Evidence that bivalve burrowing is mediated by serotonin receptors: activation of foot inflation and protrusion by serotonin, serotonergic ligands and SSRI-type antidepressants in three species of freshwater bivalve

Abstract

ABSTRACT Foot inflation and protrusion in bivalve molluscs is part of a sequence of events necessary for burrowing into the substrate. Although this process been observed after exogenous exposure to the neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] and by some selective serotonin reuptake inhibitor (SSRI) type antidepressants, no study has shown that foot inflation and protrusion as a precursor to burrowing is under serotonergic control. In order to elucidate the physiological mechanism mediating this response, we tested over 2,000 freshwater bivalves in three species by exposing them to 5-HT, two 5-HT receptor agonists, two receptor antagonists and five antidepressants. Fingernail clams (Sphaerium striatinum), Asian clams (Corbicula fluminea) and unionid mussels (Elliptio complanata) all showed significant foot inflation or foot protrusion when exposed to 5-HT and the serotonergic agonists 8-OH-DPAT and alpha-methyl 5-HT. Some SSRI-type antidepressants (fluoxetine and fluvoxamine) also significantly induced these responses in all three species but were not as potent at the concentrations tested. In S. striatinum and C. fluminea, a 2-h exposure to the 5-HT2 receptor antagonists mianserin and cyproheptadine effectively blocked foot inflation induced by 5-HT and 8-OH-DPAT. The 5-HT receptor mediating this response is thus sensitive to mammalian 5-HT1 and 5-HT2 receptor ligands, supporting the hypothesis that serotonin mediates the initial step in burrowing.