PERMEANT ION AND SUBUNIT DEPENDENCE OF EXTERNAL Mg2+ BLOCK OF NMDA RECEPTORS

摘要

N-methyl-D-aspartate (NMDA) receptors are broadly involved in the CNS physiological and pathological processes. The voltage-dependent block by external Mg2+ is a signature characteristic of the NMDA receptors and is partly responsible for the many important roles NMDA receptors play. The work included in this Dissertation was designed to advance our understanding of the mechanism of Mg2+ block of NMDA receptors by exploring the permeant ion and subunit dependence of this process. Whole-cell and outside-out patch recordings from primary cultures of rat cortical neurons or heterologous mammalian cell lines were performed in combination with kinetic modeling. I report that Mg2+ inhibition of whole-cell NMDA currents in cortical neurons, which express NMDA receptors with NR2A or NR2B NR2 subunits, is very sensitive to ionic conditions. This phenomenon can be explained by a kinetic model which incorporates external permeant ion binding sites within the pore. Permeant ions binding to these sites prevents Mg2+ blocking or unblocking the channel. The general mechanisms of Mg2+ channel block of NR1/2D receptors is fundamentally similar to that of cortical receptors. However, Mg2+ block of NR1/2D receptors is much weaker than cortical receptors, mostly due to faster Mg2+ unblocking. Permeant ions also greatly affect Mg2+ block of NR1/2D receptors. The results can be explained by a kinetic model that incorporates two external and one internal permeant ion binding sites in the channel of NMDA receptors. When these sites are occupied by permeant ions, Mg2+ blocking or unblocking is affected. Thus, the research included in this Dissertation has deepened our understanding of the mechanism of Mg2+ block . The work also provides insights into NMDA receptor structure and gating.