Gonadotropin-releasing hormone gene expression in teleosts

Expression of multiple molecular forms of gonadotropin-releasing hormone (GnRH) mRNAs and GnRH peptides were examined in the brains of tilapia (Oreochromis mossambicus) and sockeye salmon (Oncorhynchus nerka), using in situ hybridization histochemistry and immunohistochemical techniques. After otherwise identical conditions, lesser background and stronger GnRH hybridization signals were observed on cryostat vs. paraffin sections. In both fresh and Bouin's-fixed paraffin-embedded tissues, there was a good correlation between the distribution of GnRH mRNA and GnRH peptide-containing cells. Although the brains of tilapia and the sockeye were immunoreactive to three forms of the GnRH molecule (salmon, mammal, chicken-II), GnRH mRNA expression was site-specific and species-specific. In the tilapia, ganglionic cells of the nucleus olfactoretinalis, basal telencephalon and the anteroventral preoptic area were immunoreactive to salmon-, and mammalian-GnRH peptide. Neurons of the nucleus olfactoretinalis expressed cichlid-GnRH I mRNA. The preoptic neurons, despite the immunoreactivity, expressed no hybridization signals. Midbrain neurons were immunoreactive to salmon-GnRH but expressed cichlid-GnRH II beta (= chicken-GnRH II) mRNA hybridization signals. In the sockeye, ganglionic cells along the extracerebral course of the nervus terminalis were immunoreactive to mammalian-, chicken-II and salmon-GnRH. These neurons expressed only salmon-GnRH mRNA hybridization signals. Intracerebral GnRH expression in the sockeye was delayed till smoltification. The basal telencephalon and midbrain neurons immunoreactive to salmon-GnRH, formed no hybridization signals with GnRH antisense probes. Oligonucleotide probes complementary to chicken-GnRH I and mammalian-GnRH revealed no hybridization signals in the tilapia and in the sockeye brain. Fibers, immunoreactive to salmon-, mammalian-, and chicken II-GnRH were seen in close association with growth hormone cells. Chicken-GnRH II-immunoreactive fibers were also seen in close proximity to somatolactin cells in the sockeye salmon.