By Justine Rouge

SOURCE: Modulation of host learning in Aedes aegypti mosquitoes, Vinauger et al. 2018

Mosquitoes are infectious diseases vectors, well-known for their tendency to prefer certain host species more than others [1-3] in order to feed.

However, some environmental factors such as the migration can modify their host preference [2] once their sanguine host is no longer available [4-6].

Indeed, certain mosquito species such as Culex tarsalis feed primarily on birds during the summer but on both mammals and birds during the winter, while Aedes. aegypti [7-8] prefers to feed on human hosts even though they can show a variable preference for other host species [9-10].

Other factors like the defensive behaviour of their hosts can oblige mosquitoes to adapt themselves and to modify their preference in order to survive.

When they encounter a defensive host, mosquitoes are exposed to mechanical perturbations, similar to a slapping effect that can be perceived as negative reinforcement by the insect when associated with other host-related cues such as host odours.

These shifts are driven by genetic and physiological factors but also by the mosquitoes’ olfactive learning experiences [6].

In other insects, it was shown that learning and memory are guided by several neurotransmitters* such as the octopamine and the dopamine [11-15].

However, the links between of mosquitoes experience and learning mechanisms influencing host preferences, and the neurophysiological bases for learning, remained unclear until the team of JA Riffell from the University of Washington (USA) studies the question in Aedes aegypti mosquitoes.

Results were published in Current Biology [16].

In the first instance, in order to know if aversive learning could modify the mosquitoes host preference, researchers have trained these mosquitoes with a Pavlovian conditioning paradigm** composed by the association of 2 stimuli :

1) A conditioned stimulus (CS) displayed by human odour or animal odours from rat or chicken.

2) An unconditioned aversive stimulus displayed by a mechanical shock.

This stimulus was reproduced via a vortexer simulating mechanical vibrations that a mosquito can feel when confronting to a defensive host who tries to swat it, a behaviour similar to a slap.

Then, the behavioural and olfactive answers of this mosquitoes were evaluated via 2 experiments using a Y-maze olfactometer with two choice arms.

During the first experiment, one of the arms delivered a control scent and the other delivered a human host odour while during the second experiment, one of the arms delivered a rat odour and the other delivered a chicken odour.

For the first experiment, results showed that trained mosquitoes have quickly learnt the association between the host odour and the mechanical shock, avoiding the human host odour, contrary to the naïve control mosquitoes. This suggests that the previous conditioning induced an active decision-making behaviour in the choice of these mosquitoes which developed a sort of avoidance memory.

In the second experiment, the control mosquitoes were attracted by the odour of the two host species.

However, while mosquitoes trained against rat odour avoided this latter during the olfactive test, the choice of mosquitoes trained against chicken odour was not affected by the conditioning, showing a disability to learn the association between the aversive stimulus with this animal odour.

Similar results were obtained with Rhodnius prolixus, a kissing bug, vector of Chagas disease [17].

A possibility to explain these results is the fact that odours which are released by hosts (human or animal) are complex mixtures of hundreds of odorants. Some of them like octenol is very attractive for mosquitoes and are present in mammals [18-20] but absent from birds, suggesting that mosquitoes would be able to learn when this type of odorant is associated with an aversive stimulus such as a mechanical shock.

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Because of dopamine is implied in the learning of aversive stimuli in other insects, researchers tested in a second time if this neurotransmitter is also implied in the aversive learning in mosquitoes.

After the use of different approaches to inactivate the dopamine receptors of mosquitoes, followed by an aversive conditioning against the octenol (CS), mosquitoes whose the dopamine receptors were manipulated showed significant deficits in their learning abilities contrary to the control mosquitoes, suggesting that the dopamine is a key factor for the aversive learning in mosquitoes.

Thereafter, the immunohistochemistry analyse made to examine dopaminergic innervation within mosquito brain showed that dopaminergic neurons are particularly concentrated in the glomeruli*** of the antennal lobes (ALs) and lateral protocerebrum**** (Figure A), which are important units for the olfactive learning and memory in insects [21, 22].

Figure A:

Schematic of the Ae. aegypti brain superimposed on a scanned electron microscope image. Highlighted regions include the AL (multicoloured to represent individual glomeruli that receive input from olfactory receptor neurons) and the mushroom bodies (MB), implicated in learning and memory.

SOURCE: Modulation of host learning in Aedes aegypti mosquitoes, Vinauger et al. 2018

Moreover, results showed that dopaminergic innervation is heterogeneous in the AL (Figure B), with some glomeruli being more innervated than others, including the glomerulus that receives input from the octenol-sensitive neuron, which could help mosquitoes to process, and selectively learn, specific odorants from a complex bouquet emitted from individual hosts.

Figure B:

Confocal micrograph of a Ae. aegypti brain stained with antibodies against tyrosine hydroxylase (cyan) shows heterogeneous innervation of dopaminergic neurons across antennal lobe glomeruli.

SOURCE: Modulation of host learning in Aedes aegypti mosquitoes, Vinauger et al. 2018

According to the electrophysiological recordings, this process could be explained by the fact that AL represented the odorants by chemical class and activity level on the one hand, and by the neuromodulatory activity of the dopamine which increased the discrimination of certain odorants in the different glomeruli of the AL on the other hand.

In this way, dopamine would allow the mosquitoes ability to better discriminate and avoid their potential “predator” host.

These results are a new hope for the identification of the genetic and neural bases influencing the flexibility of host preference in mosquitoes and could take part in the future vector control strategies, and therefore, in order to limit the transmission of infectious diseases.

Thanks for reading

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Notes

* Neurotransmitter: Substance that transmits nerve impulses across a synapse, a contact area between 2 neurons or between a neuron and a cell.

** Pavlovian conditioning: A process of behaviour modification by which a subject comes to respond in a desired manner to a previously neutral stimulus that has been repeatedly presented along with an unconditioned stimulus that elicits the desired response.

*** Glomeruli: Neural network found in the olfactory bulb invertebrate, or antennal lobes in insects.

**** Antennal lobes: Units similar to the olfactory bulb in the vertebrate. They are organized in different glomeruli.
Protocerebrum: Anterior part of the brain in insects innervating the compound eyes.
These units are implied in the learning and memory of insects.

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