What Worms Can Tell Us About Cocaine

Cocaine is an illegal and addictive drug. It causes addiction by hijacking small molecules called chemical messengers that the cells in the brain use to communicate and work together. Most scientists have studied how cocaine affects three chemical messengers, called serotonin, dopamine, and norepinephrine, but cocaine may also affect a fourth chemical messenger called acetylcholine. To understand whether cocaine affects acetylcholine, we used the egg-laying “brain” of small worms to determine how cocaine works in humans. We found that cocaine makes these worms lay eggs, but only when the acetylcholine message is working properly. This tells us that cocaine acts in part through acetylcholine. Our study provides a first step in understanding the interaction between cocaine and acetylcholine. Treatments for cocaine addiction depend on our understanding of how the drug works, so learning how cocaine affects acetylcholine could lead to the development of effective treatments for cocaine addiction.

Cocaine is an illegal and addictive drug. It causes addiction by hijacking small molecules called chemical messengers that the cells in the brain use to communicate and work together. Most scientists have studied how cocaine a ects three chemical messengers, called serotonin, dopamine, and norepinephrine, but cocaine may also a ect a fourth chemical messenger called acetylcholine. To understand whether cocaine a ects acetylcholine, we used the egg-laying "brain" of small worms to determine how cocaine works in humans. We found that cocaine makes these worms lay eggs, but only when the acetylcholine message is working properly. This tells us that cocaine acts in part through acetylcholine. Our study provides a first step in understanding the interaction between cocaine and acetylcholine. Treatments for cocaine addiction depend on our understanding of how the drug works, so learning how cocaine a ects acetylcholine could lead to the development of e ective treatments for cocaine addiction.

COCAINE ADDICTION
Drug addiction is a medical condition in which people cannot stop using drugs despite negative consequences, such as poor physical or mental health, financial problems, or unstable relationships [ ]. People can become addicted to legal drugs, such as the alcohol in beer, wine, and spirits, or the nicotine in cigarettes and vapes. People can also become addicted to illegal drugs such as cocaine. The causes of cocaine addiction are complex and scientists want to better understand how cocaine acts on the brain, so they can develop more e ective treatments for addiction [ ]. Use of cocaine and other drugs can change the levels of substances in the brain called chemical messengers, and these changes in chemical messengers are

CHEMICAL MESSENGERS IN THE BRAIN
The brain is made up of microscopic, interconnected cells called neurons. The job of neurons is to communicate with each other NEURONS Small, interconnected cells that make up the brain. Neurons are specialized to send information to each other. so that the brain can function properly [ ]. Working together, these neurons direct our thoughts, our senses, and our movements. Neurons talk to other neurons by sending out chemical messengers.
The process of sending chemical messengers between neurons can be broken down into four steps that are kind of like sending a snap on Snapchat (Figure ). First, just like sending a snap requires taking a picture, the neuron must make the chemical messenger. Second, the neuron must release the chemical messenger, which is like sending the snap from your phone. Third, another neuron receives the chemical messenger by capturing it with a special protein on its surface called a receptor. This step is similar to your friend's Snapchat receiving your snap, so that your friend can open it. Finally, just like your friend's Snapchat automatically deletes your snap, the neuron that received the message eventually destroys the chemical messenger.

COCAINE AND THE CHEMICAL MESSENGER ACETYLCHOLINE
One reason scientists care so much about chemical messengers is that these messengers play an important role in addiction to dangerous drugs like cocaine [ ]. Researchers have found that cocaine a ects the brain by increasing the amounts of three of its chemical messengers, called dopamine, serotonin, and norepinephrine. There is additional evidence, however, that cocaine may a ect a fourth chemical messenger called acetylcholine [ ]. Acetylcholine plays an ACETYLCHOLINE A chemical messenger that neurons use to produce motivation and attention. important role in several major parts of addiction, including motivation (such as the motivation to seek out drugs) and attention (such as the attention placed on opportunities to use drugs). Studying how cocaine might a ect acetylcholine could one day help scientists kids.frontiersin.org June | Volume | Article |

Emerson and Granger
Learning About Cocaine Through Worms understand and treat cocaine addiction, but cocaine's possible e ects on acetylcholine are not yet fully understood.

WHY STUDY THE EFFECT OF COCAINE ON WORMS?
Studying e ect of cocaine on acetylcholine with human participants would take a long time, would be very expensive, and would be hard to do safely, so we need another way to know if cocaine a ects acetylcholine. There is a small worm called Caenorhabditis elegans, and even though it does not look much like a human, it has neurons that work very similarly to those of humans. Some of the worms' neurons use acetylcholine to communicate with each other, like human neurons do, so these worms provide an e cient model for MODEL A simplified version of a complicated scientific problem that helps scientists study that problem more easily, or without causing harm to people. human neurons.
In fact, worms are a particularly good model because we can observe whether cocaine is a ecting their neurons without needing a fancy microscope. Because neurons control behavior, we can observe changes to neuronal activity by looking at changes in worm behavior. Specifically, we chose to study the e ect of cocaine on the egg-laying behavior of worms because egg laying is controlled by a very small "brain" that uses acetylcholine messages [ ]. Since the neurons and chemical messengers involved in worm egg laying are very similar to those in humans, we can use the egg laying "brain" as a model for how cocaine a ects neurons in people.

COCAINE AND ACETYLCHOLINE INTERACT TO CAUSE WORM EGG LAYING
In our first experiment, we investigated how cocaine a ects worm egg laying. To do so, we put hundreds of worms in tiny baths of cocaine or sugar water (sugar does not a ect egg laying) and let them swim in the baths for h (Figure ). We found that the worms in cocaine baths laid about six eggs, while those in sugar water rarely laid any eggs. This experiment showed us that cocaine causes worms to lay eggs.

Figure
In our first experiment, we tested the e ect of cocaine on worm egg laying. The worms were placed in either sugar water baths or cocaine baths for h. During that time, the worms in cocaine baths laid about eggs each, whereas the ones in sugar water almost never laid eggs. This tells us that cocaine causes worms to lay eggs.
Next, we checked whether acetylcholine is necessary for cocaine to cause worms to lay eggs. To do this, we used a series of knockout experiments. In this type of experiment, we take something away from ball. To test your hypothesis, you could cut or "knock out" the racket's strings. If you tried to hit a ball with this racket, you would soon realize that, without good strings, you cannot properly hit the ball-so the strings must be important for hitting a tennis ball. In our knockout experiments, rather than damaging the strings on a tennis racket to see how they normally interact with a tennis ball, we damaged the worms' acetylcholine communication, to see how it normally interacts with cocaine ( Figure ).
For our second experiment, we again put hundreds of worms in tiny baths of cocaine or sugar water, but this time, some of the worms were "knockouts," missing the proteins needed to do one of the four steps of sending acetylcholine messages. We found that all worms lacking proper acetylcholine signaling laid fewer eggs in the cocaine baths compared to normal worms in cocaine baths. This suggests that acetylcholine signaling is needed for cocaine to produce egg laying. Understanding this interaction between cocaine and acetylcholine in worms may help us better understand the interaction of cocaine and acetylcholine in the human brain as well.

OUR FINDINGS COULD HELP COCAINE ADDICTION RESEARCH
E ective treatments for cocaine addiction have not been developed because scientists still do not fully understand how cocaine a ects the chemical messengers in the human brain [ , ]. The role of the Figure   Figure In our second experiment, we tested cocaine's interaction with acetylcholine. This can be compared to testing a tennis ball's interaction with racket strings. The example hypothesis is that the strings interact with the ball to produce a great shot. We could confirm this hypothesis by showing that, without strings, there is no great shot. Our actual hypothesis was that acetylcholine interacts with cocaine to cause egg laying. We confirmed our hypothesis by "knocking out" acetylcholine and showing that the acetylcholine message is in fact needed for cocaine to cause egg laying.
chemical messenger acetylcholine in cocaine addiction particularly needs more study [ ]. In our experiments, we used the egg laying "brain" of small worms as a model for how cocaine a ects acetylcholine, and our experiments showed that cocaine changes acetylcholine messages.
Our use of the worm egg laying "brain" establishes a new model to investigate how cocaine a ects acetylcholine. Our finding that cocaine interacts with acetylcholine could contribute to the development of new treatments for cocaine addiction. Scientists have tried to develop cocaine addiction treatments that block other chemical messengers, including serotonin, dopamine, and norepinephrine, but these treatments have not been very e ective [ ].
It is possible that blocking cocaine's e ect on acetylcholine could be a new way to treat cocaine addiction, which is currently an undertreated condition that a ects many people.  Emerson, S., Hay, M., Smith, M., Granger, R., Blauch, D., Snyder, N., et al. . . doi: . /frym. .

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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YOUNG REVIEWER
CHRIS, AGE: I play chess competitively and I am on a rock climbing team. My favorite subject in school is physics. When I can not be outside with my dog I like to play drums.

SOREN EMERSON
I am a first-year graduate student at Vanderbilt University, where I will study how drugs a ect the chemical messengers in the brain. I am a neuroscientist interested in understanding how di erent drugs can hurt people or treat diseases. In my free time I enjoy cooking, writing about science, and spending time outdoors. *soren.d.emerson@vanderbilt.edu RICKY GRANGER I am a first-year medical student at the University of Wisconsin Madison School of Medicine and Public Health. At my undergraduate college, I studied biology and worked in a neuroscience lab that uses worms to investigate questions about the nervous system. When I am not learning about health and science, I like watching movies with friends, walking my dog, and cycling. rhgranger@wisc.edu.