected. In 48 (69%) fibers, ethanol increased nerve activity, whereas 6 fibers (9%) exhibited suppression, which was displayed as a diminished response to mechanical stimulation. The excitatory response was characterized by regular impulse activity after a latency of 3-40 sec. With higher concentrations of ethanol, the latency became shorter, and the impulse activity evoked became higher. In many fibers the response peaked and ceased before the end of the 52-sec long-stimulation period. Most of the fibers affected by ethanol responded to light touch and cooling. During repeated touch, ethanol initially potentiated and then abolished the response to mechanical stimulation. Methanol and propanol gave similar results. Butanol only inhibited nerve activity.

A new gene (rmSTG) specific for taste buds is found by laser capture microdissection

Getting pure populations of taste buds suitable for molecular analysis has hampered the characterization of genes specifically expressed in taste cells. To solve this problem, we prepared specific cDNA libraries from small numbers of taste cells and surrounding epithelium isolated by laser capture microdissection (LCM) and report the discovery of a rhesus monkey novel gene (rmSTG) expressed specifically in taste cells, as found by differential screening of the cDNA libraries and RNA in situ hybridization. RNA in situ hybridization shows the preferential expression of this gene in taste buds from circumvallate, foliate, and fungiform papillae of the tongue. RT-PCR and Northern analysis of RNA from different non-taste organs showed no expression, pointing to a very specialized function of the protein in taste cells. Analysis of extended cDNAs and genomic DNA showed two exons and one intron. Northern analysis of circumvallate papillae showed a transcript of 1.3 kb as established in the gene model. BLAST search analysis showed that the human homolog is localized in the recently completely sequenced HLA class I region of Chromosome 6p21 and is sublocalized to the main susceptibility region for psoriasis vulgaris. The predicted gene encodes a protein of 314 amino acids with an N-terminal signal peptide and cleavage site, suggesting a membrane-bound or secreted protein with an extracellular role in taste cell physiology. The monkey, human, and mouse STG proteins contain potential O-glycosylation sites and tandem repeats inside a region showing approximately 50% similarity with prion proteins.

The taste of ethanol in a primate model. II. Glossopharyngeal nerve response in Macaca mulatta

The glossopharyngeal nerve (NG) mediates taste from the posterior part of the tongue. Here, we studied the effects of ethanol on the tongue in recordings from both the whole NG and individual taste fibers of the rhesus monkey, Macaca mulatta. The results show that the nerve activity increased at 0.7 M ethanol, reaching half maximum at around 4 M alcohol. Previously, we identified three types of taste fibers in the rhesus monkey NG: S fibers predominantly responding to sweeteners, Q fibers responding to bitter, such as quinine hydrochloride (QHCl), and M fibers responding best to monosodium glutamate, NaCl and acids [Hellekant, G., Danilova, V., & Ninomiya, Y. (1997). Primate sense of taste: behavioral and single chorda tympani and glossopharyngeal nerve fiber recordings in the rhesus monkey, Macaca mulatta. J Neurophysiol 77, 978-993]. Here, this fiber classification was used to elucidate the oral effects of ethanol and ethanol mixtures with NaCl, sucrose, citric acid and QHCl. One and three molar concentrations of ethanol stimulated all fiber types. Mixtures of ethanol with QHCl elicited a smaller response in Q fibers than did QHCl alone. In S fibers, mixtures of ethanol with sucrose gave a larger response than did sucrose alone. The variability of M fibers was too large to allow a conclusion about the effect of ethanol. These results suggest that ethanol suppresses the taste of QHCl. Similarly, the taste of sucrose might be enhanced by adding ethanol to sucrose. These effects and conclusions corroborate an earlier ethanol study of the chorda tympani (CT) nerve [Hellekant, G., Danilova, V., Roberts, T., & Ninomiya, Y. (1997). The taste of ethanol in a primate model: I. Chorda tympani nerve response in Macaca mulatta. Alcohol 14, 473-484].

Responses to umami substances in taste bud cells innervated by the chorda tympani and glossopharyngeal nerves

The chorda tympani (CT) and glossopharyngeal (GL) nerves of several mammalian species respond differently to umami substances (US) such as monosodium glutamate (MSG), disodium 5'-inosinate (IMP) and disodium 5'-guanylate (GMP). In mice and rhesus monkeys, responses to US are greater in the GL than the CT nerve, with the GL nerve containing larger numbers of MSG-sensitive fibers. Gurmarin, a sweet response inhibitor, suppresses the mouse CT responses to the mixture of MSG and IMP to approximately 65% of control levels but not to the metabotropic and ionotropic glutamate agonists 2-amino-4-phophonobutyrate and N-methyl-D-aspartate. Gurmarin does not inhibit any taste responses in the GL. In mice, CT responses to MSG may be masked by their greater sensitivity to sodium ions. Calcium imaging studies demonstrate that some mouse taste cells isolated from the fungiform papilla innervated by the CT respond selectively (as indicated by a rise in intracellular Ca(2+) concentrations) to MSG and/or IMP or GMP. These MSG responses are not suppressed notably by reducing the Ca(2+) concentration of the stimulus solution, suggesting that the observed Ca(2+) release is from intracellular stores. Measurements of second messengers in the mouse fungiform papilla have revealed consistently that MSG elicits increases in both inositol 1,4,5-trisphosphate and adenosine 3', 5'-cyclic monophosphate levels. Together, these results suggest that US may stimulate two different transduction mechanisms in the fungiform papilla. They also suggest that gurmarin-insensitive components of receptors for US, including metabotropic and ionotropic glutamate receptors, may be commonly involved in transduction for umami taste in taste cells on both anterior and posterior parts of the tongue.

Responses of single taste fibers and whole chorda tympani and glossopharyngeal nerve in the domestic pig, Sus scrofa

Whole nerve, as well as single fiber, responses in the chorda tympani proper (CT) and glossopharyngeal (NG) nerves of 1- to 7-week-old pigs were recorded during taste stimulation. In the CT acids and in the NG bitter compounds gave the largest responses. Both nerves exhibited large responses to monosodium glutamate (MSG), MSG with guanosine 5'-monophosphate (GMP) and MSG with inositine 5'-monophosphate (IMP) as well as to glycine, xylitol, sucrose, fructose and glucose. Alitame, aspartame, betaine, neohesperedin dihydrochalcone (NHDHC), super-aspartame, saccharin and thaumatin elicited no or little response. Hierarchical cluster analysis of 49 CT fibers separated four major clusters. The M cluster, comprising 28.5% of all fibers, is characterized by strong responses to MSG, KCl, LiCl and NaCl. The responses to NaCl and LiCl were unaffected by amiloride. The H cluster (24.5%) includes units responding principally to acids. The Q cluster (18.5%) responds to quinine hydrochloride (QHCl), sucrose octaacetate (SOA) and salts with amiloride. The S cluster (28.5%) exhibits strong responses to xylitol, glycine and the carbohydrates as well as to MSG alone and to MSG with GMP or IMP. In 31 NG fibers, hierarchical cluster analysis revealed four clusters: the M cluster (10%), responding to MSG and MSG with GMP or IMP; the H cluster (13%), responding to acids; the Q cluster (29%), responding strongly to QHCl, SOA and tilmicosinR; and the S cluster (48%), responding best to xylitol, carbohydrates and glycine but also to the umami compounds. Multidimensional scaling analysis across fiber responses to all stimuli showed the best separation between compounds with different taste qualities when information from both nerves was utilized.

Solution conformation of brazzein by 1H nuclear magnetic resonance: resonance assignment and secondary structure

Brazzein is a sweet-tasting protein isolated from the fruit of the West African plant Pentadiplandra brazzeana Baillon. It is the smallest and the most water-soluble sweet protein discovered so far, it is also highly thermostable. The proton NMR study of brazzein at 600 MHz (pH 3.5, 300K) is presented. Complete sequence specific assignment of the individual backbone and sidechain proton resonances were achieved using through-bond and through-space connectivities obtained from standard two-dimensional NMR techniques. The secondary structure of brazzein contains one alpha-helix (residues 21-29), one short 3(10)-helix (residues 14-17), two strands of antiparallel beta-sheet (residues 34-39, 44-50) and probably a third strand (residues 5-7) near the N-terminus.

Behavioral and single chorda tympani taste fiber responses in the common marmoset, Callithrix jacchus jacchus

Gustatory responses of the common marmoset were studied using single fiber recordings from chorda tympani (CT) nerve and two bottle preference (TBP) tests. Hierarchical cluster analysis of 43 fibers' response profiles revealed 3 major clusters of fibers characterized by predominant sensitivity to sweeteners (S cluster), bitter compounds (Q cluster) or acids (H cluster). NaCl as well as LiCl did not stimulate CT taste fibers. The TBP tests showed relationship between a compound's ability to stimulate the taste fibers and the animals' consumption. Activity in the S cluster was associated with preference, while the activity in the Q cluster was associated with rejection. Marmosets neither preferred nor rejected sweeteners which did not stimulate any CT fibers.

Taste in chimpanzees. III: Labeled-line coding in sweet taste

In peripheral taste the coding mechanism remains an enigma. Among coding theories the "across-fiber pattern" argues that activity across fibers codes for taste, whereas the "labeled line" claims that activity in a particular set of fibers underlies a taste quality. We showed previously that chimpanzee chorda tympani taste fibers grouped according to human taste qualities into an S-cluster, responding predominantly to sweet stimuli, a Q-cluster, sensitive to bitter tastants, and an N-cluster, stimulated by salts. The analysis showed that information in the S-line suffices to distinguish stimuli of one taste quality from the others. However, one condition for the labeled line remained: that blockage of activity in a particular line must cause blockage of one taste quality, but of no other, or its onset give rise to the sensation of a taste quality. Here we studied this requirement with gymnemic acids and miraculin. In humans and chimpanzees, gymnemic acids suppress the sweet taste of all sweeteners whereas miraculin adds a sweet taste quality to sour stimuli. Gymnemic acids also abolish miraculin-induced sweet taste. We found that gymnemic acids practically abolished the response to every sweetener in the chimpanzee S-cluster. Equally important, they had no effect on the responses of the Q- and N-clusters. After miraculin, the S-cluster fibers responded to acids as well as to sweeteners, although they had not responded to acids before miraculin. Gymnemic acids abolished this miraculin-induced response to acids and responses to sweeteners in the S-fibers. These results link the sweet taste quality to activity in fibers of the S-cluster. Thus the S-cluster fibers satisfy the definition of the labeled-line theory: "that activity in a particular fiber type represents a specific taste quality."

Gustatory responses of the hamster Mesocricetus auratus to various compounds considered sweet by humans

The taste of 30 compounds was studied in the golden hamster with three different methods: single-fiber recordings, two-bottle preference (TBP), and conditioned taste aversion (CTA) tests. On the whole, the results showed that the sense of taste in the hamster differs in many respects from that in humans because, of 26 tested compounds known as sweet to humans, 11 had no taste or tasted differently. The results also supported the notion that activity in S-fibers elicits liking and activity in Q- or H-fibers rejection. Specifically hierarchial cluster analysis of 36 single fibers from the chorda tympani proper nerve separated N-, H-, and S-clusters consisting of 11 sucrose-, 14 NaCl-, and 11 citric-best fibers. Ace-K, cyanosuosan, N-4-cyanophenyl-N'-cyanoguanidineacetate (CCGA), -tryptophan, N-3, 5-dichlorophenyl-N'-(S)-alpha-methylbenzylguanidineacetate (DMGA), saccharin, SC-45647, and suosan stimulated only the S-fibers, were significantly preferred in TBP tests, and generalized to sucrose in the CTA tests. Ethylene glycol stimulated the N-fibers in addition to the S-fibers. This explains its generalization to sucrose in CTA. Its toxicity may contribute to its rejection in TBP tests. Sodium cyclamate stimulated a few N- but no S-fibers, which may explain the nondiscriminatory TBP and CTA results. Glycine elicited its largest response in the S-fibers, although it also stimulated other fibers. The resulting mixed taste sensation may explain why it was not preferred in TBP, although it generalized to sucrose in the CTA. Alitame, aspartame, N-4-cyanophenylcarbamoyl--aspartyl-(R)-alpha-methylbenzylamine (CAM), N-4-cyanophenylcarbamoyl-(R, S)-3-amino-3-(3, 4-methylenedioxyphenyl) propionic acid (CAMPA), N-(S)-2-methylhexanoyl--glutamyl-5-amino-2-pyridinecarbonitrile (MAGAP), N-1-naphthoyl--glutamyl-5-amino-2-pyridinecarbonitrile (NAGAP), NHDHC, superaspartame, and thaumatin were among the compounds considered sweet by humans that gave no response, were not discriminated in the TBP test, and gave no generalization in the CTA tests.

Solution structure of the thermostable sweet-tasting protein brazzein

The fruit of Pentadiplandra brazzeana Baillon contains a small, sweet-tasting protein named brazzein. The structure of brazzein in solution was determined by proton nuclear magnetic resonance spectroscopy at pH 5.2 and 22 degrees C. The brazzein fold, which contains one alpha-helix and three strands of antiparallel beta-sheet, does not resemble that of either of the other two sweet-tasting proteins with known structures, monellin and thaumatin. Instead, the structure of brazzein resembles those of plant gamma-thionins and defensins and arthropod toxins. Sequence comparisons predict that members of a newly-identified family of serine proteinase inhibitors share the brazzein fold.

The taste of ethanol in a primate model: I. Chorda tympani nerve response in Macaca mulatta

The chorda tympani nerve (CT) mediates taste from the anterior part of the tongue. Here we studied the effects of ethanol on the tongue in recordings from both the whole CT nerve and individual taste fibers of the rhesus monkey, M. mulatta. The response to ethanol consisted of a phasic and a tonic part. At the lowest concentration tested (0.3 M) ethanol gave a response in some animals and at 0.7 M in all animals. A sigmoidal function described best the relationship between nerve response and ethanol concentrations. Hierarchial cluster analysis with 26 nonalcoholic sweet, sour, salty, and bitter stimuli had earlier identified four types of taste fibers each responding predominantly to stimuli within one of the four human taste qualities. Here were found that ethanol stimulated all sweet-best fibers and at high concentration some salt-best fibers, but never any acid-best and bitter-best fibers. This may explain the sweet taste attributed to low ethanol concentration by humans. Further, in mixtures it suppressed the responses in acid-best and bitter-best taste fibers. This may partly explain the effects of ethanol on sour and bitter taste in alcoholic beverages.

Taste in chimpanzees II: single chorda tympani fibers

Data are presented from 48 taste fibers in chorda tympani nerves of 10 chimpanzees during taste stimulation with 29 stimuli. The results demonstrated a higher taste fiber specificity than in any other mammalian species reported; breadth of tuning equals 0.3. Hierarchical cluster analysis separated an S-cluster (50% of all fibers), an N-cluster (31%), and a Q-cluster (19%). The S-cluster showed the highest specificity. Its fibers responded, with few exceptions, to every sweetener tested, including the sweet proteins brazzein and monellin. The response grew with increasing sweetener concentration. A large response to one sweetener was generally accompanied by a large response to all other sweeteners, and vice versa. Except for one broadly tuned fiber, the fibers of the S-cluster never responded to the bitter compounds. The fibers of the Q-cluster were more broadly tuned than any other fibers. Quinine hydrochloride was their best stimulus, but most fibers were also stimulated by KCl and NaCl with amiloride. Acids stimulated some of these fibers. The N-cluster could be divided into 3 subclusters: an Na-subcluster (3 fibers), Na-K subcluster (10 fibers), and M-subcluster (3 fibers). The Na-fibers responded strongly to, and were quite specific to, NaCl and LiCl stimulation but not to KCl, and fibers of the Na-K subcluster responded equally well to NaCl and KCl. The response to NaCl was suppressed by amiloride in the fibers of the Na-subcluster, but not in the fibers of the Na-K subcluster. Umami compounds elicited the strongest responses in the M-subcluster.

Primate sense of taste: behavioral and single chorda tympani and glossopharyngeal nerve fiber recordings in the rhesus monkey, Macaca mulatta

The responses of 51 chorda tympani proper (CT) and 33 glossopharyngeal (NG) neural taste units from the rhesus monkey (Macaca mulatta) were recorded during stimulation of either the anterior (CT) or posterior (NG) part of the tongue with 26 stimuli that taste salty, umami, sour, bitter, and sweet to humans. In the CT, hierarchical cluster analysis separated four major clusters. The N and S clusters were most populous, followed by the H cluster and a small Q cluster. NaCl, monosodium glutamate (MSG), and MSG with guanosine 5'-monophosphate were the best stimuli in the N cluster. Amiloride suppressed responses to NaCl. KCl did not stimulate fibers from this cluster. S cluster fibers were characterized by strong responses to all sweeteners. The H cluster responded best to acids but also to some of the sweeteners such as xylitol, fructose, and sucrose. Q fibers responded well to quinine hydrochloride (QHCl) and caffeine, but not to denatonium benzoate. In the NG, hierarchical cluster analysis separated three major clusters. Q fibers formed the largest cluster. QHCl, caffeine, and sucrose octa-acetate but not denatonium benzoate elicited very strong responses in these fibers. S fibers formed a second cluster. Although most of the sweeteners stimulated the S fibers, their responses were not so pronounced as in CT S fibers. The small M cluster was formed by fibers that responded best to MSG. They also responded to NaCl and acids. Two bottle preference tests showed a positive relationship between a sweetener's ability to stimulate the taste fibers and the animals' consumption. Thus the most-liked sweeteners stimulated the S cluster fibers of CT best, whereas less-liked sweeteners such as D-phenylalanine elicited a response in Q fibers and sodium cyclamate stimulated N fibers. The results show that both CT and NG taste fibers of M. mulatta group according to the human concepts of taste qualities.

Understanding the mechanism of sweet taste: synthesis of ultrapotent guanidinoacetic acid photoaffinity labeling reagents

Azido-functionalized analogs of potently sweet guanidinoacetic acids have been synthesized for use as sweetener receptor photoaffinity labeling reagents. These compounds have been synthesized using readily available starting materials. One of the azido-labeled guanidinoacetic acids has been evaluated in an electrophysiological model in the Rhesus monkey. We found that the photoaffinity-labeling reagent caused irreversible inhibition in electrophysiological response to sweeteners upon exposure of the monkey tongue to a combination of the reagent and UV light.

Taste in chimpanzee: I. The summated response to sweeteners and the effect of gymnemic acid

The purpose of this study was to investigate further with electrophysiological and behavioral techniques the similarity between the sense of taste of humans and chimpanzees, especially with regard to the effects of gymnemic acid. Gymnemic acid (GA) is a powerful suppressor of sweet taste in humans but lacks this ability in nonprimates and lower primates. The summated taste responses from the chorda tympani nerve were recorded in nine adult chimpanzees, Pan troglodytes. The results show that all tested compounds that taste to humans elicited nerve responses in chimpanzees. GA suppressed or abolished the response to all sweeteners, but had no effects on the responses to the nonsweet compounds. The suppression varied from complete abolishment (aspartame, saccharin), to about 50% reduction (xylitol). When the effect of GA was tested on concentration series, 20% remained of the response to sucrose, whereas the responses to aspartame and saccharin were basically abolished. Higher concentrations of GA suppressed more. The effects of GA developed also in the presence of saccharin, but seemed less pronounced. The behavioral results were obtained with a one-bottle preference technique before and after GA. The results demonstrated that after exposure of the tongue to GA, the animals' liking for sweet diminished. These results parallel psychophysical and electrophysiological findings in humans. The way GA suppressed sweet taste in chimpanzees added one more characteristic to those that set chimpanzees apart from monkeys and close to humans.

Immunohistochemical, electrophysiological, and electron microscopical study of rat fungiform taste buds after regeneration of chorda tympani through the non-gustatory lingual nerve

The sensory innervation of fungiform papillae on the rat dorsal tongue is derived from branches of two cranial nerves: the lingual branch of the trigeminal nerve which provides somatosensory innervation and the chorda tympani (CT) branch of the facial nerve, which provides innervation to the taste buds. Removal of the CT results in degeneration of the taste buds. Removal of both nerves results in reduction in size of fungiform papillae and an altered pattern of keratinization in its epithelium. Regeneration of nerves to the epithelium restores the pre-operative condition. Thus, in addition to their sensory functions, both the CT and lingual seem to exert trophic effects on the phenotypic expression of epithelial cells in the fungiform papillae. We severed both the CT and lingual nerves in rats and sutured the proximal stump of the CT to the distal stump of the lingual to promote regeneration of the CT along the lingual nerve pathway. At the same time, we prevented the proximal stump of the lingual from regenerating into the tongue. Our purpose was to determine whether and how the innervation pattern of the regenerated taste bud might be different from normal under these experimental conditions. We found that reinnervation by the CT through the lingual nerve occurs, that this restores the anatomical and functional integrity of the fungiform taste buds and papillae, and that some papillae, but not all, were richly innervated with subgemmal, extragemmal, and perigemmal neuron-specific enolase, calcitonin gene-related peptide, substance P, and neurokinin A-positive fibers. Moreover, responses to taste stimuli were recorded electrophysiologically from the CT.

Assignment of the disulfide bonds in the sweet protein brazzein

The thermostable sweet protein brazzein consists of 54 amino acid residues and has four intramolecular disulfide bonds, the location of which is unknown. We found that brazzein resists enzymatic hydrolysis at enzyme/substrate ratios (w/w) of 1:100-1:10 at 35-40 degrees C for 24-48 h. Brazzein was hydrolyzed using thermolysin at an enzyme/substrate ratio of 1:1 (w/w) in water, pH 5.5, for 6 h and at 50 degrees C. The disulfide bonds were determined, by a combination of mass spectrometric analysis and amino acid sequencing of cystine-containing peptides, to be between Cys4-Cys52, Cys16-Cys37, Cys22-Cys47, and Cys26-Cys49. These disulfide bonds contribute to its thermostability.

Bitter taste in single chorda tympani taste fibers from chimpanzee

We have found earlier that chimpanzee chorda tympani taste fibers fall into groups that conform with the human taste qualities. This study focuses on bitter taste and its relation to sweet taste. Eight fibers were classified as bitter fibers according to their responses to 31 stimuli. The stimuli included the bitter compounds quinine, denatonium benzoate and caffeine. The results indicate a clear dichotomy between the bitter and sweet fibers. Sweet fibers never responded to the bitter compounds. However, in addition to their responses to the above compounds, some of the bitter fibers were stimulated by other compounds. Most prominent were responses to NaCl-amiloride mixture, KCl and xylitol. In most cases the cause could be assumed to be a bitter taste in the compound. These results suggest that the bitter and sweet tastes are conveyed in specific and separate groups of nerve fibers in the chimpanzee. Because of the closeness between chimpanzee and human, this finding has implications on the question of taste coding in human and the concept of taste qualities.

Brazzein, a new high-potency thermostable sweet protein from Pentadiplandra brazzeana B

We have discovered a new high-potency thermostable sweet protein, which we name brazzein, in a wild African plant Pentadiplandra brazzeana Baillon. Brazzein is 2,000 times sweeter than sucrose in comparison to 2% sucrose aqueous solution and 500 times in comparison to 10% of the sugar. Its taste is more similar to sucrose than that of thaumatin. Its sweetness is not destroyed by 80 degrees C for 4 h. Brazzein is comprised of 54 amino acid residues, corresponding to a molecular mass of 6,473 Da.

Sweet taste in the calf: III. Behavioral responses to sweeteners

The hedonic response to the sweeteners acesulfame-K, aspartame, fructose, galactose, glucose, glycine, lactose, maltose, Na-saccharine, sucrose, and xylitol was recorded in five groups of 4-16-week-old calves. The compounds were presented to the calves for 12 or 24 h in two-bottle preference tests with tap water as one choice. Glycine (10 mM and higher), sucrose (20 mM and higher), and fructose concentrations were most preferred. Sodium-saccharine was highly preferred at and above 4 mM concentration, fructose and lactose were preferred above 40 mM, galactose was preferred moderately, acesulfame-K and maltose were preferred inconsistently, and aspartame and xylitol were not preferred at any concentration. The change of preference during the tests was also studied. Three types of consumption changes were observed. 1) Increased preference of the tastant during consumption, seen during sucrose and, to lesser a extent, fructose consumption. 2) Initial high preference for the tastants, diminishing during the test period, observed with fructose, galactose, glucose, glycine, lactose, and maltose. 3) Initial large fluctuations in consumption from the two bottles, but no change in overall preference. This pattern was seen with xylitol and aspartame. This technique seems to offer a method to assess the long-term preference for a compound within one relatively short two-bottle preference session.

Enhancing effects of saccharin on gustatory responses to D-phenylalanine in monkey single chorda tympani fibers

Taste enhancing effects of sodium saccharin (Sac) on D-phenylalanine (D-Phe), first found in mice, were examined by comparing single fiber responses to various taste stimuli in the monkey chorda tympani nerve. Fifteen fibers sampled were divided into the following 5 groups according to their responsiveness to 5 prototypical taste stimuli; 8 sucrose-, 2 quinine-, 2 acid-, 2 NaCl- and one monosodium glutamate (MSG)-best fibers. Out of 8 sucrose-best fibers, 5 fibers showed enhancement of D-Phe responses after the stimulation with Sac, but neither the remaining 3 sucrose-best fibers nor other fibers showed the enhancement. These results suggest that (1) the enhancement of D-Phe responses by Sac also occurs in the monkey peripheral taste system, and (2) there exist distinct receptor sites for D-Phe responsible for occurrence of the enhancement, and (3) taste cells possessing the D-Phe receptor site are innervated by a limited subpopulation of sucrose-best fibers.

Ultrastructural evidence for a binding substance to the sweet-tasting protein thaumatin inside taste bud pores of rhesus monkey foliate papillae

Thaumatin is a protein that tastes intensely sweet only to Old World monkeys and to higher primates, including man. Here we used pre-embedding ultrastructural methods to study the distribution of thaumatin in apical regions of Rhesus monkey foliate papillae, using thaumatin conjugated to 5 nm gold particles. With freeze-substitution we saw that gold-labeled thaumatin bound to an electron-opaque, sponge-like secretory substance inside the taste bud pores. Labeled thaumatin was found at the surface of the secretory substance even deep inside the pore, where other, unlabeled cellular structures surrounded the substance. With freeze-fracture deep-etching the secretory substance that bound the thaumatin-gold particles appeared coarsely granular. There was no labeling of any other taste bud pore structure, including microvilli and small membrane-lined vesicles. Pre-incubation with an excess of unlabeled thaumatin inhibited binding with gold-labeled thaumatin. The results suggest that the secretory substance had the greatest affinity of all taste pore structures to the sweet-tasting compound under our experimental conditions. Therefore, gustatory reception probably involves various taste compound binding structures, microvilli, and also secretory substances like the one described here which bound thaumatin. We speculate that the secretory substance may bind taste stimuli and serve as an intermediate between stimuli and receptors. It could be involved in stimulus removal or delivery or both.

On the taste of umami in chimpanzee

Whole and single fiber chorda tympani nerve recordings were obtained in 5 chimpanzees to stimulation with MSG (monosodium phosphate) and GMP (guanosine 5'-monophosphate, disodium salt) alone and in combination. The overall chorda tympani nerve activity was recorded to 5 concentrations of MSG, ranging from 1 to 100 mM with and without 0.3 mM GMP, and to 5 concentrations of GMP, ranging from 0.1 to 10 mM, with and without 30 mM MSG. A synergistic effect was recorded between MSG and GMP in 3 out of 4 animals. The effect of stimulation with MSG and GMP alone and mixed was studied in approximately 25 single fiber recordings against a background of the stimulating effects of 11 different sweeteners, 3 acids, 3 bitter compounds and 3 different salts. The fibers showed a high taste specificity and fell into groups which corroborated with the human concepts of the taste qualities. The umami compounds elicited moderate responses which were largest in the sweet fibers. In the 6 sweet fibers that responded to the umami compounds. 0.3 mM GMP was a more effective stimulus than 10 mM MSG. In 3 of these fibers a synergistic effect was recorded to the mixture of GMP and MSG. It is interesting that the response to GMP and MSG was unaffected by gymnemic acid, although it blocked the response to the sweet compounds. Three out of 10 salt fibers responded to MSG and GMP but no synergistic effect was recorded. No specific umami fibers were recorded. However, more data must be collected before the final conclusion on the presence or absence of specific umami fibers can be drawn.

mRNAs for PRPs, statherin, and histatins in von Ebner's gland tissues

A search was made for expression of genes for proline-rich proteins (PRPs) and other salivary-type proteins, including statherin and histatins, in taste-bud tissues of mice and primates because of previous genetic findings in mice (Azen et al., 1986) that Prp and taste genes for certain bitter substances are either the same or closely linked. Taste-bud tissues and other tissues were tested for specific mRNAs with labeled DNA probes by Northern blotting and in situ hybridization. It was found that PRP mRNAs were present in von Ebner's glands of mice and macaques, and that there was a much greater degree of PRP mRNA induction in mouse parotid (16-fold) than in von Ebner's gland (two-fold) after in vivo isoproterenol stimulation. This difference may be due, in part, to differences in autonomic nerve innervation. Statherin and histatin mRNAs were found in macaque taste-bud tissues containing von Ebner's gland, and statherin protein was found in human von Ebner's gland by immunohistochemistry. The finding of PRP gene expression in von Ebner's gland, whose secretions have been suggested to play a role in taste stimulation, adds further support to a possible function of PRPs in bitter tasting. The possible functions of statherin and histatins in von Ebner's gland secretions may be related to statherin's regulation of salivary calcium and histatins' antibacterial and antifungal properties.