Neomycin Is an Agonist at a Polyamine Site on the N‐Methyl‐D‐Aspartate Receptor

: Neomycin appears as a full agonist and spermidine as a partial agonist at the site where polyamines enhance 1‐[1‐(2‐thienyl)cyclohexyl][3H]piperidine ([3H]TCP) binding on the N‐methyl‐D‐aspartate (NMDA) receptor. Other aminoglycosides also enhance [3H]TCP binding with efficacies roughly proportional to the number of primary amine groups. The polyamine antagonists ifenprodil and arcaine inhibit enhancement of [3H]TCP binding by spermidine or neomycin. The inhibition of [3H]TCP binding by arcaine is apparently competitively reduced by neomycin and spermidine, supporting a common site. Diethylenetriamine (previously described as a polyamine antagonist) may be a partial agonist. Enhancement by neomycin or spermidine is not additive to that of Mg2+, consistent with competition of Mg2+ and spermidine or neomycin at the site where these compounds enhance [3H]TCP binding. Polyamines also enhance the binding of the competitive antagonist 2 ‐ (2 ‐ carboxypiperazin ‐ 4 ‐ yl)[3H]propyl ‐ 1 ‐ phosphonic acid ([3H]CPP). Neomycin, which does not enhance [3H]CPP binding, inhibits the enhancement by spermidine. That this site is distinct from the site where spermidine and neomycin increase [3H]TCP binding is supported by different pharmacology. Arcaine and diethylenetriamine do not inhibit spermidine enhancement of [3H]CPP binding. Mg2+ also does not compete with the spermidine enhancement of [3H]CPP binding. Ifenprodil inhibits the spermidine enhancement of [3H]CPP binding. The data suggest two or more polyamine sites, with arcaine selective for the site that enhances [3H]TCP binding. Neomycin is an agonist at one polyamine site and an antagonist to the second.