Amphibians have been well-studied models in endocrine disruption research because of their global distribution, aquatic developmental phase, global decline, and ease of husbandry. In this research we used an amphibian model system, Xenopus tropicalis, to provide a mechanism to test questions of EDCs and reproduction in a relatively short period of time. In order to develop this species as a model for endocrine disruption of behavior, we evaluated induction of male and female sexual behavior by treatment with a gonadotropin hormone releasing hormone (GnRH) agonist. During mating we observed approaches, touches, arm-waves, vocalizations, and amplexus. GnRH-agonist treatment had no impact on female behaviors, but increased male approaches, touches and amplexus. Next we exposed X. tropicalis for 30 days to 0, 10−8M estradiol (E2), or 10−7 M or 10−8M OP, and then treated with GnRH agonist. There was no impact on female behaviors, but in males, E2 and OP-7 doses increased incidence of arm waving and E2, OP-8, and OP-7 increased calling behavior. We also developed a total sexual behavior score by calculating the sum of all behaviors. We found no significant difference in the female or male total sexual behavior score. My results indicate that OP acts similar to E2 on male X. tropicalis behavior.
In our second study we evaluated body mass, snout-vent length (SVL), organ mass, gonads (nest count, lobule count, lobule size), and nuptial pad coloration in frogs treated with GnRH agonist, after exposure to OP and E2. We found no significant difference in change in SVL, body mass, or organ weights. There was no significant difference in gonad lobule size. Numbers of gonadal cysts and lobule count increased in only the OP-7 group. The number of cysts per gonad decreased with exposure to OP-7. These results indicate that unlike the estrogen like effects of OP on behavior, OP does not act like estrogen in gonad cysts and lobules. E2 also does not have an effect on cyst and lobule count. Looking specifically at the OP results I can conclude OP effects on the gonads is also dose dependant. There was also a significant difference in nuptial pad coloration. Both the E2 and OP-7 groups had decreased nuptial pad darkness compared to the control and OP-8 groups. These results indicate that OP-7 acts similar to OP-7 on nuptial pad coloration. Nuptial pad coloration, a secondary sex characteristic, is androgen dependant. OP-7 acts through anti-androgenic mechanism and is dose dependant.
The results of these studies have provided data on male sexual behaviors and OP effects on male behaviors, testes, and nuptial pad coloration. More importantly these results show that a single compound OP can act very differently on different physiological systems in a dose dependant manner. Understanding this adds complexity to understanding the effects of EDCs.