KNEBEL, SM; SAWYER, SJ; KRAJNIAK, KG; Southern Illinois University Edwardsville: Effects of serotonergic agonists and selective serotonin reuptake inhibitors on the earthworm crop-gizzard

Serotonin (5-HT) is a biogenic amine neurotransmitter whose presence has been identified in both vertebrate and invertebrate organisms. In vertebrates, serotonin receptors have been classified into seven different serotonin receptor classes based upon their pharmacological profile. In the earthworm Lumbricus terrestris, serotonin causes a dose-dependent decrease in the amplitude of contraction of the isolated, spontaneously contracting crop-gizzard. To determine whether the earthworm serotonin receptor fits into one of the seven known serotonin receoptor classes, we challenged the crop-gizzard with a series of serotonergic agonists. Of the agonists tried from the seven classes, only the serotonin receptor class 2 agonists, 1-(m-chlorophenyl)piperazine (m-CPP), [S]-1-[2,5-dimethoxy-4-iodophenyl]propan-2-amine (±DOI), and alpha-methylserotonin caused a dose-dependent decrease in amplitude of contraction similar to that of serotonin. Serotonin agonists belonging to the class 1 and class 3 categories failed to affect the amplitude of contraction of the earthwrom crop-gizzard. One serotonin reuptake inhibitor (SSRI), fluvoxamine, acted similarly to serotonin and caused a slight decrease in the amplitude of contraction of the earthworm crop-gizzard. Another SSRI, fluoxetine, failed to affect the amplitude of contraction of the earthworm crop-gizzard. Taken together, the SSRI and serotonin agonist data suggest the serotonin receptor crop-gizzard in Lumbricus terrestris has characteristics similar to well defined serotonin receptors and may belong to the class 2 receptor category.