Final Activity Report Summary - GNRH SYSTEMS IN COD (Characterization of GnRH systems in Atlantic cod: cloning, brain distribution, and mechanisms of action) Puberty in fish, as in all vertebrates, is regulated mainly through the brain-pituitary-gonad axis. In this ERG project, we have focused on the brain-pituitary axis of Atlantic cod, characterizing the stimulatory gonadotropin-releasing hormone (GnRH) systems in the cod brain and pituitary. We have characterised two cDNAs encoding two forms of preproGnRH in the Atlantic cod, sGnRH and cGnRH-II. Furthermore, we have characterised four cDNAs encoding four distinct GnRH-Rs (type 1b, 2a, 2b, and 2c, respectively). Phylogenetic analyses of the isolated genes confirm their identity with the homologous genes of other teleost fish. This molecular characterisation provides the molecular basis for our long-term research programme: development of specific riboprobes for in situ hybridisation experiments to investigate the cellular distribution of the cloned genes, development of specific primers for quantitative real-time RT-PCR assays, to monitor quantitatively the gene expression during pubertal development and following treatment with sexual steroids and manipulation of photoperiod. The impact of the identified GnRH ligands on gonadotropes in cod will also be investigated using electrophysiological (patch-clamp) techniques. Together, these investigations will contribute to a better understanding of pubertal development in teleost fish, and the Atlantic cod in particular.