Insect olfaction

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Olfactory pathway, insects Olfactory pathway insects.png
Olfactory pathway, insects

Insect olfaction refers to the function of chemical receptors that enable insects to detect and identify volatile compounds for foraging, predator avoidance, finding mating partners (via pheromones) and locating oviposition habitats. [1] Thus, it is the most important sensation for insects. [1] Most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it. [2] For example, olfaction is essential for locating host plants and hunting prey in many species of insects, such as the moth Deilephila elpenor and the wasp Polybia sericea , respectively.

Contents

The two organs insects primarily use for detecting odors are the antennae and specialized mouth parts called the maxillary palps. [3] However, a recent study has demonstrated the olfactory role of ovipositor in fig wasps. [4] Inside of these olfactory organs there are neurons called olfactory receptor neurons which, as the name implies, house receptors for scent molecules in their cell membrane. The majority of olfactory receptor neurons typically reside in the antenna. These neurons can be very abundant; for example, Drosophila flies have 2,600 olfactory sensory neurons. [3]

Insects are capable of smelling and differentiating between thousands of volatile compounds both sensitively and selectively. [1] [5] Sensitivity is how attuned the insect is to very small amounts of an odorant or small changes in the concentration of an odorant. Selectivity refers to the insects ability to tell one odorant apart from another. Among blood-feeding arthropods, these compounds are commonly broken into three classes: short chain carboxylic acids, aldehydes and low molecular weight nitrogenous compounds. [5]

Insects have been used as a model system to study mammal and especially human olfaction. Yet, unlike vertebrates who use G protein coupled receptors (GPCRs), insects express proteins including ORs (olfactory receptors), GRs (gustatory receptors) and IRs (ionotropic receptors) which are all heteromeric ligand-gated ion channels. [3] A moth species in the order of Lepidoptera known as the black cutworm moth ( Agrotis ipsilon ) produces even more proteins including OBPs (odorant-binding proteins), CSPs (chemosensory binding proteins), and SNMPs (sensory neuron membrane proteins) that help the moth recognize sex pheromones and odorants such as those released from host plants. [6] Much like in vertebrates, axons from the sensory neurons converge into glomeruli, but differ in where the glomeruli are housed. In mammals they are located in the olfactory bulbs, but in insects they are in the antennal lobe. [7]

Evolutionary history

Olfaction is metabolically costly. The evolutionary trade-offs involved require further study because as of 2016 most such research has been done under laboratory conditions with unrealistically reliable food. [8]

Olfactory cascade

Sensory neurons in the antenna, maxillary palp, and labella generate odor-specific electrical signals called spikes (action potentials) in response to binding of odors to cell surface proteins like the olfactory receptors. The sensory neurons in the antenna and maxillary send this information via their axons to the antennal lobe, [7] while sensory neuron in the labella send this information via axons to the subesophageal ganglion. [9] Inside the antennal lobe they synapse with other neurons in semidelineated (with membrane boundaries) structures called glomeruli.

Specifically the process is as follows: first the odorant wafts towards an insect's antenna or maxillary palp which is covered with hair-like projections called sensilla. [5] The odorant then enters through tiny pores in the exoskeleton (or cuticle) of that sensillum and diffuses into the fluid between the cells called extracellular fluids. [1] There the odorant molecule binds to an odorant binding protein which transports it to a receptor [1] and co-receptor (Orco) team on the surface of the olfactory receptor neuron (ORN). [1] [3] This leads to the neuron firing an action potential down the axon. [2] This signal is sent to the antennal lobe or subesophogeal ganglion of the insects brain where it can then integrate the information with other signals from other sensilla.

These ORNs are bipolar, on one end are the olfactory dendrites with the receptors for the odors and on the other end are the axons that carry the action potential to the antennal lobe of the brain. [3] The antennal lobes have two kinds of neurons, projection neurons (mostly excitatory) and local neurons (inhibitory, with some excitatory). The projection neurons send their axon terminals to a part of the insect brain called the mushroom bodies (important in regulating learned odor responses) and another part of the brain called the lateral horn (important in regulating innate odor responses [3] ). Both of these regions are part of the protocerebrum of the insect brain.

Research methods

Action potential recordings are conducted in three different ways electroantenograms, electropalpograms, and single sensillum recordings (SSR). [5] In electroantenograms (EAG) and electropalpograms (EPG) the action potentials from the entire antenna or maxillary palp, respectively, is recorded. EAGs and EPGs provide an overall view of olfaction in the respective organ. [5] During an SSR an electrode is inserted into just one sensillum and the recording is made from only the ORNs which are contained within that sensillum, providing more detailed information [5] .

Any of these methods can be combined with a high resolution gas chromatography to isolate volatile compounds from important animals or habitats. [5] For example, this method could be used to determine which compound from a particular flower is the most attractive to a bee. Recordings from projection neurons show that in some insects there is strong specialization and discrimination for the odors presented by the ORNs. This is especially true for the projection neurons of the macroglomeruli, a specialized complex of glomeruli responsible for the pheromones detection.

Repellents and attractants

Humans exploit the insect olfactory system to control agricultural and disease carrying pests. [3] For some agricultural pests manufactured sex pheromones are placed in traps to capture adults before they can oviposit (lay their eggs) leading to the hatching of their destructive larvae. [3] While there are thousands of chemicals insects can detect there is a limited range that insects use as cues to move towards or away from the source of the odorant. [5]

The art of finding an attractant or repellent for a particular insect of interest is complicated and a long, intensive process. For example, using pheromones only attracts insects in their reproductive stage, a short period in their lives. [2] While scents of food may be attractive to hungry insects they would not be effective in a field full of a crop that is palatable to that insect. [2]

Situationally-dependent attractants / repellents

Insects use the same signal for many different uses depending on the situation this is called chemical parsimony. [5] Situations that may change how an insect behaves in reaction to a scent are things like the concentration of the compound, the life stage of the insect, its mating status, other olfactory cues, the insects feeding state (hungry or full), the time of day, or even the insects body position. [2] [3] [5] For example, Drosophila are very attracted to apple cider vinegar but in very high concentrations an additional olfactory receptor (that has low affinity for the vinegar, Or85a) is activated which changes the fly’s behavior from attraction to aversion. [3] These different behaviors to the same cue is called behavioral plasticity. [2]

Carbon dioxide

Many insects are capable of detecting very minute changes in the concentration of CO2. [5] While CO2 has been found to be an attractant in every arthropod studied [5] and it is very important in mosquito monitoring and control, even this stereotyped reaction can be plastic. Drosophila avoid CO2 when walking but move towards it when in flight. [3]

DEET

Many insects (and other arthropods) have been shown to avoid areas containing N,N-diethyl-3-methylbenzamide or DEET. They innately avoid DEET, likely because it is a “confusant” that stimulates gustatory, ionotropic, and olfactory receptors and “distorts” other odorants interaction with those receptors. [3]

See also

Related Research Articles

<span class="mw-page-title-main">Antenna (biology)</span> Paired appendages used for sensing in arthropods

Antennae, sometimes referred to as "feelers", are paired appendages used for sensing in arthropods.

<span class="mw-page-title-main">Olfactory bulb</span> Neural structure

The olfactory bulb is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway. Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance to generate a biological signal. This signal may be in the form of an action potential, if the chemoreceptor is a neuron, or in the form of a neurotransmitter that can activate a nerve fiber if the chemoreceptor is a specialized cell, such as taste receptors, or an internal peripheral chemoreceptor, such as the carotid bodies. In physiology, a chemoreceptor detects changes in the normal environment, such as an increase in blood levels of carbon dioxide (hypercapnia) or a decrease in blood levels of oxygen (hypoxia), and transmits that information to the central nervous system which engages body responses to restore homeostasis.

<span class="mw-page-title-main">Mushroom bodies</span> Pair of structures in the brains of some arthropods and annelids

The mushroom bodies or corpora pedunculata are a pair of structures in the brain of insects, other arthropods, and some annelids. They are known to play a role in olfactory learning and memory. In most insects, the mushroom bodies and the lateral horn are the two higher brain regions that receive olfactory information from the antennal lobe via projection neurons. They were first identified and described by French biologist Félix Dujardin in 1850.

<span class="mw-page-title-main">Olfactory receptor neuron</span> Transduction nerve cell within the olfactory system

An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system.

<span class="mw-page-title-main">Olfactory epithelium</span> Specialised epithelial tissue in the nasal cavity that detects odours

The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 5 cm2 (0.78 sq in) and lies on the roof of the nasal cavity about 7 cm (2.8 in) above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.

<span class="mw-page-title-main">Glomerulus (olfaction)</span>

The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells. Each glomerulus is surrounded by a heterogeneous population of juxtaglomerular neurons and glial cells.

Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants which give rise to the sense of smell. Activated olfactory receptors trigger nerve impulses which transmit information about odor to the brain. These receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs). The olfactory receptors form a multigene family consisting of around 800 genes in humans and 1400 genes in mice.

<span class="mw-page-title-main">Docking theory of olfaction</span>

The docking theory of olfaction proposes that the smell of an odorant molecule is due to a range of weak non-covalent interactions between the odorant [a ligand] and one or more G protein-coupled odorant receptors. These include intermolecular forces, such as dipole-dipole and Van der Waals interactions, as well as hydrogen bonding. More specific proposed interactions include metal-ion, ion-ion, cation-pi and pi-stacking. Interactions can be influenced by the hydrophobic effect. Conformational changes can also have a significant impact on interactions with receptors, as ligands have been shown to interact with ligands without being in their conformation of lowest energy.

<span class="mw-page-title-main">Mitral cell</span>

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs is still a matter of controversy. One prominent hypothesis is that mitral cells encode the strength of an olfactory input into their firing phases relative to the sniff cycle. A second hypothesis is that the olfactory bulb network acts as a dynamical system that decorrelates to differentiate between representations of highly similar odorants over time. Support for the second hypothesis comes primarily from research in zebrafish.

The antennal lobe is the primary olfactory brain area in insects. The antennal lobe is a sphere-shaped deutocerebral neuropil in the brain that receives input from the olfactory sensory neurons in the antennae and mouthparts. Functionally, it shares some similarities with the olfactory bulb in vertebrates. The anatomy and physiology function of the insect brain can be studied by dissecting open the insect brain and imaging or carrying out in vivo electrophysiological recordings from it.

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.

Olfactory fatigue, also known as odor fatigue, olfactory adaptation, and noseblindness, is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound. For example, when entering a restaurant initially the odor of food is often perceived as being very strong, but after time the awareness of the odor normally fades to the point where the smell is not perceptible or is much weaker. After leaving the area of high odor, the sensitivity is restored with time. Anosmia is the permanent loss of the sense of smell, and is different from olfactory fatigue.

Odorant-binding proteins (OBPs) are small soluble proteins secreted by auxiliary cells surrounding olfactory receptor neurons, including the nasal mucus of many vertebrate species and in the sensillar lymph of chemosensory sensilla of insects. OBPs are characterized by a specific protein domain that comprises six α-helices joined by three disulfide bonds. Although the function of the OBPs as a whole is not well established, it is believed that they act as odorant transporters, delivering the odorant molecules to olfactory receptors in the cell membrane of sensory neurons.

Dysosmia is a disorder described as any qualitative alteration or distortion of the perception of smell. Qualitative alterations differ from quantitative alterations, which include anosmia and hyposmia. Dysosmia can be classified as either parosmia or phantosmia. Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers. Phantosmia is the perception of an odor when no odorant is present. The cause of dysosmia still remains a theory. It is typically considered a neurological disorder and clinical associations with the disorder have been made. Most cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. Dysosmia tends to go away on its own but there are options for treatment for patients that want immediate relief.

Kenyon cells are the intrinsic neurons of the mushroom body, a neuropil found in the brains of most arthropods and some annelids. They were first described by F. C. Kenyon in 1896. The number of Kenyon cells in an organism varies greatly between species. For example, in the fruit fly, Drosophila melanogaster, there are about 2,500 Kenyon cells per mushroom body, while in cockroaches there are about 230,000.

<span class="mw-page-title-main">Sense of smell</span> Sense that detects smells

The sense of smell, or olfaction, is the special sense through which smells are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.

The lateral horn is one of the two areas of the insect brain where projection neurons of the antennal lobe send their axons. The other area is the mushroom body. Several morphological classes of neurons in the lateral horn receive olfactory information through the projection neurons.

Or83b, also known as Orco, is an odorant receptor and the corresponding gene that encodes it. The odorant receptor Or83b is not exclusively expressed in insects. Though its actual function is still a mystery, the broadly expressed Or83b has been conserved across highly divergent insect populations across 250 million years of evolution.

<span class="mw-page-title-main">Reinhard F. Stocker</span> Swiss biologist

Reinhard F. Stocker is a Swiss biologist. He pioneered the analysis of the sense of smell and taste in higher animals, using the fly Drosophila melanogaster as a study case. He provided a detailed account of the anatomy and development of the olfactory system, in particular across metamorphosis, for which he received the Théodore-Ott-Prize of the Swiss Academy of Medical Sciences in 2007, and pioneered the use of larval Drosophila for the brain and behavioural sciences.

References

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  2. 1 2 3 4 5 6 Gadenne, Christophe; Barrozo, Romina B.; Anton, Sylvia (2016). "Plasticity in Insect Olfaction: To Smell or Not to Smell?". Annual Review of Entomology. 61: 317–333. doi:10.1146/annurev-ento-010715-023523. hdl: 11336/19586 . PMID   26982441.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 Li, Qian; Liberles, Stephen D. (2015). "Aversion and Attraction through Olfaction". Current Biology. 25 (3): R120–R1209. doi:10.1016/j.cub.2014.11.044. PMC   4317791 . PMID   25649823.
  4. ↑ Yadav, Pratibha; Borges, Renee M. (2017). "The insect ovipositor as a volatile sensor within a closed microcosm". Journal of Experimental Biology. 220 (9): 1554–1557. doi: 10.1242/jeb.152777 . PMID   28468812.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 Syed, Zainulabeuddin (2015). "Chemical ecology and olfaction in arthropod vectors of diseases". Current Opinion in Insect Science. 10: 83–89. doi:10.1016/j.cois.2015.04.011. PMID   29588018.
  6. ↑ Gu, Shao-Hua; et al. (2014). "Molecular Characterization and Differential Expression of Olfactory Genes in the Antennae of the Black Cutworm Moth Agrotis ipsilon". PLOS ONE. 9 (8): e103420. Bibcode:2014PLoSO...9j3420G. doi: 10.1371/journal.pone.0103420 . PMC   4118888 . PMID   25083706.
  7. 1 2 Wilson, Rachel (2013). "Early olfactory processing in Drosophila: mechanisms and principles". Annual Review of Neuroscience. 36: 217–241. doi:10.1146/annurev-neuro-062111-150533. PMC   3933953 . PMID   23841839.
  8. ↑ Gadenne, Christophe; Barrozo, Romina B.; Anton, Sylvia (2016-03-11). "Plasticity in Insect Olfaction: To Smell or Not to Smell?". Annual Review of Entomology . Annual Reviews. 61 (1): 317–333. doi:10.1146/annurev-ento-010715-023523. hdl: 11336/19586 . ISSN   0066-4170. PMID   26982441. S2CID   207568844.
  9. ↑ Riabinina, Olena; Task, Darya; Marr, Elizabeth; Lin, Chun-Chieh; Alford, Robert; O'Brochta, David A.; Potter, Christopher J. (2016-10-03). "Organization of olfactory centres in the malaria mosquito Anopheles gambiae". Nature Communications. 7: 13010. Bibcode:2016NatCo...713010R. doi:10.1038/ncomms13010. PMC   5063964 . PMID   27694947.
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