Some aspects of the hypothalamic and pituitary development, metamorphosis, and reproductive behavior as studied in amphibians

In this review article, information about the development of the hypothalamo-hypophyseal axis, endocrine control of metamorphosis, and hormonal and pheromonal involvements in reproductive behavior in some amphibian species is assembled from the works conducted mainly by our research group. The hypothalamic and pituitary development was studied using Bufo embryos and larvae. The primordium of the epithelial hypophysis originates at the anterior neural ridge and migrates underneath the brain to form a Rathke's pouch-like structure. The hypothalamo-hypophyseal axis develops under the influence of thyroid hormone (TH). For the morphological and functional development of the median eminence, which is a key structure in the transport of regulatory hormones to the pituitary, contact of the adenohypophysis with the undeveloped median eminence is necessary. For the development of proopiomelanocortin-producing cells, contact of the pituitary primordium with the infundibulum is required. The significance of avascularization in terms of the function of the intermediate lobe of the pituitary was evidenced with transgenic Xenopus frogs expressing a vascular endothelial growth factor in melanotropes. Metamorphosis progresses via the interaction of TH, adrenal corticosteroids, and prolactin (PRL). We emphasize that PRL has a dual role: modulation of the speed of metamorphic changes and functional development of organs for adult life. A brief description about a novel type of PRL (1B) that was detected was made. A possible reason why the main hypothalamic factor that stimulates the release of thyrotropin is not thyrotropin-releasing hormone, but corticotropin-releasing factor is considered in light of the fact that amphibians are poikilotherms. As regards the reproductive behavior in amphibians, studies were focused on the courtship behavior of the newt, Cynops pyrrhogaster. Male newts exhibit a unique courtship behavior toward sexually developed conspecific females. Hormonal interactions eliciting this behavior and hormonal control of the courtship pheromone secretion are discussed on the basis of our experimental results.