Pharmacological characterization of serotonin receptor in the oocyte membrane of bivalve molluscs and its formation during oogenesis

Serotonin (5-hydroxytryptamine, 5-HT) is a major neurotransmitter that triggers the spawning of bivalve molluscs and oocyte germinal vesicle breakdown (GVBD) of oocytes. The present study employed [³H] 5-HT binding assay to characterize a putative receptor on oocyte membranes prepared from scallops and oysters. Results demonstrated a profile of hormone control of 5-HT receptor formation during oocyte growth. The dissociation constants for 5-HT on the surfaces of scallop and oyster oocytes were estimated to be 2.64 μM and 2.01 μM, respectively, and the maximal binding capacity of the oocyte surface was 152 pmol/mg protein in scallops and 529 pmol/mg protein in oysters. The inhibition of [³H] 5-HT bound by various agonists and antagonists provided pharmacological evidence for a 5-HT receptor in each oocyte, a mixed profile of 5-HT₁/5-HT₂ in scallops and a single profile of 5-HT₁ in oyster. Formation of 5-HT receptor on the surface of the scallop oocyte was induced by estradiol-17β(E₂), which was suggested as a primary promoter of the effect of E₂ on sensitivity to 5-HT, resulting in the induction of spawning. This formation occurred during oocyte growth controlled by E₂.