Evidence that [3H]glutamate binding sites are masked by biologically relevant endogenous factor on cell membranes of frog spinal cord.

We identified the possible endogenous factor effective to modulate the binding of 3H]-labeled excitatory amino acid agonists and antagonists in the 100,000 x g supernatant of Triton X-100 (0.01%)-treated cell membranes from frog spinal cords. The factor inhibited the binding of 3H]glutamate to Triton X-100-treated cell membranes, to which the binding capacity of 3H]glutamate increased much more than that to intact cell membranes. The binding capacities of 3H]AMPA (an AMPA type agonist) and 3H]CPP (an NMDA type antagonist) to cell membranes remained low by Triton treatment, but they were enhanced significantly by the addition of the factor. The effect of the factor on the 3H]kainate binding was hardly observable. The factor may provide key information on receptor structures and the classification of receptor types concerning excitatory amino acids in the mammalian central nervous system.