Freeze. I’ve got a chemical

Flies using the FlyWalker. Top: normal walking at around 25 mm/s. Bottom: fly with its VNC serotonin neurons stimulated, which slows its speed to 15 mm/s.
CREDIT: Clare Howard/Mann Lab/Columbia University’s Zuckerman Institute

By Nick Carne

What causes that “deer-in-the-headlights” reflex, where people freeze momentarily in response to a potential threat?

If we’re anything like flies, the answer could be the chemical serotonin.

US researchers have discovered that when a fly experiences an unexpected change in its surroundings, such as a sudden vibration, release of serotonin helps to temporarily stop it in its tracks.

These findings, published in the journal Current Biology by a team from Columbia University, may, they say, offer broad insight into the biology of the startle response, which has been observed in virtually every animal studied to date, from flies to fish to people.

Serotonin is most closely associated with regulating mood and emotion, but previous research on flies and vertebrates has shown it can also affect the speed of an animal’s movement. 

In the new study, Richard Mann and colleagues wanted to try to understand how and why.

They first analysed fruit fly steps using FlyWalker, an apparatus developed by Mann and physicist Szabolcs Marka to track an insect’s steps on a special type of glass. 

After monitoring how the flies moved, they manipulated the levels of serotonin and another chemical, dopamine, in the fly’s ventral nerve cord (VNC), which is analogous to the vertebrate spinal cord.

Initial results revealed that activating neurons that produce serotonin in the VNC slows flies down, while silencing those same neurons speeds flies up. 

Further experiments showed serotonin levels could impact the insects’ walking speed under a variety of conditions, including different temperatures, when the flies were hungry, or while they walked upside down, all situations that normally affect walking speed. {%recommended 9928%}

“We witnessed serotonin’s biggest effects when the flies experienced rapid environmental changes,” says first author Clare Howard. “In other words, when they were startled.”

To further investigate, the team elicit a fly’s startle response. In two ways: a total blackout and a simulated earthquake. 

“We found that when a fly is startled in these scenarios, serotonin acts like an emergency brake; its release is needed for them to freeze, and that part of this response may be a result of stiffening both sides of the animal’s leg joints,” says Mann. “This co-contraction could cause the brief pause in walking, after which the insect begins to move.” 

They also noticed that even though the fly’s response in both scenarios was to cause an immediate pause, their subsequent walking speeds differed significantly.

“After being startled in the blackout scenario, the fly’s gait was slow and deliberate,” Howard says. “But the earthquake caused the flies to walk faster after the initial pause.”

While these findings are specific to fruit flies, the ubiquity of serotonin and the startle response provides clues as to the chemical and molecular processes that occur when more complex animals, including people, get startled, the researchers say.

Going forward, the researchers hope to further investigate serotonin’s role in movement, as well as what other factors may be at play.

“As we and others continue to investigate, we hope to develop a detailed, molecular blueprint for locomotion that can be applied broadly to other animals, perhaps even people,” says Mann.