Salt Receptor Cell Research Articles

Age-related electrophysiological changes in mouse taste receptor cells.

What is the central question of this study? Loss of taste or inability to distinguish between different tastes progresses with age. The purpose was to evaluate the age-dependent changes in taste by studying the electrophysiological properties of taste receptor cells. What is the main finding and its importance? Ageing decreased the voltage-gated Na+ and K+ current densities of type III cells (sour and/or salt receptor cells) but did not affect the current densities in type II cells. At the peripheral levels, the excitability of type III cells was reduced due to ageing, which may affect the signal transduction to taste nerves. The loss of taste due to normal ageing in mammals is assumed to be caused by the ageing of taste receptor cells. We examined the electrophysiological properties of taste receptor cells in the fungiform taste buds of ∼20-month-old mice in situ and subsequently identified their cell types with immunological markers: the inositol 1,4,5-trisphosphate (IP3 ) receptor (IP3 R3) for type II cells and a SNARE protein, synaptosomal-associated protein 25 (SNAP-25), for type III cells. Other cells are referred to as non-immunoreactive cells (non-IRCs). Cell types of some cells that could not be identified using cell-type markers were identified based on the electrophysiological feature of the respective cell types. All cell types generated action potentials and a variety of voltage-gated currents. The type II cells mainly expressed tetraethylammonium (TEA)-insensitive and slowly activating outwardly rectifying currents and generated tail currents in repolarization. In contrast, the type III cells expressed TEA-sensitive and faster activating K+ currents and did not generate tail currents. These cell type-specific characteristics of voltage-gated currents in ∼20-month-old mice were similar to their respective cell types in ∼2-month-old mice. Also, we showed an age-dependent decrease in Na+ and K+ current densities in type III cells and an age-dependent increase in outwardly rectifying current density in non-IRCs. Ageing did not affect the voltage-gated current densities in type II cells. The decreased Na+ and K+ current densities, i.e. the decreased excitability of type III cells, due to ageing may affect the signal transduction to taste nerves.

Quantitative Analysis of Taste Bud Cell Numbers in the Circumvallate and Foliate Taste Buds of Mice.

A mouse single taste bud contains 10-100 taste bud cells (TBCs) in which the elongated TBCs are classified into 3 cell types (types I-III) equipped with different taste receptors. Accordingly, differences in the cell numbers and ratios of respective cell types per taste bud may affect taste-nerve responsiveness. Here, we examined the numbers of each immunoreactive cell for the type II (sweet, bitter, or umami receptor cells) and type III (sour and/or salt receptor cells) markers per taste bud in the circumvallate and foliate papillae and compared these numerical features of TBCs per taste bud to those in fungiform papilla and soft palate, which we previously reported. In circumvallate and foliate taste buds, the numbers of TBCs and immunoreactive cells per taste bud increased as a linear function of the maximal cross-sectional taste bud area. Type II cells made up approximately 25% of TBCs irrespective of the regions from which the TBCs arose. In contrast, type III cells in circumvallate and foliate taste buds made up approximately 11% of TBCs, which represented almost 2 times higher than what was observed in the fungiform and soft palate taste buds. The densities (number of immunoreactive cells per taste bud divided by the maximal cross-sectional area of the taste bud) of types II and III cells per taste bud are significantly higher in the circumvallate papillae than in the other regions. The effects of these region-dependent differences on the taste response of the taste bud are discussed.

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.

Electrophysiological studies of salty taste modification by organic acids in the labellar taste cell of the blowfly.

Using the labellar salt receptor cells of the blowfly, Phormia regina, we electrophysiologically showed that the response to NaCl and KCl aqueous solutions was enhanced and depressed by acetic, succinic and citric acids. The organic acid concentrations at which the most enhanced salt response (MESR) was obtained were found to be different: 0.05-1 mM citric acid, 0.5-2 mM succinic acid and 5-50 mM acetic acid. Moreover, the degree of the salt response was not always dependent on the pH values of the stimulating solutions. The salt response was also enhanced by HCl (pH 3.5-3.0) only when the NaCl concentration was greater than the threshold, indicating that the salty taste would be enhanced by the comparatively lower concentrations of hydrogen ions. Another explanation for the enhancement is that the salty taste may also be enhanced by undissociated molecules of the organic acids, because the MESRs were obtained at the pH values lower than the pKa(1) or pKa(2) values of these organic acids. On the other hand, the salty taste could be depressed by both the lower pH range (pH 2.5-2.0) and the dissociated organic anions from organic acid molecules with at least two carboxyl groups.

Responsiveness of antennal taste hairs of the apterygotan insect, Thermobia domestica (Zygentoma); an electrophysiological investigation

Antennal taste hairs of Thermobia domestica were investigated by a modified tip recording technique. They are sensitive to aqueous salt solutions but do not show the characteristic features of a water and a salt receptor cell. Sucrose and fructose stimulated approximately half of the tested hairs. As revealed by spike amplitude histograms, in most cases two sense cells responded. Proline stimulated the hairs with a threshold concentration of about 2 mM. Again, in most cases, two sense cells responded. In relation to sucrose the major sensitivities to both fructose and proline were not found in clearly defined receptor cells as in flies, but were realized in different cells of either a higher or smaller spike amplitude. This suggests, that the number of specific binding sites varies among the receptor cells from one taste hair to the other. As in cockroaches, but unlike flies, spike series are mainly generated in the apical dendritic segments. It is suggested that these features of the sense cells in thermobian taste hairs represent a primitive condition in relation to the specification of taste hairs cells in higher insect taxa

Chemosensory control of pollen ingestion in the hoverfly Eristalis tenax by labellar taste hairs.

The labellar gustatory system of the dronefly Eristalis tenax L. (Syrphidae; Diptera) that enables the fly to discriminate between pollen and nectar is investigated, and the triggering of pollen ingestion is examined. In behavioural preference tests, exhaustively extracted pollen of the sunflower Helianthus annuus is consumed in smaller amounts than untreated pollen, indicating that water-soluble substances are important for acceptance. Dry pollen is preferred over moist pollen in which the grains stick together, suggesting that mechanical properties of the pollen also play a role in its sensory assessment. Electrophysiological studies of the labellar taste hairs reveal that aqueous extracts of pollen (2% w/v) stimulate the salt receptor cell, but not the sugar receptor cell. The response of the water receptor cell remains the same as to pure water (or standard electrolyte, 10 mmol.l-1 KCl). Of the 20 amino acids tested, the salt cell is sensitive only to proline in a submillimolar range. Behavioural experiments support the electrophysiological findings. When KCl is applied at concentrations eliciting salt-cell spike frequencies equal to those produced by pollen extract (which is often accepted), the water receptor cell is inhibited and a pronounced rejection behaviour occurs. This rejection of concentrated salt solution in Eristalis is therefore mainly mediated by the inhibition of the water cell.

Multiple receptor sites for nucleotide reception in the labellar taste receptor cells of the fleshfly Boettcherisca peregrina

Nucleotides applied to the labellar chemosensory hair of the fleshfly, Boettcherisca peregrina, stimulated the taste receptor cells. Adenosine 5′-diphosphate (ADP) evoked a large response of the sugar receptor cell (sugar response) and guanosine 5′-diphosphate (GDP) evoked a large response of the salt receptor cell (salt response), but the salt response to ADP and the sugar response to GDP were relatively small. While the sugar response to ADP was independent over a wide range, pH 5–9, the salt responses to GDP and ADP were inhibited at neutral and alkaline pH's, even though they elicited a marked salt response between pH's of about 5 and 6. Only adenine nucleotides (ADP, AMP, ATP) could stimulate the sugar receptor cell, with an order of stimulating effectiveness of ADP>>AMP≥ATP. However, the salt receptor cell could respond significantly not only to GDP but various nucleoside 5′-diphosphates, nucleoside 5′-monophosphates, cyclic nucleotides and thiamine diphosphate. These results clearly suggest that the specificity of the receptor site reacting with nucleotide in the sugar receptor cell is very different from that in the salt receptor cell.

The Effects of G Protein Modulators on the Labellar Taste Receptor Cells of the Fleshfly (Boettcherisca peregrina)

A nonhydrolyzable G protein activator (guanosine 5′-O-(3-thiotriphosphate); GTPγS) and a G protein inhibitor (guanosine 5′-O-(2-thiodiphosphate); GDPβS) were introduced into the labellar taste receptor cells of the fleshfly by treatment of their receptive membranes beneath the tip opening of the chemosensory hair with each reagent in 0.03% deoxycholate solution for 4 min. After treatment with GTPγS, the responses of the sugar receptor cell to d-glucose, d-fructose, l-phenylalanine and l-valine and that of the salt receptor cell to cyclic AMP were markedly enhanced, compared with those after treatment with deoxycholate alone. Treatment with GDPβS depressed these responses. These results strongly suggest that the responses are mediated by G protein. However, the response of the salt receptor cell to NaCl was not affected by treatment with either GTPγS or GDPβS, and thus the response to NaCl clearly is not elicited through a G protein-regulated mechanism. © 1997 Elsevier Science Ltd. All rights reserved

Receptor responses in labellar taste hairs of the housefly Musca domestica to aqueous solutions of KCl and CaCl 2

Responses of largest labellar taste hairs of the housefly Musca domestica were investigated with the tip recording technique. Aqueous solutions of KCl and CaCl 2 evoke responses in the water and/or the salt receptor cell. The water receptor is progressively inhibited by increasing concentrations of KCl. A concentration as low as 23 m m KCl results in a slight depression (compared to 12 m m KCl), 200 m m and 1 m KCl are needed for 50 and 100% inhibition, respectively. Compared to other fly species the salt receptor activity is relatively weak. The threshold lies around 200 m m KCl and there is no indication of saturation at 3 m m KCl. Therefore it is most likely the water receptor with its graded inhibition which encodes low KCl concentrations. The salt receptor is activated, and the water receptor inhibited by 100 m m CaCl 2 in dilute KCl. At comparable receptor activities, the salt receptor displays a shorter latency than the water receptor. The results are compared to data obtained from other fly species. The water and the salt receptor discharge impulses of comparable amplitude and overall shape. The assignment of spikes to the respective receptor types is therefore based on other criteria: regularity of spike trains, latency, susceptivity to CaCl 2.

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.

Simultaneous chemical and electrical stimulation of labellar taste hairs of the blowfly Calliphora vicina

When recording from the tip of insect taste hairs, responses to chemical stimulation may be influenced by electrical currents, such as the preamplifier's input bias current. The effect of electrical currents on firing frequency of the salt receptor cell to KCl and NaCl stimulation was determined in labellar ‘aboral’ and ‘adoral’ taste hairs of the blowfly Calliphora vicina. Negative currents always decreased spike frequency, whereas positive currents either increased it, or did not change it significantly. Spike frequency changed less than 1% per 5 × 10 −11 A. A consistent picture of the electrophysiology of blowfly taste hairs is given. It includes a distal pore, present in the dendrite-free lumen of the hair. It abandons the concept of a generator current that transmits excitation from the distal, chemoreceptive part of the taste cell dendrite to the action potential generator in or near the taste cell body. The experimental results are interpreted on the basis of this picture. It is concluded that the ‘electrophoretic effect’ of the electrical current is very small. Thus, the measured effect should mainly be due to a ‘direct effect’ of electrical current on electrically excitable structures in the salt receptor cell, particularly in its dendrite.

The effects of amino acids on the labellar hair chemosensory cells of the fly.

The effects of amino acids on the labellar hair chemosensory cells were examined with two kinds of flies (the fleshfly, Boettcherisca peregrina, and the blowfly, Phormia regina). As a result of this examination, the effects of amino acids were divided into four main classes. Amino acids in class 1 did not stimulate any chemoreceptor cell. Amino acids in class 2 inhibited nonspecifically the discharges from three kinds of chemosensory cells. Amino acids in class 3 stimulated the salt receptor cell. Amino acids in class 4 stimulated the sugar receptor cell. A possibility that a fourth neuron in the labellar hair chemosensory cell might be a protein or an amino acid receptor cell was eliminated.

Stimulation of a Primary Taste Receptor by Salts

A quantitative study was made of the repetitive response of the salt receptor cell of the blowfly taste receptor. The response begins at a high frequency and declines to a steady frequency during brief stimuli. The initial response was found to be a sigmoid function of the log of stimulus intensity over a short range of intensities. It was shown that a theory (Beidler, 1954; for mammalian salt receptors) that relates the magnitude of the steady response to stimulus intensity applies to this receptor. From the theory, it was calculated that the relative free energy change of the reaction between salt and receptor site was in the range 0 to -1 kcal/mole; and, therefore, the reaction probably involves weak physical forces. Evidence is given that the salt-combining sites of the receptor are anionic and strongly acidic and that consequently the cation of a salt largely dominates stimulation. Preliminary evidence suggests that the receptor has a high degree of specificity toward salts, being stimulated primarily by monovalent inorganic cations.