The role of Glycine(B) binding site and glycine transporter (GlyT1) in theregulation of [H-3]GABA and [H-3]glycine release in the rat brain

The effect of N-methyl-D-aspartic acid (NMDA), a selective glutamate recept or agonist, on the release of previously incorporated [H-3]gamma -aminobuty ric acid(GABA) was examined in superfused striatal slices of the rat. NMDA (0.01 to 1.0 mM) increased [H-3]GABA overflow with an EC50 value of 0.09 mM . The [H-3]GABA releasing effect of NMDA was an external Ca2+- dependent pr ocess and the GABA uptake inhibitor nipecotic acid (0.1 mM) potentiated thi s effect. These findings support the view that NMDA evokes GABA release fro m vesicular pool in striatal GABAergic neurons. Addition of glycine (I mM), a cotransmitter for NMDA receptor, did not influence the NMDA-induced [H-3 ]GABA overflow. Kynurenic acid (1 mM), an antagonist of glycine(B) site, de creased the [H-3]GABA-releasing effect of NMDA and this reduction was suspe nded by addition of 1 mM glycine. Neither glycine nor kynurenic acid exerte d effects on resting [H-3]GABA outflow. These data suggest that glycine(B) binding site at NMDA receptor may be saturated by glycine released from nei ghboring cells. Glycyldodecylamide (GDA) and N-dodecylsarcosine, inhibitors of glycincTl transporter, inhibited the uptake of [H-3]glycine (IC50 33 an d 16 muM) in synaptosomes prepared from rat hippocampus, When hippocampal s lices were loaded with [H-3]glycine, resting efflux was detected whereas el ectrical stimulation failed to evoke [H-3]glycine overflow. Neither GDA (0. 1 mM) nor N-dodecylsarcosine (0.3 mM) influenced [H-3]glycine efflux. Using Krebs-bicarbonate buffer with reduced Na+ for superfusion of hippocampal s lices produced an increased [H-3]glycine outflow and electrical stimulation further enhanced this release. These experiments speak for glial and neuro nal [H-3]glycine release in hippocampus with a dominant role of the former one. GDA, however, did not influence resting or stimulated [H-3]glycine eff lux even when buffer with low Na+ concentration was applied.