Some properties of ionic channels activated by excitatory amino acids in hippocampal neurons

Properties of excitation induced by various excitatory amino acids were studied in thin slices of the guinea pig hippocampus in the presence of Mn2+, tetrodotoxin and tetraethylammonium chloride. Depolarizations induced by L-glutamate (Glu), quisqualate (Quis) and D-homocysteate (DH) were accompanied consistently by decreases in neuron input resistance. In the current-voltage function, increases in input resistance were never observed at any membrane potential. The amplitude of Glu, Quis and DH responses decreased during tonic outward currents and increased during tonic inward currents. Although neuron input resistance decreased with depolarizations induced by L-aspartate (Asp) as well, the magnitude of the resistance reduction was significantly smaller than that induced by Glu. Asp responses changed in amplitude as did Glu responses during tonic inward and outward currents. Depolarizations induced by N-Methyl-D-aspartate (NMDA) were accompanied by apparent increases in input resistance, and their amplitudes increased and decreased during tonic depolarization and hyperpolarization, respectively. Mn2+ was almost without effect at the concentration used (2.7 mM) on responses induced by Glu, DH or Asp. These results suggest that Glu, Quis and DH induce depolarizations in hippocampal neurons by activating only Quis receptors, and that Asp activates Quis receptors preferentially though it activates NMDA receptors as well.