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Why Flies and Humans Freeze When Startled

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When faced with something unexpected and potentially ominous, like a sudden, loud noise or a threat of danger, humans often freeze before we act. This is colloquially referred to as the “deer in the headlights” phenomenon. The movie of fruit flies that you see above may help explain the ancient origins of the “startle response” and other biomechanical aspects of motion.

In this video, which shows a footrace between two flies (Drosophila melanogaster), there are no winners or losers. Their dash across the screen provides a world-class view of the biomechanics of walking in these tiny, 3 millimeter-long insects that just won’t sit still.

The fly at the top zips along at about 25 millimeters per second, the normal walking speed for Drosophila. As a six-legged hexapod, the fly walks with a “tripod gait,” alternating between its stance phase—right fore (RF), left middle (LM), and right hind (RH) —and its swing phase sequence of left fore (LF), right middle (RM), and left hind (LH).

The slowpoke at the bottom of the video clocks in at a mere 15 millimeters per second. This fly’s more-tentative gait isn’t due to an injury or a natural lack of speed. What is causing the delay is the rapid release of the chemical messenger serotonin into its nervous system, which models a startle response.

You may have already heard about serotonin because of its role in regulating mood and appetite in humans. Now, a team led by Richard S. Mann and Clare Howard, Columbia University’s Zuckerman Institute, New York, has discovered that fruit flies naturally release serotonin to turn on neural circuits that downshift and steady the speed of their gait.

As detailed recently in Current Biology [1], serotonin is active under myriad conditions to tell flies to slow things down. For example, serotonin helps flies weather the stress of extreme temperatures, conserve energy during bouts of hunger, and even walk upside down on the ceiling.

But the research team, which was supported by the NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative, found that serotonin’s most-powerful effect came during an actual startle response, prompted by a sudden, jolting vibration. Scientists suspect the release of serotonin activates motor neurons much like an emergency brake, stiffening and locking up the fly’s leg joints. When the researchers blocked the fly’s release of serotonin, it interrupted their normal startle response.

In years past, such a detailed, high-resolution “action video” of Drosophila, one of the most-popular model organisms in biology, would have been impossible to produce. Fruit flies are tiny and possess extremely high energy.

But a few years ago, the Mann lab developed the approach used in this video to bring the hurried gait of fruit flies into tight focus [2]. Their system combines an optical touch sensor and high-speed video imaging that records the footfalls of all six of a fly’s feet.

Then, using the lab’s unique software program called FlyWalker , the researchers can extract various biomechanical parameters of walking in time and space. These include step length, footprint alignment, and, as the letters in the video show, the natural sequence of a tripod gait.

Drosophila may be a very distant relative of humans. But these ubiquitous insects that sometimes buzz around our fruit bowls contain many fundamental clues into human biology, whether the area of research is genetics, nutrition, biomechanics, or even the underlying biology of the startle response.


[1] Serotonergic Modulation of Walking in Drosophila. Howard CE, Chen CL, Tabachnik T, Hormigo R, Ramdya P, Mann RS. Curr Biol. 2019 Nov 22.

[2] Quantification of gait parameters in freely walking wild type and sensory deprived Drosophila melanogaster. Mendes CS, Bartos I, Akay T, Márka S, Mann RS. Elife. 2013 Jan 8;2:e00231.


Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)

Mann Lab (Columbia University’s Zuckerman Institute, New York)

MouseWalker Colored Feet (YouTube)

NIH Support: National Institute for Neurological Disorders and Stroke; National Institute of General Medical Sciences

Mood-Altering Messenger Goes Nuclear

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Serotonin is best known for its role as a chemical messenger in the brain, helping to regulate mood, appetite, sleep, and many other functions. It exerts these influences by binding to its receptor on the surface of neural cells. But startling new work suggests the impact of serotonin does not end there: the molecule also can enter a cell’s nucleus and directly switch on genes.

While much more study is needed, this is a potentially groundbreaking discovery. Not only could it have implications for managing depression and other mood disorders, it may also open new avenues for treating substance abuse and neurodegenerative diseases.

To understand how serotonin contributes to switching genes on and off, a lesson on epigenetics is helpful. Keep in mind that the DNA instruction book of all cells is essentially the same, yet the chapters of the book are read in very different ways by cells in different parts of the body. Epigenetics refers to chemical marks on DNA itself or on the protein “spools” called histones that package DNA. These marks influence the activity of genes in a particular cell without changing the underlying DNA sequence, switching them on and off or acting as “volume knobs” to turn the activity of particular genes up or down.

The marks include various chemical groups—including acetyl, phosphate, or methyl—which are added at precise locations to those spool-like proteins called histones. The addition of such groups alters the accessibility of the DNA for copying into messenger RNA and producing needed proteins.

In the study reported in Nature, researchers led by Ian Maze and postdoctoral researcher Lorna Farrelly, Icahn School of Medicine at Mount Sinai, New York, followed a hunch that serotonin molecules might also get added to histones [1]. There had been hints that it might be possible. For instance, earlier evidence suggested that inside cells, serotonin could enter the nucleus. There also was evidence that serotonin could attach to proteins outside the nucleus in a process called serotonylation.

These data begged the question: Is serotonylation important in the brain and/or other living tissues that produce serotonin in vivo? After a lot of hard work, the answer now appears to be yes.

These NIH-supported researchers found that serotonylation does indeed occur in the cell nucleus. They also identified a particular enzyme that directly attaches serotonin molecules to histone proteins. With serotonin attached, DNA loosens on its spool, allowing for increased gene expression.

The team found that histone serotonylation takes place in serotonin-producing human neurons derived from induced pluripotent stem cells (iPSCs). They also observed this process occurring in the brains of developing mice.

In fact, the researchers found evidence of those serotonin marks in many parts of the body. They are especially prevalent in the brain and gut, where serotonin also is produced in significant amounts. Those marks consistently correlate with areas of active gene expression.

The serotonin mark often occurs on histones in combination with a second methyl mark. The researchers suggest that this double marking of histones might help to further reinforce an active state of gene expression.

This work demonstrates that serotonin can directly influence gene expression in a manner that’s wholly separate from its previously known role in transmitting chemical messages from one neuron to the next. And, there are likely other surprises in store.

The newly discovered role of serotonin in modifying gene expression may contribute significantly to our understanding of mood disorders and other psychiatric conditions with known links to serotonin signals, suggesting potentially new targets for therapeutic intervention. But for now, this fundamental discovery raises many more intriguing questions than it answers.

Science is full of surprises, and this paper is definitely one of them. Will this kind of histone marking occur with other chemical messengers, such as dopamine and acetylcholine? This unexpected discovery now allows us to track serotonin and perhaps some of the brain’s other chemical messengers to see what they might be doing in the cell nucleus and whether this information might one day help in treating the millions of Americans with mood and behavioral disorders.


[1] Histone serotonylation is a permissive modification that enhances TFIID binding to H3K4me3. Farrelly LA, Thompson RE, Zhao S, Lepack AE, Lyu Y, Bhanu NV, Zhang B, Loh YE, Ramakrishnan A, Vadodaria KC, Heard KJ, Erikson G, Nakadai T, Bastle RM, Lukasak BJ, Zebroski H 3rd, Alenina N, Bader M, Berton O, Roeder RG, Molina H, Gage FH, Shen L, Garcia BA, Li H, Muir TW, Maze I. Nature. 2019 Mar 13. [Epub ahead of print]


Any Mood Disorder (National Institute of Mental Health/NIH)

Drugs, Brains, and Behavior: The Science of Addiction (National Institute on Drug Abuse/NIH)

Epigenomics (National Human Genome Research Institute/NIH)

Maze Lab (Icahn School of Medicine at Mount Sinai, New York, NY)

NIH Support: National Institute on Drug Abuse; National Institute of Mental Health; National Institute of General Medical Sciences; National Cancer Institute

Measuring Brain Chemistry

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Anne Andrews

Anne Andrews
Credit: From the American Chemical Society’s “Personal Stories of Discovery”

Serotonin is one of the chemical messengers that nerve cells in the brain use to communicate. Modifying serotonin levels is one way that antidepressant and anti-anxiety medications are thought to work and help people feel better. But the precise nature of serotonin’s role in the brain is largely unknown.

That’s why Anne Andrews set out in the mid-1990s as a fellow at NIH’s National Institute of Mental Health to explore changes in serotonin levels in the brains of anxious mice. But she quickly realized it wasn’t possible. The tools available for measuring serotonin—and most other neurochemicals in the brain—couldn’t offer the needed precision to conduct her studies.

Instead of giving up, Andrews did something about it. In the late 1990s, she began formulating an idea for a neural probe to make direct and precise measurements of brain chemistry. Her progress was initially slow, partly because the probe she envisioned was technologically ahead of its time. Now at the University of California, Los Angeles (UCLA) more than 15 years later, she’s nearly there. Buoyed by recent scientific breakthroughs, the right team to get the job done, and the support of a 2017 NIH Director’s Transformative Research Award, Andrews expects to have the first fully functional devices ready within the next two years.

Finding Brain Circuits Tied to Alertness

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Everybody knows that it’s important to stay alert behind the wheel or while out walking on the bike path. But our ability to react appropriately to sudden dangers is influenced by whether we feel momentarily tired, distracted, or anxious. How is it that the brain can transition through such different states of consciousness while performing the same routine task, even as its basic structure and internal wiring remain unchanged?

A team of NIH-funded researchers may have found an important clue in zebrafish, a popular organism for studying how the brain works. Using a powerful new method that allowed them to find and track brain circuits tied to alertness, the researchers discovered that this mental state doesn’t work like an on/off switch. Rather, alertness involves several distinct brain circuits working together to bring the brain to attention. As shown in the video above that was taken at cellular resolution, different types of neurons (green) secrete different kinds of chemical messengers across the zebrafish brain to affect the transition to alertness. The messengers shown are: serotonin (red), acetylcholine (blue-green), and dopamine and norepinephrine (yellow).

What’s also fascinating is the researchers found that many of the same neuronal cell types and brain circuits are essential to alertness in zebrafish and mice, despite the two organisms being only distantly related. That suggests these circuits are conserved through evolution as an early fight-or-flight survival behavior essential to life, and they are therefore likely to be important for controlling alertness in people too. If correct, it would tell us where to look in the brain to learn about alertness not only while doing routine stuff but possibly for understanding dysfunctional brain states, ranging from depression to post-traumatic stress disorder (PTSD).

Studies of Dogs, Mice, and People Provide Clues to OCD

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Chances are you know someone with obsessive-compulsive disorder (OCD). It’s estimated that more than 2 million Americans struggle with this mental health condition, characterized by unwanted recurring thoughts and/or repetitive behaviors, such as excessive hand washing or constant counting of objects. While we know that OCD tends to run in families, it’s been frustratingly difficult to identify specific genes that influence OCD risk.

Now, an international research team, partly funded by NIH, has made progress thanks to an innovative genomic approach involving dogs, mice, and people. The strategy allowed them to uncover four genes involved in OCD that turn out to play a role in synapses, where nerve impulses are transmitted between neurons in the brain. While more research is needed to confirm the findings and better understand the molecular mechanisms of OCD, these findings offer important new leads that could point the way to more effective treatments.

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