5-nitro-6-7-8-9-tetrahydrobenzo(g)indole-2-3-dione-3-oxime and domoic-acid

1. The effect of a low concentration (1 microM) of kainate (kainic acid; KA) on presynaptic calcium (Ca2+) influx at the Schaffer collateral-commissural (SCC) synapse was examined in rat hippocampal slices. 2. Following selective loading of the presynaptic terminals with the fluorescent Ca2+ indicator rhod-2 AM, transient increases in the presynaptic Ca2+ concentration (pre[Ca2+]t) and field excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the SCC pathway were recorded simultaneously. 3. Bath application of 1 microM KA reversibly suppressed field EPSPs and pre[Ca2+]t to 37.7 +/- 4.0 % and 72.9 +/- 2.4 % of control, respectively. Excitatory postsynaptic currents (EPSCs) recorded with the use of the whole-cell patch-clamp technique were also suppressed by 1 microM KA to 42.6 +/- 6.3 % of control. A quantitative analysis of the decreases in pre[Ca2+]t and the amplitude of field EPSP during KA application suggests that KA inhibits transmission primarily by reducing the pre[Ca2+]t. 4. Consistent with a presynaptic site for these effects, paired-pulse facilitation (PPF) was enhanced by 1 microM KA. 5. A substantial KA-induced suppression of NMDA receptor-mediated EPSPs was detected when AMPA receptors were blocked by the AMPA receptor-selective antagonist GYKI 52466 (100 microM). 6. The suppressive effect of KA on field EPSPs and pre[Ca2+]t was antagonized by the KA antagonist NS-102 (10 microM). 7. These results suggest that the presynaptic inhibitory action of KA at the hippocampal CA1 synapse is primarily due to the inhibition of Ca2+ influx into the presynaptic terminals.

Topics: Action Potentials; Animals; Anti-Anxiety Agents; Benzodiazepines; Calcium; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Hippocampus; Indoles; Kainic Acid; Neuromuscular Depolarizing Agents; Oximes; Patch-Clamp Techniques; Presynaptic Terminals; Rats; Rats, Wistar; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Rhodamines; Synaptic Transmission

Our objective was to characterize the neurotoxic actions of systemically administered domoic acid on different excitatory amino acid receptors, and to compare the receptor selectivity of domoate with the related compound kainic acid. Groups of mice were injected with various ligands selective for N-methyl-D-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid-kainate (AMPA/kainate) receptors prior to injection of equitoxic doses of domoic acid or kainic acid. Domoic acid toxicity was not significantly altered by pretreatment with any NMDA receptor selective antagonists, with the exception of 3-(2-carboxypiperazine-4-yl)propyl-1 -phosphonic acid. Consistent with its characterization as an AMPA/kainate agonist, domoate toxicity was significantly antagonized by all non-NMDA receptor antagonists tested. Non-NMDA receptor antagonists that do not distinguish between high- and low-affinity [3H]kainic acid binding (i.e., quinoxalinediones) were equally effective at reducing domoic acid and kainic acid toxicity. However, the novel isatinoxime NS-102, which has been shown to interact selectively with low-affinity [3H]kainic acid binding sites, produced a selective dose-related antagonism of domoic acid toxicity relative to kainic acid. NS-102 produced significant reductions in overall toxicity, onset of motor seizures, and hippocampal CA3 cell damage induced by domoic acid at NS-102 doses that did not antagonize kainic acid induced toxicity. We conclude that domoic acid toxicity in vivo is mediated largely by a subclass of non-NMDA receptors that are selectively antagonized by NS-102.

Topics: Animals; Behavior, Animal; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Indoles; Kainic Acid; Mice; Mice, Inbred Strains; Oximes; Reaction Time

5-Nitro-6,7,8,9-tetrahydrobenzo[G]indole-2,3-dione-3-oxime (NS-102), a new competitive glutamate receptor antagonist displaced binding to non-N-methyl-D-aspartate (non-NMDA) binding sites with no activity at the NMDA and strychnine-insensitive glycine binding sites. Under experimental conditions in which both high- and low-affinity sites were labelled, NS-102 only partially inhibited the binding of [3H]kainate. Studies of NS-102 displacement of high-affinity versus low-affinity [3H]kainate binding showed a high selectivity of NS-102 for the low-affinity [3H]kainate binding site (Ki = 0.6 microM) compared to the high-affinity [3H]kainate binding site (Ki > 10 microM). NS-102 was a relatively weak inhibitor of 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) binding (IC50 = 7.2 microM). NS-102 and related compounds with similar pharmacological profiles may become valuable tools in the characterization of the functional importance of the low-affinity [3H]kainate binding site.

Topics: Animals; Binding, Competitive; Calcium; Calcium Chloride; Cerebral Cortex; Excitatory Amino Acid Antagonists; Indoles; Kainic Acid; Male; Membranes; Molecular Structure; Neuromuscular Depolarizing Agents; Oximes; Quinoxalines; Radioligand Assay; Rats; Rats, Wistar; Tritium