Upregulation of the maturation-inducing steroid membrane receptor in spotted seatrout ovaries by gonadotropin during oocyte maturation and its physiological significance

Changes in ovarian maturation-inducing steroid (MIS; 17,20 beta ,21-trihydr oxy-3-pregnen-3-one [20 beta -S]) membrane receptor concentrations during t he reproductive cycle were investigated in spotted seatrout (Cynoscion nebu losus) captured at their spawning grounds. Ovarian receptor concentrations increased gradually during ovarian recrudescence and subsequently increased rapidly during oocyte maturation, reaching 3.5-fold the prematuration valu es by the beginning of ovulation, The significant elevation of receptor con centrations by the germinal vesicle migration stage of oocyte maturation wa s accompanied by increases in circulating levels of gonadotropin (LH, GTH I I) and MIS (20 beta -S). The regulation and physiological significance of t he increase in ovarian MIS membrane receptor concentrations were investigat ed in a double in vitro incubation system, Incubation of fully grown, folli cle-enclosed oocytes with hCG (10 IU/ml) for 6 h caused a two- to fourfold increase in oocyte and ovarian MIS receptor concentrations and the developm ent of oocyte maturational competence (OMC; ability to complete oocyte matu ration in vitro in response to exogenous 20 beta -S in a second incubation) . Both upregulation of the MIS receptor and development of OMC in response to gonadotropin were blocked by coincubation with actinomycin D or cyclohex imide, which are inhibitors of mRNA and protein synthesis, respectively, bu t not by cyanoketone, which is an inhibitor of 3 beta -hydroxysteroid dehyd rogenase-dependent steroid synthesis. Incubation with a variety of steroids , including 20 beta -S, failed to increase receptor concentrations or to in duce OMC, further supporting a steroid-independent mechanism of gonadotropi n action, In contrast insulinlike growth factor I (IGF-I) mimicked the acti ons of gonadotropin, which suggests ICF-I may be a component of the hormone signaling pathway. A close correlation was found between the relative incr ease in MIS receptor concentrations and the percentage of oocytes that beca me maturationally competent after treatment with different concentrations o f gonadotropins and drugs that elevate cAMP levels. The finding that upregu lation of the MIS receptor in response to gonadotropin and other treatments is invariably associated with the development of OMC indicates that these two processes are intimately related, and it suggests that the increase in MIS receptor concentrations is a critical regulatory step in the hormonal c ontrol of oocyte maturation.