Radioligand binding studies reveal agmatine is a more selective antagonist for a polyamine-site on the NMDA receptor than arcaine or ifenprodil

Ifenprodil, arcaine and agmatine have all been reported to inhibit the NMDA receptor by actions at polyamine-sites, however the specific sites with which these compounds interact is unknown. Here we used radioligand binding of [³H]MK-801 to a membrane preparation from rat cerebral cortex to investigate the interactions of these compounds with the NMDA receptor complex. In the absence of exogenous polyamines, agmatine reduced [³H]MK-801 binding only at concentrations over 500 μM, as opposed to the putative polyamine-site antagonists arcaine and ifenprodil which directly reduce ligand binding at much lower concentrations (5 μM) in the absence of polyamines. In our studies, all three compounds significantly reduced spermidine-potentiated [³H]MK-801 binding, however agmatine was the only compound effective at concentrations below those that produced direct inhibition of [³H]MK-801 binding. Under these conditions, agmatine had a K_i=14.8 μM for spermidine-potentiated [³H]MK-801 binding and displayed characteristics of a competitive antagonist. Agmatine, as well as ifenprodil and arcaine, also displaced [³H]spermidine from rat cortical membranes at concentrations similar to those that were effective at reducing spermidine-potentiated [³H]MK-801 binding. In conclusion, these data suggest that agmatine reduces the potentiating effects of polyamines by competitive antagonism at a specific site on the NMDA receptor complex, and that these actions of agmatine differ from those of ifenprodil and arcaine.