The researchers asked whether the monoamine transporters targeted by human antidepressants are pharmacologically similar in fish, and whether human drug data could understate the concentrations that affect fish proteins.
The research question and why it matters
The researchers asked whether the monoamine transporters targeted by human antidepressants are pharmacologically similar in fish, and whether human drug data could understate the concentrations that affect fish proteins.
Antidepressants and their metabolites have repeatedly been detected downstream of wastewater treatment, and some fish experiments have reported altered behavior at environmentally relevant exposures. Earlier reviews concluded that standard toxicity endpoints often imply low risk while behavioral effects and tissue accumulation remain concerns. This study adds a cross-species molecular comparison that can help explain why human pharmacology alone may miss fish-specific responses.
What researchers found
SERTa was consistently more drug-sensitive than SERTb in both fish species. For some medicines, the concentration needed to inhibit fish SERTa by half was more than tenfold lower than for human SERT. Duloxetine, fluoxetine, citalopram and paroxetine inhibited medaka SERTa at concentrations from a few hundred to about 1,300 nanograms per liter—a range that overlaps measurements reported for severely contaminated waterways. Drugs such as mirtazapine and quetiapine, which do not primarily target human monoamine transporters, also inhibited some fish transporters in the assay.
Key results from the tested systems
fish compared
Medaka and ayu are evolutionarily distant, but both carried the same four transporter categories tested.
transporter types per species
The assays covered DAT, NET, SERTa and SERTb; mammals have lost the SERTb lineage.
largest reported sensitivity difference
Some drugs inhibited fish SERTa at less than one-tenth the concentration needed for human SERT.
medaka SERTa inhibition range
Four antidepressants reached half-inhibition within this approximate range, which overlaps reports from heavily polluted waters.
How the research worked
The team isolated genes for dopamine transporter, norepinephrine transporter and two fish serotonin-transporter lineages, SERTa and SERTb, from medaka and ayu brains. Each cloned protein was produced in cultured human cells. Researchers measured uptake of a fluorescent surrogate and then generated concentration-response curves for commonly used antidepressants and related medicines. Sequence comparisons and expression measurements were used to interpret why the two serotonin-transporter lineages behaved differently.
How to interpret this design
The design determines what kind of conclusion the evidence can support. Direct measurement strengthens the reported observation, while generalization beyond the tested subjects, material, place or conditions requires additional evidence.
The evidence comes from a controlled physical or chemical system. That control helps establish what happened under the tested conditions, while scale-up, durability, manufacturing and real-world performance remain separate questions.
What strengthens or limits the finding?
The study directly compared cloned transporters under the same controlled assay and reported drug-specific concentration-response measurements. It establishes molecular sensitivity, not toxicity in intact animals; metabolism, exposure duration, mixtures and whole-organism responses could change the result.
The work advances biological understanding before adequate human testing. Claims about treatment, prevention or human safety would go beyond this evidence.
Funding and disclosure context
The recorded funding source is: Japanese Grants-in-Aid for Scientific Research, including 20H006301, 21H02522 and 25H01183; Environment Research and Technology Development Fund grants 5-1952 and 5-2204 from Japan's Environmental Restoration and Conservation Agency and Ministry of the Environment; and additional UK–Japan research support listed in the paper. The recorded conflict information is: The authors declared no competing financial interest. Funding or a disclosed relationship does not by itself invalidate a result, but it is relevant when judging design choices, analysis and the need for independent replication.
What it means
Environmental risk assessment cannot assume that a medicine's human target profile transfers unchanged to aquatic species. The results identify particular drug–transporter combinations worth prioritizing in water monitoring and whole-animal testing, especially where wastewater creates unusually high concentrations.
Deeper analysis
A molecular warning signal is not an ecological damage estimate
The clean comparison of cloned proteins answers whether a drug can interact with a fish target. Risk in a river also depends on how much drug reaches fish tissue, how long it remains, whether the animal compensates and whether any resulting change affects survival or reproduction.
Two serotonin transporters reveal an evolutionary split
Fish retained SERTa and SERTb after an ancient gene duplication, while mammals retained only the SERTa-related form. Their different drug responses show why naming proteins by a shared function is not enough to assume identical pharmacology.
Unexpected targets complicate human-to-wildlife extrapolation
Mirtazapine and quetiapine are not chiefly classified by monoamine-transporter inhibition in people, yet they affected fish transporters in the assay. Environmental screening based only on a medicine's intended human target could therefore overlook relevant interactions.
The overlap deserves follow-up, not alarm
The reported assay concentrations intersect measurements from severely contaminated locations. That is a rational reason to prioritize monitoring and in-vivo studies, but it cannot establish how common those exposures are or whether they translate into harm.
What it does NOT prove
- It does not show that ordinary river concentrations are harming fish populations.
- It does not demonstrate behavioral, reproductive or survival effects in living medaka, ayu or any other fish.
- It does not establish a safe or harmful water-quality threshold; half-inhibition in a cell assay is not an ecological-effects limit.
- It does not mean antidepressants should be avoided by patients or that prescribed use is the only source of pharmaceutical residues.
- It does not show that every fish species is more sensitive than humans to every antidepressant.
Important limitations
- The experiments tested isolated proteins in engineered human cells rather than intact fish, so absorption, metabolism, excretion and compensatory biology were absent.
- Only two fish species were characterized directly; their evolutionary distance broadens the comparison but cannot represent the diversity of fishes.
- The environmental overlap applies to concentrations reported in heavily polluted waters, not necessarily typical rivers or treatment-plant effluent.
- Single-compound laboratory curves do not reproduce chronic exposure to changing mixtures of pharmaceuticals and other pollutants.
- A fluorescent uptake assay measures transporter function under specific conditions; its half-inhibition values are not interchangeable with whole-animal effect concentrations.
- The study identifies molecular targets but does not connect the measured inhibition to population-level outcomes such as growth, reproduction or predator avoidance.
How this fits with previous research
Antidepressants and their metabolites have repeatedly been detected downstream of wastewater treatment, and some fish experiments have reported altered behavior at environmentally relevant exposures. Earlier reviews concluded that standard toxicity endpoints often imply low risk while behavioral effects and tissue accumulation remain concerns. This study adds a cross-species molecular comparison that can help explain why human pharmacology alone may miss fish-specific responses.
Questions still unanswered
- Do the same concentration rankings appear when living fish are exposed through water over days or months?
- Which transporter changes produce measurable effects on feeding, stress responses, reproduction or predator avoidance?
- How do metabolites and mixtures of several psychoactive medicines interact at environmentally realistic levels?
- Are early life stages or particular fish lineages more vulnerable than medaka and ayu?
- How effectively do different wastewater-treatment methods remove the drugs highlighted by the assay?
Relevant U.S. government resources
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Fish transport proteins responded to some antidepressants at lower concentrations than the human version
This review was developed from the source record below and, when separately available, the primary paper or government report. The summary and analysis on this page are original editorial writing.
- Source organization
- Tokyo University of Science
- Source type
- University
- Authors
- Kikuko Honda, Minguang Han, Fuyuka Mori, Ayaka Morinaga, Yuka Nishimura, Renji Kaneko, Kie Oizumi, Hana Kajiyama, Mariko O. Ihara, Han Zhang, Daisuke Kato, Kenji Toyota, Anke Lange, Charles R. Tyler, Taisen Iguchi, Yuji Mushirobira, Masaki Nagae, Kiyoshi Soyano, Masaru Ihara and Shinichi Miyagawa
- Journal / report
- Environmental Science & Technology
- Publication date
- August 24, 2026
- DOI
- 10.1021/acs.est.6c04982
- PMID
- Not available
- Institution
- Tokyo University of Science-led collaboration with Kochi University, Kyoto University, Dalian University of Technology, Hiroshima University, University of Exeter, Yokohama City University and Nagasaki University
- Funding
- Japanese Grants-in-Aid for Scientific Research, including 20H006301, 21H02522 and 25H01183; Environment Research and Technology Development Fund grants 5-1952 and 5-2204 from Japan's Environmental Restoration and Conservation Agency and Ministry of the Environment; and additional UK–Japan research support listed in the paper
- Conflicts
- The authors declared no competing financial interest
- Open access
- Yes
- Reuse approach
- Facts and numerical ranges summarized in original language from Tokyo University of Science, the peer-reviewed paper and its open supporting information; no source wording, photographs, figures, tables, protein sequences or code reproduced.
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