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.     

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.     

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.     

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.     

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.     

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.     

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.     

Efferent fibers innervate gustatory and mechanosensitive afferent fibers in frog fungiform papillae

A possibility of efferent innervation of gustatory and mechanosensitive afferent fiber endings was studied in frog fungiform papillae with a suction electrode. The amplitude of antidromic impulses in a papillary afferent fiber induced by antidromically stimulating an afferent fiber of glossopharyngeal nerve (GPN) with low voltage pulses was inhibited for 40 s after the parasympathetic efferent fibers of GPN were stimulated orthodromically with high voltage pulses at 30 Hz for 10 s. This implies that electrical positivity of the outer surface of papillary afferent membrane was reduced by the efferent fiber-induced excitatory postsynaptic potential. The inhibition of afferent responses in the papillae was blocked by substance P receptor blocker, L-703,606, indicating that substance P is probably released from the efferent fiber terminals. Slow negative synaptic potential, which corresponded to a slow depolarizing synaptic potential, was extracellularly induced in papillary afferent terminals for 45 s by stimulating the parasympathetic efferent fibers of GPN with high voltage pulses at 30 Hz for 10 s. This synaptic potential was also blocked by L-703,606. These data indicate that papillary afferent fiber endings are innervated by parasympathetic efferent fibers.     

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.     

Electrical activity of the sympathetic ganglia during the depressor reflex

Tonic activity of neurons of the superior cervical sympathetic ganglion was recorded by the "sucrose gap" method and in the 4th and 5th lumbar sympathetic ganglia with the aid of focal nonpolarizing electrodes in acute experiments on anesthetized cats and rabbits. The preganglionic fibers of the ganglia were left intact. Stimulation of the depressor nerve not only sharply inhibited the tonic activity of the ganglia but also led to the appearance of electropositive potentials of 0.7 ± 0.2 mV in the superior cervical ganglion and 20–250 µV in the lumbar ganglia. The amplitude of this potential was unchanged by atropine (1 · 10^–6M). A similar effect occured without stimulation of the depressor nerve, after division of the preganglionic fibers or blocking of their conduction; it is attributed to the cessation of preganglionic tonic impulses which induce not only spikes, but also many EPSPs in neurons of the ganglion. Their frequency in the lumbar ganglia was 4/sec. Summation of these EPSPs leads to constant electronegativity of the ganglion surface relative to the postganglionic fibers, and its disappearance is recorded as a positive potential. Stimulation of the depressor nerve thus does not induce IPSPs in the ganglion; consequently, the inhibition of synaptic activity observed under these circumstances is located in the CNS and not in the ganglion.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 6, No. 5, pp. 519–524, September–October, 1974.     

Effects of antidromic activity in gustatory nerve fibers on taste disc cells of the frog tongue

Summary Antidromic electrical stimulation of the lingual branch of the glossopharyngeal (IX) nerve of the frog was carried out while recording intracellular potentials of taste disc cells.Antidromic activation of sensory fibers resulted in depolarization of cells of the upper layer of the disc and most commonly in hyperpolarization of the cells in the lower layer. These changes in potential exhibited latencies greater than 1 s (Fig. 3), and thus cannot be due to electrotonic effects of action potentials in terminals of IX nerve fibers, which have much shorter conduction times. These cell potentials also showed summation, adaptation and post-stimulus rebound (Figs. 3, 4).Depending upon the chemical stimulus used, antidromic activity produced either depression or enhancement of gustatory fiber discharge in response to taste stimuli (Fig. 5).Alteration of the resting membrane potential by current injection did not significantly modify the antidromically evoked potentials (Fig. 8), whereas chemical stimulation of the tongue did (Fig. 7), indicating that these potential changes are not the result of passive electrical processes.These experimental results indicate that the membrane potential of taste disc cells can be modified by antidromic activity in their afferent nerves. This mechanism may be responsible for peripheral interactions among gustatory units of the frog tongue.The research was supported in part by NIH grant NS-09168.     

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.     

Electrophysiological study on the effects of leptin in rat dorsal motor nucleus of the vagus

Immunoreactivity of leptin receptor (Ob-R) has been detected in rat dorsal motor nucleus of the vagus (DMNV). Here, we confirmed the presence of Ob-R immunoreactivity on retrograde-labeled parasympathetic preganglionic neurons in the DMNV of neonatal rats. The present study investigated the effects of leptin on DMNV neurons, including parasympathetic preganglionic neurons, by using whole cell patch-clamp recording technique in brain stem slices of neonatal rats. Leptin (30-300 nM) induced membrane depolarization and hyperpolarization, respectively, in 14 and 15 out of 80 DMNV neurons tested. Both leptin-induced inward and outward currents persisted in the presence of TTX, indicating that leptin affected DNMV neurons postsynaptically. The current-voltage (I-V) curve of leptin-induced inward currents is characterized by negative slope conductance and has an average reversal potential of -90 +/- 3 mV. The reversal potential of the leptin-induced inward current was shifted to a more positive potential level in a high-potassium medium. These results indicate that a decrease in potassium conductance is likely the main ionic mechanism underlying the leptin-induced depolarization. On the other hand, the I-V curve of leptin-induced outward currents is characterized by positive slope conductance and has an average reversal potential of -88 +/- 3 mV, suggesting that an increase in potassium conductance may underlie leptin-induced hyperpolarization. Most of the leptin-responsive DMNV neurons were identified as being parasympathetic preganglionic neurons. These results suggest that the DMNV is one of the central target sites of leptin, and leptin can regulate parasympathetic outflow from the DMNV by directly acting on the parasympathetic preganglionic neurons of the DMNV.     

Microelectrode study of the cellular reactions of the taste bud in the frog Rana temporaria

Microelectrophysiological studies reveal two types of cells in the taste bud of frog which differ by the level of their membrane potential. During vertical implantation of microelectrode through the apical part of the taste bud, the potential difference in the upper layer amounts to 15 mV. Further implantation of the electrode results in a stepwise decrease of the potential difference up to 27 mV. Cells of the deeper layer are located 12-24 micron lower from the apical surface. Stimulation of cells by solutions of chemical substances is accompanied by cell depolarization, its amplitude being proportional to stimulus concentration. The steepness of depolarization depends on the modality of the stimulus, being maximum for salts. The data obtained suggest that cells of the second layer, with a higher resting membrane potential level, are taste ones.     

Effects of Na-octanoate on potassium contractures in normal and denervated frog tonic fibres

In frog twitch muscle fibres, Na-octanoate (NaC8) shifted the relation between potassium induced tension and membrane potential to the right. The present study has been carried out to investigate the effect of this fatty acid on frog tonic fibres. Potassium contractures measured on bundles of 30-40 fibres of ileofibularis muscles were less decreased by NaC8 (2.5-10 mmol/l) than those of twitch fibre bundles. In denervated muscles the sensitivity to NaC8 was increased, probably due to the development of sodium channels in the membranes. Experiments with mixed fibre bundles also showed a lower influence of NaC8 on potassium contracture of tonic fibres. On the other hand, tonic fibres showed a lower threshold of the potassium induced tension as well as a lower K+ concentration for maximal activation. This lower threshold was further lowered by NaC8, corresponding to a shift of the relation between potassium concentration and tension to the left. The membrane resting potentials were -58 +/- 9 mV in tonic fibres and -83 +/- 5 mV in twitch fibres. Five mmol/l NaC8 only induced depolarization of the membrane of tonic fibres. This depolarization (by about 20 mV) may be responsible for the threshold shift to lower K+ concentration in NaC8-exposed tonic fibres. In addition to the effects of NaC8 on sodium channels, interactions with Ca2+ binding sites are discussed.     

Uteroglobin and other proteins in rabbit blastocyst fluid after development in vivo and in vitro

Summary The taste bud of the human fungiform papilla was examined by electron microscopy. Typical type I, type II, and type III cells were found along with contact sites with nerve endings. Vesicles in nerve fibers contacting type I and type II cells suggest that these cells may receive efferent impulses, whereas vesicles and granules in type III cells adjacent to (afferent) nerve fibers support the view that type III cells are sensory receptors. All of these features are virtually indistinguishable from those previously reported in fungiform taste buds of other mammals.Supported by fellowship from Campbell Institute for Food Research.     

Are there efferent synapses in fish taste buds?

In fish, nerve fibers of taste buds are organized within the bud's nerve fiber plexus. It is located between the sensory epithelium consisting of light and dark elongated cells and the basal cells. It comprises the basal parts and processes of light and dark cells that intermingle with nerve fibers, which are the dendritic endings of the taste sensory neurons belonging to the cranial nerves VII, IX or X. Most of the synapses at the plexus are afferent; they have synaptic vesicles on the light (or dark) cells side, which is presynaptic. In contrast, the presumed efferent synapses may be rich in synaptic vesicles on the nerve fibers (presynaptic) side, whereas the cells (postsynaptic) side may contain a subsynaptic cistern; a flat compartment of the smooth endoplasmic reticulum. This structure is regarded as a prerequisite of a typical efferent synapse, as occurring in cochlear and vestibular hair cells. In fish taste buds, efferent synapses are rare and were found only in a few species that belong to different taxa. The significance of efferent synapses in fish taste buds is not well understood, because efferent connections between the gustatory nuclei of the medulla with taste buds are not yet proved.     

The immunomorphologic analysis of innervation of chromaffin paraganglian cells of the mammalian arteries and heart

Innervation of chromaffin cells of paraganglia of the wall of mammalian large arterial vessels and heart (in rat, cat, and human) was studied by neuromorphological and immunohistochemical methods. There is established similarity in structure of specialized, "basket"-type nerve endings of the chromaffin cells (ChC) with pericellular nerve apparatuses of sympathetic and parasympathetic autonomic neurons. It is proposed to use immunohistochemical reaction for synaptophysin as method of selective detection of ChC of paraganglia and adrenal medulla. The conclusion is made that synaptophysin-positive terminals (SPPT) found on bodies of ChC and postganglionic neurons represent efferent, rather than afferent, synapses formed by myelinated axons of preganglionic fibers. It is suggested that ChC of paraganglia alongside with their characteristic endocrine function participate in complex mechanisms of chemoreceptor regulation of tissue homeostasis of mammalian blood vessels and heart.     

The immunomorphological analysis of innervation of paraganglian chromaffin cells of mammalian arteries and heart

Innervation of chromaffin cells of paraganglia of the wall of mammalian large arterial vessels and heart (in rat, cat, and human) was studied by neuromorphological and immunohistochemical methods. There is established similarity in structure of specialized, “basket“-like nerve endings of the chromaffin cells (ChC) with pericellular nerve apparatuses of sympathetic and parasympathetic autonomic neurons. It is proposed to use immunohistochemical reaction for synaptophysin as method of selective detection of ChC of paraganglia and adrenal medulla. The conclusion is made that synaptophysin-positive terminals (SPPT) found on bodies of ChC and postganglionic neurons represent efferent, rather than afferent, synapses formed by myelinated axons of preganglionic fibers. It is suggested that ChC of paraganglia alongside with their characteristic endocrine function participate in complex mechanisms of chemoreceptor regulation of tissue homeostasis of mammalian blood vessels and heart.