The reproductive state of lactation, which occurs in all mammals, is associated with an inhibition of reproductive cyclicity and ovulation due to a suppression of gonadotropin releasing hormone (GnRH), the hypothalamic neuroendocrine neurons regulating reproduction. The focus of our studies is to identify the afferent neuronal pathways activated during lactation by the suckling stimulus that are responsible for the suppression of GnRH neuronal function. This past year, we made the following observations 1) Using neuronal tract tracing techniques and confocal microscopy, we have shown that NPY neurons activated by the suckling stimulus project to areas containing GnRH neurons and make contact with GnRH neuronal processes. These results suggest a possible neuronal mechanism by which suckling inhibits GnRH neuronal activity. 2) The large increase in food intake during lactation most likely occurs in response to substances that signal changes in metabolic activity, such as leptin (a satiety-inducing substance) and agouti-related transcript (ART, induces obesity). During lactation, blood leptin levels are greatly suppressed, whereas ART expression is increased in the hypothalamus. Current studies in the laboratory are examining the relationship between the changes in leptin and ART and the increase in NPY neuronal activity and food intake. In addition, we are exploring possible interactions between the regulation of food intake and of gonadotropin releasing hormone neuronal activity during lactation. It is well established that many causes of infertility are related to changes in energy homeostatis, such as in exercised-induced amenorrhea and anorexia nervosa. An understanding of the mechanisms by which the suckling stimulus imposes an inhibition on GnRH neuronal function provides information that is relevant to primates (including humans), in which the reproductive neuroendocrine axis regulating ovarian cyclicity is also inhibited. These studies have relevance to women's reproductive health as they will increase our understanding of hypothalamic causes of infertility and provide new approaches for contraception.
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