Action of excitatory amino acids on hypodermis and the motornervous system of Ascaris suum: pharmacological evidence for a glutamate transporter

Abstract

Electrophysiological and pharmacological experiments suggest the presence of an electrogenic glutamate transporter in the motornervous system of the parasitic nematode Ascaris suum. This putative transporter occurs in hypodermis (a tissue in some respects analogous to glia) and in DE2 motorneurons, a dorsal excitatory motorneuron class which receives excitatory glutamatergic post-synaptic potentials. Glutamate application to hypodermis produced non-conductance mediated depolarizations that were smaller in amplitude and slower in rate of rise than DE2 responses where a glutamate-activated conductance occurs. The hypodermal response is sodium dependent and calcium independent. Excitatory amino acid ionotropic receptor agonists (kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid and N-methyl-D-aspartate) were ineffective in eliciting hypodermal responses. The ionotropic receptor antagonist, 6,7-dinitroquinoline-2,3-dione, had no effect on hypodermal glutamate responses. The L- and D-forms of glutamate, aspartate and homocysteate produced hypodermal and DE2 depolarizations consistent with the pharmacological profile for glutamate transporters in other systems. Glutamate transport inhibitors (L-trans-pyrrolidine-2,4-dicarboxylate and beta-hydroxyaspartate) elicited electrogenic depolarizations in hypodermis and DE2. These results suggest that the hypodermal glutamate response has an electrogenic transporter component, while the DE2 response has 2 components, one conductance-mediated and the other due to an electrogenic transporter.