[H-3] GABAPENTIN MAY LABEL A SYSTEM-L-LIKE NEUTRAL AMINO-ACID CARRIERIN BRAIN

The ability of large neutral amino acids to interact with a site in mo use and pig brain labelled by [H-3]gabapentin was examined. As previou sly described for rat tissue, [H-3]gabapentin bound to synaptic plasma membranes prepared from mouse or pig cerebral cortex with high affini ty (Kinetically derived K(D) = 14 and 17 nM for mouse and pig, respect ively). Equilibrium binding in each species was inhibited by gabapenti n 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-methion ine (IC50 = 50 nM) were the most potent of those tested. Binding was a lso inhibited by L-phenylalanine (IC50 = 380 nM), L-valine (IC50 = 310 nM) and the selective system-L substrate 2-amino-2-carboxy-bicyclohep tane (IC50 = 420 nM) but not by the sodium-dependent System-A substrat e methylaminoisobutyric acid. The presence of a submaximal concentrati on of leucine reduced [H-3]gabapentin binding affinity but did not aff ect the maximum number of binding sites, suggesting a competitive inte raction between leucine and the binding protein. The results suggest [ H-3]gabapentin may label a site in brain that resembles the large neut ral amino acid transporter described in other tissues.