Why do we freeze when we get scared? New study on flies points to serotonin

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A Columbia University study on fruit flies has identified serotonin as a chemical that triggers the body's startle response, the automatic reflection of deer in the headlights that freezes the body momentarily in response to a potential threat. Today's study reveals that when a fly experiences an unexpected change in its environment, such as a sudden vibration, the release of serotonin helps to stop the fly literally, and temporarily.

These findings, published today in Current biology, offer a broad view of the biology of the startle response, an omnipresent, but mysterious phenomenon, which has been observed in virtually all animals studied to date, from flies to fish and people.

"Imagine sitting in your living room with your family and, suddenly, the lights go out or the ground begins to shake," said Richard Mann, PhD, principal investigator at Mortimer B. Zuckerman Mind Brain Behavior of Columbia. Institute and lead author of the article. "Your response, and that of your family, will be the same: it will stop, freeze and then move to a safe place. With this study, we show in flies that a rapid release of chemical serotonin in your nervous system drives that initial freeze And because serotonin also exists in people, these findings shed light on what may be happening when we get scared too. "

In the brain, serotonin is most closely associated with the regulation of mood and emotions. But previous research on flies and vertebrates has shown that it can also affect the speed of movement of an animal. The initial objective of the Columbia researchers was to better understand how the chemical achieved this.

The team first analyzed the steps of the fruit fly using FlyWalker, an apparatus developed by Dr. Mann and the physicist from Columbia Szabolcs Marka, PhD, to track the steps of an insect in a special type of glass. After monitoring how the flies moved, the scientists manipulated the levels of serotonin, and another chemical called dopamine, in the ventral nerve cord (VNC) of the fly, which is analogous to the spinal cord of vertebrates.

Their initial results revealed that the activation of the neurons that produce serotonin in the VNC slows the flies, while silencing those same neurons accelerates the flies. Additional experiments showed that serotonin levels could affect the walking speed of insects in a wide variety of conditions, including different temperatures, when flies were hungry or while walking upside down, all situations that normally affect walking speed.

"We witnessed the greatest effects of serotonin when flies experienced rapid environmental changes," said Clare Howard, PhD, first author of the article. "In other words, when they were surprised."

To continue investigating, the research team devised two scenarios to elicit a fly's startle response. In the first, they turned off the lights: a total blackout for insects. For the second, they simulated an earthquake.

To achieve this, the scientists partnered with Tanya Tabachnik, Director of Advanced Instrumentation at the Zuckerman Institute in Columbia. Tabachnik's team of engineers and engineers works with scientists to design and build custom systems for their research. For this study, they created a miniature sand, the size of a fly, perched on specialized vibrating motors. Adjusting the motor force produced the desired seismic effect. When researchers exposed flies to blackout or earthquake scenarios, they also manipulated the fly's ability to produce serotonin.

"We discovered that when a fly is startled in these scenarios, serotonin acts as an emergency brake; its release is necessary for them to freeze, and that part of this response may be the result of stiffness on both sides of the joints of the paws of the animal. " said Dr. Mann, who is also the Higgins Professor of Biochemistry and Molecular Biophysics (in Systems Biology) at the Vagelos College of Physicians and Surgeons of Columbia. "This co-contraction could cause a brief pause while walking, after which the insect begins to move."

"We believe this pause is important," Dr. Howard added, "it could allow the fly's nervous system to gather information about this sudden change and decide how it should respond."

Interestingly, although the response of the fly in both scenarios was to cause an immediate pause, their subsequent walking speeds differed significantly.

"After startling on the blackout scenario, the fly's march was slow and deliberate," said Dr. Howard. "But the earthquake caused flies to walk faster after the initial break."

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

In the future, researchers hope to continue investigating the role of serotonin in movement, as well as what other factors may be at stake.

"Our results indicate that serotonin has the potential to interact with many different types of nerve cells in the fly's nervous system, such as those that guide movement and process sensory information," said Dr. Mann. "While we and others continue to investigate, we hope to develop a detailed molecular plan for locomotion that can be widely applied to other animals, perhaps even to people."