NMDA Biophysical Properties

Question 1

Key biophysical properties and significance

Answer 1

Slow activation and deactivation kinetics

Significance:

1. Slow NMDA receptor-mediated synaptic current
2. Permeability to calcium
3. Voltage-dependent block by magnesium

Question 2

What do NMDAR’s mediate at excitatory synapses in the nervous system?

Answer 2

Slow component of synaptic transmission

Question 3

Describe NMDAR activation time course at room temperature (rise + decay)

Answer 3

Rises slowly to peak about 20 ms after initiation and then decays with a time constant of about 100 ms

Question 4

Is NMDAR slow time course dependent on glutamate diffusion?

Answer 4

No

Kinetic properties of the receptor

Question 5

How do we know that slow time course is dependent on kinetic properties rather than glutamate diffusion?

Answer 5

Concentration-jump experiments

Lester et al., 1990

Question 6

What cell cultures did Lester use and what did he apply to their synapses?

Answer 6

Hippocampal neuron cultures

Competitive antagonist AP5

Question 7

What did Lester’s experiment with synapses show?

Answer 7

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

Question 8

What patch clamp technique did Lester use?

Answer 8

Outside-out

Question 9

What did Lester apply in his patch clamp studies and what did this show?

Answer 9

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.

Question 10

What function does slow time course allow NMDAR’s to have?

Answer 10

Coincidence detectors of pre- and postsynaptic activity

Question 11

What does the synaptic time course of NMDAR’s depend on?

Answer 11

Subunit composition

Question 12

How does decay time constant of NMDAR in the cortex differ in development and adulthood? and when does the change occur?

Answer 12

−250 ms in early development -> ∼80 ms in adult

the end of the critical period for synaptic plasticity in somatosensory cortex

Question 13

How does decay time constant relate to subunit composition and which researchers showed these changes?

Answer 13

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

Question 14

Broadly describe GluN2A and GluN2D containing receptor kinetics

Answer 14

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

Question 15

What are the main single channel conductances of the different GluN2 subunit containing receptors?

Answer 15

GluN2A + GluN2B - 50 pS
GluN2C - 36 pS
GluN2D - 18 pS

Question 16

How were activation profiles of GluN1/GluN2A and GluN1/GluN2B characterised? By who?

Answer 16

Patch clamp recording

Erreger, 2005

Question 17

What did GluN1/GluN2A and GluN1/GluN2B activation profiles show?

Answer 17

Glutamate binding (k  +1) is rapid, while unbinding (k  −1) is relatively slow

Question 18

What conformational changes of NMDAR’s underlie slow synaptic currents and which researchers demonstrated this?

Answer 18

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*)

Question 19

How long do channels stay open in NMDAR activation and do they just open once? Who showed this?

Answer 19

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

Question 20

What happens to the receptor state during structural rearrangements?

Answer 20

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.