Serotonin and acetylcholine: further analysis of praziquantel-induced contraction of magnesium-paralysed Schistosoma mansoni

Abstract

SUMMARYThe nature of stimulus-induced flaccid paralysis produced in Mg2+-paralysed Schistosoma mansoni was investigated. Serotonin induced a dose-dependent, heterologous flaccid paralysis with an IC50 of 600 nm. This flaccid paralysis was a function of the extracellular Mg2+:Ca2+ ratio and was reversible. Tonic contractions produced by phorbol-12,13- dibutyrate or 60 mM K+ were reversed by the application of serotonin and flaccid paralysis was induced. These actions of serotonin were mimicked by forskolin and synergized by IBMX but the potassium channel blocker, 3,4-DAP, did not produce flaccid paralysis. When Mg2+-paralysed parasites were stimulated with 3,4-DAP, IBMX produced a dose- dependent flaccid paralysis with an IC50 of 11 μm. Membrane permeable analogues of cAMP and cGMP did not synergize with IBMX. Cholinergic agonists, but not other inhibitory substances, prevented the serotonin- and forskolin-induced and the IBMX-synergized flaccid paralysis but not that produced by praziquantel. The possible interactions of these agents with the muscle are discussed.