SEROTONIN INHIBITION OF STEROID-INDUCED MEIOTIC MATURATION IN THE TELEOST FUNDULUS-HETEROCLITUS - ROLE OF CYCLIC-AMP AND PROTEIN-KINASES

The transduction of the serotonin (5-HT) signal in Fundulus heteroclit us ovarian follicles leading to the inhibition of oocyte meiosis reini tiation (oocyte maturation) in vitro induced by the naturally occurrin g maturation-inducing steroid 17 alpha,20 beta-dihydroxy-4-pregnen-3-o ne (17,20 beta P)was investigated. Steroid-induced oocyte maturation w as inhibited by 5-HT in a dose-dependent manner; maximum inhibition (9 0%) was observed with 10(-4) M 5-HT Groups of follicle-enclosed oocyte s were cultured in the presence of the phosphodiesterase inhibitor 3-i sobutyl-1-methylxanthine (IBMX) and treated with increasing doses of 5 -HT. Serotonin was found to slightly increase the levels of follicular 3',5'-cyclic adenosine monophosphate (cAMP) in a dose-dependent manne r; 10(-4) M 5-HT induced approximately a 3 fold increase in cAMP with respect to the controls. The changes in cAMP were then evaluated in fo llicles treated with 17,20 beta P in IBMX-free culture media in the pr esence or absence of 10(-4) M 5-HT. The exposure of follicles to 17,20 beta P alone produced a small and transient reduction in cAMP (40%) w ithin 1-3 hr of steroid stimulation, and these early changes in cAMP a ppeared associated with a high incidence of germinal vesicle breakdown (80% GVBD) by 24 hr of incubation. Under these conditions, treatment of follicles with 5-HT also increased significantly the production of cAMP, and when 5-HT was combined with 17,20 beta P, the steroid-mediat ed reduction in cAMP was prevented and the levels of GVBD inhibited by 95%. Meiosis also was reinitiated with either the protein kinase A (P KA) inhibitor H8 or the protein kinase C (PKC) activator PMA, and the 5-HT inhibitory action on GVBD was found to be 100-fold reduced or com pletely ineffective, respectively. Preincubation of follicles with the PKC inhibitor GF109203x abolished PMA-induced GVBD in a dose-dependen t manner, whereas this inhibitor had no effect on 17,20 beta P-trigger ed meiotic maturation, indicating that activation of PKC is apparently sufficient but not necessary to reinitiate meiosis. Taken together, t hese findings suggest that 5-HT may inhibit 17,20 beta P-induced meiot ic reinitiation through the activation of a cAMP-PKA transduction path way and that PKC possibly induces oocyte maturation by a different pat hway than the steroid and thus is not affected by 5-HT. (C) 1998 Wiley -Liss, Inc.