1 2004 Vol: 5(4):263-278. DOI: 10.1038/nrn1365

Genes and ligands for odorant, vomeronasal and taste receptors

The chemical senses (smell and taste) have evolved complex repertoires of chemosensory receptors — G-protein coupled receptors with a seven-transmembrane domain structure. In the mouse, 1,000 odorant receptors are dedicated to the conventional sense of smell, 300 vomeronasal receptors mediate the detection of chemical stimuli (such as pheromones) by the vomeronasal organ, and 40 taste receptors are implicated in bitter, sweet and umami taste. Nearly all receptor genes have now been identified as the result of genome sequencing, but few receptor–ligand interactions have been characterized. Targeted expression of the green fluorescent protein in chemosensory cells is a promising approach to achieve this objective.

Mentions
Figures
Figure 1: The mammalian nose.a | Schematic diagram of half a mouse head. Axons of sensory neurons in the main olfactory epithelium (MOE) project to the main olfactory bulb (MOB), and axons of sensory neurons in the vomeronasal organ (VNO) project to the accessory olfactory bulb (AOB). b | Cross-section of the main olfactory epithelium, and cross-section of the peripheral part of the main olfactory bulb. Figure 2: The mammalian tongue.a | Three types of taste receptor cell-containing papillae occupy distinct regions of the rodent tongue. b | The single circumvallate papilla contains multiple taste buds. c | A taste bud contains multiple closely appositioned taste receptor cells, some of which synapse with nerve fibres. Figure 3: Odorant and vomeronasal receptors.a | Odorant receptors and V1R vomeronasal receptors have short N-terminal extracellular domains, in contrast to V2Rs. The number of genes in each family is indicated in brackets. b | The degree of amino acid conservation in the consensus sequence of an odorant receptor is represented as a colour in the rainbow spectrum, with blue being highly conserved and red highly variable. Modified, with permission, from Ref. 46 © (2003) Academic Press. Figure 4: Wiring of the main olfactory and vomeronasal systems.a | Olfactory sensory neurons that express the same odorant receptor project to distinct glomeruli in the main olfactory bulb (MOB). b | Vomeronasal sensory neurons that express the same V1R or V2R project to multiple, small glomeruli in the accessory olfactory bulb (AOB). The apical layer of the epithelium projects to the rostral half of the AOB, and the basal layer projects to the posterior half. c | The four zones in the main olfactory epithelium (MOE) correspond roughly to four domains in the MOB. The boundaries between the domains have not been delineated precisely because not enough odorant receptors have been mapped onto the MOB. Figure 5: The odorant and vomeronasal receptor gene families.a | Distribution of odorant receptor (OR) gene clusters in the mouse genome. Data from Ref. 43. b | Unrooted phylogenetic tree of human ORs. Genes on chromosome 11 are shown in green, genes on chromosome 1 are in red, and all other genes are in black. The scale bar is equivalent to 10% sequence divergence. Modified, with permission, from Ref. 35 © (2001) BioMed Central. c | Phylogenetic tree of mouse V1R vomeronasal receptors. There are 12 phylogenetically isolated families, 10 of which contain more than one member. Modified, with permission, from Ref. 71 © (2002) Macmillan Magazines Ltd. Figure 6: Canonical pathway of signal transduction in olfactory sensory neurons.The odorant receptor defines odorant responsiveness. The heterotrimeric G-protein, the adenylyl cyclase, the cyclic-nucleotide gated Na+/Ca+ channel and the Cl- channel are thought to be common among olfactory sensory neurons. Figure 7: Homologous in vivo assay for odorant responsiveness of odorant receptors.a | Gene targeting procedure to introduce an IRES-tauGFP cassette into the M71 odorant receptor (OR) locus. Modified, with permission, from Ref. 162 © (2001) Society for Neuroscience. b | The IRES (internal ribosome entry site) sequence instructs co-translational expression of the M71 OR and the tauGFP marker, allowing identification of M71-expressing olfactory sensory neurons (OSNs) by green fluorescence. c | The main olfactory epithelium of mice that carry the targeted M71-IRES-tauGFP mutation is dissociated into single OSNs, which are loaded with the calcium indicator Fura-2. Ratiometric Ca2+ imaging provides a readout of physiological activation of the OSNs by odorants. Modified, with permission, from Ref. 31 © (2002) Society for Neuroscience. Figure 8: Odorant response relationships.Responses were observed for the mouse and rat I7 odorant receptor (OR) in different assays. Selected ligands are listed in the left-hand column. Response amplitudes are indicated by a dot area expressed as a percentage of the octanal response normalized for a given assay in a publication. For rat I7, heptanal, octanal, nonenal, decanal and citronellal responses have been corroborated by more than one assay. (More rat I7 ligands can be found in Ref. 127). So far, only the heptanal response has been corroborated for mouse I7 by more than one assay. Figure 9: The known non-chemosensory and chemosensory GPCR genes in mouse and human.Dark shade, receptors with known ligands. Light shade, receptors without identified ligands. GPCR, G-protein-coupled receptor; TR, taste receptor.
Altmetric
References
  1. Finger, T. E., Silver, W. L. & Restrepo, D. (eds) The Neurobiology of Taste and Smell (Wiley-Liss (2000) , .
  2. Firestein, S. How the olfactory system makes sense of scents. Nature 413, 211-218 (2001) , .
    • . . . How many chemicals can be detected by an animal? For air-breathing organisms such as humans, odorants2 must be sufficiently volatile to be detected by the nose . . .
    • . . . The canonical pathway of transduction in OSNs of mammals2 consists of one variable component — the OR — and four constant elements: the Golf-containing heterotrimeric G-protein; adenylyl cyclase, which produces the second messenger cAMP; a cyclic nucleotide-gated cation channel, and a chloride channel (Fig. 6) . . .
  3. Keverne, E. B. The vomeronasal organ. Science 286, 716-720 (1999) , .
    • . . . The rodent vomeronasal organ3 is specialized for the detection of pheromones (an operationally defined class of olfactory cues), but the extent of its chemosensory capacity is unclear . . .
    • . . . Most mammals possess a second olfactory system, which is termed the accessory olfactory system or the vomeronasal system3, 55, 56, 57 (Fig. 1) . . .
  4. Sam, M. et al. Odorants may arouse instinctive behaviours. Nature 412, 142 (2001) , .
    • . . . In certain conditions4, 5, its neurons respond to molecules other than pheromones, but it remains to be established whether these molecules are physiologically relevant stimuli6. . . .
    • . . . Conversely, the VNO can also detect 'common' odorants4, 5 that are not typically regarded as pheromones, although it is not always clear whether it does so in physiological circumstances6 . . .
  5. Trinh, K. & Storm, D. R. Vomeronasal organ detects odorants in absence of signaling through main olfactory epithelium. Nature Neurosci. 6, 519-525 (2003) , .
    • . . . In certain conditions4, 5, its neurons respond to molecules other than pheromones, but it remains to be established whether these molecules are physiologically relevant stimuli6. . . .
    • . . . Conversely, the VNO can also detect 'common' odorants4, 5 that are not typically regarded as pheromones, although it is not always clear whether it does so in physiological circumstances6 . . .
    • . . . The main olfactory mucosa and the VNO mucosa both respond to this molecule5 . . .
  6. Luo, M., Fee, M. S. & Katz, L. C. Encoding pheromonal signals in the accessory olfactory bulb of behaving mice. Science 299, 1196-1201 (2003) , .
    • . . . In certain conditions4, 5, its neurons respond to molecules other than pheromones, but it remains to be established whether these molecules are physiologically relevant stimuli6. . . .
    • . . . Conversely, the VNO can also detect 'common' odorants4, 5 that are not typically regarded as pheromones, although it is not always clear whether it does so in physiological circumstances6 . . .
  7. Smith, D. V. & Margolskee, R. F. Making sense of taste. Sci. Am. 284, 32-39 (2001) , .
    • . . . The tongue can identify a wide array of tastes7 that can be classified as sweet, bitter, salty or sour, with umami being increasingly recognized as a fifth taste modality . . .
  8. Buck, L. & Axel, R. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65, 175-187 (1991).Olfactory research can be divided into two eras: before and after the publication of this paper , .
    • . . . The molecular era of research into the chemical senses came of age in 1991 with the discovery of odorant receptor genes8, 9 . . .
    • . . . Relying on the novel strategy at that time, DEGENERATE POLYMERASE CHAIN REACTION (PCR), a diverse superfamily of 1,000 genes that encoded 7TM proteins was identified in the rat (Fig. 3), and RNA transcripts were localized to the olfactory mucosa8 . . .
    • . . . It took seven years from the publication of the OR discovery8 for the first unambiguous OR–ligand pair to be reported126 . . .
  9. Buck, L. B. The search for odorant receptors. Cell 116, Suppl. S117-S119 (2004) , .
    • . . . The molecular era of research into the chemical senses came of age in 1991 with the discovery of odorant receptor genes8, 9 . . .
  10. Mombaerts, P. Seven-transmembrane proteins as odorant and chemosensory receptors. Science 286, 707-711 (1999) , .
    • . . . These genes are referred to interchangeably as 'odorant receptor', 'olfactory receptor', and 'odour receptor' genes, and are abbreviated to 'OR genes'10. . . .
  11. Mombaerts, P. Odorant receptor gene choice in olfactory sensory neurons: the one receptor-one neuron hypothesis revisited. Curr. Op. Neurobiol. 14, 31-36 (2004).A critical analysis of a dogma, and the proposal of the oligogenic developmental hypothesis , .
    • . . . How is transcription of the OR gene superfamily organized across olfactory sensory neurons (OSNs)? The one gene–one neuron hypothesis — a single OR gene is expressed in each OSN — is attractive in its simplicity and has become widely popular, but I have argued elsewhere that there is no convincing proof11 . . .
    • . . . The expressed OR might exert negative feedback regulation on the expression of the remainder of the repertoire including the other allele19, 20, but it is difficult to distinguish between negative feedback at the single cell level and negative selection at the population level11. . . .
  12. Malnic, B., Hirono, J., Sato, T. & Buck, L. B. Combinatorial receptor codes for odors. Cell 96, 713-723 (1999) , .
    • . . . The best evidence is that a single OR can be amplified from an individual OSN by the combination of reverse- transcription and polymerase chain reaction (RT-PCR)12, 13, 14 . . .
    • . . . A single assay has been developed for this purpose12, 13, 14, 140 . . .
  13. Touhara, K. et al. Functional identification and reconstitution of an odorant receptor in single olfactory neurons. Proc. Natl Acad. Sci. USA 96, 4040-4045 (1999) , .
    • . . . The best evidence is that a single OR can be amplified from an individual OSN by the combination of reverse- transcription and polymerase chain reaction (RT-PCR)12, 13, 14 . . .
    • . . . First, adenovirally-mediated gene transfer into the olfactory mucosa of living rodents was successful for rat I7 (Ref. 126) and MOR23 (Refs 13,134) . . .
    • . . . A single assay has been developed for this purpose12, 13, 14, 140 . . .
    • . . . This crucial evidence has been reported for three ORs, by adenovirally-mediated infection of olfactory mucosa13 (lyral and MOR23) or transfection of HEK293 cells14, 141 (eugenol and mOR-EG; ethyl vanillin and mOR-EV) (Table 1); however, it is not routinely done. . . .
  14. Kajiya, K. et al. Molecular bases of odor discrimination: reconstitution of olfactory receptors that recognize overlapping sets of odorants. J. Neurosci. 21, 6018-6025 (2001) , .
    • . . . The best evidence is that a single OR can be amplified from an individual OSN by the combination of reverse- transcription and polymerase chain reaction (RT-PCR)12, 13, 14 . . .
    • . . . The most extensive characterization is for mOR-EG, a eugenol-responsive receptor14 . . .
    • . . . A single assay has been developed for this purpose12, 13, 14, 140 . . .
    • . . . This crucial evidence has been reported for three ORs, by adenovirally-mediated infection of olfactory mucosa13 (lyral and MOR23) or transfection of HEK293 cells14, 141 (eugenol and mOR-EG; ethyl vanillin and mOR-EV) (Table 1); however, it is not routinely done. . . .
  15. Rawson, N. E. et al. Expression of mRNAs encoding for two different olfactory receptors in a subset of olfactory receptor neurons. J. Neurochem. 75, 185-195 (2000) , .
    • . . . In one report, co-expression of two ORs in some rat OSNs was shown by IN SITU HYBRIDIZATION15 . . .
  16. Chess, A., Simon, I., Cedar, H. & Axel, R. Allelic inactivation regulates olfactory receptor gene expression. Cell 78, 823-834 (1994) , .
    • . . . What is better established is that only one of the two alleles of a given OR gene is expressed in a cell, a phenomenon that is termed monoallelic expression16, 17, 18 . . .
  17. Strotmann, J. et al. Local permutations in the glomerular array of the mouse olfactory bulb. J. Neurosci. 20, 6927-6938 (2000) , .
    • . . . What is better established is that only one of the two alleles of a given OR gene is expressed in a cell, a phenomenon that is termed monoallelic expression16, 17, 18 . . .
    • . . . OSNs that express the maternal allele and OSNs that express the paternal allele coexist in the same individual, often in comparable numbers17 . . .
  18. Ishii, T. et al. Monoallelic expression of the odourant receptor gene and axonal projection of olfactory sensory neurons. Genes Cells 6, 71-78 (2001) , .
    • . . . What is better established is that only one of the two alleles of a given OR gene is expressed in a cell, a phenomenon that is termed monoallelic expression16, 17, 18 . . .
  19. Serizawa, S. et al. Negative feedback regulation ensures the one receptor-one olfactory neuron rule in mouse. Science 302, 2088-2094 (2003) , .
    • . . . The expressed OR might exert negative feedback regulation on the expression of the remainder of the repertoire including the other allele19, 20, but it is difficult to distinguish between negative feedback at the single cell level and negative selection at the population level11. . . .
  20. Lewcock, J. W. & Reed, R. R. A feedback mechanism regulates monoallelic odorant receptor expression. Proc. Natl Acad. Sci. USA 101, 1069-1074 (2004) , .
    • . . . The expressed OR might exert negative feedback regulation on the expression of the remainder of the repertoire including the other allele19, 20, but it is difficult to distinguish between negative feedback at the single cell level and negative selection at the population level11. . . .
  21. Ressler, K. J., Sullivan, S. L. & Buck, L. B. A zonal organization of odorant receptor gene expression in the olfactory epithelium. Cell 73, 597-609 (1993) , .
    • . . . In situ hybridization indicates that a given OR gene is expressed in a small subset of OSNs21, 22, which usually reside within one of four parallel zones23 of the olfactory epithelium (Fig. 4c) . . .
  22. Vassar, R., Ngai, J. & Axel, R. Spatial segregation of odorant receptor expression in the mammalian olfactory epithelium. Cell 74, 309-318 (1993) , .
    • . . . In situ hybridization indicates that a given OR gene is expressed in a small subset of OSNs21, 22, which usually reside within one of four parallel zones23 of the olfactory epithelium (Fig. 4c) . . .
  23. Sullivan, S. L., Adamson, M. C., Ressler, K. J., Kozak, C. A. & Buck, L. B. The chromosomal distribution of mouse odorant receptor genes. Proc. Natl Acad. Sci. USA 93, 884-888 (1996) , .
    • . . . In situ hybridization indicates that a given OR gene is expressed in a small subset of OSNs21, 22, which usually reside within one of four parallel zones23 of the olfactory epithelium (Fig. 4c) . . .
  24. Iwema, C. L., Fang, H., Kurtz, D. B., Youngentob, S. L. & Schwob, J. E. Odorant receptor expression patterns are restored in lesion-recovered rat olfactory epithelium. J. Neurosci. 24, 356-369 (2004) , .
    • . . . This zonal distribution has been documented in mouse and rat, but it might be not as well-defined as was originally thought: areas of OR gene expression might overlap and be subtly distinct24 . . .
  25. Vassar, R. et al. Topographic organization of sensory projections to the olfactory bulb. Cell 79, 981-991 (1994) , .
    • . . . In situ hybridization25, 26 and genetic labelling of OSN populations by targeted mutagenesis of OR genes27 revealed that axons of OSNs that express a given OR gene project their axons to one (or a few) glomeruli in each of the two halves (medial and lateral) of an olfactory bulb (Fig. 4a) . . .
  26. Ressler, K. J., Sullivan, S. L. & Buck, L. B. Information coding in the olfactory system: evidence for a stereotyped and highly organized epitope map in the olfactory bulb. Cell 79, 1245-1255 (1994) , .
    • . . . In situ hybridization25, 26 and genetic labelling of OSN populations by targeted mutagenesis of OR genes27 revealed that axons of OSNs that express a given OR gene project their axons to one (or a few) glomeruli in each of the two halves (medial and lateral) of an olfactory bulb (Fig. 4a) . . .
    • . . . Within a bulb, OR-specific glomeruli show a roughly mirrored, symmetrical arrangement between the medial and lateral halves26, 28 . . .
  27. Mombaerts, P. et al. Visualizing an olfactory sensory map. Cell 87, 675-686 (1996).Development of the genetic approach that later allowed targeted expression of GFP in homologous cells , .
    • . . . In situ hybridization25, 26 and genetic labelling of OSN populations by targeted mutagenesis of OR genes27 revealed that axons of OSNs that express a given OR gene project their axons to one (or a few) glomeruli in each of the two halves (medial and lateral) of an olfactory bulb (Fig. 4a) . . .
    • . . . Gene targeting experiments in which the coding region of an OR gene is replaced by that of another OR gene have implicated the OR itself in axonal convergence27, 30, 31 . . .
    • . . . Specific populations of VSNs were labelled by targeted mutagenesis of three V1R genes78, 80 and one V2R gene79, using the same strategy as for OR genes27 . . .
    • . . . This is similar to the strategy that was developed to show axonal convergence to glomeruli27 . . .
  28. Nagao, H., Yoshihara, Y., Mitsui, S., Fujisawa, H. & Mori, K. Two mirror-image sensory maps with domain organization in the mouse main olfactory bulb. Neuroreport 11, 3023-3027 (2000) , .
    • . . . Within a bulb, OR-specific glomeruli show a roughly mirrored, symmetrical arrangement between the medial and lateral halves26, 28 . . .
  29. Treloar, H. B., Feinstein, P., Mombaerts, P. & Greer, C. A. Specificity of glomerular targeting by olfactory sensory axons. J. Neurosci. 22, 2469-2477 (2002) , .
    • . . . All axons that project to a given glomerulus are from OSNs that express the same OR, at least in the case of the M72 OR29 . . .
  30. Wang, F., Nemes, A., Mendelsohn, M. & Axel, R. Odorant receptors govern the formation of a precise topographic map. Cell 93, 47-60 (1998) , .
    • . . . Gene targeting experiments in which the coding region of an OR gene is replaced by that of another OR gene have implicated the OR itself in axonal convergence27, 30, 31 . . .
  31. Bozza, T., Feinstein, P., Zheng, C. & Mombaerts, P. Odorant receptor expression defines functional units in the mouse olfactory system. J. Neurosci. 22, 3033-3043 (2002).First example of analysis of OR-ligand interactions in homologous cells by gene targeting , .
    • . . . Gene targeting experiments in which the coding region of an OR gene is replaced by that of another OR gene have implicated the OR itself in axonal convergence27, 30, 31 . . .
    • . . . Third, a homologous in vivo approach has been developed, which uses gene targeting to tag OSNs that express a given OR gene endogenously31 (Fig. 7) . . .
    • . . . Green fluorescent cells from M71-IRES-tauGFP transgenic mice respond to acetophenone and benzaldehyde in a concentration-dependent fashion31 . . .
    • . . . Modified, with permission, from Ref. 31 © (2002) Society for Neuroscience. . . .
    • . . . There are some discrepancies between ligands identified in the adenoviral assay126, 127 and in genetically tagged neurons that express rat I7 (Ref. 31) . . .
    • . . . These discrepancies are also seen when mouse I7 is expressed from its endogenous locus31 . . .
    • . . . By applying a similar gene tagging strategy as for M71 and acetophenone31, it was shown that green fluorescent VSNs from mice that express V1rb2 along with GFP respond reproducibly to 2-heptanone, both by calcium imaging and by patch-clamp recording142 . . .
  32. Mombaerts, P. How smell develops. Nature Neurosci. 4, Suppl. 1192-1198 (2001) , .
    • . . . Somehow, axons of OSNs that express the same OR recognize each other, allowing them to coalesce into discrete glomeruli in an OR-dependent manner32. . . .
  33. Mombaerts, P. Molecular biology of odorant receptors in vertebrates. Annu. Rev. Neurosci. 22, 487-509 (1999) , .
    • . . . Degenerate PCR with pan-OR primers has permitted the cloning of OR genes from many vertebrate species33 . . .
  34. Glusman, G., Yanai, I., Rubin, I. & Lancet, D. The complete human olfactory subgenome. Genome Res. 11, 685-702 (2001) , .
    • . . . In human34, 35, 36, 37, 38, 50–60% of OR sequences are PSEUDOGENES . . .
  35. Zozulya, S., Echeverri, F. & Nguyen, T. The human olfactory receptor repertoire. Genome Biol. 2, 0018.1-0018.12 (2001).References 34 and 35 are the first drafts of the human OR repertoire , .
    • . . . In human34, 35, 36, 37, 38, 50–60% of OR sequences are PSEUDOGENES . . .
    • . . . Modified, with permission, from Ref. 35 © (2001) BioMed Central. c | Phylogenetic tree of mouse V1R vomeronasal receptors . . .
  36. Niimura, Y. & Nei, M. Evolution of olfactory receptor genes in the human genome. Proc. Natl Acad. Sci. USA 100, 12235-12240 (2003) , .
    • . . . In human34, 35, 36, 37, 38, 50–60% of OR sequences are PSEUDOGENES . . .
  37. Mombaerts, P. Odorant receptor genes in humans. Curr. Opin. Genet. Dev. 9, 315-320 (1999) , .
    • . . . In human34, 35, 36, 37, 38, 50–60% of OR sequences are PSEUDOGENES . . .
  38. Mombaerts, P. The human repertoire of odorant receptor genes and pseudogenes. Annu. Rev. Genomics Hum. Genet. 2, 493-510 (2001) , .
    • . . . In human34, 35, 36, 37, 38, 50–60% of OR sequences are PSEUDOGENES . . .
  39. Gilad, Y., Wiebe, V., Przeworski, M., Lancet, D. & Pääbo, S. Loss of olfactory receptor genes coincides with the acquisition of full trichromatic vision in primates. PLoS Biol. 2, 0120-0125 (2004) , .
    • . . . The massive pseudogenization of the OR repertoire in humans and Old World primates39 is preceded by a moderately high level of pseudogenes (28–36%) in lower primate species40 . . .
  40. Gilad, Y., Man, O., Pääbo, S. & Lancet, D. Human specific loss of olfactory receptor genes. Proc. Natl Acad. Sci. USA 100, 3324-3327 (2003) , .
    • . . . The massive pseudogenization of the OR repertoire in humans and Old World primates39 is preceded by a moderately high level of pseudogenes (28–36%) in lower primate species40 . . .
  41. Zhang, X. & Firestein, S. The olfactory receptor gene superfamily of the mouse. Nature Neurosci. 5, 124-133 (2002) , .
    • . . . By contrast, of the 1,300–1,500 OR sequences in the mouse genome41, 42, 43, 44, only 20% are pseudogenes, and at least 419 are expressed in the olfactory epithelium45 . . .
    • . . . The intact mouse OR genes can be grouped into families, defined by an amino-acid identity of >40% (Ref. 41) and containing between 1 and 50 member genes . . .
  42. Young, J. M. et al. Different evolutionary processes shaped the mouse and human olfactory receptor gene families. Hum. Mol. Genet. 11, 535-546 (2002).References 41 and 42 are the first two drafts of the mouse OR repertoire , .
    • . . . By contrast, of the 1,300–1,500 OR sequences in the mouse genome41, 42, 43, 44, only 20% are pseudogenes, and at least 419 are expressed in the olfactory epithelium45 . . .
  43. Zhang, X., Rodriguez, I., Mombaerts, P. & Firestein, S. Odorant and vomeronasal receptor genes in two mouse genome assemblies. Genomics DOI 10.1016/j.ygeno.2003.10.009 , .
    • . . . By contrast, of the 1,300–1,500 OR sequences in the mouse genome41, 42, 43, 44, only 20% are pseudogenes, and at least 419 are expressed in the olfactory epithelium45 . . .
    • . . . Data from Ref. 43. b | Unrooted phylogenetic tree of human ORs . . .
    • . . . Subsequently, genome-wide drafts of the mouse V1R repertoire43, 71 (150 genes) have made it clear that 'V3Rs' are simply one of 12 V1R families (family d), which are phylogenetically isolated from each other and can therefore be mistaken for new classes of receptors (Fig. 5c). . . .
    • . . . Mouse V1R and V2R genes are also scattered in clusters across several chromosomes43 . . .
  44. Godfrey, P. A., Malnic, B. & Buck, L. B. The mouse olfactory receptor gene family. Proc. Natl Acad. Sci. USA 101, 2156-2161 (2004) , .
    • . . . By contrast, of the 1,300–1,500 OR sequences in the mouse genome41, 42, 43, 44, only 20% are pseudogenes, and at least 419 are expressed in the olfactory epithelium45 . . .
  45. Young, J. M. et al. Odorant receptor expressed sequence tags demonstrate olfactory expression of over 400 genes, extensive alternate splicing and unequal expression levels. Genome Biol. 4, R71 (2003) , .
    • . . . By contrast, of the 1,300–1,500 OR sequences in the mouse genome41, 42, 43, 44, only 20% are pseudogenes, and at least 419 are expressed in the olfactory epithelium45 . . .
  46. Liu, A. H., Zhang, X., Stolovitzky, G. A., Califano, A. & Firestein, S. J. Motif-based construction of a functional map for mammalian olfactory receptors. Genomics 81, 443-456 (2003) , .
    • . . . Modified, with permission, from Ref. 46 © (2003) Academic Press. . . .
    • . . . Of the plethora of short motifs that can be discerned among mouse ORs46, certain combinations are diagnostic for ORs as opposed to other GPCRs, and this also extends to other vertebrate species . . .
  47. Quignon, P. et al. Comparison of the canine and human olfactory receptor gene repertoires. Genome Biol. 4, R80 (2003) , .
    • . . . An initial survey of the dog OR superfamily47, 48 reveals a similar number of genes and a similar fraction of pseudogenes to the mouse. . . .
  48. Olender, T. et al. The canine olfactory subgenome. Genomics 83, 361-372 (2004) , .
    • . . . An initial survey of the dog OR superfamily47, 48 reveals a similar number of genes and a similar fraction of pseudogenes to the mouse. . . .
  49. Xie, S. Y., Feinstein, P. & Mombaerts, P. Characterization of a cluster comprising 100 odorant receptor genes in mouse. Mamm. Genome 11, 1070-1078 (2000) , .
    • . . . OR genes reside in clusters of various sizes (>130 genes spread over 2.5 megabases, for instance49), with an average intergene distance of 25 kilobases . . .
  50. Bulger, M. et al. Conservation of sequence and structure flanking the mouse and human -globin loci: the -globin genes are embedded within an array of odorant receptor genes. Proc. Natl Acad. Sci. USA 96, 5129-5134 (1999) , .
    • . . . OR genes have invaded or enclosed other clustered gene families such as -globin50, T-cell receptor51 and MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) genes52 . . .
  51. Tsuboi, A. et al. Olfactory neurons expressing closely linked and homologous odorant receptor genes tend to project their axons to neighboring glomeruli on the olfactory bulb. J. Neurosci. 19, 8409-8418 (1999) , .
    • . . . OR genes have invaded or enclosed other clustered gene families such as -globin50, T-cell receptor51 and MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) genes52 . . .
  52. Younger, R. M. et al. Characterization of clustered MHC-linked olfactory receptor genes in human and mouse. Genome Res. 11, 519-530 (2001) , .
    • . . . OR genes have invaded or enclosed other clustered gene families such as -globin50, T-cell receptor51 and MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) genes52 . . .
  53. Qasba, P. & Reed, R. R. Tissue and zonal-specific expression of an olfactory receptor transgene. J. Neurosci. 18, 227-236 (1998) , .
    • . . . Consistent with the diffuse and compact genomic organization, transgenic experiments indicate that the DNA sequences that control the expression of some OR genes can be short and proximal to the coding region53, 54. . . .
  54. Vassalli, A., Rothman, A., Feinstein, P., Zapotocky, M. & Mombaerts, P. Minigenes impart odorant receptor-specific axon guidance in the olfactory bulb. Neuron 35, 681-696 (2002) , .
    • . . . Consistent with the diffuse and compact genomic organization, transgenic experiments indicate that the DNA sequences that control the expression of some OR genes can be short and proximal to the coding region53, 54. . . .
  55. Halpern, M. The organization and function of the vomeronasal system. Annu. Rev. Neurosci. 10, 325-362 (1987) , .
    • . . . Most mammals possess a second olfactory system, which is termed the accessory olfactory system or the vomeronasal system3, 55, 56, 57 (Fig. 1) . . .
  56. Halpern, M. & Martínez-Marcos, A. Structure and funtion of the vomeronasal system: an update. Prog. Neurobiol. 70, 245-318 (2003) , .
    • . . . Most mammals possess a second olfactory system, which is termed the accessory olfactory system or the vomeronasal system3, 55, 56, 57 (Fig. 1) . . .
  57. Dulac, C. & Torello, A. T. Molecular detection of pheromone signals in mammals: from genes to behaviour. Nature Rev. Neurosci. 4, 551-562 (2003) , .
    • . . . Most mammals possess a second olfactory system, which is termed the accessory olfactory system or the vomeronasal system3, 55, 56, 57 (Fig. 1) . . .
  58. Brennan, P. A. & Keverne, E. B. Something in the air? New insights into mammalian pheromones. Curr. Biol. 14, R81-R89 (2004) , .
    • . . . However, the VNO does not have a monopoly and exclusivity with regard to pheromone recognition: it is debated to what extent only the VNO can detect pheromones, and also to what extent the VNO can only detect pheromones58 . . .
  59. Karlson, P. & Lüscher, M. 'Pheromones': a new term for a class of biologically active substances. Nature 183, 55-56 (1959).The classical definition of a pheromone , .
    • . . . Historically, pheromones were defined as 'biological compounds that are secreted and have a defined physiological or behavioural effect on an individual of the same species'59 . . .
    • . . . The original definition59 was for invertebrates — animals that do not have a VNO . . .
  60. Wyatt, T. D. Pheromones and Animal Behaviour. (Cambridge Univ. Press, Cambridge, 2003) , .
    • . . . In the intervening decades, the chemical structure and biosynthetic pathways of thousands of invertebrate pheromones, but not their receptors, have become well characterized60 . . .
  61. Schaal, B. et al. Chemical and behavioural characterization of the rabbit mammary pheromone. Nature 424, 68-72 (2003) , .
    • . . . Vertebrate pheromones remain largely operationally defined and are not identified chemically, with a few exceptions61, 62. . . .
  62. Novotny, M. V. Pheromones, binding proteins and receptor responses in rodents. Biochem. Soc. Trans. 31, 117-122 (2003) , .
    • . . . Vertebrate pheromones remain largely operationally defined and are not identified chemically, with a few exceptions61, 62. . . .
    • . . . This compound was tested because of its known pheromonal effect on mice: it is present in male mouse urine, and extends oestrus in females62 . . .
  63. Wysocki, C. J. & Lepri, J. J. Consequences of removing the vomeronasal organ. J. Steroid Biochem. Mol. Biol. 4, 661-669 (1991) , .
    • . . . What is the relationship between vertebrate pheromones and the VNO? Surgical excision of the VNO produces distinct behavioral impairments in various mammalian species63, providing the strongest support for its role in pheromone detection . . .
  64. Hudson, R. & Distel, H. Pheromonal release of suckling in rabbits does not depend on the vomeronasal organ. Physiol. Behav. 37, 123-128 (1986) , .
    • . . . However, the main olfactory system can also mediate pheromone responses, for instance in the rabbit64 and pig65 . . .
  65. Dorries, K. M., Adkins-Regan, E. & Halpern, B. P. Sensitivity and behavioral responses to the pheromone androstenone are not mediated by the vomeronasal organ in domestic pigs. Brain Behav. Evol. 49, 53-62 (1997) , .
    • . . . However, the main olfactory system can also mediate pheromone responses, for instance in the rabbit64 and pig65 . . .
  66. Dulac, C. & Axel, R. A novel family of genes encoding putative pheromone receptors in mammals. Cell 83, 195-206 (1995).The discovery of V1R vomeronasal receptors , .
    • . . . In 1995, comparative hybridization of cDNA libraries constructed from single rat VSNs resulted in the discovery of a GPCR superfamily of genes that are now usually referred to as V1R genes66 . . .
    • . . . The main arguments are that VR probes anneal by in situ hybridization with small, non-overlapping subsets of VSNs, and that only one V1R or V2R gene was isolated from the single-cell cDNA libraries that led to the discovery of these superfamilies66, 67, 68 . . .
  67. Herrada, G. & Dulac, C. A novel family of putative pheromone receptors in mammals with a topographically organized and sexually dimorphic distribution. Cell 90, 763-773 (1997) , .
    • . . . In 1997, an unrelated GPCR superfamily of vomeronasal receptor genes — the V2R genes— was identified in rats and mice using a similar strategy67, 68, or by fortuitous cross-hybridization69 . . .
    • . . . An early report of topographic and sexually dimorphic expression of some rat V2Rs has not been substantiated67 . . .
    • . . . The main arguments are that VR probes anneal by in situ hybridization with small, non-overlapping subsets of VSNs, and that only one V1R or V2R gene was isolated from the single-cell cDNA libraries that led to the discovery of these superfamilies66, 67, 68 . . .
  68. Matsunami, H. & Buck, L. B. A multigene family encoding a diverse array of putative pheromone receptors in mammals. Cell 90, 775-784 (1997) , .
    • . . . In 1997, an unrelated GPCR superfamily of vomeronasal receptor genes — the V2R genes— was identified in rats and mice using a similar strategy67, 68, or by fortuitous cross-hybridization69 . . .
    • . . . The main arguments are that VR probes anneal by in situ hybridization with small, non-overlapping subsets of VSNs, and that only one V1R or V2R gene was isolated from the single-cell cDNA libraries that led to the discovery of these superfamilies66, 67, 68 . . .
  69. Ryba, N. J & Tirindelli, R. A new multigene family of putative pheromone receptors. Neuron 19, 371-379 (1997).References 67-69 describe the discovery of V2R vomeronasal receptors , .
    • . . . In 1997, an unrelated GPCR superfamily of vomeronasal receptor genes — the V2R genes— was identified in rats and mice using a similar strategy67, 68, or by fortuitous cross-hybridization69 . . .
  70. Pantages, E. & Dulac, C. A novel family of candidate pheromone receptors in mammals. Neuron 28, 835-845 (2000) , .
    • . . . The deduced amino-acid structure of V2Rs predicts a long amino (N)-terminal domain that is encoded by multiple exons, with the 7TM region contained within a single exon. 'V3R' receptors were briefly thought to form a third class of vomeronasal receptors70 . . .
  71. Rodriguez, I., Del Punta, K., Rothman, A., Ishii, T. & Mombaerts, P. Multiple new and isolated families within the mouse superfamily of V1r vomeronasal receptors. Nature Neurosci. 5, 134-140 (2002).First draft of the mouse V1R repertoire , .
    • . . . Modified, with permission, from Ref. 71 © (2002) Macmillan Magazines Ltd. . . .
    • . . . Subsequently, genome-wide drafts of the mouse V1R repertoire43, 71 (150 genes) have made it clear that 'V3Rs' are simply one of 12 V1R families (family d), which are phylogenetically isolated from each other and can therefore be mistaken for new classes of receptors (Fig. 5c). . . .
    • . . . Interestingly, the five human genes74 do not fit into any of the 12 mouse V1R families71, perhaps reflecting the evolutionary imperative to segregate species-specific reproductive behaviours by diversification of receptors for pheromones . . .
  72. Giorgi, D., Friedman, C., Trask, B. J. & Rouquier, S. Characterization of nonfunctional V1R-like pheromone receptor sequences in human. Genome Res. 10, 1979-1985 (2000) , .
    • . . . In human, the overwhelming majority (>95%) of V1R sequences are pseudogenes72 . . .
  73. Rodriguez, I., Greer, C. A., Mok, M. Y. & Mombaerts, P. A putative pheromone receptor gene expressed in human olfactory mucosa. Nature Genet. 26, 18-19 (2000) , .
    • . . . However five human V1R genes have maintained an intact open reading frame73, 74, and at least one of these is expressed at the RNA level in human olfactory mucosa73 . . .
  74. Rodriguez, I. & Mombaerts, P. Novel human vomeronasal receptor-like genes reveal species-specific families. Curr. Biol. 12, R409-411 (2002) , .
    • . . . However five human V1R genes have maintained an intact open reading frame73, 74, and at least one of these is expressed at the RNA level in human olfactory mucosa73 . . .
    • . . . Interestingly, the five human genes74 do not fit into any of the 12 mouse V1R families71, perhaps reflecting the evolutionary imperative to segregate species-specific reproductive behaviours by diversification of receptors for pheromones . . .
  75. Mundy, N. I. & Cook, S. Positive selection during the diversification of class I vomeronasal receptor-like (V1RL) genes, putative pheromone receptor genes, in human and primate evolution. Mol. Biol. Evol. 20, 1805-1810 (2003) , .
    • . . . It has been argued that evolutionary pressure has kept the five V1R genes intact in the human genome75, and, conversely, that they are non-functional remnants of a massively decayed repertoire76 . . .
  76. Zhang, J. & Webb, D. M. Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrhine primates. Proc. Natl Acad. Sci. USA 100, 8337-8341 (2003) , .
    • . . . It has been argued that evolutionary pressure has kept the five V1R genes intact in the human genome75, and, conversely, that they are non-functional remnants of a massively decayed repertoire76 . . .
  77. Martini, S., Silvotti, I., Shirazi, A., Ryba, N. J. & Tirindelli, R. Co-expression of putative pheromone receptors in the sensory neurons of the vomeronasal organ. J. Neurosci. 21, 843-848 (2001) , .
    • . . . A curious observation that has remained unexplained is that a subfamily of V2R genes is co-expressed broadly across basal VSNs, along with 'conventional' V2R genes that are expressed in small subsets of cells77. . . .
  78. Rodriguez, I., Feinstein, P. & Mombaerts, P. Variable patterns of axonal projections of sensory neurons in the mouse vomeronasal system. Cell 97, 199-208 (1999) , .
    • . . . Monoallelic expression has been demonstrated for a V1R gene78, and probably pertains to V2R genes as well79 . . .
    • . . . Specific populations of VSNs were labelled by targeted mutagenesis of three V1R genes78, 80 and one V2R gene79, using the same strategy as for OR genes27 . . .
  79. Del Punta, K., Puche, A., Adams, N. C., Rodriguez, I. & Mombaerts, P. A divergent pattern of sensory axonal projections is rendered convergent by second-order neurons in the accessory olfactory bulb. Neuron 35, 1057-1066 (2002) , .
    • . . . Monoallelic expression has been demonstrated for a V1R gene78, and probably pertains to V2R genes as well79 . . .
    • . . . Specific populations of VSNs were labelled by targeted mutagenesis of three V1R genes78, 80 and one V2R gene79, using the same strategy as for OR genes27 . . .
    • . . . The multiple dendrites of the second-order neurons can innervate glomeruli that are innervated by axons of VSNs that express the same VR, so the flow of information is reorganized anatomically79 . . .
  80. Belluscio, L., Koentges, G., Axel, R. & Dulac, C. A map of pheromone receptor activation in the mammalian brain. Cell 97, 209-220 (1999) , .
    • . . . Specific populations of VSNs were labelled by targeted mutagenesis of three V1R genes78, 80 and one V2R gene79, using the same strategy as for OR genes27 . . .
  81. Hoon, M. A. et al. Putative mammalian taste receptors: a class of taste-specific GPCRs with distinct topographic selectivity. Cell 96, 541-551 (1999).In hindsight, the first report of taste receptors , .
    • . . . In 1999, two GPCR genes, TR1 and TR2 (now renamed T1R1 and T1R2, respectively), were isolated, and were shown to be expressed in subsets of TRCs according to patterns that were intriguing but ultimately not informative as to their function81 . . .
  82. Adler, E. et al. A novel family of mammalian taste receptors. Cell 100, 693-702 (2000) , .
    • . . . In 2000, a family of GPCR genes, T2Rs (also called TRBs), was identified82, 83, 84, and three T2Rs were shown to be receptors for bitter tasting chemicals83 . . .
  83. Chandrashekar, J. et al. T2Rs function as bitter taste receptors. Cell 100, 703-711 (2000) , .
    • . . . In 2000, a family of GPCR genes, T2Rs (also called TRBs), was identified82, 83, 84, and three T2Rs were shown to be receptors for bitter tasting chemicals83 . . .
    • . . . The human receptors hT2R-4 (Ref. 83), TAS2R10 (Ref. 150) and TAS2R16 (Ref. 150) respond, respectively, to denatonium and 6-n-propyl-2-thiouracil, strychnine, and salicin and other b-glucopyranosides . . .
    • . . . The mouse receptors mT2R-8 (a probable orthologue of hT2R-4) and mT2R-5 respond to denatonium and cycloheximide, respectively83 . . .
    • . . . For instance, the mT2R-5 allele varies between 'taster' and 'non-taster' mouse strains with regard to cycloheximide taste perception, and the non-taster version shows a corresponding reduction in cycloheximide sensitivity when it is expressed in HEK293 cells83 . . .
  84. Matsunami, H., Montmayeur, J. P. & Buck, L. B. A family of candidate taste receptors in human and mouse. Nature 404, 601-604 (2000).References 82-84 describe the isolation of T2Rs and their function as bitter receptors , .
    • . . . In 2000, a family of GPCR genes, T2Rs (also called TRBs), was identified82, 83, 84, and three T2Rs were shown to be receptors for bitter tasting chemicals83 . . .
  85. Conte, C., Ebeling, M., Marcuz, A., Nef, P. & Andres-Barquin, P. J. Identification and characterization of human taste receptor genes belonging to the TAS2R family. Cytogenet. Genome Res. 98, 45-53 (2002) , .
    • . . . There are 25 and 36 putatively functional T2R genes in human and mouse, respectively85, 86, 87 . . .
  86. Conte, C., Ebeling, M., Marcuz, A., Nef, P. & Andres-Barquin, P. J. Evolutionary relationships of the Tas2r receptor gene families in mouse and human. Physiol. Genomics 14, 73-82 (2003) , .
    • . . . There are 25 and 36 putatively functional T2R genes in human and mouse, respectively85, 86, 87 . . .
  87. Shi, P., Zhang, J., Yang, H. & Zhang, Y. Adaptive diversification of bitter taste receptor genes in mammalian evolution. Mol. Biol. Evol. 20, 805-814 (2003) , .
    • . . . There are 25 and 36 putatively functional T2R genes in human and mouse, respectively85, 86, 87 . . .
  88. Max, M. et al. Tas1r3, encoding a new candidate taste receptor, is allelic to the sweet responsiveness locus Sac. Nature Genet. 28, 58-63 (2001) , .
    • . . . In 2001, six groups independently identified T1R3 (Refs 88–93), a gene at the Sac locus, which controls variations among mouse strains in thresholds that are used to distinguish sweet substances such as saccharin. . . .
  89. Montmayeur, J. P., Liberles, S. D., Matsunami, H. & Buck, L. B. A candidate taste receptor gene near a sweet taste locus. Nature Neurosci. 4, 492-498 (2001) , .
  90. Kitagawa, M., Kusakabe, Y., Miura, H., Ninomiya, Y. & Hino, A. Molecular genetic identification of a candidate receptor gene for sweet taste. Biochem. Biophys. Res. Commun. 283, 236-242 (2001) , .
  91. Sainz, E., Korley, J. N., Battey, J. F. & Sullivan, S. L. Identification of a novel member of the T1R family of putative taste receptors. J. Neurochem. 77, 896-903 (2001) , .
  92. Bachmanov, A. A. et al. Positional cloning of the mouse saccharin preference (Sac) locus. Chem. Senses 26, 925-933 (2001) , .
  93. Nelson, G. et al. Mammalian sweet taste receptors. Cell 106, 381-390 (2001).Functional evidence that T1R3 is the Sac gene , .
    • . . . The first functional evidence for a role of T1R3 in sweet taste was the rescue of a 'non-taster' mouse strain with a T1R3 transgene derived from the genome of a 'taster' mouse strain93 . . .
  94. Li, X. et al. Human receptors for sweet and umami taste. Proc. Natl Acad. Sci. USA 99, 4692-4696 (2002) , .
    • . . . The latest view is that T1R3 usually does not act alone, but forms (at least in heterologous cells) heteromers with T1R2, thereby mediating sweet taste, or combines with T1R1 to form a receptor that is broadly responsive to L-amino acids and monosodium L-glutamate (MSG), which has an umami taste94, 95 . . .
    • . . . This conundrum came to a provisional resolution by the demonstration that T1R3 can impart responsiveness to sweet tastants in HEK293 cells in combination with T1R2 (Refs 94,95) . . .
  95. Nelson, G. et al. An amino-acid taste receptor. Nature 416, 199-202 (2002) , .
    • . . . The latest view is that T1R3 usually does not act alone, but forms (at least in heterologous cells) heteromers with T1R2, thereby mediating sweet taste, or combines with T1R1 to form a receptor that is broadly responsive to L-amino acids and monosodium L-glutamate (MSG), which has an umami taste94, 95 . . .
    • . . . The orthologous T1R genes are 70% identical between rodents and humans98, and these species differences profoundly influence the sensitivity to MSG and L-amino acids95. . . .
    • . . . This conundrum came to a provisional resolution by the demonstration that T1R3 can impart responsiveness to sweet tastants in HEK293 cells in combination with T1R2 (Refs 94,95) . . .
  96. Reed, D. R. et al. Polymorphisms in the taste receptor gene (Tas1r3) region are associated with saccharin preference in 30 mouse strains. J. Neurosci. 24, 938-946 (2004) , .
    • . . . Analysis of polymorphisms in the T1R3 gene and flanking sequences in 30 mouse strains did not give a simple answer to how T1R3 controls saccharin preference; the best candidate is an isoleucine to threonine substitution in the extracellular N-terminus96 . . .
  97. Chaudhari, N., Landin, A. M. & Roper, S. D. A metabotropic glutamate receptor variant functions as a taste receptor. Nature Neurosci. 3, 113-119 (2000) , .
    • . . . Transduction of umami taste is proposed to involve T1R1 combined with T1R3, but a splice variant of the mGluR4 glutamate receptor might also function as an umami receptor97 . . .
  98. Liao, J. & Schultz, P. G. Three sweet receptor genes are clustered in human chromosome 1. Mamm. Genome 14, 291-301 (2003) , .
    • . . . The orthologous T1R genes are 70% identical between rodents and humans98, and these species differences profoundly influence the sensitivity to MSG and L-amino acids95. . . .
  99. Damak, S. et al. Detection of sweet and umami taste in the absence of taste receptor T1r3. Science 301, 850-853 (2003) , .
    • . . . Further functional insights came from the behavioural and whole-nerve electrophysiological characterization of T1R1-, T1R2- and T1R3-knockout mice99, 100 . . .
    • . . . The question of whether all sweet and umami tastes operate, respectively, through T1R2/T1R3 and T1R1/T1R3, is a subject for controversy from knockout-mouse analyses99, 100 . . .
    • . . . The answer depends on the assay — for instance, a standard long-term assay (two-bottle preference) gives a residual response99, whereas a short-term assay (immediate lick response) gives zero response100 in knockout mice . . .
  100. Zhao, G. Q. et al. The receptors for mammalian sweet and umami taste. Cell 115, 255-266 (2003).References 99 and 100 describe the first knockout mice for TR genes , .
    • . . . Further functional insights came from the behavioural and whole-nerve electrophysiological characterization of T1R1-, T1R2- and T1R3-knockout mice99, 100 . . .
    • . . . Initially, transfection of HEK293 cells with T1R1, T1R2 or T1R3 did not result in responses to any tastants, although more recent data indicate that T1R3 alone can produce responses to very high concentrations of sucrose and other natural sweet compounds100 . . .
    • . . . The question of whether all sweet and umami tastes operate, respectively, through T1R2/T1R3 and T1R1/T1R3, is a subject for controversy from knockout-mouse analyses99, 100 . . .
    • . . . The answer depends on the assay — for instance, a standard long-term assay (two-bottle preference) gives a residual response99, whereas a short-term assay (immediate lick response) gives zero response100 in knockout mice . . .
  101. Wong, G. T., Gannon, K. S. & Margolskee, R. F. Transduction of bitter and sweet taste by gustducin. Nature 381, 796-800 (1996) , .
    • . . . T2Rs are co-expressed with gustducin, a G-protein subunit whose absence results in impaired, but not abolished, sweet and bitter taste101 . . .
    • . . . Mice that lack the gene for a G-protein -subunit, gustducin, have a diminished, but not abolished, sense of taste for bitter and sweet substances101, whereas mice that lack the transient receptor potential channel TRPM5 or PLC2 are insensitive to bitter and sweet substances103 . . .
  102. Pérez, C. A. et al. A transient receptor potential channel expressed in taste receptor cells. Nature Neurosci. 5, 1169-1176 (2002) , .
    • . . . Half of all TRCs express a transient receptor potential channel, TRMP5 (Ref. 102), which is required for sweet, bitter and umami taste103, and is nearly always co-expressed with PLC2 (phosphoplipase C, 2 subunit) . . .
  103. Zhang, Y. et al. Coding of sweet, bitter, and umami tastes: different receptor cells sharing similar signaling pathways. Cell 112, 293-301 (2003) , .
    • . . . Half of all TRCs express a transient receptor potential channel, TRMP5 (Ref. 102), which is required for sweet, bitter and umami taste103, and is nearly always co-expressed with PLC2 (phosphoplipase C, 2 subunit) . . .
    • . . . Mice that lack the gene for a G-protein -subunit, gustducin, have a diminished, but not abolished, sense of taste for bitter and sweet substances101, whereas mice that lack the transient receptor potential channel TRPM5 or PLC2 are insensitive to bitter and sweet substances103 . . .
    • . . . Moreover, a controversy lingers103 about how many taste modalities are represented by individual TRCs: can one cell sense both bitter and sweet tastes? Single-cell imaging in tongue slices indicates that some cells respond to both sweet and bitter stimuli124, which conflicts with the non-overlapping expression of T1R3 and T2R genes . . .
  104. Kim, M. R. et al. Regional expression patterns of taste receptors and gustducin in the mouse tongue. Biochem. Biophys. Res. Comm. 312, 500-506 (2003) , .
    • . . . An in situ hybridization TR study of one group is in profound disagreement with many of these conclusions, but remains to be confirmed104 . . .
  105. Finger, T. E. et al. Solitary chemoreceptor cells in the nasal cavity serve as sentinels of respiration. Proc. Natl Acad. Sci. USA 100, 8981-8986 (2003) , .
    • . . . Interestingly, T2Rs are expressed along with gustducin in solitary cells in the respiratory epithelium of the rodent nasal cavity105 and, according to RT-PCR analysis, also in the stomach and duodenum106 . . .
  106. Wu, S. V. et al. Expression of bitter taste receptors of the T2R family in the gastrointestinal tract and enteroendocrine STC-1 cells. Proc. Natl Acad. Sci. USA 99, 2392-2397 (2003) , .
    • . . . Interestingly, T2Rs are expressed along with gustducin in solitary cells in the respiratory epithelium of the rodent nasal cavity105 and, according to RT-PCR analysis, also in the stomach and duodenum106 . . .
  107. Belluscio, L., Gold, G. H., Nemes, A. & Axel, R. Mice deficient in Golf are anosmic. Neuron 20, 69-81 (1998) , .
    • . . . The ANOSMIC phenotypes of mouse knockouts for Golf107, adenylyl cyclase108 and a channel subunit109, 110, 111, are consistent with a fundamental role for this canonical pathway . . .
  108. Wong, S. T. et al. Disruption of the type III adenlyl cyclase gene leads to peripheral and behavioural anosmia in transgenic mice. Neuron 27, 487-497 (2000) , .
    • . . . The ANOSMIC phenotypes of mouse knockouts for Golf107, adenylyl cyclase108 and a channel subunit109, 110, 111, are consistent with a fundamental role for this canonical pathway . . .
  109. Brunet, L. J., Gold, G. H. & Ngai, J. General anosmia caused by a targeted disruption of the mouse olfactory cyclic nucleotide-gated cation channel. Neuron 17, 681-693 (1996) , .
    • . . . The ANOSMIC phenotypes of mouse knockouts for Golf107, adenylyl cyclase108 and a channel subunit109, 110, 111, are consistent with a fundamental role for this canonical pathway . . .
  110. Baker, H. et al. Targeted deletion of a cyclic nucleotide-gated channel subunit (OCNC1): biochemical and morphological consequences in adult mice. J. Neurosci. 19, 9313-9321 (1999) , .
    • . . . The ANOSMIC phenotypes of mouse knockouts for Golf107, adenylyl cyclase108 and a channel subunit109, 110, 111, are consistent with a fundamental role for this canonical pathway . . .
  111. Zheng, C., Feinstein, P., Bozza, T., Rodriguez, I. & Mombaerts, P. Peripheral olfactory projections are differentially affected in mice deficient in a cyclic nucleotide-gated channel subunit. Neuron 26, 81-91 (2000) , .
    • . . . The ANOSMIC phenotypes of mouse knockouts for Golf107, adenylyl cyclase108 and a channel subunit109, 110, 111, are consistent with a fundamental role for this canonical pathway . . .
  112. Norlin, E. M., Gussing, F. & Berghard, A. Vomeronasal phenotype and behavioral alterations in Gi2 mutant mice. Curr. Biol. 13, 1214-1219 (2003) , .
    • . . . Although Gi2-knockout (Ref. 112) and Go-knockout113 mice display behavioural and anatomical abnormalities, participation of Gi2 in transduction within V1R-expressing cells, and of Go within V2R-expressing cells, has not been demonstrated . . .
  113. Tanaka, M., Treloar, H., Kalb, R. G., Greer, C. A. & Strittmatter, S. M. G0 protein-dependent survival of primary accessory olfactory neurons. Proc. Natl Acad. Sci. USA 96, 14106-14111 (1999) , .
    • . . . Although Gi2-knockout (Ref. 112) and Go-knockout113 mice display behavioural and anatomical abnormalities, participation of Gi2 in transduction within V1R-expressing cells, and of Go within V2R-expressing cells, has not been demonstrated . . .
  114. Liman, E. R., Corey, D. P. & Dulac, C. TRP2: a candidate transduction channel for mammalian pheromone sensory signaling. Proc. Natl Acad. Sci. USA 96, 5791-5796 (1999) , .
    • . . . The transient receptor potential channel TRP2 is expressed in VSNs but not in OSNs114 . . .
  115. Stowers, L., Holy, T. E., Meister, M., Dulac, C. & Koentges, G. Loss of sex discrimination and male-male aggression in mice deficient for TRP2. Science 295, 1493-1500 (2002) , .
    • . . . Electrophysiological responses to urine and pheromones are abolished115 or strongly reduced116 in TRP2-knockout mice, which also show abnormal behaviour . . .
  116. Leypold, B. G. et al. Altered sexual and social behaviours in trp2 mutant mice. Proc. Natl Acad. Sci. USA 99, 6376-6381 (2002) , .
    • . . . Electrophysiological responses to urine and pheromones are abolished115 or strongly reduced116 in TRP2-knockout mice, which also show abnormal behaviour . . .
  117. Lucas, P., Ukhanov, K., Leinders-Zufall, T. & Zufall, F. A diacylglycerol-gated cation channel in vomeronasal neuron dendrites is impaired in TRPC2 mutant mice: mechanism of pheromone transduction. Neuron 40, 551-561 (2003) , .
    • . . . The TRP2 channel is a crucial component of the signal transduction pathway in VSNs, and it is gated by diacylglycerol117 . . .
  118. Liman, E. R. & Innan, H. Relaxed selective pressure on an essential component of pheromone transductin in primate evolution. Proc. Natl Acad. Sci. USA 100, 3328-3332 (2003) , .
    • . . . The TRP2 gene is a pseudogene in humans and higher primates, indicating that VNO function — at least as we understand it in lower animals — is not preserved118 . . .
  119. Del Punta, K. et al. Deficient pheromone responses in mice lacking a cluster of vomeronasal receptor genes. Nature 419, 70-74 (2002).First functional evidence for mammalian pheromone receptors , .
    • . . . Functional evidence for V1R genes in pheromone detection was provided by the behavioural and physiological deficits in mice in which a cluster of 16 V1R genes was deleted119 . . .
    • . . . Complementary functional information came from experiments in which chromosome engineering144 was used to excise a cluster of 16 V1R genes from the mouse genome119 . . .
    • . . . Three compounds (6-hydroxy-6-methyl-3-heptanone, n-pentylacetate and isobutylamine) failed to stimulate the mutant VNO119 . . .
  120. Leinders-Zufall, T. et al. Ultrasensitive pheromone detection by mammalian vomeronasal neurons. Nature 405, 792-796 (2000) , .
    • . . . VSNs seem to be extremely sensitive to chemical stimuli at sub-nanomolar concentrations, and they might be 'tuned' much more narrowly than OSNs120. . . .
  121. Gilbertson, T. A., Damak, S. & Margolskee, R. F. The molecular physiology of taste transduction. Curr. Opin. Neurobiol. 10, 519-527 (2000) , .
    • . . . Taste transduction in TRCs is notorious for its apparent richness: the literature harbours evidence for anyone's favourite pathway, although it must be recognized that this evidence is spread over several model systems that might differ in fundamental ways121, 122, 123 . . .
  122. Lindemann, B. Receptors and transduction in taste. Nature 413, 219-225 (2001) , .
    • . . . Taste transduction in TRCs is notorious for its apparent richness: the literature harbours evidence for anyone's favourite pathway, although it must be recognized that this evidence is spread over several model systems that might differ in fundamental ways121, 122, 123 . . .
  123. Margolskee, R. F. Molecular mechanisms of bitter and sweet taste transduction. J. Biol. Chem. 277, 1-4 (2002) , .
    • . . . Taste transduction in TRCs is notorious for its apparent richness: the literature harbours evidence for anyone's favourite pathway, although it must be recognized that this evidence is spread over several model systems that might differ in fundamental ways121, 122, 123 . . .
  124. Caicedo, A., Kim, K. N. & Roper, S. D. Individual mouse taste cells respond to multiple chemical stimuli. J. Physiol. (Lond.) 544, 501-509 (2002) , .
    • . . . Moreover, a controversy lingers103 about how many taste modalities are represented by individual TRCs: can one cell sense both bitter and sweet tastes? Single-cell imaging in tongue slices indicates that some cells respond to both sweet and bitter stimuli124, which conflicts with the non-overlapping expression of T1R3 and T2R genes . . .
  125. Caicedo, A. & Roper, S. D. Taste receptor cells that discriminate between bitter stimuli. Science 291, 1557-1560 (2001) , .
    • . . . It was also shown that not all bitter compounds activate the same cell125, which is not consistent with co- expression of T2R genes within the same cells . . .
  126. Zhao, H. et al. Functional expression of a mammalian odorant receptor. Science 279, 237-242 (1998).First functional evidence that an OR determines responsiveness to a specific odorant , .
    • . . . It took seven years from the publication of the OR discovery8 for the first unambiguous OR–ligand pair to be reported126 . . .
    • . . . First, adenovirally-mediated gene transfer into the olfactory mucosa of living rodents was successful for rat I7 (Ref. 126) and MOR23 (Refs 13,134) . . .
    • . . . There are some discrepancies between ligands identified in the adenoviral assay126, 127 and in genetically tagged neurons that express rat I7 (Ref. 31) . . .
  127. Araneda, R. C., Kini, A. D. & Firestein, S. The molecular receptive range of an odorant receptor. Nature Neurosci. 3, 1248-1255 (2000) , .
    • . . . The molecular receptive range of rat I7 was further delineated127 using electro-olfactograms: a chain length of 10Å and an aldehyde are the first structure–function relationships to be established for an OR . . .
    • . . . There are some discrepancies between ligands identified in the adenoviral assay126, 127 and in genetically tagged neurons that express rat I7 (Ref. 31) . . .
    • . . . For rat I7, heptanal, octanal, nonenal, decanal and citronellal responses have been corroborated by more than one assay. (More rat I7 ligands can be found in Ref. 127) . . .
    • . . . Such effects were first reported with the identification of a partial agonist, citral, for the rat I7 receptor127 . . .
  128. Araneda, R. C., Peterlin, Z., Zhang, X., Chesler, A. & Firestein, S. A pharmacological profile of the aldehyde receptor repertoire in rat olfactory epithelium. J. Physiol. (Lond.) doi 10.1113/jphysiol.2003.058040 (2004) , .
    • . . . Octanal happens to be a powerful odorant — the current estimate is that the rat has between 33 and 55 octanal-responsive ORs128. . . .
    • . . . A comprehensive study of 55 octanal-responsive rat OSNs tested with eight other odorants revealed 33 'clusters' of cells that exhibited distinct pharmacological profiles128 . . .
  129. Krautwurst, D., Yau, K. W. & Reed, R. R. Identification of ligands for olfactory receptors by functional expression of a receptor library. Cell 95, 917-926 (1998) , .
    • . . . Guided by the rat I7–octanal association, a heterologous expression system was developed in human embryonic kidney cells (HEK293)129 . . .
    • . . . Screening a library of 80 chimaeric ORs against 26 odorous ligands (2080 combinations) resulted in three matches — I-D3 and carvone, I-C6 and citronellal, and I-G7 and limonene129. . . .
  130. Wetzel, C. H. et al. Specificity and sensitivity of a human olfactory receptor functionally expressed in human embryonic kidney 293 cells and Xenopus laevis oocytes. J. Neurosci. 19, 7426-7433 (1999) , .
    • . . . An analogous HEK293 expression system, but with different OR protein modifications, yielded the first ligand for a human OR: helional for OR17-40 (Ref. 130) . . .
  131. Spehr, M. et al. Identification of a testicular odorant receptor mediating human sperm chemotaxis. Science 299, 2054-2058 (2003) , .
    • . . . The same group used the HEK293 system to show responsiveness to bourgeonal with a chimaeric version of the human OR17-4 (Ref. 131) . . .
  132. McClintock, T. S. & Sammeta, N. Trafficking prerogatives of olfactory receptors. Neuroreport 14, 1547-1552 (2003) , .
    • . . . Other than these few success stories, HEK293-based systems have performed rather poorly; OR proteins tend to get stuck in their endoplasmic reticulum132 . . .
  133. Lu, M., Echeverri, F. & Moyer, B. Endoplasmic reticulum retention, degradation, and aggregation of olfactory G-protein coupled receptors. Traffic 4, 416-433 (2003) , .
    • . . . A study of the cell biology of ORs expressed in HEK293 cells indicates that poor surface expression is attributable to retention in the endoplasmic reticulum as a result of inefficient folding and poor coupling to the export machinery, combined with aggregation and degradation through both proteosomal and autophagic pathways133 . . .
  134. Touhara, K. Odor discrimination by G protein-coupled olfactory receptors. Microsc. Res. Tech. 58, 135-141 (2002) , .
    • . . . First, adenovirally-mediated gene transfer into the olfactory mucosa of living rodents was successful for rat I7 (Ref. 126) and MOR23 (Refs 13,134) . . .
  135. Murrell, J. R. & Hunter, D. D. An olfactory sensory neuron line, odora, properly targets olfactory proteins and responds to odorants. J. Neurosci. 19, 8260-8270 (1999) , .
    • . . . Second, an immortal cell line that was derived from the olfactory epithelium — odora — can be induced to differentiate into cells with the same properties as mature OSNs by modifying the culture conditions135 . . .
    • . . . Odora cells transfected with the rat U131 receptor respond to heptanoic acid135, and to helional when transfected with the human OR17-40 receptor136 . . .
  136. Levasseur, G. et al. Ligand-specific dose-response of heterologously expressed olfactory receptors. Eur. J. Biochem. 270, 2905-2912 (2003) , .
    • . . . Odora cells transfected with the rat U131 receptor respond to heptanoic acid135, and to helional when transfected with the human OR17-40 receptor136 . . .
  137. Belluscio, L., Lodovichi, C., Feinstein, P., Mombaerts, P. & Katz, L. C. Odorant receptors instruct functional circuitry in the mouse olfactory bulb. Nature 419, 296-300 (2002) , .
    • . . . In vivo intrinsic signal imaging confirmed that glomeruli that receive input from rat I7-expressing neurons are activated when the mouse is exposed to octanal, an experimental scenario that is the closest to the real situation137 . . .
  138. Oka, Y., Omura, M., Kataoka, H. & Touhara, K. Olfactory receptor antagonism between odorants. EMBO J. 23, 120-126 (2004) , .
    • . . . Methyl isoeugenol, a molecule of related chemical structure, can block eugenol responses in HEK293 cells and in native OSNs138 . . .
  139. Omura, M., Sekine, H., Shimizu, T., Kataoka, H. & Touhara, K. In situ Ca2+ imaging of odor responses in a coronal olfactory epithelium slice. Neuroreport 14, 1123-1127 (2003) , .
    • . . . Furthermore, calcium signal recordings in dissociated OSNs and epithelial slices139 indicate that other ORs probably have eugenol as an agonist and methyl isoeugenol as an antagonist . . .
  140. Hamana, H., Hirono, J., Kizumi, M. & Sato, T. Sensitivity-dependent hierarchical receptor codes for odors. Chem. Senses 28, 87-104 (2003) , .
    • . . . A single assay has been developed for this purpose12, 13, 14, 140 . . .
  141. Katada, S., Nakagawa, T., Kataoka, H. & Touhara, K. Odorant response assays for a heterologous expressed olfactory receptor. Biochem. Biophys. Res. Commun. 305, 964-969 (2003) , .
    • . . . This crucial evidence has been reported for three ORs, by adenovirally-mediated infection of olfactory mucosa13 (lyral and MOR23) or transfection of HEK293 cells14, 141 (eugenol and mOR-EG; ethyl vanillin and mOR-EV) (Table 1); however, it is not routinely done. . . .
  142. Boschat, C. et al. Pheromone detection mediated by a V1r vomeronasal receptor. Nature Neurosci. 5, 1261-1262 (2002).First reported ligand for a mammalian pheromone receptor , .
    • . . . Of the 300 V1R and V2R genes in mouse or rat, only one receptor–ligand interaction has been described to date, which is present in homologous cells142 (Table 2) . . .
    • . . . By applying a similar gene tagging strategy as for M71 and acetophenone31, it was shown that green fluorescent VSNs from mice that express V1rb2 along with GFP respond reproducibly to 2-heptanone, both by calcium imaging and by patch-clamp recording142 . . .
    • . . . By contrast, green fluorescent cells from mice that expressed GFP without V1rb2 from the V1rb2 locus (due to a knockout mutation) did not respond to 2-heptanone142 . . .
  143. Gaillard, I. et al. A single olfactory receptor specifically binds a set of odorant molecules. Eur. J. Neurosci. 15, 409-418 (2002) , .
    • . . . Interestingly, mOR912-93, a mouse OR, has also been identified as a receptor for 2-heptanone143 . . .
  144. Ramirez-Solis, R., Liu, P. & Bradley, A. Chromosome engineering in mice. Nature 378, 720-724 (1995) , .
    • . . . Complementary functional information came from experiments in which chromosome engineering144 was used to excise a cluster of 16 V1R genes from the mouse genome119 . . .
  145. Amoore, J. E. & Steinle, S. in Chemical Senses: Volume 3 - Genetics of Perception and Communication (eds C. J. Wysocki and M. R. Klare) 331-351 (Marcel Dekker, New York, 1991) , .
    • . . . This new type of sensory deficit was termed 'avnosmia' by analogy with 'anosmia' in the main olfactory system145 . . .
  146. Matsunami, H. & Amrein, H. Taste and pheromone perception in mammals and flies. Genome Biol. 4, 220.1-220.9 (2003) , .
    • . . . No ligands for V2Rs have been described146 . . .
  147. Ishii, T., Hirota, J. & Mombaerts, P. Combinatorial coexpression of neural and immune multigene families in mouse vomeronasal sensory neurons. Curr. Biol. 13, 394-400 (2003) , .
    • . . . A new family of nine non-classical class I MHC molecules is expressed in V2R-positive mouse VSNs147, 148 . . .
  148. Loconto, J. et al. Functional expression of murine V2R pheromone receptors involves selective assocation with the M10 and M1 families of MHC class Ib molecules. Cell 112, 607-618 (2003) , .
    • . . . A new family of nine non-classical class I MHC molecules is expressed in V2R-positive mouse VSNs147, 148 . . .
    • . . . These seem to be accessory proteins that load V2Rs onto the surface of VSNs and heterologous cells along with 2-microglobulin148 . . .
  149. Ueda, T., Ugawa, S., Yamamura, H., Imaizumi, Y. & Shimada, S. Functional interaction between T2R taste receptors and G-protein subunits expressed in taste receptor cells. J. Neurosci. 23, 7376-7380 (2003) , .
    • . . . Promiscuous G-proteins were included and tinkering with them might be of further help149 . . .
  150. Bufe, B., Hofmann, T., Krautwurst, D., Raguse, J. D. & Meyerhof, W. The human TAS2R16 receptor mediates bitter taste in response to -glucopyranosides. Nature Genet. 32, 397-401 (2002) , .
    • . . . The human receptors hT2R-4 (Ref. 83), TAS2R10 (Ref. 150) and TAS2R16 . . .
    • . . . The rat T2R9 receptor (homologous to mT2R-5) also responds to cycloheximide150 . . .
    • . . . Likewise, the threshold doses for taste perception in humans among various -glucopyranosides have the same relative order and similar absolute values as the half-maximal effective concentrations in T2R-transfected HEK293 cells150 . . .
  151. Kim, U. et al. Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide. Science 299, 1221-1225 (2003) , .
    • . . . Finally, classical positional cloning methods identified human TAS2R38 as the gene in a QUANTITATIVE TRAIT LOCUS that underlies taste sensitivity to phenylthiocarbamide151 . . .
  152. Mombaerts, P. Better taste through chemistry. Nature Genet. 25, 130-132 (2001) , .
    • . . . However, with a largely uncharacterized repertoire of two–three dozen T2Rs, additional receptors need to be deorphaned to generalize this conclusion and to equate 'T2Rs' with 'bitter receptors' and vice versa152 . . .
  153. Caicedo, A., Pereira, E., Margolskee, R. F. & Roper, S. D. Role of the G-protein subunit -gustducin in taste cell responses to bitter stimuli. J. Neurosci. 23, 9947-9952 (2003) , .
    • . . . For instance, single-cell physiological studies indicate that not all bitter-responsive cells express gustducin, indicating that T2Rs need not be the only bitter receptors153. . . .
  154. Buck, L. B. Information coding in the vertebrate olfactory system. Annu. Rev. Neurosci. 19, 517-544 (1996) , .
    • . . . The identification of receptor–ligand interactions is not merely a descriptive exercise, but is directed towards understanding how chemical structure relates to chemosensory quality154, 155: for example, why does acetophenone smell like almond? The Holy Grail of olfaction is within sight156, but there is a long way to go — more than half of mouse and human non-chemosensory GPCRs have known ligands157, but the vast majority of chemosensory GPCRs remain orphans (Fig. 9) . . .
  155. Buck, L. B. The molecular architecture of odor and pheromone sensing in mammals. Cell 100, 611-618 (2000) , .
    • . . . The identification of receptor–ligand interactions is not merely a descriptive exercise, but is directed towards understanding how chemical structure relates to chemosensory quality154, 155: for example, why does acetophenone smell like almond? The Holy Grail of olfaction is within sight156, but there is a long way to go — more than half of mouse and human non-chemosensory GPCRs have known ligands157, but the vast majority of chemosensory GPCRs remain orphans (Fig. 9) . . .
  156. Reed, R. R. After the holy grail: establishing a molecular basis for mammalian olfaction. Cell 116, 329-336 (2004) , .
    • . . . The identification of receptor–ligand interactions is not merely a descriptive exercise, but is directed towards understanding how chemical structure relates to chemosensory quality154, 155: for example, why does acetophenone smell like almond? The Holy Grail of olfaction is within sight156, but there is a long way to go — more than half of mouse and human non-chemosensory GPCRs have known ligands157, but the vast majority of chemosensory GPCRs remain orphans (Fig. 9) . . .
  157. Vassilatis, D. K. et al. The G protein-coupled receptor repertoires of human and mouse. Proc. Natl Acad. Sci. USA 100, 4903-4908 (2003).First drafts of GPCRs in human and mouse , .
    • . . . The identification of receptor–ligand interactions is not merely a descriptive exercise, but is directed towards understanding how chemical structure relates to chemosensory quality154, 155: for example, why does acetophenone smell like almond? The Holy Grail of olfaction is within sight156, but there is a long way to go — more than half of mouse and human non-chemosensory GPCRs have known ligands157, but the vast majority of chemosensory GPCRs remain orphans (Fig. 9) . . .
  158. Gilbert, A. N. & Firestein, S. Dollars and scents: commercial opportunities in olfaction and taste. Nature Neurosci. 5 Suppl. 1043-1045 (2002) , .
    • . . . A matrix of a large subset of interactions between ORs and odorants might underlie the rational design of odorant agonists and antagonists, and this would be of great value to the fragrance industry158. . . .
  159. Medler, K. F., Margolskee, R. F. & Kinnamon, S. C. Electrophysiological characterization of voltage-gated currents in defined taste cell types of mice. J. Neurosci. 23, 2608-2617 (2003) , .
    • . . . Initial results from gustducin–GFP transgenic mice are promising159, and they indicate that it will be possible to record responses by imaging native, single TRCs that express a particular T1R or T2R. . . .
  160. Milani, N., Guarin, E., Renfer, E., Nef, P. & Andres-Barquin, P. J. Functional expression of a mammalian olfactory receptor in Caenorhabditis elegans. Neuroreport 13, 2515-2520 (2003) , .
  161. Ivic, L., Zhang, C., Zhang, X., Yoon, S. O. & Firestein, S. Intracellular trafficking of a tagged and functional mammalian olfactory receptor. J. Neurobiol. 50, 56-68 (2002) , .
  162. Potter, S. M. et al. Structure and emergence of specific olfactory glomeruli in the mouse. J. Neurosci. 21, 9713-9723 (2001) , .
    • . . . Modified, with permission, from Ref. 162 © (2001) Society for Neuroscience. b | The IRES (internal ribosome entry site) sequence instructs co-translational expression of the M71 OR and the tauGFP marker, allowing identification of M71-expressing olfactory sensory neurons (OSNs) by green fluorescence. c | The main olfactory epithelium of mice that carry the targeted M71-IRES-tauGFP mutation is dissociated into single OSNs, which are loaded with the calcium indicator Fura-2 . . .
Expand