Climate change is causing heatwaves to become longer, more frequent and just plain hotter. For many of us, dealing with oppressive heat comes with several unpleasant challenges, from profuse sweating to feeling tired and sluggish. Humans, however, are not the only ones suffering through unprecedented heatwaves in a warming climate. In a ground-breaking study by Anna Andreassen and her team at the Norwegian University of Science and Technology, the researchers investigated what happens to the brains of larval zebrafish (Danio rerio) during extreme heatwaves, particularly as temperatures approach the upper limit of their heat tolerance. For these small fish, the consequences of unbearable heat are far worse than becoming a little sweaty.
Fish can only tolerate temperatures so high. At some point, they lose the ability to swim properly and assume the ‘belly up’ position (termed loss of equilibrium), an indicator of imminent death. For decades, biologists have wondered what happens in the body of fish at their upper temperature limit that prompts this behaviour. Andreassen and her team set out to solve this mystery by testing whether going belly up is caused by the brain not communicating properly with the rest of the body. This miscommunication is thought to occur because high temperatures stop nerve cells in the brain from functioning as they should. But first, the researchers needed a way to visualize the fish's brain activity at extremely high temperatures. Thanks to advances in genetic technology, they were able to study genetically modified zebrafish whose brains emit a fluorescent light when their neurons are active.
The researchers gradually increased the water temperature in the tank and observed the nerve cell activity of the genetically modified zebrafish using a microscope that could detect fluorescent light. At their upper temperature limit (approximately 40.9°C), the fish's brains showed very little fluorescence, indicating that the nerve cells were largely inactive. At this temperature, the larval fish were also no longer responsive to stimuli, such as bright lights flickering in front of their eyes. Interestingly, when the water temperature was raised by only 0.5°C above the upper temperature limit, something unexpected happened – the whole brain lit up as if the fish were having a seizure! This seizure-like activity was a sign that the brain had become completely depolarized, meaning that its nerve cells were firing on all cylinders, but not in a controlled manner. As this seizure-like event occurred at temperatures only slightly higher than the upper temperature limit, the team concluded that this is not what causes fish to lose equilibrium. Rather, they suggest that impaired neuron activity at the upper temperature limit is the mechanism underpinning this dangerous phenomenon.
The researchers decided to take their study one step further to determine why neurons stop working at the upper temperature limit. They had a hunch that high temperatures limit how much oxygen gets to the brain, which then impairs how its nerve cells function. Andreassen and her team repeated the heat ramping experiment, but this time at high, medium and low oxygen levels. They discovered that larvae exposed to high oxygen levels in the water did better at high temperatures than those in low oxygen; the fish had higher brain activity and recovered faster after being exposed to their upper temperature limit. Overall, these findings suggest that the amount of oxygen in the water plays an important role in controlling how the brain communicates with the body at extremely high temperatures. So, the next time you're sweating profusely in the midst of a summer heatwave, take a deep breath and be thankful that your brain function has remained intact.