Allosteric modulation of [ 3H]gabapentin binding by ruthenium red

Abstract

Gabapentin is an anticonvulsant with an unknown mechanism of action. However, it has been proposed that gabapentin acts by binding to voltage-gated calcium channels. To further characterize the interaction of gabapentin with its endogenous binding site in cerebral cortex, we tested for competitive and allosteric interactions between [ 3H]gabapentin and a variety of calcium channel binding ligands. Most ligands for voltage- or ligand-gated calcium channels (verapamil, the ω-conotoxins MVIIC and GVIA, ryanodine, caffeine, capsaicin, MK-801) had no significant effect on [ 3H]gabapentin binding. However, ruthenium red, a relatively nonselective calcium channel ligand, was found to robustly modulate [ 3H]gabapentin binding. Ruthenium red slowed the association and dissociation kinetics of [ 3H]gabapentin while increasing the number of detectable binding sites. Spermine and MgCl 2, which also bind to calcium channels and modulate [ 3H]gabapentin binding, were found to act in a similar manner. These findings support the contention that the principal endogenous binding site for gabapentin is a calcium channel; they characterize the nature of the allosteric interaction of spermine, MgCl 2 and ruthenium red with this binding site; and they suggest possible mechanisms by which gabapentin may modulate calcium channel function and ultimately produce therapeutic actions.