GV 150526A, 7-Cl-kynurenic acid and HA 966 antagonize the glycine enhancement of N-methyl-D-aspartate-induced [3H]noradrenaline and [3H]dopamine release.

The effect of selective antagonists (7-Cl-kynurenic acid, 3-amino-1-hydroxypyrrolid-2-one (HA 966) and GV 150526A) at strychnine-insensitive glycine sites was studied by measuring how much glycine potentiated the [3H]dopamine and [3H]noradrenaline release induced by 100 microM N-methyl-D-aspartate (NMDA) from superfused striatal and hippocampal synaptosomes, respectively, in a Mg2+-free buffer. Glycine, which per se had no effect on [3H]catecholamine release, concentration-dependently potentiated the effect of NMDA, with similar potency in the two brain regions (EC50 0.25 and 0.27 microM for [3H]dopamine and [3H]noradrenaline release, respectively). 7-Cl-Kynurenic acid reduced the effect of NMDA alone and antagonized the effect of 1 microM glycine, with Ki values of 1.1 and 0.6 microM for [3H]dopamine and [3H]noradrenaline release, respectively. HA 966 did not inhibit the effect of NMDA alone, but reduced the effect of glycine with Ki = 11.5 and 66 microM for [3H]dopamine and [3H]noradrenaline release. GV 150526A inhibited the effect of NMDA alone and potently antagonized the effect of glycine, with Ki = 12.4 and 17.3 nM for [3H]dopamine and [3H]noradrenaline release. Our results are consistent with the possibility that HA 966 is a partial agonist, while 7-Cl-kynurenic acid and GV 150526A are competitive antagonists at the strychnine-insensitive glycine sites. In addition HA 966 shows regional differences in its interaction with the strychnine-insensitive glycine receptor, being about six times more potent on striatal than on hippocampal synaptosomes, suggesting a possible heterogeneity of glycine sites recognized by HA 966 or different intrinsic activity in the two brain regions. The nanomolar potency of GV 150526A in reducing NMDA receptor function by competitively acting at the strychnine-insensitive glycine sites suggests that GV 150526A could be effective in vivo to reduce NMDA receptor over-stimulation, like in brain ischemia.