Role NR1/2C and NR1/2D NMDA Receptors in Cortex Function and Memantine Action

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DESCRIPTION (provided by applicant): Most excitatory synaptic excitation in the brain is mediated by glutamate receptors. NMDA receptors (NMDARs), a glutamate receptor subtype specifically activated by N-methyl-D-aspartate, are expressed on almost all mammalian central neurons. NMDARs exhibit high Ca2+ permeability and voltage-dependent channel block by Mg2+, characteristics that allow them to play central roles in synaptic plasticity and memory. NMDARs also are broadly involved in nervous system dysfunction, and have been implicated in many nervous system diseases including Alzheimer's disease (AD), Huntington's disease, schizophrenia, epilepsy, and depression. NMDARs are usually composed of two types of subunits, NR1 and NR2; there are four NR2 subunits encoded by separate genes (NR2A-NR2D), which, when combined with NR1, define four major NMDAR subtypes (NR1/2A ¿ NR1/2D). The function of the NR1/2A and NR1/2B NMDAR subtypes, which are heavily expressed in adult cortex, have been extensively investigated. The function of the NR1/2C and NR1/2D NMDAR subtypes, which also are expressed in adult cortex (especially NR1/2D), although at lower levels than the other NMDAR subtypes, are less well understood. Recent data suggest that the NR1/2C and/or NR1/2D NMDARs play an especially important role in the clinical utility of the widely-used AD drug memantine, which is an antagonist of NMDARs. It appears surprising that memantine, a drug that slows cognitive decline in AD patients, would act by inhibiting NMDARs, which are essential for memory. The paradoxical therapeutic effects of memantine have been proposed to result from selective inhibition of NR1/2C and NR1/2D receptor subtypes located on cortical interneurons, resulting in cortical disinhibition. The involvement of NR1/2C and/or NR1/2D NMDARs in activation of inhibitory neurons also may be of special significance to animal models of schizophrenia. The broad objectives of this application are to uncover the roles of NR1/2C and NR1/2D NMDARs in the cortex, and to improve understanding of the mechanism of action of memantine. These objectives will be accomplished by determining: the neuronal subtypes in cortex that express NR1/2C and/or NR1/2D NMDARs; their synaptic versus extrasynaptic location; whether the receptors contribute to tonically active glutamate currents; the effects of memantine on NMDAR responses of several neuronal subtypes in cortex; how genetic deletion of the NR2D subunit affects NMDAR responses in neuronal subtypes in cortex; and the influence of NR2D subunit genetic deletion on the behavioral effects of memantine. To achieve these goals we will apply, to both wild-type and mutant rodents, a powerful combination of approaches, including electrophysiological recordings from brain slices, receptor identification with new pharmacological tools, and analysis of animal behavior. The proposed research will provide fundamental information on cortical NMDARs with broad implications for nervous system function and dysfunction, and will help explain the therapeutic mechanism of a widely used AD drug.