"It was truly quite incredible. "The whole brain lit up," Anna Andreassen, a PhD student at the Norwegian University of Science and Technology, stated (NTNU).
Living species, whether fish or humans, tend to perform worse as the temperature rises. This is something that many people have certainly experienced on a very hot summer day. But what occurs inside the body when the temperature rises too high?
To find the solution, researchers at NTNU's Department of Biology used genetic technology with neurophysiological methodologies.
"We were interested in the factors that restrict organisms' heat tolerance." Which creatures will thrive when the Earth's temperature rises as a result of climate change, and why? "We decided to look at the brain," Andreassen explains.
Climate change causing heat waves
Heat waves that sweep across continents are becoming more common, and creatures that dwell in water are suffering dangerously high temperatures. Understanding what limits survival at extremely high temperatures is critical for forecasting how organisms will respond to climate change.
"Thermal tolerance has been studied for decades and the idea that temperature impacts brain activity is not new. "What's different is that we can now analyse the phenomenon using genetic technologies and neurophysiology," Andreassen explains.
Researchers at NTNU in Trondheim studied newly hatched zebrafish larvae's brain activity while progressively increasing the temperature around the larval fish.
"These fish have been genetically engineered such that when the neurons in the brain are active, they emit fluorescent light." While the larvae swim about, we can see this light through a microscope. These larval fish have the added benefit of being translucent. "We get to peer straight into the brains of living larvae," Andreassen explains.
Lose the ability to respond
The researchers can then monitor brain activity while gradually increasing the temperature of the water in which the fish are swimming.
"We'll see how the larvae react when the weather warms up. When it gets too hot, they lose their balance and begin swimming around in circles, belly up."
The researchers prodded the fish larvae to see how they reacted. They poked the larvae's tails, which generally causes them to swim.
But something strange happened when they increased the temperature.
"At a certain point, the fish ceased responding to the pokes." They were still living, but in an ecological sense, they were dead. "They wouldn't be able to swim away from predators or make their way to colder water in that condition in nature," Andreassen says, adding that this condition is only temporary in the small experimental fish.
"They'll be completely fine once we get them back into cooler water," Andreassen says.
Heat turns off the brain
So far, the experiments have gone exactly as expected. They could also test whether the brain was perceiving visual stimuli by flashing light in front of the fish's eyes. As the temperature increased, the brain ceased responding to inputs and became fully dormant. But something strange happened when they increased the temperature.
"My entire head lit up. "The closest I can describe what we saw is a kind of seizure," Andreassen explains.
Normally, brain activity is only seen as little spots of light in certain areas of the brain. Under the microscope, the researchers could see how the fluorescent light spread out in a matter of seconds and covered the entire brain of the microscopic fish larva.
"We know that zebrafish brains have a lot in common with human brains - 70% of the genetic material is the same," Andreassen explains.
Following that, the researchers intend to examine glial cells, a type of brain cell.
"We're particularly interested in the activity of glial cells during heating." These cells are critical to the brain's oxygen delivery since they both assess the oxygen level and regulate blood flow and the oxygen supply. Because we can see that oxygen levels affect temperature tolerance, one theory is that the brain shuts down because the glial cells no longer have the ability to regulate the oxygen level."
Differences advance evolution
To get a better understanding of what happened, Trondheim researchers began adjusting the amount of oxygen in the water the fish were swimming in while changing the temperature.
"We were surprised to see that the oxygen level played a role in modulating heat tolerance." When we introduced more oxygen, the larval fish performed better at high temperatures, had higher brain activity, and recovered from being exposed to upper thermal limits faster than the fish with low oxygen.
When evaluating the influence of oxygen content on temperature tolerance in various animals, researchers found contradictory results.
"Being "insensitive" to changes in oxygen levels may potentially provide an evolutionary advantage as the Earth's temperature rises."
"The data suggest that temperature tolerance differs between species." This could be a trait that determines whether a species can adapt to climate change or will perish as temperatures rise. Many species live in oxygen-depleted conditions where temperatures can quickly rise above ordinary. "They'll be extremely vulnerable," Andreassen predicts.
She uses species that live in shallow freshwater environments, rivers, or the intertidal zone as an example.
"These are habitats where substantial changes in oxygen levels can occur, frequently concurrently with temperature fluctuations." In these settings, fish whose heat tolerance is limited by oxygen levels are more likely to struggle than fish that are not."
Being "insensitive" to changes in oxygen levels may potentially provide an evolutionary advantage as the Earth's temperature rises.
"Animals that can retain nerve function under low oxygen levels may be the best at withstanding high temperatures," Andreassen says. (ANI)