Effects of Anti‐Glutamate‐Binding Protein Antibodies on Synaptic Membrane Ion Flux, Glutamate Transport and Release, and l‐Glutamate Binding Activities

Antibodies (Abs) raised against the l‐glutamate‐binding protein (GBP) purified from bovine brain were used to define the possible physiologic activity of GBP in synaptic membranes. Three processes were examined for their sensitivity to the Abs: the excitatory amino acid stimulation of thiocyanate (SCN⁻) flux, the transport of l‐glutamic acid across the synaptic membrane, and the depolarization‐induced release of l‐glutamate. Only the amino acid‐induced changes in ion flux were inhibited by the anti‐GBP Abs. The change in membrane potential produced by exposure of synaptic membranes to excitatory amino acids was measured as the increase in the uptake of the lipophilic anion SCN⁻. The l‐glutamate‐induced SCN⁻ influx was 40 times more sensitive to inhibition by the anti‐GBP Abs than the stimulation of ion flux by kainate, and 60 times more sensitive than that produced by quisqualate. The anti‐GBP Abs did not inhibit the activation of ion flux produced by N‐methyl‐d‐aspartate. The inhibition of glutamate‐stimulated ion fluxes by the Abs was complete, whereas the inhibition of l‐glutamate binding to either the rat or bovine brain GBP was not. The results obtained indicated that although the majority of the anti‐GBP Abs were not directed against the glutamate recognition site of the GBP and of presumed synaptic membrane receptors, they were effective in blocking the activation of receptor‐associated ion channels. Thus, the GBP may be considered a component of some excitatory amino acid receptor complexes.