Abstract
The presence of chemicals in the environment continues to garner attention as many are pharmaceuticals with specific modes of action on conserved molecular targets (e.g., enzymes or receptors), with the potential to inhibit or amplify hormones that drive development of key physiological traits in animals. For example, the pharmaceutical Finasteride (FIN), used to treat male human pattern hair loss, suppresses masculine trait development in mammals and amphibians by inhibiting the enzyme (5α-reductase; 5αR) catalyzing conversion of testosterone(T) to the potent androgen dihydrotestosterone (DHT). While the developmental model for fish has historically assumed 11-ketotestosterone as the primary androgen for this taxa, there is evidence that DHT may also fulfill a similar role in fish as it does in mammals. In vivo (living organism) assays exposing fish to compounds that inhibit 5αR, like FIN, can thus provide valuable data to better inform hazard and risk assessment of pharmaceuticals, while also helping to reduce future usage of vertebrates in ecotoxicological testing. Two distinct aquatic laboratory studies were conducted exposing adult male fathead minnow and medaka to FIN. The objective of these studies was to characterize the molecular responses and test whether FIN has similar effects in fish as in humans, thus comparing potential responses to a pharmaceutical between two phylogenetically distant species that possess different androgen receptor subtypes. Fish were exposed to a water concentration of Finasteride (1 mg/L)and a control under flow through conditions for up to 21 days. Sub-samples of fish (n=3 per replicate; n=12 per treatment/control) were sacrificed after 2, 6, 24, 48, 168, and 504 hours of exposure. In the fathead minnow study, measurements at each time point included standard length, body weight, T and DHT blood plasma concentrations, testes type-2 isoform 5αR gene expression (srd5a2), blood plasma FIN concentration, and nuptial tubercle score (504-houronly). Measurements in the medaka study included standard length, body weight, testes srd5a2expression, and papillary processes count (504-hour only). The FIN water concentration was also confirmed at each time point for both studies. Relationships between the response variables and factors such as treatment, exposure duration, and replicate were investigated via Pearson correlation analysis and mixed effect ANOVA. In fathead minnows exposed to FIN, plasma DHT concentration was significantly reduced (p<0.001) from controls; in both species, testes srd5a2 expression was significantly upregulated in exposed fish compared to controls (fathead minnow, p<0.001; Japanese medaka, p=0.006), suggesting DHT plays an important role in fishes as well as other taxa, providing evidence of conserved molecular targets in this androgen biosynthesis pathway. Mean internal plasma concentrations of FIN were higher than water concentrations after 48 hours of exposure and were nearly 1.5 times greater than the water concentration after 168- and 504-hours of exposure. Individual variability of androgens and FIN in plasma was high, suggesting that a direct dose-response between individual internal plasma and steroid concentration is difficult given the inherent variability of these hormones in fish. The steroidal and gene expression changes induced by FIN provide evidence for read across between humans and fish through 5αR inhibition. The plasma FIN concentrations highlight the importance of understanding the internal exposure when assessing pharmacological effects in fish.