CHARACTERIZATION OF [3H]GABAPENTIN BINDING TO A NOVEL SITE IN RAT-BRAIN - HOMOGENATE BINDING-STUDIES

The binding characteristics of [H-3]gabapentin, the radiolabelled analogue of the novel anticonvulsant gabapentin (1-(aminomethyl)cyclohexaneacetic acid) were studied using purified synaptic plasma membranes prepared from rat cerebral cortex. In 10 mM HEPES buffer [H-3]gabapentin bound to a single population of sites with high affinity (K(D) = 38 +/- 2.8 nM) with a maximum binding capacity of 4.6 +/- 0.4 pmol/mg protein, reaching equilibrium after 30 min at 20-degrees-C. This novel site was unique to the central nervous system with little or no specific [H-3]gabapentin binding being measurable in a range of peripheral tissues. Binding was potently inhibited by a range of gabapentin analogues and 3-alkyl substituted gamma-aminobutyric acid (GABA) derivatives although GABA itself and the selective GABA(B) receptor ligand baclofen, were only weakly active. Gabapentin itself (IC50 = 80 nM) and 3-isobutyl GABA (IC50 = 80 nM) which also has anticonvulsant properties, showed the highest affinity for the binding site. Of a wide range of other pharmacologically active compounds only the polyamines spermine and spermidine influenced [H-3]gabapentin binding, with both compounds producing a maximum of 50% inhibition of specific binding. Magnesium ions produced a similar pattern of inhibition but the effect of the polyamines and magnesium ions were not additive. The data provide evidence for the existence in brain of a novel binding site that may mediate the anticonvulsant effects of gabapentin and other potential anticonvulsant compounds.