Changes of Thyrotropin-Releasing Hormone (Trh) Levels in Brain Regions and Pituitary During Induced Metamorphosis of Ambystoma Mexicanum

Abstract

Axolotls (Ambystoma mexicanum) are neotenic amphibians: they keep larval features during their entire lifetime. However, metamorphosis can be induced by injection of thyroid hormones or thyrotropin (TSH). The role played by thyrotropin-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 (T4)-induced metamorphosis. TRH levels in skin, hypothalamus, pituitary and extrahypothalamic region of the brain as well as plasma T4 and triiodothyronine (T3) levels were measured by radioimmunoassay (RIA). Preliminary data on the expression of TRH receptors in the pituitary of a neotenic and a metamorphosed animal were obtained by in situ hybridisation 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 levels. During metamorphosis, TRH levels decreased in hypothalamus (and extrahypothalamic region) and increased in pituitary; postmetamorphic values did not differ from neotenic controls. TRH levels in skin did not change significantly. During metamorphosis, plasma thyroid hormone levels were high, followed by a decrease to premetamorphic values after metamorphosis. Preliminary in situ hybridisation results suggested that the expression of TRH receptors is low in neotenic animals and increases during T4-induced metamorphosis.