[3H]gabapentin may label a system-L-like neutral amino acid carrier in brain

The ability of large neutral amino acids to interact with a site in mouse and pig brain labelled by [3H]gabapentin was examined. As previously described for rat tissue, [3H]gabapentin bound to synaptic plasma membranes prepared from mouse or pig cerebral cortex with high affinity (Kinetically derived KD = 14 and 17 nM for mouse and pig, respectively). Equilibrium binding in each species was inhibited by gabapentin and a range of large neutral amino acids. L-leucine (IC50 = 80 nM), L-isoleucine (IC50 = 72 nM), L-norleucine (IC50 = 40 nM) and L-methionine (IC50 = 50 nM) were the most potent of those tested. Binding was also inhibited by L-phenylalanine (IC50 = 380 nM), L-valine (IC50 = 310 nM) and the selective system-L substrate 2-amino-2-carboxy-bicycloheptane (IC50 = 420 nM) but not by the sodium-dependent System-A substrate methylaminoisobutyric acid. The presence of a submaximal concentration of leucine reduced [3H]gabapentin binding affinity but did not affect the maximum number of binding sites, suggesting a competitive interaction between leucine and the binding protein. The results suggest [3H]gabapentin may label a site in brain that resembles the large neutral amino acid transporter described in other tissues.