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Prolactin (Prl) and growth hormone (Gh) as well as insulin-like growth factor 1 (Igf1) are involved in the physiological adaptation of fish to varying salinities. The Igfs have been also ascribed other physiological roles during development, growth, reproduction and immune regulation. However, the main emphasis in the investigation of osmoregulatory responses has been the endocrine, liver-derived Igf1 route and local regulation within the liver and osmoregulatory organs. Few studies have focused on the impact of salinity alterations on the Gh/Igf-system within the neuroendocrine and immune systems and particularly in a salinity-tolerant species, such as the blackchin tilapia Sarotherodon melanotheron. This species is tolerant to hypersalinity and saline variations, but it is confronted by severe climate changes in the Saloum inverse estuary. Here we investigated bidirectional effects of increased salinity followed by its decrease on the gene regulation of prl, gh, igf1, igf2, Gh receptor and the tumor-necrosis factor a. A mixed population of sexually mature 14-month old blackchin tilapia adapted to freshwater were first exposed to seawater for one week and then to fresh water for another week. Brain, pituitary, head kidney and spleen were excised at 4 h, 1, 2, 3 and 7 days after both exposures and revealed differential expression patterns. This investigation should give us a better understanding of the role of the Gh/Igf system within the neuroendocrine and immune organs and the impact of bidirectional saline challenges on fish osmoregulation in non-osmoregulatory organs, notably the complex orchestration of growth factors and cytokines.

International audience; With over 32,000 species, teleost fi sh have adapted to a tremendous range of aquatic habitats. At least 200 of them are used for aquaculture all over the world and even more are handled and transported for the ornamental fi sh market, as well as for bait or research purposes, including cytogenetic studies.

IGF-I plays a crucial role in the regulation of bony fish growth, differentiation, and reproduction. The major source of circulating IGF-I is liver, but IGF-I-producing cells also occur in other organs. Our knowledge on the presence of IGF-I in the hypothalamic-pituitary-gonadal (HPG) axis is limited. Thus, hypothalamus, pituitary and gonads of monosex breedings of male and female tilapia from 0 day post fertilization (DPF) to adulthood were investigated for the occurrence of IGF-I mRNA by in situ hybridization. In the male and female gonad anlage, IGF-I mRNA appeared in somatic cells at 7 DPF. In female germ cells IGF-I mRNA was found at 29 DPF, and in male germ cells at 51-53 DPF suggesting that the production of IGF-I in the germ cells is linked to the onset of meiosis. In the neurohypophysis, axons containing IGF-I-immunoreactivity appeared around 17 DPF but no IGF-I mRNA was detected suggesting that IGF-I mRNA containing neuronal perikarya within the hypothalamus are the source. In the adenopituitary, IGF-I mRNA was first detected at 30 DPF in some cells of the ACTH, α-MSH and GH regions and persisted throughout life constitutively in ACTH and α-MSH cells but its presence in GH cells showed marked inter-individual differences in later life, the latter likely due to the physiological status of the individual. Around 30 DPF, IGF-I mRNA appeared in cells in the gonadotropin (GTH) regions of the female and at 50 DPF of the male pituitary. During puberty (around 80 DPF), the expression of IGF-I mRNA in GTH cells was most pronounced in both sexes. It is assumed that IGF-I released from the GTH cells acts as auto/paracrine regulator of cell proliferation and enhances GTH synthesis and release during puberty and reproductive phases.