NMDA Biophysical Properties Flashcards
Key biophysical properties and significance
Slow activation and deactivation kinetics
Significance:
- Slow NMDA receptor-mediated synaptic current
- Permeability to calcium
- Voltage-dependent block by magnesium
What do NMDAR’s mediate at excitatory synapses in the nervous system?
Slow component of synaptic transmission
Describe NMDAR activation time course at room temperature (rise + decay)
Rises slowly to peak about 20 ms after initiation and then decays with a time constant of about 100 ms
Is NMDAR slow time course dependent on glutamate diffusion?
No
Kinetic properties of the receptor
How do we know that slow time course is dependent on kinetic properties rather than glutamate diffusion?
Concentration-jump experiments
Lester et al., 1990
What cell cultures did Lester use and what did he apply to their synapses?
Hippocampal neuron cultures
Competitive antagonist AP5
What did Lester’s experiment with synapses show?
Rapid application of the competitive antagonist AP5 during the synaptic current fails to block the current demonstrating that glutamate is bound to the receptor, throughout the synaptic current
What patch clamp technique did Lester use?
Outside-out
What did Lester apply in his patch clamp studies and what did this show?
Brief (1 ms duration) applications of glutamate
Resulting current was found to have the same time course as the synaptic current, indicating that the receptor kinetics determine the synaptic current time course.
What function does slow time course allow NMDAR’s to have?
Coincidence detectors of pre- and postsynaptic activity
What does the synaptic time course of NMDAR’s depend on?
Subunit composition
How does decay time constant of NMDAR in the cortex differ in development and adulthood? and when does the change occur?
−250 ms in early development -> ∼80 ms in adult
the end of the critical period for synaptic plasticity in somatosensory cortex
How does decay time constant relate to subunit composition and which researchers showed these changes?
GluN2B subunits predominate early in development
Carmingnoto and Vicini, 1992; Hestrin, 1992
GluN2A expression rises through development
Flint et al., 1997; Lu et al., 2001
Broadly describe GluN2A and GluN2D containing receptor kinetics
GluN2A containing receptors have relatively rapid deactivation kinetics and mediate relatively fast synaptic currents
GluN2D containing receptors have extremely slow deactivation kinetics with a time constant of around 4 seconds and have not yet been shown to be involved in synaptic transmission
What are the main single channel conductances of the different GluN2 subunit containing receptors?
GluN2A + GluN2B - 50 pS
GluN2C - 36 pS
GluN2D - 18 pS
How were activation profiles of GluN1/GluN2A and GluN1/GluN2B characterised? By who?
Patch clamp recording
Erreger, 2005
What did GluN1/GluN2A and GluN1/GluN2B activation profiles show?
Glutamate binding (k +1) is rapid, while unbinding (k −1) is relatively slow
What conformational changes of NMDAR’s underlie slow synaptic currents and which researchers demonstrated this?
GluN1 and GluN2 subunits adjust to the rearrangements resulting from closing of the clam shell-like domains of the agonist binding sites (Furukawa et al., 2005; Mayer, 2006), that take time, before the channel can finally open (state A2R*)
How long do channels stay open in NMDAR activation and do they just open once? Who showed this?
Each opening is relatively short ∼3.0 ms
The channel can open and close many times during the activation, with gaps between openings ranging from 10 to 100 ms
Gibb and Colquhoun, 1992
What happens to the receptor state during structural rearrangements?
The receptor may slip into one or more desensitized states (state A2RD1, A2RD2) but on exiting these states, may continue to open and close for a further burst of activity.
