On the trail of the “thermostat” in the brain

Researchers have looked into the brains of transparent zebrafish larvae to determine their temperature regulation behavior. © Image: Portugues Labor / TU Munich

To regulate our body temperature, we sweat or shiver and seek places with temperatures that are comfortable for us. Fish also use the latter strategy - and processes take place in their brains that also play a role in our thermoregulation, a study shows. This suggests that the basic "thermostat system" in the brain emerged early in evolution, the scientists say.

Certain values ​​are optimal for the body's functions - however, at extreme temperatures there is even a danger to life. That's why all animals have developed strategies to regulate their body temperature when necessary. They are particularly important for creatures that have the same temperature, because certain values ​​are important for their optimal metabolism - in our case 37 degrees Celsius. To keep them at the same temperature, the living creatures have various mechanisms: They give off excess heat through evaporative cooling or the widening of the blood vessels in the skin. However, tremors or increased combustion lead to heating if necessary. There are also behavioral adjustments: warm or cool places are sought out. This is also the only adaptation option available to cold-blooded animals such as fish, whose body temperature adapts to the ambient values. They look for places that are as close as possible to their “feel-good temperature”.

Looking into the brains of fish

Neuroscientists have already discovered a lot about the thermostat system in the brains of creatures that are kept at the same temperature. Now a research team from the Technical University of Munich (TUM) and the University of Bonn has also devoted itself to investigating the mechanisms in cold-blooded animals. “We asked ourselves to what extent they recognize optimal temperature conditions through similar brain mechanisms to humans and how these help them find the right temperature environment,” says co-author Ilona Grunwald Kadow from the University of Bonn.

As a model, the scientists chose an aquarium fish that has already provided many insights into neurological mechanisms: the zebrafish. It is particularly suitable for this because its young animals are transparent. This allows you to look into their brains as they are exposed to certain stimuli or exhibit different behaviors. That’s exactly what the researchers did in this case. “The test fish were genetically modified so that they produced a dye in their nerve cells. This caused the neurons to light up when they were active. We were therefore able to observe under the microscope which brain regions were currently working,” explains Grunwald Kadow.

In their experiments, the researchers first flowed water around the fish, the temperature of which increased or decreased over the course of the flow. “We were able to show that the fish prefer a temperature of 25.3 degrees. As soon as it was a few tenths of a degree colder or warmer, they started looking for a more comfortable place,” says first author Virginia Palieri from TUM. The researchers could see what was going on in the fish's brains from the light signals that were associated with the neuronal activity. It was found that two regions became active in response to adverse temperatures - the preoptic area of ​​the hypothalamus (PoA) and the dorsal habenula.

Similar brain mechanism to ours

This seemed to confirm the researchers' hypothesis. Mammals like humans also have a PoA and its function in thermoregulation is already known: “Even in these much more highly developed organisms, this brain region is responsible for temperature regulation. It controls autonomic measures such as sweating or muscle tremors,” says Kadow. In the case of fish, the PoA is apparently responsible for detecting deviations from the target temperature, as the researchers were able to prove through further experiments: “If we switched off the PoA in the animals, there was no search behavior even when there were larger differences from the comfortable temperature one,” says Palieri.

Although the PoA takes on slightly different functions, the researchers conclude that it is a kind of basic “thermostat” in the brain of animals. Since fish are an ancient group of animals that also represent the distant ancestors of mammals, this leads to an interesting clue: the basic thermostat system in the brain apparently emerged early in the course of evolution, say the scientists.

But what about the activity of the habenula in fish that has also been identified? The team was able to show that this brain region apparently functions as a kind of navigation system when the fish look for favorable temperatures. This is because they apparently use it to remember where in the aquarium they previously found pleasant values ​​and then swim there in a targeted manner when necessary. “With their navigation system, the animals can orientate themselves very efficiently and quickly find their way back to the place with the best temperature,” explains senior author Ruben Portugues from TUM. However, when the researchers impaired the function of this brain region, the fish lost this ability.

The researchers now plan to investigate this aspect further: They want to shed more light on how the zebrafish's surprisingly highly developed memory and navigation system works.

Source: University of Bonn, specialist article: Current Biology, doi: 10.1016/j.cub.2023.12.030

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