Ca2+-Dependent Block by Mematine and Selective Inhibition of Overactive NMDA Receptors

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PROJECT SUMMARY/ABSTRACT The research proposed here addresses drugs that inhibit N-methyl-D-aspartate receptors (NMDARs), which are 4-subunit ionotropic glutamate receptors found at most vertebrate excitatory synapses. NMDARs are involved in a remarkable range of both nervous system physiology and nervous system disorders. Ca2+ influx through NMDARs is a signal of central importance to synaptic plasticity throughout the brain. Excessive NMDAR- mediated Ca2+ influx, however, has been linked to many nervous system disorders, including Alzheimer's disease and other neurodegenerative diseases, stroke, and traumatic brain injury. It therefore would appear that NMDAR inhibitors should have wide therapeutic potential. However, most NMDAR inhibitors have been unsuccessful in clinical trials, probably because widespread inhibition of NMDARs has multiple unacceptable side effects. Memantine, however, is an NMDAR channel blocking antagonist that is one of the few drugs approved for treatment of Alzheimer's disease. The reasons why memantine is both effective and unusually well- tolerated remain under debate. An explanation is suggested by the recent observation that memantine acts to stabilizes a Ca2+-dependent desensitized state of NMDARs while blocking the NMDAR channel. As a result, memantine preferentially inhibits NMDARs that are exposed to high intracellular Ca2+ concentrations, which are the NMDARs most likely to mediate pathological Ca2+ influx. Thus, designing drugs that, like memantine, inhibit NMDARs more effectively as intracellular Ca2+ rises offers a promising new strategy for developing especially effective therapeutic agents. The goals of the proposed research are to deepen understanding of interactions between memantine and NMDARs, including of NMDARs composed of three different types of subunits, which are widely expressed by challenging to study. Binding sites on NMDARs for memantine and other channel blockers will be identified and distinguished using an advanced combination of computational chemical modeling and physiological study of wild-type and mutant NMDARs. Guided by computational models, new compounds designed to interact with NMDARs in a strongly Ca2+-dependent manner will be synthesized and used to deepen understanding of channel blocker-NMDAR interactions. The dependence on intracellular Ca2+ of inhibition by memantine and other channel blockers will be examined using neuronal preparations, and the Ca2+ dependence of their neuroprotective properties evaluated. New channel blockers with enhanced dependence on intracellular Ca2+ will serve as lead compounds for future development of more effective treatments for Alzheimer's disease and related neurodegenerative diseases.