Before we start:

What is the question?What is the role/contribution of the different subtypes of NMDA glutamate receptor to plasticity in the brain?

Why is it interesting?NMDA receptors play an important role in many forms of synaptic plasticity.

NMDA receptors are important for cell survival and cell death (ischemia, excess glutamate)

NMDA receptor composition changes during development and it has been proposed to be one of the main regulators of the critical period.

The developmental switch between NR2A and NR2B can be modulated by experience.

Glutamate Receptors: NMDA-sensitive

Tetramers with the following subunits:NR1 (always 2)

8 splice variants.

Glycine binding site.

The different splice variants differ in their conductances, their affinity for glycine, etc.

NR2 (always 2)

Glutamate binding site.

Need NR1 to form functional receptors

4 genes. (NR2A, NR2B. NR2C, NR2D)

Only NR2A and NR2B are expressed in the forebrain.

NR2A and NR2B are developmentally regulated, with NR2B starting to decrease after a few weeks.

Very little is known about the proteins that each subunit interacts with.

NMDAR Receptors: Adult distribution of NR2

Nakanishi, S., 1992., Science

Functional Properties of NR1 and NR2

Cull-candy and leszkiewicz, Science STKE, 2004

NR2 NR1Affinity for glutamate Inhibition by Protons

Modulation by Glycine

Potentiation by polyamines

Ca2+ current Inhibition by Zn+2

Channel Kinetics Some kinetics properties (very

unknown)

Dendritic transport, synaptic localization,

trafficking of receptors

Required for release from the ER; might

be important for targeting between

soma and dendrites

Deactivation Kinetics of NR2B subunits

Cull-candy and leszkiewicz, Science STKE, 2004

> Mg2+ sensitive < Mg2+ sensitive

Plasticity in the Hippocampus

Studying Long Term Potentiation (LTP) and Long Term Depression (LTD)

Time (min)

% EPSP

100 %

200 %

Baseline

Stim. Schaffer Collateral at 0.2Hz

CA1

Induction

Stim. Schaffer Collateral at 100Hz, 1sec

CA1

Post- InductionStim. Schaffer Collateral at 0.2Hz

CA1

100Hz

LTP and LTD in CA1HFS of the Schaffer collateral fibers leads to LTP in CA1.

LFS of the Schaffer collateral fibers leads to LTD in CA1.

These changes in synaptic strength depend on:

Intracellular calcium.

NMDA receptors.

Simple Model:

Modifed from Lisman, 1989; Bear and Malenka, 1994.

Ca2+

Presynaptic Postsynaptic

NMDA receptor

HFS

LFS

Support for this model

Intracellular calcium levels

Yang S-N. et al., 1999. J. Neurophysiol.

“LTD” “LTP”

Hippocampal slices, p 11-p22, WC Recording, Photolysis of EGTA

Support for this model

Degree of NMDA receptor activation Amount of calcium

Cummings et al., 1996. Neuron Nishiyama et al., 2000.

Nature

NR2A KO NR2B KO

“Genetic enhancement of learning and memory in mice”

Tang et al., Nature 1999.

“NR2B expression is downregulated during the period of transition between juvenile and adult, correlating with the gradual shortening of the EPSP duration through the NMDA channel. This could decrease NMDA-mediated plasticity, and perhaps explain decreased memory performance in adult animals…”

BUT… NR2B over-expression:

“Smart Mice”: Over-expression of NR2B

Enhanced Novel Object Recognition Tang et al., 1999. Nature.

Selective block of NR2A

In oocytes In hippocampal slices

% blockcurrent2B2A

LTD LTP

LFS

HFS

NR2A

X

LFS

HFS

LTD (1XLFS) LTP (1XHFS)

NR2B

X

Conclusions:[Ca++]

0.1uM Ca++ (resting) too low for both calcineurin & CamIIK – stable release

~1uM Ca++ (LFS=1-5 Hz) – calcineurin: LTD

>5uM Ca++ (HFS=>Hz) – calcineurin + CamIIK… CamIIK out-competes: LTP

Molecular ID of NR2

NR2A necessary for LTP, not LTD.

NR2B necessary for LTD, not LTP.

Reduction of NR2B during development could explain increased difficulty in inducing LTD.

who or where???

“Spillover” during HFS

Induction: association cooperativityspecificity

NMDAR coincidence detection: glutamate & depolarizationGlutamate release- localAMPAR depolarization- small ampl., passive propagation: small spreadBack-propagating AP- big ampl., active propagation: wide-spread

Time, space, co-incidence detection

Timecalcineurin – high affinity, on/offCamIIK- medium affinity, persistent onPKC- medium affinity, on/off

SpaceCa++ via NMDAR- local glutamate, wide-spread voltage unblockCa++ via IP3- local glutamate (mGluR)Ca++ via VG-CC- wide-spread Ca++ AP

Spill-over (2A/2B in synapse; 2B extra-synaptic)

NO- trans-cellular, on/off, intermed. distance- retrograde transmitter!!

Na+ channels

AP

NOS

Specificity - synaptic tagging

tag