SUBTYPE-SELECTIVE INHIBITION OF N-METHYL-D-ASPARTATE RECEPTORS BY HALOPERIDOL

Previous studies indicate that haloperidol, a therapeutically useful a ntipsychotic drug, inhibits neuronal N-methyl-D-aspartate (NMDA) respo nses and has neuroprotective effects against NMDA-induced brain injury . To further characterize this inhibition, we used electrical recordin gs to assay the effects of haloperidol on four diheteromeric subunit c ombinations of cloned rat NMDA receptors expressed in Xenopus laevis o ocytes: NR1A coexpressed with NR2A, NR2B, NR2C, or NR2D. Haloperidol s electively blocks NR1A/2B subunit combinations (IC50 = similar to 3 mu M; maximum inhibition, similar to 85%), whereas the other subunit com binations are greater than or equal to 100-fold less sensitive (IC50 = >300 mu M). Inhibition of NR1A/2B receptors is insurmountable with re spect to glutamate and glycine and does not exhibit voltage dependence . The splice variant combinations NR1B/2B and NR1e/2B are also blocked by haloperidol. In oocytes from some frogs, 30-100 mu M haloperidol i nduces potentiation of NR1A/2A receptor responses. NMDA responses in E 16-17 rat cortical neurons cultured for less than or equal to 10 days are inhibited by haloperidol at the same potency and to the extent as NR1/2B receptors (IC50 = similar to 2 mu M; maximum inhibition, simila r to 80%). In contrast, cells cultured for longer periods show a wide range of sensitivity. This change in pharmacology coincides with a dev elopmental switch in subunit expression; from NR1 expressed with NR2B to NR1 coexpressed with NR2A and NR2B. Inhibition of macroscopic neuro nal NMDA responses is mechanistically similar to inhibition of NR1A/2B receptors. Single-channel recordings from neurons show that antagonis m is associated with a decrease in the frequency of channel openings a nd a shortening of mean channel open time. Collectively, our experimen ts indicate that haloperidol selectively inhibits NMDA receptors compr ised of NR1 and NR2B subunits. Inhibition is consistent with action at a noncompetitive allosteric Site that is distinct from the glutamate- , glycine-, and phencyclidine-binding sites and is probably mechanisti cally related to the atypical antagonist ifenprodil. Our results sugge st that haloperidol can be used as a tool for investigating NMDA recep tor subunit composition and can serve as a structural lead for designi ng novel subtype-selective NMDA receptor ligands.