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PubMed1986

Constraints imposed on taste physiology by human taste reaction time data.

The speed with which an organism responds to stimulus events is reaction time (RT): the minimum time interval between stimulus arrival at a receptor organ, and an overt response by the organism. This time interval specifies maximum duration of all processes necessary for the RT sequence. Responses to any change in taste have RT less than 1 sec for suprathreshold concentrations. Therefore, constituent events at taste receptors, in the central nervous system (CNS), and at the response organ, must have sufficient durations less than 1 sec (Constraint 1). Taste stimulus durations of 50 msec, and therefore taste receptor events of approximately 50 msec, are sufficient for these responses (Constraint 2), as well as for taste quality identification responses (Constraint 3). Taste receptor latencies, neural conduction times, and RT response organ events are even briefer. Thus, 60% to 90% of human taste RT is CNS events. Taste receptor events remain crucial, but CNS processing is important, and apparently time limiting, in all human taste judgments.

Afferent Pathways↗
PubMed2002

Taste sensitivity to 6-n-propylthiouracil predicts acceptance of bitter-tasting spinach in 3-6-y-old children.

BACKGROUND: Understanding what motivates the preference for and selection of foods has important health implications. Research suggests that the phytochemicals present in green leafy vegetables contain anticarcinogenic properties. As a result of the bitter taste of phytochemical compounds, however, foods containing these are often not well accepted, particularly by children. OBJECTIVE: We aimed to study the relation between sensitivity to the bitter taste of 6-n-propylthiocuracil (PROP) and acceptance of bitter- and strong-tasting foods in 3-6-y-old children. DESIGN: Two independent procedures, a threshold detection and a suprathreshold intensity task, were used to measure individual sensitivity to PROP, and 3 independent tasks were used to assess food acceptance. RESULTS: Sensitivity to the bitter taste of PROP was positively correlated with dislike of the taste of raw spinach (P < 0.05). CONCLUSIONS: The acceptance of spinach may to some extent be mediated by individual taste perception and be predictable via both threshold and suprathreshold measures of PROP taste sensitivity. Furthermore, children as young as 3 y of age can partake in direct investigations of taste, reliably comply with test procedures, and accurately communicate taste perceptions and preferences under study conditions.

Child, Preschool↗
PubMed1998

The addition of CO2 to traditional taste solutions alters taste quality.

Previous studies of the effect of carbonation on taste perception have suggested that it may be negligible, manifesting primarily in increases in the perceived intensity of weak salt and sour stimuli. Assuming CO2 solutions in the mouth stimulate only trigeminal nerve endings, this result is not altogether surprising; however, there are neurophysiological data indicating that CO2 stimulates gustatory as well as trigeminal fibers. In that case, carbonation might alter the quality profile of a stimulus without producing substantial changes in overall taste intensity--much as occurs when qualitatively different taste stimuli are mixed. To address this possibility, subjects were asked to rate the total taste intensity of moderate concentrations of stimuli representing each of the basic tastes and their binary combinations, with an without added carbonation. They then subdivided total taste intensity into the proportions of sweetness, saltiness, sourness, bitterness and 'other taste qualities' they perceived. The addition of carbonation produced only small increases in ratings of total taste intensity. However, rather dramatic alterations in the quality profiles of stimuli were observed, particularly for sweet and salty tastes. The nature of the interaction is consistent with a direct effect of carbonation/CO2 on the gustatory system, although the possibility that at least some of the observed effects reflect trigeminal-gustatory interactions cannot be ruled out.

Adult↗
PubMed1996

The taste of monosodium glutamate: membrane receptors in taste buds.

Receptor proteins for photoreception have been studied for several decades. More recently, putative receptors for olfaction have been isolated and characterized. In contrast, no receptors for taste have been identified yet by molecular cloning. This report describes experiments aimed at identifying a receptor responsible for the taste of monosodium glutamate (MSG). Using reverse transcriptase (RT)-PCR, we found that several ionotropic glutamate receptors are present in rat lingual tissues. However, these receptors also could be detected in lingual tissue devoid of taste buds. On the other hand, RT-PCR and RNase protection assays indicated that a G-protein-coupled metabotropic glutamate receptor, mGluR4, also is expressed in lingual tissues and is limited only to taste buds. In situ hybridization demonstrated that mGluR4 is detectable in 40-70% of vallate and foliate taste buds but not in surrounding nonsensory epithelium, confirming the localization of this metabotropic receptor to gustatory cells. Expression of mGluR4 in taste buds is higher in preweaning rats compared with adult rats. This may correspond to the known higher sensitivity to the taste of MSG in juvenile rodents. Finally, behavioral studies have indicated that MSG and L-2-amino-4-phosphonobutyrate (L-AP4), a ligand for mGluR4, elicit similar tastes in rats. We conclude that mGluR4 may be a chemosensory receptor responsible, in part, for the taste of MSG.

Amino Acid Sequence↗
PubMed2007

Immunohistochemical localization of aromatic L-amino acid decarboxylase in mouse taste buds and developing taste papillae.

Aromatic L-amino acid decarboxylase (AADC) catalyses the decarboxylation of all aromatic L-amino acids. In mammals, AADC is expressed in many tissues besides the nervous system, and is associated with additional regulatory roles of dopamine and serotonin in a wide range of tissues. We examined the expression of AADC by using reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. RT-PCR analysis showed that mRNA of AADC was detected in the taste bud-containing epithelium of the circumvallate papilla of mice. By immunohistochemical analyses, AADC was detected in a subset of taste bud cells of fungiform, foliate, and circumvallate papillae. Double-label studies showed that AADC colocalized with serotonin, NCAM, PLCbeta2, and PGP9.5. On the other hand, AADC never colocalized with alpha-gustducin. Our results of double staining with AADC and taste cell markers indicate that only the type III cells could convert 5-hydroxytryptophan (5-HTP) to serotonin within taste buds. Taken together with previous studies, the properties of the type III cell of taste buds exactly fit into the APUD (amine and amine precursor uptake and decarboxylation) cell scheme. Furthermore, in the developing circumvallate papilla, AADC are first detected in a small number of papillary epithelial cells at E14.5. By E18.5, AADC-positive epithelial cells also express PGP9.5, which is one of marker of taste cells, and these cells have been contacted by developing nerve fibers. These results suggest that AADC expression begins at early stages of taste bud cell differentiation, and biogenic amines may act on taste bud differentiation of tongue epithelial cells, and further may regulate innervation of taste bud progenitor cells.

Animals↗
PubMed1986

Influence of taste stimulation during the suckling period on adult taste preference in rats.

Effects of taste deprivation and selective taste exposure during the suckling period on adult preference for flavored solutions was examined. Taste deprivation was achieved by rearing rat pups "artificially," in individual cups with intragastric feeding from Postnatal Days 4 through 17 or 18. In the first study the effects of artificial rearing on male Long-Evans rats were examined. In the second study female Wistar rats were exposed to a single tastant, sucrose, which was introduced on a background of taste deprivation. Results of both studies were similar. No differences in taste sensitivity or preference were detected between animals subjected to taste deprivation or selective taste exposure and their normally reared littermates. These results suggest that suckling stimulation is not necessary for normal taste development and that the taste system of the rat is not particularly susceptible to the influence of early experience. Other interpretations of these results are also discussed.

Animal Population Groups↗
PubMed2006

Imaging taste responses in the fly brain reveals a functional map of taste category and behavior.

The sense of taste allows animals to distinguish nutritious and toxic substances and elicits food acceptance or avoidance behaviors. In Drosophila, taste cells that contain the Gr5a receptor are necessary for acceptance behavior, and cells with the Gr66a receptor are necessary for avoidance. To determine the cellular substrates of taste behaviors, we monitored taste cell activity in vivo with the genetically encoded calcium indicator G-CaMP. These studies reveal that Gr5a cells selectively respond to sugars and Gr66a cells to bitter compounds. Flies are attracted to sugars and avoid bitter substances, suggesting that Gr5a cell activity is sufficient to mediate acceptance behavior and that Gr66a cell activation mediates avoidance. As a direct test of this hypothesis, we inducibly activated different taste neurons by expression of an exogenous ligand-gated ion channel and found that cellular activity is sufficient to drive taste behaviors. These studies demonstrate that taste cells are tuned by taste category and are hardwired to taste behaviors.

Animals↗
PubMed1999

The taste of monosodium glutamate (MSG), L-aspartic acid, and N-methyl-D-aspartate (NMDA) in rats: are NMDA receptors involved in MSG taste?

Monosodium glutamate (MSG) is believed to elicit a unique taste perception known as umami. We have used conditioned taste aversion assays in rats to compare taste responses elicited by the glutamate receptor agonists MSG, L-aspartic acid (L-Asp), and N-methyl-D-aspartate (NMDA), and to determine if these compounds share a common taste quality. This information could shed new light upon the receptor mechanisms of glutamate taste transduction. Taste aversions to either MSG, L-Asp or NMDA were produced by injecting rats with LiCl after they had ingested one of these stimuli. Subsequently, rats were tested to determine whether they would ingest any of the above compounds. The results clearly show that a conditioned aversion to MSG generalized to L-Asp in a dose-dependent manner. Conversely, rats conditioned to avoid L-Asp also avoided MSG. Conditioned aversions to MSG or L-Asp generalized to sucrose when amiloride was included in all solutions. Importantly, aversions to MSG or L-Asp did not generalize to NMDA, NaCl or KCl, and aversions to NMDA did not generalize to MSG, L-Asp, sucrose or KCl. These data indicate that rats perceive MSG and L-Asp as similar tastes, whereas NMDA, NaCl and KCl elicit other tastes. The results do not support a dominant role for the NMDA subtype of glutamate receptors in taste transduction for MSG (i.e. umami) in rats.

Animals↗
PubMed1999

Confusing tastes and smells: how odours can influence the perception of sweet and sour tastes.

This study investigated the relationship between perception of an odour when smelled and the taste of a solution to which the odour is added as a flavorant. In Experiment 1 (E1) sweetness, sourness, liking and intensity ratings were obtained for 20 odours. Taste ratings were then obtained for sucrose solutions to which the odours had been added as flavorants. Certain odours were found to enhance tasted sweetness while others suppressed it. The degree to which an odour smelled sweet was the best predictor of the taste ratings. These findings were extended in Experiment 2 (E2), which included a second tastant, citric acid, and employed four odours from E1. The most sweet smelling odour, caramel, was found to suppress the sourness of citric acid and, as in E1, to enhance the sweetness of sucrose. Again, odours with low sweetness suppressed the sweetness of tasted sucrose. The study demonstrated that the effects of odours on taste perception are not limited to sweetness enhancement and apply to sour as well as sweet tastes. The overall pattern of results is consistent with an explanation of the taste properties of odours in terms of prior flavour-taste associations.

Adult↗
PubMed2002

The neural differentiation gene Mash-1 has a distinct pattern of expression from the taste reception-related genes gustducin and T1R2 in the taste buds.

Taste bud cells have a limited lifespan and are continuously replaced just like other epithelial cells. Although there is some evidence that taste buds may arise from the local epithelium, taste receptor cells have neuronal properties. This implies that there must be a critical stage at which the epithelial precursor cells for taste receptor cells start to exhibit neural properties during the differentiation of the taste receptor cells. The expression of the neural-specific transcription factors Mash-1 and Prox-1 in the nervous system is transient and precedes neuronal differentiation. Therefore, we examined the expression of Mash-1 and Prox-1 in the epithelium of circumvallate papillae of the tongue in order to clarify the localization of the precursor cells with neural properties and observed that both expressions are restricted to the taste buds. Two-colour in situ hybridization showed that the signals for Mash-1 did not overlap those for taste receptor cell-specific genes such as gustducin and T1R2. In the process of development and regeneration of the taste buds, the expression of Mash-1 preceded that of gustducin and T1R2. These observations suggest that Mash-1 could be a candidate for a marker of immature taste receptor cells, including the cells that express gustducin and/or T1R2 at a later stage.

Animals↗
PubMed2004

A strong nerve dependence of sonic hedgehog expression in basal cells in mouse taste bud and an autonomous transcriptional control of genes in differentiated taste cells.

The nerve-dependency of gene expression in mouse taste bud was examined through an analysis of changes in gene expression in and around the taste buds in circumvallate papillae after surgery of cranial nerve IXth (glossopharyngeal nerve). The number of cells expressing T1r3, gustducin, Mash1 and Nkx2.2 gradually decreased after denervation. However, the expression intensity of these genes was barely influenced by denervation, and strong expression was observed at 6 days after denervation. In contrast, the basal cell-specific Sonic hedgehog (Shh) expression in the taste buds was decreased markedly at 6 h after denervation. In the regeneration process of taste buds, Shh expression was observed during a very early phase before taste bud formation. These results indicate the autonomous transcriptional control of genes in differentiated taste cells and the strong nerve-dependency of Shh expression in basal cells. Furthermore, in order to reveal the mitotic activity of Shh-expressing cells in taste buds, the BrdU-labeling experiments were performed using a combination of BrdU-immunohistochemistry and in situ hybridization. BrdU-signal was very rarely observed in Shh-expressing cells immediately after BrdU injection, and the signals were noted mainly in Ptc-expressing cells. BrdU signals rapidly increased in Shh-expressing cells in following 12 h and began to decrease after 2 days post-injection. These results suggest that most Shh-expressing cells are not mitotically active, but that Shh-expressing cells may be in the early transient developmental state of taste cells in taste buds.

Animals↗
PubMed2000

The representation of umami taste in the taste cortex.

To investigate the neural encoding of glutamate (umami) taste in the primate, recordings were made from taste-responsive neurons in the cortical taste areas in macaques. Most of the neurons were in the orbitofrontal cortex (secondary) taste area. First, it was shown that there is a representation of the taste of glutamate that is separate from the representation of the other prototypical tastants, sweet (glucose), salt (NaCl), bitter (quinine) and sour (HCl). Second, it was shown that single neurons that had their best responses to sodium glutamate also had good responses to glutamic acid. Third, it was shown that the responses of these neurons to the nucleotide umami tastant inosine 5'-monophosphate were more correlated with their responses to monosodium glutamate than to any prototypical tastant. Fourth, concentration-response curves showed that concentrations of monosodium glutamate as low as 0.001 mol/L were just above threshold for some of these neurons. Fifth, some neurons in the orbitofrontal region which responded to monosodium glutamate and other food tastes, decreased their responses after feeding with monosodium glutamate to behavioral satiety, revealing a mechanism of satiety. In some cases, this reduction was sensory-specific. Sixth, it was shown in psychophysical experiments in humans that the flavor of umami is strongest with a combination of corresponding taste and olfactory stimuli (e.g., monosodium glutamate and garlic odor). The hypothesis is proposed that part of the way in which glutamate works as a flavor enhancer is by acting in combination with corresponding food odors. The appropriate associations between the odor and the glutamate taste may be learned at least in part by olfactory to taste association learning in the primate orbitofrontal cortex.

Animals↗
PubMed2005

[Age-related changes in renewal of taste bud cells and expression of taste cell-specific proteins in mice].

It is known that taste sensitivities change throughout life and they decline with aging. We have also found that both detection and recognition thresholds of taste were significantly elevated in aged persons, while oral somatic sensations hardly changed. To elucidate the peripheral mechanisms of the decline of taste sensitivities with age, we first investigated age-associated changes in cell renewal of taste buds in the circumvallate papillae of ddY mice, because one of the major characteristic differences of taste receptors from somatosensory receptors is a continual turnover of cells. In addition, we examined the expression patterns of taste cell-specific proteins such as protein gene products 9.5 (PGP 9.5) and gustducin, and ultrastructure of taste buds which may change resulting from the change of turnover rate. As a reference, we also used mice in the developing stage. The rate of cell renewal was examined using 5-bromo-2'-deoxyuridine (BrdU), which is incorporated in DNA during the S-phase of cells. BrdU and the proteins were detected immunohistochemically and the ultrastructure was investigated by electron microscopy. The aged mice demonstrated a delayed cell renewal and highly vacuolated cytoplasm in taste buds, while they showed no change in PGP 9.5-immunoreactive cells and an increase in gustducin-immunoreactive cells compared with young adult mice. In contrast, the developing mice showed a higher rate of cell turnover and rapid increase of PGP 9.5- and gustducin-immunoreactive cells reaching the mature level in 3 weeks after birth. The changes observed in the present study in aged mice may be related to the decline in taste sensitivity with aging.

Aging↗
PubMed1994

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.

Animals↗
PubMed1978

Taste profiles from single human taste papillae.

Earlier psychophysical research on single human fungiform taste papillae employed a procedure which limited subjects to selecting only one taste to describe the sensations they experienced. That procedure precludes the possibility of determining whether single papillae can mediate complex tastes, i.e., tastes consisting of more than one sensation experienced simultaneously. By using highly trained subjects and allowing them freedom to describe all sensations simultaneously elicited by a given taste stimulus, single papilla taste profiles were obtained. It is suggested that obtaining taste profiles may increase the utility of single papillae as models for study of the taste system.

Adult↗
PubMed1991

A taste illusion: taste sensation localized by touch.

Taste sensations appear to come from all over the inner surface of the mouth, yet the taste receptors are restricted to relatively small particular areas of the oral surface. In addition, even if a relatively large (e.g., one half) proportion of the taste field is damaged, subjective taste experience may be unaffected. The touch system contributes to this constancy because taste sensations appear to be localized by touch. If a taste solution is painted from the side of the tongue (an area of low receptor density) past the tip (an area of high receptor density) and on to the second side, the taste sensation begins weak, gets stronger at the tip, and retains much of its intensity. The strong taste from the tip follows the tactile path of the stimulus sweep. This illusion occurs for all four stimuli tested: sucrose, sodium chloride, citric acid, and quinine hydrochloride.

Adolescent↗
PubMed1982

The response characteristics of rat taste cells to four basic taste stimuli.

1. The shapes of receptor potentials of rat taste cells in response to the four basic taste stimuli (0.5 M NaCl, 0.02 M quinine-HCl (Q-HCl), 0.01 M HCl and 0.5 M sucrose) were classified into three types, i.e. (1) a depolarization alone, (2) a depolarization preceded by a transient hyperpolarization and (3) hyperpolarization alone. 2. The rise and fall times of depolarizing responses to NaCl were much shorter than those to the other three stimuli. The fall time of depolarization evoked by HCl was the longest. The rise and fall times of all hyperpolarizing responses were shorter than those of all depolarizing responses. 3. The input resistance of taste cells decreased during depolarizations elicited by NaCl stimulation, but increased during depolarizations and hyperpolarizations elicited by stimulation with Q-HCl, HCl and sucrose. 4. The taste stimulus-induced input resistance change returned faster to the control in the order of NaCl greater than sucrose greater than Q-HCl greater than HCl when the stimulus was rinsed from the tongue. 5. From these response characteristics the rat taste cells responding to each of the four basic taste stimuli are largely divided into two types, low-sensitive taste cell and high-sensitive taste cell.

Animals↗
PubMed2006

Discrete innervation of murine taste buds by peripheral taste neurons.

The peripheral taste system likely maintains a specific relationship between ganglion cells that signal a particular taste quality and taste bud cells responsive to that quality. We have explored a measure of the receptoneural relationship in the mouse. By injecting single fungiform taste buds with lipophilic retrograde neuroanatomical markers, the number of labeled geniculate ganglion cells innervating single buds on the tongue were identified. We found that three to five ganglion cells innervate a single bud. Injecting neighboring buds with different color markers showed that the buds are primarily innervated by separate populations of geniculate cells (i.e., multiply labeled ganglion cells are rare). In other words, each taste bud is innervated by a population of neurons that only connects with that bud. Palate bud injections revealed a similar, relatively exclusive receptoneural relationship. Injecting buds in different regions of the tongue did not reveal a topographic representation of buds in the geniculate ganglion, despite a stereotyped patterned arrangement of fungiform buds as rows and columns on the tongue. However, ganglion cells innervating the tongue and palate were differentially concentrated in lateral and rostral regions of the ganglion, respectively. The principal finding that small groups of ganglion cells send sensory fibers that converge selectively on a single bud is a new-found measure of specific matching between the two principal cellular elements of the mouse peripheral taste system. Repetition of the experiments in the hamster showed a more divergent innervation of buds in this species. The results indicate that whatever taste quality is signaled by a murine geniculate ganglion neuron, that signal reflects the activity of cells in a single taste bud.

Afferent Pathways↗