To clarify the roles of prolactin (PRL) and GH in the control of the immune system, the effects of environmental salinity, hypophysectomy, and PRL and GH administration on several immune functions were examined in tilapia (Oreochromis mossambicus). Transfer from fresh water (FW) to seawater (SW) did not alter plasma levels of immunoglobulin M (IgM) and lysozyme. The superoxide anion (O(2)(-)) production in head kidney leucocytes accompanied by phagocytosis was elevated in SW-acclimated fish over the levels observed in FW fish. Hypophysectomy of the fish in FW resulted in a reduction in O(2)(-) production in leucocytes isolated from the head kidney, whereas there was no significant change in plasma levels of IgM or lysozyme. Treatment with tilapia GH and PRLs (PRL(177) and PRL(188)) enhanced O(2)(-) production in vitro in head kidney leucocytes in a dose-related manner. Extrapituitary expression of two PRLs, GH and IGF-I mRNA was detected in lymphoid tissues and cells such as head kidney, spleen, intestine and leucocytes from peripheral blood and head kidney. PRL-receptor mRNA was detected in head kidney leucocytes, and the level of expression was higher in SW-acclimated fish than that in FW fish. Treatment with PRL(177) caused higher production of O(2)(-) in the head kidney leucocytes isolated from SW tilapia than that from FW fish. In view of the fact that PRL acts antagonistically to osmoregulation in SW, its immunomodulatory actions in this euryhaline fish would appear to be independent of its osmoregulatory action.
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T Yada, K Uchida, S Kajimura, T Azuma, T Hirano, and EG Grau
T Yada, H Kaiya, K Mutoh, T Azuma, S Hyodo, and K Kangawa
To clarify the role of ghrelin in the fish immune system, the in vitro effect of ghrelin was examined in phagocytic leukocytes of rainbow trout (Oncorhynchus mykiss). Administration of trout ghrelin and des-VRQ-trout ghrelin, in which three amino acids are deleted from trout ghrelin, increased superoxide production in zymosan-stimulated phagocytic leukocytes from the head kidney. Gene expression of growth hormone (GH) secretagogue-receptor (GHS-R) was detected by RT–PCR in leukocytes. Pretreatment of phagocytic leukocytes with a GHS-R antagonist, [D-Lys3]-GHRP-6, abolished the stimulatory effects of trout ghrelin and des-VRQ-trout ghrelin on superoxide production. Ghrelin increased mRNA levels of superoxide dismutase and GH expressed in trout phagocytic leukocytes. Immunoneutralization of GH by addition of anti-salmon GH serum to the medium blocked the stimulatory effect of ghrelin on superoxide production. These results suggest that ghrelin stimulates phagocytosis in fish leukocytes through a GHS-R-dependent pathway, and also that the effect of ghrelin is mediated, at least in part, by GH secreted by leukocytes.