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Comparison of the uptake of [3H]‐gabapentin with the uptake of L‐[3H]‐leucine into rat brain synaptosomes
Author(s) -
Thurlow Richard J.,
Hill David R.,
Woodruff G.N.
Publication year - 1996
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1996.tb15424.x
Subject(s) - gabapentin , leucine , chemistry , ligand (biochemistry) , stereochemistry , amino acid , biochemistry , receptor , medicine , alternative medicine , pathology
1 Gabapentin is a novel anticonvulsant with an unknown mechanism of action. Homogenate binding studies described elsewhere have suggested that [ 3 H]‐gabapentin binds to a site in brain similar to the large neutral amino acid (LNAA) uptake site, termed system‐L. 2 This study describes an investigation into the uptake of [ 3 H]‐gabapentin into a crude synaptosomal preparation from cerebral cortex of rat brain. Characterization studies showed that [ 3 H]‐gabapentin is taken up into synaptosomes by a system that is similar to that responsible for the uptake of L‐[ 3 H]‐leucine. This system is sodium‐independent, temperature‐sensitive and requires ATP for function. 3 Kinetic studies of [ 3 H]‐gabapentin uptake produced a Michaelis constant ( K M = 160 μ m ) similar to that observed for L‐[ 3 H]‐leucine ( K M = 110.3 μ m ). V max values were 837.1 pmol mg −1 protein min −1 and 2.192 nmol mg −1 protein min −1 respectively. 4 Gabapentin and L‐leucine mutually inhibit their uptake. Lineweaver‐Burke plots of these data demonstrate that inhibition occurs by a competitive mechanism. Further to this the Dixon transformation of the data illustrates that these two substrates share a common uptake site by the similarity between their calculated K i and K M values (gabapentin inhibition of L‐[ 3 H]‐leucine uptake: K i = 160 μ m ; L‐leucine inhibition of [ 3 H]‐gabapentin uptake: K i = 262 μ m ). 5 Studies into the effect of gabapentin, the system‐L‐specific ligand 2‐(—)‐endoamino‐bicycloheptane‐2‐carboxylic acid (BCH), and the system‐A‐specific ligand α‐(methyl‐amino)‐isobutyric acid (MeAIB), on the initial rate of uptake of [ 3 H]‐glycine, L‐[ 3 H]‐glutamate, L‐[ 3 H]‐glutamine, and L‐[ 3 H]‐leucine were performed. At 100 μ m , gabapentin significantly inhibited initial rate of uptake of [ 3 H]‐glycine (29%), L‐[ 3 H]‐glutamate (22%) and L‐[ 3 H]‐leucine (40%). 6 Gabapentin is taken up into synaptosomes by a system similar to system‐L, responsible for the uptake of large neutral amino acids. Gabapentin will also inhibit the uptake of certain excitatory amino acids in this synaptosomal preparation. The implications of these findings for the mechanism of action for gabapentin are unclear. The data presented here may suggest an intracellular site for mechanism of action for this compound. Similarly changes in levels of amino acid pools may be involved in the mechanism of gabapentin's anticonvulsant action.