A bitter substance induces a rise in intracellular calcium in a subpopulation of rat taste cells

Generation of inositol phosphates in bitter taste transduction

It is probable that there is a diversity of mechanisms involved in the transduction of bitter taste. One of these mechanisms uses the second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Partial membrane preparations from circumvallate and foliate taste regions of mice tongues responded to the addition of known bitter taste stimuli by increasing the amount of inositol phosphates produced after 30 s incubation. Addition of both the bitter stimulus, sucrose octaacetate and the G-protein stimulant, GTP gamma S, led to an enhanced production of inositol phosphates compared with either alone. Pretreatment of the tissue samples with pertussis toxin eliminated all response to sucrose octaacetate plus GTP gamma S, whereas pretreatment with cholera toxin was without effect. Western blots of solubilized tissue from circumvallate and foliate regions probed with antibodies to the alpha-subunit of several types of G-proteins revealed bands reactive to antibodies against G alpha i1-2 and G alpha o, with no apparent activity to antibodies against G alpha i3. Given the results from the immunoblots and those of the toxin experiments, it is proposed that the transduction of the bitter taste of sucrose octaacetate in mice involves a receptor-mediated activation of a Gi-type protein which activates a phospholipase C to produce the two second messengers, IP3 and DAG.

Molecular cloning of G proteins and phosphodiesterases from rat taste cells

To identify and characterize those proteins involved in taste transduction, we cloned G proteins and phosphodiesterases from rat taste tissue. Using degenerate primers corresponding to conserved regions of G protein alpha subunits, the polymerase chain reaction was used to amplify and clone eight distinct cDNAs: alpha i-2, alpha i-3, alpha 12, alpha 14, a(s), alpha t-rod, alpha t-cone and alpha gustducin. alpha i-3, alpha 14, alpha s, and alpha t-rod are more highly expressed in taste tissue than in the surrounding nonsensory tissue. alpha gustducin is only expressed in taste cells. Rod transducin had previously been found only in the rod cells of the retina, where it converts light stimulation of rhodopsin into activation of cGMP phosphodiesterase. The primary sequence of alpha gustducin shows striking similarities to rod transducin in the receptor interaction domain and the phosphodiesterase activation site. We propose that gustducin and transducin regulate phosphodiesterase activity in taste cells and that this may promote bitter transduction and inhibit sweet transduction. Consistent with this proposal, we cloned two types of cAMP PDE from taste tissue: dnc-1 and PDE-3.

Taste perception of bitter compounds in young and elderly persons: relation to lipophilicity of bitter compounds

Threshold and suprathreshold sensitivities to 13 bitter compounds were determined for 16 young adults (mean age = 27.4 years) and 18 elderly persons (mean age = 81.3 years). Half of the subjects in each age group were tasters of the bitter compound phenylthiocarbamide (PTC) and half were nontasters. Both detection and recognition thresholds, determined by a forced-choice ascending detection method, were elevated in older subjects; there were no significant differences in threshold values between tasters and nontasters of PTC. A strong relationship between bitter threshold values and the logarithm of the octanol/water partition coefficient was found for both young and elderly subjects. For young subjects, suprathreshold bitterness ratings were more intense for tasters of PTC compared with nontasters. Significant losses in suprathreshold sensitivity to bitter tastants with age were also found. However, unlike threshold sensitivity, no relationship was found between suprathreshold bitter taste intensity and lipophilicity.

Electrophysiological and morphological properties of light and dark cells isolated from mudpuppy taste buds

Isolated Necturus taste receptor cells were studied by giga-seal whole-cell recording and electron microscopy to correlate electrophysiological properties with taste cell structural features. Dark (type I) cells were identified by the presence of dense granular packets in the supranuclear and apical regions of the cytoplasm. In response to a series of depolarizing voltage commands from a holding potential of -80 mV, these cells exhibited a transient, TTX-sensitive inward Na+ current, a sustained outward K+ current, and a slowly inactivating inward Ca++ current. Light (type II) cells were identified by a lack of granular packets and by an abundance of smooth endoplasmic reticulum distributed throughout the cell. In addition, isolated light cells had clear vesicular inclusions in the cytoplasm and blebs on the plasma membrane. Light cells were divided into two functional populations based upon electrophysiological criteria: cells with inward and outward currents, and cells with outward currents only. Light cells with inward and outward currents had voltage-activated Na+, K+, and Ca++ currents with properties similar to those of dark cells. In contrast, the second group of light cells had only voltage-activated outward K+ currents in response to depolarizing voltage commands. These data suggest that dark cells and light cells with inward and outward currents are capable of generating action potentials and releasing neurotransmitters onto gustatory afferent neurons in response to taste stimulation. In contrast, light cells with outward currents only likely serve a different function in the taste bud.

Modulators of the adenylate cyclase system can alter electrophysiological taste responses in gerbil

The adenylate cyclase system has been implicated in taste transduction. The purpose of this study was to determine whether application of modulators of the adenylate cyclase system to the tongue alter taste responses. Integrated chorda tympani (CT) recordings were made in gerbils to bitter, sweet, salty, sour, and glutamate tastants before and after a 4-min application of four types of modulators of the adenylate cyclase system. The four types of modulators tested were: a) NaF, a compound that promotes dissociation of GTP binding protein; b) forskolin, a powerful stimulant of adenylate cyclase; c) 8-bromoadenosine 3' :5'-cyclic monophosphate sodium salt (8BrcAMP) and N6,2'-O-dibutyryl-adenosine 3' :5'-cyclic monophosphate sodium salt (DBcAMP), two membrane permeable forms of cAMP; and d) 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine dihydrochloride (H-7) and N-(2-[methylamino]ethyl)-5-isoquinolinesulfonamide dihydrochloride) (H-8), which are protein kinase inhibitors. The tast compounds tested were: NaCl (30 mM), monosodium glutamate-MSG (50 mM), sucrose (30 mM), HCl (5 mM and 10 mM), KCl (300 mM), quinine HCl (30 mM), MgCl2 (30 mM), erythromycin (0.7 mM and 1 mM), HCl (5 mM and 10 mM), and urea (2 M). The main findings were as follows. NaF (20 mM) significantly inhibited responses to bitter compounds up to 35% and enhanced the response to sucrose by 30%. NaCl (20 mM), used as a control for NaF, inhibited most responses up to 78% with no enhancement of sucrose as seen with NaF. 8BrcAMP (1.16 mM) reduced the responses to bitter-tasting quinine HCl, MgCl2, and erythromycin but not to urea.(ABSTRACT TRUNCATED AT 250 WORDS)

Taste bud expression of human blood group antigens

Some human blood group antigens are expressed by rodent epithelial cells at different stages of differentiation. Since adult taste cells are continually replaced throughout life, we investigated the expression of the H, B, A and Lewisb blood group determinants by cells of the rat fungiform, foliate and vallate papillae. We employed antibodies against the trisaccharide structures of the H, B, and A blood group antigens and against the Lewisb blood group epitope in studies of normal and denervated taste buds. The antibody against the H antigen reacted with the majority of cells in all taste buds and with cells in the spinous layer of the tongue epithelium. The B antigen was expressed by the majority of taste cells but not by other epithelial cells. The expression of the A antigen was significantly less in the fungiform taste buds than in the vallate or foliate taste buds. The A antigen was also abundantly expressed in the acini of the lingual salivary glands. The Lewisb epitope was expressed by a subset of cells in taste buds of the fungiform, foliate and vallate papillae. Taste buds are trophically dependent upon gustatory nerve innervation. Transection of the chorda tympani or the IXth nerve resulted in the loss of expression of these molecules from the gustatory epithelium, indicating that they are expressed only on differentiated taste cells. The blood group antigens are lactoseries carbohydrates; they are differentially expressed in developing cochlear hair cells and olfactory neurons and may play roles in cell-cell recognition, adhesion, and other interactions important in the developing nervous system. They could have similar functions in the taste and olfactory systems, where the receptors are continually renewed and new synapses between the receptors and their neural targets continually form.

Expression and localization of amiloride-sensitive sodium channel indicate a role for non-taste cells in taste perception

Salty taste is blocked by the diuretic amiloride, which inhibits specific sodium channels. We have isolated an amiloride-sensitive sodium channel (ASSC) from taste tissues by polymerase chain reaction and screening of a cDNA library prepared from rat circumvallate papillae. Northern analysis reveals ASSC in taste and non-taste tissues with the highest level of expression of ASSC in the lung. In situ hybridization establishes ASSC localizations in the epithelia of lung and colon as well as tongue epithelial layers containing and lacking taste buds. These results support a model in which ASSC in non-taste cells regulates responses of taste cells to salt as well as other tastants.

Some taste substances are direct activators of G-proteins

Amphiphilic substances may stimulate cellular events through direct activation of G-proteins. The present experiments indicate that several amphiphilic sweeteners and the bitter tastant, quinine, activate transducin and Gi/Go-proteins. Concentrations of taste substances required to activate G-proteins in vitro correlated with those used to elicit taste. These data support the hypothesis that amphiphilic taste substances may elicit taste through direct activation of G-proteins.

Enhancing effects of saccharin on gustatory responses to D-phenylalanine in monkey single chorda tympani fibers

Taste enhancing effects of sodium saccharin (Sac) on D-phenylalanine (D-Phe), first found in mice, were examined by comparing single fiber responses to various taste stimuli in the monkey chorda tympani nerve. Fifteen fibers sampled were divided into the following 5 groups according to their responsiveness to 5 prototypical taste stimuli; 8 sucrose-, 2 quinine-, 2 acid-, 2 NaCl- and one monosodium glutamate (MSG)-best fibers. Out of 8 sucrose-best fibers, 5 fibers showed enhancement of D-Phe responses after the stimulation with Sac, but neither the remaining 3 sucrose-best fibers nor other fibers showed the enhancement. These results suggest that (1) the enhancement of D-Phe responses by Sac also occurs in the monkey peripheral taste system, and (2) there exist distinct receptor sites for D-Phe responsible for occurrence of the enhancement, and (3) taste cells possessing the D-Phe receptor site are innervated by a limited subpopulation of sucrose-best fibers.

Enhancement of murine gustatory neural responses to D-amino acids by saccharin

Taste enhancing effects of sodium saccharin (Sac) on responses to particular sweet-tasting D-amino acids were found during the recording of mouse chorda tympani nerve responses to various taste stimuli in C57BL and BALB strains. In both strains, responses to D-tryptophan and D-histidine significantly increased (167.7-216.7% of control) after the stimulation with Sac as compared with those applied before Sac. In C57BL mice, the enhancement of Sac was also observed in response to D-phenylalanine (262.5% of control), but this was not the case for BALB mice, suggesting a prominent strain difference in response to D-phenylalanine, as shown previously. Responses to other sweet-tasting D- and L-amino acids and sugars were not enhanced by Sac. Enhancement of responses to these D-amino acids by Sac was also evident when responses to a mixture of D-amino acids and Sac were compared with the sum of responses to each component, although in this response analysis, the calculated magnitude of enhancement generally become smaller (135.7-180.5% of the sum) and enhancement of D-histidine responses disappeared. Except for Sac, various sweet-tasting amino acids and sugars and NaCl also tested showed no enhancing effect on D-phenylalanine responses in C57BL mice. Sac and D-amino acids, to which responses were enhanced by Sac, possess some common molecular features, namely ring structures. This structural similarity probably relates to the occurrence of the enhancement at the receptor sites.

NCAM expression by subsets of taste cells is dependent upon innervation

The expression of the neural cell adhesion molecule (NCAM) and distinct carbohydrate groups by cells of the taste buds of the rat vallate papilla was investigated by immunohistochemical and biochemical techniques. We employed antibodies against 1) the extracellular (mAb 3F4) and cytoplasmic (mAb 5B8) portions of the NCAM polypeptide, 2) the highly sialylated form of NCAM (mAb 5A5), 3) carbohydrate epitopes associated with glycosylated NCAM forms in the rat (mAb 2B8) or frog (mAb 9-OE) olfactory system, and also 4) the Lewisb blood group carbohydrate epitope (mAb CO431). NCAM mRNA was demonstrated by polymerase chain reaction (PCR) in samples of the vallate papilla, suggesting the presence of NCAM in cells of the taste buds. Antibodies against NCAM (mAbs 3F4 and 5B8) recognized a subset (about 20%) of cells within the vallate taste buds; fibers of the glossopharyngeal nerve, including those innervating the gustatory epithelium, were NCAM immunoreactive. Taste bud cells did not express polysialic acid (mAb 5A5), but mAb 5A5 immunoreactivity was observed on fibers of the IXth nerve, including a few that entered the taste buds. All or nearly all of the cells within the vallate taste buds were immunoreactive to mAb 2B8, whereas mAbs 9-OE and CO431 reacted with subsets of cells. The carbohydrates recognized by mAbs 2B8 and 9-OE were also abundantly expressed in the ducts and acini of the lingual salivary glands. Bilateral crush of the IXth nerve resulted in the loss of expression of all of these molecules from the gustatory epithelium. If cells of the taste bud express NCAM during their final stage(s) of differentiation, then NCAM could play a role(s) in the growth of gustatory axons toward their target epithelial cells and in the recognition between the nerve fibers and mature taste receptor cells, or among the taste bud cells themselves.

Transcellular and paracellular pathways in lingual epithelia and their influence in taste transduction

The lingual epithelium is innervated by special sensory (taste) and general sensory (trigeminal) nerves that transmit information about chemical stimuli introduced into the mouth to the higher brain centers. Understanding the cellular mechanisms involved in eliciting responses from these nerves requires a detailed understanding of the contributions of both the paracellular and transcellular pathways. In this paper we focus on the contribution of these 2 pathways to the responses of salts containing sodium and various organic anions in the presence and absence of amiloride. Electrophysiological recordings from trigeminal nerves, chorda tympani nerves, and isolated lingual epithelia were combined with morphological studies investigating the location (and permeability) of tight junctions, the localization of amiloride-inhibitable channels, and Na-K-ATPase in taste and epithelial cells. Based on these measurements, we conclude that diffusion across tight junctions can modulate chorda tympani and trigeminal responses to sodium-containing salts and rationalize the enhancement of taste responses to saccharides by NaCl.

The molecular biology of taste transduction

Taste cells respond to a wide variety of chemical stimuli: certain ions are perceived as salty (Na+) or sour (H+); other small molecules are perceived as sweet (sugars) and bitter (alkaloids). Taste has evolutionary value allowing animals to respond positively (to sweet carbohydrates and salty NaCl) or aversively (to bitter poisons and corrosive acids). Recently, some of the proteins involved in taste transduction have been cloned. Several different G proteins have been identified and cloned from taste tissue: gustducin is a taste cell specific G protein closely related to the transducins. Work is under way to clone additional components of the taste transduction pathways. The combination of electrophysiology, biochemistry and molecular biology is being used to characterize taste receptor cells and their sensory transduction mechanisms.

HVEM ultrastructural analysis of mouse fungiform taste buds, cell types, and associated synapses

We have used high voltage electron microscopy and computer-generated three-dimensional reconstructions from serial sections to elucidate the structure of taste bud cells and their associated synapses in fungiform taste buds of the mouse. Five fungiform taste buds (two of which were serially sectioned) were examined with the high-voltage electron microscope (HVEM). We identified the synaptic connections from taste cells onto sensory nerve fibers and classified the presynaptic taste cells based on previously established ultrastructural criteria. From those data we have distinguished dark, intermediate, and light cells in murine fungiform taste buds. Synapses in murine fungiform taste buds are fewer in number, but contain many more vesicles than synapses in either foliate or circumvallate taste buds. Synapses in mouse circumvallate and foliate taste buds typically contain a few to several synaptic vesicles per section, whereas fungiform synapses may have in excess of 100 vesicles per profile. The significance of these differences in the numbers of synapses and synaptic structure between fungiform and circumvallate/foliate synapses is not known. Based on the small number of synapses observed in fungiform taste buds, we speculate that fungiform taste buds have only a few cells transducing sensory stimuli at any given time. Alternatively, communication of sensory information from the taste receptor cells to the afferent nerve fibers may be mediated by some other mechanism(s) in addition to classical chemical synapses.

Mechanism of the electric response of lipid bilayers to bitter substances

In order to clarify by what mechanism the lipid bilayer membrane changes its potential under the stimulation of bitter substances, a microscopic model for the effects of the substances on the membrane is presented and studied theoretically. It is assumed that the substances are adsorbed on the membrane and change the partition coefficients of ions between the membrane and the stimulation solution, the dipole orientation in the polar head, and the diffusion constants of ions in the membrane. It is shown, based on the comparison of the calculated results with the experimental ones, that the response arises mainly from a change in the partition coefficients. Protons play an essential role in the membrane potential variation due to the change in their partition coefficients. The present model reproduces the following observed unique properties in the response of lipid bilayers to bitter substances, which cannot be accounted for by the usual channel model for the membrane potential: 1) the response of the membrane potential appears even under the condition that there is no ion gradient across the membrane, 2) the response remains even when the salt in the stimulating solution is replaced with the salt made of an impermeable cation, and 3) the direction of the polarization of the potential is not reversed, even when the ion gradient across the bilayer is reversed.

The biochemistry and molecular biology of taste transduction

The recent application of molecular biological techniques to taste cells has led to the identification and cloning of a number of proteins that are involved in signal transduction. Some of these are expressed only in taste cells, whereas others are expressed at higher levels in taste cells than in other, non-sensory lingual cells. Among these proteins are several G protein alpha subunits, of which alpha gustducin is particularly interesting because of its similarity to the alpha transducins expressed in the visual system, suggesting that there are similarities in taste and visual transduction mechanisms.

The physiology of vertebrate taste reception

The study of vertebrate taste-cell physiology has advanced dramatically with the use of modern electrophysiological techniques. Recent studies show that taste cells have a wide variety of ion channels which transduce chemical stimuli and are critical to cellular function. Hormones and neurotransmitters modulate ion channel function and, in turn, may affect the performance of the gustatory system.

Multiple genes for G protein-coupled receptors and their expression in lingual epithelia

Using the polymerase chain reaction (PCR), we identified a gene family including more than 60 members which encoded similar G protein-coupled seven-transmembrane receptors. Sequence analyses of six representatives out of the 60 PCR clones showed that they had significant structural similarity to olfactory and optic receptors. Their expression is restricted in the surface of lingual epithelia.

The sense of taste: neurobiology, aging, and medication effects

The sense of taste is an oral chemical sense in mammals that is involved in the choice of foods. Initial transduction of taste stimuli occurs in taste buds, which are distributed in four discrete fields in the oral cavity. Medications can affect the taste buds and ion channels in taste-bud cell membranes involved in stimulus transduction. The sense of taste gradually declines with aging, with bitter taste most affected. Neural circuits that mediate taste in primates include cranial nerves VII, IX, and X, the solitary nucleus in the brain stem, the ventroposteromedial nucleus of the thalamus, and the insular-opercular cortex. The central taste pathways process taste information about sweet, salty, sour, and bitter stimuli serially and in parallel. Medications associated with "metallic" dysgeusia and taste losses affect the taste system via unknown mechanisms.

Gustducin is a taste-cell-specific G protein closely related to the transducins

A novel G protein alpha-subunit (alpha-gustducin) has been identified and cloned from taste tissue. alpha-Gustducin messenger RNA is expressed in taste buds of all taste papillae (circumvallate, foliate and fungiform); it is not expressed in non-sensory portions of the tongue, nor is it expressed in the other tissues examined. alpha-Gustducin most closely resembles the transducins (the rod and cone photoreceptor G proteins), suggesting that gustducin's role in taste transduction is analogous to that of transducin in light transduction.

Apical K+ channels in Necturus taste cells. Modulation by intracellular factors and taste stimuli

The apically restricted, voltage-dependent K+ conductance of Necturus taste receptor cells was studied using cell-attached, inside-out and outside-out configurations of the patch-clamp recording technique. Patches from the apical membrane typically contained many channels with unitary conductances ranging from 30 to 175 pS in symmetrical K+ solutions. Channel density was so high that unitary currents could be resolved only at negative voltages; at positive voltages patch recordings resembled whole-cell recordings. These multi-channel patches had a small but significant resting conductance that was strongly activated by depolarization. Patch current was highly K+ selective, with a PK/PNa ratio of 28. Patches containing single K+ channels were obtained by allowing the apical membrane to redistribute into the basolateral membrane with time. Two types of K+ channels were observed in isolation. Ca(2+)-dependent channels of large conductance (135-175 pS) were activated in cell-attached patches by strong depolarization, with a half-activation voltage of approximately -10 mV. An ATP-blocked K+ channel of 100 pS was activated in cell-attached patches by weak depolarization, with a half-activation voltage of approximately -47 mV. All apical K+ channels were blocked by the sour taste stimulus citric acid directly applied to outside-out and perfused cell-attached patches. The bitter stimulus quinine also blocked all channels when applied directly by altering channel gating to reduce the open probability. When quinine was applied extracellularly only to the membrane outside the patch pipette and also to inside-out patches, it produced a flickery block. Thus, sour and bitter taste stimuli appear to block the same apical K+ channels via different mechanisms to produce depolarizing receptor potentials.

Intracellular free calcium concentrations in single taste receptor cells in the guinea pig

Single, viable taste receptor cells were isolated from the tongue of the guinea pig by enzymatic digestion and mechanical dissociation. The cells could be classified into flask, spindle and intermediate shapes. The intracellular free calcium ion concentrations [(Ca2+)i] of these cells were determined using the Ca2+ sensitive dye fura-2 and digital imaging microscopy. All types of cells produced an irreversible increase in (Ca2+)i upon addition of Ca2+ ionophore ionomycin (1 microM) and denatonium (10 microM). There was no evidence of any increase in (Ca2+)i in the taste receptor cells in nominally Ca2+ free solution, and when stimulated by denatonium (10 microM). When 3 mM CaCl2 was added, the (Ca2+)i remarkably increased. This would suggest that the (Ca2+)i increase in the presence of denatonium mainly depended on calcium influx from the extracellular space. There was no increase in case of high potassium (50 mM and 150 mM) or saccharose (1 mM and 5 mM) stimulation. The hypothesis that the increase in (Ca2+)i controls biochemical mechanisms related to the bitter taste transduction process is worthy of further study.

Ca(2+)-dependent chloride conductance in Necturus taste cells

This report describes the occurrence and localization of a Ca(2+)-dependent chloride conductance in taste cells of Necturus maculosus. Lingual epithelium from Necturus was removed with blunt dissection and mounted in a modified Ussing chamber which allowed individual taste cells to be impaled with intracellular micropipettes. Solutions in the mucosal and serosal chambers could be changed independently and the properties of apical and basolateral membranes tested separately. Action potentials in taste cells, elicited by brief depolarizing current pulses passed through the intracellular recording microelectrode, provided an accurate description of whether voltage-dependent conductances had been blocked or unmasked by the experimental conditions. We found that Ca2+ influx during the action potential triggers a prolonged depolarization due to Ca(2+)-dependent conductance changes, particularly in the presence of TEA to block repolarizing K+ currents. This afterdepolarization could last up to 7 sec and is due, in part, to a Ca(2+)-dependent Cl- conductance. Other Ca(2+)-dependent channels such as Ca(2+)-dependent K+ channels or nonselective cation channels may also contribute to the afterpotential. Calcium-dependent conductance channels were situated on apical and basolateral membranes of the taste cells. We speculate that Ca(2+)-dependent Cl- channels may play a role in discriminating chloride salts from salts of other anions and may help shape receptor cell responses elicited by taste stimuli.

Cellular basis of taste reception

The recent application of precise biochemical and electrophysiological techniques to studies of taste cells has brought new insights into the cellular mechanisms of taste transduction. They have revealed that taste cells use a variety of mechanisms for transduction, including apically located ion channels, ligand-gated channels, and receptors coupled to second messenger systems.

Transduction mechanisms for the taste of amino acids

Amino acids are important taste stimuli for a variety of animals. One animal model, the channel catfish, I. punctatus, possesses sensitive taste receptor systems for several amino acids. Neurophysiological and biochemical receptor binding studies suggest the presence of at least three receptor pathways: one is a relatively nonspecific site(s) responsive to short-chain neutral amino acids such as L-alanine (L-ALA); another is responsive to the basic amino acid L-arginine (L-ARG); still another is a low affinity site for L-proline (L-PRO). Several possible transduction pathways are available in the taste system of this animal model for these amino acids. One of these, formation of inositol trisphosphate (IP3) and cyclic AMP (cAMP), is mediated by GTP-binding regulatory proteins, while another involves ion channels directly activated by stimuli. L-ALA is a potent stimulus to cAMP and IP3 accumulation, while L-ARG at low concentrations is without effect. On the other hand, L-ARG and L-PRO, but not L-ALA, are able to activate stimulus-specific and cation-selective channels in taste epithelial membranes reconstituted in phospholipid bilayers at the tips of patch pipettes. Preliminary studies using mouse taste tissue demonstrate that monosodium-L-glutamate (MSG) did not enhance production of IP3 or cAMP. However, in reconstitution experiments using taste epithelium of mouse, conductance changes due to MSG are observed. The specificity of this channel(s) and its uniqueness have yet to be determined.

Mediation of responses to calcium in taste cells by modulation of a potassium conductance

Calcium salts are strong taste stimuli in vertebrate animals. However, the chemosensory transduction mechanisms for calcium are not known. In taste buds of Necturus maculosus (mud puppy), calcium evokes depolarizing receptor potentials by acting extracellularly on the apical ends of taste cells to block a resting potassium conductance. Therefore, divalent cations elicit receptor potentials in taste cells by modulating a potassium conductance rather than by permeating the cell membrane, the mechanism utilized by monovalent cations such as sodium and potassium ions.

Electrical and morphological evidence for heterogeneous populations of cultured bovine trabecular meshwork cells

Although trabecular meshwork cells are presumed to play an important role in determining ocular aqueous outflow resistance, little is known about their membrane transport characteristics. As in vivo access by microelectrodes is difficult, we used cell culture techniques to facilitate membrane voltage recording from cultured bovine trabecular meshwork cells. Phase-contrast microscopy revealed the presence of epithelial-like and spindle-shaped cell types. The mean membrane voltage for epithelial cells was -49.7 +/- 0.8 mV (S.E.M., n = 143) and for spindle cells was -70.9 +/- 1.9 mV (S.E.M., n = 48). These cells possess an electrogenic Na+/K(+)-ATPase and a K+ conductance. K transference numbers (tk) for [K+] from 5 to 80 mM were 0.50 for epithelial cells and 0.71 for spindle cells. The epithelial cells lack the electrogenic Na+/HCO3-symport, thereby enabling their differentiation from corneal endothelial cells and confirming previous reports of differences between these cell types. A proportion of spindle cells demonstrated spontaneous and induced fluctuations of membrane voltage. One millimolar Ba2+ (n = 9) induced an immediate depolarization of membrane voltage, with the onset of 'overshooting' action potentials, which were dependent on extracellular Ca2+ and Na+ but were not blocked by tetrodotoxin, 10(-6) M. Spindle cells showed parallel alignment of intracellular smooth muscle specific alpha-isoactin filaments, whereas epithelial cells showed specks of non-fibrous staining. Electron microscopy revealed that epithelial cells had the characteristics of metabolically active cells, with few intermediate filaments (10-12 nm) and microfilaments (6-7 nm) and short cytoplasmic processes. Spindle cells had long cytoplasmic processes and abundant intermediate- and microfilaments. These data provide further evidence for multiple bovine trabecular cell types. The smooth muscle-like spindle cell may represent the previously proposed contractile element of the angle and its action could conceivably alter ocular outflow resistance.

Adenylate cyclase responses to sucrose stimulation in membranes of pig circumvallate taste papillae

1. Typical adenylate cyclase (AC) responses to guanine nucleotides were found in membranes of pig circumvallate (CV) taste papillae. 2. Sucrose stimulated AC activity in the CV membranes and this stimulation was GTP dependent and tissue specific. 3. The stimulatory effect of sucrose in the CV membranes was dependent on the concentration of membranes used in the AC assay. 4. This study provides the first biochemical data on cellular transduction of taste in the pig, compares positively to preliminary results in cattle and supports recent suggestions for a role of cAMP in sweet taste transduction.

Taste responses to electrolytes in the frog glossopharyngeal nerve: initial process of taste reception

In taste reception, it has been proposed that changes in surface potential on the apical membrane of taste cells bring about activation of taste cells during chemical stimulation. To ascertain whether changes in the surface potential are involved in taste reception of electrolytes, unitary discharges were recorded from single water fibers of the frog glossopharyngeal nerve with a suction electrode. The surface potential is a function of both the charge density of the membrane surface and the ionic strength of the medium. Low concentrations of CaCl2 (less than 1 mM) were very effective stimuli. However, 0.01-1 mM LaCl3 and HCl (pH 3.0-4.5), which alter the surface potential in the positive direction, had no excitatory effect. Transition metal cations, such as Mn2+, Co2 and Ni2+, had an excitatory effect, but the responses to these cations appeared at relatively high concentrations (greater than 5 mM), in spite of the high affinity of the receptor membrane for these cations. The results suggest that the surface charge of the apical membrane is not associated with the excitation caused by electrolytes. MgCl2 (greater than 5 mM) and NaCl (greater than 100 mM) were also effective stimuli, whereas choline Cl (100-1000 mM) had no excitatory effect. An increase in the ionic strength was achieved by the addition of 100-300 mM choline Cl to stimulating solutions of MgCl2 or NaCl. The responses to Mg2+ and Na+ were not affected by the increase in the ionic strength. The results obtained here indicate that changes in the surface potential on the surface of the apical membrane are not involved in taste reception of electrolytes. Alteration of the surface potential of the membrane in the positive direction would bring about a reduction in the local concentration of cations in the vicinity of the membrane. Hence, the presence of divalent cations in the medium may affect the response to monovalent cations. However, addition of 100 mM MgCl2 to the stimulating solution of NaCl did not affect the concentration-response curve for NaCl. This result suggests that the surface charge density of the apical membrane is very low and hence the magnitude of the surface potential is very small. The results also suggest that Mg2+ and Na+ activate the taste cells by two separate, non-interacting processes. The present study suggests that, in the initial process of taste reception, only the binding of each separate cation to its appropriate receptor site (specific receptor site) leads to activation of the receptor.

Localization of phosphatidylinositol signaling components in rat taste cells: role in bitter taste transduction

To assess the role of phosphatidylinositol turnover in taste transduction we have visualized, in rat tongue, ATP-dependent endoplasmic reticular accumulation of 45Ca2+, inositol 1,4,5-trisphosphate receptor binding sites, and phosphatidylinositol turnover monitored by autoradiography of [3H]cytidine diphosphate diacylglycerol formed from [3H]cytidine. Accumulated 45Ca2+, inositol 1,4,5-trisphosphate receptors, and phosphatidylinositol turnover are selectively localized to apical areas of the taste buds of circumvallate papillae, which are associated with bitter taste. Further evidence for a role of phosphatidylinositol turnover in bitter taste is our observation of a rapid, selective increase in mass levels of inositol 1,4,5-trisphosphate elicited by low concentrations of denatonium, a potently bitter tastant.

A stimulus-activated conductance in isolated taste epithelial membranes

Membrane vesicles isolated from the cutaneous taste epithelium of the catfish were incorporated into phospholipid bilayers on the tips of patch pipettes. Voltage-dependent conductances were observed in approximately 50% of the bilayers and single-channel currents having conductances from 8 to greater than 250 pS were recorded. In 40% of the bilayers displaying no voltage-dependent conductances, micromolar concentrations of L-arginine, a potent stimulus for one class of catfish amino acid taste receptors, activated a nonselective cation conductance. The L-arginine-gated conductance was concentration-dependent, showing half-maximal activation in response to approximately 15 microM L-arginine. L-Arginine-activated channels had unitary conductances of 40-50 pS and reversed between -6 and +18 mV with pseudointracellular solution in the pipette and Ringer in the bath. L-Alanine, a potent stimulus for the other major class of catfish amino acid taste receptors, did not alter bilayer conductance. D-Arginine, which is a relatively ineffective taste stimulus for catfish but a good cross-adapter of the L-arginine-induced neural response, had no effect on bilayer conductance at concentrations below 200 microM. However, increasing concentrations of D-arginine from 1 to 100 microM progressively suppressed the L-arginine-activated conductance, suggesting that D-arginine competed for the L-arginine receptor, but did not activate the associated cation channel. This interpretation is consonant with recent biochemical binding studies in this system. These results suggest that L-arginine taste receptor proteins in the catfish are part of or closely coupled to cation-selective channels which are opened by L-arginine binding.

Expression of catfish amino acid taste receptors in Xenopus oocytes

We demonstrate that poly (A+)RNA isolated from catfish barbels directs the expression of functional amino acid taste receptors in the Xenopus oocyte. The activity of these receptors is monitored in ovo by the two electrode voltage clamp technique. Specific conductance changes recorded in response to amino acid stimulation are analogous to those recorded electrophysiologically from intact catfish barbels. These responses exhibit specificity, reproducibility, rapid onset and termination, and desensitization to repetitive stimulation. A functional assay system that encompasses the full complement of transduction events from the ligand-receptor interaction to subsequent conductance changes is necessary to identify molecular components responsible for these events. Our results demonstrate that the Xenopus oocyte can be used to characterize and identify clones coding for amino acid taste receptors analogous to its use in studying receptor molecules for other neuroactive compounds.

Identification of electrophysiologically distinct subpopulations of rat taste cells

The gustatory sensory system provides animals with a rapid chemical analysis of a potential food substance providing information necessary to facilitate ingestion or rejection of the food. The process of gustatory transduction is initiated in the taste cells in the lingual epithelium. However, due to the small size, scarcity of the cells and their location, embedded in a keratinized squamous epithelium, it has been difficult to study the primary events in the transduction process. Recently, we have developed a preparation of dissociated rat taste cells that permits studies of the taste transduction process in single isolated cells. We have now investigated the electrophysiological properties of the rat taste cells using the patch-clamp technique. We have identified two populations of cells within the taste bud: one expressing a voltage-dependent potassium current and the second containing both voltage-dependent sodium and potassium currents. The potassium current in both cell groups is blocked by external TEA, Ba2+, and quinine. Two types of K+ channels have been identified: a 90-pS delayed rectifier K+ channel and a "maxi" calcium-activated K+ channel. The sodium current is blocked by TTX, but not by amiloride.

Analysis of taste bud innervation based on glycoconjugate and peptide neuronal markers

Primary gustatory neurons and their peripheral and central processes were evaluated histochemically in the geniculate and petrosal cranial nerve ganglia, lingual fungiform taste buds, and the nucleus of the solitary tract (NST) using 1) the plant lectin Griffonia simplicifolia I-B4, which binds specifically to D-galactose residues and selectively labels primarily nonpeptide-containing peripheral somatosensory neurons, and 2) calcitonin gene-related peptide immunoreactivity (CGRP-IR), which labels most peptidergic somatosensory neurons. Lectin reactivity was expressed by the vast majority of geniculate and petrosal ganglion cells, while CGRP-IR labeled very few cells. Peripherally, gustatory intragemmal axons penetrating fungiform taste buds were labeled only by the lectin and were depleted following chorda tympani transection. However, both lectin-labeled and CGRP-IR subpopulations of somatosensory perigemmal axons surrounding the taste buds were observed and were eliminated by section of the lingual nerve. The differing brainstem projection patterns of lectin-reactive vs. CGRP-IR central axons reflected their distinct ganglionic origins and the differential distributions of lectin reactivity and CGRP-IR among taste buds. Central lectin-reactive terminals were found throughout the entire rostrocaudal extent of the NST, including its rostral lateral "gustatory" zone; the extensive lectin-reactive visceral afferent projection can be presumed to have originated mainly from the large proportion of lectin-labeled neurons in the nodose ganglion. The lectin also prominently and selectively labeled the area postrema. CGRP-IR central terminals, however, was relatively sparse and restricted primarily to the caudal and medial "visceral" divisions of the NST. The results are discussed with respect to the possible functional implications of cell surface glycoconjugate expression by gustatory axons innervating taste bud receptor cells of the tongue.

Mechanisms of chemosensory transduction in taste cells

The application of new techniques to the study of taste cells has revealed much about both the basic physiology of these cells and also about the mechanisms of taste transduction. The taste cells are electrically excitable cells with a variety of voltage-dependent ion currents. These ionic currents have an important role in the transduction of salt taste in mammals and frogs. In mudpuppies different ion channels are involved in the transduction of acidic-sour stimuli. The role of ion currents in the transduction of sweet taste is less clear. Some proposed mechanisms suggest an important role for ion currents and others suggest that the transduction process may be a biochemical event involving cell surface receptors and intracellular second messengers, possibly cAMP. The transduction of bitter taste seems to be a biochemical event involving cell surface receptors and intracellular second messengers in the inositol trisphosphate pathway. Thus, one cannot talk about "the mechanism" of taste transduction. Different taste modalities are transduced by different mechanisms. A corollary to this is that taste cells are not a homogeneous population of cells. In order to provide animals with the ability to discriminate between different taste modalities the taste cells consist of distinct subpopulations of cells based on their primary taste modality. The primary taste modality in a given cell is determined by the receptors and transduction mechanism(s) expressed in that cell. Evidence suggests that modality-specific receptors are expressed in a segregated manner in distinct subpopulations of taste cells. Secondary responses observed in gustatory axons may arise due to a lack of absolute specificity in the transduction processes and nonspecific effects of low pH and high ionic strength and osmolarity on the taste cells. An interesting area for future work will be to elucidate the mechanism(s) by which basal cells become committed to a given taste modality and how the gustatory neurons influence this process of differentiation. The involvement of the gustatory neurons is critical as they must synapse with taste cells of the correct taste modality to preserve the integrity of the information transferred to the CNS. This process of synaptogenesis is presumably mediated by the expression of taste-modality-specific, cell surface antigens on the basolateral domain of a taste cell and receptors on the appropriate neurons, but much work will be necessary to elucidate this process.(ABSTRACT TRUNCATED AT 400 WORDS)

The applications and future implications of bitterness reduction and inhibition in food products

Numerous food and beverage products, bulking agents, and pharmaceuticals have pleasant as well as unpleasant bitter-tasting components in their taste profile. In numerous cases, the bitter taste modality is an undesirable trait of the product. Bitter characteristics found in some food systems have been removed or diminished by various known processes, but no universally applicable bitter inhibitor has ever been recognized. Some indications point to a receptor-mediated phenomenon for sweetness and bitterness. Research on sweet compounds has led to knowledge of sweetness inhibitors and could ultimately lead to bitterness inhibitors. To facilitate efforts to rationally design a universal bitter inhibitor or cocktail of such, a review of the bitter taste phenomena and known methods of bitterness reduction and inhibition have been compiled.

Chemotransduction in Necturus taste buds, a model for taste processing

The taste bud in Necturus serves as a good model for taste mechanisms in vertebrates. The large size of taste cells and relative accessibility of the tissue for detailed electrophysiological and ultrastructural studies makes this species well-suited for studying taste transduction. Important features of taste transduction that have been learned from investigations in Necturus are that voltage-gated potassium channels are preferentially distributed on the apical membrane of taste cells; voltage-gated potassium channels allow K ions to enter the cell when taste buds are stimulated with K salts; some chemical stimuli act by closing K channels, thereby eliciting depolarizing receptor potentials in taste cells. Many of these findings have been confirmed and extended in other animals, including mammals. Furthermore, recent evidence from experiments in Necturus suggests that there is a considerable degree of synaptic coupling among taste cells. This synaptic coupling could form the basis for signal processing and integration in the peripheral sensory organs of taste, the taste buds.

A method for isolating and patch-clamping single mammalian taste receptor cells

Individual taste receptor cells were isolated from the tongue of the mouse by enzymatic treatment followed by mechanical dissociation. The cells were morphologically identical with taste cells from amphibians. Whole-cell voltage-clamp recordings indicated that the murine taste cells possess a variety of voltage-dependent inward and outward currents. Delayed rectifier currents were blocked by denatonium benzoate, one of the most bitter compounds known. This preparation should permit a detailed electrophysiologcal investigation of taste transduction in mammals at the level of taste receptor cells.

Vasoactive intestinal peptide-like immunoreactivity in rodent taste cells

The neuropeptide vasoactive intestinal peptide was localized to taste buds of the posterior tongue regions of hamsters and rats by immunocytochemical techniques. Tissue sections, taken from foliate and circumvallate papillae, generally revealed taste buds in which all cells were immunoreactive; however, occasionally some taste buds were found to contain highly reactive individual cells adjacent to non-reactive cells. Additionally, some non-reactive taste buds were observed. Taste buds that displayed vasoactive intestinal peptide-like immunoreactivity usually had a tendency for much darker staining at the apical ends of the cells than the basal ends, suggesting a polar cytoplasmic distribution of the peptide. The multi-functional roles of vasoactive intestinal peptide in other physiological systems combined with both its cytoplasmic localization in taste cells and the known histochemistry/ultrastructure of taste cells raises interesting speculations of this peptide's function in gustation that include secretion, stimulation of a second messenger system, and neuromodulation.