Inhibition of Fundulus heteroclitus oocyte maturation in vitro by serotonin (5-hydroxytryptamine)

The effects of serotonin (5‐hydroxytryptamine [5‐HT]) on oocyte maturation in vitro were tested on Fundulus heteroclitus full‐grown prematurational ovarian follicles. F.heteroclitus pituitary extract (FPE)‐ and steroid (17α,20β‐dihydroxy‐4‐pregnen‐3‐one [DHP])‐induced oocyte maturation were both inhibited by 5‐HT in a dose‐dependent and reversible manner. Maximum inhibition was observed at concentrations of 0.5 and 5 μM 5‐HT, respectively. The inhibitory action was also less effective when 5‐HT was combined with human chorionic gonadotropin than with FPE, which may indicate the presence of 5‐HT and/or other inhibitory substances in the pituitary extract. Inhibition by 5‐HT was independent of FPE‐induced steroid production by the follicle, since the levels of 17β‐estradiol and DHP released into the culture medium were not modified by the presence of 5‐HT. When follicles were preincubated with DHP for increasing times, subsequently washed, and transferred to steroid‐free, 5‐HT‐containing medium, inhibition by 5‐HT was also observed in follicles preincubated for up to 4 h; however, inhibition by 5‐HT was reduced or eliminated after 8 h or 16 h preincubation with DHP, respectively. These results suggest the presence of a putative 5‐HT receptor(s) that does not affect steroidogenic pathways in the ovarian follicle yet inhibits steroid‐induced oocyte maturation at some point downstream in the steroid transduction pathway of the oocyte. © 1995 Wiley‐Liss, Inc.