When zebrafish — a small minnow common in home aquariums — encounter stress in their first few days of life, they suffer permanently altered brains, according to new research by FAU neuroscientists.
Early-life stress has fascinated scientists for decades, said Erik Duboue, Ph.D., assistant professor in the Harriet L. Wilkes Honors College and co-author on the study. For instance, research indicates that trauma and abuse in childhood can increase the likelihood of anxiety and substance abuse as an adult, he said. But, how stress developmentally affects the brain is poorly understood, Duboue explained. The zebrafish, Danio rerio, could be key to unlocking this mystery.
As a vertebrate, zebrafish retain the same primitive features of the brain as humans that drive basic behaviors, like eating and sleeping. But unlike humans or mice, which are often used as models, development is external. Zebrafish fertilize eggs outside their bodies, by releasing egg and sperm into the water column. “You can literally track them from a single cell and follow them to an adult,” Duboue said.
To conduct the research, Duboue and the scientists bred the fish in the lab, and then applied a randomized stressor to larval fish from the time they are between 2 and 6 days old. After this time, the researchers raise the fish in a stress-free environment for 60 days, at which point they are considered juvenile fish. Then, they analyze the fish’s behavior. “We can ask the question, are there changes that emerge?” Duboue said.
The findings, published in the journal Scientific Reports, reveal two ways in which fish were stressed. First, they determined stress levels physiologically. Just like humans, zebrafish release the hormone cortisol when stressed, which plays a role in metabolism, blood pressure, blood sugar, inflammation, and the flight-or-fight response. Duboue found that young fish exposed to a stress stimulus experienced a higher baseline of cortisol later in life. In essence, the fish are “chronically stressed out,” Duboue said.
Secondly, Duboue and the team examined the fish’s behavior based on the idea of exploration of a novel environment. From past research, it’s known that when put into a new environment, fish will typically hug the bottom of the tank, Duboue said. But, after a few minutes, the fish feel comfortable enough to explore the top and bottom of the tank equally. For their study, Duboue found that fish stressed in early life spent a majority of their time on the bottom of the tank.
An important finding from the study, Duboue said, is the timing of the stressor. A critical window exists where the stress has a lasting impact. For fish, it’s from two to
"What the zebrafish offers is that it provides us with a way of linking these kinds of early-life stressful events or early experience events, with how it actually affects the developmental trajectory of the brain."
— Erik Duboue, Ph.D., assistant professor in the
Harriet L. Wilkes Honors College
six days after hatching. “If you do it later in development, like two or three weeks, you’re not going to reprogram the development of the brain. As adults, they will be completely fine. So, it’s really something about changing it in early development that is altering the developmental trajectory,” Duboue said. “I don’t think anybody has ever narrowed down that critical period before.”
During this time between two to six days — when the fish are most susceptible to stress — the body is forming its ability to respond to stress, called the neuroendocrine stress axis. “That’s an interesting finding I think that emerged as well. It’s not that you’re changing the development of the neuroendocrine stress axis, but it’s actually having this effect after it’s been formed.” Once they looked at the brains of the fish and determined it was the cortisol receptor, the glucocorticoid receptor, being impacted and changing the brain, said Duboue, adding “How that’s happening, we don’t really know, but it’s something that we want to figure out.”
While drawing a comparison to humans is a natural question, it’s a difficult comparison to make, said Duboue, and, of course, the development of fish and humans are different. “But if I had to compare it to humans, I would say it’s the equivalent of early postnatal development. Yet, humans are susceptible all the way from prenatal — in utero manifestations — all the way to early adolescence.”
“What the zebrafish offers is that it provides us with a way of linking these kinds of early-life stressful events or early experience events, with how it actually affects the developmental trajectory of the brain,” Duboue said. “The goal of this work is that it’s going to inform us about basic mechanisms linking early-life stressors with development, and that it has some relevance to human populations.”
Research continues: The National Institutes of Health (National Institute of Mental Health) recently funded this work with a three-year grant of about $450,000. Now that the team of scientists have figured out that stress is hyperactivating the receptor to cortisol, they plan to continue this work to learn more about how specific regions of the brain are impacted due to stress and finding specific genes that might go awry, Duboue said.