Abstracts

RATIONALE: The NMDA receptor is a heteromeric receptor comprised of at least one NR1 subunit and any of four NR2 subunits (NR2A-D). The NR2 subunit confers different pharmacologic and kinetic properties to the receptor. NMDA receptors in the well-characterized CA3-CA1 synapse of the hippocampus are believed to play a role in seizure activity. Conantokin G (CGX-1007), a 17-amino acid polypeptide isolated from the venom of Conus geographus, is a novel NMDA receptor antagonist that is selective for the NR2B subunit. CGX-1007 has been reported to have highly potent, broad-spectrum anticonvulsant activity in animal seizure models, e.g., MES, scPTZ, amygdala kindling. The present study was designed to test the hypothesis that CGX-1007 would be effective in blocking NMDA-mediated EPSCs at the CA3-CA1 synapse in the hippocampal brain slice preparation.
METHODS: Studies employed the whole-cell patch clamp technique to record from pyramidal neurons in the CA1 region of the hippocampus in brain slices obtained from adult Sprague-Dawley rats. NMDA-mediated EPSCs were elicited a low frequency using minimal local stimulation of the Schaffer collateral pathway. EPSCs were pharmacologically isolated by perfusing the slice with oxygenated Mg2+-free Ringer[ssquote]s solution containing 10 mM NBQX and 50 mM picrotoxin. After a stable baseline of EPSCs were obtained for a minimum of 20 min, CGX-1007, which was dissolved in the Ringer[ssquote]s solution at a variety of concentrations, was bath applied for 30 minutes. CGX-1007 perfusion was followed by a washout period of at least 30 min.
RESULTS: CGX-1007 reversibly blocked NMDA-mediated EPSCs in CA1 pyramidal neurons in a concentration-dependent manner, reaching a maximum inhibition of 75% at a concentration of 10 mM. A single-binding site isotherm was sufficient to describe the block of NMDA-mediated EPSCs by CGX-1007 (IC50 = 1.1 [mu]M). In addition, CGX-1007 block was not voltage-dependent and there was no evidence that CGX-1007 altered the kinetics of the EPSCs.
CONCLUSIONS: These experiments demonstrate that CGX-1007 is an effective antagonist at NMDA receptors in the CA3-CA1 synapse and suggest that CGX-1007 may be exerting it[ssquote]s anticonvulsant actions by reducing current flow through NMDA receptors.
Support: Supported by a grant from Cognetix, Inc.
Disclosure: Salary - Cognetix, Inc. (T. McCabe); Grant - Cognetix, Inc. (H.S. White); Consulting - Cognetix, Inc. (H.S. White); Materials - Cognetix, Inc. (H.S. White); Stock - Cognetix, Inc. (T. McCabe)