BIOCHEMICAL PATHWAYS INVOLVED IN SEROTONIN-REGULATED SPISULA OOCYTE MATURATION AND FERTILIZATION

Full-grown oocytes of the surf clam Spisula solidissima are naturally arrested at the dictyate stage of meiosis and resume maturation follow ing insemination or treatment with various chemical stimuli. Serotonin or 5-hydroxytryptamine (5-HT) triggers spawning when injected into th e gonads of Spisula and induces maturation when added in vitro to a su spension of isolated oocytes. The physiological relevance of this obse rvation is supported by the identification of 5-HT or a related compou nd in the body fluid and ganglion cells of Spisula. 5-HT-induced germi nal vesicle breakdown (GVBD) is absolutely dependent on the presence o f extracellular calcium in the suspension medium. 5-HT affects Ca2+-up take mediated by voltage-gated Ca2+-channels in Spisula oocytes that l eads to increased levels of intracellular calcium. Verapamil, a voltag e-sensitive Ca2+-channel blocker, completely inhibits the actions of 5 -HT to induce GVBD, to stimulate Ca2+-uptake, and to influence intrace llular calcium concentrations [Ca2+](i). Tricyclic antidepressants blo ck neurotransmitter-induced oocyte maturation by acting as serotonin-r eceptor antagonists and thus inhibiting 5-HT-affected Ca2+-uptake by v oltage-gated Ca2+-channels. The present results suggest that 5-HT is f unctionally affecting Ca -uptake mediated by the gating of verapamil-s ensitive Ca2+-channels in Spisula oocytes, thereby activating the sign al transduction system leading eventually to the resumption of meiosis .