Immunoreactive mammalian and chicken-II GnRHs in Rana esculenta brain during development

Two forms of gonadotropin-releasing hormone (mammalian, mGnRH and chicken-II, cGnRH-II) were measured by radioimmunoassay in the nasal area (containing peripheral terminal nerve), brain and pituitary of Rana esculenta during larval development, metamorphosis, and until prior to becoming reproductively active. Small amounts of both forms of GnRH were first detected in the brain extract of early tadpoles (stage 26-27, when hindlimbs begin to develop). Later, there was a gradual, but constant, stage-dependent increase in the brain content of GnRHs, with the most remarkable increase recorded at postclimax and in young frogs. In tadpoles, postclimax froglets, and young frogs, the brain concentration of mGnRH was higher than that of cGnRH-II, with a ratio of approximately 2:1 in favor of mGnRH. In juveniles, however, the brain extract contained more cGnRH-II than mGnRH. No GnRH immunoreactivity was detected in the nasal area until stage 31. In successive stages of development, however, only mGnRH was present in the nasal area, and this confirmed our previous immunohistochemical analysis which showed that the peripheral terminal nerve contains only mGnRH-immunoreactive neurons and fibers. Although both GnRH forms were detected in the anterior (telencephalon, diencephalon) and posterior (mesencephalon, rhombencephalon) brain halves from juveniles, mGnRH content predominated in the anterior half whereas in the posterior half cGnRH-II was present in greater amounts. Pituitaries from male and female postclimax froglets and young frogs contained both forms of GnRH in a ratio of approximately 10:1 in favor of mGnRH. This finding may shed light on the question of which GnRH(s) regulate gonadotropin release from the pituitary. The developmental changes in GnRH-immunoreactive content of the brain and pituitary have been discussed in the light of functional maturation of the brain-pituitary-gonad axis.