Regulation of MT melatonin receptor expression in the foetal rat pituitary.

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 MT(1) 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 MT(1) promoter activity and therefore may represent a key inhibitor of MT(1) expression in early pituitary development. During late embryogenesis, MT(1) mRNA was expressed in both the ventral and dorsal pituitary. Analysis of a 1.5-kb fragment of the rat MT(1) 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 MT(1) promoter activity in vitro, dual-labelled in situ hybridisation revealed no colocalisation of MT(1) and Pit-1 mRNAs in vivo at e19.5. By contrast, all MT(1) positive cells colocalised with alphaGSU and most with betaTSH 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.