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Rayamajhi D, Ege M, Ukhanov K, Ringers C, Zhang Y, Jung I, D’Gama PP, Li SS, Cosacak MI, Kizil C, Park HC, Yaksi E, Martens JR, Brody SL, Jurisch-Yaksi N, Roy S. The forkhead transcription factor Foxj1 controls vertebrate olfactory cilia biogenesis and sensory neuron differentiation. PLoS Biol 2024; 22:e3002468. [PMID: 38271330 PMCID: PMC10810531 DOI: 10.1371/journal.pbio.3002468] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2023] [Accepted: 12/12/2023] [Indexed: 01/27/2024] Open
Abstract
In vertebrates, olfactory receptors localize on multiple cilia elaborated on dendritic knobs of olfactory sensory neurons (OSNs). Although olfactory cilia dysfunction can cause anosmia, how their differentiation is programmed at the transcriptional level has remained largely unexplored. We discovered in zebrafish and mice that Foxj1, a forkhead domain-containing transcription factor traditionally linked with motile cilia biogenesis, is expressed in OSNs and required for olfactory epithelium (OE) formation. In keeping with the immotile nature of olfactory cilia, we observed that ciliary motility genes are repressed in zebrafish, mouse, and human OSNs. Strikingly, we also found that besides ciliogenesis, Foxj1 controls the differentiation of the OSNs themselves by regulating their cell type-specific gene expression, such as that of olfactory marker protein (omp) involved in odor-evoked signal transduction. In line with this, response to bile acids, odors detected by OMP-positive OSNs, was significantly diminished in foxj1 mutant zebrafish. Taken together, our findings establish how the canonical Foxj1-mediated motile ciliogenic transcriptional program has been repurposed for the biogenesis of immotile olfactory cilia, as well as for the development of the OSNs.
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Affiliation(s)
- Dheeraj Rayamajhi
- Institute of Molecular and Cell Biology, Agency for Science, Technology and Research, Singapore
| | - Mert Ege
- Department of Clinical and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway
| | - Kirill Ukhanov
- Department of Pharmacology and Therapeutics, University of Florida, Gainesville, Florida, United States of America
| | - Christa Ringers
- Department of Clinical and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway
- Kavli Institute for Systems Neuroscience and Centre for Neural Computation, Norwegian University of Science and Technology, Trondheim, Norway
| | - Yiliu Zhang
- Institute of Molecular and Cell Biology, Agency for Science, Technology and Research, Singapore
| | - Inyoung Jung
- Department of Clinical and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway
- Department of Biomedical Sciences, Korea University, Ansan, South Korea
| | - Percival P. D’Gama
- Department of Clinical and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway
| | - Summer Shijia Li
- Institute of Molecular and Cell Biology, Agency for Science, Technology and Research, Singapore
| | - Mehmet Ilyas Cosacak
- German Center for Neurodegenerative Diseases (DZNE) Dresden, Helmholtz Association, Dresden, Germany
| | - Caghan Kizil
- Department of Neurology and The Taub Institute for Research on Alzheimer’s Disease and the Aging Brain, Columbia University Irving Medical Center, New York, New York, United States of America
| | - Hae-Chul Park
- Department of Biomedical Sciences, Korea University, Ansan, South Korea
| | - Emre Yaksi
- Kavli Institute for Systems Neuroscience and Centre for Neural Computation, Norwegian University of Science and Technology, Trondheim, Norway
- Koç University Research Center for Translational Medicine, Koç University School of Medicine, Istanbul, Turkey
| | - Jeffrey R. Martens
- Department of Pharmacology and Therapeutics, University of Florida, Gainesville, Florida, United States of America
| | - Steven L. Brody
- Department of Medicine, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Nathalie Jurisch-Yaksi
- Department of Clinical and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway
- Kavli Institute for Systems Neuroscience and Centre for Neural Computation, Norwegian University of Science and Technology, Trondheim, Norway
| | - Sudipto Roy
- Institute of Molecular and Cell Biology, Agency for Science, Technology and Research, Singapore
- Department of Biological Sciences, National University of Singapore, Singapore
- Department of Paediatrics, National University of Singapore, Singapore
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Pablo JL, DeCaen PG, Clapham DE. Progress in ciliary ion channel physiology. J Gen Physiol 2016; 149:37-47. [PMID: 27999145 PMCID: PMC5217089 DOI: 10.1085/jgp.201611696] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2016] [Accepted: 12/06/2016] [Indexed: 11/20/2022] Open
Abstract
Mammalian cilia are ubiquitous appendages found on the apical surface of cells. Primary and motile cilia are distinct in both morphology and function. Most cells have a solitary primary cilium (9+0), which lacks the central microtubule doublet characteristic of motile cilia (9+2). The immotile primary cilia house unique signaling components and sequester several important transcription factors. In contrast, motile cilia commonly extend into the lumen of respiratory airways, fallopian tubes, and brain ventricles to move their contents and/or produce gradients. In this review, we focus on the composition of putative ion channels found in both types of cilia and in the periciliary membrane and discuss their proposed functions. Our discussion does not cover specialized cilia in photoreceptor or olfactory cells, which express many more ion channels.
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Affiliation(s)
- Juan Lorenzo Pablo
- Howard Hughes Medical Institute, Boston Children's Hospital, Boston, MA 02115.,Department of Cardiology, Boston Children's Hospital, Boston, MA 02115.,Department of Neurobiology, Harvard Medical School, Boston, MA 02115
| | - Paul G DeCaen
- Department of Pharmacology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611
| | - David E Clapham
- Howard Hughes Medical Institute, Boston Children's Hospital, Boston, MA 02115 .,Department of Cardiology, Boston Children's Hospital, Boston, MA 02115.,Department of Neurobiology, Harvard Medical School, Boston, MA 02115
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Abstract
Cilia are highly conserved for their structure and also for their sensory functions. They serve as antennae for extracellular information. Whether the cilia are motile or not, they respond to environmental mechanical and chemical stimuli and signal to the cell body. The information from extracellular stimuli is commonly converted to electrical signals through the repertoire of ion-conducting channels in the ciliary membrane resulting in changes in concentrations of ions, especially Ca2+, in the cilia. These changes, in turn, affect motility and signaling pathways in the cilia and cell body to carry on the signal transduction. We review here the activities of ion channels in cilia from protists to vertebrates.
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Affiliation(s)
- Steven J Kleene
- Department of Molecular and Cellular Physiology University of Cincinnati Cincinnati, OH 45267-0576 USA 1-513-558-6099 (phone) 1-513-558-5738 (fax)
| | - Judith L Van Houten
- Department of Biology University of Vermont Burlington, VT 05405, USA 1-802-656-0452 (phone) 1-802-656-2914 (FAX)
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Abstract
BACKGROUND Most cells in the body possess a single primary cilium. These cilia are key transducers of sensory stimuli, and defects in cilia have been linked to several diseases. Evidence suggests that some transduction of sensory stimuli by the primary cilium depends on ion-conducting channels. However, the tiny size of the cilium has been a critical barrier to understanding its electrical properties. We report a novel method that allows sensitive, repeatable electrical recordings from primary cilia. Adherent cells were grown on small, spherical beads that could be easily moved within the recording chamber. In this configuration, an entire cilium could be pulled into a recording microelectrode. RESULTS In 47% of attempts, suction resulted in a seal with high input resistance. Single channels could be recorded while the cilium remained attached to the cell. When the pipette was raised into the air, the cell body was pulled off at the air-bath interface. The pipette retained the cilium and could then be immersed in various solutions that bathed the cytoplasmic face of the membrane. In excised cilia, ionic currents through ciliary channels were modulated by cytoplasmic Ca(2+) and transmembrane voltage. CONCLUSIONS Ciliary recording is a direct way to learn the effects of second messengers and voltage changes on ciliary transduction channels.
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Lapid H, Hummel T. Recording odor-evoked response potentials at the human olfactory epithelium. Chem Senses 2012; 38:3-17. [PMID: 22944611 DOI: 10.1093/chemse/bjs073] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Electro-olfactogram (EOG) represents the sum of generator potentials of olfactory receptor neurons in response to an olfactory stimulus. Although this measurement technique has been used extensively in animal research, its use in human olfaction research has been relatively limited. To understand the promises and limitations of this technique, this review provides an overview of the olfactory epithelium structure and function, and summarizes EOG characteristics and conventions. It describes methodological pitfalls and their possible solutions, artifacts, and questions of debate in the field. In summary, EOG measurements provide a rare opportunity of recording neuronal input from the peripheral olfactory system, while simultaneously obtaining psychophysical responses in awake humans.
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Affiliation(s)
- Hadas Lapid
- Department of Neurobiology, Hebrew University of Jerusalem, Israel.
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French DA, Badamdorj D, Kleene SJ. Spatial distribution of calcium-gated chloride channels in olfactory cilia. PLoS One 2010; 5:e15676. [PMID: 21209888 PMCID: PMC3012700 DOI: 10.1371/journal.pone.0015676] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2010] [Accepted: 11/21/2010] [Indexed: 11/18/2022] Open
Abstract
Background In vertebrate olfactory receptor neurons, sensory cilia transduce odor stimuli into changes in neuronal membrane potential. The voltage changes are primarily caused by the sequential openings of two types of channel: a cyclic-nucleotide-gated (CNG) cationic channel and a calcium-gated chloride channel. In frog, the cilia are 25 to 200 µm in length, so the spatial distributions of the channels may be an important determinant of odor sensitivity. Principal Findings To determine the spatial distribution of the chloride channels, we recorded from single cilia as calcium was allowed to diffuse down the length of the cilium and activate the channels. A computational model of this experiment allowed an estimate of the spatial distribution of the chloride channels. On average, the channels were concentrated in a narrow band centered at a distance of 29% of the ciliary length, measured from the base of the cilium. This matches the location of the CNG channels determined previously. This non-uniform distribution of transduction proteins is consistent with similar findings in other cilia. Conclusions On average, the two types of olfactory transduction channel are concentrated in the same region of the cilium. This may contribute to the efficient detection of weak stimuli.
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Affiliation(s)
- Donald A. French
- Department of Mathematical Sciences, University of Cincinnati, Cincinnati, Ohio, United States of America
| | - Dorjsuren Badamdorj
- Department of Mathematical Sciences, University of Cincinnati, Cincinnati, Ohio, United States of America
| | - Steven J. Kleene
- Department of Cancer and Cell Biology, University of Cincinnati, Cincinnati, Ohio, United States of America
- * E-mail:
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7
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Neuronal ciliary signaling in homeostasis and disease. Cell Mol Life Sci 2010; 67:3287-97. [PMID: 20544253 DOI: 10.1007/s00018-010-0425-4] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2010] [Revised: 05/24/2010] [Accepted: 05/27/2010] [Indexed: 12/15/2022]
Abstract
Primary cilia are a class of cilia that are typically solitary, immotile appendages present on nearly every mammalian cell type. Primary cilia are believed to perform specialized sensory and signaling functions that are important for normal development and cellular homeostasis. Indeed, primary cilia dysfunction is now linked to numerous human diseases and genetic disorders. Collectively, primary cilia disorders are termed as ciliopathies and present with a wide range of clinical features, including cystic kidney disease, retinal degeneration, obesity, polydactyly, anosmia, intellectual disability, and brain malformations. Although significant progress has been made in elucidating the functions of primary cilia on some cell types, the precise functions of most primary cilia remain unknown. This is particularly true for primary cilia on neurons throughout the mammalian brain. This review will introduce primary cilia and ciliary signaling pathways with a focus on neuronal cilia and their putative functions and roles in human diseases.
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Kleene SJ. Limits of calcium clearance by plasma membrane calcium ATPase in olfactory cilia. PLoS One 2009; 4:e5266. [PMID: 19390572 PMCID: PMC2668752 DOI: 10.1371/journal.pone.0005266] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2009] [Accepted: 03/24/2009] [Indexed: 12/02/2022] Open
Abstract
Background In any fine sensory organelle, a small influx of Ca2+ can quickly elevate cytoplasmic Ca2+. Mechanisms must exist to clear the ciliary Ca2+ before it reaches toxic levels. One such organelle has been well studied: the vertebrate olfactory cilium. Recent studies have suggested that clearance from the olfactory cilium is mediated in part by plasma membrane Ca2+-ATPase (PMCA). Principal Findings In the present study, electrophysiological assays were devised to monitor cytoplasmic free Ca2+ in single frog olfactory cilia. Ca2+ was allowed to enter isolated cilia, either through the detached end or through membrane channels. Intraciliary Ca2+ was monitored via the activity of ciliary Ca2+-gated Cl− channels, which are sensitive to free Ca2+ from about 2 to 10 µM. No significant effect of MgATP on intraciliary free Ca2+ could be found. Carboxyeosin, which has been used to inhibit PMCA, was found to substantially increase a ciliary transduction current activated by cyclic AMP. This increase was ATP-independent. Conclusions Alternative explanations are suggested for two previous experiments taken to support a role for PMCA in ciliary Ca2+ clearance. It is concluded that PMCA in the cilium plays a very limited role in clearing the micromolar levels of intraciliary Ca2+ produced during the odor response.
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Affiliation(s)
- Steven J Kleene
- Department of Cancer and Cell Biology, University of Cincinnati, Cincinnati, Ohio, United States of America.
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Kleene SJ. The electrochemical basis of odor transduction in vertebrate olfactory cilia. Chem Senses 2008; 33:839-59. [PMID: 18703537 DOI: 10.1093/chemse/bjn048] [Citation(s) in RCA: 143] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Most vertebrate olfactory receptor neurons share a common G-protein-coupled pathway for transducing the binding of odorant into depolarization. The depolarization involves 2 currents: an influx of cations (including Ca2+) through cyclic nucleotide-gated channels and a secondary efflux of Cl- through Ca2+-gated Cl- channels. The relation between stimulus strength and receptor current shows positive cooperativity that is attributed to the channel properties. This cooperativity amplifies the responses to sufficiently strong stimuli but reduces sensitivity and dynamic range. The odor response is transient, and prolonged or repeated stimulation causes adaptation and desensitization. At least 10 mechanisms may contribute to termination of the response; several of these result from an increase in intraciliary Ca2+. It is not known to what extent regulation of ionic concentrations in the cilium depends on the dendrite and soma. Although many of the major mechanisms have been identified, odor transduction is not well understood at a quantitative level.
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Affiliation(s)
- Steven J Kleene
- Department of Cancer and Cell Biology, University of Cincinnati, PO Box 670667, 231 Albert Sabin Way, Cincinnati, OH 45267-0667, USA.
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10
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Brauchi S, Cea C, Farias JG, Bacigalupo J, Reyes JG. Apoptosis induced by prolonged exposure to odorants in cultured cells from rat olfactory epithelium. Brain Res 2006; 1103:114-22. [PMID: 16814749 DOI: 10.1016/j.brainres.2006.05.072] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2006] [Revised: 05/12/2006] [Accepted: 05/12/2006] [Indexed: 11/26/2022]
Abstract
Multicellular organisms undergo programmed cell death (PCD) as a mechanism for tissue remodeling during development and tissue renewal throughout adult life. Overdose of some neuronal receptor agonists like glutamate can trigger a PCD process termed excitotoxicity in neurons of the central nervous system. Calcium has an important role in PCD processes, especially in excitotoxicity. Since the normal turnover of olfactory receptor neurons (ORNs) relies, at least in part, on an apoptotic mechanism and odor transduction in ORNs involves an increase in intracellular Ca2+ concentration ([Ca2+]i), we investigated the possibility that long-term exposures to odorants could trigger an excitotoxic process in olfactory epithelial cells (EC). We used single-cell [Ca2+]i determinations and fluorescence microscopy techniques to study the effects of sustained odorant exposures in olfactory EC in primary culture. Induction of PCD was evaluated successively by three independent criteria: (1) measurements of DNA fragmentation, (2) translocation of phosphatidylserine to the external leaflet of the plasma membrane, and (3) caspase-3 activation. Our results support the notion of an odorant-induced PCD in olfactory EC. This odorant-induced PCD was prevented by LY83583, an odorant response inhibitor, suggesting that ORNs are the main epithelial cell population undergoing odorant-induced PCD.
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Affiliation(s)
- Sebastian Brauchi
- Instituto de Quimica, Pontificia Universidad Catolica de Valparaiso, Casilla 4059, Valparaiso, Chile
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11
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Abstract
Olfactory cilia contain the known components of olfactory signal transduction, including a high density of cyclic-nucleotide-gated (CNG) channels. CNG channels play an important role in mediating odor detection. The channels are activated by cAMP, which is formed by a G-protein-coupled transduction cascade. Frog olfactory cilia are 25-200 microm in length, so the spatial distribution of CNG channels along the length should be important in determining the sensitivity of odor detection. We have recorded from excised cilia and modeled diffusion of cAMP into a cilium to determine the spatial distribution of the CNG channels along the ciliary length. The proximal segment, which in frog is the first 20% of the cilium, appears to express a small fraction of the CNG channels, whereas the distal segment contains the majority, mostly clustered in one region.
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Affiliation(s)
- Richard J Flannery
- Department of Cell Biology, Neurobiology, and Anatomy, University of Cincinnati, Cincinnati, Ohio 45267-0667, USA
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12
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French DA, Flannery RJ, Groetsch CW, Krantz WB, Kleene SJ. Numerical Approximation of Solutions of a Nonlinear Inverse Problem Arising in Olfaction Experimentation. ACTA ACUST UNITED AC 2006; 43:945-956. [PMID: 17401452 PMCID: PMC1540452 DOI: 10.1016/j.mcm.2005.11.010] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Identification of detailed features of neuronal systems is an important challenge in the biosciences today. Cilia are long thin structures that extend from the olfactory receptor neurons into the nasal mucus. Transduction of an odor into an electrical signal occurs in the membranes of the cilia. The cyclic-nucleotide-gated (CNG) channels which reside in the ciliary membrane and are activated by adenosine 3',5'-cyclic monophosphate (cAMP) allow a depolarizing influx of Ca(2+) and Na(+) and are thought to initiate the electrical signal.In this paper, a mathematical model consisting of two nonlinear differential equations and a constrained Fredholm integral equation of the first kind is developed to model experiments involving the diffusion of cAMP into cilia and the resulting electrical activity. The unknowns in the problem are the concentration of cAMP, the membrane potential and, the quantity of most interest in this work, the distribution of CNG channels along the length of a cilium. A simple numerical method is derived that can be used to obtain estimates of the spatial distribution of CNG ion channels along the length of a cilium. Certain computations indicate that this mathematical problem is ill-conditioned.
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Affiliation(s)
- Donald A French
- Department of Mathematical Sciences, University of Cincinnati, Cincinnati, OH 45221-0025
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Pun RYK, Kleene SJ. An estimate of the resting membrane resistance of frog olfactory receptor neurones. J Physiol 2004; 559:535-42. [PMID: 15272040 PMCID: PMC1665124 DOI: 10.1113/jphysiol.2004.067611] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The ability of a frog olfactory receptor neurone (ORN) to respond to odorous molecules depends on its resting membrane properties, including membrane resistance and potential. Quantification of these properties is difficult because of a shunt conductance at the membrane-pipette seal that is in parallel with the true membrane conductance. In physiological salines, the sum of these two conductances averaged 235 pS. We used ionic substitution and channel blockers to reduce the membrane conductance as much as possible. This yielded a lower limit for the membrane conductance of 158 pS. The upper limit of resting membrane resistance, then, is 6 GOmega. The membrane is permeable to K+ and, to a lesser extent, other cations. No resting Cl- conductance was detectable. Correcting measured zero-current potentials for distortion by the shunt suggests that the resting membrane potential is no more negative than -75 mV. The present results help to explain why frog ORNs are excitable at rest.
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Affiliation(s)
- Raymund Y K Pun
- Department of Molecular and Cellular Physiology, University of Cincinnati, PO Box 670576, Cincinnati, OH 45267-0576, USA.
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14
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Barry PH. The relative contributions of cAMP and InsP3 pathways to olfactory responses in vertebrate olfactory receptor neurons and the specificity of odorants for both pathways. J Gen Physiol 2003; 122:247-50. [PMID: 12939389 PMCID: PMC2234488 DOI: 10.1085/jgp.200308910] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Affiliation(s)
- Peter H Barry
- School of Medical Sciences, Univeristy of New South Wales, Sydney, NSW 2052, Australia
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15
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The calcium-activated chloride conductance in olfactory receptor neurons. CURRENT TOPICS IN MEMBRANES 2002. [DOI: 10.1016/s1063-5823(02)53031-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/25/2023]
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16
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Abstract
The role of ciliary geometry for transduction events was explored by numerical simulation. The changes in intraciliary ion concentrations, suspected to occur during transduction, could thus be estimated. The case of a single excised cilium, having a uniform distribution of membrane channels, voltage clamped to -80 mV, was especially investigated. The axial profile of membrane voltage was that of a leaky cable. The Ca(2+) concentration profile tended to show a maximum in proximal segments, due to a preponderance of Ca(2+) inflow over Ca(2+) export at those locations. The local increase in Ca(2+) concentration activated Cl(-) channels. The resulting current caused a local drop in Cl(-) concentration, especially at the tip of the cilium and in distal segments, accompanied by a drop in ciliary K(+) concentration. In consequence, the membrane Cl(-) current was low in distal segments but stronger in proximal segments, where resupply was sufficient. The model predicts that the Cl(-) depletion will codetermine the time course of the receptor potential or current and the ciliary stimulus-response curve. In conclusion, when modeling with transduction elements presently known to participate, the ciliary geometry has large effects on ion distributions and transduction currents because ciliary ion transport is limited by axial electrodiffusion.
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Affiliation(s)
- B Lindemann
- Department of Physiology, Universität des Saarlandes, D-66421 Homburg, Germany.
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17
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Sanhueza M, Schmachtenberg O, Bacigalupo J. Excitation, inhibition, and suppression by odors in isolated toad and rat olfactory receptor neurons. Am J Physiol Cell Physiol 2000; 279:C31-9. [PMID: 10898714 DOI: 10.1152/ajpcell.2000.279.1.c31] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Vertebrate olfactory receptor neurons (ORNs) exhibit odor-induced increases in action potential firing rate due to an excitatory cAMP-dependent current. Fish and amphibian ORNs also give inhibitory odor responses, manifested as decreases in firing rate, but the underlying mechanism is poorly understood. In the toad, an odor-induced Ca(2+)-activated K(+) current is responsible for the hyperpolarizing receptor potential that causes inhibition. In isolated ORNs, a third manner by which odors affect firing is suppression, a direct and nonspecific reduction of voltage-gated and transduction conductances. Here we show that in whole cell voltage-clamped toad ORNs, excitatory or inhibitory currents were not strictly associated to a particular odorant mixture. Occasionally, both odor effects, in addition to suppression, were concurrently observed in a cell. We report that rat ORNs also exhibit odor-induced inhibitory currents, due to the activation of a K(+) conductance closely resembling that in the toad, suggesting that this conductance is widely distributed among vertebrates. We propose that ORNs operate as complex integrator units in the olfactory epithelium, where the first events in the process of odor discrimination take place.
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Affiliation(s)
- M Sanhueza
- Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago, Chile
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18
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Kleene SJ. Both external and internal calcium reduce the sensitivity of the olfactory cyclic-nucleotide-gated channel to CAMP. J Neurophysiol 1999; 81:2675-82. [PMID: 10368387 DOI: 10.1152/jn.1999.81.6.2675] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
In vertebrate olfaction, odorous stimuli are first transduced into an electrical signal in the cilia of olfactory receptor neurons. Many odorants cause an increase in ciliary cAMP, which gates cationic channels in the ciliary membrane. The resulting influx of Ca2+ and Na+ produces a depolarizing receptor current. Modulation of the cyclic-nucleotide-gated (CNG) channels is one mechanism of adjusting olfactory sensitivity. Modulation of these channels by divalent cations was studied by patch-clamp recording from single cilia of frog olfactory receptor neurons. In accord with previous reports, it was found that cytoplasmic Ca2+ above 1 microM made the channels less sensitive to cAMP. The effect of cytoplasmic Ca2+ was eliminated by holding the cilium in a divalent-free cytoplasmic solution and was restored by adding calmodulin (CaM). An unexpected result was that external Ca2+ could also greatly reduce the sensitivity of the channels to cAMP. This reduction was seen when external Ca2+ exceeded 30 microM and was not affected by the divalent-free solution, by CaM, or by Ca2+ buffering. The effects of cytoplasmic and external Ca2+ were additive. Thus the effects of cytoplasmic and external Ca2+ are apparently mediated by different mechanisms. There was no effect of CaM on a Ca2+-activated Cl- current that also contributes to the receptor current. Increases in Ca2+ concentration on either side of the ciliary membrane may influence olfactory adaptation.
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Affiliation(s)
- S J Kleene
- Department of Cell Biology, Neurobiology, and Anatomy, University of Cincinnati, Cincinnati, Ohio 45267-0521, USA
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19
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Abstract
Cyclic AMP (cAMP) is one of the intracellular messengers that mediate odorant signal transduction in vertebrate olfactory cilia. Therefore, the diffusion coefficient of cAMP in olfactory cilia is an important factor in the transduction of the odorous signal. We have employed the excised cilium preparation from the grass frog (Rana pipiens) to measure the cAMP diffusion coefficient. In this preparation an olfactory cilium is drawn into a patch pipette and a gigaseal is formed at the base of the cilium. Subsequently the cilium is excised, allowing bath cAMP to diffuse into the cilium and activate the cyclic nucleotide-gated channels on the plasma membrane. In order to estimate the cAMP diffusion coefficient, we analyzed the kinetics of the currents elicited by step changes in the bath cAMP concentration in the absence of cAMP hydrolysis. Under such conditions, the kinetics of the cAMP-activated currents has a simple dependence on the diffusion coefficient. From the analysis we have obtained a cAMP diffusion coefficient of 2.7 +/- 0.2. 10(-6) cm2 s-1 for frog olfactory cilia. This value is similar to the expected value in aqueous solution, suggesting that there are no significant diffusional barriers inside olfactory cilia. At cAMP concentrations higher than 5 microM, diffusion slowed considerably, suggesting the presence of buffering by immobile cAMP binding sites. A plausible physiological function of such buffering sites would be to prolong the response of the cell to strong stimuli.
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Affiliation(s)
- C Chen
- Department of Physiology and Biophysics, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA
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20
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Abstract
Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.
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Affiliation(s)
- D Schild
- Physiologisches Institut, Universität Göttingen, Germany
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21
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Larsson HP, Kleene SJ, Lecar H. Noise analysis of ion channels in non-space-clamped cables: estimates of channel parameters in olfactory cilia. Biophys J 1997; 72:1193-203. [PMID: 9138566 PMCID: PMC1184503 DOI: 10.1016/s0006-3495(97)78767-8] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Ion channels in the cilia of olfactory neurons are part of the transduction machinery of olfaction. Odorant stimuli have been shown to induce a biphasic current response, consisting of a cAMP-activated current and a Ca(2+)-activated Cl- current. We have developed a noise analysis method to study ion channels in leaky cables, such as the olfactory cilium, under non-space-clamp conditions. We performed steady-state noise analysis on ligand-induced currents in excised cilia, voltage-clamped at input and internally perfused with cAMP or Ca2+. The cAMP-activated channels analyzed by this method gave results similar to those of single-channel recordings (gamma = 8.3 pS). Single-channel currents have not yet been recorded for the Ca(2+)-activated Cl- channels. Using our noise analysis method, we estimate a unit conductance, gamma = 0.8 pS, for these channels. The density of channels was found to be approximately 70 channels/micron2 for both channel species.
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Affiliation(s)
- H P Larsson
- Department of Molecular and Cell Biology, University of California Berkeley 94720, USA
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22
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Nakamura T, Lee HH, Kobayashi H, Satoh TO. Gated conductances in native and reconstituted membranes from frog olfactory cilia. Biophys J 1996; 70:813-7. [PMID: 8789098 PMCID: PMC1224981 DOI: 10.1016/s0006-3495(96)79625-x] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Although cAMP is well established as a second messenger for olfactory transduction in vertebrates, the role of inositol 1,4,5-trisphosphate (IP3) in this process remains controversial. We addressed this issue by comparing currents evoked by cAMP and IP3 in native and reconstituted membranes from olfactory cilia. We detected only a cyclic nucleotide-gated conductance in the native membrane but both cyclic nucleotide-gated and IP3-gated conductances in the reconstituted membrane. The magnitudes of the cyclic nucleotide- and IP3-gated conductances were not correlated with each other in reconstituted membranes, suggesting that cyclic nucleotide- and IP3-gated channels originate in different cellular compartments.
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Affiliation(s)
- T Nakamura
- Department of Applied Physics and Chemistry, University of Electro-Communications, Tokyo, Japan
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23
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Jorquera O, Latorre R, Labarca P. Ion channel classes in purified olfactory cilia membranes: planar lipid bilayer studies. THE AMERICAN JOURNAL OF PHYSIOLOGY 1995; 269:C1235-44. [PMID: 7491914 DOI: 10.1152/ajpcell.1995.269.5.c1235] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Ciliary membrane fragment fusion to planar lipid bilayers resulted in the insertion of four ion channel types. cAMP-activated, cation-selective channels could be detected only in the absence of Ca2+ and had a conductance of 23 pS. They exhibited an apparent dissociation constant (Kd) for the cyclic nucleotide of approximately 30 microM and an estimated permeability ratio (PNa/PK) of 2.4. The cAMP cation-selective channel coinserted with a K(+)-selective channel refractory to cAMP, Ca2+, and D-myo-inositol 1,4,5-trisphosphate. This K+ channel was voltage independent and exhibited open-conductance substates of 60 and 112 pS. cAMP was also found to modulate a novel K+ channel with a Kd = 140 microM. It displayed three nearly equally spaced open substates with conductances of 34, 80, and 130 pS. In the absence and in the presence of cAMP the probability of occurrence of the open substates was binomially distributed. A fourth channel type was a Ca(2+)-activated K+ channel with a conductance of 240 pS. It was blocked by charybdotoxin at nanomolar concentrations (Kd = 3 nM). These results add support to the idea that, besides cAMP-activated cation-selective channels, vertebrate chemosensory olfactory membranes possess an arrangement of ion channels.
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Affiliation(s)
- O Jorquera
- Centro de Estudios Científicos de Santiago, Faculty of Sciences, University of Chile, Chile
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24
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Kleene SJ. Block by external calcium and magnesium of the cyclic-nucleotide-activated current in olfactory cilia. Neuroscience 1995; 66:1001-8. [PMID: 7651604 DOI: 10.1016/0306-4522(94)00634-h] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Olfactory transduction occurs on the cilia of olfactory receptor neurons, which are in close proximity to the external environment. Transduction is mediated by cyclic AMP, which directly gates channels in the ciliary membrane. Previous evidence indicates that one environmental influence, the level of divalent cations in the mucus, may strongly influence olfactory transduction by blocking the cyclic-AMP-gated channels. In this report the effects of external calcium and magnesium on the ciliary macroscopic current activated by cytoplasmic cyclic AMP were measured. External calcium and magnesium each reduced the cyclic-AMP-activated current at both negative and positive potentials. At the neuronal resting potential (-50 mV), half-maximal inhibition of the current was produced by 250 microM calcium or 1.3 mM magnesium. Reduction in current by external calcium was strongly voltage-dependent, with larger effects at negative potentials. Reduction by magnesium was weaker and less voltage-dependent. Block of the cyclic-AMP-activated current by divalent cations in the mucus may be one element of a system that increases the signal-to-noise ratio for detection of odorants.
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Affiliation(s)
- S J Kleene
- University of Cincinnati, Department of Cell Biology, Neurobiology and Anatomy, OH 45267-0521, USA
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25
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Kurahashi T, Lowe G, Gold GH. Suppression of odorant responses by odorants in olfactory receptor cells. Science 1994; 265:118-20. [PMID: 8016645 DOI: 10.1126/science.8016645] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Odorants activate an inward current in vertebrate olfactory receptor cells. Here it is shown, in receptor cells from the newt, that odorants can also suppress this current, by a mechanism that is distinct from inhibition and adaptation. Suppression provides a simple explanation for two seemingly unrelated phenomena: the anomalously long latency of olfactory transduction and the existence of an "off response" at the end of a prolonged stimulus. Suppression may influence the perception of odorants by masking odorant responses and by sharpening the odorant specificities of single cells.
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Affiliation(s)
- T Kurahashi
- Monell Chemical Senses Center, Philadelphia, PA 19104
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26
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Abstract
1. In amphibian olfactory receptor neurones, much of the depolarizing current in response to odours is carried by cationic channels that are directly gated by cyclic AMP. The effects of four calmodulin antagonists on the cyclic AMP-activated receptor current were studied in single olfactory cilia of the frog. 2. Two antagonists, W-7 and trifluoperazine, were potent and reversible inhibitors of the cyclic AMP-activated current. IC50 values were 5 microM for W-7 and 13 microM for trifluoperazine. A third antagonist, calmidazolium, irreversibly blocked the current. The fourth, mastoparan, had little effect. 3. Calmodulin was unable to reverse the effects of W-7 and trifluoperazine, suggesting that these inhibitors act directly on the cyclic AMP-gated channels. 4. Neither W-7 nor trifluoperazine inhibited a Ca(2+)-activated Cl- current which also contributes to the odorant response. These compounds thus allow the two components of the olfactory receptor current to be discriminated.
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Affiliation(s)
- S J Kleene
- Department of Anatomy and Cell Biology, University of Cincinnati 45267-0521
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27
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Abstract
In the cilia of amphibian olfactory receptor neurons, odorants elicit a receptor current that has two components: a cationic current through cAMP-gated channels and a Cl- current. Here, a cascade of ciliary currents that accounts for the total receptor current is demonstrated. In isolated olfactory cilia, cAMP sequentially activates two currents. The first is a primary cationic current through channels directly gated by cAMP. Part of this current is carried by Ca2+, which in turn activates a Cl- current. This secondary current is eliminated by the presence of Cl- channel inhibitors, replacement of Cl- with methanesulfonate-, removal of external Ca2+, or blockers of the cAMP-gated cationic channels. When cytoplasmic Ca2+ buffering is low, small cationic currents can activate Cl- currents that are 20 times larger.
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Affiliation(s)
- S J Kleene
- Department of Anatomy and Cell Biology, University of Cincinnati, Ohio 45267-0521
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28
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Kurahashi T, Yau KW. Co-existence of cationic and chloride components in odorant-induced current of vertebrate olfactory receptor cells. Nature 1993; 363:71-4. [PMID: 7683113 DOI: 10.1038/363071a0] [Citation(s) in RCA: 216] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Odorant stimulation leads to a depolarization of olfactory receptor neurons. A mechanism underlying this transduction, which occurs in the sensory cilia, involves a G-protein-mediated increase in adenylyl cyclase activity, and therefore a rise in internal cyclic AMP and consequent opening of a cAMP-gated cation channel on the plasma membrane. Another mechanism, not as well established, involves the opening of an inositol trisphosphate-activated cation channel on the plasma membrane as a result of phospholipase C activity. In both cases, an influx of cations is thought to generate the depolarizing receptor potential. We now report, however, that the mechanism is actually more complex. The odorant-induced current appears to contain an inward chloride component also, which is triggered by calcium influx through the cation-selective channel. This newly found chloride component can be as large as the cationic component. The co-existence of cationic and chloride components in the odorant response, possibly unique among sensory transduction mechanisms, may serve to reduce variations in the transduction current resulting from changes in external ionic concentrations around the olfactory cilia. Our finding can explain the long-standing puzzle of why removal of most mucosal cations still does not diminish the amplitude of the olfactory receptor cell response.
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Affiliation(s)
- T Kurahashi
- Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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29
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Dubin AE, Dionne VE. Modulation of Cl-, K+, and nonselective cation conductances by taurine in olfactory receptor neurons of the mudpuppy Necturus maculosus. J Gen Physiol 1993; 101:469-85. [PMID: 7685047 PMCID: PMC2216775 DOI: 10.1085/jgp.101.4.469] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Odors are transduced by processes that modulate the membrane conductance of olfactory receptor neurons. Olfactory neurons from the aquatic salamander, Necturus maculosus, were acutely isolated without enzymes and studied with a resistive whole-cell method to minimize loss of soluble intracellular constituents. 55 of 224 neurons responded to the test compound taurine at concentrations between 10 nM and 100 microM. Four different conductance changes were elicited by taurine: an increased Cl- conductance (33%), an increased nonselective cation conductance (15%), a decreased Cl- conductance (15%), and a decreased K+ conductance (15%); in addition, responses too small to be characterized were elicited in some neurons. In most cases, taurine appeared to modulate only a single conductance in any particular cell. Modulation of each conductance was dose dependent, and each response ran down quickly in the normal whole-cell mode, presumably due to washout of a diffusible component in the transduction pathway. Modulation of taurine-sensitive conductances caused either inhibitory or excitatory responses. A similar diversity of responses in vivo would produce a complex pattern of electrical activity that could encode the identity and characteristics of an odor.
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Affiliation(s)
- A E Dubin
- Department of Pharmacology, University of California San Diego, La Jolla 92093-0636
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30
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Kleene SJ. The cyclic nucleotide-activated conductance in olfactory cilia: effects of cytoplasmic Mg2+ and Ca2+. J Membr Biol 1993; 131:237-43. [PMID: 7684085 DOI: 10.1007/bf02260112] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Olfactory receptor neurons depolarize in response to odorants. This depolarization is mediated by an increase in intracellular cyclic AMP, which directly gates channels in the membranes of the neuronal cilia. Previous evidence suggests that a Ca2+ influx during the odorant response may ultimately play a role in terminating the response. One way Ca2+ inside the cell could terminate the odorant response would be to directly inhibit the cAMP-gated channels. In this report the effects of cytoplasmic Ca2+ and Mg2+ on the cAMP-activated current were measured in single olfactory cilia. Near the neuronal resting potential, cytoplasmic Ca2+ and Mg2+ only slightly reduced the cAMP-activated current. Even at high levels (1.0 mM Ca2+ or 5.0 mM Mg2+), the average inhibition was only around 20%. It is therefore unlikely that an influx of divalent cations terminates the odorant response by a direct effect on the cAMP-gated channels.
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Affiliation(s)
- S J Kleene
- Department of Anatomy and Cell Biology, University of Cincinnati, Ohio 45267-0521
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31
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Abstract
A simple method is described for achieving a salt bridge within a patch pipette. The tip of the pipette is filled with a Cl(-)-free solution that bathes the membrane patch. Above this is a Cl(-)-containing solution that surrounds the recording Ag/AgCl wire electrode. Addition of 0.07% (w/v) agarose to the solution in the tip prevents bulk mixing of the 2 phases within the pipette. This configuration makes it possible to remove Cl- from the membrane surface but maintain it at the electrode surface. The presence of the Cl(-)-free phase in the bottom of the patch pipette does not reduce the electrical stability of the recording electrode.
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Affiliation(s)
- S J Kleene
- Department of Anatomy and Cell Biology, University of Cincinnati, OH 45267-0521
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32
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Abstract
The conductance of isolated frog olfactory cilia in the absence of odorants and second messengers has been measured. Current flowing through the pipette-membrane seal rather than the ciliary membrane was subtracted. In normal physiological solutions, each cilium has a conductance averaging 92 pS at the neuronal resting potential. This basal conductance allows current to be carried by K+ or Na+ but not by Cl-. In some cases, single channels with a unit conductance of 153 pS were observed. The conductance of the ciliary membrane implies a length constant for electrotonic conduction of about 160 microns. Since the reversal potential of the basal conductance is near the neuronal resting potential, it should help to stabilize the ciliary potential at some cost to stimulus transduction efficiency.
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Affiliation(s)
- S J Kleene
- Department of Anatomy and Cell Biology, University of Cincinnati, OH 45267-0521
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33
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Menco BP, Bruch RC, Dau B, Danho W. Ultrastructural localization of olfactory transduction components: the G protein subunit Golf alpha and type III adenylyl cyclase. Neuron 1992; 8:441-53. [PMID: 1550671 DOI: 10.1016/0896-6273(92)90272-f] [Citation(s) in RCA: 92] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Electron microscopy and postembedding immunocytochemistry on rapidly frozen, freeze-substituted specimens of rat olfactory epithelia were used to study the subcellular localization of the transduction proteins Golf alpha and type III adenylyl cyclase. Antibody binding sites for both of these proteins occur in the same receptor cell compartments, the distal segments of the olfactory cilia. These segments line the boundary between organism and external environment inside the olfactory part of the nasal cavity. Therefore, they are the receptor cell regions that most likely first encounter odorous compounds. The results presented here provide direct evidence to support the conclusion that the distal segments of the cilia contain the sites of the early events of olfactory transduction.
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Affiliation(s)
- B P Menco
- Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208-3520
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34
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Pun RY, Gesteland RC. Somatic sodium channels of frog olfactory receptor neurones are inactivated at rest. Pflugers Arch 1991; 418:504-11. [PMID: 1653946 DOI: 10.1007/bf00497779] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Membrane excitability of acutely isolated olfactory receptor neurones (ORNs) of the grass frog (R. pipiens) was studied with the use of the whole-cell "tight-seal" patch recording technique. ORNs of the frog had a mean resting membrane potential of -52 mV, a mean input resistance of 1-2 G omega, and a mean capacitance of 4.5 pF. In the majority of cells examined (over 70%), short duration (several milliseconds) action potentials were elicited at the end of a hyperpolarising pulse (off-spike) or following hyperpolarization of the membrane potential by injection of current. Under voltage-clamp conditions, a fast inward current followed by an outward current could be evoked upon depolarisation of the membrane. The fast inward current decayed with a time constant of 1-2 ms, with an e-fold decrease per 52 mV increase in voltage, and was blocked by the selective voltage-dependent sodium channel blocker tetrodotoxin (0.5-1 microM). Steady-state inactivation studies revealed that the mean voltage for half-inactivation (V1/2) was -82 mV (range -72 to -98 mV), which indicates that the voltage-dependent Na+ channels in the cell body or soma of frog ORNs are not available for conducting currents at the resting membrane potential. This finding raises the possibility that voltage-dependent Na+ channels may not play a significant role in sensory transduction at the soma. Our results indicate that ORNs of the frog are very efficient in transducing signals towards the brain since currents generated at the cilia will be directed towards depolarising the axons.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- R Y Pun
- Department of Physiology and Biophysics, University of Cincinnati, OH 45267
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