ZINC-DEPENDENT APPARENT DESENSITIZATION NMDA RECEPTORS

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Glutamate is the most widely used neurotransmitter in the vertebrate central nervous system. NMDA receptors, a subclass of receptors for glutamate, play a critical role in multiple forms of synaptic plasticity that lay the foundation for learning and memory. NMDA receptors are also implicated in a variety of pathological processes of the brain, including stroke-induced cell death, epilepsy, and degenerative neurological diseases. Thus, detailed knowledge about regulation of NMDA receptor function not only is important for our understanding of synaptic plasticity, but also has broad clinical implications. In the continuous presence of glutamate, NMDA receptors (in particular NR1/NR2A receptors) undergo a process called desensitization that is thought to be an important regulatory mechanism for their function. Preliminary data from this laboratory suggest that ambient zinc, in addition to acting at extracellular high-affinity site to cause voltage-independent inhibition, contributes to apparent desensitization of NR1/NR2A receptors. The goal of this study is to investigate the zinc- dependent desensitization systematically in artificial expression systems and neurons, so we could better understand the underlying mechanism and physiological implication of this phenomenon. One possible mechanism for zinc-dependent desensitization is a positive allosteric interaction between the glutamate-binding site and the extracellular zinc site. This hypothesis will be tested with several biophysical experiments, using recombinant NR1/NR2A receptors expressed in HEK 293 cells. It has also been postulated by us that the zinc-dependent desensitization and glycine-independent desensitization are mediated by distinct molecular determinants. A series of chimeric receptors and point mutants will be used to test this hypothesis. Finally, the effects of zinc on desensitization of native NMDA receptors in cultured neurons will also be investigated. The proposed study may delineate a new mechanism in the regulation of NMDA receptor function, and provide new insight on the functional roles of zinc in the brain. Furthermore, it may provide useful information for the development of new therapeutic agents for stroke, epilepsy and other neurological diseases.