Riluzole attenuates the effects of chemoconvulsants acting on glutamatergic and GABAergic neurotransmission in the planarian Dugesia tigrina

Abstract

Planarians, the non-parasitic flatworms, display dose-dependent, distinct (C-like and corkscrew-like) hyperkinesias upon exposure to 0.001–10mM aqueous solutions of glutamatergic agonists (l-glutamate and N-methyl-d-aspartate (NMDA)) and 0.001–5mM concentrations of the glutamate decarboxylase (GAD) inhibitor (semicarbazide). In the planarian seizure-like activity (PSLA) experiments the three chemoconvulsants displayed the following order of potency (EC50): l-glutamate (0.6mM)>NMDA (1.4mM)>semicarbazide (4.5mM). Planarian hyperkinesias behavior counting experiments also revealed that riluzole (0.001 to 1mM), an anti-convulsive agent, displayed no significant behavioral activity by itself, but attenuated hyperkinesias elicited by the three chemoconvulsants targeting either glutamatergic or GABAergic neurotransmission with the following order of potency (IC50): NMDA (44.7µM)>semicarbazide (88.3µM)>l-glutamate (160µM). Further, (+)-MK-801, a specific NMDA antagonist, alleviated 3mM NMDA (47%) or 3mM l-glutamate (27%) induced planarian hyperkinesias. The results provide pharmacological evidence for the presence of glutamatergic receptor-like and semicarbazide sensitive functional GAD enzyme-like proteins in planaria in addition to demonstrating, for the first time, the anti-convulsive effects of riluzole in an invertebrate model. High performance liquid chromatography coupled with fluorescence detection (HPLC-F) analysis performed on planarian extracts post no drug treatment (control) or treatment with 3mM semicarbazide, combination of 3mM semicarbazide and 0.1mM riluzole, or 0.1mM riluzole revealed that 3mM semicarbazide induced 35% decrease in the GABA levels and a combination of 3mM semicarbazide and 0.1mM riluzole induced 42% decrease in glutamate levels with respect to the control group.