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

Gabapentid is an anticonvulsant with an unknown mechanism of action. Howeve r, it has been proposed that gabapentin acts by binding to voltage-gated ca lcium channels. To further characterize the interaction of gabapentin with its endogenous binding site in cerebral cortex, we tested for competitive a nd 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, caffe ine, capsaicin, MK-801) had no significant effect on [H-3]gabapentin bindin g. However, ruthenium red, a relatively nonselective calcium channel ligand , was found to robustly modulate [H-3]gabapentin binding. Ruthenium red slo wed the association and dissociation kinetics of [H-3]gabapentin while incr easing the number of detectable binding sites. Spermine and MgCl2, which al so bind to calcium channels and modulate [H-3]gabapentin binding, were foun d to act in a similar manner. These findings support the contention that th e principal endogenous binding site for gabapentin is a calcium channel; th ey characterize the nature of the allosteric interaction of spermine, MgCl2 and ruthenium red with this binding site; and they suggest possible mechan isms by which gabapentin may modulate calcium channel function and ultimate ly produce therapeutic actions. (C) 2000 Elsevier Science Ltd. All rights r eserved.