Serotonergic Ligands Induce Spawning but not Oocyte Maturation in the Bivalve Mactra chinensis from Central Japan

We examined the spawning sensitivity to serotonin and serotonergic ligands in the Japanese bivalve Mactra chinensis. Spawning was induced by both injected and externally applied serotonin (5-hydroxytryptamine, 5-HT). The vertebrate 5- HT2 receptor agonist alpha-methyl 5-HT and the selective 5 HT1 A agonist 8-OH-DPAT were also effective at inducing spawning. However TFMPP (m-trifluoromethylphenylpiperazine, a verterbrate 5- HT1 receptor agonist) and 1-methyl-chlorophenyl biguanide (a vertebrate 5- HT3 agonist) were not effective spawning inducers. The 5-HT-induced spawning was blocked by mianserin (a vertebrate 5- HT2 antagonist). The rank order of potency of the agonists was: 5-HT > alpha-methyl 5-HT > 8-OH-DPAT >> TFMPP > 1-methyl-chlorophenyl biguanide; these data support a growing body of literature invoking a mixed 5- HT1/5- HT2 pharmacological profile for serotonin receptors mediating reproductive processes in bivalves. However, neither 5-HT nor 8-OH-DPAT induced germinal vesicle breakdown (GVBD) in Mactra oocytes. Sperm induced GVBD in a high percentage of oocytes. This is the first report of a bivalve in which spawning, but not GVBD, can be induced by 5-HT. This result might be expected because Mactra spawns germinal vesicle oocytes that normally undergo GVBD upon fertilization, but is in contrast to the case of the closely related Spisula spp. in which serotonin induces both processes. The ability of 5-HT to induce spawning but not GVBD makes Mactra chinensis a model organism for studying spawning and meiotic mechanisms in bivalves.