Reversible inhibition of intracellular calcium influx through NMDA receptors by imidazoline I2 receptor antagonists

Intracellular calcium ([Ca2+]i) influx through N-methyl-d-aspartic acid (NMDA) receptors in cortical neurons is central to NMDA receptor-mediated excitotoxicity. Drugs that uncompetitively modulate NMDA receptor-mediated [Ca2+]i influx are potential leads for development to treat NMDA receptor-mediated neuronal damage since these drugs spare NMDA receptor normal functions. Ligands to α2-adrenoceptors and imidazoline I2 receptors confer neuroprotection possibility through modulating NMDA receptor-mediated [Ca2+]i influx. Here, we investigated the characteristics of several ligands to α2-adrenoceptors and imidazoline I2 receptor, in inhibiting NMDA receptor-mediated [Ca2+]i influx in cultured cortical neurons using a ratiometric calcium imaging technique. In contrast to MK801, which non-reversibly blocks NMDA receptor-mediated [Ca2+]i influx, imidazoline I2 receptor antagonists, Idazoxan, and 2-(2-benzofuranyl)-2-imidazoline (2-BFI)-mediated inhibition of [Ca2+]i influx can be rapidly reversed when removed, in a manner similar to that of memantine, an uncompetitive antagonist to NMDA receptors. Interestingly, ligands to α2-adrenoceptors, including agmatine sulfate and yohimbine, and a ligand to the nicotinic receptor, levamisol, neither inhibited NMDA receptor-mediated [Ca2+]i influx, nor provided neuroprotection against glutamate toxicity, suggesting selective inhibition of NMDA receptor activities. The inhibition of NMDA receptor by Idazoxan and 2-BFI also led to the suppression of NMDA receptor-mediated calpain activity as a result of blocking NMDA receptor activity, rather than through direct inhibition of calpain activity. Collectively, these studies demonstrated that imidazoline I2 receptor antagonists transiently and reversibly block NMDA receptor-mediated [Ca2+]i influx. These compounds are leads for further development as uncompetitive antagonists to NMDA receptor-mediated excitotoxicity.