Zebrafish get cocaine in their eyes

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  • Published: Sep 30, 2016
  • Author: Jon Evans
  • Source: Eawag
  • Channels: Base Peak

Zebrafish larvae

MALDI-MSI images of zebrafish larvae.

Using matrix-assisted laser desorption ionization-mass spectrometry imaging (MALDI-MSI), researchers at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Zurich University have discovered that, surprisingly, cocaine accumulates in the eyes of zebrafish. This finding indicates that chemicals – especially psychoactive drugs – need to be assessed quite differently when considering waterborne exposure. In particular, the uptake mechanisms and effects of cocaine in fish cannot simply be transferred to mammals or humans.

Zebrafish larvae a few days old are frequently used in toxicology tests – e.g. to study the behavioural effects of drugs – in order to avoid experiments in mammals. Taking the example of cocaine, researchers at Eawag, together with colleagues at Zurich University, have now shown that the uptake and distribution patterns and the effects of the drug in zebrafish differ in many ways from those in mammals.

As they report in Toxicological Sciences, the study involved using MALDI-MSI to determine for the first time where cocaine accumulates in zebrafish. After being exposed to a defined concentration of the drug for eight hours, the larvae were euthanized and frozen. Tissue samples a few micrometres thick were then analyzed by MALDI-MSI.

The resultant images show that the greatest accumulation of cocaine is to be found, not in the brain, but in the eyes, where concentrations over 1500mg/kg were measured – compared to concentrations of 300–400mg/kg in the trunk and brain. This finding is striking: while increased concentrations have been observed in the head region in other fish studies, the highest concentrations were assumed (without more precise measurements) to occur in the brain. In addition, compared to mammals, these levels are very high: in mice, concentrations 100 times lower are generally lethal, and in humans, 1000 times lower.

Kristin Schirmer, an environmental toxicologist at Eawag, who co-led the project with Thomas Kraemer at the Zurich Institute of Forensic Medicine, cannot yet fully explain these findings. It is, however, clear that cocaine is taken up rapidly and continuously by zebrafish larvae, which at this early stage do not yet possess a fully-developed blood-brain barrier.

Schirmer and her team also found other major differences between the fish tests and patterns of cocaine uptake observed in mammals. While cocaine has a stimulatory action and causes hyperactivity in mammals, it suppresses locomotor activity in zebrafish. This indicates that the effects of cocaine on peripheral nerves – due to rapid uptake through the skin and gills – override any potential effects on the brain. Furthermore, in contrast to mammals, the elimination process is delayed as a result of the accumulation of cocaine in zebrafish eyes: 50% of the cocaine was eliminated after eight hours in clean water, but 30% still remained after 48 hours.

According to Schirmer, this study shows that fish tests need to be further refined if the results are to be transferable to mammals, or if they are to be used for water quality assessment: “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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