strychnine and 1-(3-chlorophenyl)piperazine

1. In order to determine whether the strychnine-insensitive glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex is fully activated in vivo, the ability of the selective glycine receptor agonist, D-serine, to modulate seizure susceptibility in the mouse has been examined. 2. D-Serine (10-200 micrograms per mouse, i.c.v.) dose-dependently increased the potency of NMDLA in inducing seizures in Swiss Webster mice by approximately 3 fold. L-Serine was without significant effect. 3. The potency of pentylenetetrazol in inducing seizures was also enhanced by D-, but not L-serine, although the magnitude of the shift (1.6 fold) was considerably less than for NMDLA. 4. Similar doses of D-serine were also able to block the anticonvulsant effect of the non-selective glycine receptor antagonist, kynurenic acid, against seizures induced by NMDLA, but were without effect on the anticonvulsant effect of the competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). 5. D-Serine completely antagonized the protective effect of the selective glycine receptor antagonist, 7-chlorokynurenic acid, against sound-induced seizures in DBA/2 mice, but was less effective in this model against the less selective antagonist, kynurenic acid. 6 The results indicate that in vivo, NMDA receptors are not maximally potentiated by endogenous glycine and suggest an important involvement of the glycine modulatory site on the NMDA receptor/ion channel complex in the pathophysiology of epilepsy.

Topics: Acoustic Stimulation; Animals; Aspartic Acid; Diazepam; Injections, Intraventricular; Ion Channels; Kynurenic Acid; Male; Mice; Mice, Inbred DBA; N-Methylaspartate; Pentylenetetrazole; Piperazines; Receptors, Glycine; Receptors, N-Methyl-D-Aspartate; Receptors, Neurotransmitter; Seizures; Serine; Strychnine

We have examined the actions of putative antagonists of the strychnine-insensitive glycine-mediated modulation of the N-methyl-D-aspartate (NMDA) receptor using [3H]MK801 binding, Ca2+ influx and [3H]GABA release assays. Kynurenic acid and HA-966 inhibited [3H]MK801 binding, NMDA and glycine induced Ca2+ influx measured using fura-2 and NMDA and glycine simulated [3H]GABA release. The effects of kynurenic acid could be partially overcome by the addition of excess glutamate and glycine, indicating limited selectivity for the glycine binding site. In addition, a component of the action of kynurenic acid was insensitive to agonist concentration, indicating a third action of kynurenic acid at high concentrations. In contrast, HA-966 was 100-fold selective for the glycine compared to the NMDA site. HA-966 only partially inhibited [3H]MK801 binding (IC50 19.7 microM), NMDA-induced Ca2+ influx and neurotransmitter release. The failure of HA-966 to completely block NMDA responses, even at high concentrations, suggests that glycine may not be an absolute requirement for the activation of NMDA receptors under these experimental conditions.

Topics: Animals; Anticonvulsants; Aspartic Acid; Brain; Calcium; Desipramine; Dibenzocycloheptenes; Dizocilpine Maleate; gamma-Aminobutyric Acid; Ketamine; Kynurenine; Membranes; N-Methylaspartate; Piperazines; Pyrrolidinones; Radioligand Assay; Rats; Receptors, Glycine; Receptors, N-Methyl-D-Aspartate; Receptors, Neurotransmitter; Strychnine

The role of endogenous glycine in supporting N-methyl-D-aspartate (NMDA)-evoked neurotransmitter release was investigated. HA-966 (1-hydroxy-3-aminopyrrolidone-2) inhibited NMDA-evoked release of [3H]norepinephrine from rat hippocampal brain slices, but was much less effective in inhibiting [3H]norepinephrine release evoked by kainic acid (KA). Glycine (1 mM) reversed the HA-966 (1 mM) antagonism of NMDA-evoked release of [3H]norepinephrine. Strychnine (10 microM) had no effect on the ability of glycine to reverse HA-966 antagonism of NMDA-evoked neurotransmitter release. Other amino acids were also capable of reversing the HA-966 antagonism of NMDA-evoked [3H]norepinephrine release with a rank order of potency: D-serine greater than or equal to glycine much greater than L-serine approximately beta-alanine. These same compounds inhibited strychnine-insensitive [3H]glycine binding to rat cortical membrane fragments with a rank order of potency: glycine greater than D-serine much greater than L-serine greater than or equal to beta-alanine. In addition, HA-966 inhibited [3H]glycine binding (IC50 = 8.5 microM). The results suggest that HA-966 antagonism of NMDA-evoked neurotransmitter release is due to the inhibition of endogenous glycine acting at a strychnine-insensitive modulatory glycine site associated with the NMDA receptor/ionophore complex.

Topics: Animals; Aspartic Acid; Binding Sites; Dibenzocycloheptenes; Dizocilpine Maleate; Glycine; Hippocampus; In Vitro Techniques; Male; N-Methylaspartate; Neurotransmitter Agents; Norepinephrine; Piperazines; Pyrrolidinones; Rats; Strychnine