DEVELOPMENT AND DISTRIBUTION OF GONADOTROPIN-RELEASING-HORMONE NEURONAL SYSTEMS IN THE FROG (RANA-ESCULENTA) BRAIN - IMMUNOHISTOCHEMICAL ANALYSIS

The ontogenesis of the gonadotropin-releasing hormone (GnRH) neuronal systems was studied in the brain of the frog, Rana esculenta. Attentio n was also focussed on the differential distribution of molecular form s of GnRH during development. The first GnRH-immunoreactive neurons ap pear in the mesencephalon of posterior limb-stage tadpoles. These neur ons are shown to contain only chicken [His(5),Trp(7),Tyr(8)]GnRH (cGnR H-II). Later in development, mammalian [Tyr(5),Leu(7),Arg(8)] GnRH (mG nRH)-like peptide-containing neurons appear simultaneously in the term inal nerve as well as in the anterior preoptic area of the telencephal on. Subsequently, only after metamorphosis, mGnRH-containing neurons a ppear in the medial septal area of the posterior telencephalon. It is here shown that neurons containing the two forms of GnRH are distribut ed in distinct brain areas during development and in the adult: mGnRH- immunoreactive neurons in the terminal nerve, olfactory bulb, mediobas al telencephalon, medial septal area, anterior preoptic area, ventrola teral thalamus and infundibulum, whereas cGnRH-II neurons are located in the mesencephalon. We hypothesize that the terminal nerve/forebrain -located GnRH neurons express immunohistochemically late in developmen t and originate extracranially migrating centrally, along the terminal nerve, during development, whereas those located in the mesencephalon express earlier and may have an intracranial site of origin.