Fluoxetine treatment affects nitrogen waste excretion and osmoregulation in a marine teleost fish

Measurable quantities of the selective serotonin reuptake inhibitor (SSRI), fluoxetine, have been found in surface waters and more recently in the tissues of fish. This highly prescribed pharmaceutical inhibits the reuptake of the monoamine, serotonin (5-HT; 5-hydroxytryptamine), causing a local amplification of 5-HT concentrations. Serotonin is involved in the regulation of many physiological processes in teleost fish including branchial nitrogen excretion and intestinal osmoregulation. Since the gill and intestine are directly exposed to the environment, environmental exposure to fluoxetine has the potential of affecting both these mechanisms. In the present study, we test the potential sensitivity of these processes to fluoxetine by implanting gulf toadfish, Opsanus beta, intraperitoneally with different concentrations of fluoxetine (0 (control), 25, 50, 75 and 100 μg g^-1). Fluoxetine treatments of 25 and 50 μg g^-1 were sub-lethal and were used in subsequent experiments. Fish treated with both 25 and 50 μg g^-1 fluoxetine had significantly higher circulating levels of 5-HT than control fish, suggesting that any 5-HT sensitive physiological process could potentially be affected by these two fluoxetine doses. However, only fish treated with 25 μg g^-1 fluoxetine showed a significant increase in urea excretion. A similar increase was not measured in fish treated with 50 μg g^-1 fluoxetine, likely because of their high circulating levels of cortisol which inhibits urea excretion in toadfish. Intestinal fluid absorption appeared to be stimulated in fish treated with 25 μg g^-1 fluoxetine but inhibited in 50 μg g^-1 treated fish. Despite these differing responses, both doses of fluoxetine resulted in lowered plasma osmolality values, which was expected based on the stimulation of fluid absorption in the 25 μg g^-1 fluoxetine-treated fish but is surprising with the 50 μg g^-1 treated fish. In the case of the latter, the corresponding stress response invoked by this level of fluoxetine may have resulted in an additional osmoregulatory response which accounts for the lowered plasma osmolality. Our findings suggest that branchial urea excretion and intestinal osmoregulation are responsive to the SSRI, fluoxetine, and further investigation is needed to determine the sensitivity of these processes to chronic waterborne fluoxetine contamination.