GnRH systems of Cichlasoma dimerus (Perciformes, Cichlidae) revisited: A localization study with antibodies and riboprobes to GnRH-associated peptides

The distribution of cells that express three prepro-gonadotropin-releasing hormones (GnRH), corresponding to salmon GnRH, sea bream GnRH (sbGnRH), and chicken II GnRH, was studied in the brain and pituitary of the South American cichlid fish, Cichlasoma dimerus. Although the ontogeny and distribution of GnRH neuronal systems have previously been examined immunohistochemically with antibodies and antisera against the various GnRH decapeptides, we have used antisera against various perciform GnRH-associated peptides (GAPs) and riboprobes to various perciform GnRH+GAPs. The results demonstrate that: (1) the GnRH neuronal populations in the forebrain (salmon and sea bream GAPs; sGAP and sbGAP, respectively) show an overlapping pattern along the olfactory bulbs, nucleus olfacto-retinalis, ventral telencephalon, and preoptic area; (2) projections with sGAP are mainly located in the forebrain and contribute to the pituitary innervation, with projections containing chicken GAP II being mainly distributed along the mid and hindbrain and not contributing to pituitary innervation, whereas sbGAP projections are restricted to the ventral forebrain, being the most important molecular form in relation to pituitary innervation; (3) sbGnRH (GnRH I) neurons have an olfactory origin; (4) GAP antibodies and GAP riboprobes are valuable tools for the study of various GnRH systems, by avoiding the cross-reactivity problems that occur when using GnRH antibodies and GnRH riboprobes alone.