Characterization of serotonin receptor mediating intracellular calcium increase in meiosis-reinitiated oocytes of the bivalve Ruditapes philippinarum from Central Japan

The serotonin (5-HT) receptor subtypes mediating germinal vesicle breakdown (GVBD) and the release of intracellular calcium, [Ca²⁺]i, in the bivalve Ruditapes philippinarum were investigated by examining the efficacy of serotonergic ligands at inducing or inhibiting GVBD and [Ca²⁺]i release. 5-HT, alpha, methyl-5-HT (a 5-HT₂ receptor agonist), and 8-OH-DPAT (5-HT(1A)) were the most potent agonists inducing a high percentage of oocytes to undergo GVBD. These three agents also significantly induced spawning in male clams. Stimulation of fura-2 injected oocytes by these compounds resulted in a large calcium transient peak seconds after agonist application, followed by one to several smaller transients. Maximum peak height, mean peak height, and time to initial peak were dose dependent for the tested agonists. This is in contrast to earlier published reports of [Ca²⁺]i release in this species. The rank order of potency for agonists was 5-HT > alpha-methyl-5-HT > 8-OH-DPAT > TFMPP (5-HT₁) > 1-m-chlorophenylbiguanide (5-HT₃). For antagonist effects on GVBD and release of [Ca²⁺]i, the 5-HT₂ receptor ligand cyproheptadine was the most effective blocker. Metoclopramide (5-HT₃) and mianserin (5-HT₂) also significantly inhibited the above processes. Propranolol (5-HT₁) was marginally effective. The rank order of potency for antagonists was cyproheptadine > metaclopramide = mianserin > propranolol. Although the pharmacology of GVBD has been previously reported in R. philippinarum, different ligands were tested. Thus, we conclude that the 5-HT receptor mediating GVBD and intracellular [Ca²⁺]i release is sensitive mainly to 5-HT₂ receptor ligands and presents a pharmacological profile unlike any yet described.