Disruption of the Endocrine Control of Final Oocyte Maturation in Teleosts by Xenobiotic Chemicals

Final oocyte maturation (FOM) in fish and other vertebrates is under precise endocrine control and involves changes in hormone secretion at all levels of the hypothalamus-pituitary-gonadal axis. Several potential sites and mechanisms of chemical disruption of the endocrine system controlling FOM by are discussed. Neurotoxic chemicals such as lead and PCBs can alter monoamine neurotransmitter function and xenoestrogens can interfere with steroid feedback mechanisms at the hypothalamus and pituitary to impair the neuroendocrine control of gonadotropin secretion. Chemicals which disrupt calcium homeostasis such as cadmium can interfere with calcium-dependent signal transduction pathways activated by reproductive hormones in the pituitary and gonads. Other xenobiotics may disrupt maturation-inducing steroid (MIS) function by impairing its synthesis or receptor binding. The problems in assessing endocrine disruption of FOM are discussed. The relatively few investigatons reported in the literature on endocrine disruption of FOM in fishes by chemicals indicate that organochlorine and organophosphorus pesticides at concentrations less than one ppb can impair induction of FOM in response to gonadotropin and the MIS. Moreover, evidence is presented that certain organochlorine pesticides block MIS action by binding to the MIS receptor which is localized on the oocyte plasma membrane. Steroid membrane receptor function may be particularly susceptible to interference by hydrophilic chemicals. Finally, an in vitro bioassay capable of screening many chemicals simultaneously for their ability to disrupt the endocrine control of FOM is described.