Biphasic effect of quinolinate on frog spinal, but not rat cortical, neurones: N-methyl-D-aspartate-like depolarisation and a novel type of hyperpolarisation.

Using grease seal techniques on rat cerebral cortical slices and on frog hemisected spinal cords, bath applied N-methyl-D-aspartate (NMDA; 10-160 microM) and quinolinate (0.25-8 mM) induced dose-dependent depolarisations. These depolarising responses, but not those to quisqualate or kainate, were reduced by 2-amino-5-phosphono-valerate (2-AP5; 10-100 microM), magnesium (0.1-10 mM) and ketamine (10-100 microM). A novel dose-dependent hyperpolarising response to quinolinate (0.1-2 mM) but not to NMDA was observed only on frog motoneurones, and this was not reduced by classical amino acid receptor antagonists.