Thyrotropin-releasing hormone: its distribution and metabolism during development in bullfrog (Rana catesbeiana).

The development changes in the metabolism of thyrotropin-releasing hormone (TRH), cyclo (His-Pro) formation from TRH, and the levels of endogenous TRH in frog brain and skin were determined. The results indicated that TRH concentrations were considerably higher in brain than in skin, and in both of these structures TRH content increased significantly following metamorphosis to adulthood. This increase in TRH concentration is probably a reflection of a marked decrease in TRH-metabolism in adult frogs compared to tadpoles. However, the formation of cyclo (His-Pro) from TRH increased during the developmental period reaching to a maximum in adulthood. The possible role of cyclo (His-Pro) in the amphibian developmental process is discussed in relation to our recent observation showing cyclo (His-Pro) inhibition of prolactin secretion.