A recent study in Nature conducted an experiment to understand the response of the sleeping fruit fly (Drosophila melanogaster) to olfactory or smell stimuli, and the results were quite telling. Since its first use in 1910, D. melanogaster has become one of the most commonly used model organisms for experiments in genetics, one of the reasons being the ease with which it can be bred in even the most basic of laboratories.

Nobody knows what the function of sleep is, but we know that it cannot be achieved without disconnecting us from the external world. Sleep is not compatible with a fully conscious experience. Why? We investigate this in our latest paper on @nature 🧵1/25https://t.co/0wh3fOtknw pic.twitter.com/9zQebr5mV1

— Giorgio Gilestro (@giorgiogilestro) September 29, 2021

The first author of the study Alice French, from the Department of Life Sciences at Imperial University, said in a release: “Sleep puts you in a vulnerable position, such as being at risk of predation. Therefore, we need to be able to respond to potential threats so that we wake up and act. If it’s a sound, like a loud bang, the processing our brains need to do is relatively simple. However, to actively decode the sounds and smells around us that may or may not be relevant to us, different parts of the brain must remain alert.”

The team found that much like humans, fruit flies are able to discern the nature of the stimulus.

Vinegar experiment

The experiment involved observing fruit flies that were asleep being treated with bubbles of vinegar (acetic acid) solution of varying concentrations.

Another parallel experiment treated asleep fruit flies with odourless bubbles in order to assess a baseline response, with which to compare the former group.

Acetic acid was chosen for the study as it is a strong attractant for the fruit fly. At lower concentrations, acetic acid can stimulate the expulsion of the egg from the oviduct and also promotes gathering. However, at higher concentrations, acetic acid mimics the smell of rotten food and can be quite repugnant to fruit flies.

When fruit flies were exposed to varying concentrations of acetic acid – attractive (1-5%), repulsive (10%), and neutral (10%) – they exhibited different reactions.

Attractive odours elicited the strongest response while the repulsive and neutral ones triggered an inferior response.

The response also is contingent on the time of the day. Fruit flies experience deep sleep early in the night, but light sleep later at night. Quite expectedly, their awakening in response to the stimuli was more likely later at night than earlier.

Drunk vs sleep-deprived state

In order to broaden the ambit of the study beyond just acetic acid, further experiments were conducted with 12 more compounds, in different concentrations (i.e. 26 odour conditions in all).

Then the same was tested on sets of fruit flies that were either sleep deprived, starved, or sedated with ethanol vapours. This was done in order to assess the relationship between the response and the internal state of the flies.

It was observed that flies became less responsive to stimuli when sedated with alcohol than they ‘would have woken them up when they were sober’.

Similarly, sleep deprivation had a similar effect in that flies subjected to ‘forced wakefulness’ were less responsive to attractive odours (1-5% acetic acid) than their rested counterparts.

A fly brain with the neurons involved in sleep-smelling highlighted. (www.imperial.ac.uk)

What is happening in the fruit fly’s brain?

The study also sheds light on an intricate neural architecture that governs sense perception, one that discriminates between important and irrelevant stimuli, depending on our internal state.

Two parts of the fly brain are involved here:

*the mushroom body, which plays an important role in insect olfaction or sensing of smells
*the fan-shaped body, which plays a crucial role in sleep

A set of neurons transmits the output from the mushroom body to neurons in the fan-shaped body. This mechanism, the authors suggest, could underpin the ability of insects to sense olfaction while asleep.

Further, the paper identifies two ‘gates’ that are responsible for governing this behaviour. The first ‘gate’ consists of a set of neurons that detects odour, while the second one releases dopamine and forms connections with the output neurons of the mushroom body. Suppression of either of these two gates markedly reduced the response of flies to food stimuli.

When such serial awakening experiments in humans were conducted in the 1960s, humans exhibited similar behaviour. “We can remain sound asleep in front of a TV playing an action movie, and yet wake upon the perception of a quieter but relevant stimulus, such as the sound of our own name being called or a baby cooing,” the paper states.

Other organisms subject to these experiments also showed a similar pattern. Dogs, while asleep, woke up to relevant acoustic stimuli such as food reward; cats were also able to filter out irrelevant stimuli from insignificant ones, an observation shared by rats as well.

– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)