Estimation of the junctional resistance between electrically coupled receptor cells in Necturus taste buds

Changes of the biophysical properties of voltage-gated Na+ currents during maturation of the sodium-taste cells in rat fungiform papillae

Taste cells are a heterogeneous population of sensory receptors that undergo continuous turnover. Different chemo-sensitive cell lines rely on action potentials to release the neurotransmitter onto nerve endings. The electrical excitability is due to the presence of a tetrodotoxin-sensitive, voltage-gated sodium current (I_Na ) similar to that found in neurons. Since the biophysical properties of neuronal I_Na change during development, we wondered whether the same also occurred in taste cells. Here, we used the patch-clamp recording technique to study I_Na in salt-sensing cells (sodium cells) of rat fungiform papillae. We identified these cells by exploiting the known blocking effect of amiloride on ENaC, the sodium (salt) receptor. Based on the amplitude of I_Na , which is known to increase during development, we subdivided sodium cells into two groups: cells with small sodium current (SSC cells; I_Na  < 1 nA) and cells with large sodium current (LSC cells; I_Na  > 1 nA). We found that: the voltage dependence of activation and inactivation significantly differed between these subsets; a slowly inactivating sodium current was more prominent in LSC cells; membrane capacitance in SSC cells was larger than in LSC cells. mRNA expression analysis of the α-subunits of voltage-gated sodium channels in fungiform taste buds supported the functional data. Lucifer Yellow labelling of recorded cells revealed that our electrophysiological criterion for distinguishing two broad groups of taste cells was in good agreement with morphological observations for cell maturity. Thus, all these findings are consistent with developmental changes in the voltage-dependent properties of sodium-taste cells. KEY POINTS: Taste cells are sensory receptors that undergo continuous turnover while they detect food chemicals and communicate with afferent nerve fibres. The voltage-gated sodium current (I_Na ) is a key ion current for generating action potentials in fully differentiated and chemo-sensitive taste cells, which use electrical signalling to release neurotransmitters. Here we show that, during the maturation of rat taste cells involved in salt detection (sodium cells), the biophysical properties of I_Na , such as voltage dependence of activation and inactivation, change significantly. Our results help reveal how taste cells gain electrical excitability during turnover, a property critical to their operation as chemical detectors that relay sensory information to nerve fibres.

Chemical and electrical synaptic interactions among taste bud cells

Chemical synapses between taste cells were first proposed based on electron microscopy of fish taste buds. Subsequently, researchers found considerable evidence for electrical coupling in fish, amphibian, and possibly mammalian taste buds. The development lingual slice and isolated cell preparations allowed detailed investigations of cell-cell interactions, both chemical and electrical, in taste buds. The identification of serotonin and ATP as taste neurotransmitters focused attention onto chemical synaptic interactions between taste cells and research on electrical coupling faded. Findings from Ca²⁺ imaging, electrophysiology, and molecular biology indicate that several neurotransmitters, including ATP, serotonin, GABA, acetylcholine, and norepinephrine, are secreted by taste cells and exert paracrine interactions in taste buds. Most work has been done on interactions between Type II and Type III taste cells. This brief review follows the trail of studies on cell-cell interactions in taste buds, from the initial ultrastructural observations to the most recent optogenetic manipulations.

Electrical coupling and its channels

Harris explores the development of our current understanding of electrical coupling between cells and the channels that mediate it, highlighting the contributions of the Journal of General Physiology.

As the physiology of synapses began to be explored in the 1950s, it became clear that electrical communication between neurons could not always be explained by chemical transmission. Instead, careful studies pointed to a direct intercellular pathway of current flow and to the anatomical structure that was (eventually) called the gap junction. The mechanism of intercellular current flow was simple compared with chemical transmission, but the consequences of electrical signaling in excitable tissues were not. With the recognition that channels were a means of passive ion movement across membranes, the character and behavior of gap junction channels came under scrutiny. It became evident that these gated channels mediated intercellular transfer of small molecules as well as atomic ions, thereby mediating chemical, as well as electrical, signaling. Members of the responsible protein family in vertebrates—connexins—were cloned and their channels studied by many of the increasingly biophysical techniques that were being applied to other channels. As described here, much of the evolution of the field, from electrical coupling to channel structure–function, has appeared in the pages of the Journal of General Physiology.

ATP activates P2X receptors to mediate gap junctional coupling in the cochlea

ATP is an important extracellular signaling molecule and can activate both ionotropic (P2X) and metabotropic purinergic (P2Y) receptors to influence cellular function in many aspects. Gap junction is an intercellular channel and plays a critical role in hearing. Here, we report that stimulation of ATP reduced gap junctional coupling between cochlear supporting cells. This uncoupling effect could be evoked by nanomolar physiological levels of ATP. A P2X receptor agonist benzoylbenzoyl-ATP (BzATP) but not a P2Y receptor agonist UTP stimulated this uncoupling effect. Application of P2X receptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 50μM) or oxidized ATP (oATP, 0.1mM) eliminated this uncoupling effect. We further found that ATP activated P2X receptors in the cochlear supporting cells allowing Ca(2+) influxing, thereby increasing intracellular Ca(2+) concentration to mediate gap junctions. These data suggest that ATP can mediate cochlear gap junctions at the physiological level by the activation of P2X receptors rather than P2Y receptors. This P2X receptor-mediated purinergic control on the cochlear gap junctions may play an important role in the regulation of K(+)-recycling for ionic homeostasis in the cochlea and the reduction of hearing sensitivity under noise stress for protection.

Cellular mechanisms in taste buds

In the soft palate, tongue, pharynx and larynx surrounding the oral region, taste buds are present, allowing the sensation of taste. On the tongue surface, 3 kinds of papillae are present: fungiform, foliate, and circumvallate. Approximately 5,000 taste buds cover the surface of the human tongue, with about 30% fungiform, 30% foliate and 40% circumvallate papillae. Each taste bud comprises 4 kinds of cells, namely high dark (type I), low light (type II), and intermediate (type III) cells in electron density and Merkel-like taste basal cells (type IV) located at a distance from taste pores. Type II cells sense taste stimuli and type III cells transmit taste signals to sensory afferent nerve fibers. However, type I and type IV cells are not considered to possess obvious taste functions. Synaptic interactions that mediate communication in taste cells provide signal outputs to primary afferent fibers. In the study of taste bud cells, molecular functional techniques using single cells have recently been applied. Serotonin (5-HT) plays a role in cell-to-cell transmission of taste signals. ATP fills the criterion of a neurotransmitter that activates receptors of taste nerve fibers. Findings on 5-HT and ATP suggest that various different transmitters and receptors are present in taste buds. However, no firm evidence for taste-evoked release from type III cells has been identified, except for 5-HT and ATP. These results suggest that different transmitters and receptors may not be present in taste buds. Accordingly, an understanding of how transmitters might function remains elusive.

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.

Cell communication in taste buds

Taste bud cells communicate with sensory afferent fibers and may also exchange information with adjacent cells. Indeed, communication between taste cells via conventional and/or novel synaptic interactions may occur prior to signal output to primary afferent fibers. This review discusses synaptic processing in taste buds and summarizes results showing that it is now possible to measure real-time release of synaptic transmitters during taste stimulation using cellular biosensors. There is strong evidence that serotonin and ATP play a role in cell-to-cell signaling and sensory output in the gustatory end organs.

Using biosensors to detect the release of serotonin from taste buds during taste stimulation

CHO cells transfected with high-affinity 5HT receptors were used to detect and identify the release of serotonin from taste buds. Taste cells release 5HT when depolarized or when stimulated with bitter, sweet, or sour tastants. Sour- and depolarization-evoked release of 5HT from taste buds is triggered by Ca2+ influx from the extracellular fluid. In contrast, bitter- and sweet-evoked release of 5HT is triggered by Ca2+ derived from intracellular stores.

Electrical coupling in sustentacular cells of the mouse olfactory epithelium

Sustentacular cells (SCs) line the apical surface of the olfactory epithelium (OE) and provide trophic, metabolic, and mechanical support for olfactory receptor neurons. Morphological studies have suggested that SCs possess gap junctions, although physiological evidence for gap junctional communication in mammalian SCs is lacking. In the present study we investigated whether coupling exists between SCs situated in tissue slices of OE from neonatal (P0-P4) mice. Using whole cell and cell-attached patch recordings from SCs, we demonstrate that SCs are electrically coupled by junctional resistances on the order of 300 M(omega). Under whole cell recording conditions, Alexa 488 added to the pipette solution failed to reveal dye coupling between SCs. Electrical coupling was deduced from the biexponential decay of capacitive currents recorded from SCs and from the bell-shaped voltage dependency of a P2Y-receptor-activated current, both of which were abolished by 18beta-glycyrrhetinic acid (20-50 microM), a blocker of gap junctions. These data provide strong evidence for functional coupling between SCs, the physiological importance of which is discussed.

Determination of cell capacitance using the exact empirical solution of partial differential Y/partial differential Cm and its phase angle

Measures of membrane capacitance offer insight into a variety of cellular processes. Unfortunately, popular methodologies rely on model simplifications that sensitize them to interference from inevitable changes in resistive components of the traditional cell-clamp model. Here I report on a novel method to measure membrane capacitance that disposes of the usual simplifications and assumptions, yet is immune to such interference and works on the millisecond timescale. It is based on the exact empirical determination of the elusive partial derivative, partial differential Y/partial differential C(m), which heretofore had been approximated. Furthermore, I illustrate how this method extends to the vesicle fusion problem by permitting the determination of partial differential Y(v)/partial differential C(v), thereby providing estimates of fusion pore conductance and vesicle capacitance. Finally, I provide simulation examples and physiological examples of how the method can be used to study processes that are routinely interrogated by measures of membrane capacitance.

Electrophysiology of Necturus taste cells

Taste buds are sensory end organs that detect chemical substances occurring in foodstuffs and relay the relative information to the brain. The mechanisms by which the chemical stimuli are converted into biological signals represent a central issue in taste research. Our understanding of how taste buds accomplish this operation relies on the detailed knowledge of the biological properties of taste bud cells-the taste cells-and of the functional processes occurring in these cells during chemostimulation. The amphibian Necturus maculosus (mudpuppy) has proven to be a very useful model for studying basic cellular processes of vertebrate taste reception, some of which are still awaiting to be explored in mammals. The main advantages offered by Necturus are the large size of its taste cells and the relative accessibility of its taste buds, which can therefore be handled easily for experimental manipulations. In this review, I summarize the functional properties of Necturus taste cells studied with electrophysiological techniques (intracellular recordings and patch-clamp recordings). My focus is on ion channels in taste cells and on their role in signal transduction, as well as on the functional relationships among the cells inside Necturus taste buds. This information has revealed to be well suited to outline some of the general physiological processes occurring during taste reception in vertebrates, including mammals, and may represent a useful framework for understanding how taste buds work.

Postnatal development of membrane excitability in taste cells of the mouse vallate papilla

The mammalian peripheral taste system undergoes functional changes during postnatal development. These changes could reflect age-dependent alterations in the membrane properties of taste cells, which use a vast array of ion channels for transduction mechanisms. Yet, scarce information is available on the membrane events in developing taste cells. We have addressed this issue by studying voltage-dependent Na+, K+, and Cl- currents (I(Na), I(K), and I(Cl), respectively) in a subset of taste cells (the so-called "Na/OUT" cells, which are electrically excitable and thought to be sensory) from mouse vallate papilla. Voltage-dependent currents play a key role during taste transduction, especially in the generation of action potentials. Patch-clamp recordings revealed that I(Na), I(K), and I(Cl) were expressed early in postnatal development. However, only I(K) and I(Cl) densities increased significantly in developing Na/OUT cells. Consistent with the rise of I(K) density, we found that action potential waveform changed markedly, with an increased speed of repolarization that was accompanied by an enhanced capability of repetitive firing. In addition to membrane excitability changes in putative sensory cells, we observed a concomitant increase in the occurrence of glia-like taste cells (the so called "leaky" cells) among patched cells. Leaky cells are likely involved in dissipating the increase of extracellular K+ during action potential discharge in chemosensory cells. Thus, developing taste cells of the mouse vallate papilla undergo a significant electrophysiological maturation and diversification. These functional changes may have a profound impact on the transduction capabilities of taste buds during development.

Postnatal development of membrane excitability in taste cells of the mouse vallate papilla

The mammalian peripheral taste system undergoes functional changes during postnatal development. These changes could reflect age-dependent alterations in the membrane properties of taste cells, which use a vast array of ion channels for transduction mechanisms. Yet, scarce information is available on the membrane events in developing taste cells. We have addressed this issue by studying voltage-dependent Na+, K+, and Cl- currents (I(Na), I(K), and I(Cl), respectively) in a subset of taste cells (the so-called "Na/OUT" cells, which are electrically excitable and thought to be sensory) from mouse vallate papilla. Voltage-dependent currents play a key role during taste transduction, especially in the generation of action potentials. Patch-clamp recordings revealed that I(Na), I(K), and I(Cl) were expressed early in postnatal development. However, only I(K) and I(Cl) densities increased significantly in developing Na/OUT cells. Consistent with the rise of I(K) density, we found that action potential waveform changed markedly, with an increased speed of repolarization that was accompanied by an enhanced capability of repetitive firing. In addition to membrane excitability changes in putative sensory cells, we observed a concomitant increase in the occurrence of glia-like taste cells (the so called "leaky" cells) among patched cells. Leaky cells are likely involved in dissipating the increase of extracellular K+ during action potential discharge in chemosensory cells. Thus, developing taste cells of the mouse vallate papilla undergo a significant electrophysiological maturation and diversification. These functional changes may have a profound impact on the transduction capabilities of taste buds during development.

Gap junction communication and connexin 43 gene expression in a rat granulosa cell line: regulation by follicle-stimulating hormone

Follicle-stimulating hormone is the major regulator of growth and development of antral follicles in the ovary. Granulosa cells (GCs) in these follicles are coupled via gap junctions (GJs) consisting of connexin 43 (Cx 43). Because we and others have found that Cx 43 and GJs, respectively, are more abundant in large antral follicles compared with small antral and preantral follicles, we hypothesized that FSH may control Cx 43 gene expression, GJ formation, and intercellular communication. To directly address these points, we chose a rat GC line (GFSHR-17) expressing the FSH receptor and the Cx 43 gene. The functionality of FSH receptors was shown by the effects of porcine FSH, namely cell rounding, reduced cellular proliferation, and stimulation of progesterone production of GFSHR-17 cells, which are effects that were detectable within hours. Treatment with FSH also statistically significantly increased Cx 43 mRNA levels, as shown after 6 to 9 h in Northern blots. These effects were antedated by altered GJ communication, which was observed within seconds. Using a single-cell/whole-cell patch clamp technique, we showed that FSH rapidly and reversibly enhanced electrical cell coupling of GFSHR-17 cells. Increased GJ communication was associated with statistically significantly decreased phosphorylation of Cx 43, which was observed within 10 min after FSH addition, during immunoprecipitation experiments. Our results demonstrate, to our knowledge for the first time, that the gonadotropin FSH acutely and directly stimulates intercellular communication of GFSHR-17 cells through existing GJs. Moreover, FSH also increases levels of Cx 43 mRNA. These changes are associated with reduced proliferation and enhanced differentiation of GFSHR-17 cells. In vivo factors in addition to FSH may be involved in the regulation of GJ/GJ communication between GCs in the follicle, but our results suggest that improved cell-to-cell coupling, enhanced Cx 43 gene expression, and possibly, formation of new GJs are direct consequences of FSH receptor activation and may antedate and/or initiate the pivotal effects of FSH on GCs.

Neuromodulatory effects of gonadotropin releasing hormone on olfactory receptor neurons

The terminal nerve is an anterior cranial nerve that innervates the lamina propria of the chemosensory epithelia of the nasal cavity. The function of the terminal nerve is ambiguous, but it has been suggested to serve a neuromodulatory role. We tested this hypothesis by exposing olfactory receptor neurons from mudpuppies (Necturus maculosus) to a peptide, gonadotropin releasing hormone (GnRH), that is found in cells and fibers of the terminal nerve. We used voltage-clamped whole-cell recordings to examine the effects of 0. 5-50 micrometer GnRH on voltage-activated currents in olfactory receptor neurons from epithelial slices. We found that GnRH increases the magnitude, but does not alter the kinetics, of a tetrodotoxin-sensitive inward current. This increase in magnitude generally begins 5-10 min after initial exposure to GnRH, is sustained for at least 60 min during GnRH exposure, and recovers to baseline within 5 min after GnRH is washed off. This effect occurred in almost 60% of the total number of olfactory receptor neurons examined and appeared to be seasonal: approximately 67% of neurons responded to GnRH during the courtship and mating season, compared with approximately 33% during the summer, when the sexes separate. GnRH also appears to alter an outward current in the same cells. Taken together, these data suggest that GnRH increases the excitability of olfactory receptor neurons and that the terminal nerve functions to modulate the odorant sensitivity of olfactory receptor neurons.

Neuromodulatory effects of gonadotropin releasing hormone on olfactory receptor neurons

The terminal nerve is an anterior cranial nerve that innervates the lamina propria of the chemosensory epithelia of the nasal cavity. The function of the terminal nerve is ambiguous, but it has been suggested to serve a neuromodulatory role. We tested this hypothesis by exposing olfactory receptor neurons from mudpuppies (Necturus maculosus) to a peptide, gonadotropin releasing hormone (GnRH), that is found in cells and fibers of the terminal nerve. We used voltage-clamped whole-cell recordings to examine the effects of 0. 5-50 micrometer GnRH on voltage-activated currents in olfactory receptor neurons from epithelial slices. We found that GnRH increases the magnitude, but does not alter the kinetics, of a tetrodotoxin-sensitive inward current. This increase in magnitude generally begins 5-10 min after initial exposure to GnRH, is sustained for at least 60 min during GnRH exposure, and recovers to baseline within 5 min after GnRH is washed off. This effect occurred in almost 60% of the total number of olfactory receptor neurons examined and appeared to be seasonal: approximately 67% of neurons responded to GnRH during the courtship and mating season, compared with approximately 33% during the summer, when the sexes separate. GnRH also appears to alter an outward current in the same cells. Taken together, these data suggest that GnRH increases the excitability of olfactory receptor neurons and that the terminal nerve functions to modulate the odorant sensitivity of olfactory receptor neurons.

Effect of membrane tension on gap junctional conductance of supporting cells in Corti's organ

The effects of turgor pressure-induced membrane tension on junctional coupling of Hensen cell isolates from the inner ear were evaluated by input capacitance or transjunctional conductance measurement techniques. Turgor pressure was altered by changing either pipette pressure or the osmolarities of extracellular solutions. Both positive pipette pressure and extracellular applications of hypotonic solutions, which caused cell size to concomitantly increase, uncoupled the cells as indicated by reduced input capacitance and transjunctional conductance. These changes were, in many cases, reversible and repeatable. Intracellular application of 50 microM H-7, a broad-based protein kinase inhibitor, and 10 mM BAPTA did not block the uncoupling effect of positive turgor pressure on inner ear gap junctions. The transjunctional conductance at a holding potential of -80 mV was 53.6 +/- 5.8 nS (mean +/- SEM, n = 9) and decreased approximately 40% at a turgor pressure of 1.41 +/- 0.05 kPa. Considering the coincident kinetics of cell deformation and uncoupling, we speculate that mechanical forces work directly on gap junctions of the inner ear. These results suggest that pathologies that induce imbalances in cochlear osmotic pressure regulation may compromise normal cochlear homeostasis.

Acute loss of cell-cell communication caused by G protein-coupled receptors: a critical role for c-Src

Gap junctions mediate cell-cell communication in almost all tissues, but little is known about their regulation by physiological stimuli. Using a novel single-electrode technique, together with dye coupling studies, we show that in cells expressing gap junction protein connexin43, cell-cell communication is rapidly disrupted by G protein-coupled receptor agonists, notably lysophosphatidic acid, thrombin, and neuropeptides. In the continuous presence of agonist, junctional communication fully recovers within 1-2 h of receptor stimulation. In contrast, a desensitization-defective G protein-coupled receptor mediates prolonged uncoupling, indicating that recovery of communication is controlled, at least in part, by receptor desensitization. Agonist-induced gap junction closure consistently follows inositol lipid breakdown and membrane depolarization and coincides with Rho-mediated cytoskeletal remodeling. However, we find that gap junction closure is independent of Ca2+, protein kinase C, mitogen-activated protein kinase, or membrane potential, and requires neither Rho nor Ras activation. Gap junction closure is prevented by tyrphostins, by dominant-negative c-Src, and in Src-deficient cells. Thus, G protein-coupled receptors use a Src tyrosine kinase pathway to transiently inhibit connexin43-based cell-cell communication.

Mechanisms of taste transduction

Taste cells use a wide variety of mechanisms for transduction. Ionic stimuli, such as salts and acids, interact directly with ion channels to depolarize taste cells. More complex stimuli, such as sugars and amino acids, utilize apically located receptors for transduction. Recent molecular biological results suggest that the metabotropic glutamate receptor mGluR4 may function in glutamate taste transduction. New biochemical studies have identified a bitter-responsive receptor that activates gustducin. Unexpected results with knockout mice suggest that gustducin may be directly involved in both bitter and sweet transduction. Electrophysiological experiments indicate that both inositol trisphosphate and cyclic nucleotides function in both bitter and sweet transduction events.