Cl-/Ca2+-dependent L-glutamate binding sites do not correspond to 2-amino-4-phosphonobutanoate-sensitive excitatory amino acid receptors.

A series of phosphono and phosphino analogues of glutamate were used to compare the pharmacological properties of (a) Cl-/Ca2+-dependent, 2-amino-4-phosphonobutanoate (AP4)-sensitive L-[3H]-glutamate binding sites in rat brain synaptic plasma membranes (SPMs) and (b) AP4-sensitive excitatory synaptic responses by use of electrophysiological techniques. In the presence of Cl- and Ca2+, L-[3H]-glutamate bound to SPMs with Kd 804 nM and Bmax 53 pmol mg-1 protein. The AP4-sensitive (Ki 7.3 microM) population of binding sites represented 61% of L-glutamate specifically bound. omega-Substituted analogues of AP4 were potent inhibitors of L-[3H]-glutamate binding (Ki values 2.4-38 microM), whereas N-substituted compounds or propionic acid derivatives were inactive. Experiments with AP4 alone and in combination with other analogues demonstrated that the primary target of all substances was the AP4-sensitive population of L-glutamate binding sites. In the hippocampal slice in vitro, AP4 antagonized lateral perforant path-evoked field potentials with an IC50 of 2.7 microM. In contrast to their actions at AP4-sensitive L-glutamate binding sites, all other compounds (except for the omega-carboxymethylphosphino analogue, IC50 19 microM) were weak or inactive as antagonists of this synaptic response (IC50 values greater than 100 microM). Inactive compounds which exhibited activity in the binding assay did not reverse the synaptic depressant effects of AP4, indicating that they were neither agonists nor antagonists at AP4-sensitive synapses. 4 The lack of correspondence between (a) the Cl- /Ca2 -dependent, AP4-sensitive population of L- [3H]-glutamate binding sites and (b) AP4-sensitive synaptic responses indicates that these binding sites are not the receptors through which AP4 exerts its neuropharmacological effects. The possibility that Cl- /Ca2+-dependent 'binding sites' represent transport into resealed SPM vesicles is discussed. 5 Electrophysiological data demonstrate that AP4-sensitive synaptic receptors display a high degree of ligand selectivity. High antagonist potency is shown only by glutamate analogues with unmodified alpha-amino and alpha-carboxyl groups, and with a bifunctional (dianionic) omega-terminal.