Dopamine modulates odour learning in mosquitoes

Researchers have long been fascinated with figuring out how mosquitoes choose a host to feed on and how they can change their preferences. One important missing piece of the puzzle may be the way that mosquitoes learn different smells. Gabriella Wolff and colleagues from the University of Washington, USA, had previously found that yellow fever mosquitoes could form memories associated with odours related to their favourite food – blood – but not with odours that had nothing to do with their life history. These mosquitoes could be trained to avoid those odours by associating them with swatting. However, it was still unknown whether other mosquito species could learn to avoid the same odours and whether dopamine, a chemical messenger important for learning, could be responsible for this olfactory learning. To figure this out, the team set out to train four different species of mosquitoes.Wolff and colleagues exposed the mosquitoes to the scents of human skin, chicken feathers and rose oil. Two of the mosquito species feed on humans [the yellow fever mosquito (Aedes aegypti) and the Asian malaria mosquito (Anopheles stephensi)], another, the southern house mosquito (Culex quinquefasciatus), feeds on birds, and the elephant mosquito (Toxorhynchites amboinensis) dines on flowers. The team found that all the mosquito species had the strongest attraction to the scent of the hosts that they preferred to feed on.To test whether multiple mosquito species could learn to recognize the same odours, the team trained female mosquitoes from each species to avoid different scents by exposing them to scent components from human odours (octenol and hexanoic acid) or a flower scent component (linalool), regardless of whether they naturally feed on humans or flowers, and then they mimicked swatting at the insects to make the scent unattractive. The team then investigated each mosquito’s preference by putting them into a maze that provided them with a choice between either a host odour (octenol, hexanoic acid or linalool) or no scent. The team found that the mosquitoes learned to avoid the odours associated with their favourite food, but not other odours. For example, the yellow fever mosquito, which dines on people, learned to avoid octenol and hexanoic acid, but not linalool. Meanwhile, nectar-feeding elephant mosquitoes only learned to avoid linalool, but not octenol and hexanoic acid.To see whether these differences in species learning were due to the effects of dopamine, the researchers investigated where dopamine was localized in the brain of each of these species. Wolff and colleagues found that dopamine was localized in the antennal lobe and mushroom bodies – structures in the brain that are important for learning and memory. However, the dopamine was localized in different areas of these structures from species to species, which could be important for differences in their learning abilities. To see whether dopamine was necessary for this olfactory learning, the team also trained yellow fever mosquitoes and the Asian malaria mosquitoes, which feed on mammals, to avoid an odour associated with their preferred diets, and found that when they blocked the dopamine receptor, they eliminated their ability to learn to avoid the odour. Last, the team found that areas in the antennal lobe that had an increased response to these scents also had a raised level of dopamine.Learning the basis of how mosquitoes smell and the impact on their host preference is vital, as the insects can be responsible for the spread of some terrible diseases. The researchers highlight that being able to change dopamine inputs without having to change brain structure may give mosquitoes an evolutionary advantage and could be a way for mosquitoes to change their host preferences quickly.

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