Evidence for the presence of glutamatergic receptors in adult Schistosoma mansoni.

Several studies have suggested that L-glutamate is a putative neurotransmitter in helminths. The present study investigated the presence of non-N-methyl-D-aspartate (NMDA) ionotropic receptors for glutamate in four subcellular fractions from adult male Schistosoma mansoni. Low-affinity (K(d)=221+/-80 nM) binding sites for [3H]kainic acid (KA) were detected in the heterogeneous (P(1)) fraction, which contains pieces of unbroken worm tissues, tegument, nuclei, and some vesicles. This binding was inhibited by classical glutamatergic ligands in the following order of potency: KA>L-glutamate>alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)>quisqualate congruent with 6,7-dinitroquinoline-2,3-dione (DNQX). However, neither NMDA, a selective agonist for NMDA receptors, nor DL-threo-beta-hydroxyaspartate (THA) and 1-trans-pyrollidine-2-dicarboxylic acid (PDC), inhibitors of high-affinity glutamate transporters, modified [3H]KA binding to the P(1) fraction. In addition, no specific binding for 10nM [3H]AMPA was detected in any subcellular fraction from S. mansoni. These results suggested the presence of KA receptors in adult male worms. This is supported by the evidence that direct application of 10 microM KA to whole worms produced a corkscrew-like coiling of their bodies, modifying the motility of the worms. The KA-induced response, measured as a decrease of the body area, was time-dependent and reversible. PDC was ineffective at blocking the KA effects, indicating that KA does not depend on high-affinity glutamate transporters to reach its site of action. On the other hand, DNQX, the non-NMDA antagonist, was able to partially inhibit KA-induced responses. As a whole, the present data support the presence of a glutamatergic signaling pathway in this parasite.