A serotonin-regulating protein in two fish species responded to some antidepressants at lower concentrations than its human equivalent in laboratory tests. Several effective concentrations overlapped with levels previously recorded in heavily polluted waters, raising questions about how pharmaceutical residues are assessed for environmental risk.
The research, published in Environmental Science & Technology on August 24, examined medaka, or Japanese rice fish, and ayu. Tokyo University of Science publicized the findings on September 9.
The experiments measured the activity of fish proteins produced in cultured cells. Harm to the behavior, reproduction or survival of fish living at those water concentrations remains to be established.
Testing the proteins that recycle chemical signals
Serotonin, dopamine and norepinephrine are neurotransmitters, chemicals that carry signals between nerve cells. Transporter proteins help clear these messengers after a signal by moving them back into cells.
Many antidepressants act on that recycling process. Fish have related proteins, but a drug designed around human biology may interact differently with another species.
The team, led by Professor Shinichi Miyagawa of Tokyo University of Science and including Professor Masaru Ihara of Kochi University, examined four transporters from each fish species. Two handle serotonin and are called SERTa and SERTb; the others handle dopamine and norepinephrine.
According to the university’s account of the work, researchers produced the fish proteins in genetically engineered human cells. They used a fluorescent marker to measure how strongly medicines inhibited transport activity.
SERTa was more sensitive than SERTb in both species. Compared with the human serotonin transporter, fish SERTa also responded to several medicines at substantially lower concentrations.
The laboratory result is not a river safety limit
The researchers compared their results with published measurements from aquatic environments. For medaka SERTa, the concentration producing 50% inhibition was approximately 357 nanograms per liter for duloxetine and 835 nanograms per liter for fluoxetine.
A nanogram is one-billionth of a gram. The 50% inhibition measure, known as IC50, describes a response in the particular laboratory assay. It is not a threshold at which half the fish die or a legal water-quality limit.
The overlap concerned the upper end of concentrations reported in heavily affected aquatic systems. It should not be read as evidence that those levels occur throughout rivers and lakes.
A living fish also absorbs, distributes, breaks down and excretes chemicals. The amount reaching a protein inside its body can differ from the concentration in surrounding water. Longer exposure and mixtures of substances add further questions that this cell-based comparison cannot resolve.
The researchers call for studies in living animals under environmentally realistic exposure conditions. Their molecular results give those studies a target to investigate.
Wastewater carries medicines beyond their intended use
The OECD’s report on pharmaceutical residues identifies untreated household wastewater and discharges from municipal treatment plants as major routes into the environment. Manufacturing and farming can also create local pollution hotspots.
Some active medicine and its breakdown products leave the body in excreted waste. Treatment can reduce contamination, but removal is not complete for every substance.
The OECD also identifies chemical mixtures as a source of uncertainty. Testing one medicine at a time leaves open how substances sharing a biological target behave together in wastewater.
Who pays for more thorough treatment?
In Europe, the financing question is already written into law. The revised Urban Wastewater Treatment Directive requires countries to establish producer-responsibility arrangements by December 31, 2028, covering pharmaceuticals and cosmetics, subject to exemptions.
Those producers must fund at least 80% of specified costs for advanced treatment to remove micropollutants and associated monitoring. The requirement concerns this additional treatment, including investment and operating costs, rather than 80% of every wastewater expense.
As we reported in our coverage of industrial water reuse, the treatment required and the cost of delivering it are central to water investment decisions. Ecological protection adds a further question: which substances need closer control in the receiving water?
The fish study neither calculates treatment costs nor establishes a new discharge standard. It offers evidence for choosing substances and species for further assessment.
Under the EU directive, each producer’s contribution is to reflect both the quantity of substances placed on the market and their hazardousness in wastewater. Better evidence about effects on wildlife may therefore inform how the costs are allocated, as well as what treatment plants need to remove.
