Göran Hellekant

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.

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.

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°C for 4 h. Brazzein is comprised of 54 amino acid residues, corresponding to a molecular mass of 6,473 Da.

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 taste responses in primates to the proteins thaumatin and monellin and their phylogenetic implications

Electrophysiological and behavioural methods have been applied to 34 species of the primates and, for comparison, to the Madagascan hedgehog to determine their responses to the proteins thaumatin and monellin. These substances elicit an intensely sweet taste sensation in man. All Catarrhina prefer monellin to water. The responses of the Prosimii as well as those of the South American primates to monellin are different, some species show a reaction, other species are not sensitive. In the case of thaumatin neither the Prosimii--including Tupaia and Tarsius--nor the South American primates show any response to this protein. Only the Cercopithecidae, the Hylobatidae and the Pongidae respond to this protein like man and prefer this substance to water. This physiological aspect of taste constitutes a clear dichotomy within the order Primates. This capability to taste thaumatin probably developed as long as 38 million years ago.

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 sweetness‐inducing effect of miraculin; behavioural and neurophysiological experiments in the rhesus monkey Macaca mulatta

1. The gustatory effects of miraculin, the sweetness‐inducing protein from the miracle fruit Synsepalum dulcificum, was studied in the rhesus monkey, Macaca mulatta.

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.