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Daghfous G, Auclair F, Clotten F, Létourneau JL, Atallah E, Millette JP, Derjean D, Robitaille R, Zielinski BS, Dubuc R. GABAergic modulation of olfactomotor transmission in lampreys. PLoS Biol 2018; 16:e2005512. [PMID: 30286079 PMCID: PMC6191151 DOI: 10.1371/journal.pbio.2005512] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2018] [Revised: 10/16/2018] [Accepted: 09/18/2018] [Indexed: 12/21/2022] Open
Abstract
Odor-guided behaviors, including homing, predator avoidance, or food and mate searching, are ubiquitous in animals. It is only recently that the neural substrate underlying olfactomotor behaviors in vertebrates was uncovered in lampreys. It consists of a neural pathway extending from the medial part of the olfactory bulb (medOB) to locomotor control centers in the brainstem via a single relay in the caudal diencephalon. This hardwired olfactomotor pathway is present throughout life and may be responsible for the olfactory-induced motor behaviors seen at all life stages. We investigated modulatory mechanisms acting on this pathway by conducting anatomical (tract tracing and immunohistochemistry) and physiological (intracellular recordings and calcium imaging) experiments on lamprey brain preparations. We show that the GABAergic circuitry of the olfactory bulb (OB) acts as a gatekeeper of this hardwired sensorimotor pathway. We also demonstrate the presence of a novel olfactomotor pathway that originates in the non-medOB and consists of a projection to the lateral pallium (LPal) that, in turn, projects to the caudal diencephalon and to the mesencephalic locomotor region (MLR). Our results indicate that olfactory inputs can induce behavioral responses by activating brain locomotor centers via two distinct pathways that are strongly modulated by GABA in the OB. The existence of segregated olfactory subsystems in lampreys suggests that the organization of the olfactory system in functional clusters may be a common ancestral trait of vertebrates.
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Affiliation(s)
- Gheylen Daghfous
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
- Groupe de Recherche en Activité Physique Adaptée, Département des sciences de l'activité physique, Université du Québec à Montréal, Montréal, Québec, Canada
| | - François Auclair
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
| | - Felix Clotten
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
| | - Jean-Luc Létourneau
- Groupe de Recherche en Activité Physique Adaptée, Département des sciences de l'activité physique, Université du Québec à Montréal, Montréal, Québec, Canada
| | - Elias Atallah
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
| | - Jean-Patrick Millette
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
| | - Dominique Derjean
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
- Groupe de Recherche en Activité Physique Adaptée, Département des sciences de l'activité physique, Université du Québec à Montréal, Montréal, Québec, Canada
| | - Richard Robitaille
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
| | - Barbara S. Zielinski
- Department of Biological Sciences, University of Windsor, Windsor, Ontario, Canada
- Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Ontario, Canada
| | - Réjean Dubuc
- Groupe de Recherche sur le Système Nerveux Central, Département de neurosciences, Université de Montréal, Montréal, Québec, Canada
- Groupe de Recherche en Activité Physique Adaptée, Département des sciences de l'activité physique, Université du Québec à Montréal, Montréal, Québec, Canada
- * E-mail:
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Biochemical Evidence for a Putative Inositol 1,3,4,5-Tetrakisphosphate Receptor in the Olfactory System of Atlantic Salmon (Salmo salar). NEUROSCIENCE JOURNAL 2013; 2013:460481. [PMID: 26317094 PMCID: PMC4437272 DOI: 10.1155/2013/460481] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/25/2012] [Revised: 02/01/2013] [Accepted: 02/04/2013] [Indexed: 11/28/2022]
Abstract
Olfactory receptor neurons in Atlantic salmon (Salmo salar) appear to use a phosphoinositide-directed phospholipase C (PLC) in odorant signal transduction. The consequences of odor-activated PLC depend on its product, inositol 1,4,5-trisphosphate (IP3). Therefore, a plasma membrane rich (PMR) fraction, previously characterized from salmon olfactory rosettes, was used to study binding sites for IP3 and its phosphorylation product, inositol 1,3,4,5-tetrakisphosphate (IP4). Binding sites for IP3 were present at the lower limit for detection in the PMR fraction but were abundant in a microsomal fraction. Binding sites for IP4 were abundant in the PMR fraction and thus colocalized in the same subcellular fraction with odorant receptors for amino acids and bile acids. Binding of IP4 was saturable and high affinity (Kd = 83 nM). The rank order for potency of inhibition of IP4 by other inositol polyphosphates (InsPx) followed the phosphorylation number with InsP6 > InsP5 > other InsP4 isomers > InsP3 isomers > InsP2 isomers, with the latter showing no activity. The consequences of PLC activity in this system may be dictated in part by a putative receptor for IP4.
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Hegg CC, Jia C, Chick WS, Restrepo D, Hansen A. Microvillous cells expressing IP3 receptor type 3 in the olfactory epithelium of mice. Eur J Neurosci 2010; 32:1632-45. [PMID: 20958798 DOI: 10.1111/j.1460-9568.2010.07449.x] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Microvillous cells of the main olfactory epithelium have been described variously as primary olfactory neurons, secondary chemosensory cells or non-sensory cells. Here we generated an IP3R3(tm1(tauGFP)) mouse in which the coding region for a fusion protein of tau and green fluorescent protein replaces the first exon of the Itpr3 gene. We provide immunohistochemical and functional characterization of the cells expressing IP3 receptor type 3 in the olfactory epithelium. These cells bear microvilli at their apex, and we therefore termed them IP3R3 MV cells. The cell body of these IP3R3 MV cells lies in the upper third of the main olfactory epithelium; a long thick basal process projects towards the base of the epithelium without penetrating the basal lamina. Retrograde labeling and unilateral bulbectomy corroborated that these IP3R3 MV cells do not extend axons to the olfactory bulb and therefore are not olfactory sensory neurons. The immunohistochemical features of IP3R3 MV cells varied, suggesting either developmental stages or the existence of subsets of these cells. Thus, for example, subsets of the IP3R3 MV cells make contact with substance P fibers or express the purinergic receptor P2X3. In addition, in recordings of intracellular calcium, these cells respond to ATP and substance P as well as to a variety of odors. The characterization of IP3R3 MV cells as non-neuronal chemoresponsive cells helps to explain the differing descriptions of microvillous cells in the literature.
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Affiliation(s)
- Colleen C Hegg
- Department of Pharmacology and Toxicology, Michigan State University, East Lansing, MI, USA
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Cummins SF, De Vries MR, Hill KS, Boehning D, Nagle GT. Gene identification and evidence for expression of G protein alpha subunits, phospholipase C, and an inositol 1,4,5-trisphosphate receptor in Aplysia californica rhinophore. Genomics 2007; 90:110-20. [PMID: 17498918 DOI: 10.1016/j.ygeno.2007.03.020] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2007] [Revised: 03/28/2007] [Accepted: 03/28/2007] [Indexed: 11/29/2022]
Abstract
In the marine mollusk Aplysia californica, waterborne protein pheromones that are released during egg laying act in concert to stimulate mate attraction. However, molecular information concerning the cellular receptors and signaling mechanisms that may be involved in waterborne peptide and protein pheromonal communication is lacking. As a first step toward examining whether members of the G protein family and phosphoinositide signaling pathway are present in the primary peripheral chemosensory organs (i.e., rhinophores), we isolated five full-length cDNA clones from an A. californica central nervous system cDNA library. These clones encoded (1) the G protein alpha subunits of the Gq, Gi, and Go families, (2) a protein with homology to phospholipase C (PLC) isoforms, and (3) an inositol 1,4,5-trisphosphate receptor (IP3R). The expression of these genes was examined using laser capture microdissection/reverse transcription-polymerase chain reaction and in situ hybridization. All of them are expressed in the rhinophore sensory epithelium, suggesting that Galphaq, Galphai, Galphao, PLC-like protein, and IP3R may be involved in waterborne protein pheromone detection in Aplysia-possibly via a phosphoinositide signaling mechanism.
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Affiliation(s)
- Scott F Cummins
- Department of Neuroscience and Cell Biology, University of Texas Medical Branch, Galveston, TX 77555, USA.
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Lin W, Arellano J, Slotnick B, Restrepo D. Odors detected by mice deficient in cyclic nucleotide-gated channel subunit A2 stimulate the main olfactory system. J Neurosci 2004; 24:3703-10. [PMID: 15071119 PMCID: PMC6729751 DOI: 10.1523/jneurosci.0188-04.2004] [Citation(s) in RCA: 110] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
It is believed that odor transduction in the mammalian main olfactory system only involves the cAMP-signaling pathway. Here, we report on odor responsiveness in mice with a disrupted cyclic nucleotide-gated (CNG) channel subunit A2. Several odorants, including putative pheromones, can be detected and discriminated by these mice behaviorally. These odors elicit responses in the olfactory epithelium, main olfactory bulb, and olfactory (piriform) cortex of CNGA2 knock-out mice. In addition, responses to odors detected by CNGA2 knock-out mice are relatively insensitive to inhibitors of the cAMP pathway. These results provide strong evidence that cAMP-independent pathways in the main olfactory system of mammals participate in detecting a subset of odors.
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Affiliation(s)
- Weihong Lin
- Department of Cell and Developmental Biology, Neuroscience Program and Rocky Mountain Taste and Smell Center, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA
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Odors detected by mice deficient in cyclic nucleotide-gated channel subunit A2 stimulate the main olfactory system. J Neurosci 2004. [PMID: 15071119 DOI: 10.1523/jneurosci.0188‐04.2004] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
It is believed that odor transduction in the mammalian main olfactory system only involves the cAMP-signaling pathway. Here, we report on odor responsiveness in mice with a disrupted cyclic nucleotide-gated (CNG) channel subunit A2. Several odorants, including putative pheromones, can be detected and discriminated by these mice behaviorally. These odors elicit responses in the olfactory epithelium, main olfactory bulb, and olfactory (piriform) cortex of CNGA2 knock-out mice. In addition, responses to odors detected by CNGA2 knock-out mice are relatively insensitive to inhibitors of the cAMP pathway. These results provide strong evidence that cAMP-independent pathways in the main olfactory system of mammals participate in detecting a subset of odors.
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Abstract
The olfactory epithelium of fish contains three intermingled types of olfactory receptor neurons (ORNs): ciliated, microvillous, and crypt. The present experiments were undertaken to test whether the different types of ORNs respond to different classes of odorants via different families of receptor molecules and G-proteins corresponding to the morphology of the ORN. In catfish, ciliated ORNs express OR-type receptors and Galpha(olf). Microvillous ORNs are heterogeneous, with many expressing Galpha(q)/11, whereas crypt ORNs express Galpha(o). Retrograde tracing experiments show that ciliated ORNs project predominantly to regions of the olfactory bulb (OB) that respond to bile salts (medial) and amino acids (ventral) (Nikonov and Caprio, 2001). In contrast, microvillous ORNs project almost entirely to the dorsal surface of the OB, where responses to nucleotides (posterior OB) and amino acids (anterior OB) predominate. These anatomical findings are consistent with our pharmacological results showing that forskolin (which interferes with Galpha(olf)/cAMP signaling) blocks responses to bile salts and markedly reduces responses to amino acids. Conversely, U-73122 and U-73343 (which interfere with Galpha(q)/11/phospholipase C signaling) diminish amino acid responses but leave bile salt and nucleotide responses essentially unchanged. In summary, our results indicate that bile salt odorants are detected predominantly by ciliated ORNs relying on the Galpha(olf)/cAMP transduction cascade. Nucleotides are detected by microvillous ORNs using neither Galpha(olf)/cAMP nor Galpha(q)/11/PLC cascades. Finally, amino acid odorants activate both ciliated and microvillous ORNs but via different transduction pathways in the two types of cells.
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Frontini A, Zaidi AU, Hua H, Wolak TP, Greer CA, Kafitz KW, Li W, Zielinski BS. Glomerular territories in the olfactory bulb from the larval stage of the sea lamprey Petromyzon marinus. J Comp Neurol 2003; 465:27-37. [PMID: 12926014 DOI: 10.1002/cne.10811] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
The goal of this study was to investigate the spatial organization of olfactory glomeruli and of substances relevant to olfactory sensory neuron activity in the developing agnathan, the sea lamprey Petromyzon marinus. A 45-kD protein immunoreactive to G(olf), a cAMP-dependent olfactory G protein, was present in the ciliary fraction of sea lamprey olfactory epithelium and in olfactory sensory neurons of larval and adult sea lampreys. This result implies that G(olf) expression was present during early vertebrate evolution or evolved in parallel in gnathostome and agnathostome vertebrates. Serial sectioning of the olfactory bulb revealed a consistent pattern of olfactory glomeruli stained by GS1B(4) lectin and by anterograde labeling with fluorescent dextran. These glomerular territories included the dorsal cluster, dorsal ring, anterior plexus, lateral chain, medial glomeruli, ventral ring, and ventral cluster. The dorsal, anterior, lateral, and ventral glomeruli contained olfactory sensory axon terminals that were G(olf)-immunoreactive. However, a specific subset, the medial glomeruli, did not display this immunoreactivity. Olfactory glomeruli in the dorsal hemisphere of the olfactory bulb, the dorsal cluster, dorsal ring, anterior plexus, lateral chain, and medial glomeruli, were seen adjacent to 5HT-immunoreactive fibers. However, glomeruli in the ventral hemisphere, the ventral ring, and ventral cluster did not display this association. The presence of specific glomerular territories and discrete glomerular subsets with substances relevant to olfactory sensory neuron activity suggest a spatial organization of information flow in the lamprey olfactory pathway.
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Affiliation(s)
- Andrea Frontini
- Department of Biological Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
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Menco BP, Carr VM, Ezeh PI, Liman ER, Yankova MP. Ultrastructural localization of G-proteins and the channel protein TRP2 to microvilli of rat vomeronasal receptor cells. J Comp Neurol 2001; 438:468-89. [PMID: 11559902 DOI: 10.1002/cne.1329] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Microvilli of vomeronasal organ (VNO) sensory epithelium receptor cells project into the VNO lumen. This lumen is continuous with the outside environment. Therefore, the microvilli are believed to be the subcellular sites of VNO receptor cells that interact with incoming VNO-targeted odors, including pheromones. Candidate molecules, which are implicated in VNO signaling cascades, are shown to be present in VNO receptor cells. However, ultrastructural evidence that such molecules are localized within the microvilli is sparse. The present study provides firm evidence that immunoreactivity for several candidate VNO signaling molecules, notably the G-protein subunits G(ialpha2) and G(oalpha), and the transient receptor potential channel 2 (TRP2), is localized prominently and selectively in VNO receptor cell microvilli. Although G(ialpha2) and G(oalpha) are localized separately in the microvilli of two cell types that are otherwise indistinguishable in their apical and microvillar morphology, the microvilli of both cell types are TRP2(+). VNO topographical distinctions were also apparent. Centrally within the VNO sensory epithelium, the numbers of receptor cells with G(ialpha2)(+) and G(oalpha)(+) microvilli were equal. However, near the sensory/non-sensory border, cells with G(ialpha2)(+) microvilli predominated. Scattered ciliated cells in this transition zone resembled neither VNO nor main olfactory organ (MO) receptor cells and may represent the same ciliated cells as those found in the non-sensory part of the VNO. Thus, this study shows that, analogous to the cilia of MO receptor cells, microvilli of VNO receptor cells are enriched selectively in proteins involved putatively in signal transduction. This provides important support for the role of these molecules in VNO signaling.
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Affiliation(s)
- B P Menco
- Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208-3520, USA.
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Kang WS, Lee WK, Seo MS, Kim JK, Han JW, Moon YW, Cho KK, Kim MJ, Lee KH, Kim YS, Jo YH, Choi CR. Cellular distribution of isozymes of protein kinase C in septal olfactory epithelium of mice. Neurosci Lett 2000; 288:143-6. [PMID: 10876081 DOI: 10.1016/s0304-3940(00)01230-1] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
To determine the presence of protein kinase C (PKC) isozymes in the septal olfactory epithelium of mice (mSOE), western blotting and immunohistochemistry were performed using antibodies against PKC isozymes. With the exception of PKC-betaI, all of the PKC isozymes were detected in the whole lysate of septal tissue layer and apparent molecular weights for each isoform were found. PKC-alpha, PKC-gamma and PKC-epsilon were detected in the olfactory glandular cells of the lamina propria, and PKC-betaI and PKC-betaII were located in the microvillar cells. Neither novel PKC nor atypical PKC was detected in olfactory glandular cells or microvillar cells, except for PKC-epsilon. PKC-lambda was localized in the mucous layer of the mSOE. Meanwhile, PKC-delta and PKC-xi were distributed in the receptor cells in the mSOE. These data demonstrate the isoform-specific expression of PKC in mSOE and suggest a role for the novel and atypical types of PKC in olfactory transduction.
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Affiliation(s)
- W S Kang
- Department of Biology, College of Medicine, The Catholic University of Korea, Seoul, South Korea
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Okada Y, Fujiyama R, Miyamoto T, Sato T. Comparison of a Ca(2+)-gated conductance and a second-messenger-gated conductance in rat olfactory neurons. J Exp Biol 2000; 203:567-73. [PMID: 10637185 DOI: 10.1242/jeb.203.3.567] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The effect of a rise in intracellular Ca(2+) concentration was analyzed in isolated rat olfactory neurons using a whole-cell patch-clamp technique. Intracellular dialysis of 1 mmol l(−)(1) Ca(2+) in a standard-K(+), low-Cl(−) internal solution (E(Cl)=−69 mV) from the patch pipette into the olfactory neurons induced a sustained outward current of 49+/−5 pA (N=13) at −50 mV in all the cells examined. The outward currents were inhibited by external application of 100 micromol l(−)(1) 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB). External application of a Ca(2+) ionophore, 3 micromol l(−)(1) ionomycin, induced an inward current in three of eight cells whose voltages were clamped using the gramicidin-perforated technique, but ionomycin elicited an outward current in the other five cells, suggesting that natural intracellular Cl(−) concentration in the olfactory neurons was heterogeneous. While intracellular dialysis of 50 micromol l(−)(1) inositol 1,4,5-trisphosphate (1,4,5-InsP(3)) in the standard-K(+), low-Cl(−) internal solution induced the NPPB-sensitive outward current in 31 % of cells, and 500 micromol l(−)(1) cAMP induced it in 21 % of cells, a large proportion of the cells displayed an inward current in response to 1,4,5-InsP(3) and cAMP. The results suggest that 1,4,5-InsP(3) and cAMP can elicit Ca(2+)-dependent Cl(−) conductance and Ca(2+)-independent cation conductance in rat olfactory neurons.
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Affiliation(s)
- Y Okada
- Department of Physiology, Nagasaki University School of Dentistry, Nagasaki 852-8588, Japan.
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Abstract
A number of controversial issues in olfactory transduction are discussed including the matter of multiple transduction pathways, with a new experiment proposed. Evidence is reviewed concerning the fact that cyclic AMP is the only pathway mediating olfactory transduction. Two knockout mice have been produced: a knockout for a cyclic nucleotide-gated channel and a G(olf) knockout. The results obtained with both mice are consistent with cyclic AMP being the only second messenger. The evidence for gaseous second channel messengers is also reviewed. Slow gating kinetics of the cyclic nucleotide-gated channel and the detection of single-odorant molecules are reviewed. A new phenomenon in which odorants can block odorant responses is discussed.
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Affiliation(s)
- G H Gold
- Monell Chemical Senses Center, Philadelphia, Pennsylvania 19104-3308, USA
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Schandar M, Laugwitz KL, Boekhoff I, Kroner C, Gudermann T, Schultz G, Breer H. Odorants selectively activate distinct G protein subtypes in olfactory cilia. J Biol Chem 1998; 273:16669-77. [PMID: 9642220 DOI: 10.1074/jbc.273.27.16669] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Chemoelectrical signal transduction in olfactory neurons appears to involve intracellular reaction cascades mediated by heterotrimeric GTP-binding proteins. In this study attempts were made to identify the G protein subtype(s) in olfactory cilia that are activated by the primary (odorant) signal. Antibodies directed against the alpha subunits of distinct G protein subtypes interfered specifically with second messenger reponses elicited by defined subsets of odorants; odor-induced cAMP-formation was attenuated by Galphas antibodies, whereas Galphao antibodies blocked odor-induced inositol 1,4, 5-trisphosphate (IP3) formation. Activation-dependent photolabeling of Galpha subunits with [alpha-32P]GTP azidoanilide followed by immunoprecipitation using subtype-specific antibodies enabled identification of particular individual G protein subtypes that were activated upon stimulation of isolated olfactory cilia by chemically distinct odorants. For example odorants that elicited a cAMP response resulted in labeling of a Galphas-like protein, whereas odorants that elicited an IP3 response led to the labeling of a Galphao-like protein. Since odorant-induced IP3 formation was also blocked by Gbeta antibodies, activation of olfactory phospholipase C might be mediated by betagamma subunits of a Go-like G protein. These results indicate that different subsets of odorants selectively trigger distinct reaction cascades and provide evidence for dual transduction pathways in olfactory signaling.
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Affiliation(s)
- M Schandar
- Universität Stuttgart-Hohenheim, Institut für Physiologie, 70593 Stuttgart, Germany
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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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