Effects of bath-applied excitatory amino acids and their analogs on spinal interneurons of the lamprey.

Depolarizations, conductance increases and time courses of the responses to bath application of glutamate, aspartate, DL-homocysteate, N-methyl-DL-aspartate (NMDLA), quisqualate and kainate were determined in interneurons of the isolated spinal cord of the lamprey, one of the most primitive vertebrates. Conductance increases produced by these excitants in perfusate containing tetrodotoxin (0.5 microgram/ml), 4-aminopyridine (1 mM) and without Ca2+ were very small in comparison with those produced by glycine or GABA. NMDLA-induced depolarizations were associated with conductance decreases and rhythmic oscillations in membrane potentials in this perfusate. Quisqualate was strongest among these amino acids in producing depolarizations and conductance increases. Responses induced by analogs were slower than those produced by glutamate and aspartate. Phylogenetic distribution of N-methyl-D-aspartate receptors on neurons and muscles is discussed.