The taste of polycose in hamsters

Hamsters show a preference for Polycose, a mixture of starch-derived glucose polymers, that is as strong as their preference for sucrose. However, in the hamster, taste aversions to Polycose may be less easily acquired than taste aversions to sucrose and the qualitative aspects of Polycose are unknown in this species. In order to examine the taste of Polycose in the hamster, we utilized a taste-aversion protocol with two conditioning trials. Animals were trained to avoid one of three different conditioning stimuli: 50 mM sucrose, 100 mM Polycose and a mixture of 50 mM sucrose with 100 mM Polycose. Control animals were conditioned with deionized water. After the second conditioning trial, generalization testing began for the three conditioning stimuli plus 3 mM citric acid, 300 mM KCI and 30 mM NaCl. The results showed that aversions to Polycose, sucrose or the Polycose/sucrose mixture cross- generalized, demonstrating that Polycose and sucrose share a common taste percept in the hamster. None of the aversions generalized to NaCl, citric acid or KCI. In addition, comparisons among the patterns of taste generalizations indicated that the tastes of Polycose and sucrose also had distinct qualitative components. Finally, although the taste of 100 mM Polycose was more salient than the taste of 50 mM sucrose, the taste of sucrose could still be detected in a mixture with Polycose.      

Generalization of learned taste aversions in rabbits: similarities among gustatory stimuli

Although rabbits have been used in a number of electrophysiologicaland anatomical studies on the gustatory system, there have beenfew behavioral experiments on these animals and these have beenlimited to studies of taste preference. The similarities amonga number of gustatory stimuli were assessed in rabbits by measuringthe generalization patterns in a conditioned taste aversionexperiment. Rabbits were trained to take their daily rationof water within a 30-min session, during which the number oflicks per 10-s presentation of a drinking tube could be recorded.During one of these sessions, one of 12 stimuli (sucrose, fructose,Na-saccharin, NaCI, NaNO₃, Na₂SO₄, KC1, NH₄C1, CaCl₂, HC1, QHC1or urea) was presented, followed by i.p. injection of LiCl toproduce a conditioned taste aversion. Animals were then testedwith all of the stimuli and the amount of suppression of lickingwas used as a measure of stimulus generalization. The patternsof generalization were compared for the test and conditioningstimuli separately. Some nonreciprocities were seen betweenthe conditioning and test stimuli, which reflected the occurrenceof multiple taste qualities and the tendency for aversions togeneralize more to stronger stimuli than to weaker ones. Principalcomponents analysis of the stimulus relationships showed thatrabbits responded to these stimuli in a fashion similar to thatof other mammals, including humans. Within the principal componentssolution, there were strong similarities among the sugars, thesodium salts, the nonsodium salts and the bitter-tasting stimuli.     

Rapid stimulus-bound suppression of intake in response to an intraduodenal nonnutritive sweetener after training with nutritive sugars predicting malaise

In a previous report (Schier et al., Am J Physiol Regul Integr Comp Physiol 301: R1557-R1568, 2011), we demonstrated with a new behavioral procedure that rats exhibit stimulus-bound suppression of intake in response to an intraduodenal (ID) bitter tastant predicting subsequent malaise. With the use of the same modified taste aversion procedure, the present experiments evaluated whether the sweet taste properties of ID stimuli are likewise detected and encoded. Thirsty rats licked at sipper spouts for hypotonic NaCl for 30 min and received brief (first 6 min) yoked ID infusions of either the same NaCl or an isomolar lithium chloride (LiCl) solution in each session. An intestinal taste cue was mixed directly into the LiCl infusate for aversion training. Results showed that rats failed to detect intestinal sweet taste alone (20 mM Sucralose) but clearly suppressed licking in response to a nutritive sweet taste stimulus (234 mM sucrose) in the intestine that had been repeatedly paired with LiCl. Rats trained with ID sucrose in LiCl subsequently generalized responding to ID Sucralose alone at test. Replicating this, rats trained with ID Sucralose in compound with 80 mM Polycose rapidly suppressed licking to the 20 mM Sucralose alone in a later test. Furthermore, ID sweet taste signaling did not support the rapid negative feedback of sucrose or Polycose on intake when their digestion and transport were blocked. Together, these results suggest that other signaling pathways and/or transporters engaged by caloric carbohydrate stimuli potentiate detection of sweet taste signals in the intestine.     

The sweet-bitter taste of alcohol: aversion generalization to various sweet-quinine mixtures in the rat

Rats were trained to avoid a 5% alcohol solution and then testedwith either sweet + quinine hydrochloride solutions (Experiment1) or sweet + hydrochloric acid solutions (Experiment 2). Thesweet stimuli used were sucrose, glucose, fructose and saccharin.Significant aversion generalization was found only in Experiment1 where trained rats generalized to all four test stimuli, thussuggesting that alcohol has a sweet taste (in combination withbitter) not specific to one sweetener. No significant aversiongeneralization was noted in Experiment 2 when sweet + hydrochloricacid solutions were tested. In Experiment 3, rats were trainedto avoid 6% alcohol and tested with sucrose + quinine hydrochloridemixtures with varying concentrations of each component. In general,rats showed generalization of the alcohol aversions across thevarious concentrations of sucrose and quinine hydrochloridetested.     

The psychophysical relationship between bitter taste and burning sensation: evidence of qualitative similarity

Although it has long been studied as a pure sensory irritant, the ability of capsaicin to evoke, mask, and desensitize bitter taste suggests that burning sensations and bitter taste might be closely related perceptually. The current study investigated the psychophysical relationship between bitterness and burning using 2 different approaches. In Experiment 1, spatial discrimination of 4 taste stimuli was measured in the presence or absence of capsaicin. The subjects' task was to report which of 3 swabs, spaced 1 cm apart and presented to the tongue tip, contained a taste stimulus when 1) water was presented on the other 2 swabs or 2) when 10 muM capsaicin was presented on all 3 swabs. The presence of capsaicin did not change performance on the 3 alternative forced-choice (3-AFC) task for sweet, sour, and salty stimuli, while the localization error for 1.8 mM quinine sulfate (QSO(4)) increased significantly. In Experiment 2, the perceptual similarity/dissimilarity of taste stimuli and capsaicin was measured directly using pairs of stimuli applied to opposite sides of the tongue tip on swabs separated by 2 cm. Multidimensional scaling analyses showed that capsaicin fell nearer to QSO(4) than to any other taste stimulus. Cluster analysis corroborated this finding: capsaicin was closely linked with QSO(4) and the capsaicin-QSO(4) group was separated from the other taste stimuli. The latter result indicated that bitterness was more similar to burning than to the other tastes. These findings imply that despite being mediated by different sensory modalities, bitterness and burn are qualitatively similar. We speculate that this similarity reflects a common function of these 2 sensations as sensory signals of potentially harmful stimuli.     

Sweetness intensity enhancement by pulsatile stimulation: effects of magnitude and quality of taste contrast

Upon stimulation with continuously alternating (pulsatile) taste concentrations, humans report higher average taste intensities than for continuous stimulation with the same average tastant concentration. We investigated the effect of the magnitude of concentration changes (concentration contrast) and the effect of taste quality changes (quality contrast) between alternating tastants on sweet taste enhancement. The perceived sweetness intensity increased with the magnitude of the sucrose concentration contrast: The pulsatile stimulus with the highest concentration difference (average sucrose concentration: 60 g/L) was rated as the sweetest in spite of the fact that the gross sucrose concentrations were identical over stimuli. Moreover, this stimulus was rated equally sweet as a continuous reference of 70 g/L sucrose. On alternation of sucrose with the qualitatively different citric acid, sweet taste enhancement remained at the level observed for alternation with water at citric acid concentration levels up to 3 times its detection threshold. Alternation of a sucrose solution with a citric acid solution at 9 times its threshold concentration, resulted in an attenuation of the pulsation-induced enhancement effect. Upon alternation of citric acid pulses at concentrations around the threshold with water intervals only, no taste enhancement was observed compared with continuous citric acid stimuli of the same net concentration. We propose that the magnitude of pulsation-induced taste enhancement is determined by the absolute rather than relative change of tastant concentration. This explains why 1) pulsation-induced sweet taste enhancement is determined by the magnitude of the sucrose pulse-interval contrast and 2) the alteration of citric acid with water does not enhance taste intensity at detection threshold level.     

Monosodium glutamate and sweet taste: discrimination between the tastes of sweet stimuli and glutamate in rats

Generalization of a conditioned taste aversion (CTA) is based on similarities in taste qualities shared by the aversive substance and another taste substance. CTA experiments with rats have found that an aversion to a variety of sweet stimuli will cross-generalize with monosodium glutamate (MSG) when amiloride, a sodium channel blocker, is added to all solutions to reduce the taste of sodium. These findings suggest that the glutamate anion elicits a sweet taste sensation in rats. CTA experiments, however, generally do not indicate whether two substances have different taste qualities. In this study, discrimination methods in which rats focused on perceptual differences were used to determine if they could distinguish between the tastes of MSG and four sweet substances. As expected, rats readily discriminated between two natural sugars (sucrose, glucose) and two artificial sweeteners (saccharin, SC45647). Rats also easily discriminated between MSG and glucose, saccharin and, to a lesser extent, SC45647 when the taste of the sodium ion of MSG was reduced by the addition of amiloride to all solutions, or the addition of amiloride to all solutions and NaCl to each sweet stimulus to match the concentration of Na+ in the MSG solutions. In contrast, reducing the cue function of the Na+ ion significantly decreased their ability to discriminate between sucrose and MSG. These results suggest that the sweet qualities of glutamate taste is not as dominate a component of glutamate taste as CTA experiments suggest and these qualities are most closely related to the taste qualities of sucrose. The findings of this study, in conjunction with other research, suggest that sweet and umami afferent signaling may converge through a taste receptor with a high affinity for glutamate and sucrose or a downstream transduction mechanism. These data also suggest that rats do not necessarily perceive the tastes of these sweet stimuli as similar and that these sweet stimuli are detected by multiple sweet receptors.     

Basic taste stimuli elicit unique responses in facial skin blood flow

Facial expression changes characteristically with the emotions induced by basic tastes in humans. We tested the hypothesis that the five basic tastes also elicit unique responses in facial skin blood flow. Facial skin blood flow was measured using laser speckle flowgraphy in 16 healthy subjects before and during the application of basic taste stimuli in the oral cavity for 20 s. The skin blood flow in the eyelid increased in response to sweet and umami taste stimuli, while that in the nose decreased in response to a bitter stimulus. There was a significant correlation between the subjective hedonic scores accompanying these taste stimuli and the above changes in skin blood flow. These results demonstrate that sweet, umami, and bitter tastes induce unique changes in facial skin blood flow that reflect subjective hedonic scores.     

Taste confusions following chlorhexidine treatment.

Chlorhexidine, a bitter bis-biguanide antiseptic, is the only known blocker of the human salty taste. In order to characterize the effects of chlorhexidine on stimulus identification, taste confusion matrix (TCM) performance was measured for subjects treated with 1.34 mM chlorhexidine gluconate (n = 9) and water controls (n = 9). Ten stimuli [water, 0.1 M NaCl, 0.1 M KCl, 0.1 mM quinine-HCl (QHCl), 0.1 M monosodium glutamate (MSG), 3 mM citric acid, 0.3 M sucrose and mixtures of NaCl, QHCl and citric acid with sucrose] were presented in 10 replicates for identification from a list of 10 stimulus names. T(10), a measure of performance consistency from information theory, was lower for chlorhexidine-treated subjects (2.02 +/- 0.11 bits) than controls (2.73 +/- 0.11 bits) (P < 0.0001). T(2), an indirect measure of pairwise stimulus discrimination, approached chance levels (0.40 bit) in chlorhexidine-treated subjects for all possible pairs of NaCl, KCl, QHCl and water, as well as pairs composed of sucrose and the NaCl-sucrose and quinine-sucrose mixtures. In controls T(2) values approached perfect scores (1.00 bit) for all stimulus pairs except NaCl-KCl and NaCl-MSG. The results demonstrate a decreased ability to identify taste stimuli that is consistent with alterations in the ability of stimuli to elicit salty and bitter taste perceptions. As a selective, effective, persistent and reversible blocker of taste perceptions, chlorhexidine should prove useful in defining taste mechanisms in humans.     

Drosophila melanogaster prefers compounds perceived sweet by humans

To understand the functional similarities of fly and mammalian taste receptors, we used a top-down approach that first established the fly sweetener-response profile. We employed the fruit fly Drosophila melanogaster, an omnivorous human commensal, and determined its sensitivity to an extended set of stimuli that humans find sweet. Flies were tested with all sweeteners in 2 assays that measured their taste reactivity (proboscis extension assay) and their ingestive preferences (free roaming ingestion choice test). A total of 21 sweeteners, comprised of 11 high-potency sweeteners, 2 amino acids, 5 sugars, 2 sugar alcohols, and a sweet salt (PbCl2), were tested in both assays. We found that wild-type Drosophila responded appetitively to most high-potency sweeteners preferred by humans, even those not considered sweet by rodents or new world monkeys. The similarities in taste preferences for sweeteners suggest that frugivorous/omnivorous apes and flies have evolved promiscuous carbohydrate taste detectors with similar affinities for myriad high-potency sweeteners. Whether these perceptual parallels are the result of convergent evolution of saccharide receptor-binding mechanisms remains to be determined.     

Parabrachial unit activities after the acquisition of conditioned taste aversion to a non-preferred HCl solution in rats

Abstract In a behavioral experiment, rats reliably acquired a taste aversion to non-preferred 0.01 M HCl that had been previously paired with intraperitoneal injection of 0.15 M LiCl. These rats showed aversions to other acidic solutions such as malic acid and tartaric acid. In a neurophysiological experiment, the neuronal activities of the parabrachial nucleus (PBN) were recorded after the acquisition of conditioned taste aversion (CTA) to 0.01 M HCl in urethane-anesthetized rats. Neuronal responses to the conditioned stimulus (CS) did not change on the whole but decreased in the dorsal region to the brachium conjunctivum. The proportion of HCl-best to NaCl-best units was lower in the CTA group than in controls. The spontaneous firing rate was lower in the CTA group than in controls. Correlation coefficients between the HCl CS and normally preferred tastes (sucrose and NaCl) were more negative and those between HCl and quinine were more positive in the CTA group than in the controls. These results may be explained by the notion that gustatory responses of PBN neurons are concerned with alterations in taste hedonics after the acquisition of conditioned taste aversions.     

Generalization of Human Fear Acquisition and Extinction within a Novel Arbitrary Stimulus Category

Adaptive anxiety relies on a balance between the generalization of fear acquisition and fear extinction. Research on fear (extinction) generalization has focused mostly on perceptual similarity, thereby ignoring the importance of conceptual stimulus relations in humans. The present study used a laboratory procedure to create de novo conceptual categories of arbitrary stimuli and investigated fear and extinction generalization among these stimuli. A matching-to-sample task produced two four-member categories of abstract figures. Next, a member from one category was coupled with an aversive electrical stimulation, while a member from the other category was presented alone. As expected, conditioned fear responses generalized to the other members of the first category (skin conductance and online shock-expectancy). Subsequent extinction of the conditioned member also generalized to the other members. However, extinguishing a non-conditioned member failed to reduce fear of the conditioned member itself. We conclude that fears generalize readily across conceptually related stimuli, but that the degree of extinction generalization depends on the stimulus subjected to extinction.     

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.     

Monosodium glutamate and sweet taste: generalization of conditioned taste aversion between glutamate and sweet stimuli in rats

Even though monosodium glutamate (MSG) is a prototypical umami substance, previous studies reported that a conditioned taste aversion (CTA) to MSG, mixed with amiloride to block the taste of sodium, generalizes to sucrose. These findings suggest that the taste of glutamate mimics the taste of sucrose and raise the question of whether glutamate has a broadly tuned sweet taste component. To test this hypothesis, CTA experiments were conducted to test for generalization between MSG and several sweet stimuli: sucrose, glucose, maltose, saccharin and SC-45647. Strong bidirectional generalization was seen between MSG mixed with amiloride and sucrose, glucose, saccharin and SC-45647. Weak generalization was seen between MSG and maltose, and sucrose and maltose. None of the CTAs generalized to NMDA. These findings support the hypothesis that the taste of MSG has broadly tuned, sweet-like characteristics, possibly due to the convergence of afferent signals for MSG, natural sugars and artificial sweeteners.     

Generalization of conditioned taste aversions in hamsters: evidence for multiple bitter receptor sites

Multiple bitter receptor sites appear to exist within the hamstergustatory system supporting the data of other investigatorson humans, rats and frogs. The sodium salts of four anions,m-nitrobenzene sulfonate (NBSA), picrate, m-nitrobenzoate (NBA)and cholate, were tested in two-bottle preference tests andfor generalization to a variety of stimuli in a conditionedtaste aversion (CTA) paradigm. All four of these anions arebitter to humans. One, NBSA, generalized to sucrose suggestinga sweet taste, while the remaining three appear to be bitterwith varying degrees of saltiness. The bitterness of these threeanions to hamsters appears to be perceptually different froma quinine-type bitterness. Separate bitter receptor sites areindicated for quinine and urea, plus a third site acceptingNBA, picrate and cholate. More bitter sites are plausible. Separatesites for quinine and urea appear to occur across species. Itwas also concluded that quinine does not serve as a prototypicbitter stimulus for all bitters in the CTA test.     

The selective serotonin reuptake inhibitor paroxetine does not alter consummatory concentration-dependent licking of prototypical taste stimuli by rats

Serotonin and the 5HT(1A) receptor are expressed in a subset of taste receptor cells, and the 5HT(3) receptor is expressed on afferent fibers innervating taste buds. Exogenous administration of the selective serotonin reuptake inhibitor, paroxetine, has been shown to increase taste sensitivity to stimuli described by humans as sweet and bitter. Serotonergic agonists also decrease food and fluid intake, and it is possible that modulations of serotonin may alter taste-based hedonic responsiveness; alternatively, or in combination, serotonin may interact with physiological state to impact ingestive behavior. In this study, the unconditioned licking of prototypical taste stimuli by rats in brief-access taste tests was assessed following paroxetine administration (0.3-10 mg/kg intraperitoneal). We also measured sucrose licking by rats in different deprivation states after paroxetine (5 mg/kg). In neither experiment did we find any evidence of an effect of paroxetine on licking relative to water to any of the taste stimuli in the brief-access test at doses that decreased food intake. However, in some conditions, paroxetine decreased trials initiated to tastants. Therefore, a systemic increase in serotonin via paroxetine administration can decrease appetitive behavior in brief-access tests but is insufficient to alter taste-guided consummatory behavior.     

TIME-INTENSITY PROPERTIES OF SWEET AND BITTER STIMULI: IMPLICATIONS FOR SWEET AND BITTER TASTE CHEMORECEPTION

Interindividual differences in sweet and bitter taste sensitivity were investigated using time-intensity (TI) measurements and multivariate statistics. TI profiles were obtained in triplicate from 25 subjects for 23 sweet and/or bitter stimuli first matched to be approximately equi-intense to 200 mM NaCl. Sweet stimuli, except for the larger sweeteners, were less persistent, and required less time to reach maximum intensity than bitter stimuli. The results of principal component (PCA) and cluster (CA) analyses of the stimuli X subjects matrices for maximum intensity (Imax), time to maximum intensity (Tmax), total duration (Tdur), and area under the curve (Area) suggest that sweet and bitter stimuli do not share common receptors; and that there are at least two receptor mechanisms each for sweet taste (one for sugars and other small compounds, and the other for large sweeteners) and bitter taste (one for PTC/PROP and one for other bitter compounds).     

Receptors for bitter and sweet taste

The identification of two families of receptors, T1Rs and T2Rs, for sweet and bitter taste stimuli has opened the door to understanding some of the basic mechanisms underlying taste transduction in mammals. Studies of the functions of these receptors and their patterns of expression provide important information regarding the detection of structurally diverse taste compounds and the manner in which different taste qualities are encoded in the mouth.     

Responses of the rat chorda tympani nerve to glutamate-sucrose mixtures

Monosodium glutamate (MSG) has a multifaceted, unusual taste to humans. Rats and other rodents also detect a complex taste to MSG. Responses of the chorda tympani nerve (CT) to glutamate applied to the front of the tongue were recorded in 13 anesthetized rats. Whole-nerve responses to 30 mM, 100 mM and 300 mM MSG mixed with 300 mM sucrose were recorded before and after adding 30 micro M amiloride to the rinse and stimulus solutions. Responses of CT single fibers were also recorded. Predictions from models of whole-nerve responses to binary mixtures were compared to the observed data. Results indicated that MSG-elicited CT responses have multiple sources, even in an amiloride-inhibited environment in rats. Those sources include responses of sucrose-sensitive CT neural units, which may provide the substrate for a sucrose-glutamate perceptual similarity, and responses of sucrose-insensitive CT neural units, which may respond synergistically to MSG-sucrose mixtures.     

Estradiol benzoate can function as an unconditioned stimulus in a conditioned taste aversion paradigm

The extent to which gonadal steroid hormones can serve as unconditioned stimuli in a conditioned taste aversion paradigm was examined in Rockland-Swiss albino mice. With saccharin serving as the conditioned stimulus, subcutaneously injected estradiol benzoate, but not progesterone or testosterone propionate, was found to be a potent unconditioned stimulus in both male and female mice. Dose-response effects were also observed; increasing dosages of estradiol benzoate led to increasingly stronger conditioned aversions in both males and females. The aversion detected in males was more resistant to extinction than that seen in females. Prepubertal gonadectomy reversed the sex-dependent effects of estradiol benzoate in learned aversions in adulthood; castration of males promoted the extinction process, whereas ovariectomy of females retarded extinction. The results may be useful for our understanding of the mechanisms involved in conditioned taste aversion learning as well as a wide array of hormone-dependent behavioral responses.