[125I]Ifenprodil: a convenient radioligand for binding and autoradiographic studies of the polyamine-sensitive site of the NMDA receptor.

Iodination of ifenprodil, a non-competitive NMDA antagonist, with Na125I/Chloramin-T gave a radioligand which bound rapidly and saturably to a single population of sites (dissociation constant 145 nM) in membranes of rat cerebral cortex. In competition studies, specific binding of [125I]-ifenprodil was inhibited by analogues of ifenprodil, as well as by spermine and spermidine. Binding was sensitive to Ca2+, Mg2+ and Zn2+. [125I]-Ifenprodil labelled a population of binding sites, which was topographically distributed in rat forebrain, as shown by autoradiography. [125I]Ifenprodil is a useful radioligand for the investigation of the polyamine site of the N-methyl-D-aspartate (NMDA) receptor-complex.