Characterization of the binding of dl‐[3H]‐2‐amino‐4‐phosphonobutyrate to l‐glutamate‐sensitive sites on rat brain synaptic membranes

Abstract

1 The binding of dl-[3,4-³H] 2-amino-4-phosphonobutyric acid dl[³H]-APB to rat whole brain synaptic membranes was investigated. 2 Binding was linear with membrane protein concentration, and optimal at physiological pH and temperature. The association rate was rapid, achieving equilibrium within 10 min. Prolonged incubations (> 20 min) revealed additional sites, which apparently possessed identical binding characteristics to those detected with 10 min incubations. 3 Binding of dl-[³H]-APB was enriched in synaptic membrane fractions, and assessment of the regional distribution, indicated greatest binding in those areas with a rich glutamatergic innervation. 4 The binding of dl-[³H]-APB in HEPES-KOH buffer exhibited an absolute requirement for Cl⁻. The addition of Ca²⁺ resulted in a further enhancement of binding. 5 Saturation analysis revealed the presence of specific glutamate-sensitive dl-[³H]-APB binding sites, with a K_D = 1.26 μm and B_max = 12.08 pmol mg⁻¹ protein. A Hill plot revealed a slope slightly greater than unity, which could possibly be a reflection of a contribution to binding of a further site which is relatively insensitive to glutamate. Analysis of 60 min incubation data indicated an approximately 3 fold increase in the capacity of the system, but a relatively unchanged K_D. 6 Examination of the pharmacological specificity of binding, showed that for both agonist and antagonist molecules, the l-enantiomers were invariably more active than the d-forms. For example, the l-(+)-2-amino-4-phosphonobutyrate isomer was 15 times more active than the d-(−)-form in inhibiting the binding of dl-[³H]-APB. This is in close agreement with the ability of these compounds to produce depression of synaptic transmission. The most potent inhibitor of binding was quisqualate. It is suggested that APB may interact with a quisqualate-preferring class of excitatory amino acid receptors, possibly localised predominantly on presynaptic terminals.