The adaptation of the frog tongue to bitter solutions: Enhancing effect on gustatory neural response to salt stimuli

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.

Depression of Gustatory Receptor Potential in Frog Taste Cell by Parasympathetic Nerve-Induced Slow Hyperpolarizing Potential

Parasympathetic nerve (PSN) innervates taste cells of the frog taste disk, and electrical stimulation of PSN elicited a slow hyperpolarizing potential (HP) in taste cells. Here we report that gustatory receptor potentials in frog taste cells are depressed by PSN-induced slow HPs. When PSN was stimulated at 30 Hz during generation of taste cell responses, the large amplitude of depolarizing receptor potential for 1 M NaCl and 1 mM acetic acid was depressed by approximately 40% by slow HPs, but the small amplitude of the depolarizing receptor potential for 10 mM quinine-HCl (Q-HCl) and 1 M sucrose was completely depressed by slow HPs and furthermore changed to the hyperpolarizing direction. The duration of the depolarizing receptor potentials depressed by slow HPs prolonged with increasing period of PSN stimulation. As tastant-induced depolarizing receptor potentials were increased, the amplitude of PSN-induced slow HPs inhibiting the receptor potentials gradually decreased. The mean reversal potentials of the slow HPs were approximately -1 mV under NaCl and acetic acid stimulations, but approximately -14 mV under Q-HCl and sucrose stimulations. This implies that when a slow HP was evoked on the same amplitude of depolarizing receptor potentials, the depression of the NaCl and acetic acid responses in taste cells was larger than that of Q-HCl and sucrose responses. It is concluded that slow HP-induced depression of gustatory depolarizing receptor potentials derives from the interaction between gustatory receptor current and slow hyperpolarizing current in frog taste cells and that the interaction is stronger for NaCl and acetic acid stimulations than for Q-HCl and sucrose stimulations.

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.

Off-depolarization and off-hyperpolarization after termination of quinine-HCl stimulation in frog taste cells

Off-responses in frog taste cells evoked by a Ringer rinse following 1 mM quinine-HCI (Q-HCl) stimulation were investigated with an intracellular recording technique. Three types of off-responses were found; a transient off-depolarization, a rebound-type off-depolarization and a transient off-hyperpolarization. The time to peak and duration of off-responses were in the order of rebound type-off-depolarization > transient off-depolarization > transient off-hyperpolarization. The reversal potential for the rebound-type off-depolarization existed in more positive level than the resting potential. The reversal potential for the transient off-depolarization was around 0 mV, and that for the transient off-hyperpolarization was -58 mV. These three off-responses in frog taste cells may be initiated by an increase in permeability of the apical receptive membrane to Na+, K+ and CI-. A kind of principal ion is dependent on off-response types.

Centrifugal decrement in diameter of myelinated afferent fibres innervating frog taste organ

1. Regional changes in the diameter of single myelinated afferent nerve fibres innervating the taste disc of the fungiform papillae on the bullfrog tongue were investigated morphologically and functionally. 2. The diameter of myelinated afferents in the medial lingual branch of the glossopharyngeal nerve averaged 8.4 microns at the proximal end of the tongue and gradually decreased at the rate of 0.8 micron/cm length of the fibres as they ran in the apical direction of the tongue. 3. The conduction velocity of single myelinated afferent fibres within the tongue decreased gradually as they ran peripherally. 4. Electrophysiological inspection of neural connections between the fungiform papillae suggests that a gradual centrifugal decrease in the diameter of a single myelinated afferent fibre is not due to multiple bifurcations of the fibre at various sites within the tongue, but due to a natural gradual decrease in the thickness of the myelin sheath and the diameter of axon.

Latency of gustatory neural impulses initiated in frog tongue

Latencies of gustatory neural impulses evoked by stimulation of the bullfrog tongue with the 4 basic taste substances (NaCl, acetic acid, quinine-HCl(Q-HCl), sucrose), CaCl2 and water were studied by recording antidromic impulses conducted to the fungiform papillae. Mean latencies of the impulses ranged from 58 to 107 ms when very strong stimuli, such as 2 M NaCl, 0.1 M acetic acid, 0.1 M Q-HCl, 2 M sucrose and 1 M CaCl2, were applied. Mean latency in response to water was 2.41 s. The time required for arrival of an applied taste stimulus on the taste receptor membrane was a mean of 20.1 ms. The time required for antidromic conduction from the impulse initiation site to the recording site was a mean of 2.4 ms. Electrical stimulation of the fungiform papilla with a strong intensity produced the impulse with a long and fluctuating latency. The mean minimum latency of the fluctuating impulse, from which the conduction time was subtracted, was 5.3 ms. Mechanical destruction of the taste disk situated at the top of the fungiform papilla resulted in a disappearance of the fluctuating impulse, suggesting that this was initiated synaptically via a depolarization of taste cells by electrical current. The minimum 5.3-ms latency was likely to be the time required from the onset of taste cell depolarization to the initiation of an impulse at the first node of Ranvier of myelinated gustatory fiber. These results indicate that the latencies of 58-107 ms by strong taste stimulation were composed of the 30- to 79-ms latency of taste cell receptor potential and the remaining 28 ms latency, which was the sum of the time of stimulant diffusion, the time from taste cell depolarization to the first impulse and the time of impulse conduction.

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.