Effects of binary taste stimuli on the neural activity of the hamster chorda tympani

Binary mixtures of taste stimuli were applied to the tongue of the hamster and the reaction of the whole corda tympani was recorded. Some of the chemicals that were paired in mixtures (HCl, NH4Cl, NaCl, CaCl2, sucrose, and D-phenylalanine) have similar tastes to human and/or hamster, and/or common stimulatory effects on individual fibers of the hamster chorda tympani; other pairs of these chemicals have dissimilar tastes and/or distinct neural stimulatory effects. The molarity of each chemical with approximately the same effect on the activity of the nerve as 0.01 M NaCl was selected, and an established relation between stimulus concentration and response allowed estimation of the effect of a "mixture" of two concentrations of one chemical. Each mixture elicited a response that was smaller than the sum of the responses to its components. However, responses to some mixtures approached this sum, and responses to other mixtures closely approached the response to a "mixture" of two concentrations of one chemical. Responses of the former variety were generated by mixtures of an electrolyte and a nonelectrolyte and the latter by mixtures of two electrolytes or two nonelectrolytes. But, beyond the distinction between electrolytes and nonelectrolytes, the whole-nerve response to a mixture could not be predicted from the known neural or psychophysical effects of its components.     

Effects of amiloride on salt taste in humans

Amiloride reduces several responses of the taste system to NaCl.These effects are crucial support for the ion transport theoryof salt taste. Here, three experiments tested for the inhibitoryeffect of amiloride on salt taste in normal, whole-mouth tastingin humans. There was no evidence that amiloride reduces thesaltiness of NaCl when used as a pretreatment, mixed into solutionwith NaCl, or both. In a fourth experiment, pretreating thetongue with amiloride-soaked filter paper and presenting NaClon filter paper within the treated area also had no effect onsaltiness. In addition, amiloride, a bitter compound, did notsuppress the taste of sucrose, and caffeine did not enhancethe taste of NaCl. These results suggest the ion transport theoryof NaCl taste needs to be modified or extended to account fornormal whole-mouth tasting of NaCl in humans. ¹ Present address: 1605 Harbor Court Tower, 10 East Lee St.,Baltimore, MD 21202, USA     

Sensitivities of single nerve fibers in the hamster chorda tympani to mixtures of taste stimuli

Responses of three groups of neural fibers from the chorda tympani of the hamster to binary mixtures of taste stimuli applied to the tongue were analyzed. The groups displayed different sensitivities to six chemicals at concentrations that had approximately equal effects on the whole nerve. Sucrose-best fibers responded strongly only to sucrose and D-phenylalanine. NaCl-best and HCl-best fibers, responded to four electrolytes: equally to CaCl2 and nearly equally to HCl, but the former responded more to NaCl, and the latter responded more to NH4Cl. The groups of fibers dealt differently with binary mixtures. Sucrose- best fibers responded to a mixture of sucrose and D-phenylalanine as if one of the chemicals had been appropriately increased in concentration, but they responded to a mixture of either one and an electrolyte as if the concentration of sucrose or D-phenylalanine had been reduced. NaCl- best fibers responded to a mixture as if it were a "mixture" of two appropriate concentrations of one chemical, or somewhat less. But, responses of HCl-best fibers to mixtures were greater than that, approaching a sum of responses to components. These results explain effects on the whole nerve, suggest that the sensitivity of a mammalian taste receptor to one chemical can be affected by a second, which may or may not be a stimulus for that receptor, and suggest that some effects of taste mixtures in humans may be the result of peripheral processes.     

Effects of pharmacologic reductions in salivary flow on judgments of suprathreshold taste stimuli

The effects of short term salivary flow reductions on suprathresholdjudgments of taste intensity were measured using both a seriesof taste solutions and a comparable series of tastants driedon a filter paper base. Decreases in salivary flow were producedby the oral administration of either Elavil^®, Benadryl^®or atropine. The pharmacologic agents produced a 25 –82% reduction in salivary flow during the period that tastetesting occurred but no measurable effects on perceived tasteintensity were found. The exponents of power functions describingmolar concentration and perceived intensity were unchanged aswas the spread in perceived intensity between aqueous and drystimuli. Significant relationships between individual differencesin salivary flow and suprathreshold taste parameters also failedto emerge.     

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.     

Attention and the detectability of weak taste stimuli

Subjects detected weak solutions of sucrose or citric acid under conditions in which attention was directed toward one of the tastants or the other. Detection thresholds were measured using an adaptive, forced-choice procedure, with a three-down one-up rule, which computer simulations suggest should be more reliable than the popular two-down one-up rule. The thresholds were modestly but systematically lower for attended tastants than for unattended ones. Similar results have been reported in other sense modalities, including vision (greater sensitivity to stimuli presented to attended versus unattended spatial locations) and hearing (greater sensitivity to stimuli presented at attended versus unattended sound frequencies). Taken together, the findings are consistent with a general hypothesis regarding attention in sensory systems: gains or losses in detectability occur when a central attentional mechanism (or, conceivably, a preattentive mechanism) selectively and preferentially monitors signals arising from particular subsets of peripheral neural inputs.      

Studies on after-taste of various taste stimuli in humans

This series of experiments was performed in order to evaluatethe physiological characteristics and patterns of after-tasteof various taste substances. The durations of after-taste followingmonosodium glutamate (MSG), inosine-5'-monophosphate (IMP) andguanosine-5'-monophosphate (GMP) (umami substances) were longerthan those for sucrose, NaCl, tartaric acid and quinine-HCIat concentrations corresponding to the recognition threshold.The periods of after-taste of solutions of MSG and IMP, andMSG and GMP, were longer than those for the single componentsolutions. Most subjects recognized sucrose as sweet, NaCl assalty, tartaric acid as sour and quinine-HCI as bitter, bothin terms of immediate taste and after-taste. According to thepatterns of after-taste for umami substances, the subjects weredivided into three groups. In group A, umami (appealing, savorytaste in Japanese cuisine) was the main quality of the after-tastesensation; in group B, an indefinite, equivocal taste was thecharacteristic quality of the aftertaste; and no differencewas reported in group C between the immediate taste and after-taste.These results suggest that the characteristics of after-tastefor MSG, IMP and GMP are different from those of the four basictastes.     

Salivary ions and neural taste responses in the hamster

Saliva is a chemically complex fluid that bathes oral surfacesand may affect early events in mammalian gustation. We measuredchorda tympani responses to taste stimuli in hamsters (Mesocricetusauratus) while their tongues were adapted to either water, artificialsaliva or natural saliva. Artificial saliva on the tongue loweredneural responses to taste stimuli that were present in the artificialsaliva and to those stimuli that cross-adated with salivarycomponents. Changing from a water-adapted tongue to one soakedwith pilocarpine-stimulated saliva from donor hamsters led tosignificantly smaller responses to NaCl. Responses to sucrose,NH₄Cl and quinine were unaffected. Chemical analysis of hamstersaliva revealed ‘normal’ mammalian levels of K⁺,Ca²⁺ and Mg²⁺, but unexpectedly low levels of Na⁺ and Cl^–.     

Study of the effect of gymnemic acid on taste in hamster

Behavioral and electrophysiological experiments with 10 sweetenershave been made to test if gymnemic acid (GA) is able to blockthe response to sweet stimuli in single taste fibres of thechorda tympani proper nerve in hamsters. The hamster was chosenbecause earlier studies show that it is more sensitive to GAthan any other non-human species. Since GA has been shown toaffect the sweetness of many different substances, its effectswere studied on an array of sweeteners. To avoid, however, theinclusion of sweeteners unpalatable to the hamster, the hamsters'liking of this array was tested with two-bottle preference techniquefor –24 h. It was found that acesulfam-K, fructose, glucose,sucrose and xylitol were strongly liked, while the animals showedno preference for aspartame, D-tryptophane, sodium cyclamate,sodium saccharin and thaumatin over water. The summated nerveresponse of these stimuli was then recorded. It was found thatneither thaumatin nor aspartame elicited a response, while theother stimuli gave a good response. Finally, the sweetenerswhich were both preferred in the two-bottle tests and gave anerve response were used as taste stimuli in single fibre experimentstogether with sodium chloride, quinine hydrochloride, citricacid and saccharin. The single fibre recordings were made beforeand after application of 5 mg GA for 3 min on the tongue. Itwas found that GA did not cause any dramatic decrease or disappearanceof the responses to either the sweet or the non-sweet substances.The responses to the sweeteners, however, were more depressedthan those to the non-sweet stimuli.     

Topographic distribution of taste responsiveness in the hamster medulla

The purpose of the present investigation was to map the multiunitresponsiveness of the gustatory portion of the nucleus of thesolitary tract (NTS) in the hamster, elicited by chemical stimulationof oral taste receptors. Neural responsiveness to four stimuli(0.1 M sucrose, 0.03 M NaCl, 0.003 M HCl, 0.001 M QHCl) deliveredto either the anterior tongue or other parts of the oral cavitywas examined at 37 NTS recording sites. Gustatory responseswere shown-to depend collectively upon the stimulus, the receptivearea being stimulated, and the location of the recording sitewithin the NTS. By comparing the proportional magnitudes ofintegrated responses across recording sites, unique topographicpatterns of responsiveness were demonstrated for sucrose, NaCIand QHCl. Responses to HCl and NaCl generated similar patterns.Further, the response patterns for each stimulus differed followingstimulation of the anterior tongue or posterior oral cavity.Spatial differences in NTS responsiveness arise as a resultof differences in peripheral gustatory nerve sensitivities andprovide a possible substrate for the coding of taste quality.     

The effects of amiloride on the labellar taste receptor cells of the fleshfly Boettcherisca peregrina

Amiloride is known to inhibit the taste response of vertebrates to salt by blocking the amiloride-sensitive sodium channel. In this study, we investigated electrophysiologically the effect of amiloride on the taste response of the fleshfly Boettcherisca peregrina. When 0.5 mM amiloride was included in taste solutions, the response of the salt receptor cell (salt response) to sodium chloride (NaCl) was not depressed but those of the sugar receptor cell (sugar responses) to sucrose, glucose, fructose, l-valine (l-Val) and l-phenylalanine (l-Phe) were strongly depressed. An inhibitory effect of amiloride on the concentration-response relationship for both sucrose and l-Phe was clearly revealed, but not at high concentrations of sucrose. After pretreatment of a chemosensory seta with 0.15 mM amiloride for 10 min, the salt response to NaCl was not affected. On the other hand, the sugar responses to sucrose, fructose, l-Val and l-Phe were depressed just after amiloride pretreatment. The sugar response to adenosine 5’-diphosphate (ADP) mixed with 0.5 mM amiloride was not depressed, but the response to ADP alone was depressed after amiloride pretreatment. It was therefore observed that amiloride depressed the responses to all stimulants that react with each of the receptor sites of the sugar receptor cell.     

Effects of various taste stimuli on heart rate in humans

Relationships between taste stimuli and heart rate were evaluated in 29 healthy university students. The test solutions were sucrose, NaCl, citric acid, quinine-HCl and monosodium glutamate (MSG). Heart rate increased by 7.1-13.6% for all the taste stimuli after use as compared with pre-stimuli values. The maximum increases in heart rate came approximately 25 s after the taste stimuli. After the increases, heart rate returned to pre-stimuli levels after between 80 and 100 s. Heart rate reached its maximum with citric acid. Recovery from the heart-rate increase was more delayed for quinine-HCl and MSG than for the other stimuli. Except for sucrose, increases in heart rate and the hedonic scale values of the taste solutions showed significant negative correlation. These findings show that the taste stimuli solutions increased the heart rate and that the increase differed with the concentration and taste solution used.     

The influence of taste stimuli and illumination on electrogastrogram measurements

An electrogastrogram (EGG) is considered to be an index to the autonomic nervous system of the digestive organs. In the present study, we attempted to clarify whether or not an EGG can be used to evaluate the influence of illumination, and what kinds of effect taste stimuli and illumination have on the autonomic nervous system. In this study, we used the ratio of the normal wave component of the EGG (EGG-NR: 2-4.5 cpm power/1-10 cpm power) and the amplitude of a normal wave (EGG-NI: integrated EGG of 2-4.5 cpm). Thirteen healthy males participated in 16 experimental conditions (4 lighting conditionsx4 taste stimuli). The four lighting conditions were set by combinations of illuminance levels of 200 and 1500 lx and color temperatures of 3000 and 7500 K. The four taste stimuli were sweet (glucose), salty (salt), sour (acetic acid), and bitter (quinine). The changes in EGG-NR and EGG-NI were compared for different taste conditions. The results showed that EGG-NI was not significantly affected by the different taste conditions. However, the main effect of taste on EGG-NR was significant: sweet and salty tastes were significantly higher than the bitter taste. EGG-NR and EGG-NI in different lighting conditions were also compared. The main effect of different color temperatures was also significant, but the illuminance level did not affect EGG-NR. EGG-NR increased significantly at the lower color temperature. On the other hand, EGG-NI significantly increased at the lower illuminance. These results suggest that parasympathetic nervous activity has a predominant effect on gastric activity in different lighting environments. Therefore, EGG measurements may be useful indicators for illumination environment studies.     

Iontophoretic application of bitter taste stimuli in hamsters

Recent electrophysiological studies on the iontophoretic applicationof taste stimuli by weak electric currents using rodents andfrogs have produced stimuli which appear to mimic the actionof salty, sour and sweet solutions. However, there has beenno report of an ionic stimulus which might serve as a bitteriontophoretic probe. Many common bitter stimuli are either uncharged(e.g. quinine, urea) or have mixed quality sensations (e.g.the bitter salts KCl, MgCl₂) and therefore are unsuitable. Thisreport investigates the use of four organic anions, all of whichare bitter to humans, which may serve as potential bitter stimulifor iontophoretic application to the tongue of the hamster whilerecording electrophysiologically from its chorda tympani nerve.These anions are m-nitrobenzene sulfonate (NBSA), picrate, cholateand m-nitrobenzoate (NBA). The electrophysiological responsesto cathodal polarization via these four anions plus saccharin,an effective cathodal stimulus in the hamster, form the sameefficacy series as chemical (i.e. normal sapid) presentationsof sodium salts of these anions, i.e. saccharin > NBSA >picrate > NBA > cholate. Behavioral evidence suggeststhat NBSA is sweet to hamsters while the latter three anions,picrate, NBA and cholate, are bitter. Electrophyiological observations,based on magnitude of response, appear to support these behavioralfindings. It was concluded that picrate, NBA and cholate mayserve as useful bitter stimulus probes for ionto-phoretic applicationin the hamster.     

Effect of non-well-mixed compartment and bulk flow on diffusion through a pore

The assumptions of well-mixed and zero bulk flow in compartmental analysis are reexamined. Using Poiseuille flow inside and radial flow outside a pore, the mass-transfer equations are solved by perturbations on simple diffusion. Formulas are obtained for solute distribution, total mass transfer, and apparent permeability. The effects of non-well mixing and bulk flow are discussed.