Interaction Between Gustatory Depolarizing Receptor Potential and Efferent-Induced Slow Depolarizing Synaptic Potential in Frog Taste Cell

Electrical stimulation of parasympathetic nerve (PSN) efferent fibers in the glossopharyngeal nerve induced a slow depolarizing synaptic potential (DSP) in frog taste cells under hypoxia. The objective of this study is to examine the interaction between a gustatory depolarizing receptor potential (GDRP) and a slow DSP. The amplitude of slow DSP added to a tastant-induced GDRP of 10 mV was suppressed to 60% of control slow DSPs for NaCl and acetic acid stimulations, but to 20–30% for quinine–HCl (Q-HCl) and sucrose stimulations. On the other hand, when a GDRP was induced during a prolonged slow DSP, the amplitude of GDRPs induced by 1 M NaCl and 1 M sucrose was suppressed to 50% of controls, but that by 1 mM acetic acid and 10 mM Q-HCl unchanged. It is concluded that the interaction between GDRPs and efferent-induced slow DSPs in frog taste cells under hypoxia derives from the crosstalk between a gustatory receptor current across the receptive membrane and a slow depolarizing synaptic current across the proximal subsynaptic membrane of taste cells.     

Characteristics of biphasic slow depolarizing and slow hyperpolarizing potential in frog taste cell induced by parasympathetic efferent stimulation

When the velocity of capillary blood flow in the frog tongue declined to an intermediate range of 0.2-0.7 mm/s, the glossopharyngeal nerve stimulation induced a biphasic slow depolarizing and slow hyperpolarizing potential (HP) in taste cells. The objective of this work was to examine the generative mechanisms of the biphasic slow potentials. The biphasic slow response was always preceded by a slow depolarizing potential (DP) component and followed by a slow HP component. Intravenous injection of tubocurarine completely blocked the biphasic slow responses, suggesting that both components of the biphasic slow potentials are evoked by the parasympathetic nerve (PSN) fibers. Membrane conductance of taste cells increased during slow DPs and decreased during slow HPs. The reversal potential of either component of a biphasic slow response was the almost same value of -12 mV. An antagonist, L-703,606, for neurotransmitter substance P neurokinin(1) receptor completely blocked both components of the biphasic slow responses. An antagonist, flufenamic acid, for nonselective cation channels on the taste cell membrane completely blocked the biphasic slow responses. These results suggest that PSN-induced biphasic slow responses are postsynaptically elicited in taste cells by releasing substance P at the PSN axon terminals. It is concluded that the slow DP component may be generated by opening one type of nonselective cation channel on taste cells and that the slow HP component may be generated by closing the other type of nonselective cation channel. We discussed that a second messenger inositol 1,4,5-trisphosphate might be related to a slow DP component and another second messenger diacylglycerol might be related to a slow HP component.     

Electrical properties and gustatory responses of various taste disk cells of frog fungiform papillae

We compared the electrical properties and gustatory response profiles of types Ia cell (mucus cell), Ib cell (wing cell), and II/III cell (receptor cell) in the taste disks of the frog fungiform papillae. The large depolarizing responses of all types of cell induced by 1 M NaCl were accompanied by a large decrease in the membrane resistance and had the same reversal potential of approximately +5 mV. The large depolarizing responses of all cell types for 1 mM acetic acid were accompanied by a small decrease in the membrane resistance. The small depolarizing responses of all cell types for 10 mM quinine-HCl (Q-HCl) were accompanied by an increase in the membrane resistance, but those for 1 M sucrose were accompanied by a decrease in the membrane resistance. The reversal potential of sucrose responses in all cell types were approximately +12 mV. Taken together, depolarizing responses of Ia, Ib, and II/III cells for each taste stimulus are likely to be generated by the same mechanisms. Gustatory depolarizing response profiles indicated that 1) each of Ia, Ib, and II/III cells responded 100% to 1 M NaCl and 1 mM acetic acid with depolarizing responses, 2) approximately 50% of each cell type responded to 10 mM Q-HCl with depolarizations, and 3) each approximately 40% of Ia and Ib cells and approximately 90% of II/III cells responded to 1 M sucrose with depolarizations. These results suggest that the receptor molecules for NaCl, acid, and Q-HCl stimuli are equivalently distributed on all cell types, but the receptor molecules for sugar stimuli are richer on II/III cells than on Ia and Ib cells. Type III cells having afferent synapses may play a main role in gustatory transduction and transmission.     

Non-synaptic transformation of gustatory receptor potential by stimulation of the parasympathetic fiber of the frog glossopharyngeal nerve

When the glossopharyngeal nerve (GP) in the frog was strongly stimulated electrically, slow potentials were elicited from the tongue surface and taste cells in the fungiform papillae. Injection of atropine completely blocked these slow potentials. The present and previous data indicate that the slow potentials induced in the tongue surface and taste cells are due to a liquid junction potential between saliva secreted from the lingual glands due to parasympathetic fiber activity and an adapting solution on the tongue surface. Intracellularly recorded depolarizing receptor potentials in taste cells induced by 0.5 M NaCl and 3 mM acetic acid were enhanced by depolarizing slow potentials induced by GP nerve stimulation, but were depressed by the hyperpolarizing slow potentials. On average, the receptor potential of taste cells for 0.5 M NaCl was increased by 25% by the GP nerve-induced slow potential, but the receptor potential of taste cells for 3 mM acetic acid was decreased by 1% by the slow potential. These transformations of receptor potentials in frog taste cells were not due to a synaptic event initiated between taste cells and the efferent nerve fiber, but due to a non-synaptic event, a lingual junction potential generated in the dorsal lingual epithelium by GP nerve stimulation.     

Taste cell responses in the frog are modulated by parasympathetic efferent nerve fibers

We studied the anatomical properties of parasympathetic postganglionic neurons in the frog tongue and their modulatory effects on taste cell responses. Most of the parasympathetic ganglion cell bodies in the tongue were found in extremely small nerve bundles running near the fungiform papillae, which originate from the lingual branches of the glossopharyngeal (GP) nerve. The density of parasympathetic postganglionic neurons in the tongue was 8000-11,000/mm(3) of the extremely small nerve bundle. The mean major axis of parasympathetic ganglion cell bodies was 21 microm, and the mean length of parasympathetic postganglionic neurons was 1.45 mm. Electrical stimulation at 30 Hz of either the GP nerve or the papillary nerve produced slow hyperpolarizing potentials (HPs) in taste cells. After nicotinic acetyl choline receptors on the parasympathetic ganglion cells in the tongue had been blocked by intravenous (i.v.) injection of D-tubocurarine (1 mg/kg), stimulation of the GP nerve did not induce any slow HPs in taste cells but that of the papillary nerve did. A further i.v. injection of a substance P NK-1 antagonist, L-703,606, blocked the slow HPs induced by the papillary nerve stimulation. This suggests that the parasympathetic postganglionic efferent fibers innervate taste cells and are related to a generation of the slow HPs and that substance P is released from the parasympathetic postganglionic axon terminals. When the resting membrane potential of a taste cell was hyperpolarized by a prolonged slow HP, the gustatory receptor potentials for NaCl and sugar stimuli were enhanced in amplitude, but those for quinine-HCl and acetic acid stimuli remained unchanged. It is concluded that frog taste cell responses are modulated by activities of parasympathetic postganglionic efferent fibers innervating these cells.     

Membrane resistance change of the frog taste cells in response to water and Nacl

The electrical properties of the frog taste cells during gustatory stimulations with distilled water and varying concentrations of NaCl were studied with intracellular microelectrodes. Under the Ringer adaptation of the tongue, two types of taste cells were distinguished by the gustatory stimuli. One type, termed NaCl-sensitive (NS) cells, responded to water with hyperpolarizations and responded to concentrated NaCl with depolarizations. In contrast, the other type of cells, termed water-sensitive (WS) cells, responded to water depolarizations and responded to concentrated NaCl with hyperpolarizations. The membrane resistance of both taste cell types increased during the hyperpolarizing receptor potentials and decreased during the depolarizing receptor potentials, Reversal potentials for the depolarizing and hyperpolarizing responses in each cell type were a few millivolts positive above the zero membrane potential. When the tongue was adapted with Na-free Ringer solution for 30 min, the amplitude of the depolarizing responses in the NS cells reduced to 50% of the control value under normal Ringer adaptation. On the basis of the present results, it is concluded (a) that the depolarizing responses of the NS and WS cells under the Ringer adaptation are produced by the permeability increase in some ions, mainly Na+ ions across the taste cell membranes, and (b) that the hyperpolarizing responses of both types of taste cells are produced by a decrease in the cell membrane permeability to some ions, probably Na+ ions, which is slightly enhanced during the Ringer adaptation.     

Electrical responses of supporting cells in the frog taste organ to chemical stimuli

1. The mean resting potential of supporting cells in the frog taste organ was -19.1 mV. The supporting cells responded to the four basic taste stimuli with a depolarization but responded to water with a depolarization or a hyperpolarization. 2. The membrane resistances of supporting cells decreased during stimulation with sucrose, NaCl and acetic acid, but increased during stimulation with Q-HCl and water. 3. Reversal potential of the depolarizing response for 0.5 M NaCl in supporting cells was +7.6 mV. The depolarizing responses for Q-HCl and acetic acid were independent of the membrane potential level. 4. These results suggest that the characteristics of taste responses in supporting cells are similar to those in taste cells.     

Analysis of slow hyperpolarizing potentials in frog taste cells induced by glossopharyngeal nerve stimulation

Electrical stimulation of the frog glossopharyngeal (GP) nerve evoked slow hyperpolarizing potentials (HPs) in taste cells. This study aimed to clarify whether slow HPs were postsynaptically induced in taste cells. The slow HPs were recorded intracellularly with a microelectrode. When Ca2+ concentration in the blood plasma was decreased to approximately 0.5 mM, the amplitude of slow HPs reduced and their latency lengthened. When the Ca2+ concentration was increased to approximately 20 mM, the amplitude of slow HPs increased and their latency shortened. Addition of Cd2+ to the plasma greatly reduced the amplitude of slow HPs and lengthened their latency. These data suggest that the slow HPs are dependent on presynaptic activities in the GP nerve terminals in the taste disk. Of various antagonists injected intravenously for blocking receptors of neurotransmitter biogenic amines and peptides, only antagonists for substance P blocked the slow HPs at 2-4 mg/kg body wt. Application of substance P of 2 mg/kg to the plasma induced hyperpolarizing responses in taste cells, whose amplitude was the same as that of the slow HPs induced by GP nerve stimulation. Application of a nonselective cation channel antagonist, flufenamic acid, to the plasma blocked the slow HPs. These results suggest that the slow HPs are generated by closing the nonselective cation channels in the postsynaptic membrane of taste cells following possible release of substance P from the GP nerve terminals in the taste disk.     

Analysis of slow depolarizing potential in frog taste cell induced by parasympathetic efferent stimulation under hypoxia

Strong electrical stimulation (ES) of the frog glossopharyngeal (GP) efferent nerve induced slow depolarizing potentials (DPs) in taste cells under hypoxia. This study aimed to elucidate whether the slow DPs were postsynaptically induced in taste cells. After a block of parasympathetic nerve (PSN) ganglia by tubocurarine, ES of GP nerve never induced slow DPs in the taste cells, so slow DPs were induced by PSN. When Ca(2+) in the blood plasma under hypoxia was decreased to approximately 0.5 mM, the slow DPs reduced in amplitude and lengthened in latency. Increasing the normal Ca(2+) to approximately 20 mM increased the amplitude of slow DPs and shortened the latency. Addition of Cd(2+) to the plasma greatly reduced the amplitude of slow DPs and lengthened the latency. These data suggest that the slow DPs depend on Ca(2+) and Cd(2+) concentration at the presynaptic PSN terminals of taste disk. Antagonists, [D-Arg(1), D-Trp(7,9), Leu(11)]-substance P and L-703 606, of neurotransmitter substance P neurokinin(1) receptor completely blocked the slow DPs. Intravenous application of substance P induced a DP of approximately 7 mV and a reduction of membrane resistance of approximately 48% in taste cells. A nonselective cation channel antagonist, flufenamic acid, completely blocked the slow DPs. These findings suggest that the slow DPs are postsynaptically initiated in frog taste cells under hypoxia by opening nonselective cation channels on the postsynaptic membrane after substance P is probably released from the presynaptic PSN axon terminals.     

Response characteristics of rat taste cells to potassium benzoate

The rat taste cells responded to K-benzoate solutions higher than the threshold concentrations (0.03-0.3 M) with a depolarizing receptor potential, but they responded to K-benzoate lower than the thresholds with a hyperpolarizing receptor potential. In either depolarizing or hyperpolarizing receptor potentials the rise time decreased with increasing amplitude, but the fall time increased with increasing amplitude. During generation of either depolarizing or hyperpolarizing receptor potentials the input resistance of taste cells decreased with increasing amplitude. Application of the mixtures of various concentrations of NaCl and 0.05 M K-benzoate resulted in a reduction of receptor potential amplitude, as compared with that evoked by application of NaCl alone. It is concluded that a depression of gustatory neural impulse frequency by low concentrations of K-benzoate is mainly due to the hyperpolarizing receptor potential of taste cells elicited by the K-benzoate solutions.     

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.     

Topographical difference in taste organ density and its sensitivity of frog tongue

Distribution density of the taste disks of the fungiform papillae in the frog tongue was larger at the proximal portion than at the apical and middle portions. The number of myelinated afferent nerve fibres and taste cells per cm2 area of the tongue increased in the order of proximal greater than middle greater than apical portion. The amplitudes of gustatory neural responses for 0.5 M NaCl, 0.5 M KCl, 0.5 M NH4Cl, 0.05 M CaCl2, 1 mM acetic acid and 1 mM quinine-HCl (Q-HCl) were significantly larger with lingual stimulation of the proximal region than with the stimulation of the apical region. With these stimuli the mean ratio of the apical response to the proximal response was 1.00:1.54. On the other hand, this ration with deionized water was 1.00:5.00. The mean magnitudes of receptor potentials in taste cells for 1 mM acetic acid and 10 mM Q-HCl were the same among the apical, middle and proximal portions of the tongue. The mean magnitudes of receptor potentials for 0.5 M NaCl were significantly larger at the apical portion than at the other portions, whereas those for deionized water tended to be the largest at the proximal portion. It is concluded that the larger magnitude of the gustatory neural responses at the proximal portion of the tongue is due to morphological and physiological properties of the taste organ.     

Arterial perfusion of frog tongue for intracellular recording of taste cell receptor potential

The frog tongue was perfused through its artery with a Ringer solution using a peristaltic pump, and a method was developed to record stable intracellular receptor potentials of taste cells. Perfusing at 0.05 ml/min with a Ringer solution containing 5% dextran did not cause tongue edema, but perfusing at the same rate with Ringer without dextran caused edema. After perfusion at 0.05 ml/min with 100 mM K Ringer, the membrane potential of taste cells gradually decreased and reached a constant level in about 30 min, indicating that the intercellular fluid of the tongue could be replaced within this time period. While the artery of the frog tongue was perfused at 0.05 ml/min with Ringer containing 5% dextran, intracellular receptor potentials of taste cells elicited by four basic taste stimuli (1 M NaCl, 10 mM quinine-HCl (Q-HCl), 1 mM acetic acid and 1 M galactose) were similar to those obtained from the control taste cells under normal blood flow.     

Multiple sensitivity of single taste cells of the frog tongue to four basic taste stimuli

The electrical response of the taste cells of the frog fungiform papillae to four fundamental taste solutions (NaCl, acetic acid, quinine-HCl and sucrose) was studied by using the intracellular recording technique. The average value of resting membrane potential was 22.5 mV, inside negative. Each of the four taste solutions applied to the tongue produced a slow depolarizing potential, the receptor potential, on which no spike potential was superimposed. The amplitude of the receptor potentials increased linearly as a function of the logarithm of the concentration of the stimulus. Amplitudes of depolarizations to a given taste stimulation varied from one cell to another even within a single taste bud. Most of the cells responded to more than two of the four basic taste solutions. Sensitivity patterns in terms of the number of effective solutions and the relative effectiveness of different kinds of solutions were variable among cells. Statistical analysis suggests that at the receptor membranes of the taste cells, the sensitivities for the four basic stimuli are independent and random.     

Tonic activity of parasympathetic efferent nerve fibers hyperpolarizes the resting membrane potential of frog taste cells

We investigated the relationship between the membrane potential of frog taste cells in the fungiform papillae and the tonic discharge of parasympathetic efferent fibers in the glossopharyngeal (GP) nerve. When the parasympathetic preganglionic fibers in the GP nerve were kept intact, the mean membrane potential of Ringer-adapted taste cells was -40 mV but decreased to -31 mV after transecting the preganglionic fibers in the GP nerve and crushing the postganglionic fibers in the papillary nerve. The same result occurred after blocking the nicotinic acetylcholine receptors on parasympathetic ganglion cells in the tongue and blocking the substance P neurokinin-1 (NK-1) receptors in the gustatory efferent synapses. This indicates that the parasympathetic nerve (PSN) hyperpolarizes the membrane potential of frog taste cells by -9 mV. Repetitive stimulation of a transected GP nerve revealed that a -9-mV hyperpolarization of taste cells maintained under the intact GP nerve derives from an approximately 10-Hz discharge of the PSN efferent fibers. The mean frequency of tonic discharges extracellularly recorded from PSN efferent fibers of the taste disks was 9.1 impulses/s. We conclude that the resting membrane potential of frog taste cells is continuously hyperpolarized by on average -9 mV by an approximately 10-Hz tonic discharge from the parasympathetic preganglionic neurons in the medulla oblongata.     

Effect of Gap Junction Blocker β-Glycyrrhetinic Acid on Taste Disk Cells in Frog

A gap junction blocker, 18β-glycyrrhetinic acid (β-GA), increased the membrane resistance of Ia, Ib and II/III cells of frog taste disk by 50, 160, and 300 MΩ, respectively, by blocking the gap junction channels and hemichannels. The amplitudes of gustatory depolarizing potentials in the disk cells for 4 basic taste stimuli were reduced to 40–60% after intravenous injection of β-GA at 1.0 mg/kg. β-GA of 1.0 mg/kg did not affect the resting potentials and the reversal potentials for tastant-induced depolarizing potentials in any taste disk cells. The percentage of cells responding to each of 4 basic taste stimuli and varying numbers of 4 taste qualities did not differ between control and β-GA-treated taste disk cells. This implies that gustatory depolarizing response profiles for 4 basic taste stimuli were very similar in control and β-GA-treated taste disk cells. It is concluded that β-GA at 1.0 mg/kg reduced the amplitude of gustatory depolarizing potentials in taste disk cells by strongly blocking depolarizing currents flowing through the gap junction channels and hemichannels, but probably weakly affected the gustatory transduction mechanisms for 4 taste stimuli.     

The adaptation of the frog tongue to various taste solutions: the effect on gustatory neural responses to bitter stimuli

1. After the frog tongue was adapted for 10 sec to various salts and sugars, the initial phasic component of gustatory neural responses to almost all of quinine hydrochloride (Q-HCl), quinine sulfate (Q-H2SO4). Brucine, caffeine and picric acid was suppressed. 2. Following 10 sec adaptation to acetic acid, the phasic responses to Q-HCl and Q-H2SO4 were unchanged, those to brucine and caffeine were enhanced, and that to picric acid was depressed slightly. 3. The response to any one of Q-HCl, Q-H2SO4. brucine and caffeine was suppressed after adaptation to the other three, while those to picric acid and nicotine were unchanged or enhanced after adaptation to another bitter solution.     

The origin of slow potentials on the tongue surface induced by frog glossopharyngeal efferent fiber stimulation

When the glossopharyngeal (GP) nerve of the frog was stimulated electrically, electropositive slow potentials were recorded from the tongue surface and depolarizing slow potentials from taste cells in the fungiform papillae. The amplitude of the slow potentials was stimulus strength- and the frequency-dependent. Generation of the slow potentials was not related to antidromic activity of myelinated afferent fibers in the GP nerve, but to orthodromic activity of autonomic post-ganglionic C fibers in the GP nerve. Intravenous injection of atropine abolished the positive and depolarizing slow potentials evoked by GP nerve stimulation, suggesting that the slow potentials were induced by the activity of parasympathetic post-ganglionic fibers. The amplitude and polarity of the slow potentials depended on the concentration of adapting NaCl solutions applied to the tongue surface. These results suggest that the slow potentials recorded from the tongue surface and taste cells are due to the liquid junction potential generated between saliva secreted from the lingual glands by GP nerve stimulation and the adapting solution on the tongue surface.     

Vasopressin increases frog gustatory neural responses elicited by NaCl and HCl.

1. The effect of arginine vasopressin (AVP) on frog gustatory responses was investigated by recording integrated responses of the whole glossopharyngeal nerve by stimulation of the tongue with tastants. 2. After AVP (100 mUnits/ml) was perfused to the basolateral side of taste cells through the lingual artery, gustatory neural responses for NaCl and hydrochloric acid (HCl) stimuli were greatly enhanced, but the responses for CaCl2, quinine hydrochloride (Q-HCl) and galactose were not affected. 3. Three hours after the onset of AVP perfusion, the responses for NaCl and HCl increased to 260% and 270% of the respective controls. 4. The NaCl response which was insensitive to amiloride during normal saline perfusion became sensitive to amiloride during AVP perfusion. 5. When membrane-permeable 8-bromo-cyclic AMP (8-Br-cAMP, 0.1 mM) was perfused to the basolateral side of taste cells, the responses for NaCl and HCl decreased to 41 and 63% of the respective controls. 6. These results suggest that AVP may regulate the gustatory responses for monovalent salts and acids by a mechanism which is not necessary to activate adenylate cyclase.     

Dye-coupling among frog (Rana catesbeiana) taste disk cells

• 1.1. Dye-coupling among taste disk cells in the bullfrog fungiform papillae was examined histologically by injecting a fluorescent dye (Lucifer yellow) into the cell, and the effects of the dye-coupling on depolarizing responses induced by taste stimuli were studied electrophysiologically.
• 2.2. With dye injection into a taste cell, dye-coupling was found between taste cells (23%) or between taste cell and supporting cell (28%). With dye injection into a supporting cell, dye-coupling was found between supporting cells (34%) or between supporting cell and taste cell (27%).
• 3.3. Depolarizing responses recorded from either a taste cell or a supporting cell to stimulation with 0.5 M NaCl or 10 mM quinine-HCl were the same in amplitude whether the dye-coupling to another cell was present or not. On the other hand, depolarizing responses recorded from a taste cell for 0.5 mM acetic acid became significantly larger when dye-coupled to a supporting cell.
• 4.4. It is concluded that gustatory transduction for acid stimuli is influenced by supporting cells coupled to taste cells.