Reception and transduction of the serotonin signal responsible for oocyte meiosis reinitiation in bivalves

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Abstract
Prophase-arrested oocytes of Spisula solidissima complete maturation either upon fertilization, KCl treatment or serotonin (=5 HT, for 5-hydroxytryptamine). In Ruditapes philippinarum, 5 HT stimulation is a necessary prerequisite to get further development since prophase-arrested oocytes are not fertilizable. The neurohormone drives them to asecond arrest in metaphase 1, at which time they can be activated by sperm or excess KCl. These contrasted situations rely on a difference in the kinetics of appearance of L-type voltage-dependent calcium channels which are required for ensuring sperm or KCl-dependent activations. Serotonin-specific receptors have been characterized in both species and seem to mediate maturation by mobilizing internal and external Ca2+ ions. In Ruditapes, transduction of the serotonin signal involves a coupling of the receptor to G-proteins, which would activate phospholipase C, producing a transient peak of inositol trisphosphate (IP3) and an increase in the intracellular Ca2+ concentration. Experimental arguments suggest that diacylglycerol exerts a negative retrocontrol on the calcium response by stimulating kinase C. Protein kinases activation and conditions responsible for the metaphase arrest are also discussed.