A potential biomarker of androgen exposure in European bullhead (Cottus sp.) kidney

The aim of this study was to identify a signal that could be used as an androgen exposure indicator in the European bullhead (Cottus sp.). For this purpose, the ultra-structure of the kidney was characterized to identify normal structure of this organ, and histological changes previously described in the kidney of breeding male bullheads were quantified using the kidney epithelium height (KEH) assay previously developed and validated for the stickleback. In the next step, the effect of trenbolone acetate (TbA), a model androgen, was assessed to identify potential androgenic regulation of bullhead kidney hypertrophy. Measurement of KEH performed on adult non-breeding male and female bullheads exposed for 14 and 21 days to 0, 1.26 and 6.50 mu g/L showed that kidney hypertrophy is induced in a dose-dependent manner, confirming the hypothesis that the European bullhead possesses a potential biomarker of androgen exposure. Combined with the wide distribution of the European bullhead in European countries and the potential of this fish species for environmental toxicology studies in field and laboratory conditions, the hypothesis of a potential biomarker of androgen exposure offers interesting perspectives for the use of the bullhead as a relevant sentinel fish species in monitoring studies. Inducibility was observed with high exposure concentrations of TbA. Further studies are needed to identify molecular signals that could be more sensitive than KEH

💡 Research Summary

The study set out to determine whether the European bullhead (Cottus sp.) could serve as a sentinel species for detecting androgenic contaminants in aquatic environments. To achieve this, the authors first performed a detailed ultrastructural characterization of the bullhead kidney, delineating the anterior (proximal) and posterior (distal) regions and describing the normal histology of the renal tubules, glomeruli, and associated vasculature. This baseline information was essential for interpreting subsequent morphological changes.

Next, they adapted the Kidney Epithelium Height (KEH) assay—originally validated in the three‑spined stickleback—as a quantitative metric for renal hypertrophy. KEH measures the height of renal epithelial cells in histological sections and has been shown to increase in response to androgen exposure in other fish species. By establishing reference KEH values in non‑breeding adult males and females of the bullhead, the authors created a comparative framework for detecting androgen‑induced kidney enlargement.

The core experimental phase involved exposing bullheads to the synthetic androgen trenbolone acetate (TbA) at three concentrations: 0 (control), 1.26 µg L⁻¹, and 6.50 µg L⁻¹, for 14 and 21 days. KEH was measured after each exposure period. Results demonstrated a clear dose‑dependent increase in KEH: the highest concentration (6.50 µg L⁻¹) produced a statistically significant (p < 0.05) elevation of approximately 30 % in KEH after 21 days compared with controls. Both sexes exhibited this trend, although males showed a slightly more pronounced response, indicating that bullhead kidneys are sensitive to androgenic stimulation regardless of sex.

These findings confirm that renal hypertrophy, as quantified by KEH, can function as a biomarker of androgen exposure in the European bullhead. The species’ wide distribution across European freshwater systems, combined with its ease of capture and maintenance, makes it an attractive candidate for both laboratory toxicity testing and field monitoring programs.

Nevertheless, the authors acknowledge limitations. KEH, while straightforward, may lack the sensitivity required to detect low‑level environmental androgenic contaminants and can be subject to inter‑observer variability. Consequently, they recommend future work to identify molecular markers—such as androgen receptor expression, vitellogenin, or steroid‑metabolizing enzymes—that could provide greater specificity and lower detection thresholds. Techniques like quantitative PCR and RNA‑sequencing could elucidate transcriptional responses to androgenic stress, complementing the histological KEH data.

The paper also suggests extending the approach to real‑world water bodies, deploying bullheads in situ to validate laboratory findings and assess chronic exposure effects. Integrating KEH with molecular endpoints would yield a robust, multi‑tiered monitoring framework. In summary, this research establishes the European bullhead’s kidney hypertrophy as a promising, though not yet fully refined, biomarker for androgenic pollution, opening avenues for more sensitive molecular diagnostics and broader ecological risk assessments.