Combinatorial and chemotopic odorant coding in the zebrafish olfactory bulb visualized by optical imaging

A spatial map of olfactory receptor expression in the Drosophila antenna

Insects provide an attractive system for the study of olfactory sensory perception. We have identified a novel family of seven transmembrane domain proteins, encoded by 100 to 200 genes, that is likely to represent the family of Drosophila odorant receptors. Members of this gene family are expressed in topographically defined subpopulations of olfactory sensory neurons in either the antenna or the maxillary palp. Sensory neurons express different complements of receptor genes, such that individual neurons are functionally distinct. The isolation of candidate odorant receptor genes along with a genetic analysis of olfactory-driven behavior in insects may ultimately afford a system to understand the mechanistic link between odor recognition and behavior.

Combinatorial receptor codes for odors

The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. Here we used a combination of calcium imaging and single-cell RT-PCR to identify odorant receptors (ORs) for odorants with related structures but varied odors. We found that one OR recognizes multiple odorants and that one odorant is recognized by multiple ORs, but that different odorants are recognized by different combinations of ORs. Thus, the olfactory system uses a combinatorial receptor coding scheme to encode odor identities. Our studies also indicate that slight alterations in an odorant, or a change in its concentration, can change its "code," potentially explaining how such changes can alter perceived odor quality.

Characterizing complex chemosensors: information-theoretic analysis of olfactory systems

The mechanisms that underlie a wine lover's ability to identify a favorite vintage and a dog's ability to track the scent of a lost child are still deep mysteries. Our understanding of these olfactory phenomena is confounded by the difficulty encountered when attempting to identify the parameters that define odor stimuli, by the broad tuning and variability of neurons in the olfactory pathway,and by the distributed nature of olfactory encoding. These issues pertain to both biological systems and to newly developed 'artificial noses' that seek to mimic these natural processes. Information theory, which quantifies explicitly the extent to which the state of one system (for example, the universe of all odors) relates to the state of another (for example, the responses of an odor-sensing device),can serve as a basis for analysing both natural and engineered odor sensors. This analytical approach can be used to explore the problems of defining stimulus dimensions, assessing strategies of neuronal processing, and examining the properties of biological systems that emerge from interactions among their complex components. It can also serve to optimize the design of artificial olfactory devices for a variety of applications, which include process control, medical diagnostics and the detection of explosives.

Current-source density analysis in the rat olfactory bulb: laminar distribution of kainate/AMPA- and NMDA-receptor-mediated currents

The one-dimensional current-source density method was used to analyze laminar field potential profiles evoked in rat olfactory bulb slices by stimulation in the olfactory nerve (ON) layer or mitral cell layer (MCL) and to identify the field potential generators and the characteristics of synaptic activity in this network. Single pulses to the ON evoked a prolonged (>/=400 ms) sink (S1ON) in the glomerular layer (GL) with corresponding sources in the external plexiform layer (EPL) and MCL and a relatively brief sink (S2ON) in the EPL, reversing in the internal plexiform and granule cell layers. These sink/source distributions suggested that S1ON and S2ON were generated in the apical dendrites of mitral/tufted cells and granule cells, respectively. The kainate/AMPA-receptor antagonist CNQX (10 microM) reduced the early phase of S1ON, blocked S2ON, and revealed a low amplitude, prolonged sink at the location of S2ON in the EPL. Reduction of Mg2+, in CNQX, enhanced both the CNQX-resistant component of S1ON and the EPL sink. This EPL sink reversed below the MCL, suggesting it was produced in granule cells. The NMDA-receptor antagonist APV (50 microM) reversibly blocked the CNQX-resistant field potentials in all layers. Single pulses were applied to the MCL to antidromically depolarize the dendrites of mitral/tufted cells. In addition to synaptic currents of granule cells, a low-amplitude, prolonged sink (S1mcl) was evoked in the GL. Corresponding sources were in the EPL, suggesting that S1mcl was generated in the glomerular dendritic tufts of mitral/tufted cells. Both S1mcl and the granule cell currents were nearly blocked by CNQX (10 microM) but enhanced by subsequent reduction of Mg2+; these currents were blocked by APV. S1mcl also was enhanced by gamma-aminobutyric acid-A-receptor antagonists applied to standard medium; this enhancement was reduced by APV. ON activation produces prolonged excitation in the apical dendrites of mitral/tufted cells, via kainate/AMPA and NMDA receptors, providing the opportunity for modulation and integration of sensory information at the first level of synaptic processing in the olfactory system. Granule cells respond to input from the lateral dendrites of mitral/tufted cells via both kainate/AMPA and NMDA receptors; however, in physiological concentrations of extracellular Mg2+, NMDA-receptor activation does not contribute significantly to the granule cell responses. The glomerular sink evoked by antidromic depolarization of mitral/tufted cell dendrites suggests that glutamate released from the apical dendrites of mitral/tufted cells may excite the same or neighboring mitral/tufted cell dendrites.

Calcium Imaging in the Olfactory System: New Tools for Visualizing Odor Recognition

Transient elevations of intracellular Ca2+play an important role in regulating the sensitivity of the sense of smell, both at the level of signal transduction within the cilia of olfactory receptor neurons and at presynaptic sites in the olfactory bulb, but such elevations have not been demonstrated previously because of the small size of these neuronal com partments. Here, we summarize recent progress employing high resolution Ca2+-imaging techniques that permit the visualization of odor-induced neural activity in these critical subcellular compartments of the vertebrate olfactory system. In olfactory neurons, Ca2+permeable cyclic nucleotide-gated (CNG) cation channels mediate the initial Ca2+entry during odor transduction. The surprisingly widespread distribution of members of the CNG channel family in the mammalian brain suggests that CNG channel-mediated Ca2+entry contributes to signal transduction in many CNS neurons. NEUROSCIENTIST 5:4- 7, 1999

Chemotopic, combinatorial, and noncombinatorial odorant representations in the olfactory bulb revealed using a voltage-sensitive axon tracer

Odor information is first represented in the brain by patterns of input activity across the glomeruli of the olfactory bulb (OB). To examine how odorants are represented at this stage of olfactory processing, we labeled anterogradely the axons of olfactory receptor neurons with the voltage-sensitive dye Di8-ANEPPQ in zebrafish. The activity induced by diverse natural odorants in afferent axons and across the array of glomeruli was then recorded optically. The results show that certain subregions of the OB are preferentially activated by defined chemical odorant classes. Within these subregions, "ordinary" odorants (amino acids, bile acids, and nucleotides) induce overlapping activity patterns involving multiple glomeruli, indicating that they are represented by combinatorial activity patterns. In contrast, two putative pheromone components (prostaglandin F2alpha and 17alpha, 20beta-dihydroxy-4-pregnene-3-one-20-sulfate) each induce a single focus of activity, at least one of which comes from a single, highly specific and sensitive glomerulus. These results indicate that the OB is organized into functional subregions processing classes of odorants. Furthermore, they suggest that individual odorants can be represented by "combinatorial" or "noncombinatorial" (focal) activity patterns and that the latter may serve to process odorants triggering distinct responses such as that of pheromones.

Cloning and localization of two multigene receptor families in goldfish olfactory epithelium

Goldfish reproduction is coordinated by pheromones that are released by ovulating females and detected by males. Two highly potent pheromones, a dihydroxyprogesterone and a prostaglandin, previously have been identified, and their effects on goldfish behavior have been studied in depth. We have cloned goldfish olfactory epithelium cDNAs belonging to two multigene G-protein coupled receptor families as a step toward elucidating the molecular basis of pheromone recognition. One gene family (GFA) consists of homologs of putative odorant receptors (approximately 320 residues) found in the olfactory epithelium of other fish and mammals. The other family (GFB) consists of homologs of putative pheromone receptors found in the vomeronasal organ (VNO) of mammals and also in the nose of pufferfish. GFB receptors (approximately 840 residues) are akin to the V2R family of VNO receptors, which possess a large extracellular N-terminal domain and are homologs of calcium-sensing and metabotropic glutamate receptors. In situ hybridization showed that the two families of goldfish receptors are differentially expressed in the olfactory epithelium. GFB mRNA is abundant in rather compact cells whose nuclei are near the apical surface. In contrast, GFA mRNA is found in elongated cells whose nuclei are positioned deeper in the epithelium. Our findings support the hypothesis that the separate olfactory organ and VNO of terrestrial vertebrates arose in evolution by the segregation of distinct classes of neurons that were differentially positioned in the olfactory epithelium of a precursor aquatic vertebrate.

Differential projections of ciliated and microvillous olfactory receptor cells in the catfish, Ictalurus punctatus

The primary olfactory projections of channel catfish Ictalurus punctatus have been examined with postmortem tracing by using either 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate or 1,1-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). Following DiI deposition into small areas in different parts of the posterior olfactory bulb, olfactory sensory neurons always were labeled throughout the olfactory epithelium. No obvious topographical mapping exists between the epithelium and olfactory bulb. The different dye placements, however, did result in labeling of different morphologies of receptor cells, depending on the site of injection. Retrogradely labeled neurons in the olfactory epithelium were classified into three types on the basis of their height: tall, intermediate, and short. Tall olfactory sensory neurons had perikarya at the bottom one-fourth of the epithelium, extended slender dendrites to the epithelial surface, and possessed numerous cilia on the apical dendritic tips. These tall olfactory sensory neurons were labeled predominantly following DiI applications to the ventral part of the posterior olfactory bulb. In contrast, the short olfactory sensory neurons had perikarya situated within the superficial half of the epithelium and with short apical dendrites bearing microvilli. These short olfactory sensory neurons projected predominantly to the dorsal, posterior olfactory bulb. Thus, short microvillous receptor cells and tall ciliated receptor cells connect to different parts of the olfactory bulb, although the receptor cells are intermingled within the olfactory epithelium. Because different parts of the olfactory bulb are thought to respond preferentially to different classes of odorants, these results suggest that receptor cell morphology may be related to odorant quality detection. In addition, to compare this study with previous in vivo studies, Fluoro-Gold was injected in vivo into either the olfactory bulb or intraperitoneally. These in vivo studies show that so-called "type II ciliar receptor cells" of the nonsensory epithelium are labeled nonselectively by blood-borne substances, but they are not labeled by postmortem injections of DiI anywhere in the olfactory bulb.

Odour coding is bilaterally symmetrical in the antennal lobes of honeybees (Apis mellifera)

The primary olfactory neuropil, the antennal lobe (AL) in insects, is organized in glomeruli. Glomerular activity patterns are believed to represent the across-fibre pattern of the olfactory code. These patterns depend on an organized innervation from the afferent receptor cells, and interconnections of local interneurons. It is unclear how the complex organization of the AL is achieved ontogenetically. In this study, we measured the functional activity patterns elicited by stimulation with odours in the right and the left AL of the same honeybee (Apis mellifera) using optical imaging of the calcium-sensitive dye calcium green. We show here that these patterns are bilaterally symmetrical (n=25 bees). This symmetry holds true for all odours tested, irrespective of their role as pheromones or as environmental odours, or whether they were pure substances or complex blends (n=13 odours). Therefore, we exclude that activity dependent mechanisms local to one AL determine the functional glomerular activity. This identity is genetically predetermined. Alternatively, if activity dependent processes are involved, bilateral connections would have to shape symmetry, or, temporal constraints could lead to identical patterns on both sides due to their common history of odour exposure.

The area code hypothesis revisited: olfactory receptors and other related transmembrane receptors may function as the last digits in a cell surface code for assembling embryos

Recent evidence emerging from several laboratories, integrated with new data obtained by searching the genome databases, suggests that the area code hypothesis provides a good heuristic model for explaining the remarkable specificity of cell migration and tissue assembly that occurs throughout embryogenesis. The area code hypothesis proposes that cells assemble organisms, including their brains and nervous systems, with the aid of a molecular-addressing code that functions much like the country, area, regional, and local portions of the telephone dialing system. The complexity of the information required to code cells for the construction of entire organisms is so enormous that we assume that the code must make combinatorial use of members of large multigene families. Such a system would reuse the same receptors as molecular digits in various regions of the embryo, thus greatly reducing the total number of genes required. We present the hypothesis that members of the very large families of olfactory receptors and vomeronasal receptors fulfill the criteria proposed for area code molecules and could serve as the last digits in such a code. We discuss our evidence indicating that receptors of these families are expressed in many parts of developing embryos and suggest that they play a key functional role in cell recognition and targeting not only in the olfactory system but also throughout the brain and numerous other organs as they are assembled.

Discrimination of pheromonal cues in fish: emerging parallels with insects

Many fish species employ hormonal products as sex pheromones, and these cues are often mixtures that are released with a temporal pattern. This behavior is strikingly similar to that of insects, as moths use precise blends of odorants as sex pheromones and are skillful at tracking them in spite of changes in odor intensity associated with aerial dispersal. New studies in both groups of animals suggest many parallels in the functional anatomy of olfactory pathways and the organization of information-coding circuits.

The specification of olfactory neurons

Recent studies have shed light on the different relationships between odorant receptor expression and the specification of neural identity in the olfactory systems of vertebrates and invertebrates. In mice, neuronal identity and axon guidance are specified by the single expressed olfactory receptor, whereas in C. elegans, neuronal identity appears to be independent of receptor expression.

Odorant response properties of convergent olfactory receptor neurons

Information about odorant stimuli is thought to be represented in spatial and temporal patterns of activity across neurons in the olfactory epithelium and the olfactory bulb (OB). Previous studies suggest that olfactory receptor neurons (ORNs) distributed in the nasal cavity project to localized regions in the glomerular layer of the OB. However, the functional significance of this convergence is not yet known, and in no studies have the odorant response properties of individual ORNs projecting to defined OB regions been measured directly. We have retrogradely labeled mouse ORNs connecting to different glomeruli in the dorsal OB and tested single cells for responses to odorants using fura-2 calcium imaging. ORNs that project to clusters of dorsomedial (DM) glomeruli exhibit different odorant response profiles from those that project to dorsolateral (DL) glomeruli. DL-projecting ORNs showed responses to compounds with widely different structures, including carvone, eugenol, cinnamaldehyde, and acetophenone. In contrast, DM-projecting neurons exhibited responses to a more structurally restricted set of compounds and responded preferentially to organic acids. These data demonstrate that ORN afferents segregate by odorant responsiveness and that the homogeneity of ORN and glomerular input varies with different OB regions. The data also demonstrate that a subpopulation of ORNs projecting to DM glomeruli is functionally similar.

Pathfinding of olfactory neuron axons to stereotyped glomerular targets revealed by dynamic imaging in living zebrafish embryos

In the vertebrate olfactory system, sensory neurons with common odorant specificities project to specific glomeruli in the olfactory bulb. How do olfactory sensory neurons find their glomerular targets? To address this question, we have visualized the genesis of the peripheral olfactory system in living zebrafish embryos. Dye labelings reveal that a primordial yet stereotyped map of glomeruli is apparent during embryogenesis. By labeling a small number of cells with an ectopically expressed green fluorescent protein reporter, we can observe the dynamic growth behaviors of individual olfactory neuron growth cones as they project to their glomeruli. We find that olfactory axons extend directly to their partner glomeruli, suggesting that these cells' growth cones rely upon pathfinding cues to reach their targets.

Olfactory processing: a time and place for everything

The behavioral effects of pharmacologically desynchronizing neuronal firing in the brain of the honeybee provide new evidence that the oscillatory synchronization of neuronal activity plays an important role in fine olfactory discrimination.

Odorant receptors govern the formation of a precise topographic map

Olfactory neurons expressing a given odorant receptor project with precision to 2 of the 1800 glomeruli within the olfactory bulb to create a topographic map of odor quality. We demonstrate that deletions or nonsense mutations in the P2 odorant receptor gene cause the axons of these cells to wander rather than converge on a specific glomerulus. Receptor substitution experiments that replace the P2 gene with the coding region of the P3 gene result in the projection of P3-->P2 axons to a glomerulus touching the wild-type P3 glomerulus. These data, along with additional receptor substitutions, indicate that the odorant receptor plays an instructive role in the establishment of the topographic map.

Drugs affecting phospholipase C-mediated signal transduction block the olfactory cyclic nucleotide-gated current of adult zebrafish

Amino acid and bile salt odorants are detected by zebrafish with relatively independent odorant receptors, but the transduction cascade(s) subsequently activated by these odorants remains unknown. Electro-olfactogram recording methods were used to determine the effects of two drugs, reported to affect phospholipase C (PLC)/inositol tripohsphate (IP3)-mediated olfactory transduction in other vertebrate species, on amino acid and bile salt-evoked responses. At the appropriate concentrations, either an IP3-gated channel blocker, ruthenium red (0.01-0.1 microM), or a PLC inhibitor, neomycin (50 microM), reduced amino-acid-evoked responses to a significantly greater extent than bile salt-evoked responses. Excised patch recording techniques were used to measure the affects of these drugs on second-messenger-activated currents. Ruthenium red and neomycin are both effective blockers of the olfactory cyclic nucleotide-gated (CNG) current. Both drugs blocked the CNG channel in a voltage-dependent and reversible manner. No IP3-activated currents could be recorded. The differential effects of ruthenium red and neomycin on odor-evoked responses suggest the activation of multiple transduction cascades. The nonspecific actions of these drugs on odor-activated transduction pathways and our inability to record an IP3-activated current do not permit the conclusion that zebrafish, like other fish species, use a PLC/IP3-mediated transduction cascade in the detection of odorants.

How simple is the organization of the olfactory glomerulus?: the heterogeneity of so-called periglomerular cells

Recent progress in the studies of the olfactory system, especially in the molecular biological studies, makes it one of the useful sensory model systems for understanding neural mechanisms for the information processing. In the olfactory bulb, the primary center of the olfactory system, glomeruli are regarded as important functional units in the transmission of odorant signals and in processing the olfactory information, but have been believed to be composed by only a small number of neuronal types and thus to be simple in their neuronal and synaptic organization. However, accumulating morphological data reveal that each type of neurons might further consist of several different subpopulations, indicating that the organization of glomeruli might not be so simple as it was believed. Here we describe an aspect of the structural organization of glomeruli, focusing on the heterogeneities of periglomerular neurons in mammalian main olfactory bulb.

Mice deficient in G(olf) are anosmic

We have used gene targeting to examine the role of the G alpha subunit, G(olf), in olfactory signal transduction. Mice homozygous for a null mutation in G(olf) show a striking reduction in the electrophysiological response of primary olfactory sensory neurons to a wide variety of odors. Despite this profound diminution in response to odors, the topographic map of primary sensory projections to the olfactory bulb remains unaltered in G(olf) mutants. Greater than 75% of the G(olf) mutant mice are unable to nurse and die within 2 days after birth. Rare surviving homozygotes mate and are fertile, but mutant females exhibit inadequate maternal behaviors. Surviving homozygous mutant mice also exhibit hyperactive behaviors. These behavioral phenotypes, taken together with the patterns of G(olf) expression, suggest that G(olf) is required for olfactory signal transduction and may also function as an essential signaling molecule more centrally in the brain.

A new multigene family of putative pheromone receptors

The vomeronasal organ (VNO) mediates detection of pheromones related to social and reproductive behavior in most terrestrial vertebrates. We have identified a new multigene family of G protein-linked receptors (V2Rs) that are specifically expressed in the VNO. V2Rs have no significant homology to other putative pheromone receptors (V1Rs) or to olfactory receptors but are related to the Ca2+-sensing receptor and metabotropic glutamate receptors. V2Rs are expressed at high levels in small subpopulations of VNO neurons. V2Rs are primarily expressed in a different layer of VNO neurons from V1Rs, thus both gene families are likely to encode mammalian pheromone receptors.