P. THOMAS AND I. A. KHAN, The University of Texas at Austin, Marine Science Institute, 750 Channelview Drive, Port Aransas, Texas 78373-5015, USA

Teleost reproduction is primarily controlled by hormones secreted by the hypothalamus-pituitary-gonadal (HPG) axis. Chemicals can potentially act at numerous sites on the HPG axis and by a variety of mechanisms to alter hormone secretion, action or metabolism and disrupt reproductive function. Some mechanisms of endocrine disruption which have been identified in Atlantic croaker Micropogonias undulatus are discussed. Evidence is presented that Kepone (chlordecone) can exert estrogenic effects at the liver to stimulate vitellogenesis in vitro by binding to the hepatic estrogen receptor. Other in vitro studies show that cadmium can act directly at the pituitary and at the ovary to stimulate hormone secretion. These results indicate that the stimulatory effect of cadmium on gonadal steroidogenesis is mediated by activation of the adenylate cyclase second messenger system. Finally, we show that the impairment of reproductive function in male croaker after chronic exposure to lead in vitro is associated with a lack of pituitary response to stimulation by luteinizing-hormone releasing hormone (LHRH). Interestingly, the decline in gonadotropin secretion was accompanied by decreases in the hypothalamic content of serotonin, a neurotransmitter which has been shown to augment the gonadotropic response to LHRH in this species. Taken together, these results suggest that lead impairs reproductive function by disrupting the serotonergic system in the hypothalamus that regulates gonadotropin secretion. It is concluded that chemicals can act via a variety of mechanisms to disrupt reproductive endocrine function in teleosts.