Regulation of melatonin receptor expression by gonadotrophin-releasing hormone (GnRH)

The hormone melatonin has many important functions in biology, many of which involve the regulation of other hormonal systems. Most of our understanding of melatonin biology comes from studies of adult animals, in which melatonin only acts on a restricted number of tissues throughout the body. However, it is known that more tissues in the body are sensitive to melatonin in foetuses and newborns than in adults. To date, the biological significance of the broader sensitivity to melatonin during development is not known. One way in which we can start to explore these undefined roles of melatonin is to gain a clear understanding of the nature of the processes that underlie these developmental changes. This project will build upon recent studies by the applicant and help us to understand how tissues change their sensitivity to melatonin during development. The applicant has recently published multiple research papers investigating the developmental regulation of melatonin sensitivity in gonadotroph cells of the pituitary gland, which is the best-characterised model for studying the developmental biology of melatonin. Pituitary gonadotroph cells are essential for reproduction function, including pubertal development. We have provided preliminary evidence to suggest that the developmental secretion of a brain-derived chemical, gonadotrophin-releasing hormone (GnRH), is the primary stimulus for decreasing sensitivity to melatonin in gonadotroph cells. Furthermore, the effects of GnRH appear to be mediated by a cellular signalling molecule called Egr-1. The current project will be the first to provide a detailed investigation into the role of GnRH in determining melatonin sensitivity. We will use a range of complementary experimental techniques to manipulate both GnRH and Egr-1 activity and then evaluate the effects of these manipulations on melatonin sensitivity. These techniques will include the use of isolated cells grown in laboratory conditions, in order to minimise the use of animals. The results of this work will have important implications for scientists and the public. On completion of the project, we will have a greater understanding of the biological role of melatonin and, more generally, an insight into developmental changes in hormone-secreting cells. As melatonin is available over-the-counter as a food supplement in some countries, this work will also help us to understand potential health risks of taking melatonin in certain situations e.g. during pregnancy.

Although the melatonin receptors are highly regionalised in adult mammals, their expression exhibits a wider distribution in perinatal mammals. This not only suggests novel physiological roles for melatonin during development, but also the presence of as yet undefined mechanisms that modify the physiology of multiple cell types by regulation of their sensitivity to melatonin. The best studied model of perinatal melatonin receptor expression is the neonatal pituitary gonadotroph. Melatonin inhibits gonadotrophin-releasing hormone (GnRH) induced luteinising hormone (LH) secretion and has been proposed as a regulator of puberty. In recent work, we have undertaken analysis of the MT1 melatonin receptor, which is the predominant subtype in the neuroendocrine system. Promoter analysis identified Pitx-1 and Egr-1 as potent regulators of MT1 gene expression in COS-7 cells. Interestingly, Egr-1 is an early response gene following GnRH stimulation of gonadotroph cells. Moreover, mice lacking GnRH express more pituitary MT1 mRNA. This proposal will use complementary in vivo and in vitro models to directly test the hypothesis that GnRH down-regulates MT1 via Egr-1 induction. By the end of this project, we will have an increased understanding of MT1 promoter regulation, particularly the importance of putative Egr-1 cis-elements. We will be able to compare the expression of MT1 mRNA in mice lacking Egr-1 with mice lacking GnRH. Finally, we will know the effect of reduced GnRH signalling, by both pharmacological and physiological means, on the expression of both Egr-1 and MT1 mRNA. Together, these experiments will provide important advances in our understanding of how neuroendocrine signalling regulates the sensitivity of 'peripheral' endocrine cells to melatonin. The work will therefore have major benefit to both the understanding of melatonin physiology but also the developmental plasticity of neuroendocrine systems.