Imidazoline-induced neuroprotective effects result from blockade of NMDA receptor channels in neuronal cultures

Imidazolines have been shown to be neuroprotective in focal and global isch emia in the rat. However, their mechanism of action is still unclear. We ha ve studied the neuroprotective effects of imidazolines against NMDA-induced neuronal death and hypoxic insult in cerebellar and striatal neuronal cult ures. All of the imidazolines tested decreased the NMDA-mediated neurotoxic ity in a non-competitive manner. Antazoline was the most effective (IC50 of 5 mu M, maximal neuroprotection reaching 90% at 100 mu M). The neuroprotec tive effects were still present when the imidazolines were applied during t he post-insult period. Antazoline, idazoxan and guanabenz also showed neuro protective effects against hypoxia-induced neuronal death (neuroprotection reaching 95% for antazoline at 100 mu M). Antazoline was still active if ap plied during the reoxygenation period (15% neuroprotection). To determine t he mechanism of the neuroprotective effects, the possible interaction of im idazolines with NMDA receptors was studied. Imidazolines dose-dependently a nd non-competitively inhibited NMDA currents. As found for the neuroprotect ive effects, antazoline was the most effective imidazoline, with an IC50 of 4 mu M and a maximal inhibition of 90% at 100 mu M. This blockade was rapi d, reversible and voltage-dependent. We compared these effects to those of the classical non-competitive antagonist of NMDA channels, MK-801. In contr ast to imidazolines, blockade of the NMDA current by MK-801 was voltage-ind ependent and reversible only at positive potentials. When co-applied with M K-801, antazoline prevented the long lasting blockade of the NMDA current b y MK-801. These results are consistent with the existence of overlapping bi nding sites for these drugs on the NMDA receptor channel. They indicate tha t imidazolines exert a strong neuroprotective effect against excitotoxicity and hypoxia in cerebellar and striatal primary neuronal cultures by inhibi ting NMDA receptors. Since these effects were non-competitive, imidazolines appear to he interesting new drugs with therapeutic potential. (C) 2000 El sevier Science Ltd. All rights reserved.