Vicktoria Danilova

ATP Signaling Is Crucial for Communication from Taste Buds to Gustatory Nerves

Taste receptor cells detect chemicals in the oral cavity and transmit this information to taste nerves, but the neurotransmitter(s) have not been identified. We report that adenosine 5′-triphosphate (ATP) is the key neurotransmitter in this system. Genetic elimination of ionotropic purinergic receptors (P2X2 and P2X3) eliminates taste responses in the taste nerves, although the nerves remain responsive to touch, temperature, and menthol. Similarly, P2X-knockout mice show greatly reduced behavioral responses to sweeteners, glutamate, and bitter substances. Finally, stimulation of taste buds in vitro evokes release of ATP. Thus, ATP fulfils the criteria for a neurotransmitter linking taste buds to the nervous system.

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

BackgroundRecent 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.ResultsIn 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.ConclusionThese 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.

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.

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.

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

BackgroundPsychophysically, 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.ResultsWe 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.ConclusionIn 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.

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.

Species differences toward sweeteners

Abstract The understanding of the sense of taste in mammals has over the last few decades slowly changed from the misconception that all mammals are equal with regard to taste to a realization that there are profound differences between species. These differences probably pertain to all basic tastes, but have been especially documented with regard to the sweet taste. This study addresses two issues: the difference in taste fiber specificity between mammals and the related issue of species differences in ability to taste sweeteners. These issues are illustrated by single taste fiber recordings from hamster, pig, rhesus monkey and chimpanzee. The hamster, a rodent, is used as an animal model in taste research because of its especially well developed sweet taste sensitivity, but this study shows that many sweeteners do not taste sweet to the hamster. The same is true for the pig, an ungulate, and from this point of view quite unrelated to the human, but with similar internal anatomy, food preferences and diets, and therefore extensively used as an animal model. Even the rhesus monkey, an old world primate belonging to the same superfamily as human, Catarrhina, shows some differences in its sweet tasting ability and taste fibers specificity although much less so than the previously mentioned species. The only species in which studies of its sense of taste have not yet revealed any differences from the human sense of taste, is the chimpanzee, which by most accounts is our closest relative.

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].

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.

The evolution of taste perception: psychophysics and taste nerves tell the same story in human and non-human primates

Abstract This paper presents a parallel analysis of the results of recent studies of taste responses of humans and phylogenetically different non-human primates, in order to provide evidence of the evolutionary history of taste perception, and to discuss its present significance. A cluster analysis of the signals recorded on isolated nerve fibres of the various non-human primates shows additive trees representations, corresponding to a two-way system that allows discriminating what is beneficial from what could be toxic or anti-nutrient (sugars vs. alkaloids and tannins) among a large number of chemical stimuli. In humans, using psychophysical data (recognition taste thresholds) with a similar method of analysis, we observed the same dichotomy in the additive tree and similar covariation between some tastes thresholds, notably those of alkaloids and tannins. The converging results obtained by both methods show that the target of evolutionary pressure was a set of taste receptors that were not initially tuned to respond to what is presently described as basic tastes. To cite this article: C.-M. Hladik et al., C. R. Palevol 2 (2003).