NMDA Mg2+ + Ca2+ Flashcards
Briefly outline NMDAR activation under physiological conditions
Opening of the NMDA receptor ion channel requires concurrent binding of glycine and L-glutamate, and relief of magnesium block at the ion channel pore by membrane depolarization. The resulting calcium flux triggers a cascade of signal transduction necessary for synaptic plasticity.
What is the importance of steep voltage dependence of Mg2+ block?
Underlies the crucial role that it plays in imparting NMDA receptors with the property of ‘coincidence detectors’ in the nervous system
What does selectivity for Mg2+ and Ca2+ depend on and who showed this?
Asparagine residue located in the apex of the M2 segment of the GluN1 subunit and the second of a pair of asparagines in the GluN2 subunit (Wollmuth et al., 1998).
What is the significance of asparagine placement in GluN subunits and who showed this?
Asymmetry provides channel traversing of Ca2+, but not passage of the smaller Mg2+ ions (Wollmuth & Sobolevsky, 2004)
Why was there controversy over asparagine residue placement in the channel? (mention researchers)
Structural predictions suggest this asparagine residue should be located about halfway through the channel (Wollmuth et al., 1998), but the steep voltage-dependence of Mg2+ block infers that the Mg2+ ion senses about 90% of the membrane electric field at its binding site.
How was the issue over asparagine residue resolved
Discovery that occupancy of permeant ion binding sites enhances voltage-dependence of Mg2+ block, placing Mg2+ ion binding site at a much shallower position (∼50 per cent) in the membrane electric field
What residue is important in defining Mg2+ block affinity? who and why?
A serine(S)/leucine(L) residue was identified to be crucial in defining the Mg2+ block affinity of NMDAR subtypes (Siegler Retchless et al., 2012) through interacting with a tryptophan residue in GluN1
What are the other functions of S/L in GluN1 besides Mg2+ block?
Also influences differing Ca2+ permeability, single channel conductance and agonist potency of NMDAR subtypes.
How is Ca2+ permeability affected by subunit?
Ca2+ permeability is higher in GluN2A/GluN2B containing NMDARs, and lower in those that do not.
How was NMDAR conductance first studied?
recording single-channel currents from outside-out patches of Glutamate-stimulated Xenopus oocytes expressing different combinations of recombinant NMDARs consisting of GluN1/GluN2A, GluN1/GluN2B and GluN2/GluN2C
What are the conductances of GluN1/GluN2A and GluN1/GluN2B combinations?
main conductance level of about 50 pS, with subconductance levels no lower than 38 pS (Colquhoun, 1992)
What are the conductances of GluN2C combinations?
Main conductance level of 36 pS, with a subconductance level around 19 pS (Stern et al., 1992)
What are the conductances of GluN2D combinations?
GluN1a/GluN2D were similar to GluN2C containing receptors, with a main level conductance of 35 pS, and sublevel conductance of 17pS (Wyllie et al., 1996).
How does Mg2+ block vary between different subunit containing NMDAR’s - and what experiments was used to show this?
Currents recorded from Xenopus oocytes expressing wild-type NMDAR channels in the presence of differing concentrations of extracellular Mg2+ and at different voltages showed that channels containing GluN2A and GluN2B subunits were blocked more strongly by Mg2+ than those with GluN2C or GluN2D subunits (Kuner and Schoepfer, 1996)
How does the concentration of extracellular Mg2+ and voltage-dependence required vary between subtypes?
Less extracellular Mg2+ is required for effective block of current through GluN2A/GluN2B containing receptors, and does so at lower voltages, compared to those containing GluN2C/GluN2D
