Odor Coding Research Articles

Odor representation and coding by the mitral/tufted cells in the olfactory bulb.

The olfactory bulb (OB) is the first relay station in the olfactory system and functions as a crucial hub. It can represent odor information precisely and accurately in an ever-changing environment. As the only output neurons in the OB, mitral/tufted cells encode information such as odor identity and concentration. Recently, the neural strategies and mechanisms underlying odor representation and encoding in the OB have been investigated extensively. Here we review the main progress on this topic. We first review the neurons and circuits involved in odor representation, including the different cell types in the OB and the neural circuits within and beyond the OB. We will then discuss how two different coding strategies-spatial coding and temporal coding-work in the rodent OB. Finally, we discuss potential future directions for this research topic. Overall, this review provides a comprehensive description of our current understanding of how odor information is represented and encoded by mitral/tufted cells in the OB.

Ring-shaped odor coding in the antennal lobe of migratory locusts

The representation of odors in the locust antennal lobe with its >2,000 glomeruli has long remained a perplexing puzzle. We employed the CRISPR-Cas9 system to generate transgenic locusts expressing the genetically encoded calcium indicator GCaMP in olfactory sensory neurons. Using two-photon functional imaging, we mapped the spatial activation patterns representing a wide range of ecologically relevant odors across all six developmental stages. Our findings reveal a functionally ring-shaped organization of the antennal lobe composed of specific glomerular clusters. This configuration establishes an odor-specific chemotopic representation by encoding different chemical classes and ecologically distinct odors in the form of glomerular rings. The ring-shaped glomerular arrangement, which we confirm by selective targeting of OR70a-expressing sensory neurons, occurs throughout development, and the odor-coding pattern within the glomerular population is consistent across developmental stages. Mechanistically, this unconventional spatial olfactory code reflects the locust-specific and multiplexed glomerular innervation pattern of the antennal lobe.

Pheromone representation in the ant antennal lobe changes with age

While the neural basis of age-related decline has been extensively studied,1,2,3 less is known about changes in neural function during the pre-senescent stages of adulthood. Adult neural plasticity is likely a key factor in social insect age polyethism, where individuals perform different tasks as they age and divide labor in an age-dependent manner.4,5,6,7,8,9 Primarily, workers transition from nursing to foraging tasks,5,10 become more aggressive, and more readily display alarm behavior11,12,13,14,15,16 as they get older. While it is unknown how these behavioral dynamics are neurally regulated, they could partially be generated by altered salience of behaviorally relevant stimuli.4,6,7 Here, we investigated how odor coding in the antennal lobe (AL) changes with age in the context of alarm pheromone communication in the clonal raider ant (Ooceraea biroi).17 Similar to other social insects,11,12,16 older ants responded more rapidly to alarm pheromones, the chemical signals for danger. Using whole-AL calcium imaging,18 we then mapped odor representations for five general odorants and two alarm pheromones in young and old ants. Alarm pheromones were represented sparsely at all ages. However, alarm pheromone responses within individual glomeruli changed with age, either increasing or decreasing. Only two glomeruli became sensitized to alarm pheromones with age, while at the same time becoming desensitized to general odorants. Our results suggest that the heightened response to alarm pheromones in older ants occurs via increased sensitivity in these two core glomeruli, illustrating the importance of sensory modulation in social insect division of labor and age-associated behavioral plasticity.

Age-related differences in perception and coding of attractive odorants in mice

Hedonic perception deeply changes with aging, significantly impacting health and quality of life in elderly. In young adult mice, an odor hedonic signature is represented along the antero-posterior axis of olfactory bulb, and transferred to the olfactory tubercle and ventral tegmental area, promoting approach behavior. Here, we show that while the perception of unattractive odorants was unchanged in older mice (22 months), the appreciation of some but not all attractive odorants declined. Neural activity in the olfactory bulb and tubercle of older mice was consistently altered when attraction to pleasant odorants was impaired while maintained when the odorants kept their attractivity. Finally, in a self-stimulation paradigm, optogenetic stimulation of the olfactory bulb remained rewarding in older mice even without ventral tegmental area’s response to the stimulation. Aging degrades behavioral and neural responses to some pleasant odorants but rewarding properties of olfactory bulb stimulation persisted, providing new insights into developing novel olfactory training strategies to elicit motivation even when the dopaminergic system is altered as observed in normal and/or neurodegenerative aging.

Evolution at multiple processing levels underlies odor-guided behavior in the genus Drosophila

Olfaction is a fundamental sense guiding animals to their food. How the olfactory system evolves and influences behavior is still poorly understood. Here, we selected five drosophilid species, including Drosophila melanogaster, inhabiting different ecological niches to compare their olfactory systems at multiple levels. We first identified ecologically relevant natural food odorants from every species and established species-specific odorant preferences. To compare odor coding in sensory neurons, we analyzed the antennal lobe (AL) structure, generated glomerular atlases, and developed GCaMP transgenic lines for all species. Although subsets of glomeruli showed distinct tuning profiles, odorants inducing species-specific preferences were coded generally similarly. Species distantly related or occupying different habitats showed more evident differences in odor coding, and further analysis revealed that changes in olfactory receptor (OR) sequences partially explain these differences. Our results demonstrate that genetic distance in phylogeny and ecological niche occupancy are key determinants in the evolution of ORs, AL structures, odor coding, and behavior. Interestingly, changes in odor coding among species could not be explained by evolutionary changes at a single olfactory processing level but rather are a complex phenomenon based on changes at multiple levels.

Porphyrinoids-Junctions Based Chemical Sensors

The three main elements that form the porphyrinoids (aromatic ring, metal atom, and lateral compounds) provide the key to direct the chemical reactivity of the molecule towards desired classes of compounds. This property gives rise to a manifold of molecular architectures each with distinct features, that, in many cases, make them suitable for technological applications, among the others, chemical sensing.The gas sensing properties of porphyrinoids involves the interaction with airborne molecules [1,2]. These are driven by a multiplicity of mechanisms including hydrogen bond, Van der Waals forces, and coordination that are active at once. Coordinating molecules, such as amines, have a preferential absorption onto porphyrins solid state layers, however the response to other molecules is not negligible so, eventually porphyrin sensors are seldom selective. However, the pattern of sensitivity strongly depends on slight changes in the molecular structure. This facilitate the development of sensor arrays that can easily implement the combinatorial selectivity principle of olfaction [3]. This property has been exploited to prepare electronic noses for various applications, not least medical diagnosis from the analysis of the volatile portion of the human metabolites [4–6].These results were obtained coating with porphyrin layers the surface of quartz microbalance sensors. These are mass sensitive piezoelectric devices where the crystal resonance frequency is matched with the electric resonance. Mass sensors are quite suitable to prepare sensors that preserve the global sensitivity pattern of porphyrinoids, thus enabling an efficient combinatorial selectivity suitable for those applications which are characterized by patterns of volatile compounds. In spite of these positive properties, quartz microbalances are bulky and costly devices not suitable for low-costs, miniaturized devices. Thus, the current challenge is the development of impedance-based sensors that can preserve the wide sensitivity pattern of porphyrinoids.To this regard, we have been interested to study the combination of porphyrnoids with conductive materials such as inorganic, and organic semiconductors.In this presentation some relevant results will be present in particular about porphyrinoids coated ZnO nanostructures [7], the functionalization of the internal surface of face-masks with blends of porphyrinoids and PEDOT [8]and the heterojunctions formed by corrole polymer and phthalocyanine [9]. The sensitivity patterns of these devices approaches the behavior of quartz microbalance sensors paving the way for an effective large-scale deployment of porphyrinoids based artificial olfaction systems.Reference[1] R. Paolesse, Porphyrinoids for Chemical Sensor Applications, Chem. Rev. 117 (2017) 2517–2583.[2] C. Di Natale, C.P. Gros, R. Paolesse, Corroles at work: a small macrocycle for great applications, Chem. Soc. Rev. 51 (2022) 1277–1335.[3] I. Manzini, D. Schild, Di Natale C., Principles of odor coding in vertebrates and artificial chemosensory systems, Physiol. Rev. 102 (2022) 61–154.[4] R. Gasparri, R. Capuano, A. Guaglio, V. Caminiti, F. Canini, A. Catini, G. Sedda, R. Paolesse, C. Di Natale, L. Spaggiari, Volatolomic urinary profile analysis for diagnosis of the early stage of lung cancer, J. Breath Res. 16 (2022).[5] Y.K. Mougang, L. Di Zazzo, M. Minieri, R. Capuano, A. Catini, J.M. Legramante, R. Paolesse, S. Bernardini, C. Di Natale, Sensor array and gas chromatographic detection of the blood serum volatolomic signature of COVID-19, IScience. 24 (2021) 102851.[6] M. Murdocca, F. Torino, S. Pucci, M. Costantini, R. Capuano, C. Greggi, C. Polidoro, G. Somma, V. Pasqualetti, Y. Ketchanji Mougang, A. Catini, G. Simone, R. Paolesse, A. Orlandi, A. Mauriello, M. Roselli, A. Magrini, G. Novelli, C. Di Natale, F.C. Sangiuolo, Urine LOX-1 and Volatilome as Promising Tools towards the Early Detection of Renal Cancer, Cancers (Basel). 13 (2021) 4213.[7] G. Magna, M. Muduganti, M. Stefanelli, Y. Sivalingam, F. Zurlo, E. Di Bartolomeo, A. Catini, E. Martinelli, R. Paolesse, C. Di Natale, Light-Activated Porphyrinoid-Capped Nanoparticles for Gas Sensing, ACS Appl. Nano Mater. 4 (2021) 414–424.[8] L. Di Zazzo, G. Magna, M. Lucentini, M. Stefanelli, R. Paolesse, C. Di Natale, Sensor-embedded face masks for detection of volatiles in breath: A proof of concept study, Chemosensors. 9 (2021).[9] L. Di Zazzo, A. Kumar, R. Meunier-Prest, C. Di Natale, R. Paolesse, M. Bouvet, Electrosynthesized copper polycorroles as versatile materials in double lateral heterojunctions (submitted) 2022.

High-precision mapping reveals the structure of odor coding in the human brain.

Odor perception is inherently subjective. Previous work has shown that odorous molecules evoke distributed activity patterns in olfactory cortices, but how these patterns map on to subjective odor percepts remains unclear. In the present study, we collected neuroimaging responses to 160 odors from 3 individual subjects (18 h per subject) to probe the neural coding scheme underlying idiosyncratic odor perception. We found that activity in the orbitofrontal cortex (OFC) represents the fine-grained perceptual identity of odors over and above coarsely defined percepts, whereas this difference is less pronounced in the piriform cortex (PirC) and amygdala. Furthermore, the implementation of perceptual encoding models enabled us to predict olfactory functional magnetic resonance imaging responses to new odors, revealing that the dimensionality of the encoded perceptual spaces increases from the PirC to the OFC. Whereas encoding of lower-order dimensions generalizes across subjects, encoding of higher-order dimensions is idiosyncratic. These results provide new insights into cortical mechanisms of odor coding and suggest that subjective olfactory percepts reside in the OFC.

Chronic exposure to odors at naturally occurring concentrations triggers limited plasticity in early stages of Drosophila olfactory processing.

In insects and mammals, olfactory experience in early life alters olfactory behavior and function in later life. In the vinegar fly Drosophila, flies chronically exposed to a high concentration of a monomolecular odor exhibit reduced behavioral aversion to the familiar odor when it is reencountered. This change in olfactory behavior has been attributed to selective decreases in the sensitivity of second-order olfactory projection neurons (PNs) in the antennal lobe that respond to the overrepresented odor. However, since odorant compounds do not occur at similarly high concentrations in natural sources, the role of odor experience-dependent plasticity in natural environments is unclear. Here, we investigated olfactory plasticity in the antennal lobe of flies chronically exposed to odors at concentrations that are typically encountered in natural odor sources. These stimuli were chosen to each strongly and selectively excite a single class of primary olfactory receptor neuron (ORN), thus facilitating a rigorous assessment of the selectivity of olfactory plasticity for PNs directly excited by overrepresented stimuli. Unexpectedly, we found that chronic exposure to three such odors did not result in decreased PN sensitivity but rather mildly increased responses to weak stimuli in most PN types. Odor-evoked PN activity in response to stronger stimuli was mostly unaffected by odor experience. When present, plasticity was observed broadly in multiple PN types and thus was not selective for PNs receiving direct input from the chronically active ORNs. We further investigated the DL5 olfactory coding channel and found that chronic odor-mediated excitation of its input ORNs did not affect PN intrinsic properties, local inhibitory innervation, ORN responses or ORN-PN synaptic strength; however, broad-acting lateral excitation evoked by some odors was increased. These results show that PN odor coding is only mildly affected by strong persistent activation of a single olfactory input, highlighting the stability of early stages of insect olfactory processing to significant perturbations in the sensory environment.

Ocean acidification alters foraging behaviour in Dungeness crab through impairment of the olfactory pathway.

Crustacean olfaction is fundamental to most aspects of living and communicating in aquatic environments and more broadly, for individual- and population-level success. Accelerated ocean acidification from elevated CO2 threatens the ability of crabs to detect and respond to important olfactory-related cues. Here, we demonstrate that the ecologically and economically important Dungeness crab (Metacarcinus magister) exhibits reduced olfactory-related antennular flicking responses to a food cue when exposed to near-future CO2 levels, adding to the growing body of evidence of impaired crab behaviour. Underlying this altered behaviour, we find that crabs have lower olfactory nerve sensitivities (twofold reduction in antennular nerve activity) in response to a food cue when exposed to elevated CO2 . This suggests that near-future CO2 levels will impact the threshold of detection of food by crabs. We also show that lower olfactory nerve sensitivity in elevated CO2 is accompanied by a decrease in the olfactory sensory neuron (OSN) expression of a principal chemosensory receptor protein, ionotropic receptor 25a (IR25a) which is fundamental for odorant coding and olfactory signalling cascades. The OSNs also exhibit morphological changes in the form of decreased surface areas of their somata. This study provides the first evidence of the effects of high CO2 levels at multiple levels of biological organization in marine crabs, linking physiological and cellular changes with whole animal behavioural responses.

New insights on the adsorption of floral odorants on Apis cerana cerana olfactory receptor AcerOr1: Theoretical modeling and thermodynamic study

Apis cerana cerana counted on its sensitive olfactory system to make survival activities in the surrounding environment and the olfactory receptors can be considered as a primary requirement of odorant detection, recognition and coding. Indeed, the exploitation of the olfactory system of insects in particular the Asian honeybee “Apis cerana cerana” can be the best experimental model to investigate the essentials of the chemosensitivity and may help to better understand the olfactory perception in insects. Hence, an advanced statistical physics modeling via the monolayer model with single energy (n ≠ 1) of the three dose-olfactory responses curves indicated that undecanoic acid, 1-octyl alcohol and 1-nonanol were docked with a mixed parallel and non-parallel orientation on AcerOr1. Furthermore, in the present work, the Apis cerana cerana olfactory receptor AcerOr1 showed high sensitivity and discrimination power to detect undecanoic acid, 1-octyl alcohol and 1-nonanol with concentrations at half saturations values of 10−7 mol/L and the molar adsorption energy values obtained from data fitting results, which were ranged from 17.91 to 24.00 kJ/mol, confirmed the exothermic and the physisorption nature of the adsorption of the studied floral odorants on AcerOr1. The studied experimental dose-response curves of undecanoic acid, 1-octyl alcohol and 1-nonanol provided access to quantitative (i.e., stereographic and energetic) characterizations of AcerOr1 via the determination of the olfactory receptor site size distributions (RSDs) and the adsorption energy distributions (AEDs). The stereographic characterization showed RSDs spread out from 0.20 to 8 nm presenting average values corresponding to the maximum of the peaks at 1.50 nm, at 1.10 nm and at 1.04 nm for undecanoic acid, 1-octyl alcohol and 1-nonanol, respectively. The energetic characterization presented AEDs ranged from 0 to 40 kJ/mol showing an approximate adsorption energy bands defined between 7.50 and 27.50 kJ/mol, between 15 and 33 kJ/mol and between 13.50 and 34.50 kJ/mol for undecanoic acid, 1-octyl alcohol and 1-nonanol, respectively. The utilization of the analytical expression of the olfactory threshold allowed giving important and helpful informations about the occupation rate of AcerOr1 binding sites that fired a minimal olfactory response at a honeybee olfactory receptor. Hence, the olfactory response can be detected only when 1.97 %, 1.13 % and 2.00 % of AcerOr1 binding sites were occupied by undecanoic acid, 1-octyl alcohol and 1-nonanol, respectively. Lastly, by means of the selected model, the thermodynamic potentials, such as the adsorption entropy, the Gibbs free enthalpy and the internal energy could be calculated and interpreted.

Geosmin suppresses defensive behaviour and elicits unusual neural responses in honey bees

Geosmin is an odorant produced by bacteria in moist soil. It has been found to be extraordinarily relevant to some insects, but the reasons for this are not yet fully understood. Here we report the first tests of the effect of geosmin on honey bees. A stinging assay showed that the defensive behaviour elicited by the bee’s alarm pheromone component isoamyl acetate (IAA) is strongly suppressed by geosmin. Surprisingly, the suppression is, however, only present at very low geosmin concentrations, and disappears at higher concentrations. We investigated the underlying mechanisms at the level of the olfactory receptor neurons by means of electroantennography, finding the responses to mixtures of geosmin and IAA to be lower than to pure IAA, suggesting an interaction of both compounds at the olfactory receptor level. Calcium imaging of the antennal lobe (AL) revealed that neuronal responses to geosmin decreased with increasing concentration, correlating well with the observed behaviour. Computational modelling of odour transduction and coding in the AL suggests that a broader activation of olfactory receptor types by geosmin in combination with lateral inhibition could lead to the observed non-monotonic increasing–decreasing responses to geosmin and thus underlie the specificity of the behavioural response to low geosmin concentrations.

Mechanisms and functions of respiration-driven gamma oscillations in the primary olfactory cortex.

Gamma oscillations are believed to underlie cognitive processes by shaping the formation of transient neuronal partnerships on a millisecond scale. These oscillations are coupled to the phase of breathing cycles in several brain areas, possibly reflecting local computations driven by sensory inputs sampled at each breath. Here, we investigated the mechanisms and functions of gamma oscillations in the piriform (olfactory) cortex of awake mice to understand their dependence on breathing and how they relate to local spiking activity. Mechanistically, we find that respiration drives gamma oscillations in the piriform cortex, which correlate with local feedback inhibition and result from recurrent connections between local excitatory and inhibitory neuronal populations. Moreover, respiration-driven gamma oscillations are triggered by the activation of mitral/tufted cells in the olfactory bulb and are abolished during ketamine/xylazine anesthesia. Functionally, we demonstrate that they locally segregate neuronal assemblies through a winner-take-all computation leading to sparse odor coding during each breathing cycle. Our results shed new light on the mechanisms of gamma oscillations, bridging computation, cognition, and physiology.

Multisite imaging of neural activity using a genetically encoded calcium sensor in the honey bee.

Understanding of the neural bases for complex behaviors in Hymenoptera insect species has been limited by a lack of tools that allow measuring neuronal activity simultaneously in different brain regions. Here, we developed the first pan-neuronal genetic driver in a Hymenopteran model organism, the honey bee, and expressed the calcium indicator GCaMP6f under the control of the honey bee synapsin promoter. We show that GCaMP6f is widely expressed in the honey bee brain, allowing to record neural activity from multiple brain regions. To assess the power of this tool, we focused on the olfactory system, recording simultaneous responses from the antennal lobe, and from the more poorly investigated lateral horn (LH) and mushroom body (MB) calyces. Neural responses to 16 distinct odorants demonstrate that odorant quality (chemical structure) and quantity are faithfully encoded in the honey bee antennal lobe. In contrast, odor coding in the LH departs from this simple physico-chemical coding, supporting the role of this structure in coding the biological value of odorants. We further demonstrate robust neural responses to several bee pheromone odorants, key drivers of social behavior, in the LH. Combined, these brain recordings represent the first use of a neurogenetic tool for recording large-scale neural activity in a eusocial insect and will be of utility in assessing the neural underpinnings of olfactory and other sensory modalities and of social behaviors and cognitive abilities.

Olfactory sensitivity differentiates morphologically distinct worker castes in Camponotus floridanus

BackgroundCamponotus floridanus ant colonies are comprised of a single reproductive queen and thousands of sterile female offspring that consist of two morphologically distinct castes: smaller minors and larger majors. Minors perform most of the tasks within the colony, including brood care and food collection, whereas majors have fewer clear roles and have been hypothesized to act as a specialized solider caste associated with colony defense. The allocation of workers to these different tasks depends, in part, on the detection and processing of local information including pheromones and other chemical blends such as cuticular hydrocarbons. However, the role peripheral olfactory sensitivity plays in establishing and maintaining morphologically distinct worker castes and their associated behaviors remains largely unexplored.ResultsWe examined the electrophysiological responses to general odorants, cuticular extracts, and a trail pheromone in adult minor and major C. floridanus workers, revealing that the repertoire of social behaviors is positively correlated with olfactory sensitivity. Minors in particular display primarily excitatory responses to olfactory stimuli, whereas major workers primarily manifest suppressed, sub-solvent responses. The notable exception to this paradigm is that both minors and majors display robust, dose-dependent excitatory responses to conspecific, non-nestmate cuticular extracts. Moreover, while both minors and majors actively aggress non-nestmate foes, the larger and physiologically distinct majors display significantly enhanced capabilities to rapidly subdue and kill their adversaries.ConclusionsOur studies reveal the behavioral repertoire of minors and majors aligns with profound shifts in peripheral olfactory sensitivity and odor coding. The data reported here support the hypothesis that minors are multipotential workers with broad excitatory sensitivity, and majors are dedicated soldiers with a highly specialized olfactory system for distinguishing non-nestmate foes. Overall, we conclude that C. floridanus majors do indeed represent a physiologically and behaviorally specialized soldier caste in which caste-specific olfactory sensitivity plays an important role in task allocation and the regulation of social behavior in ant colonies.

If Proust had whiskers: Recalling locations with smells.

Recent research suggests that the piriform cortex simultaneously represents spatial and olfactory information. These findings may provide further insight into the non-topographic principles of odor coding.

Sensory biology: Olfactory crosstalk reshapes odor coding

New research uncovers a novel form of crosstalk between olfactory pathways in the antennal lobe, the first olfactory center of the fly brain. This crosstalk reshapes odor coding and may explain how carbon dioxide can elicit either attraction or aversion.

More than meets the AI: The possibilities and limits of machine learning in olfaction.

Can machine learning crack the code in the nose? Over the past decade, studies tried to solve the relation between chemical structure and sensory quality with Big Data. These studies advanced computational models of the olfactory stimulus, utilizing artificial intelligence to mine for clear correlations between chemistry and psychophysics. Computational perspectives promised to solve the mystery of olfaction with more data and better data processing tools. None of them succeeded, however, and it matters as to why this is the case. This article argues that we should be deeply skeptical about the trend to black-box the sensory system’s biology in our theories of perception. Instead, we need to ground both stimulus models and psychophysical data on real causal-mechanistic explanations of the olfactory system. The central question is: Would knowledge of biology lead to a better understanding of the stimulus in odor coding than the one utilized in current machine learning models? That is indeed the case. Recent studies about receptor behavior have revealed that the olfactory system operates by principles not captured in current stimulus-response models. This may require a fundamental revision of computational approaches to olfaction, including its psychological effects. To analyze the different research programs in olfaction, we draw on Lloyd’s “Logic of Research Questions,” a philosophical framework which assists scientists in explicating the reasoning, conceptual commitments, and problems of a modeling approach in question.

Disorder and the Neural Representation of Complex Odors.

Animals smelling in the real world use a small number of receptors to sense a vast number of natural molecular mixtures, and proceed to learn arbitrary associations between odors and valences. Here, we propose how the architecture of olfactory circuits leverages disorder, diffuse sensing and redundancy in representation to meet these immense complementary challenges. First, the diffuse and disordered binding of receptors to many molecules compresses a vast but sparsely-structured odor space into a small receptor space, yielding an odor code that preserves similarity in a precise sense. Introducing any order/structure in the sensing degrades similarity preservation. Next, lateral interactions further reduce the correlation present in the low-dimensional receptor code. Finally, expansive disordered projections from the periphery to the central brain reconfigure the densely packed information into a high-dimensional representation, which contains multiple redundant subsets from which downstream neurons can learn flexible associations and valences. Moreover, introducing any order in the expansive projections degrades the ability to recall the learned associations in the presence of noise. We test our theory empirically using data from Drosophila. Our theory suggests that the neural processing of sparse but high-dimensional olfactory information differs from the other senses in its fundamental use of disorder.

Non-canonical odor coding in the mosquito

SUMMARYAedes aegypti mosquitoes are a persistent human foe, transmitting arboviruses including dengue when they feed on human blood. Mosquitoes are intensely attracted to body odor and carbon dioxide, which they detect using ionotropic chemosensory receptors encoded by three large multi-gene families. Genetic mutations that disrupt the olfactory system have modest effects on human attraction, suggesting redundancy in odor coding. The canonical view is that olfactory sensory neurons each express a single chemosensory receptor that defines its ligand selectivity. We discovered that Ae. aegypti uses a different organizational principle, with many neurons co-expressing multiple chemosensory receptor genes. In vivo electrophysiology demonstrates that the broad ligand-sensitivity of mosquito olfactory neurons depends on this non-canonical co-expression. The redundancy afforded by an olfactory system in which neurons co-express multiple chemosensory receptors may increase the robustness of the mosquito olfactory system and explain our long-standing inability to disrupt the detection of humans by mosquitoes.

Cracking the encoding of human scent in the mosquito brain

In a recent study, Zhao et al. decipher how the olfactory system encodes human versus animal odors in the mosquito Aedes aegypti. By combining genome engineering, invivo calcium imaging, advanced chemistry, and behavioral analysis, the authors provide compelling evidence that the discriminatory coding of host odors is surprisingly simple - and bridges labeled line with combinatorial coding.