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Bi/CNS 150 Lecture 7

Monday October 14, 2013

Structure & function of glutamate receptors

Henry Lester

Chapter 10 (211-227)

Superfamilies of ligand-gated ion channels that are synaptic receptors

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A. ACh, Serotonin 5-HT3, GABA, (invert. GluCl, dopamine, tyrosine) receptor-channels

Most

^

Modified from Figure 10-7

The Hippocampus: a favorite model system for neuroscience

The “tri-synaptic pathway”3See also Chapter 67

Electron micrograph of hippocampal synapse

“Map” of micrograph to the left

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400 nm

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A postsynaptic density, with a cartoon of important proteins

Ionotropic Glutamate Receptors:3 transmembrane helices plus a selectivity filter per subunit

x 4 subunits

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Selectivity Filter

There are also G protein-coupled glutamate receptors

Figure 10-8

3 families of Ionotropic Glutamate Receptorsare named by their selective synthetic agonists

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AMPA GluR1-4 (Most are GluR1/2 or GluR2/3)

Kainate GluR5-7

NMDA NR1* (*obligatory) NR2A or B or C or D

Family Subunits

Ion Selectivity of Glutamate Receptor Channels

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AMPA and Kainate Receptors: With GluR2 subunit: • permeable only to K+ and Na+

Without GluR2 subunit: • Ca2+-permeable (and K+, Na+)

NMDA Receptors:

• Permeable to K+, Na+, Ca2+

RNA Editing Determines Ca2+ Permeability of AMPA Receptors #1

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Transcribed codon: CAGEdited codon: CIG

Right: Modified from Zigmond et al. (Eds.) Fundamental Neuroscience, Sinauer (1999)

Q=

R=

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RNA Editing Determines Ca2+ Permeability of AMPA Receptors #2

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Isolating AMPA-Receptor and NMDA-Receptor CurrentsWith Selective Blockers #1

There is outward NMDA receptor current

Nestler, Hyman, & Malenka, Molecular Neuropharmacology

Nestler, Hyman, & Malenka, Molecular Neuropharmacology

Isolating AMPA-Receptor and NMDA-Receptor CurrentsWith Selective Blockers #2

blocks NMDA receptors

blocks AMPA receptors

There is **no** inward NMDA receptor current

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AMPA response is faster than NMDA response

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A. Are composed of NR1 subunits and four different NR2 subunits; NR2A, NR2B, NR2C, and NR2D.

B. They contain two NR1 subunits, and a pair of NR2 subunits, which can be identical or mixed.

C. NR1 subunits are similar in size to GluR1-4; they are necessary to form the receptor channel, and they bind the co-ligands glycine or d-serine.

D. NR2 subunits are approximately twice as long as NR1 subunits. They bind glutamate, and their very long cytosolic tails bind signal transduction molecules and link the receptors to the postsynaptic density scaffold.

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NMDA Receptors

Na+ , K+1 ns

(~ 109/s) Na+ , K+, and Ca2+ can flow through single channels at rates > 1000-fold greater than Mg2+

Ca2+5 ns

(2 x 108/s)

Mg2+10 ms

(105/s)As the most charge-dense cation, Mg2+ holds its waters of hydration most tightly.

Time required to exchange waters of hydration

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From Lecture 1

1. Binding of glutamate

2. Strong postsynaptic membrane depolarization

(as by an action potential)

The depolarization relieves block by Mg2+

Modified from Zigmond et al. (Eds.) Fundamental Neuroscience, Sinauer (1999)

The coincidence readout:

NMDA receptors are very permeable to Ca2+16

Greater detail on learning & memory in a later lecture

In NMDA receptors, the selectivity filter also serves as a Mg 2+ binding site, producing a “coincidence detector”.

Their channel opens only when two events happen concurrently:

Behavior of the NMDA receptor current

Single channels Macroscopic I-V relations

Channels are blocked by Mg2+ at negative potentials

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Figure 10-5

From Grynkiewicz, Poenie, and Tsien (1985) J. Biol. Chem. 260, 3440.

Fura-2

Synthetic fluorescent dyes such as Fura-2

Detect intracellular Ca2+ transients

“Ratio Imaging”

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NMDA Receptors Mediate Synaptic Ca2+ Entry

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Lisman et al. Nature Rev. Neurosci. 3: 175 (2002)

(repeated glutamate pulses)

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In green fluorescent protein (GFP), the fluorophore is well protected from the environment, by a “can” of β strands.

If we render this protection sensitive to a olecule or condition, we have a good fluorescent sensor. .

21Akerboom, (30 other authors), Looger, J Neurosci 2012

Genetically encoded Ca2+ sensors

Circularly permuted enhanced green fluorescent protein

Calmodulin

Control of Synaptic Plasticity by NMDA Receptors(thought to underlie some aspects of memory and learning)

I. The central role of Ca2+ in initiation of long-term plastic changesA. The “Ca2+ hypothesis” for control of synaptic plasticityB. Measurement of cytosolic Ca2+ with fluorescent dyes.C. Control of postsynaptic Ca2+ by “spike timing”

II. Structure and behavior of the NMDA receptorA. Subunit compositionB. Gating (“coincidence detection”)C. Ion selectivity (Na+, K+, Ca2+)D. Kinetics, NMDA receptors are slower than AMPA receptorsE. PharmacologyF. The NMDA receptor is also a “scaffold.”

III. The postsynaptic densityA. LTP and LTD are triggered by Ca2+-sensitive signaling machinery

located near the mouth of the NMDA receptor.B. Critical components of the postsynaptic densityC. Biochemical pathways mediating changes in synaptic strength

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Greater detail in a later lecture

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End of Lecture 7

Henry Lester’s office hoursMon, Fri

1:15 – 2 Red Door