Meiosis reinitiation in surf clam oocytes is mediated via a 5-hydroxytryptamine5 serotonin membrane receptor and a vitelline envelope-associated high affinity binding site.

The neurohormone serotonin (5-hydroxytryptamine, 5HT) triggers meiosis reinitiation in prophase-arrested oocytes of Spisula solidissima. The original pharmacological profile of this response prompted us to examine whether it involved a novel type of 5HT receptor. In order to characterize these putative receptors, we performed [3H]5HT binding assays. Given the complexity of oocyte surface architecture, [3H]5HT-specific binding was measured in both plasma membrane and vitelline envelope fractions. Our data reveal the existence of a single class of 5HT-specific binding sites in each fraction. These binding sites appear distinct by their kinetic properties: 1) plasma membrane binding sites are of lower affinity but are more numerous than vitelline envelope sites; 2) [3H]5HT binding is more rapid in membrane fraction. The pharmacological profiles of both binding sites were defined in competition experiments by using sixteen 5HT-related and unrelated compounds. Both membrane and vitelline envelope sites displayed novel profiles. However, only the pharmacological profile determined for the plasma membrane sites corresponds to that observed for 5HT-induced meiosis reinitiation. These new 5HT5 binding sites are therefore the physiological receptors that mediate the effects of 5HT in Spisula oocytes. The putative role of vitelline envelope sites is discussed.