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 ischemia in the rat. However, their mechanism of action is still unclear. We have studied the neuroprotective effects of imidazolines against NMDA-induced neuronal death and hypoxic insult in cerebellar and striatal neuronal cultures. All of the imidazolines tested decreased the NMDA-mediated neurotoxicity in a non-competitive manner. Antazoline was the most effective (IC(50) of 5 microM, maximal neuroprotection reaching 90% at 100 microM). The neuroprotective effects were still present when the imidazolines were applied during the post-insult period. Antazoline, idazoxan and guanabenz also showed neuroprotective effects against hypoxia-induced neuronal death (neuroprotection reaching 95% for antazoline at 100 microM). Antazoline was still active if applied during the reoxygenation period (15% neuroprotection). To determine the mechanism of the neuroprotective effects, the possible interaction of imidazolines with NMDA receptors was studied. Imidazolines dose-dependently and non-competitively inhibited NMDA currents. As found for the neuroprotective effects, antazoline was the most effective imidazoline, with an IC(50) of 4 microM and a maximal inhibition of 90% at 100 microM. This blockade was rapid, reversible and voltage-dependent. We compared these effects to those of the classical non-competitive antagonist of NMDA channels, MK-801. In contrast to imidazolines, blockade of the NMDA current by MK-801 was voltage-independent and reversible only at positive potentials. When co-applied with MK-801, antazoline prevented the long lasting blockade of the NMDA current by MK-801. These results are consistent with the existence of overlapping binding sites for these drugs on the NMDA receptor channel. They indicate that imidazolines exert a strong neuroprotective effect against excitotoxicity and hypoxia in cerebellar and striatal primary neuronal cultures by inhibiting NMDA receptors. Since these effects were non-competitive, imidazolines appear to be interesting new drugs with therapeutic potential.