Characterisation of [ 3H]gabapentin binding to a novel site in rat brain: homogenate binding studies

The binding characteristics of [ 3H]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 [ 3H]gabapentin bound to a single population of sites with high affinity (K D = 38 ± 2.8 mM) with a maximum binding capacity of 4.6 ± 0.4 pmol/mg protein, reaching equilibrium after 30 min at 20°C. This novel site was unique to the central nervous system with little or no specific [ 3H]gabapentin binding being measurable in a range of peripheral tissues. Binding was potently inhibited by a range of gabapentin analogues and 3-alkyl substituted γ-aminobutyric acid (GABA) derivatives although GABA itself and the selective GABA B receptor ligand baclofen, were only weakly active. Gabapentin itself (IC 50 = 80 nM) and 3-isobutyl GABA (IC 50 = 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 [ 3H]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.