Changes of thyrotropin-releasing hormone (TRH) levels in brain regions andpituitary during induced metamorphosis of Ambystoma mexicanum

Axolotls (Ambystoma mexicanum) are neotenic amphibians: they keep larval fe atures during their entire lifetime. However, metamorphosis can be induced by injection of thyroid hormones or thyrotropin (TSH). The role played by t hyrotropin-releasing hormone (TRH) in amphibian metamorphosis is not clear, since TRH treatment never succeeded in inducing metamorphosis in amphibian larvae. We studied the changes of TRH and plasma thyroid hormone levels in axolotls during thyroxine (T-4)-induced metamorphosis. TRH levels in skin, hypothalamus, pituitary and extrahypothalamic region of the brain as well as plasma T-4 and triiodothyronine (T-3) levels were measured by radioimmun oassay (RIA). Preliminary data on the expression of TRH receptors in the pi tuitary of a neotenic and a metamorphosed animal were obtained by in situ h ybridisation with an antisense oligonucleotide probe, based on nucleotides 1011 to 1060 of chicken TRH receptor mRNA. In general, TRH RIA showed that hypothalamus, extrahypothalamic region and pituitary contained high TRH lev els. During metamorphosis, TRH levels decreased in hypothalamus land extrah ypothalamic region) and increased in pituitary; postmetamorphic values did not differ from neotenic controls TRH levels in skin did not change signifi cantly. During metamorphosis. plasma thyroid hormone levels were high, foll owed by a decrease to premetamorphic values after metamorphosis. Preliminar y in situ hybridisation results suggested that the expression of TRH recept ors is low in neotenic animals and increases during T-4-induced metamorphos is.