Diltiazem and MK-801 but not APV Act Synergistically to Protect Rat Hippocampal Slices against Hypoxic Damage

Abstract

The noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 {(+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine maleate, dizocilpine} potently protects hippocampal slices against hypoxic neuronal damage, while the competitive NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) has a much weaker neuroprotective potency.1,2 In addition, MK-801 protects hippocampal slices against the combined effects of hypoxia + NMDA, and hypoxia + kainate (KA) or α-amino-3-hydroxy-5-methyl-4-isoxazolpropionate (AMPA). Furthermore, MK-801 was found to be a better antagonist of KA (or AMPA)-enhanced hypoxic neuronal damage than the specific KA/AMPA antagonist GYKI 52466 [1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine HC1]. The combination of APV and 7-chlorokynurenate could not abolish the protective effect of MK-801 against hypoxia, a effect one should expect if MK-801 exerts its protective action via an open NMDA receptor.3 Thus, at least in the continuously-perfused hippocampal slice preparation, where hypoxiainduced glutamate accumulation cannot reach excitotoxic levels, the antihypoxic effect of MK-801 does not appear to be mediated through the NMDA receptor. Several other recent studies also raise questions in regard to MK-801 mode of action. One study4 concluded that ketamine, an anesthetic known to compete for MK-801 binding sites,5 is an inhibitor of L-type calcium channels, while another study6 points to both similarities and differences between these two compounds depending on the method of administration of ketamine. Yet, another study7 demonstrated the ability of diltiazem (DILT), an L-type calcium channel blocker,8–10 to inhibit [3H]MK-801 binding to hippocampal synaptic membranes at IC50 value which is very similar to the IC50 of unlabelled MK-801. These studies, along with our own results that showed DILT to provide a significant protection against hypoxia-induced neuronal damage,1 led us to suggest that MK-801 may possess L-type calcium channel blocking properties.