Stimulation of a Primary Taste Receptor by Salts

A quantitative study was made of the repetitive response of the salt receptor cell of the blowfly taste receptor. The response begins at a high frequency and declines to a steady frequency during brief stimuli. The initial response was found to be a sigmoid function of the log of stimulus intensity over a short range of intensities. It was shown that a theory (Beidler, 1954; for mammalian salt receptors) that relates the magnitude of the steady response to stimulus intensity applies to this receptor. From the theory, it was calculated that the relative free energy change of the reaction between salt and receptor site was in the range 0 to -1 kcal/mole; and, therefore, the reaction probably involves weak physical forces. Evidence is given that the salt-combining sites of the receptor are anionic and strongly acidic and that consequently the cation of a salt largely dominates stimulation. Preliminary evidence suggests that the receptor has a high degree of specificity toward salts, being stimulated primarily by monovalent inorganic cations.     

Electrical activity in the chemoreceptors of the blowfly. II. Responses to electrical stimulation

An analysis of the various parts of the electrical responses to the chemical and electrical stimulation of a single labellar chemosensory hair of the blowfly, Phormia regina, indicates that the recording conditions for the spike potentials approximate the intracellular recordings made in other types of sense cells. The large positive resting potential probably arises from the basement membrane of the hypodermal cells and neurilemma rather than from the neurons at the base of the chemosensory hair. The responses to polarizing currents passed through single chemosensory hairs support this analysis. The behavioral responses to similar polarizing currents are shown to result from the action of the current on the neurons at the bases of the adjacent chemosensory hairs. The reported neural interaction of the two chemosensory neurons associated with the chemosensory hair is probably due to the physical-chemical attributes of the stimulating solution rather than to any real neural interaction. Observations on the latency of the initial nerve impulse in response to chemical stimulation indicate that the chemosensory neurons are normally free from spontaneous spike activity.     

Innervation of the cercal sensilla on the ovipositor of the Australian sheep blowfly (Lucilia cuprina)

ABSTRACT. The ovipositor of the female sheep blowfly, Lucilia cuprina (Wied.) (Diptera: Calliphoridae), has a complement of cercal sensilla that includes long, medium and short tactile hairs, two campaniform domes, four olfactory pegs, and ten double-channelled gustatory hairs. This sensory array is suited to assess oviposition site resources, prior to and during the laying of an egg batch.
The tactile hairs and campaniform sensilla are each innervated by a single, tubular body containing dendrite. The olfactory pegs are each innervated by a single, moderately branched dendrite, which gains access to the external environment via pores at the bottom of deep grooves in the peg wall. The gustatory hairs fall into two categories. Four hairs have a single, tubular body containing dendrite at their base, and four unbranched dendrites running up to the hair tip which has a terminal pore. Six of the taste hairs have no tubular body containing dendrite at the base, and three unbranched dendrites running up to a terminal pore.     

Proton currents through amiloride-sensitive Na channels in hamster taste cells. Role in acid transduction.

The activity of taste cells maintained in the intact hamster tongue was monitored in response to acid stimulation by recording action currents from taste receptor cells with an extracellular "macro" patch pipette: a glass pipette was pressed over the taste pore of fungiform papillae and perfused with citric acid, hydrochloric acid, or NaCl. Because this technique restricted stimulus application to the small surface area of the apical membranes of the taste cells, many nonspecific, and potentially detrimental, effects of acid stimulation could be avoided. Acid stimulation reliably elicited fast transient currents (action currents of average amplitude, 9 pA) which were consistently smaller than those elicited by NaCl (29 pA). The frequency of action currents elicited by acid stimuli increased in a dose-dependent manner with decreasing pH from a threshold of about pH 5.0. Acid-elicited responses were independent of K+, Na+, Cl-, or Ca2+ at physiological (salivary) concentrations, and were unaffected by anthracene-9-carboxylic acid, tetraethylammonium bromide, diisothiocyanate-stilbene-2,2'-disulfonic acid, vanadate, or Cd2+. In contrast, amiloride (< or = 30 microM) fully and reversibly suppressed acid-evoked action currents. At submaximal amiloride concentrations, the frequency and amplitude of the action currents were reduced, indicating a reduction of the taste cell apical conductance concomitant with a decrease in cell excitation. Exposure to low pH elicited, in addition to transient currents, an amiloride-sensitive sustained d.c. current. This current is apparently carried by protons instead of Na+ through amiloride-sensitive channels. When citric acid was applied while the taste bud was stimulated by NaCl, the action currents became smaller and the response resembled that produced by acid alone. Because of the strong interdependence of the acid and salt (NaCl) responses when both stimuli are applied simultaneously, and because of the similarity in the concentration dependence of amiloride block, we conclude that amiloride-sensitive Na+ channels on hamster taste receptor cells are permeable to protons and may play a role in acid (sour) taste.     

Transduction ion channels directly gated by sugars on the insect taste cell

Insects detect sugars and amino acids by a specialized taste cell, the sugar receptor cell, in the taste hairs located on their labela and tarsi. We patch-clamped sensory processes of taste cells regenerated from the cut end of the taste hairs on the labelum of the flashfly isolated from the pupa approximately 20 h before emergence. We recorded both single channel and ensemble currents of novel ion channels located on the distal membrane of the sensory process of the sugar receptor cell. In the stable outside-out patch membrane excised from the sensory processes, we could repeatedly record sucrose-induced currents for tens of minutes without appreciable decrease. An inhibitor of G-protein activation, GDP-beta-S, did not significantly decrease the sucrose response. These results strongly suggested that the channel is an ionotropic receptor (a receptor/channel complex), activated directly by sucrose without mediation by second messengers or G protein. The channel was shown to be a nonselective cation channel. Analyses of single channel currents showed that the sucrose-gated channel has a single channel conductance of approximately 30 pS and has a very short mean open time of approximately 0.23 ms. It is inhibited by external Ca(2+) and the dose-current amplitude relation could be described by a Michaelis-Menten curve with an apparent dissociation constant of approximately 270 mM. We also report transduction ion channels of the receptor/channel complex type directly gated by fructose and those gated by L-valine located on the sensory process.     

Responsiveness of antennal taste hairs of the apterygotan insect, Thermobia domestica (Zygentoma); an electrophysiological investigation

Antennal taste hairs of Thermobia domestica were investigated by a modified tip recording technique. They are sensitive to aqueous salt solutions but do not show the characteristic features of a water and a salt receptor cell. Sucrose and fructose stimulated approximately half of the tested hairs. As revealed by spike amplitude histograms, in most cases two sense cells responded. Proline stimulated the hairs with a threshold concentration of about 2 mM. Again, in most cases, two sense cells responded. In relation to sucrose the major sensitivities to both fructose and proline were not found in clearly defined receptor cells as in flies, but were realized in different cells of either a higher or smaller spike amplitude. This suggests, that the number of specific binding sites varies among the receptor cells from one taste hair to the other. As in cockroaches, but unlike flies, spike series are mainly generated in the apical dendritic segments. It is suggested that these features of the sense cells in thermobian taste hairs represent a primitive condition in relation to the specification of taste hairs cells in higher insect taxa     

Location and dynamic properties of the spike generator in an insect mechanosensory neuron

1.The cereal bristle hairs of the cockroach, Periplaneta americana, are each innervated by one mechanosensory cell and 1–5 chemosensory cells. In transepithelial recordings, chemo- and mechanosensory spikes could be discriminated from each other by their relative amplitude. 2. When current steps were applied via the sensory hair, trains of impulses were triggered whatever the polarity of the current. 3. All responses adapted to the current, but the time course of adaptation was fitted by a power law for outward currents and an exponential law for inward currents. 4. During application of outward currents, the spikes showed a negative initial phase on which a small positive component was superimposed; strong polarizations produced purely negative spikes. More classical spikes with a positive initial phase were induced by inward currents. 5. The present work supports the hypothesis of a direct excitability of the apical dendrite in cereal bristle mechanoreceptors and confirms previous results suggesting that spikes are normally triggered within that region during mechanical stimulations. It is also established, for the first time, that adaptation to currents may be different in the apical dendrite and in more basal regions of the same mechanosensory neuron.Abbreviation RP receptor potential - TEV transepithelial voltage - TTX tetrodotoxin     

刺激鲫鱼小脑腹外侧区诱发的Mauthner细胞兴奋性突触后电位

实验采用微电极胞内记录技术探查鲫鱼Mauthner细胞(M-细胞)对小脑刺激的电反应特征。电刺激鲫鱼小脑腹外侧部,可在双侧M-细胞胞体、腹侧树突和外侧树突近端记录到一种复合性兴奋性突触后电位(小脑诱发性EPSP)。小脑诱发性EPSP潜伏期较短(0.63±0.09 ms),持续时间较长(5.49±1.13 ms),幅度分级和刺激频率依从等特征。以较高强度刺激小脑常引起M-细胞顺向激活。多点胞内连续穿刺实验显示小脑诱发性EPSP起源于腹侧树突远端。实验结果提示,小脑-M-细胞通路可能包含一组长短不等的神经元链,它们根据链的短或长,由近及远依次投射在腹侧树突远端。     

Electrical activity in the chemoreceptors of the blowfly. I. Responses to chemical and mechanical stimulation

The electrical responses of the neurons associated with the various types of chemosensory hairs of the blowfly, Phormia regina Meigen, following stimulation by chemical and mechanical means have been studied. The singly innervated chemosensory hairs on the ovipositor, maxillary palpi, and antennae respond vigorously to chemical stimulation, but not to mechanical stimulation. The triply innervated chemosensory hairs on the labellum, tarsus, and wing have two neurons which respond only to chemical stimuli. The third neuron responds only to mechanical stimulation. The differential responses of the two chemosensory neurons to various chemical stimuli following the removal of the tip of the hair suggest that the structures responsible for chemoreception are located throughout the distal processes of these neurons. The response of the third neuron to mechanical stimulation is similar to the response recorded from the neuron associated with one type of tactile hair which responds to motion and not to steady deformation. Recordings have been made from the neurons associated with purely tactile hairs using the cut hair as an extension of the micropipette. The mechanosensory neuron of the wing chemosensory hair is capable of responding at the rate of at least 600 impulses per sec. and may serve to indicate changes in air flow over the wing surfaces during flight to enable the fly to correct the wing camber and attack angle.     

Alterations of ultrastructure and physiology of chemoreceptor dendrites in blowfly taste hairs treated with vinblastine and colchicine

Vinblastine (10^?3 M) and colchicine (10^?2 M) dissolved in fly Ringer solution were applied to chemoreceptor dendrites in individual tarsal taste hairs of blowflies (Phaenicia serricata Meigen), and the resultant changes in the morphology and physiological responsiveness of the receptors were examined. While Ringer solution alone did not have any morphological or physiological effects, treatment with drugs dissolved in Ringer solution increased the latency of responses to sucrose and NaCl and decreased spike amplitude and frequency. The drugs also disrupted microtubules and caused the destruction of the distal end of the dendrites. The observations provide evidence that the microtubules have a role in maintaining the structural integrity of the dendrites. Anodal current applied along with a chemical stimulus (NaCl or sucrose) increased the action potential frequency of treated and untreated receptors, and the electrical current elicited spikes only from those receptors which were activated by the chemical stimulus alone. When drug treatment resulted in a complete cessation of response to chemical stimuli, the receptors were also unresponsive to electrical stimulation. The results of this study are discussed in relation to hypotheses of transduction in chemoreception.     

Modeling and simulation of ion channels and action potentials in taste receptor cells

Based on patch clamp data on the ionic currents of rat taste receptor cells, a mathematical model of mammalian taste receptor cells was constructed to simulate the action potentials of taste receptor cells and their corresponding ionic components, including voltage-gated Na⁺ currents and outward delayed rectifier K⁺ currents. Our simulations reproduced the action potentials of taste receptor cells in response to electrical stimuli or sour tastants. The kinetics of ion channels and their roles in action potentials of taste receptor cells were also analyzed. Our prototype model of single taste receptor cell and simulation results presented in this paper provide the basis for the further study of taste information processing in the gustatory system.     

Comparison of the ultrastructure of stimulated and unstimulated mechanoreceptors in the taste hairs of the blowfly Phaenicia serricata

The structure of mechanoreceptors at the base of labeilar taste hairs of the blowfly Phaenicia serricata were examined in stimulated and unstimulated conditions (i.e. with the hair bent or unbent). Physiological recordings from the mechanoreceptor showed that the receptors responded when the hair is bent dorsally or ventrally and when the hair is bent at extreme angles. These conditions are the same as those placed on hairs in the anatomical studies. Bending the hair toward the ventral labellar surface caused the hair base to compress and indent the tubular body and its surrounding membrane and sheath at the distal end of the mechanoreceptor dendrite. In compressed tubular bodies, microtubules oriented longitudinally were bent and separated a greater distance from each other. Separation as much as 70 nm was observed in compressed tubular bodies as compared with a maximum of 26 nm between microtubules in tubular bodies of unbent hairs. The dense amorphous material between microtubules of compressed tubular bodies formed prominent bridges 18 nm thick connecting the microtubules at intervals of 48–74 nm. Thin 10 nm filaments were also evident in the spaces between microtubules. When the hair was bent toward the proximal end of the proboscis, the tip of the tubular body was bent about 15 °. The tubular body appears to function as a firm but resilient structure over which the dendritic membrane can be stretched during mechanostimulation. Comparison of morphology of bent and unbent hairs suggests a means by which mechanical force from the movement of the hair is transferred to the receptors by structures in the hair socket region. No differences were found in ciliary structures of stimulated and unstimulated receptors.     

Chromosaponin I stimulates the sugar taste receptor cells of the blowfly, Phormia regina

Chromosaponin I (CSI), a gamma-pyronyl-triterpenoid saponin isolated from pea and other leguminous plants, stimulates the growth of roots in a variety of plants. In the present work, we introduce CSI as a sugar taste substance for the blowfly, Phormia regina. The blowfly has taste chemosensilla on the labellum. The sensory receptor cells in the chemosensillum are highly specialized for the tastes of sugar, salt and water, respectively. Application of CSI induced the feeding response of blowflies including full proboscis extension. CSI also induced impulses of the sugar taste receptor cell in the LL-type sensillum. The optimum concentration of CSI in these responses was 0.1 mM which is much lower than that of sucrose. Based on the comparison of dose-response relationships, CSI is 100 times more effective than sucrose in stimulating the sugar taste receptor cells. CSI-induced impulses appeared after a significant latency compared with sucrose. As far as we know, this is the first report describing that a natural saponin induces sugar responses in insects. CSI is a unique saponin because of its bifunctional property in plants and insects.     

Serotonergic Agonists Inhibit Calcium-Activated Potassium and Voltage-Dependent Sodium Currents in Rat Taste Receptor Cells

Recently we reported that rat taste receptor cells respond to the neurotransmitter serotonin with an inhibition of a calcium-activated potassium current [17]. In the present study, this observation is confirmed and extended by studying the effects of an array of serotonergic agonists on membrane properties, calcium-activated potassium current, and voltage-dependent sodium current in taste receptor cells using the patch-clamp recording technique in the whole-cell configuration. Serotonergic inhibition of calcium-activated potassium current was mimicked by the agonists N-(3-trifluoromethylphenyl)piperazine and by (±)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene. Both produced reversible inhibition of K _Ca as well as significantly increasing the input resistance of the cell. The agonists 1-(1-naphthyl)piperazine and buspirone (both serotonin receptor 1A agonists) were similarly effective in reducing K _Ca . Outward current was unaffected by application of phenylbiguanide, a serotonin receptor 3 agonist, though current was affected by subsequent application of (±)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene. Two agonists—N-(3-trifluoromethylphenyl)piperazine and (±)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene—were also tested on voltage-dependent sodium currents; both were effective and reversible in reducing its magnitude at a variety of applied potentials. These data are consistent with the notion that serotonin effects in rat taste receptor cells are mediated by serotonin 1A receptors, though other receptor subtypes may be additionally expressed. Serotonin may affect the taste cell electrical properties during active stimulation in a paracrine fashion. Received: 10 May 1999/Revised: 27 September 1999     

Electrophysiological responses of the housefly, Musca domestica nebula (Fr), taste receptors to KCl

This paper aims at studying the response of the housefly labellar chemoreceptor to stimulation by KCl. These studies are limited to testing various concentrations of KCl solutions (0.005-1 M), using the electrophysiological "tip recording" method and examining the various spike characteristics such as threshold, phasic and tonic response, spike amplitudes, frequency and adaptation; for the anion, cation and water receptors. The large, medium and small hairs all respond to salt. The frontal large, medium and small hairs show a higher frequency response to equimolar KCl than the distal hairs. There is a bilateral symmetry in response pattern. The results are compared with the reported work on Phormia and Calliphora species. The Biedler's hypothesis is also tested for the salt receptors of Musca.     

Triterpenoid saponins stimulate the sugar taste receptor cell through a G protein-mediated mechanism in the blowfly,Phormia regina

The blowfly has taste chemosensilla on the labellum. The sensory receptor cells in the chemosensillum are highly specialized for the tastes of sugar, salt and water, respectively. Previously we introduced chromosaponin I (CSI) and glycyrrhizin (GL), as sweet substances for the blowfly, Phormia regina. Application of these triterpenoid saponins induced feeding responses as well as impulses of the sugar taste receptor cell in the LL-type sensillum at a much lower concentration than that of sucrose. In the present paper, we show the involvement of G protein-mediated cascade in the CSI- and GL-responses as well as in sugar responses. CSI activates the sugar signal transduction cascade after penetrating through the membrane. On the other hand, GL exerts dual effects to stimulate the sugar signal transduction possibly by activating it inside the cell and also by interacting with the pyranose sugar receptor site. A non hydrolyzable G protein inhibitor guanosine 5′-O-(2-thiodiphosphate), GDPβS, markedly decreased the responses of the sugar receptor cell to the two triterpenoid saponins as well as the response to sucrose and fructose. These results suggest that CSI and GL are direct activators of G protein.     

Effects of Polarization of the Receptor Membrane and of the First Ranvier Node in a Sense Organ

It has previously been shown that the site of production of the generator potential in Pacinian corpuscles is the receptor membrane of the non-myelinated ending, and the site of initiation of the nerve impulse, the adjacent (first) Ranvier node. Effects of membrane polarization of these sites were studied in the present work. Nerve ending and first Ranvier node were isolated by dissection, electric activity was recorded from, and polarizing currents were passed through them. All observations were done at steady levels of polarization, seconds after onset of current flow. The following results were obtained: The amount of charge transferred through the excited receptor membrane is a function of the electrical gradients across the membrane. The generator potential in response to equal mechanical stimuli increases with resting potential of the receptor membrane. The refractory state of the generator potential is not affected by polarization. The electrical threshold for impulse firing at the first Ranvier node (measured by the minimal amplitude of generator potential which elicits a nodal impulse) is nearly minimal at normal resting potential of the node. Both, hyperpolarization and depolarization lead to a rise in nodal threshold. For any level of polarization of nodal and receptor membrane, the threshold for production of impulses by adequate (mechanical) stimulation appears determined by the generator potential-stimulus strength relation and by the electrical threshold of the node.     

Intrinsic nitric oxide regulates the taste response of the sugar receptor cell in the blowfly, Phormia regina

The taste organ in insects is a hair-shaped taste sensory unit having four functionally differentiated contact chemoreceptor cells. In the blowfly, Phormia regina, cGMP has been suggested to be a second messenger for the sugar receptor cell. Generally, cGMP is produced by membranous or soluble guanylyl cyclase (sGC), which can be activated by nitric oxide (NO). In the present paper, we electrophysiologically showed that an NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO), an NO donor, 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC 7) or an NO synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) specifically affected the response in the sugar receptor cell, but not in other receptor cells. PTIO, when introduced into the receptor cells in a sensillum aided by sodium deoxycholate (DOC, pH 7.2), depressed the response of sugar receptor cells to sucrose but did not affect those of the salt or water receptor cells. NOC 7, given extracellularly, latently induced the response of sugar receptor cells; and L-NAME, when introduced into the receptor cells, depressed the response of sugar receptor cells. The results clearly suggest that NO, which may be produced by intrinsic NOS in sugar receptor cells, participates in the transduction cascade of these cells in blowfly.     

Biochemical and physiological evidence that calmodulin is involved in the taste response of the sugar receptor cells of the blowfly, Phormia regina

The gustatory system is essential for almost all animals. However, the signal transduction mechanisms have not yet been fully elucidated. We isolated labellar chemosensilla from blowfly, Phormia regina, and purified calcium binding proteins from the water soluble fraction. The most abundant calcium-binding protein was calmodulin. To investigate the role of calmodulin in taste transduction, electrophysiological responses were recorded with the calmodulin inhibitor, W-7. When we stimulated the labellar chemosensillum with sucrose plus W-7, a dose-dependent decrease of impulse frequency was observed when the concentration was <50 microM. In addition, when W-7 at 50 microM or higher concentration was added, an initial short-term impulse generation from the sugar receptor cell was observed, but this was followed by a silent period. When the sensillum was stimulated with W-7 plus a membrane-permeable cGMP analog, dibtyryl-cGMP or 8-bromo-cGMP, impulses of the sugar receptor cell were induced but the frequency was decreased. By the sidewall-recording method, we observed that the receptor potential induced by sucrose stimulation was decreased by W-7 in the sugar receptor cell, and corresponded with a disappearance of impulses. These data strongly suggest that the cGMP-gated channel generating receptor potential in the sugar receptor cell requires calmodulin for its gating.