Mosquitoes on the hunt: the role of olfaction in finding human hosts

Rohitesh Gupta & Debraj Manna

A recent study led by Nitin Gupta, Associate Professor, Indian Institute of Technology (IIT), Kanpur, have questioned how mosquitoes can detect hundreds of odourants to find human hosts. In contrast to dedicated neural pathways for every odourant, the team of researchers discovered that a combinatorial action of multiple neurons for each odourant is the key to solving the puzzle. 

Mosquitoes on the hunt: the role of olfaction in finding human hosts. Image for reference only.
Mosquitoes on the hunt: the role of olfaction in finding human hosts. Image for reference only. 

While large wild animals seem deadly and ferocious, one of the major causes of death in humans is a tiny insect primarily found in tropical and subtropical regions. Yes, you guessed it right! Mosquitoes cause a large number of deaths in most low & middle-income countries. In India alone, yearly, millions of people get infected by mosquito-borne diseases such as malaria, dengue, Zika, chikungunya, etc.

Aedes aegypti is a mosquito species that senses its host by temperature, humidity, visual cues, and odour. Mainly, odour released from the skin or found in the breath plays a vital role in guiding mosquitoes to their host. In this study led by Nitin Gupta, Associate Professor, IIT Kanpur, the primary focus has been to unravel whether insect olfaction or sense of smell” is represented by an alternative mechanism as opposed to the commonly understood process of odour-encoding by dedicated pathways (labelled lines) in neuronal circuits. 

The labelled lines approach activates a dedicated neural pathway for each odour an organism responds to. However, Gupta pointed out, For a large number of odours, the labelled lines approach does not apply; thus, it is necessary to know how neurons in the mosquito brain, particularly in the smell-processing centres of the brain such as the antennal lobe [the main brain area responding to smell in insects], respond innately to different smells”.

The insect antennal lobe has two types of neurons: projection and local neurons. While local neurons interact laterally with the antennal lobe, the projection neurons carry the output [signal] from the antennal lobe to the deeper brain regions involved in encoding olfactory behaviours,” explained Pranjul Singh, the first author of the study. 

The authors developed an in vivo patch clamp electrophysiology technique to determine the morphological and physiological properties of the antennal lobe. This technique allowed exposure of the mosquitoes’ olfactory systems to various odours while giving access to their brains for electrical recording. This technique also helped visualise individual neuronal morphology in response to multiple odours. The group studied the brains of 1250 mosquitoes to accomplish this task.

Picture Credits: Singh et al (From Gupta lab)
Picture Credits: Singh et al (From Gupta lab)

The results from this technique helped identify a detailed analysis of projection neuron activity. The researchers found that each odourant responded to multiple projection neurons and vice-versa. Among the local neurons that responded to odours, some were associated with a neurotransmitter, gamma-aminobutyric acid. This association suggested excitatory or inhibitory crosstalk of local neurons with projection neurons, which could have behavioural implications. Since both local and projection neurons responded to various odours, the researchers concluded that the mosquito odour response followed a combinatorial code.

The authors didn’t stop here but custom-designed a T‑maze behavioural assay to correlate the projection neuron pattern to the insect’s behavioural preference. Different odours were presented to the insect to identify its attractive or repulsive action. The authors found that an individual projection neuron may respond to multiple odours, but their activity is unique as a group. This suggests that projection neurons follow unique patterns to discriminate each scent.

Can we adopt mosquitoes as a model organism to study vector-borne disorders? Mosquitoes are becoming established as a model organism, not only for studying malaria, dengue, etc. but also to understand the basic biology of these insects. Another advantage of experiments on mosquitoes is that the data obtained enable comparative studies with Drosophila,” said Gupta.

All experiments in this study were conducted on female mosquitoes because only they bite hosts. It would be interesting to address how blood-fed females respond to certain human odours differently than male mosquitoes,” said Gaurav Das, Scientist, NCCS, Pune, who studies behaviour in flies and is not associated with the study. According to him, other open questions that need answers are whether mosquitoes can distinguish between two humans based on their smell and could one blood source be preferable to another. Ram Swaroop Gupta, a retired Paediatrician at MBS Hospital, Rajasthan, added, As of today, there is no speciality treatment for dengue and chikungunya, and no vaccination is available to cure mosquito-borne diseases. This study will help in developing newer and safer repellents, further preventing humans from mosquito-borne diseases”.