An Introduction to Zebrafish Brain

What is consciousness? How do we learn complex coordinated movements like riding a bicycle and never forget afterwards? How does your brain know where your body ends and bicycle starts? The humble Zebrafish has all the clues to these questions and more. Systems simple enough to understand and complex enough to make generalizations always attracted scientists. The 80,000-neurons in the brain of the developing the zebrafish embryos provides one of those ideal systems to understand vertebrate brain evolution and function. Brain is a very expensive organ. The brain of a sleeping human baby consumes 60 percent of the total energy budget of the body. So how does the zebrafish help?

The zebrafish is native to tropical southeast Asia. Adult forms are familiar to many by their bluish stripes that run along the length of the body. Since the 1960s the Zebrafish (Danio rerio) has been widely used as an animal model in biology. Its cells are exceptionally transparent making it ideal to observe embryonic development. The high regenerative capacity of its tissues render it especially interesting among vertebrates with regenerative potentials such as salamanders. Zebrafish have already contributed to study a number of molecular biological events behind neuromuscular diseases such as muscular dystrophy. Sleep has been shown to be an effective way to repair genetic damage in zebrafish brain. It serves as an important model for understanding cancer as well. The genome sequence of the zebrafish was published in 2013. Its genome is 1,505,581,940 base pairs (1,5 Gb) long.

video source

This annotated video from zebrafishbrain.org refers to many anatomical terms of locations such as rostral, caudal, dorsal, ventral. Brain tissues have been histochemically labeled with two fluorescent antibodies (1) anti-acetylated tubulin antibody in green targeting axon tracts and (2) anti-synaptic vesicle protein 2 (SV2) in magenta.

Thanks to cerebral models such as this one researchers can observe brain activity and map reactions on it. For instance these fishes are visual predators hunting prey such as parameceum. Therefore finding and tracking prey requires formation of a search image and perform cognitive abilities such as context generalization (filtering background noise in order to pay particular attention to an object) and positional invariance (recognizing same object no matter how far).

Researchers can immobilize young fish and modify their surroundings using video screens around the chamber holding the animal. Such virtual reality projections into fish eye allows observation of single-neuron level activity when fish react. Although the fish are immobilized with drugs, their neurons innervating the muscles work just like a free swimming animal. By animating a virtual fast or slow moving landscape they could obtain fast or slow swimming response by the fish.

This type of high-resolution global observations allow mapping the regions of the brain communicating with each other. Zebrafish has quite a predictive innate behavior called opto-motor response and light seeking phototaxis behavior. These are adaptive sets of behavior that prevent young fish to avoid hazards such as being washed away downstream into a river. Just like mice fish are terrified when a shadow gets larger (looming) which is what happens when a predator such as a kingfisher approaches. Researchers instigate anti-predatory reactions by creating a black dot growing in diameter and record cerebral activity before during and after.

For obvious reasons brain has always been a curious organ for study. Brain structures called mushroom bodies were first noticed in Hymenopterans (ants, bees, wasps and sawflies) by Felix Dujardin in 1850. Mushroom bodies are now suspected to have the same ancestral origin with the cerebral cortex of the mammalian brain responsible from higher cognitive abilities. Solitary wasps are also known for their unusually large brains.

Back to Zebrafish: A single pair of zebrafish can produce up to 300 offspring. Therefore an interesting genetic character can be passed on to a large population of fish. In this manner, the power of genetic analysis increases by simply increasing sample numbers. Moreover, mature zebrafish have an integrated nervous system with homologous brain structures to those of humans. Zebrafish in this respect is a powerful model organism for studying cognitive aging.

zebrafish