Pysicochemical studies of taste reception. V. Suppressive effect of salts on sugar response of the frog

Hypertonicity augments bullfrog taste nerve responses to inorganic salts

The tonicity of taste stimulating solutions has been usually ignored, though taste substances themselves yielded the tonicity. We investigated the effect of hypertonicity on bullfrog taste nerve responses to inorganic salts by adding nonelectrolytes such as urea and sucrose that elicited no taste nerve responses. Here, we show that hypertonicity alters bullfrog taste nerve-response magnitude and firing pattern. The addition of urea or sucrose enhances the taste nerve-response magnitude to NaCl and shifts the concentration-response curve to the left. The effect of hypertonicity on responses to CaCl(2) is bimodal; hypertonicity suppresses CaCl(2) responses at concentrations less than ~30 mM and enhances them at concentrations greater than ~30 mM. The hypertonicity also enhances response magnitude to other monovalent salts. The extent of the enhancing effects depends on the difference between the mobility of the cation and anion in the salt. We quantitatively suggest that both the enhancing and suppressing effects result from the magnitude and direction of local circuit currents generated by diffusion potentials across tight junctions surrounding taste receptor cells.

Rat taste nerve responses to salts carrying cations of large molecular size; are the taste responses to the salts induced by cation transport across apical membranes of taste cells?

1. The responses of rat chorda tympani nerve to various salts carrying cations of large molecular size which have small permeability were measured. 2. Salts carrying polyvalent cations such as Fe3+ or La3+ elicited much larger responses than NaCl or KCl. 3. Ammonium chloride derivatives having methyl or ethyl groups and salts carrying other organic cations of large molecular size elicited the responses comparable to that induced by NH4Cl or NaCl. 4. It was suggested that the taste responses to the salts carrying the cations of large molecular size are induced not by the cation transport but by adsorption of the cations on the membranes.

Taste responses to electrolytes in the frog glossopharyngeal nerve: initial process of taste reception

In taste reception, it has been proposed that changes in surface potential on the apical membrane of taste cells bring about activation of taste cells during chemical stimulation. To ascertain whether changes in the surface potential are involved in taste reception of electrolytes, unitary discharges were recorded from single water fibers of the frog glossopharyngeal nerve with a suction electrode. The surface potential is a function of both the charge density of the membrane surface and the ionic strength of the medium. Low concentrations of CaCl2 (less than 1 mM) were very effective stimuli. However, 0.01-1 mM LaCl3 and HCl (pH 3.0-4.5), which alter the surface potential in the positive direction, had no excitatory effect. Transition metal cations, such as Mn2+, Co2 and Ni2+, had an excitatory effect, but the responses to these cations appeared at relatively high concentrations (greater than 5 mM), in spite of the high affinity of the receptor membrane for these cations. The results suggest that the surface charge of the apical membrane is not associated with the excitation caused by electrolytes. MgCl2 (greater than 5 mM) and NaCl (greater than 100 mM) were also effective stimuli, whereas choline Cl (100-1000 mM) had no excitatory effect. An increase in the ionic strength was achieved by the addition of 100-300 mM choline Cl to stimulating solutions of MgCl2 or NaCl. The responses to Mg2+ and Na+ were not affected by the increase in the ionic strength. The results obtained here indicate that changes in the surface potential on the surface of the apical membrane are not involved in taste reception of electrolytes. Alteration of the surface potential of the membrane in the positive direction would bring about a reduction in the local concentration of cations in the vicinity of the membrane. Hence, the presence of divalent cations in the medium may affect the response to monovalent cations. However, addition of 100 mM MgCl2 to the stimulating solution of NaCl did not affect the concentration-response curve for NaCl. This result suggests that the surface charge density of the apical membrane is very low and hence the magnitude of the surface potential is very small. The results also suggest that Mg2+ and Na+ activate the taste cells by two separate, non-interacting processes. The present study suggests that, in the initial process of taste reception, only the binding of each separate cation to its appropriate receptor site (specific receptor site) leads to activation of the receptor.

Receptive fields and gustatory responsiveness of frog glossopharyngeal nerve. A single fiber analysis

Receptive fields and responsiveness of single fibers of the glossopharyngeal (IXth) nerve were investigated using electrical, gustatory (NaCl, quinine HCl, acetic acid, water, sucrose, and CaCl2), thermal, and mechanical stimulation of the single fungiform papillae distributed on the dorsal tongue surface in frogs. 172 single fibers were isolated. 58% of these fibers (99/172) were responsive to at least one of the gustatory stimuli (taste fibers), and the remaining 42% (73/172) were responsive only to touch (touch fibers). The number of papillae innervated by a single fiber (receptive field) was between 1 and 17 for taste fibers and between 1 and 10 for touch fibers. The mean receptive field of taste fibers (X = 6.6, n = 99) was significantly larger than that of touch fibers (X = 3.6, n = 73) (two-tailed t test, P less than 0.001). In experiments with natural stimulation of single fungiform papillae, it was found that every branch of a single fiber has a similar responsiveness. Taste fibers were classified into 14 types (Type N, Q, A, NA, NCa, NCaA, NCaW, NCaAW, NCaWS, NQ, NQA, NQAS, NQWarm, Multiple) on the basis of their responses to gustatory and thermal stimuli. The time course of the response in taste fibers was found to be characteristic of their types. For example, the fibers belonging to Type NQA showed phasic responses, those in Type NCa showed tonic responses, etc. These results indicate that there are several groups of fibers in the frog IXth nerve and that every branch of an individual fiber has a similar responsiveness to the parent fiber.

Large enhancement of canine taste responses to sugars by salts

The effects of changed ionic environments on the canine taste responses to sugars were examined by recording the activity of the chorda tympani nerve. a) The responses to various sugars were greatly enhanced by the presence of salts having monovalent cations such as Na+, K+, choline+, or Tris+. The responses to sugars were suppressed by high concentrations of salts. (b) The presence of 100 mM NaCl in fructose solution did not affect the maximal response and changed the Hill constant for the concentration-response relationship from 1.3 to 2.4. (c) CaCl2 greatly enhanced the response to fructose, while MgCl2 exhibited practically no effect. The presence of 20 mM CaCl2 in fructose solution changed the Hill constant from 1.2 to 2.4. (d) CaCl2 suppressed the responses to 0.5 M sugars except for fructose and sucrose and enhanced the responses to all sugars examined at 1 M. In the glucose response, the slope of the concentration-response curve was increased by the presence of CaCl2. Here the curve in the absence of CaCl2 intersected with that in the presence of CaCl2, indicating that CaCl2 suppressed the response to glucose of low concentrations and enhanced that of high concentrations. (e) The enhancement of the sugar responses by salts was not simply explained in terms of ionic permeability at the apical membranes of taste cells. The enhanced and suppressed effects of salts on the sugar responses were interpreted in terms of the cooperativity between receptor molecules for sugars.

Multimodal responses of taste neurons in the frog nucleus tractus solitarius

The responses of 216 neurons in the nucleus tractus solitarius (NTS) of the American bullfrog were recorded following taste, temperature, and tactile stimulation. Cells were classified on the basis of their responses to 5 taste stimuli: 0.5 M NaCl, 0.0005 M quinine-HCl (QHCl), 0.01 M acetic acid, 0.5 M sucrose, and deionized water (water). Neurons showing excitatory responses to 1, 2, 3, or 4 of the 5 kinds of taste stimuli were named Type I, II, III, or IV, respectively. Cells whose spontaneous rate was inhibited by taste and/or tactile stimulation of the tongue were termed Type V. Type VI neurons were excited by tactile stimulation alone. Of the 216 cells, 115 were excited or inhibited by taste stimuli (Types I-V), with 35 being Type I, 34 Type II, 40 Type III, 2 Type IV and 4 Type V. The remaining 101 cells were responsive only to tactile stimulation (Type VI). Of those 111 cells excited by taste stimulation (Types I-IV), 106 (95%) responded to NaCl, 66 (59%) to acetic acid, 44 (40%) to QHCl, 10 (9%) to water, and 9 (8%) to warming. No cells responded to sucrose. Of the 111 cells of Types I-IV, 76 (68%) were also sensitive to mechanical stimulation of the tongue. There was some differential distribution of these neuron types within the NTS, with more narrowly tuned cells (Type I) being located more dorsally in the nucleus than the more broadly tuned (Type III) neurons. Cells responding exclusively to touch (Type VI) were also more dorsally situated than those responding to two or more taste stimuli (Types II and III).

Contribution of electrostatic and hydrophobic interactions of bitter substances with taste receptor membranes to generation of receptor potentials

The effects of changed ionic environments on the frog taste nerve responses to the bitter substances were examined. The responses to quinine and strychnine carrying a positive charge were suppressed by an increase in ionic strength of stimulating solutions. It was concluded that electrostatic interaction of these positive bitter substances with the receptor membranes greatly contributes to the adsorption of the substances on the membranes and that this interaction was suppressed by an increase in ionic strength. The responses to neutral bitter substances (caffeine and theophylline) were unchanged by an increase in salt concentration. The zeta potential of the mouse neuroblastoma (N-18 clone), which was depolarized by various bitter substances similarly to a taste cell, was measured in the presence of the bitter substances. The zeta potential was a little changed by quinine and practically unchanged by strychnine, caffeine and theophylline. The membrane fluidity of the N-18 cell monitored with 2-(9-anthroyloxy)stearic acid was changed in response to the bitter substances, while the fluidity monitored with 12-(9-anthroyloxy)stearic acid or 1,6-diphenyl-1,3,5-hexatriene was unchanged. This suggested that the bitter substances are adsorbed on the hydrophobic region near the surface and induce a conformational change at the region. The depolarization by the bitter substances seems to stem from changes in the "boundary potential" at the region near the surface within the membrane interior.

Neuroblastoma cell as a model for a taste cell: mechanism of depolarization in response to various bitter substances

The mouse neuroblastoma cell (N-18 clone) was used as a model for a taste cell. The N-18 cell was found to be reversibly depolarized by various bitter substances. The minimum concentrations of bitter substances which induced depolarization (threshold concentration) varied greatly with the type of the substance. There was a good correlation between the threshold concentrations for various bitter substances in the N-18 cell and those in the human taste responses. The input membrane resistance was little changed during the depolarization induced by the bitter substances. Replacement of Na+ and Cl- with impermeable ions had practically no effect on the depolarization response to the bitter substances and reduction of calcium concentration from 1.8 to 0.2 mM led to a slight increase in the responses. It was suggested that the depolarization of the N-18 cell by bitter substances mainly stems from changes in the phase-boundary potential at the outer surface of the cell.

Selective enhancement and suppression of frog gustatory responses to amino acids

Properties of the receptor sites for L-amino acids in taste cells of the bullfrog (Rana catesbeiana) were examined by measuring the neural activities of the glossopharyngeal nerve under various conditions. (a) The frogs responded to 12 amino acids, but the responses to the amino acids varied with individual frogs under natural conditions. The frog tongues, however, exhibited similar responses after an alkaline treatment that removes Ca2+ from the tissue. The variation in the responses under natural conditions was apparently due to the variation in the amount of Ca2+ bound to the receptor membrane. (b) The responses to hydrophilic L-amino acids (glycine, L-alanine, L-serine, L-threonine, L-cysteine, and L-proline) were of a tonic type, but those to hydrophobic L-amino acids (L-valine, L-leucine, L-isoleucine, L-methionine, L-phenylalanine, and L-tyrptophan) were usually composed of both phasic and tonic components. (c) The properties of the tonic component were quite different from those of the phasic component: the tonic component was largely enhanced by the alkaline treatment and suppressed by the acidic treatment that increases binding of Ca2+ to the tissue. Also, the tonic component was suppressed by the presence of low concentrations of salts, or the action of pronase E, whereas the phasic component was unchanged under these conditions. These properties of the phasic component were quite similar to those of the response to hydrophobic substances such as quinine. These results suggest that the hydrophilic L-amino acids stimulate receptor protein(s) and that the hydrophobic L-amino acids stimulate both the receptor protein and a receptor site similar to that for quinine. (d) On the basis of the suppression of the responses to amino acids by salts, the mechanism of generation of the receptor potential is discussed.