Serotonin Injections Induce Metamorphosis in Larvae of the Gastropod Mollusc Ilyanassa obsoleta

Bath-applied serotonin (5-HT) induces competent larvae of the marine snail Ilyanassa obsoleta to metamorphose. Previously, the mode of action of 5-HT, whether as an external ligand or as an internal neurotransmitter, was unknown. Larvae were injected with 10-4 M 5-HT and other pharmacological agents to provide evidence that serotonergic neurons are necessary for metamorphosis in Ilyanassa larvae and that serotonin functions as a neurotransmitter or neuromodulator during this process. About 50% of 5-HT-injected animals metamorphose within 48 hours. Fluoxetine, a 5-HT reuptake inhibitor, and alpha-methyl-5-hydroxytryptamine (αm5HT), a 5-HT agonist, were also effective inducers of metamorphosis. Gramine (3-[dimethylaminomethyl]indole), a 5-HT antagonist, inhibited the inductive activity of 5-HT, while the amino acid gammaaminobutyric acid (GABA) resulted in rates of morphological restructuring similar to those of controls. Collectively, the results of our experiments support the idea that serotonergic neurons are active during larval metamorphosis of Ilyanassa and that 5-HT does not induce metamorphosis by binding to epidermal chemoreceptors.