Effects of exposure to microcystin-LR at environmentally relevant concentrations on the metabolism of thyroid hormones in adult zebrafish (Danio rerio)

Microcystin-LR (MC-LR) has the potential to disturb thyroid hormone homeostasis. However, the effects of MC-LR at environmentally relevant concentrations on the thyroid system in adult fish are still unclear. In this study, adult zebrafish were exposed to 0, 1, 5, and 25 mu g/L MC-LR for 7, 14, 21, and 28 days. Whole body thyroid hormones (THs) levels and thyroid follicle histology were used to assess thyroid function. The transcription of corticotropin-releasing hormone (crh), thyroid-stimulating hormone (tsh), transthyretin (ttr), thyroid hormone receptors (trs) genes, and the activities of iodothyronine deiodinases (IDs) were investigated to study the process of TH metabolism disruption. No differences in the histopathology of thyroid follicles and unchanged T-4 levels were observed in adult zebrafish. A significant decline in T-3 levels associated with a decrease in ID2 activity in male zebrafish was observed at 21 days exposure. Moreover, the mRNA expression of tsh, ttr and trs appeared to be a dynamic process as expression first decreased and then increased with continued exposure. These results indicated that exposure to MC-LR did not inhibit the production of TH. The decrease in ID2 activity may be an important factor in the decline of T-3 levels. Furthermore, it seems that the fish triggered a compensatory mechanism to maintain TH homeostasis in respond to environmental concentrations of MC-LR which induced TH disruption. (C) 2016 Elsevier Ltd. All rights reserved.