Physicochemical studies of taste reception. III. Interpretation of the water response in taste reception

Time-dependent expression of hypertonic effects on bullfrog taste nerve responses to salts and bitter substances

We previously showed that the hypertonicity of taste stimulating solutions modified tonic responses, the quasi-steady state component following the transient (phasic) component of each integrated taste nerve response. Here we show that the hypertonicity opens tight junctions surrounding taste receptor cells in a time-dependent manner and modifies whole taste nerve responses in bullfrogs. We increased the tonicity of stimulating solutions with non-taste substances such as urea or ethylene glycol. The hypertonicity enhanced phasic responses to NaCl>0.2M, and suppressed those to NaCl<0.1M, 1mM CaCl2, and 1mM bitter substances (quinine, denatonium and strychnine). The hypertonicity also enhanced the phasic responses to a variety of 0.5M salts such as LiCl and KCl. The enhancing effect was increased by increasing the difference between the ionic mobilities of the cations and anions in the salt. A preincubation time >20s in the presence of 1M non-taste substances was needed to elicit both the enhancing and suppressing effects. Lucifer Yellow CH, a paracellular marker dye, diffused into bullfrog taste receptor organs in 30s in the presence of hypertonicity. These results agreed with our proposed mechanism of hypertonic effects that considered the diffusion potential across open tight junctions.

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.

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.

Mechanism of the water response in frog gustation: possible significance of surface potential

The frog taste response to deionized water (water response) after adaptation of the tongue to salts was recorded from the glossopharyngeal nerve under various conditions. It was found that the frog water response exhibits different behavior from the carp water response examined in a previous paper13. (a) The frog water response did not decline during stimulation and lasted for at least 3 min, while the carp water response declined within 10 s after stimulation to a spontaneous level. (b) The frog water response was practically independent of species and concentrations of salts in adapting solutions when the tongue was adapted to salts of monovalent cations, while the carp water response was highly dependent of salt concentration in adapting solution. (c) The water response was increased with an increase of CaCl2 concentration in adapting solution, while it was decreased with an increase of MgCl2 concentration. (d) The water response was suppressed by the presence of electrolytes in stimulating solution: the data obtained with different species of salts were described by a single curve as a function of the ionic strength. (e) The mechanism of the frog water response together with the carp water response was explained in terms of the surface potential.

Different receptor sites for Ca2+ and Na+ in single water fibers of the frog glossopharyngeal nerve

Unitary discharges were recorded from single water-sensitive fibers (water fibers) of the frog glossopharyngeal nerve during stimulation of the tongue with chemical stimuli. Low CaCl2 (1 mM CaCl2) and relatively high NaCl (500 mM NaCl) are effective stimuli which excite water fibers. To learn whether or not Ca2+ and Na+ react with different receptor sites, a proteolytic enzyme was topically applied to the tongue dorsum. After treatment of the tongue with 0.05% pronase E, the magnitude of the NaCl response remained unchanged, but the magnitude of the CaCl2 response markedly decreased. The selective elimination by the pronase E treatment indicates that there exist different receptor sites for Ca2+ and Na+ in single water fibers of the frog glossopharyngeal nerve. The effect of pronase E treatment was due to the proteolytic action. The results suggest that the Ca2+ receptor site may be composed of a protein.

Two different receptor sites for Ca2+ and Na+ in frog taste responses

Distilled water, 1 mM CaCl2 and 500 mM NaCl (pH 4.5) are effective stimuli which excite chemoreceptors of the frog tongue. To learn whether or not these taste stimuli react with different taste receptor sites, a proteolytic enzyme was topically applied to the tongue dorsum. Responses were recorded from the frog glossopharyngeal nerve during taste stimulation. After application of 0.1% pronase E to the dorsal tongue surface, the magnitude of the NaCl response remained unchanged, but the magnitude of the water and CaCl2 responses was markedly decreased. The selective suppression by the pronase E treatment indicates that there are two different receptor sites for Ca2+ and Na+ in the frog taste receptor cell and the receptor sites responsible for the generation of the water and the Ca2+ response may be composed of a protein.

Enhancement of taste responses to acids by calcium ions

The frog taste nerve responses to HCl and acetic acids were greatly enhanced by increasing calcium concentration in a solution to which the tongue had adapted when the lingual artery was perfused with Ringer solution. Enhancement was not seen in the responses to other taste stimuli.

Mechanism of the water response in carp gustatory receptors: independent generation of the water response from the salt response

The carp gustatory responses to various salts and those to distilled water after adaptation of the receptors to the salts were recorded from the palatine nerve under varying conditions. (a) As far as Na4Fe(CN)6 solution prepared freshly was used, any peak in the dose-response curve was not observed, while the solution stored overnight induced a large peak response at the low concentration region as Konishi reported. The magnitude of the water response after adaptation to the stored solution was practically equal to that after adaptation to the fresh solution, suggesting that the receptor site for the water response is different from that for the dilute salts. (b) The responses to salts depended largely on the species of both cations and anions of the salts. The responses were deceased with an increase in the lyotropic number of the anions and increased with an increase in the number above 11. The response to distilled water was practically independent of the species of both monovalent cations and anions of the salts used for the adaptation. The pH dependence of the response to 10 mM NaCl was largely different from that to distilled water after adaptation to 10 mM NaCl. (c) The water response was suppressed by the presence of electrolytes in stimulating solution; the data obtained with different species of salts were described by a single curve as a function of the ionic strength. (d) The mechanism to explain how distilled water leads to depolarization of the taste cell was discussed in terms of the electric double-layer potential.

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

The tast responses of frog to various kinds of sugars were measured quantitatively by use of the glossopharyngeal nerve activity under an appropriate condition where the water response was completely suppressed. The concentration dependences of response of frog tongue to D-fructose, D-glucose, and sucrose were almost the same, D-galactose, however, elicited a much larger response in comparison with the other sugars in the whole range of concentrations examined. The sugar response was suppressed extensively by the presence of small amount of salts in the stimulating sugar solution. The suppressive effects of NaCl, KCl, MgCl2, MgSO4, and K4Fe(CN)6 were examined with a fixed concentration of sugar. The results obtained with these salts, added in various concentrations, fell on a single curve when the data were plotted against the ionic strength in the stimulating solution. The present results were consistent with the notion that the taste receptor potential for salts or acids is attributable to a change in the phase boundary potential at the membrane-solution interface as proposed in the previous papers of this series.