Pharmacology of the serotonergic inhibition of steroid-induced reinitiation of oocyte meiosis in the teleost Fundulus heteroclitus

Serotonin (5‐HT) was found to inhibit steroid (17α,20β‐dihydroxy‐4‐pregnen‐3‐one; 17,20βP)‐induced resumption of oocyte meiosis (oocyte maturation) in vitro in the teleost Fundulus heteroclitus. Serotonin inhibited both follicle‐enclosed and denuded oocytes, which indicates the presence of oocyte‐associated 5‐HT sensitive sites. The response of oocytes to 5‐HT was characterized pharmacologically, i.e., the capacity of serotonergic agonists and antagonists to mimic or block the 5‐HT inhibition of the steroid‐induced oocyte maturation was assessed by the changes in the percentage of oocyte germinal vesicle breakdown (GVBD). Dose‐response curves for each compound were drawn and compared. The rank order of potency among the agonists was: 5‐HT > 5‐methoxytryptamine > tryptamine = 5,6‐diHT = 5‐carboxidotryptamine > 5,7‐diHT = 5‐methoxy‐dimethyltryptamine > α‐methyl‐5‐HT > 2‐methyl‐5‐HT. Incubation of ovarian follicles with high doses of some antagonists (mianserin and metergoline) induced oocyte GVBD, although this effect was associated with high levels of oocyte atresia during GVBD or shortly after maturation. Consequently, doses of the antagonist too low to induce GVBD were tested for their ability to block the 5‐HT inhibitory action; the rank order of potency was: MDL‐72222 = metoclopramide > metergoline > propanolol > ketanserin. Dopamine, acetylcholine, epinephrine, and norepinephrine could also inhibit 17,20βP‐induced GVBD, although at doses much higher than those of 5‐HT; melatonin and histamine had no effect on oocyte maturation. These results suggest that specific receptors mediate the inhibitory action of 5‐HT on the steroid‐triggered meiosis resumption. The pharmacological profile of these 5‐HT receptors is different from those of any known mammalian 5‐HT receptor, although they showed some similarities to the 5‐HT1A, 5‐HT2, and 5‐HT3 receptors, as well as to 5‐HT receptors on oocytes of some bivalve molluscs. Mol. Reprod. Dev. 48:282–291, 1997. © 1997 Wiley‐Liss, Inc.