Blockade by polyamine NMDA antagonists related to ifenprodil of NMDA-induced synthesis of cyclic GMP, increases in calcium and cytotoxicity in cultured neurones.

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 microM)-stimulated production of guanosine 3':5'-cyclic monophosphate (cyclic GMP) was blocked by N-1([thienyl]-cyclohexyl)-piperidine (1 microM) and by the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (100 microM). 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 microM)-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 > threo-tibalosine. 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 microM, 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 amino alcohols described herein, likeifenprodil, block NMDA-mediated elevation of intracellular NO and calcium, two key events in the excitotoxic cascade, and are cytoprotective.