Encoding olfactory signals via multiple chemosensory systems

Most animals have evolved multiple olfactory systems to detect general odors as well as social cues. The sophistication and interaction of these systems permit precise detection of food, danger, and mates, all crucial elements for survival. In most mammals, the nose contains two well described chemosensory apparatuses (the main olfactory epithelium and the vomeronasal organ), each of which comprises several subtypes of sensory neurons expressing distinct receptors and signal transduction machineries. In many species (e.g., rodents), the nasal cavity also includes two spatially segregated clusters of neurons forming the septal organ of Masera and the Grueneberg ganglion. Results of recent studies suggest that these chemosensory systems perceive diverse but overlapping olfactory cues and that some neurons may even detect the pressure changes carried by the airflow. This review provides an update on how chemosensory neurons transduce chemical (and possibly mechanical) stimuli into electrical signals, and what information each system brings into the brain. Future investigation will focus on the specific ligands that each system detects with a behavioral context and the processing networks that each system involves in the brain. Such studies will lead to a better understanding of how the multiple olfactory systems, acting in concert, offer a complete representation of the chemical world.     

The combined role of the main olfactory and vomeronasal systems in social communication in mammals

The main olfactory and the vomeronasal systems are the two systems by which most vertebrates detect chemosensory cues that mediate social behavior. Much research has focused on how one system or the other is critical for particular behaviors. This has lead to a vision of two distinct and complexly autonomous olfactory systems. A closer look at research over the past 30 years reveals a different picture however. These two seemingly distinct systems are much more integrated than previously thought. One novel set of chemosensory cues in particular (MHC Class I peptide ligands) can show us how both systems are capable of detecting the same chemosensory cues, through different mechanisms yet provide the same general information (genetic individuality). Future research will need to now focus on how two seemingly distinct chemosensory systems together detect pheromones and mediate social behaviors. Do these systems work independently, synergistically or competitively in communicating between individuals of the same species?     

Molecular and cellular organization of insect chemosensory neurons

Animals use their chemosensory systems to detect and discriminate among chemical cues in the environment. Remarkable progress has recently been made in our knowledge of the molecular and cellular basis of chemosensory perception in insects, based largely on studies in Drosophila. This progress has been possible due to the identification of gene families for olfactory and gustatory receptors, the use of electro-physiological recording techniques on sensory neurons, the multitude of genetic manipulations that are available in this species, and insights from several insect model systems. Recent studies show that the superfamily of chemoreceptor proteins represent the essential elements in chemosensory coding, endowing chemosensory neurons with their abilities to respond to specific sets of odorants, tastants or pheromones. Investigating how insects detect chemicals in their environment can show us how receptor protein structures relate to ligand binding, how nervous systems process complex information, and how chemosensory systems and genes evolve.     

The combined role of the main olfactory and vomeronasal systems in social communication in mammals

The main olfactory and the vomeronasal systems are the two systems by which most vertebrates detect chemosensory cues that mediate social behavior. Much research has focused on how one system or the other is critical for particular behaviors. This has lead to a vision of two distinct and complexly autonomous olfactory systems. A closer look at research over the past 30 years reveals a different picture however. These two seemingly distinct systems are much more integrated than previously thought. One novel set of chemosensory cues in particular (MHC Class I peptide ligands) can show us how both systems are capable of detecting the same chemosensory cues, through different mechanisms yet provide the same general information (genetic individuality). Future research will need to now focus on how two seemingly distinct chemosensory systems together detect pheromones and mediate social behaviors. Do these systems work independently, synergistically or competitively in communicating between individuals of the same species?     

The vomeronasal system in mice: from the nose to the hypothalamus- and back!

The vomeronasal pathway in rodents runs parallel to the main olfactory pathway and mediates responses to different classes of chemosensory stimuli. Both olfactory systems can converge and synergize to control reproductive behaviors and hormonal changes triggered by chemosensory cues. Novel experimental approaches expressing genetic transneuronal tracers in hypothalamic neurons regulating reproduction have set the stage to analyze how chemosensory inputs are integrated in the brain to elicit reproductive behaviors and hormonal changes, and how neuroendocrine status might modulate susceptibility to chemosensory cues.     

Olfactory networks: from sensation to perception

Olfactory networks, comprised of sensory neurons and interneurons, detect and process changes in the chemical environment to drive animal behavior. Recent studies combining genetics with behavioral analyses and imaging in worms, flies and mice have revealed new insights into the mechanisms of olfaction. In this discussion, we focus on three interesting findings. First, sensory neuron responses to odor are modulated by neuropeptides. This modulation might serve to extend the range of responses of the sensory neurons and also to integrate internal state information into the chemosensory circuit. Second, genetic tracing studies in mice and flies have shown that the first layer of connections in chemosensory circuits from olfactory epithelium to the glomeruli are stereotyped, while the subsequent connections to higher order sensory processing regions are not. Distributed connectivity to the higher order sensory processing regions has profound implications for how odors are represented in those regions. Third, recent work has revealed that odors are surprisingly sparsely represented in the piriform cortex. The sparse coding in the higher brain centers implies a much greater role for experience and learning in mediating responses to olfactory cues. Analyzing olfactory network function in various species provides us with fascinating clues about how sensory information is acquired, processed and represented at multiple levels within the nervous system.     

The effects of sex on chemosensory communication in a terrestrial salamander (Plethodon shermani)

Although much evidence reveals sexually dimorphic processing of chemosensory cues by the brain, potential sex differences at more peripheral levels of chemoreception are understudied. In plethodontid salamanders, the volume of the vomeronasal organ (VNO) is almost twice as large in males as compared to females, both in absolute and relative size. To determine whether the structural sexual dimorphism in VNO volume is associated with sex differences in other peripheral aspects of chemosensation, we measured sex differences in chemo-investigation and in responsiveness of the VNO to chemosensory cues. Males and females differed in traits influencing stimulus access to VNO chemosensory neurons. Males chemo-investigated (“nose tapped”) neutral substrates and substrates moistened with female body rinses more than did females. Compared to females, males had larger narial structures (cirri) associated with the transfer of substrate-borne chemical cues to the lumen of the VNO. These sex differences in chemo-investigation and narial morphology likely represent important mechanisms for regulating sex differences in chemical communication. In contrast, males and females did not differ in responsiveness of VNO chemosensory neurons to male mental gland extract or female skin secretions. This important result indicates that although males have a substantially larger VNO compared to females, the male VNO was not more responsive to every chemosensory cue that is detected by the VNO. Future studies will determine whether the male VNO is specialized to detect a subset of chemosensory cues, such as female body rinses or female scent marks.     

Olfactory receptor gene evolution is unusually rapid across Tetrapoda and outpaces chemosensory phenotypic change

Chemosensation is the most ubiquitous sense in animals, enacted by the products of complex gene families that detect environmental chemical cues and larger-scale sensory structures that process these cues. While there is a general conception that olfactory receptor (OR) genes evolve rapidly, the universality of this phenomenon across vertebrates, and its magnitude, are unclear. The supposed correlation between molecular rates of chemosensory evolution and phenotypic diversity of chemosensory systems is largely untested. We combine comparative genomics and sensory morphology to test whether OR genes and olfactory phenotypic traits evolve at faster rates than other genes or traits. Using published genomes, we identified ORs in 21 tetrapods, including amphibians, reptiles, birds, and mammals and compared their rates of evolution to those of orthologous non-OR protein-coding genes. We found that, for all clades investigated, most OR genes evolve nearly an order of magnitude faster than other protein-coding genes, with many OR genes showing signatures of diversifying selection across nearly all taxa in this study. This rapid rate of evolution suggests that chemoreceptor genes are in “evolutionary overdrive,” perhaps evolving in response to the ever-changing chemical space of the environment. To obtain complementary morphological data, we stained whole fixed specimens with iodine, µCT-scanned the specimens, and digitally segmented chemosensory and nonchemosensory brain regions. We then estimated phenotypic variation within traits and among tetrapods. While we found considerable variation in chemosensory structures, they were no more diverse than nonchemosensory regions. We suggest chemoreceptor genes evolve quickly in reflection of an ever-changing chemical space, whereas chemosensory phenotypes and processing regions are more conserved because they use a standardized or constrained architecture to receive and process a range of chemical cues.     

The morphology and ultrastructure of the peripheral olfactory organ in newly metamorphosed coral-dwelling gobies, Paragobiodon xanthosomus Bleeker (Gobiidae, Teleostei)

We examined the peripheral olfactory organ in newly metamorphosed coral-dwelling gobies, Paragobiodon xanthosomus (SL=5.8mm+/-0.8mm, N=15), by the aid of electron microscopy (scanning and transmission) and light microscopy. Two bilateral olfactory placodes were present in each fish. They were oval-shaped and located medio-ventrally, one in each of the olfactory chambers. Each placode had a continuous cover of cilia. The placode epithelium contained three different types of olfactory receptor neurons: ciliated, microvillous and crypt cells. The latter type was rare. Following a pelagic larval phase, P. xanthosomus settle to the reef and form an obligate association with one species of coral, Seriatopora hystrix. Their well-developed olfactory organs likely enable larvae of P. xanthosomus to detect chemical cues that assist in navigating towards and selecting appropriate coral habitat at settlement. Our findings support past studies showing that the peripheral olfactory organ develops early in coral reef fishes.     

Role of proneural genes in the formation of the larval olfactory organ of <Emphasis Type="Italic">Drosophila</Emphasis>

In this paper, we address the role of proneural genes in the formation of the dorsal organ in the Drosophila larva. This organ is an intricate compound comprising the multineuronal dome—the exclusive larval olfactory organ—and a number of mostly gustatory sensilla. We first determine the numbers of neurons and of the different types of accessory cells in the dorsal organ. From these data, we conclude that the dorsal organ derives from 14 sensory organ precursor cells. Seven of them appear to give rise to the dome, which therefore may be composed of seven fused sensilla, whereas the other precursors produce the remaining sensilla of the dorsal organ. By a loss-of-function approach, we then analyze the role of atonal, amos, and the achaete-scute complex (AS-C), which in the adult are the exclusive proneural genes required for chemosensory organ specification. We show that atonal and amos are necessary and sufficient in a complementary way for four and three of the sensory organ precursors of the dome, respectively. AS-C, on the other hand, is implicated in specifying the non-olfactory sensilla, partially in cooperation with atonal and/or amos. Similar links for these proneural genes with olfactory and gustatory function have been established in the adult fly. However, such conserved gene function is not trivial, given that adult and larval chemosensory organs are anatomically very different and that the development of adult olfactory sensilla involves cell recruitment, which is unlikely to play a role in dome formation. N. Grillenzoni and V. de Vaux contributed equally to this work.     

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.     

Taste and pheromone perception in mammals and flies

The olfactory systems of insects and mammals have analogous anatomical features and use similar molecular logic for olfactory coding. The molecular underpinnings of the chemosensory systems that detect taste and pheromone cues have only recently been characterized. Comparison of these systems in Drosophila and mouse uncovers clear differences and a few surprising similarities.     

Emerging views on the distinct but related roles of the main and accessory olfactory systems in responsiveness to chemosensory signals in mice

In rodents, the nasal cavity contains two separate chemosensory epithelia, the main olfactory epithelium, located in the posterior dorsal aspect of the nasal cavity, and the vomeronasal/accessory olfactory epithelium, located in a capsule in the anterior aspect of the ventral floor of the nasal cavity. Both the main and accessory olfactory systems play a role in detection of biologically relevant odors. The accessory olfactory system has been implicated in response to pheromones, while the main olfactory system is thought to be a general molecular analyzer capable of detecting subtle differences in molecular structure of volatile odorants. However, the role of the two systems in detection of biologically relevant chemical signals appears to be partially overlapping. Thus, while it is clear that the accessory olfactory system is responsive to putative pheromones, the main olfactory system can also respond to some pheromones. Conversely, while the main olfactory system can mediate recognition of differences in genetic makeup by smell, the vomeronasal organ (VNO) also appears to participate in recognition of chemosensory differences between genetically distinct individuals. The most salient feature of our review of the literature is that there are no general rules that allow classification of the accessory olfactory system as a pheromone detector and the main olfactory system as a detector of general odorants. Instead, each behavior must be considered within a specific behavioral context to determine the role of these two chemosensory systems. In each case, one system or the other (or both) participates in a specific behavioral or hormonal response.     

The Molecular Phylogeny of a Nematode-Specific Clade of Heterotrimeric G-Protein α-Subunit Genes

In animal olfactory systems, odorant molecules are detected by olfactory receptors (ORs). ORs are part of the G-protein-coupled receptor (GPCR) superfamily. Heterotrimeric guanine nucleotide binding G-proteins (G-proteins) relay signals from GPCRs to intracellular effectors. G-proteins are comprised of three peptides. The G-protein α subunit confers functional specificity to G-proteins. Vertebrate and insect Gα-subunit genes are divided into four subfamilies based on functional and sequence attributes. The nematode Caenorhabditis elegans contains 21 Gα genes, 14 of which are exclusively expressed in sensory neurons. Most individual mammalian cells express multiple distinct GPCR gene products, however, individual mammalian and insect olfactory neurons express only one functional odorant OR. By contrast C. elegans expresses multiple ORs and multiple Gα subunits within each olfactory neuron. Here we show that, in addition to having at least one member of each of the four mammalian Gα gene classes, C. elegans and other nematodes also possess two lineage-specific Gα gene expansions, homologues of which are not found in any other organisms examined. We hypothesize that these novel nematode-specific Gα genes increase the functional complexity of individual chemosensory neurons, enabling them to integrate odor signals from the multiple distinct ORs expressed on their membranes. This neuronal gene expansion most likely occurred in nematodes to enable them to compensate for the small number of chemosensory cells and the limited emphasis on cephalization during nematode evolution. [Reviewing Editor: Dr. John Oakeshott] Damien M. O’Halloran and David A. Fitzpatrick contributed equally to this work.     

Quantitative comparative analysis of the nasal chemosensory organs of anurans during larval development and metamorphosis highlights the relative importance of chemosensory subsystems in the group

The anuran peripheral olfactory system is composed of a number of subsystems, represented by distinct neuroepithelia. These include the main olfactory epithelium and vomeronasal organ (found in most tetrapods) and three specialized epithelia of anurans: the buccal‐exposed olfactory epithelium of larvae, and the olfactory recess and middle chamber epithelium of postmetamorphic animals. To better characterize the developmental changes in these subsystems across the life cycle, morphometric changes of the nasal chemosensory organs during larval development and metamorphosis were analyzed in three different anuran species (Rhinella arenarum , Hypsiboas pulchellus , and Xenopus laevis ). We calculated the volume of the nasal chemosensory organs by measuring the neuroepithelial area from serial histological sections at four different stages. In larvae, the vomeronasal organ was relatively reduced in R. arenarum compared with the other two species; the buccal‐exposed olfactory epithelium was absent in X. laevis , and best developed in H. pulchellus . In postmetamorphic animals, the olfactory epithelium (air‐sensitive organ) was relatively bigger in terrestrial species (R. arenarum and H. pulchellus ), whereas the vomeronasal and the middle chamber epithelia (water‐sensitive organs) was best developed in X. laevis . A small olfactory recess (likely homologous with the middle chamber epithelium) was found in R. arenarum juveniles, but not in H. pulchellus . These results support the association of the vomeronasal and middle chamber epithelia with aquatic olfaction, as seen by their enhanced development in the secondarily aquatic juveniles of X. laevis . They also support a role for the larval buccal‐exposed olfactory epithelium in assessment of oral contents: it was absent in X. laevis , an obligate suspension feeder, while present in the two grazing species. These initial quantitative results give, for the first time, insight into the functional importance of the peripheral olfactory subsystems across the anuran life cycle.     

Immunocytochemical localization of serotonin in the central and peripheral chemosensory system of mosquitoes

Female mosquitoes depend on blood to complete their reproductive cycle and rely mainly on chemosensory systems to obtain blood meals. An immunocytochemical analysis reveals a number of serotonin-immunoreactive neurons that innervate the chemosensory systems, suggesting a potential role of serotonin in modulating chemosensory processes. In the primary olfactory system, we identify a single ipsilateral centrifugal neuron with arborizations in higher brain centers; the varicosities of this neuron display volumetric changes in response to both blood feeding and during a circadian rhythm. Six to eight pairs of serotonin-immunoreactive neurons are identified in the primary gustatory neuropil, including the subesophageal ganglion and tritocerebrum. The peripheral chemosensory organs, i.e. the antenna, the maxillary palp and the labium, are described as having extensive serotonergic neurohemal plexi. In addition, we describe the presence of serotonin-immunoreactive fibers in the mechanosensory Johnston's organ. Taking these results together, we discuss the potential role of serotonin as a neuromodulator in the chemosensory system of disease vector mosquitoes.     

A broadly tuned odorant receptor in neurons of trichoid sensilla in locust,Locusta migratoria

Insects have evolved sophisticated olfactory reception systems to sense exogenous chemical signals. Odorant receptors (ORs) on the membrane of chemosensory neurons are believed to be key molecules in sensing exogenous chemical cues. ORs in different species of insects are diverse and should tune a species to its own specific semiochemicals relevant to their survival. The orthopteran insect, locust (Locusta migratoria), is a model hemimetabolous insect. There is very limited knowledge on the functions of locust ORs although many locust OR genes have been identified in genomic sequencing experiments. In this paper, a locust OR, LmigOR3 was localized to neurons housed in trichoid sensilla by in situ hybridization. LmigOR3 was expressed as a transgene in Drosophila trichoid olfactory neurons (aT1) lacking the endogenous receptor Or67d and the olfactory tuning curve and dose-response curves were established for this locust receptor. The results show that LmigOR3 sensitizes neurons to ketones, esters and heterocyclic compounds, indicating that LmigOR3 is a broadly tuned receptor. LmigOR3 is the first odorant receptor from Orthoptera that has been functionally analyzed in the Drosophila aT1 system. This work demonstrates the utility of the Drosophila aT1 system for functional analysis of locust odorant receptors and suggests that LmigOR3 may be involved in detecting food odorants, or perhaps locust body volatiles that may help us to develop new control methods for locusts.     

Organization of the Olfactory and Respiratory Skeleton in the Nose of the Gray Short-Tailed Opossum Monodelphis domestica

The internal nasal skeleton in Monodelphis domestica, the gray short-tailed opossum, primarily supports olfactory and respiratory epithelia, the vomeronasal organ, and the nasal gland. This scaffold is built by the median mesethmoid, and the paired vomer and ethmoid bones. The mesethmoid ossifies within the nasal septum cartilage. The bilateral ethmoid segregates respiratory and olfactory regions, and its geometry offers insight into the functional, developmental, and genomic organization of the nose. It forms through partial coalescence of separate elements known as turbinals, which in Monodelphis comprise the maxilloturbinal, nasoturbinal, five endoturbinals, and two ectoturbinals. Geometry of the ethmoid increases respiratory mucosal surface area by a factor of six and olfactory mucosal surface by nearly an order of magnitude. Respiratory epithelium warms and humidifies inspired air, recovers moisture as air is exhaled, and may help mediate brain temperature. In contrast, the olfactory skeleton functions as a series of small funnels that support growth of new olfactory neurons throughout life. Olfactory mucosa lines the mouth of each funnel, forming blind olfactory recesses known as the ethmoid cells, and neuronal axons are funneled from the epithelium through tiny olfactory foramina in the cribriform plate, into close proximity with target glomeruli in the olfactory bulb of the brain where each axon makes its first synapse. The skeleton may thus mediate topological correspondence between odorant receptor areas in the nose with particular glomeruli in the olfactory bulb, enabling growth throughout life of new olfactory neurons and proper targeting by their axons. The geometric arrangement of odorant receptors suggests that a measure of volatility may be a component in the peripheral olfactory code, and that corresponding glomeruli may function in temporal signal processing. Supporting visualizations for this study are available online at www.DigiMorph.org.     

Projection pattern of nerve fibers from the septal organ: DiI-tracing studies with transgenic OMP mice

The septal organ represents one of the three chemosensory subsystems found in most vertebrate species. Analyzing the projection pattern of septal organ neurons using the OMP-GFP transgenic mouse line revealed that axons navigate in highly variable fiber tracks across the main olfactory epithelium toward the main olfactory bulb. All septal organ axons cross through the cribriform plate at a spatially defined site and terminate exclusively in the posterior, ventromedial aspect of the bulb. Here, one portion of axons forms a dense network on the medial side where they apparently enter glomeruli which are mainly innervated by axons of olfactory sensory neurons from the main olfactory epithelium. Another significant portion of the axons targets a few glomeruli which appear to receive input exclusively from the septal organ neurons.     

The rodent accessory olfactory system

The accessory olfactory system contributes to the perception of chemical stimuli in the environment. This review summarizes the structure of the accessory olfactory system, the stimuli that activate it, and the responses elicited in the receptor cells and in the brain. The accessory olfactory system consists of a sensory organ, the vomeronasal organ, and its central projection areas: the accessory olfactory bulb, which is connected to the amygdala and hypothalamus, and also to the cortex. In the vomeronasal organ, several receptors—in contrast to the main olfactory receptors—are sensitive to volatile or nonvolatile molecules. In a similar manner to the main olfactory epithelium, the vomeronasal organ is sensitive to common odorants and pheromones. Each accessory olfactory bulb receives input from the ipsilateral vomeronasal organ, but its activity is modulated by centrifugal projections arising from other brain areas. The processing of vomeronasal stimuli in the amygdala involves contributions from the main olfactory system, and results in long-lasting responses that may be related to the activation of the hypothalamic–hypophyseal axis over a prolonged timeframe. Different brain areas receive inputs from both the main and the accessory olfactory systems, possibly merging the stimulation of the two sensory organs to originate a more complex and integrated chemosensory perception.