Odor maps in the brain: Spatial aspects of odor representation in sensory surface and olfactory bulb

The olfactory sense detects and distinguishes a multitude of different odors. Recent progress in molecular as well as physiological approaches has elucidated basic principles of neuronal encoding of odorants, common to insects and vertebrates. The construction of neuronal representations for odors begins with the task of mapping the multidimensional odor space onto the two-dimensional sensory surface, and subsequently onto the olfactory bulb or antennal lobe. A distributed expression of odorant receptors, albeit restricted to subregions of the sensory surface (large, intermediate or small for zebrafish, mouse or drosophila, respectively), ensures a robust representation, insensitive to mechanical insult. Olfactory receptor neurons expressing the same odorant receptors converge to form a receptotopic map in the olfactory bulb or antennal lobe. The emerging coding principle is a chemotopic representation of odorants at the first brain level, realized either as combinatorial or as monospecific representation, depending on the odorant.     

Mammalian olfactory receptors: pharmacology, G protein coupling and desensitization

The vertebrate olfactory system recognizes and discriminates between thousands of structurally diverse odorants. Detection of odorants in mammals is mediated by olfactory receptors (ORs), which comprise the largest superfamily of G protein-coupled receptors (GPCRs). Upon odorant binding, ORs couple to G proteins, resulting in an increase in intracellular cAMP levels and subsequent receptor signaling. In this review, we will discuss recently published studies outlining the molecular basis of odor discrimination, focusing on pharmacology, G protein activation, and desensitization of ORs. A greater understanding of the molecular mechanisms underlying OR activity may help in the discovery of agonists and antagonists of ORs, and of GPCRs with potential therapeutic applications.     

Olfactory receptors

Olfaction is an ancient sensory system allowing an organism to detect chemicals in its environment. The first step in odor transduction is mediated by binding odorants to olfactory receptors (ORs) which belong to the heptahelical G-protein-coupled receptor (GPCR) superfamily. Mammalian ORs are disposed in clusters on virtually all chromosomes. They are encoded by the largest multigene family (1000 members) in the genome of mammals and Caenorhabditis elegans, whereas Drosophila contains only 60 genes. Each OR specifically recognizes a set of odorous molecules that share common molecular features. In mammals, signal transduces through the G-protein-dependent signal pathway in the olfactory sensory neurons that synapse ultimately in the glomeruli of the olfactory bulb, and is finally processed in higher brain structures. The expression of a given OR conditions neuron and glomerulus choices. To date, the processes which monitor OR expression and axon wiring have emerged but are not completely elucidated.Received 9 July 2003; reiceived after revision 3 Ocotober 2003; accepted 22 Ocotober 2003     

The vomeronasal system

In addition to the main olfactory system, many vertebrates possess a vomeronasal system that conveys more specialized chemosensory information. Unlike the airborne, volatile stimuli detected by the main olfactory system, vomeronasal stimuli are typically proteins of the lipocalin family which bind small, volatile ligands. Despite the smaller number of vomeronasal receptor types, the projection patterns of the vomeronasal receptor neurons to multiple glomeruli in the accessory olfactory bulb appear to be more complicated than those of the main olfactory system. The vomeronasal system has a direct sub-neocortical projection to hypothalamic areas that mediates specific behavioural and hormonal responses to pheromonal stimuli. However, the integration and transmission of this information can be modulated by learning mechanisms. The aim of this article is to outline some of the functions of the vomeronasal system, and in particular to comment on recent advances in our understanding of how vomeronasal information is coded and processed.     

Spatial variation in response to odorants on the rat olfactory epithelium

We have measured the electro-olfactogram produced by four odorants, nicotine, i-pentyl acetate, i-pentanoic acid and cineole from twelve positions on an in vitro preparation of rat olfactory tissue. Each odorant shows a different pattern of response over the twelve positions which can be explained by differences in olfactory receptor populations between regions of the rat olfactory epithelium. The result for nicotine is further evidence that there are olfactory receptors which are stimulated by nicotine when it is presented as a vapour.     

Evidence for an olfactory receptor which responds to nicotine--nicotine as an odorant

The tobacco alkaloid (S)(-)-nicotine, when applied as a vapour to an in vitro head preparation, stimulates the olfactory epithelium in three strains of rats and to a lesser extent in two strains of mice. The electro-olfactogram (EOG) generated by nicotine has similar characteristics to the EOGs produced by known odorants. The nicotine EOG increases with increasing concentration of nicotine vapour (1-100 nM) applied to the olfactory epithelium. Differential reduction of the nicotine EOG by the lectin concanavalin A is seen in Wistar and Lister Hooded rats. The reduction of the nicotine EOG by concanavalin A is prevented by adding alpha-methyl-D-mannoside to the lectin superfusion medium. This suggests that there is a glyco-moiety associated with at least one olfactory receptor responding to nicotine. Our results suggest that rat olfactory epithelium has receptor sites for nicotine. Nicotine is an unusual compound because it shows both odorant and pharmacological properties.     

Spatial variation in response to odorants on the rat olfactory epithelium

Summary We have measured the electro-olfactogram produced by four odorants, nicotine, i-pentyl acetate, i-pentanoic acid and cineole from twelve positions on an in vitro preparation of rat olfactory tissue. Each odorant shows a different pattern of response over the twelve positions which can be explained by differences in olfactory receptor populations between regions of the rat olfactory epithelium.The result for nicotine is further evidence that there are olfactory receptors which are stimulated by nicotine when it is presented as a vapour.     

Evidence for an olfactory receptor which responds to nicotine — nicotine as an odorant

Summary The tobacco alkaloid (S)(–)-nicotine, when applied as a vapour to an in vitro head preparation, stimulates the olfactory epithelium in three strains of rats and to a lesser extent in two strains of mice. The electro-olfactogram (EOG) generated by nicotine has similar characteristics to the EOGs produced by known odorants. The nicotine EOG increases with increasing concentration of nicotine vapour (1–100 nM) applied to the olfactory epithelium.Differential reduction of the nicotine EOG by the lectin concanavalin A is seen in Wistar and Lister Hooded rats. The reduction of the nicotine EOG by concanavalin A is prevented by adding alpha-methyl-D-mannoside to the lectin superfusion medium. This suggests that there is a glyco-moiety associated with at least one olfactory receptor responding to nicotine.Our results suggest that rat olfactory epithelium has receptor sites for nicotine. Nicotine is an unusual compound because it shows both odorant and pharmacological properties.22 September 1986     

Facile functional analysis of insect odorant receptors expressed in the fruit fly: validation with receptors from taxonomically distant and closely related species

With the advent of genomic sequences and next-generation sequencing technologies (RNA-Seq), multiple repertoires of olfactory proteins in various insect species are being unraveled. However, functional analyses are lagging behind due in part to the lack of simple and reliable methods for heterologous expression of odorant receptors (ORs). While the Xenopus oocyte recording system fulfills some of this lacuna, this system is devoid of other olfactory proteins, thus testing only the “naked” ORs. Recently, a moth OR was expressed in the majority of neurons in the antennae of the fruit fly using Orco-GAL4 to drive expression of the moth OR. Electroantennogram (EAG) was used to de-orphanize the moth OR, but generic application of this approach was brought to question. Here, we describe that this system works with ORs not only from taxonomically distant insect species (moth), but also closely related species (mosquito), even when the fruit fly has highly sensitive innate ORs for the odorant being tested. We demonstrate that Orco-GAL4 flies expressing the silkworm pheromone receptor, BmorOR1, showed significantly higher responses to the sex pheromone bombykol than the control lines used to drive expression. Additionally, we show that flies expressing an OR from the Southern house mosquito, CquiOR2, gave significantly stronger responses to the cognate odorants indole and 2-methylphenol than the “background noise” recorder from control lines. In summary, we validate the use of Orco-GAL4 driven UAS-OR lines along with EAG analysis as a simple alternative for de-orphanization and functional studies of insect ORs in an intact olfactory system.     

Olfactory navigation in birds

Summary Many bird species rely on an osmotactic mechanism to find food sources even at a considerable distance. Pigeons also rely on local odours for homeward orientation, and they integrate those perceived during passive transportation with those at the release site. It is possible to design experiments in which birds are given false olfactory information, and predictions about the effects of this can be made and tested. Pigeons build up their olfactory map by associating wind-borne odours with the directions from which they come; this was shown by experiments which aimed at preventing, limiting or altering this association. Some objections have been made to this conclusion; namely that even anosmic pigeons are sometimes homeward oriented, that they may be demotivated in flying or disturbed in their general behaviour, and that olfactory cues may be only one component of pigeo's navigational repertoire. The most recent experiments, however, confirm that pigeons derive directional information from atmospheric odouts. The lack of any knowledge about the chemical nature and distribution of the odorants which allow pigeons to navigate hinders progress in this area of research.     

Functional study of the rat olfactory bulb by a 14C-2-deoxyglucose autoradiographic technic]

The autoradiographic method has been used in the Rat to map active regions in the olfactory bulb after a pulse of 14C-2-deoxyglucose with electrical stimulation of the lateral olfactory tract. The highest optical densities were found at the external plexiform, mitral, internal plexiform and granular layers; the lowest was found in the glomerular layer.     

Establishment of left–right asymmetry in vertebrate development: the node in mouse embryos

Establishment of vertebrate left–right asymmetry is a critical process for normal embryonic development. After the discovery of genes expressed asymmetrically along the left–right axis in chick embryos in the mid 1990s, the molecular mechanisms responsible for left–right patterning in vertebrate embryos have been studied extensively. In this review article, we discuss the mechanisms by which the initial symmetry along the left–right axis is broken in the mouse embryo. We focus on the role of primary cilia and molecular mechanisms of ciliogenesis at the node when symmetry is broken and left–right asymmetry is established. The node is considered a signaling center for early mouse embryonic development, and the results we review here have led to a better understanding of how the node functions and establishes left–right asymmetry.     

Coordinated shift of olfactory amino acid responses and V2R expression to an amphibian water nose during metamorphosis

All olfactory receptors identified in teleost fish are expressed in a single sensory surface, whereas mammalian olfactory receptor gene families segregate into different olfactory organs, chief among them the main olfactory epithelium expressing ORs and TAARs, and the vomeronasal organ expressing V1Rs and V2Rs. A transitional stage is embodied by amphibians, with their vomeronasal organ expressing more ‘modern’, later diverging V2Rs, whereas more ‘ancient’, earlier diverging V2Rs are expressed in the main olfactory epithelium. During metamorphosis, the main olfactory epithelium of Xenopus tadpoles transforms into an air-filled cavity (principal cavity, air nose), whereas a newly formed cavity (middle cavity) takes over the function of a water nose. We report here that larval expression of ancient V2Rs is gradually lost from the main olfactory epithelium as it transforms into the air nose. Concomitantly, ancient v2r gene expression begins to appear in the basal layers of the newly forming water nose. We observe the same transition for responses to amino acid odorants, consistent with the hypothesis that amino acid responses may be mediated by V2R receptors.     

Signaling in the Chemosensory Systems

Of all five senses, olfaction is the most complex molecular mechanism, as it comprises hundreds of receptor proteins enabling it to detect and discriminate thousands of odorants. Until lately, the understanding of this highly sophisticated sensory neuronal pathway has been rather sketchy. The sequencing of the human genome and the consequent advent of new genomic tools have opened new opportunities to better understand this multifaceted biological system. Here, we present the relevant progresses made in the last decade and highlight the possible genetic mechanisms of human olfactory variability.     

Changes in olfactory perception during the menstrual cycle

Summary The aim of the study was to find correlations between changes in olfactory sensitivity and the menstrual cycle. 14 young, healthy volunteers participated in the experiments. Subjects menstruated regularly and did not use oral contraceptives. Three odorants were investigated: phenylethyl alcohol, androstenone, and nicotine. Dilution series of the odorants were prepared, and presented to the subjects in order to determine the detection thresholds (triple forced choice). Additionally, the subjects' hedonic estimates of the odorants were measured, and mood states as well as hormonal levels of LH and estrogen were determined. Before the actual experiments started, subjects participated in three training sessions.One experiment was subdivided into 5 phases (two pre- and two postovulatory phases; one ovulatory phase). Only with regard to androstenone did trend analyses reveal a significant quadratic relationship between hedonic estimates and phases of the menstrual cycle, peaking at ovulation. Olfactory sensitivity was not significantly influenced by the menstrual cycle.     

Access to the odor world: olfactory receptors and their role for signal transduction in insects

The sense of smell enables insects to recognize and discriminate a broad range of volatile chemicals in their environment originating from prey, host plants and conspecifics. These olfactory cues are received by olfactory sensory neurons (OSNs) that relay information about food sources, oviposition sites and mates to the brain and thus elicit distinct odor-evoked behaviors. Research over the last decades has greatly advanced our knowledge concerning the molecular basis underlying the reception of odorous compounds and the mechanisms of signal transduction in OSNs. The emerging picture clearly indicates that OSNs of insects recognize odorants and pheromones by means of ligand-binding membrane proteins encoded by large and diverse families of receptor genes. In contrast, the mechanisms of the chemo-electrical transduction process are not fully understood; the present status suggests a contribution of ionotropic as well as metabotropic mechanisms. In this review, we will summarize current knowledge on the peripheral mechanisms of odor sensing in insects focusing on olfactory receptors and their specific role in the recognition and transduction of odorant and pheromone signals by OSNs.     

Topography of the tecto-tectal component of the main ipsilateral visual pathway of the frog (Rana esculenta L.)]

The detailed topography of the tecto-tectal component of the Frog's ipsilateral visual pathway is electrophysiologically obtained by mapping the optic lobes. This linkage transfers the visual information explored along a transversal tectal row on to an homologous line oriented at 130 degrees on the opposite tectum. The ipsilateral projection of the temporo-nasal axis of the retina, but not of the antero-posterior axis of the visual field, is reversed compared to its contralateral projection. Finally, the majority of the homologous tectal points are asymmetrical with respect to the animal's sagittal axis.     

Changes in olfactory perception during the menstrual cycle

The aim of the study was to find correlations between changes in olfactory sensitivity and the menstrual cycle. 14 young, healthy volunteers participated in the experiments. Subjects menstruated regularly and did not use oral contraceptives. Three odorants were investigated: phenylethyl alcohol, androstenone, and nicotine. Dilution series of the odorants were prepared, and presented to the subjects in order to determine the detection thresholds (triple forced choice). Additionally, the subjects' hedonic estimates of the odorants were measured, and mood states as well as hormonal levels of LH and estrogen were determined. Before the actual experiments started, subjects participated in three training sessions. One experiment was subdivided into 5 phases (two pre- and two postovulatory phases; one ovulatory phase). Only with regard to androstenone did trend analyses reveal a significant quadratic relationship between hedonic estimates and phases of the menstrual cycle, peaking at ovulation. Olfactory sensitivity was not significantly influenced by the menstrual cycle.     

Structural determinants of a conserved enantiomer-selective carvone binding pocket in the human odorant receptor OR1A1

Chirality is a common phenomenon within odorants. Most pairs of enantiomers show only moderate differences in odor quality. One example for enantiomers that are easily discriminated by their odor quality is the carvones: humans significantly distinguish between the spearmint-like (R)-(?)-carvone and caraway-like (S)-(+)-carvone enantiomers. Moreover, for the (R)-(?)-carvone, an anosmia is observed in about 8% of the population, suggesting enantioselective odorant receptors (ORs). With only about 15% de-orphaned human ORs, the lack of OR crystal structures, and few comprehensive studies combining in silico and experimental approaches to elucidate structure–function relations of ORs, knowledge on cognate odorant/OR interactions is still sparse. An adjusted homology modeling approach considering OR-specific proline-caused conformations, odorant docking studies, single-nucleotide polymorphism (SNP) analysis, site-directed mutagenesis, and subsequent functional studies with recombinant ORs in a cell-based, real-time luminescence assay revealed 11 amino acid positions to constitute an enantioselective binding pocket necessary for a carvone function in human OR1A1 and murine Olfr43, respectively. Here, we identified enantioselective molecular determinants in both ORs that discriminate between minty and caraway odor. Comparison with orthologs from 36 mammalian species demonstrated a hominid-specific carvone binding pocket with about 100% conservation. Moreover, we identified loss-of-function SNPs associated with the carvone binding pocket of OR1A1. Given carvone enantiomer-specific receptor activation patterns including OR1A1, our data suggest OR1A1 as a candidate receptor for constituting a carvone enantioselective phenotype, which may help to explain mechanisms underlying a (R)-(?)-carvone-specific anosmia in humans.     

The insulin-degrading enzyme is an allosteric modulator of the 20S proteasome and a potential competitor of the 19S

The interaction of insulin-degrading enzyme (IDE) with the main intracellular proteasome assemblies (i.e, 30S, 26S and 20S) was analyzed by enzymatic activity, mass spectrometry and native gel electrophoresis. IDE was mainly detected in association with assemblies with at least one free 20S end and biochemical investigations suggest that IDE competes with the 19S in vitro. IDE directly binds the 20S and affects its proteolytic activities in a bimodal fashion, very similar in human and yeast 20S, inhibiting at (IDE)?≤?30 nM and activating at (IDE)?≥?30 nM. Only an activating effect is observed in a yeast mutant locked in the “open” conformation (i.e., the α-3ΔN 20S), envisaging a possible role of IDE as modulator of the 20S “open”–”closed” allosteric equilibrium. Protein–protein docking in silico proposes that the interaction between IDE and the 20S could involve the C-term helix of the 20S α-3 subunit which regulates the gate opening of the 20S.