Changes in plasma thyroid hormones and cortisol levels in crucian carp (Carassius auratus) exposed to the extracted microcystins

The endocrine response of crucian carp injected intraperitoneally with extracted microcystins (MC) was investigated in this study. Fish were injected intraperitoneally either with 0.75% NaCl (control) and Microcystis extract corresponding to 150 and 600 μg microcystins per kg body weight. The plasma levels of triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4), and cortisol were determined at 0, 1, 3, 12, 24, and 48 h post-administration of MC-containing extract. Treated fish displayed abnormal behaviors, such as a startle response and disoriented swimming, as well as changes in ventilation rates. Plasma cortisol concentrations of fish in both dose groups significantly increased after administration of extracted MC and remained high throughout the experiment, which suggested that MC elicited a stress response in treated fish. The profiles of cortisol changes in treated fish appeared to be dose dependent, indicating that fish in the high dose group experienced greater MC-induced disturbance. Mortality occurred after 12 h in the high dose group. Plasma levels of T4, T3, FT4, and FT3 did not vary significantly between the control fish. In contrast to this, fish exposed to MC-containing extract showed significant declines in T3, FT4, and FT3 levels in a dose-dependent manner throughout the experiment. Plasma T4 levels, however, did not vary significantly in the low dose group, whereas they decreased significantly at 48 h post injection in the high dose group. This study demonstrates that administration of microcystins-containing extract causes a stress response and reduces the plasma levels of thyroid hormones in crucian carp. These results illustrate that microcystins exerted potent effects on the endocrine system of crucian carp, through activating their hypothalamus-pituitary- interrenal axis and disturbing thyroid function.