Leptin regulates prothyrotropin-releasing hormone biosynthesis -: Evidence for direct and indirect pathways

The hypothalamic-pituitary-thyroid axis is down-regulated during starvation, and falling levels of leptin are a critical signal for this adaptation, acting to suppress preprothyrbtropin-releasing hormone (prepro-TRH) mRNA expression in the paraventricular nucleus of the hypothalamus, This study addresses the mechanism for this regulation, using primary cultures of fetal rat hypothalamic neurons as a model system. Leptin dose-dependently stimulated a 10-fold increase in pro-TRH biosynthesis, with a maximum response at 10 nM, TRH release was quantified using immunoprecipitation, followed by isoelectric focusing gel electrophoresis and specific TRH, radioimmunoassay, Leptin stimulated TRH release by 7-fold. Immunocytochemistry revealed that a substantial population of cells expressed TRH or leptin receptors and that 8-13% of those expressing leptin receptors coexpressed TRH. Leptin produced a 5-fold induction of luciferase activity in CV-1 cells transfected with a TRH promoter and the long form of the leptin receptor cDNA, Although the above data are consistent with a direct ability of leptin to promote TRH biosynthesis through actions on TRH neurons, addition of alpha -melanocyte-stimulating hormone produced a 3.5-fold increase in TRH biosynthesis and release, whereas neuropeptide Y treatment suppressed pro-TRH biosynthesis similar to3-fold. Furthermore, the melanocortin-4 receptor antagonist SHU9119 partially inhibited leptin-stimulated TRH release from the neuronal culture, Consequently, our data suggest that leptin regulates the TRH neurons through both direct and indirect pathways.