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.