The inhibitory effects of spermidine, putrescine and magnesium on [3H]TCP binding in crude synaptosomes and well washed buffy coat membranes were studied. We report that the IC50 values for the drugs tested varied depending on the state of activation of the channel, being lowest when the channel is maximally activated and highest when the channel is least activated in the control buffy coat preparation. Comparison of the inhibitory characteristics of these 3 agents when activated by glutamate, glycine, and low concentrations of spermidine or magnesium strongly suggest that putrescine and magnesium share a common inhibitory mechanism, perhaps mediated by the voltage-dependent channel site for magnesium. Inhibition by spermidine, however, is clearly mediated by a distinct site, non-identical to the phencyclidine receptor. Glutamate appears to uncouple this inhibition in about one-half of the N-methyl-d-aspartate (NMDA) complexes, suggesting the possibility of distinct subpopulations of NMDA-operated ion channels. The potential physiological relevance of these findings is discussed.
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