Changes of pituitary proopiomelanocortin mRNA levels during metamorphosis of the bullfrog larvae

Adrenal corticoids accelerate metamorphosis of amphibians by potentiating the action of thyroid hormone. Adrenal corticoid secretion is considered to be controlled mainly by adrenocorticotropic hormone generated from proopiomelanocortin (POMC) in the anterior lobe of the pituitary. In order to assess the changes in POMC mRNA levels during metamorphosis, a cDNA for POMC was isolated from a cDNA library constructed from bullfrog (Rana catesbeiana) pituitary polyadenylated RNA. Northern blot analysis using the POMC cDNA as a probe revealed that POMC mRNA levels in the anterior lobe were relatively low during premetamorphosis, rose during prometamorphosis, reached the maximum at the end of prometamorphosis and remained very high during climax. The POMC mRNA levels of the intermediate lobe, where cc-melanophore-stimulating hormone is generated from POMC, were also determined in metamorphosing tadpoles. The POMC mRNA levels of the intermediate lobe increased as metamorphosis progressed and were maximal at mid-climax. High POMC mRNA levels were observed even in larvae that had adapted to a white background. The significance of these findings and their relationships to the hormonal requirements during metamorphosis are discussed.