OOCYTE SENSITIVITY TO SEROTONERGIC REGULATION DURING THE FOLLICULAR CYCLE OF THE TELEOST FUNDULUS-HETEROCLITUS

In the teleost Fundulus heteroclitos, serotonin (5-HT) reversibly inhi bits oocyte maturation induced in vitro by the maturation-inducing ste roid (MIS) 17,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P). The 5- HT inhibition of 17,20 beta P-induced meiotic maturation was examined in ovarian follicles at different developmental stages or isolated at different times during the follicular cycle. Steroid treatment of late vitellogenic and early maturing follicles (1.2- to 1.7-mm diameter) p romoted oocyte maturation in a size-dependent manner, and this maturat ion was inhibited by 5-HT in follicles of < 1.6- to 1.7-mm diameter. T hus, the 5-HT inhibition progressively decreased as follicles develope d the ability to mature in the absence of 17,20 beta P. The effectiven ess of 5-HT to increase follicular cAMP remained similar within the sa me developmental stages, indicating that the reduction of 5-HT inhibit ory action was not related to the competence of 5-HT to activate inhib itory signals in the oocyte. During the follicular cycle, fully grown follicles (1.3- to 1.4-mm diameter) showed a decreased maturational co mpetence in response to gonadotropin or MIS stimulation after the foll icular recruitment into maturation and spawning occurred, which coinci ded with an increase of the effectiveness of 5-HT at inhibiting 17,20 beta P-induced maturation. In further experiments, preincubation of fo llicles with hCG was found to reduce 5-HT inhibitory action, but when follicles were incubated with either hCG in the presence of a steroido genesis inhibitor or estradiol-17 beta (E-2), the 5-HT inhibition was unaffected. These findings suggest that 5-HT inhibition of the MIS-ind uced meiotic maturation is not under direct gonadotropin or E-2 regula tion but that it might be regulated in vivo by changes in the competen ce of the oocytes to undergo oocyte maturation after MIS stimulation.