BLOCKADE BY POLYAMINE NMDA ANTAGONISTS RELATED TO IFENPRODIL OF NMDA-INDUCED SYNTHESIS OF CYCLIC-GMP, INCREASES IN CALCIUM AND CYTOTOXICITY IN CULTURED NEURONS

1 Antagonists acting at the polyamine site of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor, including a number of heterocyclic aminoalcohols related to ifenprodil, were investigated to establish their functional interaction at the NMDA receptor and their neuroprotective profile. 2 In murine cultured neocortical neurones, NMDA (100 mu M)-stimulated production of guanosine 3':5'-cyclic monophosphate (cyclic GMP) was blocked by N-1([thienyl]-cyclohexyl)-piperidine (1 mu M) and by the nitric oxide (NO) synthase inhibitor N-G-nitro-L-arginine (100 mu M). Ifenprodil and structurally related heterocyclic aminoalcohols inhibited in a concentration-dependent manner the NMDA-stimulated, NO-dependent production of cyclic GMP; rank potency order was: ifenprodil > 2309 BT > tibalosine > threo-tibalosine > 840S. 3 All of the polyamine NMDA antagonists blocked NMDA (300 mu M)-stimulated increases in intracellular calcium concentrations as measured by changes in the fluorescence of pre-loaded fluo-3-acetoxy methyl ester. Rank potency order was: ifenprodil > 2309 BT > 840S > tibalosine > threotibalosine. 4 In a series of experiments to evaluate the effectiveness of the polyamine NMDA antagonists as blockers of NMDA-induced cytotoxicity, all of the drugs were found to inhibit the leakage of lactate dehydrogenase after the exposure of the murine neocortical cultures to NMDA (100 mu M, 5 h). Rank potency order was: 2309 BT > ifenprodil > tibalosine > threo-tibalosine > 840S. 5 These results provide direct evidence that polyamine NMDA antagonists produce a functional blockade of the NMDA receptor complex. The heterocyclic aminoalcohols described herein, like ifenprodil, block NMDA-mediated elevation of intracellular NO and calcium, two key events in the excitotoxic cascade, and are cytoprotective.