How do mosquitoes make sense of the hundreds of odorants present in human smell? We report @NatureComms a combinatorial code for odors in the #mosquito #brain. Experiments led by @pranjul8765 @IITKanpur and funded by @India_Alliance @serbonline @IndiaDST🧵
nature.com/articles/s41467-0…
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2. #Mosquitoes find humans for biting using a variety of cues: smell of our skin, carbon dioxide, body heat, humidity, and visual cues. Among these, the smell plays an important role. Mosquitoes also use their sense of smell for finding other sources of food or egg-laying sites.
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3. For some ecologically relevant odorants, insects have dedicated neural pathways (“labeled lines”) from the sensory neurons to deeper areas in the brain, hardwired for specific behaviors.
Jun 15, 2023 · 8:04 PM UTC
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4. But the “labeled line” mechanism seems inadequate for the large number of different odorants that mosquitoes can detect and for each of which they have specific innate preferences. Another alternative is a combinatorial code.
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5. To test these ideas, we developed an experimental approach to record the electrical activity of two types of neurons (PNs and LNs) in the antennal lobe, which is an important region for the processing of smells in the insect brain.
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6. With dye-fills, imaging, and luck, we could see the structures of some of the recorded neurons, and therefore assign them identities using a reference atlas by @RickardIgnell and @billhansson. We also identified different sub-types of PNs and LNs in the mosquito antennal lobe.
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7. Overall, we obtained recordings along with structural identification from 208 PNs and 53 LNs. But successful recordings did not come easy. We poked the brains of nearly 1250 mosquitoes in the process!
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8. Dye-fills don’t always work. Using our large dataset of PNs and LNs, we found that even in the absence of a dye-fill, it is possible to tell apart PNs from LNs just by looking at their electrophysiological properties.
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9. So how do these neurons represent different odorants? We found a given PN could respond to multiple odorants and a given odorant could activate multiple PNs. The temporal patterns of spikes varied for different PN-odorant combinations.
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10. LNs also responded broadly to many odorants. We found some LNs are GABA-positive and some GABA-negative. Thus, LNs can potentially provide both excitatory and inhibitory crosstalk between PNs.
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11. We found evidence for excitatory crosstalk between two PNs that were recorded simultaneously, in a herculean experiment by @pranjul8765. Current injection in one PN increased the firing rate of the second PN, but not vice-versa.
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12. The fact that PNs and LNs responded broadly to odors suggests a combinatorial code for odors. We could classify odorants using responses of PN populations with good accuracy, which improved with the number of PNs used in the classification analysis.
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13. Next we wanted to check the relationship between the PN population response and the behavioral preference elicited by an odor in mosquitoes. We designed a custom T-maze behavioral assay to reveal both attractive and aversive preferences.
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14. By comparing the behavioral data and PN responses for different pairs of odorants, we found that behavioral similarity and PN response similarity were moderately correlated. These correlations increased as more PNs were used in the analysis.
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15. In summary, our results suggest that the odor identity and its valence are encoded in the PN population response in the antennal lobe. Future studies including more odorants, at more concentrations, and more neurons will help in refining these ideas further.
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16. Huge effort by Pranjul Singh @pranjul8765, with help from Shefali Goyal, Smith Gupta, Abhinav Tiwari, Varad Rajput @varadsaynothing, Alex Bates @as_bates and Arjit Gupta. Thanks to @BSBEIITK1 and @mfcem_iitk for infrastructure.
How do mosquitoes make sense of the hundreds of odorants present in human smell? We report @NatureComms a combinatorial code for odors in the #mosquito #brain. Experiments led by @pranjul8765 @IITKanpur and funded by @India_Alliance @serbonline @IndiaDST🧵
nature.com/articles/s41467-0…
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