COMPARISON OF THE UPTAKE OF [H-3] GABAPENTIN WITH THE UPTAKE OF L-[H-3]-LEUCINE INTO RAT-BRAIN SYNAPTOSOMES

1 Gabapentin is a novel anticonvulsant with an unknown mechanism of ac tion. Homogenate binding studies described elsewhere have suggested th at [H-3]-gabapentin binds to a site in brain similar to the large neut ral amino acid (LNAA) uptake site, termed system-L. 2 This study descr ibes an investigation into the uptake of [H-3]-gabapentin into a crude synaptosomal preparation from cerebral cortex of rat brain. Character ization studies showed that [H-3]-gabapentin is taken up into synaptos omes by a system that is similar to that responsible for the uptake of L-[H-3]leucine. This system is sodium-independent, temperature-sensit ive and requires ATP for function. 3 Kinetic studies of [H-3]-gabapent in uptake produced a Michaelis constant (K-M=160 mu M) Similar to that observed for L-[H-3]-leucine (K-M=110.3 mu M). V-max values were 837. 1 pmol mg(-1) protein min(-1) and 2.192 nmol mg(-1) protein min(-1) re spectively. 4 Gabapentin and L-leucine mutually inhibit their uptake. Lineweaver-Burke plots of these data demonstrate that inhibition occur s by a competitive mechanism. Further to this the Dixon transformation of the data illustrates that these two substrates share a common upta ke site by the similarity between their calculated K-i, and K-M values (gabapentin inhibition of L-[H-3]-leucine uptake: K-i=160 mu M; L-leu cine inhibition of [H-3]-gabapentin uptake: K-i=262 mu M). 5 Studies i nto the effect of gabapentin, the system-A-specific ligand 2-(-)-endoa mino-bicycloheptane-2-carboxylic acid (BCH), and the system-A-specific ligand alpha-(methyl-amino)-isobutyric acid (MeAIB), on the initial r ate of uptake of [H-3]-glycine, L-[H-3]-glutamate, L-[H-3]-glutamine, and L-[H-3]-leucine were performed. At 100 mu M, gabapentin significan tly inhibited initial rate of uptake of [H-3]-glycine (29%), L-[H-3]-g lutamate (22%) and L-[H-3]-leucine (40%). 6 Gabapentin is taken up int o synaptosomes by a system similar to system-L, responsible for the up take of large neutral amino acids. Gabapentin will also inhibit the up take of certain excitatory amino acids in this synaptosomal preparatio n. The implications of these findings for the mechanism of action for gabapentin are unclear. The data presented here may suggest an intrace llular site for mechanism of action for this compound. Similarly chang es in levels of amino acid pools may be involved in the mechanism of g abapentin's anticonvulsant action.