Metallic taste from electrical and chemical stimulation

A series of three experiments investigated the nature of metallictaste reports after stimulation with solutions of metal saltsand after stimulation with metals and electric currents. Tostimulate with electricity, a device was fabricated consistingof a small battery affixed to a plastic handle with the anodeside exposed for placement on the tongue or oral tissues. Intensityof taste from metals and batteries was dependent upon the voltageand was more robust in areas dense in fungiform papillae. Metallictaste was reported from stimulation with ferrous sulfate solutions,from metals and from electric stimuli. However, reports of metallictaste were more frequent when the word ‘metallic’was presented embedded in a list of choices, as opposed to simplefree-choice labeling. Intensity decreased for ferrous sulfatewhen the nose was occluded, consistent with a decrease in retronasalsmell, as previously reported. Intensity of taste evoked bycopper metal, bimetallic stimuli (zinc/copper) or small batteries(1.5–3 V) was not affected by nasal occlusion. This differencesuggests two distinct mechanisms for evocation of metallic tastereports, one dependent upon retronasal smell and a second mediatedby oral chemoreceptors.     

Using biosensors to detect the release of serotonin from taste buds during taste stimulation

CHO cells transfected with high-affinity 5HT receptors were used to detect and identify the release of serotonin from taste buds. Taste cells release 5HT when depolarized or when stimulated with bitter, sweet, or sour tastants. Sour- and depolarization-evoked release of 5HT from taste buds is triggered by Ca2+ influx from the extracellular fluid. In contrast, bitter- and sweet-evoked release of 5HT is triggered by Ca2+ derived from intracellular stores.     

Acid stimulation (sour taste) elicits GABA and serotonin release from mouse taste cells

Several transmitter candidates including serotonin (5-HT), ATP, and norepinephrine (NE) have been identified in taste buds. Recently, γ-aminobutyric acid (GABA) as well as the associated synthetic enzymes and receptors have also been identified in taste cells. GABA reduces taste-evoked ATP secretion from Receptor cells and is considered to be an inhibitory transmitter in taste buds. However, to date, the identity of GABAergic taste cells and the specific stimulus for GABA release are not well understood. In the present study, we used genetically-engineered Chinese hamster ovary (CHO) cells stably co-expressing GABA(B) receptors and Gαqo5 proteins to measure GABA release from isolated taste buds. We recorded robust responses from GABA biosensors when they were positioned against taste buds isolated from mouse circumvallate papillae and the buds were depolarized with KCl or a stimulated with an acid (sour) taste. In contrast, a mixture of sweet and bitter taste stimuli did not trigger GABA release. KCl- or acid-evoked GABA secretion from taste buds was Ca(2+)-dependent; removing Ca(2+) from the bathing medium eliminated GABA secretion. Finally, we isolated individual taste cells to identify the origin of GABA secretion. GABA was released only from Presynaptic (Type III) cells and not from Receptor (Type II) cells. Previously, we reported that 5-HT released from Presynaptic cells inhibits taste-evoked ATP secretion. Combined with the recent findings that GABA depresses taste-evoked ATP secretion, the present results indicate that GABA and 5-HT are inhibitory transmitters in mouse taste buds and both likely play an important role in modulating taste responses.     

Mechanism of stimulation of insect taste cells by organic substances

A plasma membrane is considered to have as its framework a bimolecular leaflet built up mainly from phospholipids but in which helical segments of proteins may also be present.     

N-substituted maleimide inactivation of the response to taste cell stimulation

N-Ethylmaleimide (NEM) irreversibly inactivates the response of gustatory cells to stimulation by NaCl, sucrose and hydrogen ions. The rate of inactivation can be measured by monitoring the decay of NaCl-stimulated summated electrophysiological activity at the chorda tympani nerve in the presence of NEM. The observed pseudo first-order rate constants are linear with NEM concentration, and the second-order rate constant is 0.38 M^?1 sec^?1. Other N-substituted maleimides, such as N-methylmaleimide and N-butylmaleimide, which have ether:water partition coefficients similar to NEM, inactivate the NaCl-stimulated response at rates comparable to NEM. However, the hydrophobic derivative, 4-(N-maleimido)phenyltrimethylammonium has a significantly lower ether:water partition coefficient and is essentially ineffective as an inactivator of the NaCl response. These results, together with the observation that the inactivation rate is independent of pH between 4.5 and 7.0, indicate the inactivation site is either intracellular or buried within the cell membrane at a locus inaccessible to most extracellular fluids. The rate of inactivation of the sucrose and HCl responses were measured indirectly and found to be comparable to the NaCl-stimulated inactivation rate, indicating the inhibited event is common to the transduction of the response for all of the stimuli examined. Possible sites of inactivation by N-Substituted maleimides are considered in the context of the results. This kinetic approach should have application in searching for and characterizing receptor-specific as well as other classes of taste cell inhibitors.     

A question of taste

A career in science is shaped by many factors, one of the most important being our tastes in research. These typically form early and are shaped by subsequent successes and failures. My tastes run to microscopes, chemistry, and spatial organization of cytoplasm. I will try to identify where they came from, how they shaped my career, and how they continue to evolve. My hope is to inspire young scientists to identify and celebrate their own unique tastes.     

Bilateral taste stimulation: spatial summation with weak NaCl stimuli

Recently, we reported that doubling the stimulated area on oneside of the tongue increased the detection probabilities oflow intensity NaCl stimuli according to a simple probabilitysummation model. In the present study spatial summation wasinvestigated with both ipsilateral and bilateral stimulation.For thirteen subjects the sensitivity to and the detection probabilitiesof five low intensity NaCl stimuli were determined. Stimulationwas performed with filter papers of two different sizes underthree main conditions: a circular area on either of the twotongue sides, half that circle on either of the two tongue sidesand two half circles on both sides of the tongue simultaneously.The half circles were presented in two orientations, a medialand a lateral one. The results showed that (a) doubling thestimulated area yielded the same increase in sensitivity forthe ipsilateral conditions as for the bilateral conditions;(b) the detection probabilities for both the ipsilateral andthe bilateral double-sized stimuli could be predicted accordingto the probability summation hypothesis and (c) the medial andlateral parts of the tongue were equally sensitive to NaCl inthe ipsilateral case, but showed significant differences withbilateral stimulation.     

N-substituted maleimide inactivation of the response to taste cell stimulation.

N-Ethylmaleimide (NEM) irreversibly inactivates the response of gustatory cells to stimulation by NaCl, sucrose and hydrogen ions. The rate of inactivation can be measured by monitoring the decay of NaCl-stimulated summated electrophysiological activity at the chorda tympani nerve in the presence of NEM. The observed pseudo first-order rate constants are linear with NEM concentration, and the second-order rate constant is 0.38 M-1 sec-1. Other N-substituted maleimides, such as N-methylmaleimide and N-butylmaleimide, which have ether:water partition coefficient and is essentially ineffective as an inactivator of the NaCl response. These results, together with the observation that the inactivation rate is independent of pH between 4.5 and 7.0, indicate the inactivation site is either intracellular or buried within the cell membrane at a locus inaccessible to most extracellular fluids. The rate of inactivation of the sucrose and HCl responses were measured indirectly and found to be comparable to the NaCl-stimulated inactivation rate, indicating the inhibited event is common to the transduction of the response for all of the stimuli examined. Possible sites of inactivation by N-substituted maleimides are considered in the context for and characterizing receptor-specific as well as other classes of taste cell inhibitors.     

Slow potentials in taste cells induced by frog glossopharyngeal nerve stimulation

Intracellular recordings were made from the taste cells of atropinized bullfrogs while the glossopharyngeal (GP) nerve fibres were electrically stimulated. Two types of slow potential, slow hyperpolarizing potentials (HPs) and slow depolarizing potentials (DPs), were induced in the taste cells. The slow HPs appeared when the lingual capillary blood flow was kept above 0.7 mm/s, whereas the slow DPs appeared when the blood flow was slowed down below 0.7 mm/s. The membrane resistance of a taste cell increased during the generation of a slow HP, but decreased during the generation of a slow DP. The reversal potentials for the slow HPs and the slow DPs were recorded at the same membrane potential (-11 to approximately -13 mV). Activation of non-selective cation channels possibly induced the slow DP and inactivation of those channels possibly induced the slow HP in the taste cell membrane. Electrical stimulation of the GP nerve activated a population of C fibres in the nerve and possibly released neurotransmitters from the nerve terminals. Released neurotransmitters might cause modulation of the membrane conductance in taste cells that leads to generation of the slow potentials. The present data suggest that slow HPs and slow DPs evoked in the taste cells of atropinized frogs by GP nerve stimulation are induced by putative neurotransmitters in the taste disc.     

Selective procaine inhibition of rat chorda tympani responses to electric taste stimulation

1. The lingual treatment of 1% procaine for 10 min selectively suppressed responses of the rat chorda tympani nerve to anodal current applied to the tongue with NaCl in the bathing medium to about 50% of control but the drug produced no significant suppression in responses to chemical taste stimuli. 2. The magnitude of suppression of response to anodal current varied with concentration of procaine and kind of bathing medium for the current stimulation (larger in the order of NaCl greater than KCl greater than CaCl2 greater than HCl). 3. Such ion specificity in procaine suppression suggests that responses of the chorda tympani nerve to anodal current are provoked through the taste cell (not direct action on the taste nerve), and that the receptor mechanisms for anodal current are at least partly different from that for chemical taste stimuli.     

Quantum theory of time-varying stimulation in nerve

The equation for the quantum transitions (spontaneous and stimulated) of membrane dipoles is solved for the various forms of time-varying stimulation in nerve. From the condition of ever-increasing dipole population in the upper state, the threshold for excitation is determined in each case. The results obtained are in agreement with the established facts. The optimum frequency for stimulation is given asv ₀=0.0615/T ₂ whereT ₂ is the dipole relaxation time. The feature of the theory is that the mathematical formulation is based upon a physical mechanism and the results can thus provide some understanding in the observed phenomena.     

A physico-chemical theory of sweet and bitter taste excitation based on the properties of the plasma membrane

Summary This paper is concerned with physical and chemical changes occurring in the taste buds due to the presence of sapid substances, and not with the nervous mechanism transmitting impulses to the brain. From this standpoint taste phenomena are closely allied with general cell physiology or pharmacology. It has been shown that the great majority of anesthetic or toxic substances have a bitter taste and it is considered that the same structural changes occur in the cell membranes in taste buds as in other cells of the body when such substances are present. From consideration of a large amount of experimental data on the taste of organic compounds it is shown that thesweet and bitter substances are chemically very closely related and there is no line of demarcation between them. In many homologous series there is a continuous (rather than discontinuous) transition from sweet to bitter. Sweet substances having a bitter after-taste are also common.It has been pointed out that it is a general property of anesthetics to be stimulants (i. e. to accelerate cell activities)when in low concentrations. The continuous transition from stimulant to narcotic action is correlated with the sweet-bitter transition. FollowingClowes, Lillie and others the cell membrane is looked upon as a balanced emulsion partly of the water in oil type and partly of the reverse type.The phenomena cited above have been explained in terms of adsorption and surface tension action of substances on this emulsion system and the consequent changes in cell membrane permeability due to changes in the phase ratio of the emulsion types.     

Salivary secretion elicited by taste stimulation with umami substances in human adults

Salivary secretion from the parotid gland was measured whenumami substances (monosodium L-glutamatc, disodium 5'-inosinateand disodium 5'-guanylate) were applied to the anterior or posteriorparts of the tongue in human adults. The amount of secretioninduced by a low concentration (0.01 M) of umami substanceswas very small and similar to that elicited by 1 M sucrose.Salivation induced by a high concentration (0.1 M) of umamisubstances was about twice that induced by the low concentration,but less than that induced by 0.1 M NaCl. The volume of secretioninduced by a mixture containing the low concentration of bothmonosodium L-glutamate and disodium 5'-inosinate (or disodium5'-guanylate) approximated the arithmetic sum of secretionsinduced by the individual components, whereas a mixture of thehigh concentration of these substances elicited much less salivationthan the sum of secretions for the components. More copiousflow of parotid saliva tended to be induced from stimulatingthe posterior tongue compared to the anterior tongue. The presentresults are discussed in terms of the taste properties of umamisubstances as shown in previous psychophysical and elcctrophysiologicalstudies.