Selective enhancement and suppression of frog gustatory responses to amino acids

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.

Influence of sweet suppressing agent on gustatory brain evoked potentials generated by taste stimuli

A measurement system was employed to detect gustatory evoked potentials from human scalp by stimulus of a taste solution with the use of a laser beam device. The evoked potentials for four taste qualities (i.e., sweet-sucrose, salty-sodium chloride, sour-tartaric acid, and bitter-quinine-HCl) were measured before and after treatment with a sweet suppressing agent (i.e., gymnema sylvestre extract) to the tongue of a human. The solution was given to the chorda tympani nerve located 20 mm from the apex of the tongue and 15 mm from the left side of the center line. The maximum potential level and its latency were evaluated. Artificial saliva was used as a control solution. The evoked potentials obtained were averaged by eight evoked potentials to detect the peak of the evoked potential more clearly. The latencies for taste stimuli were found on two kinds of peaks at approximately 50 ms and 180 ms. These peaks are P1 and P2. The purpose of this study is to investigate the influence of sweet suppressing agent on P1 and P2. The influence of the sweet suppressing agent to evoked potential by salty, sour, and bitter taste stimuli was not recognized, but the responses to sweet (sucrose) were abolished after treatment with a sweet suppressing agent. It was recognized that the peak P2 originated from the taste stimulus. The peak P1 did not suffer the influence of the sweet suppression, so it was considered that the response to P1 was due to sensations other than the gustatory response, such as somatosense.

Specific inhibitor for bitter taste: inhibition of frog taste nerve responses and human taste sensation to bitter stimuli

Among various taste stimuli, bitter substances are most abundant and their chemical structures are greatly diverse from each other. It has been known that there are multiple receptor sites and transduction mechanisms. Already in neonates, bitter stimuli elicit rejection responses, indicating strong negative hedonic tone. Bitter taste is decidedly unpleasant when the sensation is strong. The development of a method to mask bitterness has widely been required in pharmaceutical sciences and food sciences. To mask bitterness, a specific bitterness inhibitor would be most useful. Such an inhibitor would also be useful in elucidating the receptor mechanisms of bitter substances. No inhibitor has, however, been available. Recently we found that a lipoprotein, PA-LG made of phosphatidic acid (PA) and beta-lactoglobulin (beta-LG), selectively suppresses the taste responses to bitter substances. In this paper we describe the protocol used for inhibition of the frog taste (glossopharyngeal) nerve responses to bitter stimuli by the lipoprotein. The frog taste system is used because it is sensitive to various bitter substances and surgery of the animal for the electrophysiological recording is rather easy. We also describe the protocol used for inhibition of human taste sensation to bitter stimuli.

Lipoprotein that selectively inhibits taste nerve responses to bitter substances

The development of a specific inhibitor for bitter taste has been widely required in the fields of taste physiology and pharmaceutical sciences, but no inhibitor has been available. We found that lipoproteins, PA-LG composed of phosphatidic acid (PA) and beta-lactoglobulin (LG) and PA-LA composed of PA and alpha-lactalbumin (LA) reversibly suppressed the responses of the frog glossopharyngeal nerve to the bitter substances. The frog tongue was treated with PA-LG solution for 10 min and then stimulated by a stimulus dissolved in water. The responses to the bitter substances such as quinine hydrochloride, papaverine hydrochloride, caffeine and L-leucine were completely suppressed by PA-LG, while those to the salt type bitter substances such as CsCl, MgCl2 and tetraethylammonium chloride were not suppressed. The responses to NaCl, galactose, acetic acid and L-alanine were unchanged or only slightly increased. The results suggested that binding of PA-LG to the hydrophobic region of the receptor membranes leads to suppression of the responses to the bitter substances. It was pointed out that PA-LG is useful not only for elucidating the receptor mechanisms of bitter substances, but also can be safely used to mask the bitter taste of foods and drugs, since PA, LG and LA are prepared from foods (soybean and milk).

The frog taste disc: a prototype of the vertebrate gustatory organ

The frog taste disc (TD) is apparently the largest gustatory organ found in vertebrates and seems to differentiate into a specialized variety of the prototypic scheme of the taste bud. An explanation for this unusual organization is lacking although it is possible to speculate the existence of environmental and nutritional requirements. Up to the present time, the most common model of the TD was based on two main cell types (sensory and sustentacular). This model may oversimplify the morphology of this structure since more numerous cell types have been described. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years. The main conclusions are the following: (1) the TD is a pluristratified epithelium with a general organization similar to that of the olfactory and vomeronasal epithelium; (2) it has skeleton composed of three different types of epithelial cells; (3) the chemoreceptorial surface is covered by different microenvironments; (4) three different types of neuro-epithelial systems are present; the type II is an 'open' sensory cell with axonal contacts devoid of vesicles; the type III is an 'open' sensory cell with synaptic-like junctions; the type i.v. is a 'closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fibers in the basal plexus are mostly cholinergic while the peridiscal nerve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electrophysiological studies or as a source of isolated cells, and their complexity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated with taste as very simple: an idea that probably must be revised. A taste organ is a highly complex structure composed of several sensory systems and a comparative approach can aid comprehension of its general organization. The study of the 'large taste organs' present in some species of amphibians can provide useful data for knowledge of the gustatory system of vertebrates.

Taste responses of bullfrog to pungent stimuli

Taste responses of bullfrogs to various pungent compounds and taste substances were electrophysiologically recorded from glossopharyngeal nerves. The threshold concentrations were approximately 10(-7) M for piperine, approximately 10(-6) M for capsaicin and approximately 10(-4) M for allyl isothiocyanate. At any concentration examined, piperine was more potent than capsaicin. Both piperine and capsaicin elicited desensitizing responses, but the taste receptors recovered from the desensitization within 10 min after washing with deionized water. Cross-adaptation experiments revealed that capsaicin only partially desensitizes receptors for piperine, L-leucine, HCl or quinine. Perfusion of the lingual artery with a solution containing no added Ca decreased the responses to capsaicin. Such a solution has been shown to suppress the taste nerve responses by blocking synaptic transmissions between taste cells and taste nerves [8]. These results suggest that the gustatory effects of capsaicin are different from its pharmacological effects on sensory neurons. It is likely that capsaicin and other pungent compounds, when they act as seasonings, stimulate taste cells rather than the free nerve endings of the sensory neurons.

Inhibition of salt-induced gustatory responses in the frog (Rana catesbeiana) by 5'-GMP

Millimolar concentration of sodium 5'-guanylate (5'-GMP), a 'umami' substance, inhibited salt-induced gustatory neural responses, particularly tonic components, of the bullfrog when the tongue was adapted to a low salt (5 mM NaCl) solution but not when adapted to normal saline that contained 115 mM NaCl. The result suggests that 5'-GMP is a modulator of adaptation process in salt response in the bullfrog taste system.

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.

Transport pathways in rat lingual epithelium

Measurements of ion transport across isolated lingual epithelium of rat were correlated with electrophysiological recordings from taste nerves. At hyperosmotic concentrations of NaCl, sodium ions enter the mucosal membrane of the isolated epithelium partially through an amiloride-inhibitable pathway and exit the serosal membrane through a Na+-K+-ATPase. At hyposmotic concentrations of KCl, potassium ions enter the mucosal membrane through a K+ pathway that is inhibited by 4-aminopyridine and exit at the serosal membrane through a K+ pathway that is inhibited by BaCl2. The inhibition of sodium transport by amiloride and potassium transport by 4-aminopyridine is consistent with previously published electrophysiological recordings from the chorda tympani nerve bundle (CT) and recordings from nucleus of the solitary tract (NST) obtained here. The responses to NaCl are greater than the responses to KCl at equimolar concentrations over the entire concentration range both in epithelial and neural measurements. At hyposmotic concentrations of NaCl the epithelial responses include inward sodium and outward chloride components. Isolated rat tongue is only slightly stimulated by D-glucose or sucrose as are the CT and NTS responses. These data suggest that events in taste transduction can be understood, in part, by measuring the epithelial responses of isolated rat tongue.

Patch-clamp study of isolated taste receptor cells of the frog

Taste discs were dissected from the tongue of R. ridibunda and their cells dissociated by a collagenase/low Ca/mechanical agitation protocol. The resulting cell suspension contained globular epithelial cells and, in smaller number, taste receptor cells. These were identified by staining properties and by their preserved apical process, the tip of which often remained attached to an epithelial (associated) cell. When the patch pipette contained 110 mM KCl and the cells were superfused with NaCl Ringer's during whole-cell recording, the mean zero-current potential of 22 taste receptor cells was -65.2 mV and the slope resistance 150 to 750 M omega. Pulse-depolarization from a holding voltage of -80 mV activated a transient TTX-blockable inward Na current. Activation became noticeable at -25 mV and was half-maximal at -8 mV. Steady-state inactivation was half-maximal at -67 mV and complete at -50 mV. Peak Na current averaged -0.5 nA/cell. The Ca-ionophore A23187 shifted the activation and inactivation curve to more negative voltages. Similar shifts occurred when the pipette Ca was raised. External Ni (5 mM) shifted the activation curve towards positive voltages by 10 mV. Pulse depolarization also activated outward K currents. Activation was slower than that of Na current and inactivation slower still. External TEA (7.5 mM) and 4-amino-pyridine (1 mM) did not block, but 5 mM Ba blocked the K currents. K-tail currents were seen on termination of depolarizing voltage pulses. A23187 shifted the IK(V)-curve to more negative voltages. Action potentials were recorded when passing pulses of depolarizing outward current. Of the frog gustatory stimulants, 10 mM Ca caused a reversible 5- to 10-mV depolarization in the current-clamp mode. Quinine (0.1 mM, bitter) produced a reversible depolarization accompanied by a full block of Na current and, with slower time-course, a partial block of K currents. Cyclic AMP (5 mM in the external solution or 0.5 microM in the pipette) caused reversible depolarization (to -40 to -20 mV) due to partial blockage of K currents, but only if ATP was added to the pipette solution. Similar responses were elicited by stimulating the adenylate cyclase with forskolin. Blockage of cAMP-phosphodiesterase enhanced the response to cAMP. These results suggest that cAMP may be one of the cytosolic messengers in taste receptor cells. Replacement of ATP by AMP-PNP in the pipette abolished the depolarizing response to cAMP.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Taste responses to amino acids in the southern leopard frog, Rana sphenocephala

Integrated taste recordings of the glossopharyngeal (IX) nerve innervating the tongue of the southern leopard frog were studied in response to various amino acids and quinine hydrochloride. Amino acids and quinine hydrochloride elicited primarily phasic taste responses. Acidic (L-aspartic and L-glutamic) and basic (L-lysine and L-arginine) amino acids, adjusted to pH8, were effective taste stimuli. All glossopharyngeal nerve twigs that responded to amino acid stimuli also responded to quinine; however, not all quinine-sensitive IX nerve bundles were responsive to amino acids. Electrophysiological thresholds for amino acids were estimated to be 2.5-10 mM, whereas threshold for quinine hydrochloride averaged approximately 10 microM.

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.