Abnormal neurotransmission in mice lacking synaptic vesicle protein 2A (SV2A).

Synaptic vesicle protein 2 (SV2) is a membrane glycoprotein common to all synaptic and endocrine vesicles. Unlike many proteins involved in synaptic exocytosis, SV2 has no homolog in yeast, indicating that it performs a function unique to secretion in higher eukaryotes. Although the structure and protein interactions of SV2 suggest multiple possible functions, its role in synaptic events remains unknown. To explore the function of SV2 in an in vivo context, we generated mice that do not express the primary SV2 isoform, SV2A, by using targeted gene disruption. Animals homozygous for the SV2A gene disruption appear normal at birth. However, they fail to grow, experience severe seizures, and die within 3 weeks, suggesting multiple neural and endocrine deficits. Electrophysiological studies of spontaneous inhibitory neurotransmission in the CA3 region of the hippocampus revealed that loss of SV2A leads to a reduction in action potential-dependent gamma-aminobutyric acid (GABA)ergic neurotransmission. In contrast, action potential-independent neurotransmission was normal. Analyses of synapse ultrastructure suggest that altered neurotransmission is not caused by changes in synapse density or morphology. These findings demonstrate that SV2A is an essential protein and implicate it in the control of exocytosis.

The methylglutamate, SYM 2081, is a potent and highly selective agonist at kainate receptors.

The methylglutamate analog (2S,4R)-4-methylglutamate (SYM 2081) has been shown to potently displace high affinity [3H]kainate binding to cortical tissue and to recombinant kainate receptors, and to evoke rapidly desensitizing responses in electrophysiological recordings. We have used two electrode voltage clamp recordings to compare the potency and efficacy of SYM 2081 with other alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate receptor agonists at homomeric kainate and AMPA receptors expressed in Xenopus oocytes. In the presence of concanavalin A to reduce agonist induced desensitization at kainate receptors, SYM 2081 was a potent agonist at homomeric kainate receptors composed of the GluR5 and GluR6 subunit, with an EC50 of 0.12 +/- 0.02 and 0.23 +/- 0.01 microM, respectively. SYM 2081 was highly selective for kainate receptors, the EC50 for activation of AMPA receptors composed of the GluR1 and GluR3 subunits was 132 +/- 44 and 453 +/- 57 microM, respectively. Other methylglutamate analogs were tested for kainate receptor agonist activity. Methylglutamate compounds with the methyl group at the 2 or 3 position of glutamate were inactive indicating that positioning of the methyl group at the 4 position was essential for agonist activity. Of the four stereoisomers of 4-methylglutamate, SYM 2081 (2S,4R) was the most potent agonist. The (2R,4R) isomer was estimated to be 20-fold and the (2S,4S)-isomer approximately 1000-fold less potent than SYM 2081. These results indicate that SYM 2081 is a potent and selective agonist at kainate receptors, and thus will be a useful ligand for evaluating the role of kainate receptors in central nervous system function and disease.