Two recent studies have reported decreased electrophysiological responses to odorants when an OBP was targeted with RNAi 48 , 49 , and variations in behavioral responses to odorants have been associated with polymorphisms in OBP genes 50 , OBPs may play especially critical roles in the reception of atypical odorants. However, it is not clear how broadly such strong dependence applies to other insect pheromones.
Some studies have reported responses to the silk moth pheromone bombykol without the cognate OBP 24 , 54 ; others report that moth OBPs make a crucial contribution to pheromone sensitivity in a ligand-specific manner Thus, the precise role of OBPs in odorant reception remains an intriguing problem in the field, one that merits extensive analysis of the physiological and behavioral effects of manipulating individual OBPs in vivo.
It is localized to the cilia and dendrites of ORNs, and its sequence is similar to that of CD36, a vertebrate receptor that binds both proteins and fatty acids. It was proposed to interact with odorant—OBP complexes and enhance the delivery of odorants to receptors. How is odorant identity encoded by this repertoire of receptors and neurons? Thus, the identity of an odorant may be encoded largely in the identity of the Ors that it activates and by extension, in the identity of the ORNs that express those Ors.
Although ORNs expressing a given Or are widely distributed across the antenna, their axons converge in the antennal lobe AL of the brain in spherical modules called glomeruli 59 Fig. The organization of the larval olfactory circuit of Drosophila is similar to that of the adult, but because each Or is expressed in only one ORN, there is no convergence In both adults and larvae, this anatomic organization suggests that odorant identity is encoded largely by the particular combination of glomeruli that are activated. Indeed, imaging studies in ants, moths, honey bees, flies, and other insects have confirmed that individual odorants generate complex and distinct patterns of activated glomeruli ORNs expressing an individual odorant receptor same color send axons to an individual glomerulus in the antennal lobe.
In the antennal lobe, the ORNs form synaptic connections with projection neurons, which send axons to Kenyon cells of the mushroom bodies and then to the lateral horn red and blue axons , or directly to the lateral horn green axon. ORNs also form synapses with local neurons in the antennal lobe. Although some olfactory stimuli activate many classes of ORNs and their cognate glomeruli, other stimuli are more specific. Moth sex pheromones activate selectively tuned neurons on the male antenna and their cognate glomeruli 64 , CO 2 also activates strongly only one narrowly tuned ORN class and the corresponding glomerulus in D.
Such a coding strategy, in which an odorant activates a single narrowly tuned ORN class, is called a labeled line, and it may be used to encode odorants of particular biological significance. Indeed, moth pheromones robustly activate mating behavior 69 , cVA acts in D. Much insight into the molecular basis of odor coding has come from functional studies of insect odorant receptors. The Or repertoire of D. This system is based on a D. Individual Or genes were systematically expressed in this neuron and were found in most cases to confer odor response profiles that matched those of individual ORN classes of the WT fly The matches permitted the construction of a receptor to neuron map and provided evidence that one Or is sufficient to account for the response specificity of most ORNs.
These misexpression experiments also showed that, in addition to the odor response spectrum, the spontaneous firing rate, temporal dynamics, and response mode inhibitory vs. Many intriguing questions arise as to how the structural features of an Or contribute to characteristics such as odorant specificity and temporal dynamics, and these questions will be an important direction for future work. Several fundamental principles of olfactory coding by the Or repertoire were revealed by additional analysis in the empty neuron system 20 , 46 , 73 , Individual odorants activated subsets of receptors, consistent with a combinatorial model of odor coding Fig.
Individual receptors responded to overlapping subsets of odorants. Some receptors were broadly tuned, being strongly excited by a large proportion of the odorants tested, whereas others seemed more narrowly tuned, activated by just a few odorants Fig. There was a smooth continuum in receptor-tuning breadth rather than a discrete division of receptors into specialists and generalists. These principles—combinatorial coding, variation in receptor-tuning breadth, and a continuum in tuning breadths across the receptor repertoire—were found to apply to both the adult and larval Or repertoire of Drosophila.
Odor coding by a receptor repertoire. A Combinatorial coding of odors by receptors of the Drosophila larva. B Tuning curves for a narrowly tuned receptor, Or82a, and a broadly tuned receptor, Or67a. The odorants are listed along the x axis according to the magnitudes of the responses that they elicit from each receptor.
The odorants that elicit the strongest responses are placed near the center of the distribution, whereas those odorants eliciting weak responses are at the edges. The order of odors is different for the two receptors. Negative values represent inhibitory responses. We note that, in the preceding analyses, Ors were expressed in the empty neuron and tested with a panel of general odorants. When certain moth or fly pheromone receptors were expressed in the empty neuron, responses were observed with the cognate pheromones, but stronger responses were observed when these receptors were expressed in a different Drosophila neuron that is sensitive to fly pheromones 54 , 75 , 76 , consistent with a role for additional factors, including PBPs, in the detection of certain olfactory stimuli.
How is the primary representation of an odorant transformed by the downstream neuronal circuitry? The first relay in the olfactory circuit is in the antennal lobe, where the many ORNs that express a given Or converge in the same glomerulus Fig. At this location, ORNs synapse onto a smaller number of secondary neurons called projection neurons PNs Electrophysiological studies have shown that many PNs are more broadly tuned than their cognate ORNs 67 , 78 , This feature of PN tuning derives, in part, from excitatory interneurons with multiglomerular processes, which can transmit signals from an ORN in one glomerulus to a PN in another glomerulus 80 , PN responses show complex temporal features that may encode odorant identity and intensity Recent work has shown that PN dynamics are shaped largely by the temporal dynamics of ORN responses The PN responses occur in a milieu of oscillatory, synchronized neuronal activity 85 , The oscillations are believed to arise from a feedback loop between PNs and inhibitory interneurons within the AL 77 , 87 , One study showed that disrupting these oscillations impairs olfactory discrimination The precise function of these oscillations is an outstanding question in the field, and there is a pressing need for additional studies to define the role of synchronized neuronal activity in olfactory behaviors.
From the antennal lobe, PNs send axons to the mushroom body MB , a higher brain region associated with olfactory learning and memory, and the lateral horn LH , a region associated with innate olfactory behaviors Fig. PNs from multiple glomeruli synapse onto an individual KC, suggesting a role for KCs as coincidence detectors that integrate information from multiple ORN classes Consistent with this notion, KCs are much more narrowly tuned than their inputs. Their selectivity depends on strong inhibitory inputs that are overcome only by coincident excitatory inputs 92 , An intriguing question is whether PN-KC projections are stereotyped or plastic, which might be expected for a region associated with learning and memory.
This variability may be experience-dependent and may play a crucial role in olfactory learning and memory. By contrast, PN projections to the LH seem to be more stereotyped 95 , 98 , For example, PNs that respond to the D.
Remarkably little is known about the olfactory circuit beyond the third-order neurons of the MB and LH. However, a recent study extended the mapping of an olfactory circuit in Drosophila A cVA-responsive neural circuit was traced from the ORN across three synapses to the ganglia of the ventral nerve cord, where it likely initiates motor programs. This work invites a detailed definition of the behavior elicited by the circuit, and it sets an important precedent for the analysis of other insect olfactory circuits.
In addition to the feed-forward circuitry detailed above, centrifugal projections have been described in some insects. In the moth, serotonergic neurons project into the AL and may modulate PN responses in pheromone-responsive glomeruli, suggesting a mechanism by which sensitivity to olfactory cues may be subject to central control Consistent with this proposed mechanism, exogenously applied serotonin modulates PN responses in D.
These results illustrate that, although great progress has recently been made in understanding the principles of olfactory circuitry, there are major limitations to our knowledge. Remaining elements in the circuit need to be defined anatomically. Detailed knowledge of connectivity is necessary to understand the flow of olfactory information, but it is not sufficient. The polarity, strength, and modulation of synapses within the circuit must be elucidated to understand the genesis of olfactory behavior. The mapping and functional definition of ORNs and odorant receptors has permitted precise genetic manipulations of the olfactory circuit in Drosophila.
Such manipulations are elucidating the links between olfactory stimulus and behavioral response. Olfactory input and output are tightly coupled by dedicated circuits in some cases. Conversely, artificially stimulating the CO 2 -sensitive ORNs is sufficient to trigger the avoidance response 71 , Thus, one population of ORNs is both necessary and sufficient to elicit a robust behavioral response in this case.
Mechanistic insights into the tight coupling between olfactory input and behavioral output in these cases are emerging. Electrophysiological studies of PNs in the cVA-responsive glomerulus revealed that they are not more broadly tuned than their cognate ORNs in contrast to other classes of PNs This finding is consistent with the segregation of cVA-responsive ORNs into a discrete processing pathway. Furthermore, projections of these PNs to the LH are sexually dimorphic as are subsequent elements of the circuit, which may explain the sex-specific behavioral responses of D.
Behavioral responses to some odorants are driven by multiple receptors expressed in multiple ORNs. Two Ors of the larval repertoire confer robust physiological responses to the fruit odorant ethyl acetate in the empty neuron system, but the two receptors differ in their sensitivity Deletion of each Or revealed that the receptors with high and low thresholds for ethyl acetate mediate behavioral responses to high and low concentrations, respectively, of this odorant. In this case, by integrating the responses of multiple receptors, the animal can extend the dynamic range of the response and evaluate odor intensity more precisely.
The use of multiple receptors for an odorant may, thereby, allow the insect to navigate up an odor gradient more effectively. The combinatorial coding of odorants through the activation of multiple receptors and ORNs is supported by an analysis of three Ors in adult Drosophila Polymorphisms in all three of these genes were associated with variation in behavioral response to benzaldehyde. Deletion analysis of other receptor genes, such as Or43b , suggested redundancy in the olfactory system: Similarly, selective silencing of certain ORNs in Drosophila larvae resulted in subtle behavioral deficits These examples show that the circuit diagrams underlying responses to different odorants can vary markedly.
An important direction for future work is to delineate these circuits in greater anatomical and physiological detail, which will facilitate our understanding of the mechanisms by which they drive behavior. Such understanding will also aid in defining the molecular and cellular basis of plasticity in these circuits , Hundreds of millions of people suffer from vector-borne diseases every year. These diseases include malaria, yellow fever, dengue, trypanosomiasis, and leishmaniasis, which are spread by mosquitoes, tsetse flies, sandflies, or other insects Fig.
Insect vectors of disease rely on their sense of smell to locate hosts, find mates, and select egg-laying sites 4. Malaria-vector mosquitoes, for example, may fly upwind to host volatiles from up to 70 m away ; triatomine bugs, the vectors of Chagas disease, leave their resting sites when they sense the CO 2 exhaled by their sleeping hosts Culex quinquefasciatus mosquitoes, vectors of filariasis and West Nile Virus, are attracted to oviposition sites by a pheromone released from maturing eggs that signals the suitability of the site Progress in the understanding of olfaction in moths, locusts, and flies is rapidly advancing the study of olfaction in vector insects, and reciprocally, advances made in vector insects are making important contributions to our understanding of these other insect systems.
Insect vectors of disease. C Tsetse fly Glossina morsitans morsitans is a vector of trypanosomiasis. Image courtesy of Geoffrey Attardo. A nocturnal blood-feeder, A. Given the devastating impact of this mosquito's olfactory behaviors, there is great interest in elucidating the molecular mechanisms that underlie them. A family of 79 A. Functional characterization of the AgOrs was carried out in the empty neuron system Both AgOrs responded to aromatic odorants, one of which, 4-methylphenol, is a component of human sweat and oviposition sites Interestingly, some of the most narrowly tuned AgOrs responded strongly to components of human sweat and oviposition site volatiles.
The systematic characterization of both the A. Odorants are differentially encoded by the two species in ways that seem consistent with their ecological needs For example, no AgOr was narrowly tuned to esters or aldehydes, which dominate the headspace of many fruits. By contrast, of the most narrowly tuned fruit fly Ors, in most cases, the strongest responses are to esters or a compound that contains an ester group. A number of Ors are expressed in A. Recently, orthologs of the D. Because the IRs likely mediate the responses of double-walled sensilla in adult antennae to other polar compounds such as ammonia and lactic acid 37 , known mosquito attractants, there is great interest in characterizing the IRs of the adult mosquito antenna.
There are three A. All three contribute to CO 2 detection in the empty neuron system The CO 2 -sensitive Grs are expressed in the A. A number of accessory olfactory proteins have been identified in A. RNAi experiments have shown a role for AgamOBP1 in the response to indole 48 , an oviposition site compound and human volatile. An SNMP ortholog has also been identified , inviting a renewed search for hydrocarbon pheromones, which have not been identified in this species.
Chemical communication among A. It is a vector of filariasis, a disfiguring and debilitating disease that infects over million people worldwide and can cause elephantiasis It is also a vector of encephalitis viruses such as West Nile. Host odors are attractive to C.
The olfactory basis of C. Gravid females deposit large numbers of eggs in one location, thereby providing a ready target for insect control. An oviposition pheromone, released by maturing eggs, attracts gravid females and increases egg deposits Certain aromatic compounds released from decaying organic matter in the mosquito's preferred oviposition sites have similar effects , Olfactory sensilla responsive to these volatiles have been identified , , The first olfactory protein identified in C.
More recently, a family of 53 OBPs was identified bioinformatically, and many of its members are expressed in olfactory tissues CquiOBP1 binds the oviposition pheromone as well as other odorants , In a recent study, RNAi knockdown of CquiOBP1 resulted in a reduced electrophysiological response to oviposition pheromone and some, but not all, of the nonpheromonal odorants that bind to this OBP This result provides an interesting opportunity to investigate the role of an OBP in the behavioral response to nonpheromonal odorants, an unresolved question in the field.
The first Or to be identified in C. Two canonical Ors , CqOr2 and CqOr10 , are expressed in olfactory organs and were found in a Xenopus expression system to be narrowly tuned to the oviposition volatiles indole and 3-methylindole , The olfactory organs of A. However, despite the global impact of the diseases that it transmits, remarkably little is known about the molecular basis of olfaction in A.
Before this work, only the ortholog of Orco and a small number of OBPs , had been identified. However, the contribution that these OBPs make to odorant reception in vivo has not been determined. Four odorant receptors, AaOr2, AaOr8, AaOr9, and AaOr10, have been functionally characterized in heterologous expression systems, and responses to host volatiles were found , The need for additional investigation of olfaction in A. There are many more diseases transmitted by insect vectors, including sleeping sickness transmitted by tsetse flies , river blindness transmitted by flies of the Simulium genus , and Chagas disease transmitted by triatomine bugs.
For these and other vector insects, olfactory cues play a role in locating hosts and in other critical aspects of the life cycle. In these species, olfactory behavior, anatomy, and physiology have been examined, but the molecular mechanisms of olfaction remain largely unexplored. Genome projects are currently underway and should accelerate investigation of tsetse flies and the Chagas disease vector Rhodnius prolixus as well as the Lyme disease vector Ixodes scapularis. One particularly intriguing problem is the molecular and cellular basis of differences in host preferences among related vector insects.
For example, some species of Anopheles are anthropophilic, whereas others are classified as zoophilic 4. It will be interesting to determine whether differences in the response spectra of odorant receptors underlie such behavioral preferences. The great advances of the last decade in defining basic mechanisms and principles of insect olfaction have provided an exciting opportunity. The molecular and cellular insight has laid a foundation for the development of olfactory-based insect control technology. The timing is auspicious: There is added urgency to vector control efforts because of the predicted effects of climate change on the geographical distribution of many of these insects Olfactory behaviors, particularly host seeking and oviposition, offer opportunities to disrupt the disease-transmission process.
In this section, we consider how recent advances can be applied to the problem of vector control and how some limitations might be overcome through basic research. Work of the past decade has identified molecular targets that may be useful in developing insect control strategies. A number of Ors, Grs, and IRs are promising targets for manipulating the olfactory-guided behaviors of insects.
Compounds that excite or inhibit these receptors and that are inexpensive, stable, and nontoxic could provide effective and environmentally friendly means of controlling insect vectors and pests. The identification of molecular targets may greatly increase the efficiency of screens for activators of either attraction or avoidance circuits; high-throughput cell-based expression systems can be used to screen large chemical libraries and rapidly identify candidate compounds. Certain odorant receptors may be prime targets.
Among these receptors are several that are specifically and sensitively tuned to components of human odor and may report the presence of a blood-meal host. Four broad classes of odorants may be useful in insect control.
Insect olfaction: receptors, signal transduction, and behavior.
First, odorants that activate some receptors may drive attraction behaviors and could be used as lures in traps. Odorants identified in electrophysiological screens of tsetse olfactory organs have been used in this manner A blend of electrophysiologically active odorants and visual cues was highly attractive to tsetse flies, and the traps have been used successfully in Africa.
Similarly, volatiles from oviposition site material that elicited robust electrophysiological responses in C. Recently, an electrophysiologically active host odor has been shown to be effective in trapping C. Second, some odorants may activate receptors that drive avoidance circuits.
There is evidence in C. If the cognate receptor for this ORN can be identified, the development of new repellents could be significantly advanced. Third, some odorants may inhibit excitatory responses elicited by attractive human odors. Such compounds may be useful as masking agents that could be applied topically for personal protection. Indeed, one study suggested that the repellent effect of DEET is mediated through such a mechanism , although there is evidence for other mechanisms Recently, an odorant that inhibits the Gr -mediated response to CO 2 in D.
Insect olfaction: receptors, signal transduction, and behavior. - Semantic Scholar
Finally, compounds that alter the temporal dynamics of an ORN response could be useful in insect control. The importance of temporal dynamics in odor coding has been described above, and manipulations that alter the temporal structure of an odorant response affect olfactory behavior Compounds that generate unusually prolonged responses, for example, may disrupt host-seeking behavior. When the silk moth Bombyx mori was exposed to a structural analog of the mating pheromone bombykol that causes a persistent response in the bombykol-sensitive ORN, the normal attractive response to this pheromone was abolished The coreceptor Orco could, in principle, be a useful target for manipulating insect behavior on account of its essential role in the olfactory response of many ORNs 21 , 25 , The phylogenetic conservation of Orco sequence and function 23 suggests that compounds that affect it in one vector species may also affect orthologs in other vector species.
A concern, however, is the potential of such compounds to disrupt the behavior of beneficial insects as well. Insects are essential to the pollination of many crops and are vital to ecosystems; thus, species-specific disruption strategies may be preferable. The functional comparison of A. Such receptors might be exploited to manipulate olfactory behavior in a species-specific manner.
OBPs constitute another class of potential targets for modifying olfactory behaviors. Because RNAi knockdown of an OBP resulted in decreased electrophysiological responses to some odorants 48 , 49 , it will be interesting to examine the behavioral effects of such manipulations from the standpoint of technology as well as science. Targeting of OBPs that bind pheromones deserves special attention; precedent for this approach comes from the phenotypic effects observed from genetic disruption of LUSH 52 and CquiOBP1, which binds the oviposition pheromone of C.
Screens for molecules that interact with such OBPs could yield agents that disrupt mating or oviposition behavior. Rapid increases in the power of genomics and proteomics seem likely to identify additional targets. A recent analysis of the proteome of the D. As such technologies become less expensive, they may be applied to nonmodel insects more readily. One of the greatest hurdles in developing olfactory-based insect control technology is also one of the most fascinating scientific challenges.
It is difficult to predict how the activation or inhibition of a particular olfactory receptor or neuron will affect a particular olfactory behavior.
Even in the highly tractable model organism D. It is even more challenging to predict the behavioral effects of olfactory stimuli in vector insects such as A. A blend of carboxylic acids has been variously shown to be attractive to female A. The difficulty in drawing simple conclusions about the relationship between an odor stimulus and the behavior that it drives may be caused, in part, by the plasticity of olfactory behaviors and the sensitivity of the behaviors to other factors that are difficult to control.
Experimental design often varies between studies, and in laboratory-based studies, the limited numbers of available blood-feeding insects impose difficulties in obtaining a robust database. These difficulties illustrate one of the greatest challenges in the field of insect olfaction: Ideally, such paradigms should measure robust behaviors that simulate those the olfactory system has evolved to drive in its natural environment. Furthermore, they should maximize the information that can be garnered from the sometimes limited number of animals available.
Automated tracking of the movements of individual animals has improved markedly in recent years and reduces the numbers of animals needed for study Automated tracking technology has been used to analyze the behavior of larval and adult-stage mosquitoes , and could be adapted to monitor the behavior of other vector insects.
Although insects in their natural environment are exposed to complex mixtures of odorants, laboratory studies have generally used monomolecular odor stimuli. To understand and control insect behavior in nature, it will be necessary to devote increased attention to the mechanisms by which complex odor stimuli are encoded and processed. Their studies showed that the genes were active in sensory neurons in the antenna--but only in neurons that lacked the well-known odorant receptors.
By attaching a tiny tungsten needle to the region of the antenna where the receptors were found, they found that certain odors could trigger activity in those nerve cells. Stronger evidence that the genes functioned in olfaction came when the researchers genetically manipulated the neurons by inserting a particular receptor into a type of nerve cell in which it was not normally found. The neuron that is normally home to a receptor named IR84a is sensitive to a chemical known as phenylacetaldehyde. This molecule has an aroma similar to a combination of honey and grass. When the team put the gene for IR84a into neurons that normally do not respond to phenylacetaldehyde, they began to respond to the chemical, triggering a nerve impulse when it was near.
A weaker, but similar, effect was found when they shifted the receptor from ammonia-sensitive cells to new neurons. In , Vossshall's lab published evidence that odorant receptors work as channels that open when an odorant binds to let ions into the cell. The IRs, too, appear to be ion channels that sit in a cell's membrane. But that's where the similarities end, Vosshall said. And there's already pretty good evidence that they smell mostly non-overlapping smells. So you have embedded in the antennae two different ways to smell mostly non-overlapping odors.
It's not clear yet how odors detected through this alternate method influence behavior--whether, for example, the IRs help hungry mosquitoes locate their next meal. Vosshall says uncovering the link between IRs and behavior will be crucial if researchers hope to stem infectious disease transmission by manipulating insects' ability to smell. Skip to main content. Skip to navigation Skip to main content Skip to footer.
About About HHMI is a science philanthropy whose mission is to advance basic biomedical research and science education for the benefit of humanity.
Programs Programs HHMI empowers exceptional scientists and students to pursue fundamental questions in basic science. Education Education HHMI believes every student and citizen can experience science in a meaningful way. Jan 09 Research. Summary A novel kind of odor-detecting protein may explain some of the gaps in researchers' knowledge of how insects smell. For More Information Jim Keeley