INTERACTIONS BETWEEN IFENPRODIL AND THE NR2B SUBUNIT OF THE N-METHYL-D-ASPARTATE RECEPTOR

Ifenprodil is an atypical noncompetitive modulator of the N-methyl-D-a spartate (NMDA) receptor (NR) which demonstrates a 140-fold preference for NR2B over NR2A subunits, although the molecular basis for this su bunit specificity is unknown. We have made chimeric receptors by fusin g the murine forms of NR2A (epsilon(1)) and NR2B (epsilon(2)) to local ize the high affinity determinants of ifenprodil inhibition on the 2B subunit. Binding experiments with I-125-MK-801 implicated the region b etween amino acids 198 and 356 of NR2B for high affinity ifenprodil in teraction. Site-directed mutants at Arg-337 showed that this residue i s absolutely required for high affinity ifenprodil inhibition. Polyami nes also modulate the NMDA receptor with a preference for NR2B subunit s, and the pharmacology of these agents overlaps with ifenprodil. Alth ough the determinants of the polyamine enhancement of iodo-MK-801 bind ing also localize to the NH2 terminus of NR2B, the point mutants at Ar g-337 form receptors that are polyamine-stimulated at wild type levels . In addition, polyamine stimulation depends on the expression of NR1 splice variants, whereas high affinity ifenprodil inhibition is indepe ndent of NR1 isoform expression. These studies provide evidence that i fenprodil and polyamines interact at discrete sites on the NR2B subuni t.