The frog gonadotropin-releasing hormone-I (GnRH-I) gene has a mammalian-like expression pattern and conserved domains in GnRH-associated peptide, but brain onset is delayed until metamorphosis.

Recent evidence indicates a localized origin in the olfactory placode for the mammalian forebrain neurons that express GnRH. To identify the cellular and molecular signals that induce the GnRH phenotype, we cloned and characterized a cDNA encoding the GnRH prohormone, the precursor for both GnRH-I and GnRH-associated peptide in the frog, Xenopus laevis, an embryonic model accessible to experimental manipulation. The 396-base cDNA represented a single mRNA species encoding an 89-amino acid prepro-GnRH that, unlike a recently cloned fish GnRH gene, was identical to both the mammalian GnRH decapeptide as well as multiple domains within GnRH-associated peptide. Serial section in situ hybridization histochemistry and immunocytochemistry in adult frog localized a forebrain system comprising 250-350 cell bodies whose overall neuroanatomy, including fiber projections, was very similar to that described for mammals. However, neither Northern nor in situ hybridization detected GnRH expression in midbrain, arguing that another frog gene encodes the midbrain GnRH-II expression pattern described by many others using antisera directed against the fish GnRH-I or chicken GnRH-II decapeptides. In contrast to mammals and birds, in which GnRH-expressing cells migrate into embryonic forebrain, frog GnRH cells were first detected after they reached their final position in the preoptic area during the late larval period. Thus, although previous studies proposed a complex organization for the GnRH system in the frog, our findings show that similar to mammals, there is a single gene that can account for the continuum of GnRH-I cells spanning frog forebrain. However, unlike mammals, in frogs, for unknown reasons, GnRH-I gene expression is suppressed until metamorphic climax.