Cilia-driven rotational behavior in gastropod (Physa elliptica) embryos induced by serotonin and putative serotonin reuptake inhibitors (SSRIs).

We characterized the serotonin (5-hydroxytryptamine; 5-HT) receptor mediating cilia-driven rotational movement in embryos of the freshwater gastropod Physa elliptica. In addition, putative serotonin reuptake inhibitors (SSRIs), previously shown to induce other 5-HT-mediated processes in molluscs, were tested for their ability to induce rotation. As in previous studies with other freshwater gastropods, 5-HT induced a significant dose-dependent increase in rotation from 10(-6) to 10(-4) M. The 5-HT(1A) agonist 8-OH-DPAT produced a similar dose-dependent increase in rotation. However, the 5-HT(2) agonist alpha-CH3-serotonin evoked a significant rotational response only at the highest concentration of 10(-4) M. The 5-HT(2) receptor antagonist mianserin not only blocked 5-HT-induced rotation, it reduced rotation rate below that of baseline. However, two other antagonists, cyproheptadine (5-HT(2)) and propranolol (5-HT(1)), caused similar responses that consisted of an initial rotational surge followed by reduced rotation. Thus, these drugs appear to act as partial agonists. The putative SSRI fluvoxamine exhibited a significant dose-dependent increase in positive rotation as that seen with 5-HT. The SSRIs paroxetine and fluoxetine both caused an increase in rotation at 10(-6) and 10(-5) M but reduced rotation rate below that of baseline at 10(-4) M. These results agree with other studies on aquatic molluscs, suggest a 5-HT receptor with a mixed 5-HT(1)/5-HT(2) pharmacological profile and add to a now growing body of literature on the pharmacology of molluscan 5-HT receptors. In addition, all the tested putative SSRIs induced cilia-driven rotation in Physa embryos, indicating either the presence of 5-HT reuptake transporters or that these compounds act as 5-HT receptor ligands. J. Exp. Zool. 286:414-421, 2000.