Interactions between salt and acid stimuli: a lesson in gustation from simultaneous epithelial and neural recordings.
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Acids↗
Biomedical subjects
S A Simon
Publications and source records attributed to S A Simon.
Acidic stimuli activates two distinct pathways in taste receptor cells from rat fungiform papillae.
A sour taste sensation may be produced when acidic stimuli interact with taste receptor cells (TRCs) on the dorsal surface of the tongue. We have searched for pathways in TRCs that may be activated by acidic stimuli using RT-PCR and changes in intracellular calcium (Ca(2+)(I)) induced by acidic stimuli in rat fungiform papillae. RT-PCR revealed the presence of proton-gated subunits ASIC-beta and VR1. Ca(2+) imaging measurements of the TRCs revealed two distinct responses to acidic stimuli: Ca(2+)(i) was increased in 9% (28/308; Type I) and was decreased in 39% (121/308; Type II). Neither of these responses was affected by the removal of extracellular Ca(2+), indicating that the changes arise from the release and sequestration of Ca(2+) from intracellular stores. These responses were also not inhibited by the vanilloid receptor antagonist, capsazepine, suggesting they do not arise from the activation of vanilloid receptors. The Type I, but not the Type II response was inhibited by amiloride. Dose-response measurements for Types I and II responses yielded pH(50%) of 4.8 and 4.9, respectively. Type II responses were inhibited by pertussis toxin, suggesting G-protein involvement. TRCs that exhibit Type II responses could also be activated by quinine (which increased Ca(2+)(I)) thus suggesting a mechanism by which the addition of acid may be suppressive to other chemical stimuli.
Acid Sensing Ion Channels↗
Dynamic and multimodal responses of gustatory cortical neurons in awake rats.
To investigate the dynamic aspects of gustatory activity, we recorded the responses of small ensembles of cortical neurons to tastants administered to awake rats. Multiple trials of each tastant were delivered during recordings made in oral somatosensory (SI) and gustatory cortex (GC). When integrated tastant responses (firing rates averaged across 2.5 sec) were compared with water responses, 14.4% (13/90) of the GC neurons responded in a taste-specific manner. When time was considered as a source of information, however, the incidence of taste-specific firing increased: as many as 41% (37/90) of the recorded GC neurons exhibited taste-specific patterns of response. For 17% of the neurons identified as responding with taste-specific patterns, the stimulus that caused the most significant response was a function of the time since stimulus delivery. That is, a single neuron might respond most strongly to one tastant in the first 500 msec of a response and then respond most strongly to another tastant later in the response. Further analysis of the time courses of GC and SI cortical neural responses revealed that modulations of GC firing rate arose from three separable processes: early somatosensory input (less than approximately 0.2 sec post-stimulus), later chemosensory input ( approximately 0.2-1 sec), and delayed somatosensory input related to orofacial responses (more than approximately 1.0 sec). These data demonstrate that sensory information is available in the time course of GC responses and suggest the viability of views of gustatory processing that treat the temporal structure of cortical responses as an integral part of the neural code.
Action Potentials↗
The energetics of binding of a signal peptide to lipid bilayers: the role of bilayer properties.
The interactions between a signal peptide and a variety of lipid bilayers have been studied with direct binding assays, CD and isothermal titration calorimetry. We find that the binding of this peptide is influenced by charges and dipoles located in the bilayer interfacial region, as well as by the presence of cholesterol in the bilayer. These studies show that bilayer compositional and mechanical variations found in different biological membranes can affect the partitioning of peptides into the bilayer.
Amino Acid Sequence↗
Capsaicin inhibits activation of voltage-gated sodium currents in capsaicin-sensitive trigeminal ganglion neurons.
Capsaicin, the pungent ingredient in hot pepper, activates nociceptors to produce pain and inflammation. However, repeated exposures of capsaicin will cause desensitization to nociceptive stimuli. In cultured trigeminal ganglion (TG) neurons, we investigated mechanisms underlying capsaicin-mediated inhibition of action potentials (APs) and modulation of voltage-gated sodium channels (VGSCs). Capsaicin (1 microM) inhibited APs and VGSCs only in capsaicin-sensitive neurons. Repeated applications of capsaicin produced depolarizing potentials but failed to evoke APs. The capsaicin-induced inhibition of VGSCs was prevented by preexposing the capsaicin receptor antagonist, capsazepine (CPZ). The magnitude of the capsaicin-induced inhibition of VGSCs was dose dependent, having a K(1/2) = 0.45 microM. The magnitude of the inhibition of VGSCs was proportional to the capsaicin induced current (for -I(CAP) < 0.2 nA). Capsaicin inhibited activation of VGSCs without changing the voltage dependence of activation or markedly changing channel inactivation and use-dependent block. To explore the changes leading to this inhibition, it was found that capsaicin increased cAMP with a K(1/2) = 0.18 microM. At 1 microM capsaicin, this cAMP generation was inhibited 64% by10 microM CPZ, suggesting that activation of capsaicin receptors increased cAMP. The addition of 100 microM CPT-cAMP increased the capsaicin-activated currents but inhibited the VGSCs in both capsaicin-sensitive and -insensitive neurons. In summary, the inhibitory effects of capsaicin on VGSCs and the generation of APs are mediated by activation of capsaicin receptors. The capsaicin-induced activation of second messengers, such as cAMP, play a part in this modulation. These data distinguish two pathways by which neuronal sensitivity can be diminished by capsaicin: by modulation of the capsaicin receptor sensitivity, since the block of VGSCs is proportional to the magnitude of the capsaicin-evoked currents, and by modulation of VGSCs through second messengers elevated by capsaicin receptor activation. These mechanisms are likely to be important in understanding the analgesic effects of capsaicin.
Animals↗
The activation mechanism of rat vanilloid receptor 1 by capsaicin involves the pore domain and differs from the activation by either acid or heat.
The recently cloned rat vanilloid receptor, VR1, can be activated by capsaicin, acid, and heat. To determine the molecular mechanisms facilitating channel opening in response to these stimuli, VR1 and six channels containing charge neutralization point mutations surrounding the putative channel pore domain were expressed and characterized in Xenopus laevis oocytes. Steady-state dose-response relationships, current-voltage relationships, ionic selectivities, and single-channel properties were recorded using voltage-clamp techniques. Three of the mutant channels are significantly more sensitive to capsaicin than is wild-type VR1, whereas none differed in their activation by acidic pH or temperature. Furthermore, one of the mutants has lost all positive cooperativity for capsaicin activation (Hill coefficient congruent with 1, VR1 congruent with 2), is much more selective for Ca(2+), and exhibits a lower efficacy for acid than for capsaicin activation. Single-channel recordings show that capsaicin- and acid-activated channels have the same conductance, that the three mutants with increased capsaicin sensitivity exhibit higher open probabilities at submaximal capsaicin concentrations, and that the gating properties of capsaicin activation differ from those of acid activation. These data indicate that VR1 undergoes conformational changes upon capsaicin binding that it does not undergo in response to activation by protons or thermal stimuli. Furthermore, these structural rearrangements include the putative pore domain and reveal the location of an intracellular domain that contributes to the positive cooperativity seen for capsaicin activation.
Acids↗
Vanilloid (capsaicin) receptors influence inflammatory sensitivity in response to particulate matter.
The signs of airway inflammation and hyperresponsiveness that occur in animals exposed to air pollutants are often strain- and species-specific. To investigate the underlying causes of this phenomenon, BALB/c and C57bl/6 mice were exposed intratracheally to residual oil fly ash (ROFA, 3 mg/kg) and examined after 24 h for signs of airway inflammation. BALB/c showed significantly higher numbers of neutrophils and increased airway hyperresponsiveness in response to methacholine challenge, whereas B6 mice showed no significant change in either inflammatory endpoint. To determine the underlying cause of this strain specificity, cultures of dorsal root ganglion (DRG) sensory neurons, which innervate the upper airways in situ, were explanted from both BALB/c and B6 fetal mice. After 5-7 days in culture, they were exposed to ROFA, other urban and industrial particulate matter (PM; e.g., oil fly ash, woodstove, Mt. St. Helen, St. Louis, Ottawa, coal fly ash) or to prototype irritants (e.g., capsaicin 3-10 microM, pH 5.0 and 6.5). In all instances (except for woodstove), DRG neurons from BALB/c mice released significantly higher levels of the pro-inflammatory cytokine IL-6 into their nutrient media relative to neurons from B6 mice. This cytokine release could be significantly reduced for all PM treated cultures (except woodstove) by pretreatment of cultures with capsazepine (CPZ), a competitive antagonist of vanilloid receptors. DRG neurons, cultured from BALB/c and B6 neonates, were loaded with Fluo-3 AM and exposed to the prototype irritants, acid pH (5.0, 6.5), or capsaicin (3, 10 microM). Analysis of their increases in intracellular calcium showed that significantly higher numbers of BALB/c neurons responded to these prototype irritants, relative to B6 neurons. Morphometric analysis of BALB/c neurons, histochemically stained with cobalt to label neurons bearing capsaicin-sensitive receptors, showed a significantly higher level of stained neurons relative to B6 neurons. Finally, semiquantitative RT-PCR showed a higher expression of VR1 receptor mRNA in DRG and spinal cord taken from neonatal BALB/c mice relative to B6 mice. Taken together, these data suggest that capsaicin and acid-sensitive irritant receptors, located on somatosensory cell bodies and their nerve fiber terminals, subserve PM-induced airway inflammation and are quantitatively different in responsive and nonresponsive mouse strains.
Air Pollutants↗
Synthesis, structure, and thermal properties of 1,2-dipalmitoylgalloylglycerol (DPGG), a novel self-adhering lipid.
A novel diacyl glycerol-based lipid with a polyphenolic head group has been synthesized and characterized. X-ray diffraction experiments show that this lipid, 1,2-dipalmitoylgalloylglycerol (DPGG), hydrates to form gel phase bilayers at 20 degrees C with extremely narrow interbilayer fluid separations, indicating that apposing DPGG bilayers strongly adhere to each other. Differential scanning calorimetry shows that fully hydrated DPGG exhibits a pretransition exotherm (3.7 kcal/mol) at 52 degrees C and a high enthalpy (11.3 kcal/mol) main endothermic transition at 69 degrees C. These thermal properties are similar to those of galactosylceramides with similar hydrocarbon chain compositions. The adhesive and thermal properties of DPGG are likely due to both intermolecular hydrogen-bonding and hydrophobic interactions between the aromatic rings on the gallic acids.
Calorimetry, Differential Scanning↗
Capsaicin, acid and heat-evoked currents in rat trigeminal ganglion neurons: relationship to functional VR1 receptors.
Activation of primary trigeminal (TG) neurons by protons, capsaicin, or heat can evoke a variety of sensations, including tingling, stinging, warmth, and burning. Capsaicin and acid are trigeminal stimulants that are important in gustatory physiology. These stimuli can activate H(+)-gated ion channels and heterologously expressed VR1 receptors (vanilloid receptor 1). We have obtained evidence by using electrophysiological and pharmacological measurements on TG neurons that these three stimuli can activate many receptors, and we have determined the extent they behave similarly to VR1 receptors and H(+)-gated channels from the DEGenerin/ENaC superfamily. Whole-cell recordings from rat TG neurons revealed that protons evoked transient (Tp), sustained (Sp), and biphasic (TSp) currents. Tp currents had reversal potentials (Vr) of 24-45 mV, a pH(0.5) range from 5.5 to 6.5, and were inhibited by amiloride, suggesting the presence of functional H(+)-gated channels. Sp currents were inhibited by the VR1 antagonist capsazepine, had Vr's approximately 0 mV, and had pH(0.5) = 6.4. Capsaicin also activated transient (Tc), sustained (Sc), and biphasic (TSc) currents. At pH 5.9, the sensitivity of the Sc currents increased by about a factor of 10, which may partially account for the synergistic responses of acid in foods containing capsaicin. Heating TG neurons evoked a thermally active, capsazepine-inhibitable current with threshold temperature of 43 degrees C and Vr = 5 mV that is also present in neurons activated by and protons (Sp) and capsaicin (Sc). These data suggest that TG neurons have functional receptors that behave similarly to VR1. Activation of such receptors should result in a burning sensation, whereas activation of the transient and biphasic currents should result in other taste descriptors.
Acids↗
Nutrient tasting and signaling mechanisms in the gut. IV. There is more to taste than meets the tongue.
The tongue is the principal organ that provides sensory information about the quality and quantity of chemicals in food. Other information about the temperature and texture of food is also transduced on the tongue, via extragemmal receptors that form branches of the trigeminal, glossopharyngeal, and vagal nerves. These systems, together with information from the gastrointestinal (GI) system, interact to determine whether or not food is palatable. In this themes article, emphasis is placed on the integrative aspects of gustatory processing by showing the convergence of gustatory information with somatosensory, nociceptive, and visceral information (from the GI system) on the tongue and in the brain. Our thesis is that gustation should be thought of as an integral part of a distributed, interacting multimodal system in which information from other systems, including the GI system, can modulate the taste of food.
Animals↗
Residual oil fly ash and charged polymers activate epithelial cells and nociceptive sensory neurons.
Residual oil fly ash (ROFA) is an industrial pollutant that contains metals, acids, and unknown materials complexed to a particulate core. The heterogeneous composition of ROFA hampers finding the mechanism(s) by which it and other particulate pollutants cause airway toxicity. To distinguish culpable factors contributing to the effects of ROFA, synthetic polymer microsphere (SPM) analogs were synthesized that resembled ROFA in particle size (2 and 6 microm in diameter) and zeta potential (-29 mV). BEAS-2B human bronchial epithelial cells and dorsal root ganglion neurons responded to both ROFA and charged SPMs with an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and the release of the proinflammatory cytokine interleukin-6, whereas neutral SPMs bound with polyethylene glycol (0-mV zeta potential) were relatively ineffective. In dorsal root ganglion neurons, the SPM-induced increases in [Ca(2+)](i) were correlated with the presence of acid- and/or capsaicin-sensitive pathways. We hypothesized that the acidic microenvironment associated with negatively charged colloids like ROFA and SPMs activate irritant receptors in airway target cells. This causes subsequent cytokine release, which mediates the pathophysiology of neurogenic airway inflammation.
Acids↗
Orientation of LamB signal peptides in bilayers: influence of lipid probes on peptide binding and interpretation of fluorescence quenching data.
The orientation in lipid bilayers of the signal sequence of the bacterial protein LamB was studied using binding, circular dichroism, and fluorescence quenching experiments. Measurements were made of binding modifications caused by the incorporation of lipid probes (brominated or nitroxide-labeled phospholipids) used in the parallax fluorescence quenching method of determining peptide penetration depth [Abrams, F. S., and London, E. (1992) Biochemistry 31, 5312-5322]. The signal peptide bound to a similar extent to neutral bilayers composed of either egg phosphatidylcholine (EPC) or phosphatidylcholines brominated at various positions on their acyl chains. The fluorescence of a tryptophan in either the 18 or 24 position of the peptide was quenched more by bromines in the 6 and 7 than in the 9 and 10 positions on the lipid hydrocarbon chain. Parallax calculations showed that tryptophan-18 was located only 4 A from the hydrocarbon-water interface, consistent with the peptide adopting a "hammock" configuration in the bilayer, with both termini exposed to the aqueous phase and the central alpha-helix located near the hydrocarbon-water interface. In contrast, the incorporation of 10% nitroxide-labeled lipids into EPC bilayers modified peptide binding in a manner dependent on the position of the nitroxide on the hydrocarbon chain; 7-Doxyl PC reduced the percent peptide bound by about one-half, whereas 12-Doxyl PC had little effect on binding. These binding differences modified tryptophan quenching by these probes, making parallax analysis problematical. In the presence of the positively charged LamB peptide, the incorporation of negatively charged phospholipids into EPC bilayers increased the level of peptide binding and modified tryptophan quenching by nitroxide probes. These results suggest that the nitroxide probe could be partially excluded from negatively charged lipid domains where the peptide preferentially bound. Quite different binding and quenching results were obtained with a negatively charged peptide analogue, showing that the charge on both the peptide and bilayer affects peptide-nitroxide probe interactions.
Amino Acid Sequence↗
Polyphenols increase adhesion between lipid bilayers by forming interbilayer bridges.
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Flavonoids↗
Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.
It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.
Animals↗
Neuropeptides and capsaicin stimulate the release of inflammatory cytokines in a human bronchial epithelial cell line.
The role of neuropeptides in initiating and modulating airway inflammation was examined in a human bronchial epithelial cell line (i.e. BEAS-2B). At a range of concentrations, exposure of BEAS-2B cells to Substance P (SP) or calcitonin gene related protein resulted in immediate increases in intracellular calcium ([Ca(2+)](i)), the synthesis of the transcripts for the inflammatory cytokines, IL-6, IL-8 and TNFalpha after 2 h exposure, and the release of their proteins after 6 h exposure. Addition of thiorphan (100 nM), an inhibitor of neutral endopeptidase, enhanced the levels of SP-stimulated cytokine release. Stimulation of IL-6 by SP occurred in a conventional receptor-mediated manner as demonstrated by its differential release by fragments SP 4-11 and SP 1-4 and by the blockage of IL-6 release with the non-peptide, NK-1 receptor antagonist, CP-99 994. In addition to the direct stimulation of inflammatory cytokines, SP (0.5 microM), in combination with TNFalpha (25 units/ml), synergistically stimulated IL-6 release. BEAS-2B cells also responded to the botanical irritant, capsaicin (10 microM) with increases in [Ca(2+)](i) and IL-8 cytokine release after 4 h exposure. The IL-8 release was dependent on the presence of extracellular calcium. Capsaicin-stimulated increases of [Ca(2+)](i) and cytokine release could be reduced to control levels by pre-exposure to capsazepine, an antagonist of capsaicin (i.e. vanilloid) receptor(s) or by deletion of extracellular calcium from the exposure media. The present data indicate that the BEAS-2B human epithelial cell line expresses neuropeptide and capsaicin-sensitive pathways, whose activation results in immediate increases of [Ca(2+)](i) stimulation of inflammatory cytokine transcripts and the release of their cytokine proteins.
Bronchi↗
Simultaneous reorganization in thalamocortical ensembles evolves over several hours after perioral capsaicin injections.
Reorganization of the somatosensory system was quantified by simultaneously recording from single-unit neural ensembles in the whisker regions of the ventral posterior medial (VPM) nucleus of the thalamus and the primary somatosensory (SI) cortex in anesthetized rats before, during, and after injecting capsaicin under the skin of the lip. Capsaicin, a compound that excites and then inactivates a subset of peripheral C and Adelta fibers, triggered increases in spontaneous firing of thalamocortical neurons (10-15 min after injection), as well as rapid reorganization of the whisker representations in both the VPM and SI. During the first hour after capsaicin injection, 57% of the 139 recorded neurons either gained or lost at least one whisker response in their receptive fields (RFs). Capsaicin-related changes continued to emerge for >/=6 h after the injection: Fifty percent of the single-neuron RFs changed between 1-2 and 5-6 h after capsaicin injection. Most (79%) of these late changes represented neural responses that had remained unchanged in the first postcapsaicin mapping; just under 20% of these late changes appeared in neurons that had previously shown no plasticity of response. The majority of the changes (55% immediately after injection, 66% 6 h later) involved "unmasking" of new tactile responses. RF change rates were comparable in SI and VPM (57-49%). Population analysis indicated that the reorganization was associated with a lessening of the "spatial coupling" between cortical neurons-a significant reduction in firing covariance that could be related to distances between neurons. This general loss of spatial coupling, in conjunction with increases in spontaneous firing, may create a situation that is favorable for the induction of synaptic plasticity. Our results indicate that the selective inactivation of a peripheral nociceptor subpopulation can induce rapid and long-evolving (>/=6 h) shifts in the balance of inhibition and excitation in the somatosensory system. The time course of these processes suggest that thalamic and cortical plasticity is not a linear reflection of spinal and brainstem changes that occur following the application of capsaicin.
Analysis of Variance↗
Leptin binding activity changes with age: the link between leptin and puberty.
The timing of the physical transition from child to adult is determined by a biological clock that switches off the pituitary gonadal axis during infancy until puberty. Body composition (and in particular, fat mass), through leptin, are critical signals to this clock. However, no direct relationship between leptin and puberty has been demonstrated. Leptin is bound in the circulation by a high-affinity binding protein, which has been identified as a soluble leptin receptor. We found circulating levels of leptin binding activity (LBA) to be low at birth, to be high in the prepubertal years, to fall through puberty, and then to remain stable during adult life. LBA correlated with pubertal status in both boys and girls. We postulate that the fall in LBA, associated with increasing age and puberty, reflects a reduction in expression of truncated leptin receptors, and leptin is then available to the full-length receptor, which transmits the biological signal for leptin. The high levels of LBA occur during the years when the pituitary gonadal axis is quiescent. Thus, the change in LBA could explain how leptin regulates puberty.
Adolescent↗
The influence of removing extracellular Ca2+ in the desensitization responses to capsaicin, zingerone and olvanil in rat trigeminal ganglion neurons.
Desensitization is a process that describes the diminishing effect of a drug upon repeated applications. In regard to capsaicin, the pungent compound in hot pepper, it is well established that removal of extracellular calcium markedly diminishes desensitization. To explore whether this behavior extends to other analogues of capsaicin, we have determined the effect of removing extracellular calcium with capsaicin analogues, zingerone and olvanil, by whole-cell patch clamping cultured rat trigeminal ganglion neurons. Zingerone, like capsaicin, is pungent but has a shorter acyl chain, whereas olvanil is non-pungent and has a longer acyl chain. The currents evoked by 30-s applications of 30 mM zingerone or 1 microM olvanil repeated every 3 min differ in two important ways from the responses evoked by 1 microM capsaicin under these same conditions. In the presence of extracellular calcium, repeated applications of zingerone and olvanil produce nearly complete desensitization. Also in contrast to capsaicin, removing extracellular calcium for these two agonists does not diminish desensitization. These data analyses suggest the existence of calcium-independent pathways that can result in desensitization, and that pungency is not related to the phenomenon of desensitization.
Animals↗