Glycine enhances extracellular 45Ca2+ response to NMDA application investigated with microdialysis of rabbit hippocampus in vivo.

This study evaluates the role of glycine in in vivo modulation of the activity of excitatory amino acid receptors sensitive to N-methyl-D-aspartate (NMDA) in the rabbit hippocampus. Changes of extracellular calcium concentration were studied by the microdialysis technique combined with 45Ca-utilizing clearance method. The steady state level of amino acids in the dialysate was analyzed with HPLC. Pharmacologically active substances were applied directly to the hippocampus via the dialysis medium. Glycine concentration in the extracellular fluid of the hippocampus was in the range of 10(-5) M. Application of NMDA induced a drop of calcium concentration in the extracellular compartment of the hippocampus. This effect was inhibited by 7-Cl-kynurenic acid, an antagonist of glycine binding site on the NMDA receptors. Application of glycine and D-serine to the dialysis medium did not interfere with the basal level of extracellular calcium in the hippocampus, but resulted in an enhancement of the NMDA-induced decrease of calcium concentration. These results indicate that in the hippocampus the NMDA receptors are under constant positive modulation by endogenous glycine. Since glycine binding sites on the NMDA receptors are not saturated by an endogenous ligand, glycine may play a role in regulation of the NMDA receptor activity in the hippocampus in vivo.