In modern ecosystems, fish are exposed to myriad contaminants from human activities. The effects of some of those, such as mercury, are well-known. Less studied are the accumulations of pharmaceuticals that escape wastewater treatment filters and end up in our waterways.
A recent study in the journal Environmental Science & Technology by environmental scientists from Japan and the U.K. showed that fish are particularly sensitive to antidepressants that reach natural waterways at concentrations varying from just nanograms per liter to as high as micrograms per liter. Prior studies had found evidence of changes in neural, metabolic, and endocrine functions in fish exposed to antidepressant contaminants. The new study hoped to uncover the molecular mechanism behind the changes.
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The researchers focused on monoamine transporters, which control three neurotransmitters—dopamine, serotonin, and norepinephrine—in the brains of humans and other vertebrates, including fish. Many pharmaceuticals used to treat depression, anxiety, and other mental disorders block them to maintain higher concentrations of mood-regulating neurotransmitters.
Through a molecular characterization of monoamine transporters from two distantly related species—medaka (Oryzias latipes) and ayu (Plecoglossus altivelis)—the researchers assessed how antidepressants acted on fish neurotransmitters. First, they cloned fish genes that encode for dopamine, serotonin, and norepinephrine. Then, they genetically engineered the fish genes into human cells and exposed them to a range of common antidepressants like fluoxetine and citalopram.
The fish monoamine transporters, especially serotonin, which is implicated in aspects of mood like sleep and appetite, proved more sensitive than those of humans to antidepressants. In the fish-engineered cells, the serotonin transporter was shut down by drug concentrations up to 10 times lower than in the human ones.
“By demonstrating that key molecular targets in fish can be more sensitive than their human equivalents, our work offers crucial insights into the potential risks of pharmaceutical exposure to aquatic wildlife,” remarked senior study author Shinichi Miyagawa of the Tokyo University of Science in a press release.
Whether antidepressant contaminants are elevating the mood of fish remains to be determined. But without a doubt, they’re getting a real taste of our medicine. 
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Lead Image: Professor Shinichi Miyagawa from Tokyo University of Science, Japan, and Professor Masaru Ihara from Kochi University, Japan
