RESUMO
Mammalian taste buds are highly regenerative and can restore themselves after normal wear and tear of the lingual epithelium or following physical and chemical insults, including burns, chemotherapy, and nerve injury. This is due to the continual proliferation, differentiation, and maturation of taste progenitor cells, which then must reconnect with peripheral gustatory neurons to relay taste signals to the brain. The turnover and re-establishment of peripheral taste synapses are vital to maintain this complex sensory system. Over the past several decades, the signal transduction and neurotransmitter release mechanisms within taste cells have been well delineated. However, the complex dynamics between synaptic partners in the tongue (taste cell and gustatory neuron) are only partially understood. In this review, we highlight recent findings that have improved our understanding of the mechanisms governing connectivity and signaling within the taste bud and the still-unresolved questions regarding the complex interactions between taste cells and gustatory neurons.
Assuntos
Papilas Gustativas , Paladar , Papilas Gustativas/citologia , Papilas Gustativas/fisiologia , Animais , Humanos , Paladar/fisiologia , Neurônios/fisiologia , Neurônios/metabolismo , Transdução de SinaisRESUMO
Many common chemotherapeutics produce disruptions in the sense of taste which can lead to loss of appetite, nutritional imbalance, and reduced quality of life, especially if taste loss persists after treatment ends. Cyclophosphamide (CYP), an alkylating chemotherapeutic agent, affects taste sensitivity through its cytotoxic effects on mature taste receptor cells (TRCs) and on taste progenitor cell populations, retarding the capacity to replace TRCs. Mechanistic studies have focused primarily on taste cells, however, taste signaling requires communication between TRCs and the gustatory nerve fibers that innervate them. Here, we evaluate cyclophosphamide's effects on the peripheral gustatory nerve fibers that innervate the taste buds. Following histological analysis of tongue tissues, we find that CYP reduces innervation within the fungiform and circumvallates taste buds within 4 days after administration. To better understand the dynamics of the denervation process, we used 2-photon intravital imaging to visualize the peripheral gustatory nerve fibers within individual fungiform taste buds up to 20 days after CYP treatment. We find that gustatory fibers retract from the taste bud properly but are maintained within the central papilla core. These data indicate that in addition to TRCs, gustatory nerve fibers are also affected by CYP treatment. Because the connectivity between TRCs and gustatory neurons must be re-established for proper function, gustatory fibers should continue to be included in future studies to understand the mechanisms leading to chemotherapy-induced persistent taste loss.
Assuntos
Ageusia , Papilas Gustativas , Animais , Camundongos , Papilas Gustativas/fisiologia , Qualidade de Vida , Língua , Ciclofosfamida/farmacologia , PaladarRESUMO
Gustatory information is relayed from the anterior tongue by geniculate ganglion neurons and from the posterior tongue by neurons of the petrosal portion of the jugular/nodose/petrosal ganglion complex. Here, we use in vivo calcium imaging in mice to compare the encoding of taste information in the geniculate and petrosal ganglia, at single-neuron resolution. Our data support an anterior/posterior specialization of taste information coding from the tongue to the ganglia, with petrosal neurons more responsive to umami or bitter and less responsive to sweet or salty stimuli than geniculate neurons. We found that umami (50 mM MPG + 1 mM IMP) promotes salivation when applied to the posterior, but not anterior, tongue. This suggests a functional taste map of the mammalian tongue where the anterior and posterior taste pathways are differentially responsive to specific taste qualities, and differentially regulate downstream physiological functions of taste, such as promoting salivation.
RESUMO
Sour, the taste of acids, provides important sensory information to prevent the ingestion of unripe, spoiled, or fermented foods. In mammals, acids elicit disgust and pain by simultaneously activating taste and somatosensory neurons innervating the oral cavity. Early researchers detected electrical activity in taste nerves upon presenting acids to the tongue, establishing this as the bona fide sour taste. Recent studies have made significant contributions to our understanding of the mechanisms underlying acid sensing in the taste receptor cells at the periphery and the neural circuitry that convey this information to the brain. In this chapter, we discuss the characterization of sour taste receptor cells, the twists and turns eventually leading to the identification of Otopetrin1 (OTOP1) as the sour taste receptor, the pathway of sour taste signaling from the tongue to the brainstem, and other roles sour taste receptor cells play in the taste bud.
Assuntos
Papilas Gustativas , Paladar , Animais , Encéfalo/fisiologia , Humanos , Mamíferos , Neurônios , Paladar/fisiologia , Papilas Gustativas/metabolismo , Língua/metabolismoRESUMO
Within the last ten years, advances in genetically encoded calcium indicators (GECIs) have promoted a revolution in in vivo functional imaging. Using calcium as a proxy for neuronal activity, these techniques provide a way to monitor the responses of individual cells within large neuronal ensembles to a variety of stimuli in real time. We, and others, have applied these techniques to image the responses of individual geniculate ganglion neurons to taste stimuli applied to the tongues of live anesthetized mice. The geniculate ganglion is comprised of the cell bodies of gustatory neurons innervating the anterior tongue and palate as well as some somatosensory neurons innervating the pinna of the ear. Imaging the taste-evoked responses of individual geniculate ganglion neurons with GCaMP has provided important information about the tuning profiles of these neurons in wild-type mice as well as a way to detect peripheral taste miswiring phenotypes in genetically manipulated mice. Here we demonstrate the surgical procedure to expose the geniculate ganglion, GCaMP fluorescence image acquisition, initial steps for data analysis, and troubleshooting. This technique can be used with transgenically encoded GCaMP, or with AAV-mediated GCaMP expression, and can be modified to image particular genetic subsets of interest (i.e., Cre-mediated GCaMP expression). Overall, in vivo calcium imaging of geniculate ganglion neurons is a powerful technique for monitoring the activity of peripheral gustatory neurons and provides complementary information to more traditional whole-nerve chorda tympani recordings or taste behavior assays.
Assuntos
Cálcio/metabolismo , Gânglio Geniculado/fisiologia , Neurônios/fisiologia , Paladar/fisiologia , Anestesia , Animais , Gânglio Geniculado/cirurgia , Imobilização , Camundongos , Estimulação Física , Papilas Gustativas/fisiologia , TraqueotomiaRESUMO
BACKGROUND: Previous studies indicate that taste dysfunction occurs early in the development of Alzheimer's disease. It is debatable whether the deficit in taste is due primarily to peripheral sensory mechanisms or to central processing, or a combination of the two. OBJECTIVE: The aim of our current study is to combine behavior and histological data in APP/PS1 transgenic mice to determine whether APP/PS1 transgenic mice show deficits in unconditioned taste preference and avoidance behaviors and whether taste impairments are due to defects in the peripheral taste system and/or problems with central processing of taste information. METHODS: The APP/PS1 transgenic mutant mice were used as a model of Alzheimer's disease. We employed a brief-access gustometer test to assess immediate orosensory taste responses of APP/PS1 mice. We used immunohistochemistry to examine tongue, gustatory ganglion, and brain tissues to determine a cytological basis for behavioral deficits. RESULTS: There is a significant, selective reduction of bitter taste sensitivity in APP/PS1 mice. These mice also have a loss of TRPM5-expressing taste receptor cells in the circumvallate papillae of the tongue. While we observed no overt loss of neuron cell bodies within the primary gustatory sensory neurons, degeneration of the neurons' peripheral axons innervating the taste bud may play a role in the observed loss of TRPM5-expressing taste receptor cells. CONCLUSION: This data supports a potential role for peripheral taste dysfunction in AD through the selective loss of taste receptor cells. Further study is necessary to delineate the mechanisms and pathological significance of this deficit in AD.
Assuntos
Precursor de Proteína beta-Amiloide/genética , Mutação/genética , Presenilina-1/genética , Distúrbios do Paladar/genética , Paladar/genética , Animais , Relação Dose-Resposta a Droga , Feminino , Humanos , Masculino , Camundongos , Camundongos Transgênicos , Quinina/administração & dosagem , Sacarose/administração & dosagem , Canais de Cátion TRPM/deficiência , Canais de Cátion TRPM/genética , Paladar/efeitos dos fármacos , Distúrbios do Paladar/fisiopatologiaRESUMO
In mammals, taste buds typically contain 50-100 tightly packed taste-receptor cells (TRCs), representing all five basic qualities: sweet, sour, bitter, salty and umami. Notably, mature taste cells have life spans of only 5-20 days and, consequently, are constantly replenished by differentiation of taste stem cells. Given the importance of establishing and maintaining appropriate connectivity between TRCs and their partner ganglion neurons (that is, ensuring that a labelled line from sweet TRCs connects to sweet neurons, bitter TRCs to bitter neurons, sour to sour, and so on), we examined how new connections are specified to retain fidelity of signal transmission. Here we show that bitter and sweet TRCs provide instructive signals to bitter and sweet target neurons via different guidance molecules (SEMA3A and SEMA7A). We demonstrate that targeted expression of SEMA3A or SEMA7A in different classes of TRCs produces peripheral taste systems with miswired sweet or bitter cells. Indeed, we engineered mice with bitter neurons that now responded to sweet tastants, sweet neurons that responded to bitter or sweet neurons responding to sour stimuli. Together, these results uncover the basic logic of the wiring of the taste system at the periphery, and illustrate how a labelled-line sensory circuit preserves signalling integrity despite rapid and stochastic turnover of receptor cells.
Assuntos
Células-Tronco/citologia , Células-Tronco/metabolismo , Papilas Gustativas/citologia , Papilas Gustativas/metabolismo , Paladar/fisiologia , Animais , Antígenos CD/metabolismo , Gânglios/citologia , Camundongos , Neurônios/efeitos dos fármacos , Neurônios/metabolismo , Semaforina-3A/deficiência , Semaforina-3A/metabolismo , Semaforinas/metabolismo , Células-Tronco/efeitos dos fármacos , Edulcorantes/farmacologia , Paladar/efeitos dos fármacos , Papilas Gustativas/efeitos dos fármacosRESUMO
Determining the pattern of activity of individual connections within a neural circuit could provide insights into the computational processes that underlie brain function. Here, we develop new strategies to label active synapses by trans-synaptic fluorescence complementation in Drosophila. First, we demonstrate that a synaptobrevin-GRASP chimera functions as a powerful activity-dependent marker for synapses in vivo. Next, we create cyan and yellow variants, achieving activity-dependent, multi-colour fluorescence reconstitution across synapses (X-RASP). Our system allows for the first time retrospective labelling of synapses (rather than whole neurons) based on their activity, in multiple colours, in the same animal. As individual synapses often act as computational units in the brain, our method will promote the design of experiments that are not possible using existing techniques. Moreover, our strategies are easily adaptable to circuit mapping in any genetic system.
Assuntos
Drosophila/fisiologia , Neurônios/química , Coloração e Rotulagem/métodos , Sinapses/química , Animais , Drosophila/química , Fluorescência , Proteínas de Fluorescência Verde/química , Proteínas de Fluorescência Verde/genética , Proteínas de Fluorescência Verde/metabolismo , Microscopia Confocal , Neurônios/fisiologia , Coloração e Rotulagem/instrumentação , Sinapses/fisiologiaRESUMO
In Drosophila, rapid temperature changes are detected at the periphery by dedicated receptors forming a simple sensory map for hot and cold in the brain. However, flies show a host of complex innate and learned responses to temperature, indicating that they are able to extract a range of information from this simple input. Here we define the anatomical and physiological repertoire for temperature representation in the Drosophila brain. First, we use a photolabelling strategy to trace the connections that relay peripheral thermosensory information to higher brain centres, and show that they largely converge onto three target regions: the mushroom body, the lateral horn (both of which are well known centres for sensory processing) and the posterior lateral protocerebrum, a region we now define as a major site of thermosensory representation. Next, using in vivo calcium imaging, we describe the thermosensory projection neurons selectively activated by hot or cold stimuli. Fast-adapting neurons display transient ON and OFF responses and track rapid temperature shifts remarkably well, while slow-adapting cell responses better reflect the magnitude of simple thermal changes. Unexpectedly, we also find a population of broadly tuned cells that respond to both heating and cooling, and show that they are required for normal behavioural avoidance of both hot and cold in a simple two-choice temperature preference assay. Taken together, our results uncover a coordinated ensemble of neural responses to temperature in the Drosophila brain, demonstrate that a broadly tuned thermal line contributes to rapid avoidance behaviour, and illustrate how stimulus quality, temporal structure, and intensity can be extracted from a simple glomerular map at a single synaptic station.
Assuntos
Encéfalo/fisiologia , Drosophila melanogaster/fisiologia , Vias Neurais , Temperatura , Sensação Térmica/fisiologia , Animais , Encéfalo/anatomia & histologia , Encéfalo/citologia , Mapeamento Encefálico , Cálcio/análise , Cálcio/metabolismo , Drosophila melanogaster/citologia , Corpos Pedunculados/inervação , Neurônios/metabolismo , Sinapses/metabolismo , Termorreceptores/metabolismo , Fatores de TempoRESUMO
Many visual animals have innate preferences for particular wavelengths of light, which can be modified by learning. Drosophila's preference for UV over visible light requires UV-sensing R7 photoreceptors and specific wide-field amacrine neurons called Dm8. Here we identify three types of medulla projection neurons downstream of R7 and Dm8 and show that selectively inactivating one of them (Tm5c) abolishes UV preference. Using a modified GRASP method to probe synaptic connections at the single-cell level, we reveal that each Dm8 neuron forms multiple synaptic contacts with Tm5c in the center of Dm8's dendritic field but sparse connections in the periphery. By single-cell transcript profiling and RNAi-mediated knockdown, we determine that Tm5c uses the kainate receptor Clumsy to receive excitatory glutamate input from Dm8. We conclude that R7sâDm8âTm5c form a hard-wired glutamatergic circuit that mediates UV preference by pooling â¼16 R7 signals for transfer to the lobula, a higher visual center.
Assuntos
Visão de Cores/fisiologia , Transdução de Sinal Luminoso/fisiologia , Rede Nervosa/fisiologia , Células Fotorreceptoras de Invertebrados/fisiologia , Receptores de Glutamato/metabolismo , Vias Visuais/citologia , Análise de Variância , Animais , Animais Geneticamente Modificados , Mapeamento Encefálico , Visão de Cores/efeitos da radiação , Drosophila , Proteínas de Drosophila/genética , Regulação da Expressão Gênica/fisiologia , Regulação da Expressão Gênica/efeitos da radiação , Proteínas de Fluorescência Verde/genética , Transdução de Sinal Luminoso/efeitos da radiação , Rede Nervosa/efeitos da radiação , Optometria , Células Fotorreceptoras de Invertebrados/classificação , Interferência de RNA/fisiologia , Receptores de Glutamato/genética , Raios Ultravioleta , Vias Visuais/fisiologia , Vias Visuais/efeitos da radiaçãoRESUMO
Thermosensation is an indispensable sensory modality. Here, we study temperature coding in Drosophila, and show that temperature is represented by a spatial map of activity in the brain. First, we identify TRP channels that function in the fly antenna to mediate the detection of cold stimuli. Next, we identify the hot-sensing neurons and show that hot and cold antennal receptors project onto distinct, but adjacent glomeruli in the Proximal-Antennal-Protocerebrum (PAP) forming a thermotopic map in the brain. We use two-photon imaging to reveal the functional segregation of hot and cold responses in the PAP, and show that silencing the hot- or cold-sensing neurons produces animals with distinct and discrete deficits in their behavioral responses to thermal stimuli. Together, these results demonstrate that dedicated populations of cells orchestrate behavioral responses to different temperature stimuli, and reveal a labeled-line logic for the coding of temperature information in the brain.
Assuntos
Drosophila/fisiologia , Animais , Encéfalo/fisiologia , Temperatura Baixa , Proteínas de Drosophila/metabolismo , Temperatura Alta , Células Receptoras Sensoriais/fisiologia , Canais de Cátion TRPP/metabolismo , Sensação TérmicaAssuntos
Canais de Cálcio/metabolismo , Proteínas de Drosophila/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Dor/metabolismo , Canais de Cátion TRPC/metabolismo , Canais de Potencial de Receptor Transitório/metabolismo , Animais , Drosophila , Humanos , Canais Iônicos , Canal de Cátion TRPA1RESUMO
Tissue damage and its downstream consequences are experimentally assayed by formaldehyde application, which indiscriminately modifies proteins and is presumed to cause pain through broadly acting mechanisms. Here we show that formaldehyde activates the ion channel TRPA1 and that TRPA1-deficient mice exhibit dramatically reduced formaldehyde-induced pain responses. 4-Hydroxynonenal, a reactive chemical produced endogenously during oxidative stress, and other related aldehydes also activate TRPA1 in vitro. Furthermore, painful responses to iodoacetamide, a nonspecific cysteine-alkylating compound, are abolished in TRPA1-deficient mice. Therefore, although these reactive chemicals modify many proteins, the associated pain appears mainly dependent on a single ion channel.
Assuntos
Dor/induzido quimicamente , Dor/metabolismo , Canais de Potencial de Receptor Transitório/fisiologia , Aldeídos/toxicidade , Animais , Linhagem Celular , Formaldeído/toxicidade , Humanos , Iodoacetamida/toxicidade , Camundongos , Camundongos Knockout , Medição da Dor/métodos , Canal de Cátion TRPA1 , Canais de Potencial de Receptor Transitório/deficiência , Canais de Potencial de Receptor Transitório/genéticaRESUMO
Six highly temperature-sensitive ion channels of the transient receptor potential (TRP) family have been implicated to mediate temperature sensation. These channels, expressed in sensory neurons innervating the skin or the skin itself, are active at specific temperatures ranging from noxious cold to burning heat. In addition to temperature sensation thermoTRPs are the receptors of a growing number of environmental chemicals (chemesthesis). Recent studies have provided some striking new insights into the molecular mechanism of thermal and chemical activation of these biological thermometers.
Assuntos
Células Quimiorreceptoras/fisiologia , Termorreceptores/fisiologia , Sensação Térmica/fisiologia , Canais de Potencial de Receptor Transitório/fisiologia , Animais , Humanos , Modelos Moleculares , Canais de Potencial de Receptor Transitório/classificaçãoRESUMO
The nervous system senses peripheral damage through nociceptive neurons that transmit a pain signal. TRPA1 is a member of the Transient Receptor Potential (TRP) family of ion channels and is expressed in nociceptive neurons. TRPA1 is activated by a variety of noxious stimuli, including cold temperatures, pungent natural compounds, and environmental irritants. How such diverse stimuli activate TRPA1 is not known. We observed that most compounds known to activate TRPA1 are able to covalently bind cysteine residues. Here we use click chemistry to show that derivatives of two such compounds, mustard oil and cinnamaldehyde, covalently bind mouse TRPA1. Structurally unrelated cysteine-modifying agents such as iodoacetamide (IA) and (2-aminoethyl)methanethiosulphonate (MTSEA) also bind and activate TRPA1. We identified by mass spectrometry fourteen cytosolic TRPA1 cysteines labelled by IA, three of which are required for normal channel function. In excised patches, reactive compounds activated TRPA1 currents that were maintained at least 10 min after washout of the compound in calcium-free solutions. Finally, activation of TRPA1 by disulphide-bond-forming MTSEA is blocked by the reducing agent dithiothreitol (DTT). Collectively, our data indicate that covalent modification of reactive cysteines within TRPA1 can cause channel activation, rapidly signalling potential tissue damage through the pain pathway.
Assuntos
Cisteína/metabolismo , Dissulfetos/metabolismo , Ativação do Canal Iônico/efeitos dos fármacos , Noxas/farmacologia , Canais de Potencial de Receptor Transitório/agonistas , Canais de Potencial de Receptor Transitório/metabolismo , Acroleína/análogos & derivados , Acroleína/química , Acroleína/metabolismo , Acroleína/farmacologia , Animais , Cisteína/química , Dissulfetos/química , Ditiotreitol/farmacologia , Condutividade Elétrica , Metanossulfonato de Etila/análogos & derivados , Metanossulfonato de Etila/química , Metanossulfonato de Etila/metabolismo , Metanossulfonato de Etila/farmacologia , Humanos , Camundongos , Mostardeira/química , Mostardeira/metabolismo , Noxas/química , Noxas/metabolismo , Dor/induzido quimicamente , Dor/fisiopatologia , Óleos de Plantas/química , Óleos de Plantas/metabolismo , Óleos de Plantas/farmacologia , Canais de Potencial de Receptor Transitório/químicaRESUMO
Several temperature-activated transient receptor potential (thermoTRP) ion channels are the molecular receptors of natural compounds that evoke thermal and pain sensations. Menthol, popularly known for its cooling effect, activates TRPM8--a cold-activated thermoTRP ion channel. However, human physiological studies demonstrate a paradoxical role of menthol in modulation of warm sensation, and here, we show that menthol also activates heat-activated TRPV3. We further show that menthol inhibits TRPA1, potentially explaining the use of menthol as an analgesic. Similar to menthol, both camphor and cinnamaldehyde (initially reported to be specific activators of TRPV3 and TRPA1, respectively) also modulate other thermoTRPs. Therefore, we find that many "sensory compounds" presumed to be specific have a promiscuous relationship with thermoTRPs.
Assuntos
Antipruriginosos/farmacologia , Mentol/farmacologia , Neurônios Aferentes/efeitos dos fármacos , Neurônios Aferentes/fisiologia , Temperatura , Animais , Animais Recém-Nascidos , Cálcio/metabolismo , Cânfora/farmacologia , Capsaicina/farmacologia , Células Cultivadas , Cricetinae , Cricetulus , Diagnóstico por Imagem/métodos , Relação Dose-Resposta a Droga , Relação Dose-Resposta à Radiação , Interações Medicamentosas , Estimulação Elétrica/métodos , Gânglios Espinais/citologia , Queratinócitos , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Camundongos , Técnicas de Patch-Clamp/métodos , Ratos , Transfecção/métodos , Canais de Potencial de Receptor Transitório/genética , Canais de Potencial de Receptor Transitório/fisiologiaRESUMO
Garlic's pungent flavor has made it a popular ingredient in cuisines around the world and throughout history. Garlic's health benefits have been elevated from folklore to clinical study. Although there is some controversy as to the efficacy of garlic, garlic products are one of the most popular herbal supplements in the U.S. Chemically complex, garlic contains different assortments of sulfur compounds depending on whether the cloves are intact, crushed, cooked, or raw. Raw garlic, when cut and placed on the tongue or lips, elicits painful burning and prickling sensations through unknown mechanisms. Here, we show that raw but not baked garlic activates TRPA1 and TRPV1, two temperature-activated ion channels that belong to the transient receptor potential (TRP) family. These thermoTRPs are present in the pain-sensing neurons that innervate the mouth. We further show that allicin, an unstable component of fresh garlic, is the chemical responsible for TRPA1 and TRPV1 activation and is therefore likely to cause garlic's pungency.