Involvement of direct inhibition of NMDA receptors in the effects of σ- receptor ligands on glutamate neurotoxicity in vitro

Abstract This study was performed to examine the roles of the N -methyl- d -aspartate (NMDA) receptor/phencyclidine (PCP) channel complex in the protective effects of σ-receptor ligands against glutamate neurotoxicity in cultured cortical neurons derived from fetal rats. A 1-h exposure of cultures to glutamate caused a marked loss of viability, as determined by Trypan blue exclusion. This acute neurotoxicity of glutamate was prevented by NMDA receptor antagonists. Expression of σ 1 receptor mRNA in cortical cultures was confirmed by reverse transcription polymerase chain reaction (RT-PCR). σ Receptor ligands with affinity for NMDA receptor channels including the PCP site, such as (+)- N -allylnormetazocine ((+)-SKF10,047), haloperidol, and R (−)- N -(3-phenyl-1-propyl)-1-phenyl-2-aminopropane ((−)-PPAP), prevented glutamate neurotoxicity in a concentration-dependent manner. In contrast, other σ-receptor ligands without affinity for NMDA receptors, such as carbetapentane and R (+)-3-(3-hydroxyphenyl)- N -propylpiperidine ((+)-3-PPP), did not show neuroprotective effects. Putative endogenous σ receptor ligands such as pregnenolone, progesterone, and dehydroepiandrosterone did not affect glutamate neurotoxicity. The protective effects of (+)-SKF10,047, haloperidol, and (−)-PPAP were not affected by the σ 1 receptor antagonist rimcazole. These results suggested that a direct interaction with NMDA receptors but not with σ receptors plays a crucial role in the neuroprotective effects of σ receptor ligands with affinity for NMDA receptors.