The Transduction of Sweet Tastants in
Humans
Sean Patterson
Spring 2003
A critical Literature Review submitted in partial
fulfillment of the requirement for the Senior Research
Thesis.
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Taste Transduction: Sweet and Bitter Intensities, Receptor Sites, and Interactions - Prof., Papers of Psychology
The process of taste transduction, focusing on the differences in sweet and bitter intensities, the role of taste receptor sites, and the interactions between sweeteners and other taste properties. The text also discusses experiments that have contributed to our understanding of these phenomena in both humans and lab animals.
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The Transduction of Sweet Tastants in
Humans
Sean Patterson Spring 2003
A critical Literature Review submitted in partial fulfillment of the requirement for the Senior Research Thesis.
ABSTRACT
This literature review will explain the variables that affect the taste system. This review will explain the interactions of sugar with bitter, salt and fat in the taste cell. The different transduction mechanisms will also be illustrated within this review. Differences of intensities and threshold will be displayed and explained through experiments in various research articles. The effect of Sweeteners will explain the concept of synergy. Prop Sensitivity will also be investigated, from this investigation the three types of tasters will be defined. Sweet taste transduction in humans is also explained in more detail by experiments that received similar result in animals. This generalization leads to the belief that taste systems and mechanisms in lab animals are similar to those in humans.
proceed to areas of the cortex where they are labeled as the qualities as sweet, sour, salty or bitter.
There are individual differences in taste perception and humans have been grouped into types of tasters; non- tasters, tasters and super-tasters ( Drewnowski A et al, 1997). This classification is based on the ability to taste or not to taste Propylthiouracil (PROP). Non- tasters have a low ability to detect the PROP solution. Tasters detect the PROP solution more intensely than non- tasters, with the detection threshold is much lower for tasters than non-tasters. Super-tasters can taste the PROP solution even more intensely and have the lowest detection threshold. Super tasters also have a greater general taste sensitivity than the tasters or non-taters. One important difference between the types of tasters is the number of taste receptor sites. Therefore humans with more taste receptor sites have higher detection capabilities for sweets and other tastants compared to those with less receptor sites.
Taste Transduction in Humans
Taste transduction is the process of a physical chemical stimulus becoming a neural signal. Taste transduction occurs through two main mechanisms, ion channels and metabotropically. Activation of ion channels are usually perceived as a salty or sour taste. Metabotropic transduction is usually perceived as a sweet or bitter neural taste. Ionic transduction occurs when salty chemicals in food or drink enter the taste cell and depolarize the cell by making the cell more positive. Sour chemicals produce depolarization by blocking potassium(K+) ionotropic channels in the taste cells. These increases in voltage make the cell more positive until a specific voltage is reached at which neurotransmitters are released. Threshold is the level at which neural signals can be first produced.
second messenger system (G-Protein) that uses cAMP. cAMP is an intracellular messenger that is elevated when sweeteners bind to cell surface receptors. Simultaneously when cAMP levels are elevated potassium channels are inactivated in the cell (Shiffman SS et al, 1993). This binding process causes GDP to move from the binding site to be replaced by GTP. GTP in turn binds with another chemical then binds to the adenylate cyclase. PKA (protein kinase) is the end product of this series of binding, which depolarizes the cell (Shiffman SS et al, 1994). Sweeteners and inhibitors of the sweet taste were manipulated in pigs and in gerbils. The findings were concurrent with the findings of previous experiments. Adenylate cyclase can enhance or inhibit the perception of sweet tastants in both. Some of the modulators made the sweet tastant more sweet and others made the tastant less sweet. The author thought that these findings would be helpful humans that suffer from hypogusia because of its sweetening effect (Shiffman SS et al, 1994). The experiments in pigs and gerbils have similar findings allowing the inference to be drawn that the transduction mechanism works similarly in most animals.
Influence on the Perception of Sweet
Interactions of Bitter and Sweet Bitter and sweet tastants compete with each other due to their proximal relationship on the taste cell. Bitter and sweet are both transduced metbotropically on the apical portion (receptor level) of the tongue. A less intense taste perception is detected due to this competition in the taste cells. Hyman & Frank found that the chorda tempani nerve response was smaller in mixtures of the sweet and bitter compounds opposed to when they are presented separately (Schiffman SS et al, 1994). This is because one stimulus with the absence of the other would have a stronger effect on a taste receptor cell due to the lack of competition. This provides evidence for the theory of competition. Sweeteners have a greater effect on bitters that are supra-threshold. The higher the sweet intensity the bigger the impact the sweetener has on the detection level of the bitter compound. The intensity of a sweet concentration is rated much higher than most bitter concentrations at threshold. Sweeteners at threshold for certain bitter concentrations effected some bitter concentrations, but most were not. Concentration of a sweetener is important to the taste quality of bitter. The higher the concentration of a
Interaction of Salts with Sweets
Selective inhibition occurs through the tranduciton of sweet neural signals by blocking channels with a sodium salt compound. Some compounds have been found to inhibit the sweet taste in humans. Na-PMP is +- 2-(4- Methoxyphenooxy) propanoic acid is one compound that has been found to inhibit the sweet taste in humans. Another example of selective inhibition comes from studies of bitter and sweet tastants (Schiffman SS et al, 1999). It is proven that these taste properties are independent of each other because single pappilia produced bitter sensations from quinine salt, but didn’t respond to sweet stimuli. A separate experiment was conducted to determine the effect of Na-PMP on sweeteners in humans. The experiment shows that Na-PMP had an inhibitory effect on all but three of the tastants in the experiment. In another study from the same experiment Na-PMP enhanced the taste of sweet when a 30 second pre-rinse of the Na-PMP solution was administered prior to exposure to the tastant. The data showed that some sweet tastants were excited and somewhere inhibited by Na-PMP. From this the conclusion is drawn that selective inhibition is occurring. The notion
of more than one transduction mechanism in the taste cells comes from this data. It can be concluded that the sodium salt compound Na-PMP is an inhibitor of sweet taste. The findings also illustrate that there must be at least two sweet receptors or mechanisms because different responses were elicited for similar tastants.
Interaction of Fat with Sweet Fat and sucrose come together and form a taste that is pleasing to the senses (Warwick ZS, Schiffman SS,1990). High concentrations of fats in liquids are not pleasing to the senses alone, but once sucrose is added to the liquid- fat solution the taste gets better. The more sucrose that is added the better the taste of the fat concentration. This more pleasant taste leads to overeating. The overeating of these high fat foods lead to obesity in humans (Warwick ZS, Schiffman SS,1990).
Infleunce of Synergy with Sweet The intensity of stimuli can be increased without increasing its molartiy by combining a similar stimulant. This process is called synergy. Synergy can occur in two different ways. The first way being that when the taste of
It has been found that many things can affect the sweetness in taste stimuli (Shiffman SS et al, 2002). Temperature has been found to have a statistical difference in sweeteners. Heat has been found to enhance sweet stimuli while sweetness is significantly suppressed at cold temperatures.
PH The pH had no statistical difference in the effect on sweetness intensity in humans (Shiffman SS et al, 2002). The pH levels were tested at five different levels. Sourness and astringency increased as the pH decreased with concentration. Ions were thought to have a significant effect on perceived sweetness intensity (Shiffman SS et al, 2002). A study was conducted and no significant effect was found between the groups by the addition of ions. The sweetness intensity of sodium saccharin and sucralose had a longer delay to the onset of maximum sweetness intensity( Shiffman SS et al, 2002). AGE The bitter compound is tranduced metabotropically through receptor sites on the taste buds of the tongue. When K+ ion channels are blocked by H+ ions on the apical
portion of the tongue the cell then becomes depolarized. The cell then becomes so positive that it signals a neural transmitter to be release. Fungiform pappilia are found on the anterior two-thirds of the apical portion of the tongue. They are a key aspect of the detection of bitter. This study also attributes the differences to the changes in the hippocampus and the amygdala. Another aspect of older people's lower detection level is that memory loss may make judgment more difficult on the magnitude scale harder. The difficulty is that older people forget the previous intensity level and fail to have anything to
compare to the latest stimuli. ( FIX NO SWEETENERS LOWERED
DETECTION OF THE THRESHOLD (SCHIFFMAN SSET AL, 1994). SWEETENERS
HAD NO EFFECT ON DETECTION THRESHOLD IN OLDER PEOPLE.) HUMANS HAVE
SHOWN THAT THERE IS AN EFFECT ON AGE AND WEIGHT IN THE DETECTION OF
FAT-SALT AND FAT-SUCROSE MIXTURES. SOLID FAT RICH STIMULI ALSO
INCREASE IN PALATABILITY WHEN SUCROSE IS ADDED (WARWICK ZS,
SHIFFMAN SS, 1990). IN THIS STUDY THE MEAN AGE OF THE YOUNG GROUP
WAS 22.4 AND THE ELDERLY PERSONS MEAN AGE WAS 82.3. THE CRITERION
FOR OVERWEIGHT PEOPLE IS 15% OVER THE IDEAL WEIGHT ACCORDING TO
THE METROPOLITAN LIFE INSURANCE TABLE IN 1983(WARWICK ZS, SHIFFMAN
SS, 1990).
orders in intensities were not different. Both groups young and old ranked the tastants in the same order. In young, overweight people a significant effect was found in sucrose, but no significant effect was found in fat or fat-sucrose interaction. The fat-salt mixtures show that in younger obese subjects illustrates salt and fat combined had a significant effect on dietary intake (Warwick ZS, Shiffman SS, 1990). In elderly people both overweight and normally weighted subjects detected palatability in fat-sucrose mixtures (Warwick ZS, Shiffman SS, 1990). A significant effect was not found in the fat or sucrose content in the mixture, nor to the interaction of fat and sucrose. The salt content was related to preference in fat-salt mixtures in older subjects. Fat content had no significant effect on fat-salt mixtures. The author thought that the reason for discrepancies in older and younger subjects was due to the discrimination of the various viscosities, rather than cues that are from sight or smell (Warwick ZS, Shiffman SS, 1990). It was found that fat has an effect on pleasantness in fat-sucrose mixtures in older people. The quantity of salt has the effect on fat-salt mixture in older people. In contrast, the interaction of fat-sucrose makes food more pleasant in fat-sucrose ratings in young people. Fat and
salt concentration cause a more pleasant perception of stimuli in fat-salt mixtures. It is assumed that no effect was found within either fat-salt or fat-sucrose interactions because both experiments were conducted suprathreshold.
WEIGHT In normal weighted people fat-sucrose mixtures have a significant interaction. There is not a significant interaction with either sucrose or fat in conjunction to dietary intake. A significant effect was shown that preference for fat-salt mixtures depends on the concentration of salt and fat. There was not a significant effect between the fat-salt interactions in normal weighted people. RACE In previous studies African Americans were found to have had a greater preference for sweeter and saltier liquid solutions and food than adults and white children their age (Bacon AW et al, 1993). This study found that African Americans preferred more sweetness in fruit drinks. There was no difference in preference between the two races for bacon and Pork bellies (Bacon AW et al, 1993). One peculiarity was that all children tested lived in the
experiments in lab animals and humans have advanced our knowledge of the transduction of the sweet taste tremendously.
Sitations
Bacon AW, Miles JS, Schiffman SS (1993) Effect of Race on Perception of Fat Alone and in Combination With Sugar.Physiol Behav (55)3:603-
Drewnowski A, Henderson SA, Shore AB, Barratt-Fornell A. (1997) Nontasters, Tasters, Supertasters of 6- n - Propylthiouracil(PROP) and Hedonic Response to Sweet.Physiol Behav 62(3):649-
Froloff N, Lloret E, Martinez JM, Faurion A (1998) Cross- adaptation and Molecular Modeling Study of Receptor Mechanisms Common to Four Taste Stimuli in Humans.Chem Sense (23) 197-
Kinnamon SC, Getchell TV, (1991) Sensory Transduction in Olfactory receptor Neurons and Gustatory Receptor Cells.Smell Taste in Health and Disease .(LOOK UP ABBREV.)
Schiffman SS, Booth BJ, Losee ML, Pecore SD, Warwick ZS(1994) Bitterness of Sweeteners as a Function of Concentration. Brain Res Bull(36)5:505- Schiffman SS, Booth BJ, Sattely-Miller, EA, Graham BG, Gibes KM (1999) Selective Inhibition of Sweetness by theSodium Salt of + Chem Senses:439-447-^ 2-(4-Methoxyphenooxy) propanoic acid.
Schiffman SS, Gatlin LA, Suggs MS, Heiman SA, Stagner WC,Erickson RP (1993) Modulators of the Adenylare Cyclase System Can Alter Electrophysiological Taste Response in Gerbil. Pharmacol Biochem Behav(48)4:991- Schiffman SS, Gatlin LA, Sattely-Miller EAHeiman SA, Stagner WC, Erickson RP (1994) The Effect of Graham BG, Sweeteners on Bitter Taste in Young and Elderly Subjects. Brain Res Bull(35)3:189- Schiffman SS, Lindley MG, Clark TB, Makino C (1981)Molecular Mechanism of Sweet Taste: Relationship of Hydrogen Bonding to Taste Sensitivity fr Both Young and Elderly. Neurobiol of Aging(2):173-