Ann Jane Tierney, professor of psychology and neuroscience, knows what to do when one of her study subjects grabs her with its powerful claws. “You just dip your hand in water and they immediately let go,” she says. “It doesn’t do any good to flail. They just hold on tighter.”
Her hard-earned wisdom comes from many years of working with crayfish in the lab. In a recent study published in the journal Behavioral Neuroscience, Tierney explored the role of serotonin in controlling the appetites of these crustaceans. She found that this brain signaling molecule makes crayfish act less hungry and eat less food — similar to its effect in humans and other mammals.
Their pinchy tendencies aside, crayfish have long been a favorite subject for Tierney. For one thing, “They’re really a classic model in neuroscience,” she says. Their brain cells are relatively large in size, but also (sorry, crayfish) small in number. That makes it easy for scientists to identify and study individual neurons, and to observe how chemicals affect their actions.
“You can learn about how neurons function together to produce behaviors,” Tierney says. Crayfish and their larger (but also sparsely endowed with brain cells) cousins, the lobsters, “have been really important in understanding neural circuits,” she says.
In addition to being a classic study organism, crayfish are also convenient, Tierney says. “They’re inexpensive; I can catch them myself; they’re easy to maintain. They do interesting behaviors. Students can work with them pretty easily.” She can purchase crayfish from a supplier when she needs a particular species, or in the winter, but in warmer months, she can also collect the animals from local creeks.
Tierney has used these accommodating crustaceans for research on a variety of brain science topics, including the role of serotonin. “Serotonin is this fascinating, sort of universal neurotransmitter. It’s been around for hundreds of millions of years,” she says. “All animals have serotonin; all animals have serotonin receptors. But it doesn’t always do the same thing.”
In humans, this multitasking molecule is famous for mood regulation. But it also has a variety of other roles in our bodies, including regulating sleep, digestion, and blood-clotting. In crayfish, researchers have studied the role of serotonin in aggression, anxiety-like behaviors, and movement (serotonin makes crayfish act more sluggish, Tierney says). For her new study, Tierney was curious about what serotonin has to do with crayfish feeding themselves.
“Feeding is interesting — it’s something animals do every single day. It’s essential to survival, obviously,” Tierney says.
Scientists had previously seen hints that serotonin was related to feeding in crayfish. The animals assume a distinctive pose when they’re injected with serotonin: they stand up tall on their legs, instead of crouching low to the ground, and hold their tails flexed underneath them. Crayfish also stand this way after they’ve eaten a big meal.
To learn more, Tierney injected crayfish with either serotonin or a saltwater control. Then she added fishy-smelling water to a crayfish’s tank and observed its behavior: Did it walk around and try to climb the tank walls, searching for food and trying to eat? Finally, she added food pellets to the tank and recorded how much the crayfish gobbled up.
In another set of experiments, Tierney injected the crayfish with one of two different drugs that attach to serotonin receptors. There can be many types of serotonin receptors in an animal’s body, Tierney says, and although serotonin itself binds to all of them, drugs may be specific to just one type of receptor.
She saw that, when crayfish were injected with serotonin, they acted less hungry around fishy-smelling water. They also ate fewer food pellets.
As for the drugs that bind to serotonin receptors, Tierney found that these also reduced how much the crayfish ate — but didn’t make much difference in how the crayfish acted in food-scented water. “This suggests that different receptors mediate these two different behaviors,” Tierney says.
Serotonin also dials down the appetites of mammals, including humans. In certain other invertebrates, though, such as nematodes and leeches, it has the opposite effect, making them hungrier.
The results illustrate the complex role of this neurotransmitter across different life forms, Tierney says, which makes simple answers hard to come by for researchers. “That’s one of the good and the bad things. Serotonin does a lot. And, of course, in humans, too.”
In the near future, she’ll continue to tease out those complexities using a different animal: crickets. With the Robert H.N. Ho Center for Mind, Brain, and Behavior under construction in Olin Hall, the usual home of Tierney’s lab, she’s had to downsize not just her working space, but also the animals in her experiments.
She’s excited for her new study species — like crayfish, they’re “fun to watch,” Tierney says. One thing she hopes to learn is whether the insects can, in a way, medicate themselves. “This is a little bit speculative,” Tierney says. “But I am interested in if what they eat can have direct effects on the nervous system.”
Animals can recognize and choose certain compounds in the natural world that they need, Tierney says. What if they can also choose foods that affect their neurotransmitters? After all, humans do this all the time, such as by seeking out drugs (like antidepressants, or certain recreational drugs) that increase the serotonin in their brains.
The question isn’t just an academic one. Think of the insects that have some of the largest impacts on human society, Tierney says: Bees, which pollinate flowers to grow the food we depend on. Mosquitoes, which transmit deadly illnesses while sipping our blood.
“They are beneficial, and harmful, because of what they eat,” Tierney says. So understanding what brain signals and chemicals drive bugs to select the foods they do could lead to a better grasp of processes that affect the whole world. “It’s kind of an insect planet,” Tierney says.
Even so, once construction is over, she may return to her first study subjects, the crayfish — claws and all.