Regulation of MT1 Melatonin Receptor Expression in the Foetal Rat Pituitary

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Abstract
During development, melatonin receptors are transiently expressed in multiple neuroendocrine tissues, suggesting a novel role for melatonin in developmental physiology. The best characterised model of melatonin signalling during development is the pars distalis of the rat pituitary. However, although many studies have characterised the postnatal decline of melatonin receptors in the rat pars distalis, the mechanism(s) that time the developmental onset of receptor expression during embryogenesis are unknown. Analysis of these mechanisms may yield important information regarding the putative role of melatonin in neuroendocrine development. Here, we report the expression of MT1 melatonin receptor mRNA in the rat pituitary from embryonic day 15.5 (e15.5). Prior to e15.5, the homeodomain transcription factor Msx‐1, an inhibitor of cellular differentiation, is widely expressed throughout the pituitary. In transient transfection experiments, Msx‐1 potently inhibited pituitary homeobox‐1 (Pitx‐1)‐induced MT1 promoter activity and therefore may represent a key inhibitor of MT1 expression in early pituitary development. During late embryogenesis, MT1 mRNA was expressed in both the ventral and dorsal pituitary. Analysis of a 1.5‐kb fragment of the rat MT1 promoter revealed four putative cis‐elements for the POU domain factor Pit‐1, which is associated with mid‐dorsal cell lineages. Although Pit‐1 induced a strong, dose‐dependent stimulation of MT1 promoter activity in vitro, dual‐labelled in situ hybridisation revealed no colocalisation of MT1 and Pit‐1 mRNAs in vivo at e19.5. By contrast, all MT1 positive cells colocalised with αGSU and most with βTSH mRNA. Our data therefore implicate the decline of Msx‐1 expression as a key event that times the onset of melatonin receptor expression to the differentiation of endocrine cells types in the developing pituitary gland, and suggest that the melatonin‐sensitive cells in the embryonic pituitary are primarily Pit‐1‐independent thyrotrophs in the rostral pituitary, with a secondary population of pars distalis gonadotrophs.