Differential effect of insulin-like growth factor I on in vitro gonadotropin (I and II) and growth hormone secretions in rainbow trout (Oncorhynchus mykiss) at different stages of the reproductive cycle

The short-term effect of insulin-like growth factor I (IGF-I) on GTH. I (FS H-like), GTH II (LH-like), and GH production by cultured rainbow trout pitu itary cells was studied in immature fish of both sexes, at early gametogene sis and in spermiating and periovulatory animals. IGF-I had no effect on ba sal GTH I and GTH II release, whereas it always inhibited basal GH, showing decreasing intensity with the gonad maturation. In absence of IGF-I, GTH I and GTH II cells were always responsive to GnRH, whereas no response was o bserved for GH cells whatever the sexual stage. The action of IGF-I on the sensitivity to GnRH differs between GTH and GH cells. The former requires a coincubation with IGF-I (10(-6) M)/GnRH to show an increase in sensitivity , independent of the sexual stage. To be responsive to GnRH, the GH cells r equire longer exposure to IGF-I, the efficiency of which decreases with gon ad maturation. The action of IGF-I (10(-6) M) on GTH cell sensitivity to Gn RH does not seem to be related to a mitogenic effect or to an improvement i n cell survival. It seems to be IGF-I specific, not passing via the insulin receptor. Certain hypotheses on the putative role of IGF-I and GnRH as a l ink between growth and puberty are suggested.