Effects of Serotonin Receptor Agonists on the Helical Swimming and Body Reversal Escape Behaviors in the Aquatic Annelid Lumbriculus variegatus

Previous work in our laboratory on the aquatic worm Lumbriculus variegatus demonstrated that serotonin is necessary for the production of two touch-evoked escape behaviors, helical swimming and body reversal.  The goal of this study was to determine if particular types of serotonin receptors are required to produce these behaviors.  In otherwise untreated worms the 5‑HT_1/2 agonist m-CPP [1‑(3‑chlorophenyl) piperazine] reduced swimming and reversal responses in a concentration-dependent manner.  Worms depleted of all amine neurotransmitters by reserpine treatment have severely reduced ability to produce swimming and reversal behaviors.    In such worms, m-CPP had no effect on helical swimming; however, the reversal response was restored at 1 mM. The 5‑HT₁ agonist, 8-OH-DPAT [8‑hydroxy-2-(dipropylamino) tetralin] eliminated swimming and reversal responses in normal worms.  In reserpine-treated worms exposed to 8-OH-DPAT, there was no restoration of the swimming response; however, the reversal response was restored at 1 mM. Reserpine-treated worms treated with DOI (2, 5‑dimethoxy-4-iodoamphetamine), a 5-HT₂ agonist, showed no restoration of either response, but both behaviors were inhibited in normal worms.  Swimming and reversal behaviors were unaffected in normal worms treated with either m-CPBG (m-chlorophenyl- biguanide), a 5-HT₃ agonist or 5-MeOT (5‑methoxytryptamine), a 5-HT_1/2/4/6/7 agonist, at concentrations of 1 mM or below; however, both behaviors were reduced at 10 mM.  Experiments with 5-MeOT and m-CPBG in reserpine-treated worms are ongoing.   The conflict between stimulation of swimming and reversal behaviors by serotonin, but inhibition by specific agonists is an enigma; however, it may indicate both excitatory and inhibitory serotoninergic control of these behaviors.