Taste responses of human chorda tympani nerve

Records from humans of summated action potential dischargesof the chorda tympani nerve were examined. The magnitudes ofneural and psychophysical responses were well related only whenthe comparison was made within a given taste quality. The responseto a mixture of 0 02 M citric acid and 0.5 M sucrose was lessthan the sum of the separate responses to the mixture components.Citric acid failed to cross-adapt the response to sucrose, implyingthe receptor sites for sucrose are independent of citric acid.The human chorda tympani nerve shows vigorous responses to mechanicalstimulation and cooling of the tongue that are maintained aftertreatment of the tongue with a water extract of the herb Gymnemasylvestre. Gymnema extract selectively suppressed the responseto all sweeteners tested (sucrose, fructose, saccharin and cyclamate)and also suppressed by – 50% the water-after-citric-acidresponse which has a predominantly sweet taste. Gymnema suppressedby 0 – 10% the water-rinse response following NaCl. fructoseand sucrose that have a predominantly bitter-sour taste. Water-rinseresponses were present even when mechanical and thermal stimulationof the tongue were minimized. The human chorda tympani nerveappears to have positive water-rinse taste responses. Theseare solute-specific off-responses that are probably mediatedby receptor sites independent of those responsible for the on-responseto the given solute.     

Hodulcin: selective sweetness-reducing principle from Hovenia dulcis leaves

An aqueous extract of Hovenia dulcis leaves selectively reducedsweetness perception in humans. The taste-active principle,‘hodulcin’, was partially purified and comparedchromatographically with similarly prepared samples of the selective,sweetness-reducing compounds, gymnemic acids and ziziphins.Hodulcin appears to be a triterpene saponin glycoside, as arethe gymnemic acids and ziziphins. Nuclear magnetic resonance(NMR) spectra indicated an hodulcin aglycone structure differentfrom the gymnemic acids aglycone, and similar to, but not thesame as the ziziphins aglycone. Future comparative studies ofthe actions of hodulcin, gymnemic acids and ziziphins couldelucidate physiological mechanisms for the transduction andidentification of sweet stimuli and aid the development of newapproaches to the sweetening of foods.     

Inhibition of hamster chorda tympani neural response by copper chloride

Copper chloride was evaluated as a specific inhibitor of neuralresponses to sweet taste stimuli in the goldern hamster (Mesocricetusauratus). The chorda tympani whole-nerve response to taste stimuliwas recorded before and after the tongue was treated for 30s with 0.01, 0.1 and 1 mM CuCl₂. Sweet stimuli [sucrose, fructose,saccharin (calcium salt), D-phenylalanine], which primarilystimulate chorda tympani S fibers, and non-sweet stimuli (NaCl,NH₄Cl) were used. At 0.01 mM, copper chloride had little effect.At 0.10 mM it partially inhibited responses to sucrose and saccharin,but had little effect on responses to D-Phe, fructose, NaCl,NH₄Cl, or a mixture of sucrose plus L-Phe. L-Phe, which hasthe same chelating properties as D-Phe, is not an S-fiber stimulusand likely reduced sucrose inhibiton by chelating the cupricion.Analysis of concentration–response functions revealedthat 0.1 mM copper chloride inhibited the neural response tolow concentrations of sucrose by about 25%, but did not significantlyinhibit high concentrations of surcrose, suggesting competitiveinhibition. In contrast, 0.1 mM CuCl₂ reduced saccharin responsesby 25% throughtout the effective range, suggesting non-competitiveinhibition. Occupation of a saccharide receptor site by coppermay interfere with dimer but not monomer reception and distortthe saccharin receptor site. At 1 mM, CuCl₂ non-competitivelyinhibited responses to sucrose, fructose, saccharin and thenon-sweet NaCl (an N-fiber stimulus), but not NH₄Cl (an H-fiberstimulus). The mechanisms of copper chloride inhibition aredifficult to establish because its effects are weak at concentrationswhere they are specific.     

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.     

Taste responses of the gerbil IXth nerve

Summated taste responses to 12 taste solutions were recordedfrom the IXth (glossopharyngeal) nerve of 38 Mongolian gerbils(Meriones unguiculatus). 0.3 M NH₄Cl was the most effectivestimulant. The relative magnitude of the peak summated responsewas a positively accelerated function of log molar concentration.Absolute thresholds were determined for three chemicals: 0.002M NaCl, 0.0003 M HCl, and 0.002 M sucrose. The relative magnitudesof the responses to quinine, NH₄Cl, and KCl were greater forthe IXth nerve than for the chorda tympani nerve, whereas NaClwas more effective for the chorda tympani. A similar patternis seen in the rat. Acetic and citric acid may bind to commonreceptor sites. NH₄Cl, KCl, and HCl may also have receptor sitesin common.     

Genetic Sensitivity to 6-n-Propylthiouracil (PROP) and Hedonic Responses to Bitter and Sweet Tastes

Genetically mediated sensitivity to the bitter taste of 6-n-propylthiouracil(PROP) has been associated with greater acuity for bitter andfor some sweet tastes. Thus far, few studies have explored therelationship between PROP taste sensitivity and hedonic responsesto bitter and sweet. In this study, 87 normal-weight young womenwere divided into PROP non-tasters (n = 18), regular tasters(n = 49), and supertasters (n = 20), based on their PROP detectionthresholds and the scaling of five suprathreshold solutionsof PROP and NaCl. Non-tasters had thresholds >1.8 x 10^–4mol/l PROP. Supertasters had thresholds <3.2 x 10^–5mol/l PROP and PROP/NaCl ratios >1.70. As expected, dislikeof the bitter taste of PROP was determined by its perceivedintensity, which was greater among supertasters than among regulartasters or non-tasters. Significant correlations were observedbetween PROP taste thresholds and the sum of intensity ratings(r = –0.61) and between summed intensity and summed hedonicratings (r = –0.80). PROP taste sensitivity was weaklylinked to enhanced perception of sweet taste, but did not predicthedonic responses to sucrose or to saccharin solutions. Giventhat the dislike of PROP solutions is determined by their perceivedintensity, hedonic responses to PROP solutions may provide arapid way of screening for PROP taster status. Chem. Senses22: 27–37, 1997.     

Information transmission in tarsal sugar receptors of the blowfly

Responses of the sugar receptors in the tarsal D hairs of theblowfly, Phormia regina, were examined electrophysiologically.The ability of these receptors to code information about sucroseconcentration was investigated using the concepts and methodsof information theory. The stimulus-response function was linearwith respect to the logarithm of sucrose concentration between0.01 and 1.0 M. When responses were pooled from several preparations,the amount of variability in response to each sucrose concentrationallowed only 3.8 discriminable levels of sucrose within thistwo log step range. However, when responses were obtained fromindividual receptors to repeated stimulations with a given sucroseconcentration, there was much less variability, allowing forgreater information transmission, providing for the discriminationof 5.3 levels of sugar between 0.1 M and 1.0 M. If a singleprothoracic leg was treated as a sensory channel, the combinedresponses of its D-hair sugar receptors provided for 4.0 discerniblelevels of sucrose within this range. Examination of the channelcapacities of these receptors for transmitting information aboutsucrose intensity suggested that the fly should be able to discriminatebetween 3.8 and 5.3 levels of sucrose intensity between 0.01M and 1.0 M, depending upon the patterns of neural convergencein the central nervous system. Examination of the stimulus-responsefunctions of these receptors over time demonstrated that informationtransmission increased markedly over the initial 500 ms of theneural response and tended to stabilize between 500 and 1000ms. Behaviorally, it has been shown that Phormia can discriminateabout four levels of sucrose within this two log step rangeof concentrations.     

Triterpenoid saponins stimulate the sugar taste receptor cell through a G protein-mediated mechanism in the blowfly,Phormia regina

The blowfly has taste chemosensilla on the labellum. The sensory receptor cells in the chemosensillum are highly specialized for the tastes of sugar, salt and water, respectively. Previously we introduced chromosaponin I (CSI) and glycyrrhizin (GL), as sweet substances for the blowfly, Phormia regina. Application of these triterpenoid saponins induced feeding responses as well as impulses of the sugar taste receptor cell in the LL-type sensillum at a much lower concentration than that of sucrose. In the present paper, we show the involvement of G protein-mediated cascade in the CSI- and GL-responses as well as in sugar responses. CSI activates the sugar signal transduction cascade after penetrating through the membrane. On the other hand, GL exerts dual effects to stimulate the sugar signal transduction possibly by activating it inside the cell and also by interacting with the pyranose sugar receptor site. A non hydrolyzable G protein inhibitor guanosine 5′-O-(2-thiodiphosphate), GDPβS, markedly decreased the responses of the sugar receptor cell to the two triterpenoid saponins as well as the response to sucrose and fructose. These results suggest that CSI and GL are direct activators of G protein.     

Activity in salt taste fibers: peripheral mechanism for mediating changes in salt intake

In order to study the role of peripheral taste sensitivity inmediating increases in salt intake of the rat, the effects ofsodium deprivation and adrenalectomy on chorda tympani nerveresponses to taste stimulation were determined. Sodium deprivationresulted in a reduction in whole nerve responsivity to suprathresholdNaCl concentrations requiring a 10-fold increase in concentrationto elicit the same neural signal of control preparations. Saltintake of sodium deprived rats was predicted by adjusting datain a 10-min intake test from control rats for the reduced neuralsignal and lower salivary sodium levels of sodium deprived rats.The whole nerve responses to LiCl and KCl, as well as to NaCl,were reduced after sodium deprivation and adrenalectomy. Themultifiber response of the chorda tympani is comprised of theindividual responses of NaCl sensitive N-best fibers and HCl/NaClsensitive H-best fibers. After sodium deprivation N-best fibers'responses to suprathreshold concentrations of NaCl were reduced;H-best fibers' responses were not affected by sodium deprivation.Future studies will determine the effect of KCl and other saltson responses of N-best and H-best fibers. Applying Beidler'sbiophysical model to the single fiber data suggests that sodiumdeprivation influences receptor mechanisms for NaCl of N-bestfibers and not H-best fibers. Because repeated NaCl stimulationresulted in increased chorda tympani responsivity to NaCl, wesuggest that sodium deprivation may alter the salt receptorsimply by disuse. Altered receptor sensitivity may be an adaptivemechanism to influence salt consumption by a shift in suprathresholdNaCl intensity.     

Decline of IXth nerve taste responses following nerve transection

Summated impulse discharges to taste solutions were recordedfrom intact and transected IXth nerves in the Mongolian gerbil(Meriones unguiculatus). Five taste stimuli were used: 0.3 MNH₄Cl, 0.3 M NaCl, 0.01 M HCl, 0.01 M quinine hydrochloride,and 0.5 M sucrose. 0.3 M NH₄Cl was the most effective stimulus.Taste responses from intact nerves were stable for more than10 hours. Following IXth nerve transection, the peak summatedresponse to 0.3 M NH₄Cl declined by 50% in a mean of 119 min.(Some animals failed to show this taste response decline inthe winter months.) The transected IXth nerve's spontaneousactivity and responses to other taste solutions also typicallydeclined. The continued presence of normal compound action potentialsindicated that the transection-induced decline in taste responsesdid not result from a failure of impulse propagation mechanismsin the nerve trunk. The results are consistent with the propositionthat transection interferes with axonal transport of materialsvital to the short-term maintenance of taste responses.     

Effects of cations on the sugar receptor-site of the blowfly, Phormia

1) Ca^{+ +} (1 to 10 mM) lowered the binding affinity of sugarreceptor-site for sucrose in the labellar sugar receptor ofthe blowfly, Phormia regina, without changing the maximum-responseamplitude. It also elevated the values of the Hill coefficient(n_H) in some degrees. 2) Other divalent cations such as Mg^{+ +}, Ba^{+ +} or Cd^{+ +} alsoshowed almost the same property as above. The sequence of theeffect is as follows: Ba^{+ +}, Mg^{+ +} x Ca^{+ +} x Cd^{+ +}. Trivalentcation, La^{+ + +} (1 mM), changed the value of n_H from 1 (La^{++ +}-free) to 2. 3) On the contrary, the action of monovalent cations such asK⁺ or Na⁺, of which ionic strength was made the same as thatof the divalents hardly suppressed the response. 4) The results obtained do not support the hypothesis, at leaston the sugar receptor of the fly, that the receptor potentialis attributable to a change of the surface potential (zeta potential)as is proposed for the frog sugar receptor.     

Labellar lobe spreading in the blowfly: Regulation by taste and satiety

Summary Lobe spreading behavior was studied by recording electromyograms from the muscles which spread the labellar lobes, the retractors of the furca (RF) inPhormia regina. RF responses and lobe spreading could be elicited by stimulating labellar, but not tarsal, taste hairs with sucrose (Fig. 3). RF activity was important to spread the lobes at the beginning of a meal, but was not necessary for continued feeding (Fig. 4).Temporal summation between sugar receptor spikes was necessary to elicit RF responses. Central response decrement occurs independently for different labellar hairs and may participate in the termination of motor responses.RF responses were more probable and more intense when either the sucrose concentration of the stimulus or the number of hairs stimulated was increased (Fig. 7). Stimulation with NaCl had no effect on the response to simultaneous sucrose stimulation of other hairs (Table 1).Feeding caused decreases in the probability and intensity of motor responses, but did not alter chemosensory responses (Figs. 8 and 9). Section of either the recurrent or median abdominal nerves prevented this postingestional inhibition of lobe spreading (Fig. 9).These results are discussed with regard to the possible role that regulation of lobe spreading may play in the control of food intake.This work was supported by United States Public Health Service Training Grant 5T01 GM 00457-13S2 and by a grant from the National Science Foundation to Dr. Vincent G. Dethier. I wish to thank Dr. Dethier for his support and encouragement.     

Depression of gustatory receptor potential in frog taste cell by parasympathetic nerve-induced slow hyperpolarizing potential

Parasympathetic nerve (PSN) innervates taste cells of the frog taste disk, and electrical stimulation of PSN elicited a slow hyperpolarizing potential (HP) in taste cells. Here we report that gustatory receptor potentials in frog taste cells are depressed by PSN-induced slow HPs. When PSN was stimulated at 30 Hz during generation of taste cell responses, the large amplitude of depolarizing receptor potential for 1 M NaCl and 1 mM acetic acid was depressed by approximately 40% by slow HPs, but the small amplitude of the depolarizing receptor potential for 10 mM quinine-HCl (Q-HCl) and 1 M sucrose was completely depressed by slow HPs and furthermore changed to the hyperpolarizing direction. The duration of the depolarizing receptor potentials depressed by slow HPs prolonged with increasing period of PSN stimulation. As tastant-induced depolarizing receptor potentials were increased, the amplitude of PSN-induced slow HPs inhibiting the receptor potentials gradually decreased. The mean reversal potentials of the slow HPs were approximately -1 mV under NaCl and acetic acid stimulations, but approximately -14 mV under Q-HCl and sucrose stimulations. This implies that when a slow HP was evoked on the same amplitude of depolarizing receptor potentials, the depression of the NaCl and acetic acid responses in taste cells was larger than that of Q-HCl and sucrose responses. It is concluded that slow HP-induced depression of gustatory depolarizing receptor potentials derives from the interaction between gustatory receptor current and slow hyperpolarizing current in frog taste cells and that the interaction is stronger for NaCl and acetic acid stimulations than for Q-HCl and sucrose stimulations.     

Behavioural evidence for the existence of sugar,salt and amino acid taste receptor cells and some of their properties in Drosophila larvae

Independence between taste receptors in Drosophila larvae was studied using behavioural choice experiments. Independence of two receptors for A and B was assessed by testing whether a low concentration of A could be detected in the presence of a high concentration of B. For example, 0.01 M sucrose and fructose could no longer be detected in the presence of 1.3 M glucose, although 0.9 M glucose did not mask responses to the other sugars. On the other hand, 0.01 M NaCl could be detected even in the presence of 1.3 M glucose. These results were interpreted as showing that the receptor sites for sucrose, fructose and glucose were not so independent as those for glucose and NaCl.From such experiments it was concluded that larvae possess at least three taste receptor cells; sugar, salt and amino acid receptor cells. Two different binding sites seem to exist on the sugar receptor cell; one for sucrose and fructose, the other for glucose. Anionic and cationic sites appear to exist on the single salt receptor cell. In addition, the larvae exhibited avoidance responses to all of the six amino acids examined.     

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^–.     

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     

Competition of polysaccharides with sugars for the pyranose and the furanose sites in the labellar sugar receptor cell of the blowfly, Phormia regina

In the labellar sugar receptor cell of the blowfly, Phormia regina, soluble starch and dextran T500 inhibited the response to sucrose, to maltose or to glucose, but did not inhibit that to fructose. On the other hand, inulin inhibited the response to fructose, but did not inhibit that to sucrose. These results suggest that both soluble starch and dextran T500 compete with sucrose, with maltose or with glucose for the pyranose site (P site), and that inulin competes with fructose for the furanose site (F site) in a single sugar receptor cell. Each inhibition constant (K_i) was estimated to be 0.6–0.7% for soluble starch. about 4.5% for dextran T500, and about 1.3% for inulin.     

Salt taste and salt sensitivity in black adolescents

Limited support for the postulated association between tasteand blood pressure responses to NaCl has prompted investigationsof selected subgroups of subjects more likely to display a relationship.The present study included 20 black adolescents who were diagnosedas salt sensitive (n = 10) or salt insensitive (n = 10) basedupon blood pressure responses to 14 days of dietary salt loading(10 g NaCl/day). Each was administered a battery of taste tests,including measures of detection and recognition threshold, suprathresholdintensity responses and preference tests. No significant groupdifference was observed on any taste measure. While explanationsfor the failure to note differences between these individualswith marked variabilty in systemic reactivity to NaCl are offered,the present findings question the continued study of this issue.     

Sugar taste reception in mammals

This review evaluates behavioral and electrophysiological investigationsin an attempt to show correlation between the taste responsesof humans and other mammals to various sugar sweeteners. Methodologiesinclude whole nerve, single neuron and single cell studies.In addition, to enhance understanding of the mechanism of sweettaste reception, attention is focused on inhibition studies,mixture experiments, pH effects, analyses of concentration –response curves and structure–activity comparisons. Resultsshow that sucrose is the most potent naturally occurring disaccharidestimulant, while chloro-sucrose is the most potent syntheticdisaccharide. Among the monosaccharides, fructose is the mostpotent stimulant. Mixture experiments with sucrose and othersugars suggest that these compounds are interacting at a singlereceptor site whereas mixtures of sucrose and saccharin suggestdifferent receptor sites. Inhibitor experiments suggest thatthere are at least two major sucrose receptor sites. Singleneuron and single cell studies suggest a number of other sugarreceptors as well as a saccharin receptor. Concentration–responsecurve analyses show sigmoidally shaped curves throughout, andpH studies indicate that sucrose and saccharin are interactingwith their respective receptors by a different mechanism. Fromstructure–activity comparisons, we conclude that the bindingmechanism for sugars in humans and other mammals is very similar.     

Pain-reducing Properties of Sucrose in Human Newborns

To assess the characteristics of sucrose as a pain-reducingsubstance, crying in 72 newborn humans during and after bloodcollection via heel prick was determined. In the first studyinfants drank 2 ml of water or 2 ml of a 0.17-0.34- or 0.51-Msucrose solution 1 min prior to blood collection. In the secondexperiment, a delay of 30, 60, 90, 120 or 240 s was imposedbetween sucrose intake and the initiation of blood collection.The dose–response function for concentration was flat.The most effective time delay was 120 s. The effectiveness ofthe 2-min interval accords with previous findings of endogenousopioid release caused by sucrose taste. The flat dose–responsefunction extends findings in rats and humans that the calmingand pain-reducing effects of sucrose are not influenced by eitherconcentration or volume, suggesting that the transduction fromgustatory afferent to opioid-mediated efferent is of an on-offnature and not graded.