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The chorda tympani (CT) nerve is exceptionally responsive to NaCl. Amiloride, an epithelial Na+ channel (ENaC) blocker, consistently and significantly decreases the NaCl responsiveness of the CT but not the glossopharyngeal (GL) nerve in the rat. Here, we examined whether amiloride would suppress the NaCl responsiveness of the CT when it cross-reinnervated the posterior tongue (PT). Whole-nerve electrophysiological recording was performed to investigate the response properties of the intact (CTsham), regenerated (CTr), and cross-regenerated (CT-PT) CT in male rats to NaCl mixed with and without amiloride and common taste stimuli. The intact (GLsham) and regenerated (GLr) GL were also examined. The CT responses of the CT-PT group did not differ from those of the GLr and GLsham groups, but did differ from those of the CTr and CTsham groups for some stimuli. Importantly, the responsiveness of the cross-regenerated CT to a series of NaCl concentrations was not suppressed by amiloride treatment, which significantly decreased the response to NaCl in the CTr and CTsham groups and had no effect in the GLr and GLsham groups. This suggests that the cross-regenerated CT adopts the taste response properties of the GL as opposed to those of the regenerated CT or intact CT. This work replicates the 5 decade-old findings of Oakley and importantly extends them by providing compelling evidence that the presence of functional ENaCs, essential for sodium taste recognition in regenerated taste receptor cells, depends on the reinnervated lingual region and not on the reinnervating gustatory nerve, at least in the rat. [ABSTRACT FROM AUTHOR]

In a series of behavioral experiments in the 1960s, G.R. Morrison identified several unique features of the taste of Na2CO3 to rats; namely, it is 1) considerably more intense than NaCl at isomolar concentrations, 2) avoided at 10 times lower concentrations than NaCl to thirsty rats, 3) preferred at 10 times lower concentrations than NaCl in sodium-depleted rats. He also demonstrated its qualitatively similarity to NaCl. In Experiment 1, we confirmed and extended many of Morrison's observations. Rats were injected with furosemide on 3 occasions to stimulate a sodium appetite. After each depletion, rats were given a brief-access taste test in a lickometer presenting, in random order, water and 7 concentrations of salt. One test used NaCl (0.028-0.89 M, quarter log steps), another used Na2CO3, and the third used Na2CO3, but at a tenfold lower concentration range (0.0028-0.089 M). Rats licked NaCl in an inverted-U shaped concentrationresponse function peaking at 0.158-0.281 M. As Morrison's results predicted, rats licked Na2CO3 in nearly identical fashion, but at a tenfold lower concentration range (peak at 0.0158-0.028 M). In a second experiment, furosemide-treated rats were repeatedly tested with the lower Na2CO3 range but mixed in the epithelial sodium channel blocker amiloride at various concentrations (3-300 µM, half log steps). Amiloride reduced licking for Na2CO3 and shifted the peak response rightward up to about half a log unit. Thus, this "super-saltiness" of Na2CO3 to rats is at least partly amiloride-dependent. [ABSTRACT FROM AUTHOR]

Three experiments assessed potential changes in the rat's perception of sodium chloride (NaCl) during a state of sodium appetite. In Experiment 1, sodium-sufficient rats licking a range of NaCl concentrations (0.028-0.89 M) in 15 s trials showed an inverted U-shaped concentration response function peaking at 0.281 M. Depleted rats (furosemide) showed an identical function, merely elevated, suggesting altered qualitative or hedonic perception but no change in perceived intensity. In Experiment 2, sodium-depleted rats were tested with NaCl, sodium gluconate, and potassium chloride (KCl; 0.028-0.89 M) similar to Experiment 1. KCl was licked at the same rate as water except for a slight elevation at 0.158; sodium gluconate and NaCl were treated similarly, but rats showed more licking for hypertonic sodium gluconate than hypertonic NaCl. Sodium-depleted rats were also tested with NaCl mixed in amiloride (10-300 μM). Amiloride reduced licking but did not alter the shape of the concentration-response function. Collectively, these results suggest that transduction of sodium by epithelial sodium channels (which are blocked by amiloride and are more dominant in sodium gluconate than NaCl transduction) is crucial for the perception of sodium during physiological sodium depletion. In Experiment 3, sodium-deplete rats were tested with NaCl as in Experiment 1 but after taste aversion conditioning to 0.3 M NaCl or sucrose. Rats conditioned to avoid NaCl but not sucrose failed to express a sodium appetite, strongly suggesting that NaCl does not undergo a change in taste quality during sodium appetite-rats show no confusion between sucrose and NaCl in this paradigm. [ABSTRACT FROM AUTHOR]

Different parts of the mouth vary in their taste responsiveness and gustatory transduction components. However, there have been few attempts to consider regional variation among areas innervated by a single nerve branch or containing only one type of gustatory papilla. Here, we examined whether taste-elicited responses of a single nerve, the chorda tympani (CT), depend on where taste solutions are delivered on the tongue in mice. In experiment 1, multiunit CT responses to NaCl and sucrose were larger if sapid taste solutions were applied to the tongue tip, which contains the anterior-most fungiform papillae, than if they were flowed over fungiform and foliate papillae on the posterior tongue. Further, the epithelial sodium channel (ENaC) blocker amiloride suppressed NaCl responses to a greater degree for the tongue tip. In experiment 2, CT nerve responses were compared between the tongue tip and a region further back that contained only fungiform papillae. NaCl and sucrose solutions applied to posterior fungiform papillae produced smaller responses than did those elicited by the same taste stimuli applied to anterior fungiform papillae on the tongue tip. Amiloride suppressed the response to NaCl delivered to the anterior fungiform but not posterior fungiform papillae. These results indicate that the CT response is tongue-region dependent in the mouse. Furthermore, the spatial location of a fungiform papilla provides important information about its properties, such as whether sodium taste transduction is mediated by amiloride-sensitive ENaCs. [ABSTRACT FROM AUTHOR]

In behavioral experiments, rats perceive sodium carbonate (Na2CO3) as super salty. In fact, when the dissociated Na+ ions are accounted for, rats perceive Na2CO3 as 5× saltier than equinormal concentrations of NaCl. The chorda tympani nerve (CT) responds to salts through at least two receptor mechanisms and is a model system for understanding how salt taste is transmitted to the brain. Here, we recorded CT nerve activity to a broad range of NaCl (3-300 mM) and Na2CO3 (3-300 mN) to investigate why Na2CO3 tastes so salty to rats. Benzamil, a specific epithelial sodium channel (ENaC) antagonist, was used to determine the relative contribution of apical ENaCs in Na2CO3 transduction. The benzamil-insensitive component of CT nerve responses was enhanced by increasing the adapted tongue temperature from 23°C to 30°C. Na2CO3 solutions are alkaline, so we compared neural responses (with and without benzamil) to 100 mM NaCl alone (6.2 pH) and at a pH (11.2 pH) that matched 100 mN Na2CO3. As expected, NaCl responses increased progressively with increasing concentration and temperature. Responses to 3 mN Na2CO3 were greater than 3 mM NaCl with and without benzamil, but the shape of the first log-fold range of was relatively flat. Adjusting the pH of NaCl to 11.2 abolished the thermal enhancement of 100 mN NaCl through the benzamil-insensitive pathway. Rinsing Na2CO3 off the tongue resulted in robust aftertaste that was concentration dependent, thermally sensitive, and benzamil-insensitive. Responses to alkaline NaCl did not recapitulate Na2CO3 responses or aftertaste, suggesting multiple transduction mechanisms for the cations (2Na+) and anion (CO3-2)., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The primary taste of dried bonito dashi is thought to be umami, elicited by inosine 5′-monphosphate (IMP) and l-amino acids. The present study compared the taste qualities of 25% dashi with 5 basic tastes and amino acids using conditioned taste aversion methods. Although wild-type C57BL/6J mice with compromised olfactory systems generalized an aversion of dashi to all 5 basic tastes, generalization was greater to sucrose (sweet), citric acid (sour), and quinine (bitter) than to NaCl (salty) or monosodium l-glutamate (umami) with amiloride. At neutral pH (6.5–6.9), the aversion generalized to l-histidine, l-alanine, l-proline, glycine, l-aspartic acid, l-serine, and monosodium l-glutamate, all mixed with IMP. Lowering pH of the test solutions to 5.7–5.8 (matching dashi) with HCl decreased generalization to some amino acids. However, adding lactic acid to test solutions with the same pH increased generalization to 5′-inosine monophosphate, l-leucine, l-phenylalanine, l-valine, l-arginine, and taurine but eliminated generalization to l-histidine. T1R1 knockout mice readily learned the aversion to dashi and generalized the aversion to sucrose, citric acid, and quinine but not to NaCl, glutamate, or any amino acid. These results suggest that dashi elicits a complex taste in mice that is more than umami, and deleting T1R1 receptor altered but did not eliminate their ability to taste dashi. In addition, lactic acid may alter or modulate taste transduction or cell-to-cell signaling. [ABSTRACT FROM AUTHOR]

Insect odorant receptors (ORs) function as heteromeric odorant-gated ion channels consisting of a conserved coreceptor, Orco, and an odorant-sensitive tuning subunit. Although some OR modulators have been identified, an extended library of pharmacological tools is currently lacking and would aid in furthering our understanding of insect OR complexes. We now demonstrate that amiloride and several derivatives, which have been extensively used as blockers for various ion channels and transporters, also block odorant-gated currents from 2 OR complexes from the malaria vector mosquito Anopheles gambiae. In addition, both heteromeric and homomeric ORs were susceptible to amiloride blockade when activated by VUAA1, an agonist that targets the Orco channel subunit. Amiloride derivatives therefore represent a valuable class of channel blockers that can be used to investigate the pharmacological and biophysical properties of insect OR function. [ABSTRACT FROM AUTHOR]

Heteromeric insect odorant receptors (ORs) form ligand-activated nonselective cation channels in recombinant expression systems. We performed a pharmacological characterization of Drosophila melanogaster and Bombyx mori ORs expressed in the Xenopus laevis oocyte expression system and characterized them using the 2-electrode voltage clamp. We identified amiloride derivatives as high-affinity blockers, which inhibit the ion current through the channel in a low micromolar range. For the heteromeric Drosophila Or47a + DmelOrco receptor, the potency sequence (IC50) is HMA [5-(N,N-hexamethylene)amiloride] (3.9 µM), MIA [5-(N-methyl-N-isobutyl)amiloride] (11.0 µM), and DMA [5-(N,N-dimethyl)amiloride] (113.3 µM). Amiloride itself is nearly ineffective. Other tested insect ORs (Drosophila Or49b + DmelOrco, B. mori BmorOr1 + BmorOrco) were blocked in a similar fashion suggesting that the amiloride derivatives were potential general blockers of all receptor combinations. Our results suggest that pyrazine derivatives of amiloride are useful probes to study the mechanism of chemosensory transduction in insects in more detail. [ABSTRACT FROM AUTHOR]

The objective of this study was to determine whether single nucleotide polymorphisms (SNPs) in the SCNN1A (3), SCNN1B (12), SCNN1G (6), and TRPV1 (10) genes affect salt taste perception. Participants were men (n = 28) and women (n = 67) from the Toronto Nutrigenomics and Health study aged 21–31 years. Taste thresholds were determined using a 3-alternative forced-choice staircase model with solutions ranging from 9×10−6 to 0.5mol/L. Suprathreshold taste sensitivity to 0.01–1.0mol/L salt solutions was assessed using general labeled magnitude scales. None of the SNPs in the SCNN1A and SCNN1G genes were significantly associated with either outcome. In the SCNN1B gene, 2 SNPs in intronic regions of the gene modified suprathreshold taste sensitivity (mean iAUC ± SE). Those homozygous for the A allele of the rs239345 (A>T) polymorphism and the T allele of the rs3785368 (C>T) polymorphism perceived salt solutions less intensely than carriers of the T or C alleles, respectively (rs239345: 70.82±12.16 vs. 96.95±3.75, P = 0.02; rs3785368: 57.43±19.85 vs. 95.57±3.66, P = 0.03) In the TRPV1 gene, the rs8065080 (C>T, Val585Ile) polymorphism modified suprathreshold taste sensitivity where carriers of the T allele were significantly more sensitive to salt solutions than the CC genotype (98.3±3.8 vs. 74.1±8.3, P = 0.008). Our findings show that variation in the TRPV1 and the SCNN1B genes may modify salt taste perception in humans. [ABSTRACT FROM PUBLISHER]

We have demonstrated in humans that Na+ evokes changes in the lingual surface potential (LSP) using a custom chamber. To assess whether a relationship exists between the Na+-evoked changes in the LSP and the intensity of salt taste, we measured the LSP and the intensity of salt taste simultaneously in 7 subjects using test solutions (50, 100, 300, and 1000 mM NaCl) presented in random order. The evoked LSPs and intensity scores correlated with one another well (r2 = 0.992, P < 0.01). We then screened 14 subjects for their ability to discriminate between 100 and 300 mM NaCl using the chamber. Three subjects were consistently capable of distinguishing the salt concentrations. In these 3 subjects, an inhibitor of the epithelial sodium channel, amiloride (10 μM), blocked the ability to distinguish salt concentrations and affected the LSP. These data suggest that the LSP may be a component of the signal transduction system involved in human salt taste. In adept salt tasters, an amiloride-sensitive mechanism appears to have a role in distinguishing salt concentrations. [ABSTRACT FROM PUBLISHER]

The combination of dietary sodium depletion and unilateral chorda tympani (CT) nerve section decreases sodium taste function in the intact CT nerve. However, functional changes have not been examined prior to day 4 postsectioning, even though degenerative and inflammatory responses are robust during that period. Rats received unilateral CT section and/or dietary sodium depletion, accomplished by 2 injections of furosemide and a sodium-restricted diet, on day 0. Surgical controls received sham nerve sectioning. At days 1, 2, 3, or 4, taste responses were recorded from the intact nerve. Functional changes were rapid and unexpected. At day 1 postsectioning, neural responses from the uninjured CT of both control-fed and sodium-depleted animals were reduced. By day 2, however, normal function was restored in control-fed rats, whereas functional deficits persisted in depleted animals. Sodium depletion alone also induced a transient decrease in sodium responses at days 2–3 after furosemide injection. These results demonstrate that distant neural injury can elicit gustatory plasticity regardless of the dietary environment, but normal responses can be restored. We suggest that neutrophils mediate the initial postinjury deficits in taste function, whereas macrophages promote the recovery of normal function. [ABSTRACT FROM AUTHOR]

Amiloride and its derivatives inhibit a number of sensory transduction processes, including some types of chemosensory transduction. Here, we report that pyrazine derivatives of amiloride reversibly inhibit odorant-evoked activity in lobster olfactory receptor neurons. The potency sequence is as follows—(IC50, mM): 5-(N,N-hexamethylene)amiloride (0.015) ∼ 5-(N-methyl-N-isobutyl)amiloride (0.02) ∼ 5-(N-ethyl-N-isopropyl)amiloride (0.03) > 5-(N,N-dimethyl)amiloride (0.48); 3′,4′-dichlorobenzamil (0.4), phenamil (0.5), and amiloride itself (2) are ineffective. The same derivatives with the similar potency sequence also block a presumptive transient receptor potential (TRP) channel that is the likely downstream target of phosphoinositide signaling in these cells. Our results suggest that pyrazine derivatives of amiloride are useful probes to study more detailed mechanisms of chemosensory transduction in this system and possibly in other chemosensory systems in which TRP channels are the known or suspected downstream effector. [ABSTRACT FROM AUTHOR]

Inosine monophosphate (IMP) and guanosine monophosphate (GMP) elicit an umami taste in humans and synergistically increase the intensity of the umami taste of monosodium glutamate (MSG). Conditioned taste aversion (CTA) studies in rodents indicate that these nucleotides and MSG elicit quite similar tastes, but recent physiological evidence suggests that these nucleotides and MSG may not activate the same population of taste receptors and therefore may not elicit identical taste qualities. This study reports the findings of several behavioral experiments with rats that compared the taste properties of IMP and GMP with each other and with those of MSG. Well-trained rats were able to detect both nucleotides at nanomolar concentrations, but they did not respond to either nucleotide in two-bottle preference tests or brief-access CTA tests at concentrations less than 0.5 mM. Discrimination experiments found that the tastes of these nucleotides could not be discriminated from each other, but both could be discriminated from MSG, even when the taste of Na+ was controlled. Overall, these experiments indicate the taste properties of the two 5′-ribonucleotides are quite similar to each other, and even though they may elicit an umami sensation, these sensations are not identical to the taste of MSG. [ABSTRACT FROM AUTHOR]

Both amiloride-sensitive and -insensitive mechanisms contribute to NaCl taste transduction. The amiloride-sensitive mechanism relies on the epithelial Na+ channel ENaC, which is widely expressed on the apical membrane of fungiform taste cells. The amiloride-insensitive mechanism, which predominates in circumvallate and foliate taste buds, was recently reported to involve a variant of the nonselective cation channel TRPV1. We performed 2-bottle preference and threshold experiments with TRPV1 knockout mice and wild-type (C57BL/6J) controls to test for NaCl preference and detection thresholds in the presence and absence of amiloride. Surprisingly, TRPV1 knockout mice not only detected NaCl in the presence of amiloride but they preferred NaCl over water at concentrations avoided by the wild-type mice. NaCl detection thresholds were between 2 and 3 mM for both genotypes. Amiloride increased the detection thresholds of wild-type mice but not knockout mice. The knockout mice also preferred 100 mM KCl compared with wild-type controls, suggesting that TRPV1 receptors may mediate a general aversive response to salts. Analyses of consumption data also revealed that TRPV1 knockout mice ingested more of the NaCl, with and without amiloride, and KCl solutions than the wild-type mice. However, comparisons of preference ratios and consumption volumes indicated that both wild-type and TRPV1 knockout mice avoided citric acid in quite a similar manner, suggesting that TRPV1 receptors do not mediate the detection of citric acid. These data, taken together, suggest that additional mechanisms must contribute to the amiloride-insensitive NaCl response. [ABSTRACT FROM AUTHOR]

Amiloride, a sodium channel blocker, is known to suppress NaCl responses of the chorda tympani (CT) nerve in various mammalian species. In mice, the NaCl suppressing effect of amiloride is reported to differ among strains. In C57BL mice, amiloride inhibits NaCl responses to about 50% of control, whereas no such clear suppression was evident in prior studies with 129 mice. However, evidence from behavioral studies is not entirely consistent with this. Recently, it has been found that genetic backgrounds of 129 mice differ within substrains. 129X1/SvJ (formerly 129/SvJ) mice differ from the 129P3/J (formerly 129/J) strain by 25% of sequence length polymorphisms. Therefore, we examined possible substrain difference between 129P3/J and 129X1/SvJ mice in the amiloride sensitivity of electrophysiologically recorded NaCl responses. Amiloride significantly suppressed CT responses to NaCl without affecting responses to KCl both in 129P3/J and 129X1/SvJ mice. However, the magnitude of the amiloride inhibition was significantly larger (∼50% of control in response to 0.01–1.0 M NaCl by 100 μM amiloride) in 129X1/SvJ than in 129P3/J mice (∼20% of control in response to 0.03–0.3 M NaCl by 100 μM amiloride). Threshold amiloride concentration for suppression of responses to 0.3 M NaCl was 30 μM in 129P3/J mice, which was higher than that in 129X1/SvJ mice (10 μM). In 129X1/SvJ mice, the threshold amiloride concentration eliciting inhibition of NaCl responses and the magnitude of the inhibition were comparable with those in C57BL/6 mice. These results suggest that amiloride sensitivity of NaCl responses differs even among the 129 substrains, 129P3/J and 129 X1/SvJ, and the substrain difference of 129 mice in amiloride sensitivity is as large as that between two inbred strains (129P3/J and C57BL/6). [ABSTRACT FROM AUTHOR]

Our previous electrophysiological study demonstrated that amiloride-sensitive (AS) and -insensitive (AI) components of NaCl responses recovered differentially after the mouse chorda tympani (CT) was crushed. AI responses reappeared earlier (at 3 weeks after the nerve crush) than did AS ones (at 4 weeks). This and other results suggested that two salt-responsive systems were differentially and independently reformed after nerve crush. To investigate the molecular mechanisms of formation of the salt responsive systems, we examined expression patterns of three subunits (α, β and γ) of the amiloride-sensitive epithelial Na+ channel (ENaC) in mouse taste cells after CT nerve crush by using in situ hybridization (ISH) analysis. The results showed that all three ENaC subunits, as well as α-gustducin, a marker of differentiated taste cells, were expressed in a subset of taste bud cells from an early stage (1–2 weeks) after nerve crush, although these taste buds were smaller and fewer in number than for control mice. At 3 weeks, the mean number of each ENaC subunit and α-gustducin mRNA-positive cells per taste bud reached the control level. Also, the size of taste buds became similar to those of the control mice at this time. Our previous electrophysiological study demonstrated that at 2 weeks no significant response of the nerve to chemical stimuli was observed. Thus ENaC subunits appear to be expressed prior to the reappearance of AI and AS neural responses after CT nerve crush. These results support the view that differentiation of taste cells into AS or AI cells is initiated prior to synapse formation. [ABSTRACT FROM AUTHOR]

Recent research has implicated T1R1/T1R3 as the primary taste receptor in mammals for detecting l-amino acids, including l-monosodium glutamate (MSG) and l-alanine. Previous behavioral studies with rodents found only minimal evidence that these two substances share perceptual qualities, but those studies did not control for the taste of sodium associated with MSG. This study used several behavioral methods to compare the perceptual qualities of MSG and l-alanine in rats, using amiloride (a sodium channel blocker) to reduce the sodium component of MSG taste. Detection thresholds of l-alanine in rats ranged between 0.4 and 2.5 mM, with or without amiloride added, which are similar to threshold estimates for MSG. Conditioned taste aversion (CTA) found that rats showed strong cross-generalization of CTA between MSG and l-alanine when mixed with amiloride, indicating the two substances have similar perceptual qualities. Discrimination methods showed that rats easily discriminated between l-alanine and MSG unless the cue function of sodium was reduced. The discrimination became significantly more difficult at concentrations <100 mM when amiloride was added to all stimuli and became even more difficult when NaCl was also added to l-alanine solutions to match the sodium concentrations of MSG. These results indicate that, perceptually, MSG and l-alanine have quite similar taste qualities and support the hypothesis that these two l-amino acids activate a common taste receptor. The differences in perceptual qualities also suggest separate afferent processing of one or both substances may also be involved. [ABSTRACT FROM AUTHOR]

Dehydrated toads absorb water by pressing a specialized (seat patch) area of the skin to moist surfaces. This behavior, the water absorption response (WR), is preceded by periods of more limited skin contact (seat patch down, SPD) in which the suitability of the rehydration source is evaluated. WR and SPD behaviors were suppressed on 250 mM NaCl and 200 mM KCl solutions. Ten micromolar amiloride partially restored SPD and WR on NaCl solutions. The addition of 5 mM La3+ also partially restored the initiation of WR and this effect was additive to the effect of amiloride, suggesting transcellular and paracellular pathways exist in parallel. Similarly, 5 mM La3+ partially restored the initiation of WR on KCl solutions, to levels comparable to those with K+gluconate, suggesting a paracellular pathway for detection of K+. Hyperosmotic (250 mM) NaCl solutions bathing the mucosal surface rapidly and reversibly increased the paracellular conductance of isolated skin and this increase was partially inhibited by 5 mM La3+. These results suggest that the regulation of tight junctions has a chemosensory role in toad skin. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 µM 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. [ABSTRACT FROM AUTHOR]

Taste aversion studies have demonstrated that rats conditioned to avoid monosodium glutamate (MSG) with amiloride added to reduce the intensity of the sodium component of MSG taste, generalize this aversion to aspartic acid and to L-AP4, but not to ionotropic glutamate receptor agonists. That is, MSG, L-AP4 and aspartate have similar tastes to rats. However, conditioned taste aversion methods are unable to show to what extent the tastes of two substances are different. If two substances activate the same afferent processes (e.g. taste receptors), they are likely to produce the same tastes, but if they activate different afferent processes, the subject may detect differences between the tastes of the substances. In this study, rats were tested to determine if they could discriminate between the tastes of these agonists and MSG. We also established the detection thresholds for NMDA, aspartic acid and L-AP4, with and without amiloride (a sodium channel antagonist). Taste threshold values were 1–4 mM for NMDA and aspartic acid and 0.5–2.5 µM for L-AP4. None were affected by 30 µM amiloride. Rats could readily distinguish between the tastes of MSG and NMDA but they had difficulty discriminating between the tastes of aspartic acid and MSG. Rats could also easily distinguish between 10–100 mM MSG and 0.01–5 mM L-AP4. However, in two separate experiments error rates increased significantly when L-AP4 concentrations were between 10–100 mM, indicating that the tastes of L-AP4 and MSG were similar at these concentrations. [ABSTRACT FROM AUTHOR]

The epithelial sodium channel (ENaC) blocker amiloride has been shown to increase the behaviorally measured NaCl detection threshold in mice. In this study, a conditioned taste aversion (CTA) paradigm was used to examine whether 100 µM amiloride has a perceptible taste that could contribute to this observed decrease in behavioral responsiveness. Eighty-four C57BL/6J (B6) and 64 DBA/2J (D2) mice were divided into eight groups (n = 8–12 per group), in which half received an injection of 0.15 M LiCl (2 mEq/kg) and the other half an equivalent saline injection, in three conditioning trials. The four conditioned stimuli were 100 µM amiloride hydrochloride, water, 0.1 and 0.3 M NaCl. Neither strain demonstrated acquisition of a CTA to amiloride in a brief-access (BA) taste test (5 s trials in the gustometer). Although 0.3 M NaCl is inherently aversive, its pairing with LiCl led to significantly further decreases in licking during the BA test on salt trials in both strains. The D2 strain clearly avoided 0.1 M NaCl, whereas avoidance of this stimulus was more equivocal in B6 mice. The inefficacy of amiloride to serve as a conditioned stimulus in taste aversion learning involving three LiCl pairings suggests that the effects of this ENaC blocker on taste-related behavioral responses to NaCl are likely due to its pharmacological interference with sodium taste transduction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Sodium taste transduction is thought to occur via an amiloride-sensitive, sodium-selective pathway and an amiloride-insensitive, cation nonselective, anion-dependent pathway(s). It has been shown by others that amiloride, an epithelial sodium channel (ENaC) blocker, significantly reduces the chorda tympani nerve response to lingually applied NaCl in C57BL/6 (B6) mice but not in DBA/2 (D2) mice, suggesting that the latter strain might not possess functional ENaCs in taste receptor cells. We psychophysically measured and compared taste detection thresholds of NaCl and sodium gluconate (NaGlu) prepared with and without 100 μM amiloride in these two strains (eight/strain). Mice were trained and tested in a two-response operant signal detection procedure conducted in a gustometer. Surprisingly, no strain effect was found for the detection thresholds of both salts (∼0.05–0.06 M). Moreover, these thresholds were increased by almost an order of magnitude by amiloride adulteration of the solutions. This marked effect of amiloride on sodium detection thresholds suggests that ENaCs are necessary for normal sensitivity to sodium salts in both strains. In addition, because NaGlu is thought to stimulate primarily the amiloride-sensitive pathway, especially at low concentrations, the similarity of NaCl and NaGlu thresholds (r > 0.81 both strains) suggests that ENaCs are also sufficient to support the detection of sodium in weak solutions by B6 and D2 mice. [ABSTRACT FROM AUTHOR]

Abstract [ABSTRACT FROM AUTHOR]

Studies in the last two decades have shown that amiloride-sensitive Na+ channels play a role in NaCl transduction in rat taste receptors. However, this role is not readily generalized for salt taste transduction in vertebrates, because functional expression of these channels varies across species and also in development in a species. Glossopharyngeal nerve responses to sodium and potassium salts were recorded in larval and metamorphosed salamanders and compared before and after the oral floor was exposed to amiloride, a blocker of Na+ channels known to be responsible for epithelial ion transport. Pre-exposure to amiloride (100 μM) did not affect salt taste responses in both axolotls (Ambystoma mexicanum) and larval Ezo salamanders (Hynobius retardatus). In contrast, in metamorphosed Ezo salamanders the nerve responses to NaCl were significantly reduced by amiloride. In amphibians amiloride-sensitive components in salt taste transduction seem to develop during metamorphosis. [ABSTRACT FROM AUTHOR]

The effects of inhibitors [acetazolamide, an inhibitor of carbonic anhydrase; amiloride, an inhibitor of the Na channel; furosemide, an inhibitor of the Na/K/2Cl transporter; 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS), an inhibitor of the Cl channel] on the water response in the superior laryngeal nerve (SLN) were investigated using whole nerve recordings from the SLN of anesthetized and paralyzed rats. Changes in spontaneous activity in the SLN after i.v. injection of a hypo- or hypertonic solution were also investigated. The water response to higher concentration amiloride solutions (0.1, 1, 5 and 10 mM) were significantly smaller in comparison with the control, i.e. the water response to deionized water (88–59% of the control, Fisher’s PLSD, P < 0.05). DIDS suppressed the water response significantly at concentrations of 0.5 and 2 mM by 18 and 33%, respectively (P < 0.05). Likewise, acetazolamide (2 mM) and furosemide (5 mM) significantly suppressed the water response by 9 and 40%, respectively (P < 0.05). An i.v. bolus injection of a hypertonic solution (1 ml of 1.5 M NaCl or 1.0 M mannitol) depressed spontaneous activity of the SLN. In contrast, an i.v. injection of a hypotonic solution (0.015 M NaCl) increased spontaneous activity. These results suggest that several ion transporters and ion channels, as well as carbonic anhydrase, that may exist in the dorsal surface in the epiglottis may regulate the water response in the SLN and that osmotic changes in the dorsal surface of the epiglottis and in the interstitial space can affect nerve activity in the SLN. [ABSTRACT FROM AUTHOR]

The oral sensation elicited by carbonated water is reduced by capsaicin and by blockers of carbonic anhydrase. We have investigated the temporal profile of this sensation and its cross-desensitization by capsaicin. We additionally tested if the sensation is influenced by amiloride. Following pretreatment of half of the dorsal tongue with 33 p.p.m. capsaicin, carbonated water was flowed over the tongue bilaterally for 5, 15 or 60 s. Subjects then performed a two-alternative forced choice test by indicating which side of the tongue had a stronger sensation and separately rated the sensory intensity on each side. Capsaicin significantly reduced the intensity of sensation elicited by carbonated water, consistent with cross-desensitization. This effect was weaker at 60 s because of a significant decline (desensitization) in ratings of the intensity of carbonated water on both sides of the tongue. Pretreatment with amiloride resulted in a small but significant increase in the intensity of the sensation elicited by the 15 s carbonated water stimulus, suggesting an amiloride-sensitive transduction mechanism. [ABSTRACT FROM AUTHOR]

The transduction mechanism of salt-induced responses of mouse taste cells was investigated using the patch clamp and the local stimulation techniques under quasi-natural conditions. Apically applied NaCl induced a voltage-independent current, which was partially suppressed by amiloride and Cd[sup 2+]. In contrast, apically applied 0.5 M KCl induced an inwardly rectifying current (KCl-induced I [sub ir]). The KCl-induced I [sub ir] was unaffected by amiloride. The I [sub ir] was suppressed not only by external Ba[sup 2+] and Cs[sup +], but also by a Cl[sup –] channel blocker, niflumic acid. The E [sub r] of the KCl-induced response was independent of the apical ionic concentration, but rather was close to the equilibrium potential of Cl[sup –] (E [sub Cl]) at the basolateral membrane. The KCl-induced I [sub ir] displayed a fast run-down under the conditions of the conventional whole cell clamp method, but not under the perforated patch conditions. Immunohistochemical localization of an inwardly rectifying Cl[sup –] channel protein, ClC-2, was observed in taste bud cells of the fungiform papillae. It is concluded that the transduction mechanism of NaCl-induced responses is completely different from that of KCl-induced responses in mouse taste cells. [ABSTRACT FROM AUTHOR]

In a first experiment, human subjects used a bipolar scale to rate the irritant sensation elicited by 10 sequentially repeated applications of either 3 ppm capsaicin or 250 mM citric acid on one side of the dorsal surface of the tongue, at 1 min intervals (30 s inter-stimulus interval). Citric acid-evoked irritation significantly increased across trials, consistent with sensitization. With capsaicin there was a large degree of inter- and intra-individual variation in successive ratings with no overall sensitization. Following the sequential stimulation series and a 10 min rest period, self- and cross-desensitization effects were tested in a two-alternative forced choice (2-AFC) paradigm by placing either citric acid or capsaicin on both sides of the tongue and asking subjects to indicate which side of the tongue yielded a stronger irritant sensation. Subjects also gave separate intensity ratings for irritation on each side of the tongue. Capsaicin self-desensitization was confirmed, while cross-desensitization to citric acid was not observed. In addition, citric acid self-desensitization and cross-desensitization to capsaicin were observed. In a second experiment a stronger capsaicin solution (33 ppm) was applied to one side of the tongue using cotton swabs. After the burning sensation elicited by capsaicin had disappeared, citric acid was applied bilaterally and cross-desensitization was observed using the same 2-AFC and rating procedures. This was followed by repeated re-application of citric acid at 1 min intervals to the capsaicin-treated side. The irritant sensation elicited by citric acid increased significantly, indicating a ‘cross-stimulus-induced recovery’ from capsaicin desensitization. In a final experiment we investigated the effect of the sodium channel blocker amiloride on the perceived irritation elicited by citric acid or capsaicin. Following application of amiloride to one side of the tongue with cotton swabs, either citric a... [ABSTRACT FROM AUTHOR]