Mosquito Olfactory Receptor Neurons and Host Seeking Behavior

Joseph Brodsky, Allison Parker, Victoria Wong

I. Introduction

In this podcast we will discuss mosquito olfactory receptor neurons and their essential role in allowing female mosquitoes to find a host bloodmeal.

II. Podcast

Click here to listen to the audio file via SoundCloud.

III. Transcript

RHEA: Good morning, and welcome to our Insect Physiology podcast! Today, we will be discussing mosquito olfactory receptor neurons and their essential role in allowing female mosquitoes to find a host bloodmeal.

JOSEPH: Have you ever noticed that certain people tend to be bitten by mosquitoes more than others? It might be because they have smelly feet! One study showed that mosquitoes find the smell of stinky feet attractive. The scientists used naked, motionless, human volunteers to see where mosquitoes tended to bite humans the most. The results showed that mosquitoes tend to bite around the feet and ankles more than any other location on the body. They then tested if mosquitoes were attracted to Limburger cheese because it contains a microorganism that is similar to one we have between our toes that is the culprit for both the smell in the cheese and our feet. And they were! This study may seem silly, but it is rooted in the mosquito’s physiology; more specifically within their nervous system.

A female yellowfever mosquito probes a piece of Limburger cheese, one of few known mosquito attractants. Photo by Peggy Greb. (ars.usda.gov)

RHEA: That’s right. Female mosquitoes use odors to find their hosts. Even though they do not have noses like humans or birds do, mosquitoes use their antennae and maxillary palps to “smell.” Mosquitoes have olfactory receptor neurons that allow them to detect certain compounds. Olfactory receptor neurons are located on cuticular sensilla that are present on their antennae and maxillary palps. There are five morphologically different types of sensilla. Trichoid sensilla and grooved peg sensilla account for 90% of the sensilla found on mosquito antennae. Each sensillum contains one to three olfactory receptor neurons. Each antenna of a mosquito contains eight hundred trichoid sensilla, and house approximately sixteen hundred olfactory receptor neurons!

JOSEPH: Axons from sensory neurons converge into glomeruli in the antennal lobes. Glomeruli are areas of the brain that are full of synaptic contacts. One study identified 18 different ORN types that are located in 10 different types of sensilla and an additional 4 ORNs that have been non responsive in studies. Each ORN type targets a single glomerulus, which means there are at least 22 total olfactory glomeruli that receive ORN axons.

RHEA: Different parts of the maxillary palps and antennae respond to different compounds. ORNs located on grooved peg sensilla on the antennae respond to lactic acid and ammonia while trochid sensilla respond to carboxylic fatty acids. Clubbed-shaped, capitate peg sensilla on the maxillary palps detect carbon dioxide.

JOSEPH: That’s a lot of olfactory receptor neurons! No wonder mosquitoes are so good at find hosts! In most insects, ORNs typically respond to a single odor compound, but research has shown that ORNs in mosquitoes are able to respond to multiple compounds! Female mosquitoes are attracted to carbon dioxide, acetone, and many compounds resulting from sweat including ammonia, lactic acid, indole and carboxylic fatty acid to name a few.

RHEA: Let’s talk more about carbon dioxide detection. ORNs of female mosquitoes respond to small increases in carbon dioxide concentrations compared to background carbon dioxide amounts. Females can respond to a single, low concentration pulse of carbon dioxide that is given off from a host.

JOSEPH: Did you know that in a recent paper, the same receptors that detect carbon dioxide also are able to detect compounds given off by human skin? Not only do these ORNs detect multiple odorants, but the olfactory receptors on these neurons don’t work quite like the other ORs. In insects, ORs are odorant-gated ion channels that require a coreceptor called orco to function. The CO2 detecting neurons express three olfactory receptors that combined detect CO2. However, even without orco, mosquitoes can find hosts with CO2, although they have trouble telling the difference between humans and other hosts. Thus, mosquitoes use a combination of odors to find their preferred hosts. That means these particular sensilla are very important in future behavioral research on mosquito host seeking behavior.

RHEA: That’s so interesting. I was just thinking about the practical applications for this research. Mosquitoes are vectors of many diseases including malaria, west nile virus and dengue virus. Knowing the specific compounds that are attractive to mosquitoes allows scientists to create effective traps that can be used to decrease the mosquito population in areas of disease outbreak, as well as, allow researchers to sample mosquitoes for disease prevalence. The more we know about olfactory receptor neurons, the better able we are to combat mosquito disease transmission around the globe.

IV. References

Anton, S., van Loon, JJ., Meijerink, J., Smid, HM., Takken, W., Rospars, JP. (2003). Central projections of olfactory receptor neurons from single antennal and palpal sensilla in mosquitoes. Arthropod structure & development, 32(4), 319-327.

Bosch, OJ., Geier, M., Boeckh, J. (2000) Cntribution of fatty acids to olfactory host finding of female Aedes aegypti. Journal of Chemical Senses. 25(3); 323-330.

Ghaninia, M., Ignell, R., Hansson, BS. (2007). Functional classification and central nervous projections of olfactory receptor neurons housed in antennal trichoid sensilla of female yellow fever mosquitoes, Aedes aegypti. European Journal of Neuroscience, 26(6), 1611-1623.

Knols, BJK. (1996) On human odour, malaria mosquitoes, and Limburger cheese. The Lancet. 384(9037); 1322.

Martin, JP., Beyerlein, A., Dacks, AM., Reisenman, CE., Riffell, JA., Lei, H., Hilderbrand, JG. (2011) The neurobiology of insect olfaction: Sensory processing in a comparative context. Progress in Neurobiology. 95(3); 427-447.

Tauxe, GM., MacWilliam, D., Boyle, SM., Guda, T., Ray, A. (2013). Targeting a Dual Detector of Skin and CO2 to Modify Mosquito Host Seeking. Cell, 155(6), 1365-1379.

Tchouassi, D. P., Sang, R., Sole, C. L., Bastos, A. D., Teal, P. E., Borgemeister, C., & Torto, B. (2013). Common host-derived chemicals increase catches of disease-transmitting mosquitoes and can improve early warning systems for rift valley fever virus. PLoS neglected tropical diseases, 7(1), e2007.