Zebrafish are tiny than your little finger and have a brain that is around half the size of a pinhead. However, these animals have an efficient navigation system that allows them to return to areas of the water where the temperature is optimal for them.
All creatures must regulate their body temperature and cannot survive for long if it becomes too high or low. Warm-blooded organisms, such as humans, have a variety of strategies to accomplish this. Sweating or widening blood vessels in their skin helps them release heat, but shivering or burning fat in their brown adipose tissue does the opposite.
Cold-blooded species, such as the zebrafish, cannot do any of these things, so they use a different technique. They look for spots nearby that are at their “comfortable temperature,” just as we might go outside into the sun when we are cold or seek cover when it is too hot. “We formed the idea that cold-blooded organisms use similar brain mechanisms to humans to find the ideal temperature conditions for them and that these help them know where to go,” explains Professor Ilona Grunwald Kadow of the Institute of Physiology II at the University of Bonn and the University Hospital Bonn.
The animals had been genetically modified to make their nerve cells produce a dye. This caused their neurons to light up when they were active, enabling us to see under the microscope which areas of their brains were working at that precise moment.
Grunwald Kadow
Fish larvae observed “thinking”
The zebrafish is ideal for testing this hypothesis because its larvae are transparent. This permits scientists to peer into their subjects’ brains while they execute certain activities in the lab, which is exactly what these researchers performed. “The animals had been genetically modified to make their nerve cells produce a dye,” explains Grunwald Kadow from the University Hospital Bonn, who is also a member of the University of Bonn’s Transdisciplinary Research Area “Life & Health”. “This caused their neurons to light up when they were active, enabling us to see under the microscope which areas of their brains were working at that precise moment.”
In their experiments, the researchers surrounded the animals with water that they made hotter or colder. “Then we watched to see how they’d react,” Virginia Palieri explains. For her doctorate at TUM, she studied the degree of similarity between the mechanisms for regulating body temperature used by cold-blooded animals and their warm-blooded counterparts such as humans. “This told us that the fish prefer a temperature of 25.3 degrees Celsius. As soon as it got a few tenths of a degree cooler or warmer, they began to seek out more comfortable surroundings.”
“Satnav” increases the chances of finding ideal living conditions quickly
This procedure activates two areas of the brain: the preoptic area of the hypothalamus (POA) and the dorsal habenula. The POA appears to be largely responsible for detecting variations from the fish’s optimal temperature. “When we switched off the animals’ POA, they stopped embarking on searches, even when the temperature of their water was some way off what made them comfortable,” Palieri said. Interestingly, mammals like humans have a POA.
“And this region of the brain is likewise responsible for regulating temperature, even in these much more highly developed organisms,” Grunwald Kadow explains. “In them, however, it’s mainly responsible for automatic actions such as sweating or shivering — behavior less so.” Nevertheless, the study has revealed that the brain’s “thermostat” is extremely ancient, meaning that it developed very early on in the evolutionary process.
The habenula, for its part, clearly acts as a kind of “satnav,” showing the fish where it can locate a comfortable temperature and guiding it straight back there. “Thanks to their navigation system, the animals can find their way around very efficiently and make their way back quickly to the spot with the best temperature,” adds Professor Ruben Portugues from the Institute of Neuroscience at TUM and researcher in the Cluster of Excellence “SyNergy,” who led the study together with Ilona Grunwald Kadow.
Deactivating the habenula region robs the fish of its ability to find its way around and forces it to adopt a different search strategy similar to bacteria and other single-celled organisms: it swims in a straight line for a while and then checks whether the temperature has changed to its liking. If so, it carries on in the same direction; if not, it picks a different direction at random and swims off again, repeating the process until it has found somewhere with a more suitable temperature.
Although we still don’t know exactly how the zebrafish’s navigation system works, it is thought to use unique “compass cells.” The habenula may be able to store its location and reconstitute movement sequences. “We now want to examine this hypothesis more closely,” Professor Portugues explains. What’s especially noteworthy is that this navigation system appears to be used for more than just finding spots with the proper temperature. It also aids in the relocation of places with enough salt levels, pH, or other conditions or supplies required by fish to survive.
This shows how efficient the brain is: once it has found a solution to a certain problem, it is only too happy to use it for other, similar tasks as well. And this is not just true of individual species, since these solutions have been retained and improved on over the course of evolution.
