Researchers are turning to zebrafish to unlock the secrets of place cells, which play a key role in forming mental maps of space, social networks, and abstract relationships. Until now, place cells have only been found in mammals and birds, leaving the question of how other species represent the external world internally largely unanswered. A research team at the Max Planck Institute for Biocybernetics has found the first compelling evidence for the presence of place cells in the brains of tiny zebrafish larvae.
When exploring a new city, we use a variety of cues (landmarks, a sense of how far we've walked in one direction, rivers we can't cross, etc.) to create an internal map of our environment. A set of place cells located deep inside the brain in a structure called the hippocampus plays a key role in creating an internal map of the outside world. These place cells activate when we are in a particular place in space and can self-organize into an array of different mental maps.
We know a little about mammals, including humans, and even birds, but the existence of place cells in other species has been a matter of debate. A research group led by Jennifer Lee and Drew Robson at the Max Planck Institute for Biocybernetics in Tübingen, Germany, has found the first conclusive evidence for the existence of place cells in zebrafish.
The study has been published in the journal Nature.
Whole-brain recordings during natural behavior
The researchers recorded brain activity in young zebrafish as they explored their environment. The fish become completely transparent just a few days after birth, allowing researchers to see their tiny brains, which contain just 100,000 cells.
Because all neuronal activity is associated with fluctuations in calcium ion concentration, it is even possible to light up individual active neurons using fluorescent calcium indicators.Lee and Robson's earlier important invention was a tracking microscope that moves with freely swimming fish, which was essential for observing brain activity during navigation.
Using this experimental design, the team analysed what spatial information was encoded in each neuron in the fish's brain. They identified a population of around 1,000 place cells in each fish, most of which fire only when the fish is in a specific location, but a small number respond to multiple regions. “The place cells collectively encode spatial information,” explains Jennifer Lee. “From the firing patterns of the place cells, we were able to decode each fish's location over time, with an accuracy of just a few millimetres.”
Surprisingly, the majority of place cells were located in the telencephalon, a region of the zebrafish forebrain whose exact function has been a subject of debate for decades. “The concentration of place cells in the telencephalon may support long-held speculation that this brain region is a miniature functional analogue of the mammalian hippocampus,” commented Drew Robson.
A flexible mechanism for integrating various inputs
But Lee and Robson needed more evidence to conclude that the cells they had identified were indeed similar to mammalian place cells. The first feature they tested was whether place cells use self-motion or external cues. In human experience, cues such as “I have been walking briskly in a straight line for about a minute” rely on self-motion, whereas “I can see the Eiffel Tower” is an external cue.
In a series of experiments, the researchers manipulated both sources of information — removing fish from their environment and then returning them, removing cues from the cue pads, rotating behavioral chambers, etc. — and found that the fish integrated both external and self-movement cues to create an internal map, just like humans do.
Fish not only refine their spatial representation maps as they become accustomed to an unfamiliar environment, but they are also able to adapt to change, using the same neural circuits to memorize the second environment. When they return to the first environment, they do not need to build the map from scratch, but can partially restore the representation map they created earlier. Thus, place cell populations demonstrate a flexible memory system, a further feature of mammalian place cells.
A new model organism for complex neural networks
The study authors plan to use zebrafish as a new model organism to unravel the mysteries of place cells. In addition to their role in creating mental maps of space, place cells are also essential for forming maps of social networks and abstract relationships, as well as for memory and planning. Mammalian place cells have been intensively studied since their Nobel Prize-winning discovery more than 50 years ago, but scientists still do not fully understand the neural networks that generate place cells or how they support such a wide range of mental functions.
A major challenge is the complexity and sheer scale of mammalian place cell networks, making it extremely difficult to study the entire network simultaneously. In contrast, the larval zebrafish brain is one of the smallest biological systems capable of generating place cells.
“Using this new minimal model, future studies may be able to trace all of the inputs to each place cell and create a detailed model of how place cells acquire all of their unique properties,” Robson concludes.
More information:
Jennifer Lee, A population code for spatial representations in the zebrafish telencephalon; Nature (2024). DOI: 10.1038/s41586-024-07867-2. www.nature.com/articles/s41586-024-07867-2
Courtesy of the Max Planck Institute for Biocybernetics
Citation: How zebrafish map their environment: Spatial mechanisms are strikingly similar to ours (August 28, 2024) Retrieved August 28, 2024 from https://medicalxpress.com/news/2024-08-zebrafish-environment-spatial-mechanisms-similar.html
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