Comparison of [3H]-(2S,4R)-4-methylglutamate and [3H]d-aspartate as ligands for binding and autoradiographic analyses of glutamate transporters

While studies with [3H]d-aspartate ([3H]d-Asp) illustrate specific interactions with excitatory amino acid transporters (EAATs), new insights into the pharmacological characteristics and localization of specific EAAT subtypes depend upon the availability of novel ligands. One such ligand is [3H]-(2S,4R)-4-methylglutamate ([3H]4MG) which labels astrocytic EAATs in homogenate binding studies. This study examined the utility of [3H]4MG for binding and autoradiography in coronal sections of rat brain. Binding of [3H]4MG was optimal in 5 mM HEPES buffer containing 96 mM NaCl, pH 7.5. Specific binding of [3H]4MG exhibited two components, but was to a single site when glutamate receptor (GluR) sites were masked with kainate (KA; 1 μM): t1/2 ∼ 5 min, Kd 250 nM and Bmax 5.4 pmol/mg protein. Pharmacological studies revealed that [3H]4MG, unlike [3H]d-Asp, labeled both EAAT and ionotropic GluR sites. Further studies employed 6-cyano-7-nitroquinoxaline (30 μM) to block GluR sites, but selective EAAT ligands displayed lower potency than expected for binding to transporters relative to drugs possessing mixed transporter/receptor activities. Autoradiography in conjunction with densitometry with [3H]4MG and [3H]d-Asp revealed wide, but discrete distributions in forebrain; significant differences in binding levels were found in hippocampus, nucleus accumbens and cortical sub-areas. Although EAAT1 and EAAT2 components were detectable using 3-methylglutamate and serine-O-sulphate, respectively, the majority of [3H]4MG binding was to KA-related sites. Overall, in tissue sections [3H]4MG proved unsuitable for studying the autoradiographic localization of EAATs apparently due to its inability to selectively discriminate Na+-dependent binding to Glu transporters.