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

Gabapentid 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 [H-3]gabapentin and a variety of calcium channel binding ligands, Most ligands for voltage- or ligand-gated calcium channels (verapamil, the omega-conotoxins MVIIC and GVIA, ryanodine, caffeine, capsaicin, MK-801) had no significant effect on [H-3]gabapentin binding. However, ruthenium red, a relatively nonselective calcium channel ligand, was found to robustly modulate [H-3]gabapentin binding. Ruthenium red slowed the association and dissociation kinetics of [H-3]gabapentin while increasing the number of detectable binding sites. Spermine and MgCl2, which also bind to calcium channels and modulate [H-3]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, MgCl2 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. (C) 2000 Elsevier Science Ltd. All rights reserved.