Excitatory amino acid-induced responses of frog motoneurones bathed in low Na+ media: an intracellular study.

Motoneurones of the frog spinal cord slice preparation were impaled with microelectrodes and superfused at 7 degrees C with the excitatory amino acids glutamate, quisqualate or N-methyl-D-aspartate. The role of Na+ in the action of these amino acids was studied by comparing amplitude matched depolarizations obtained in control Ringer solution with the responses recorded from the same cells after replacing (86-100%) Na+ by choline or glucosamine. Effective replacement of extracellular Na+ proved to be a rather slow process requiring 30-60 min. In glucosamine solution depolarizations evoked by glutamate, N-methyl-D-aspartate or quisqualate were abolished or strongly reduced with recovery following return to control Ringer. In choline solution, glutamate and N-methyl-D-aspartate effects were blocked whereas the quisqualate response was surprisingly unaffected. Mn2+ (2 mM) added to choline solution strongly diminished the action of quisqualate. These results suggest that Na+ was important in mediating amino acid responses and that quisqualate activated an additional conductance mechanism (perhaps to Ca2+) unmasked only in choline-containing solution.