Sometimes, in the name of science, you have to give cocaine to a fish. But when you do, be prepared for something a little strange to happen.

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Researchers in Switzerland have recently discovered that when zebrafish are given the white stuff, it bypasses their brains and accumulates in their eyes. Though this might not sound like a world-changing revelation, it could actually have a major impact on the way that new medicines are studied in the future.

Despite the fact that zebrafish don’t have much in common with people, they are at least vertebrates, and therefore have a central nervous system that scientists can play around with and test all sorts of experimental drugs on. As such, they are often used as aquatic guinea pigs, particularly when researchers want to observe the effects of psychoactive substances that they don’t feel comfortable using on humans.

However, reporting their findings in the journal Toxicological Sciences, the study authors provide clear evidence that some drugs – in this case cocaine – do not have the same effect on zebrafish as they do on mammals. Indeed, cocaine primarily acts on the brain when taken by those of us who live on dry land, yet heads straight to the eyes in zebrafish. This means that any data obtained using fish may not in fact be relevant to humans, thereby ruling out the possibility of using certain marine species to study drugs.

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To conduct their investigation, the team placed zebrafish larvae in tanks containing water contaminated with various amounts of cocaine, before using an imaging technique called Matrix-assisted laser desorption ionization-mass spectrometry to observe where in their bodies the drug accumulated. Amazingly, they found that only a small amount ended up in the brain, although massive concentrations – sufficient to kill a person – built up in the eyes.

Though the researchers aren’t entirely sure why this is the case, they speculate that cocaine may have a chemical affinity for melanin, which is found in the eyes.

In contrast, when people take cocaine it acts on the brain, triggering the release of neurotransmitters like dopamine, resulting in hyperactivity. Yet the drug has the exact opposite effect on zebrafish, causing them to become less active. The study authors suggest this is because it acts like an anaesthetic when it comes into contact with the skin, gills, and peripheral nervous system.

Study co-author Kristin Schirmer explained in a statement that “if we want to have a better knowledge of the effects of such chemicals on the ecosystem, we need a more detailed understanding of the processes of uptake through water – they’re quite different from when drugs are inhaled or injected.”

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