IVMS-NMDA Receptors, Glutamate, Glycine, Serine and Neurobiology

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NMDA RECEPTORS AND GLUTAMATE

The N-methyl-D-aspartate receptor(also known as the NMDA receptororNMDAR),a glutamate receptor, is the predominantmolecular device for controlling synapticplasticity and memoryfunction.[1]The NMDAR is a specific type of ionotropic glutamatereceptor.NMDA (N-methyl-D-aspartate) is the name of a selective agonist that binds to

NMDA receptors but not to other 'glutamate' receptors.Activation of NMDA receptorsresults in the opening of an ion channel that is nonselective to cations with anequilibrium potential near 0 mV.A property of the NMDA receptor is its voltage-dependent activation, aresult of ion channel block by extracellular Mg2+ ions. Thisallows theflow of Na+ and small amounts of Ca2+ ions into the cell and K+ out ofthecell to be voltage-dependent

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NMDA receptor 1

NMDA receptor

NMDA

Glutamic acid

Stylised depiction of an activated NMDAR.

Glutamate is in the glutamate-binding site and

glycine is in the glycine-binding site. Allosteric

sites that would cause inhibition of the receptor

are not occupied. NMDARs require the binding

of two molecules of glutamate or aspartate and

two of glycine.[]

The N-methyl-D-aspartate receptor (also known as the NMDA

receptor or NMDAR), a glutamate receptor, is the predominant

molecular device for controlling synaptic plasticity and memory

function.[1]

The NMDAR is a specific type of ionotropic glutamate receptor.

NMDA (N-methyl-D-aspartate) is the name of a selective agonist that

binds to NMDA receptors but not to other 'glutamate' receptors.

Activation of NMDA receptors results in the opening of an ion channel

that is nonselective to cations with an equilibrium potential near 0 mV.

A property of the NMDA receptor is its voltage-dependent activation, a

result of ion channel block by extracellular Mg2+

ions. This allows the

flow of Na+

and small amounts of Ca2+

ions into the cell and K+

out of

the cell to be voltage-dependent.

[][][][]

Calcium flux through NMDARs is thought to be critical in synaptic

plasticity, a cellular mechanism for learning and memory. The NMDA

receptor is distinct in two ways: first, it is both ligand-gated and

voltage-dependent; second, it requires co-activation by two ligands:

glutamate and either d-serine or glycine.[2]

Structure

The NMDA receptor forms a heterotetramer between two GluN1 and

two GluN2 subunits (the subunits were previously denoted as NR1 and

NR2), two obligatory NR1 subunits and two regionally localized NR2

subunits. A related gene family of NR3 A and B subunits have an

inhibitory effect on receptor activity. Multiple receptor isoforms with

distinct brain distributions and functional properties arise by selective

splicing of the NR1 transcripts and differential expression of the NR2

subunits.

Each receptor subunit has modular design and each structural module

also represents a functional unit:

The extracellulardomain contains two globular structures: amodulatory domain and a ligand-binding domain. NR1 subunits

bind the co-agonist glycine and NR2 subunits bind the

neurotransmitter glutamate.

The agonist-binding module links to a membrane domain, which

consists of three trans-membrane segments and a re-entrant loop

reminiscent of the selectivity filter of potassium channels.

The membrane domain contributes residues to the channel pore and is responsible for the receptor's high-unitary

conductance, high-calcium permeability, and voltage-dependent magnesium block.

Each subunit has an extensive cytoplasmic domain, which contain residues that can be directly modified by aseries of protein kinases and protein phosphatases, as well as residues that interact with a large number of

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NMDA receptor 2

structural, adaptor, and scaffolding proteins.

The glycine-binding modules of the NR1 and NR3 subunits and the glutamate-binding module of the NR2A subunit

have been expressed as soluble proteins, and their three-dimensional structure has been solved at atomic resolution

by x-ray crystallography. This has revealed a common fold with amino acid-binding bacterial proteins and with the

glutamate-binding module of AMPA-receptors and kainate-receptors.

Variants

GluN1

There are eight variants of the NR1 subunit produced by alternative splicing of GRIN1:[]

NR1-1a, NR1-1b; NR1-1a is the most abundantly expressed form.

NR1-2a, NR1-2b;

NR1-3a, NR1-3b;

NR1-4a, NR1-4b;

GluN2

NR2 subunit in vertebrates (left) and

invertebrates (right). Ryan et al., 2008

While a single NR2 subunit is found in invertebrate organisms, four

distinct isoforms of the NR2 subunit are expressed in vertebrates and

are referred to with the nomenclature NR2A through D(coded by

GRIN2A, GRIN2B, GRIN2C, GRIN2D). Strong evidence shows that

the genes coding the NR2 subunits in vertebrates have undergone at

least two rounds of gene duplication.[3]

They contain the binding-site

for the neurotransmitter glutamate. More importantly, each NR2

subunit has a different intracellular C-terminal domain that can interact

with different sets of signalling molecules.[4] Unlike NR1 subunits,

NR2 subunits are expressed differentially across various cell types and

control the electrophysiological properties of the NMDA receptor. One

particular subunit, NR2B, is mainly present in immature neurons and

in extrasynaptic locations, and contains the binding-site for the selective inhibitor ifenprodil.

Whereas NR2B is predominant in the early postnatal brain, the number of NR2A subunits grows, and eventually

NR2A subunits outnumber NR2B. This is called NR2B-NR2A developmental switch, and is notable because of the

different kinetics each NR2 subunit lends to the receptor.[]

For instance, greater ratios of the NR2B subunit leads to

NMDA receptors which remain open longer compared to those with more NR2A.[5]

This may in part account for

greater memory abilities in the immediate postnatal period compared to late in life, which is the principle behindgenetically-altered 'doogie mice'.

There are three hypothetical models to describe this switch mechanism:

Dramatic increase in synaptic NR2A along with decrease in NR2B

Extrasynaptic displacement of NR2B away from the synapse with increase in NR2A

Increase of NR2A diluting the number of NR2B without the decrease of the latter.

The NR2B and NR2A subunits also have differential roles in mediating excitotoxic neuronal death.[]

The

developmental switch in subunit composition is thought to explain the developmental changes in NMDA

neurotoxicity.[]

Disruption of the gene for NR2B in mice causes perinatal lethality, whereas the disruption of NR2A

gene produces viable mice, although with impaired hippocampal plasticity.[6]

One study suggests that reelin may

play a role in the NMDA receptor maturation by increasing the NR2B subunit mobility.[]

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NMDA receptor 3

NR2B to NR2C switch

Granule cell precursors (GCPs) of the cerebellum, after undergoing symmetric cell division[]

in the external

granule-cell layer (EGL), migrate into the internal granule-cell layer (IGL) where they downregulate NR2B and

activate NR2C, a process that is independent of neuregulin beta signaling through ErbB2 and ErbB4 receptors.[]

Ligands

Agonists

Activation of NMDA receptors requires binding of glutamate or aspartate (aspartate does not stimulate the receptors

as strongly).[]

In addition, NMDARs also require the binding of the co-agonist glycine for the efficient opening of

the ion channel, which is a part of this receptor.

D-serine has also been found to co-agonize the NMDA receptor with even greater potency than glycine.[]

D-serine is

produced by serine racemase, and is enriched in the same areas as NMDA receptors. Removal of D-serine can block

NMDA-mediated excitatory neurotransmission in many areas. Recently, it has been shown that D-serine can be

released both by neurons and astrocytes to regulate NMDA receptors.

In addition, a third requirement is membrane depolarization. A positive change in transmembrane potential will

make it more likely that the ion channel in the NMDA receptor will open by expelling the Mg2+

ion that blocks the

channel from the outside. This property is fundamental to the role of the NMDA receptor in memory and learning,

and it has been suggested that this channel is a biochemical substrate of Hebbian learning, where it can act as a

coincidence detector for membrane depolarization and synaptic transmission.

Known NMDA receptor agonists include:

Aminocyclopropanecarboxylic acid

D-Cycloserine

cis-2,3-Piperidinedicarboxylic acid

L-aspartate

Quinolinate

Homocysterate

D-serine

ACPL

L-alanine

Partial agonists

N-Methyl-D-aspartic acid (NMDA)

3,5-dibromo-L-phenylalanine

[7]

GLYX-13

Antagonists

Antagonists of the NMDA receptor are used as anesthetics for animals and sometimes humans, and are often used as

recreational drugs due to their hallucinogenic properties, in addition to their unique effects at elevated dosages such

as dissociation. When certain NMDA receptor antagonists are given to rodents in large doses, they can cause a form

of brain damage called Olney's Lesions. NMDA receptor antagonists that have been shown to induce Olney's

Lesions include Ketamine, Phencyclidine, Dextrorphan (a metabolite of Dextromethorphan), and MK-801, as well as

some NDMA receptor antagonists used only in research environments. So far, the published research on Olney's

Lesions is inconclusive in its occurrence upon human or monkey brain tissues with respect to an increase in the

presence of NMDA receptor antagonists.[]

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NMDA receptor 4

Common NMDA receptor antagonists include:

Amantadine[]

Ketamine

Methoxetamine

Phencyclidine (PCP)

Nitrous oxide (laughing gas) Dextromethorphan and dextrorphan

Memantine

Ethanol

Riluzole (used in ALS)[8]

Xenon

HU-211 (also a cannabinoid)

Lead (Pb2+)[9]

Conantokins

Huperzine A

Atomoxetine[]

Dual opioid and NMDA receptor antagonists:

Ketobemidone

Methadone

Dextropropoxyphene

Tramadol

Kratom alkaloids

Ibogaine

Modulators

The NMDA receptor is modulated by a number of endogenous and exogenous compounds:[]

Mg2+

not only blocks the NMDA channel in a voltage-dependent manner but also potentiates NMDA-induced

responses at positive membrane potentials. Treatment with forms magnesium glycinate and magnesium taurinate

has been used to produce rapid recovery from depression.[]

Na+, K

+and Ca

2+not only pass through the NMDA receptor channel but also modulate the activity of NMDA

receptors.

Zn2+

and Cu2+

generally block NMDA current activity in a noncompetitive and a voltage-independent manner.

However zinc may potentiate or inhibit the current depending on the neural activity. (Zinc and Copper Influence

Excitability of Rat Olfactory Bulb Neurons by Multiple Mechanisms|http://jn.physiology.org/content/86/4/

1652.short)

Pb2+

lead is a potent NMDAR antagonist. Presynaptic deficits resulting from Pb2+ exposure during

synaptogenesis are mediated by disruption of NMDAR-dependent BDNF signaling.

It has been demonstrated that polyamines do not directly activate NMDA receptors, but instead act to potentiate

or inhibit glutamate-mediated responses.

Aminoglycosides have been shown to have a similar effect to polyamines, and this may explain their neurotoxic

effect.

The activity of NMDA receptors is also strikingly sensitive to the changes in H+

concentration, and partially

inhibited by the ambient concentration of H+

under physiological conditions.[citation needed]

The level of inhibition

by H+ is greatly reduced in receptors containing the NR1a subtype, which contains the positively charged insert

Exon 5. The effect of this insert may be mimicked by positively charged polyamines and aminoglycosides,

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NMDA receptor 5

explaining their mode of action.

NMDA receptor function is also strongly regulated by chemical reduction and oxidation, via the so-called "redox

modulatory site."[]

Through this site, reductants dramatically enhance NMDA channel activity, whereas oxidants

either reverse the effects of reductants or depress native responses. It is generally believed that NMDA receptors

are modulated by endogenous redox agents such as glutathione, lipoic acid, and the essential nutrient

pyrroloquinoline quinone. Src kinase enhances NMDA receptor currents.

[]

Reelin modulates NMDA function through Src family kinases and DAB1.[]

significantly enhancing LTP in the

hippocampus.

CDK5 regulates the amount of NR2B-containing NMDA receptors on the synaptic membrane, thus affecting

synaptic plasticity.[][]

Proteins of the major histocompatibility complex class I are endogenous negative regulators of

NMDAR-mediated currents in the adult hippocampus,[10]

and modify NMDAR-induced changes in AMPAR

trafficking[10]

and NMDAR-dependent synaptic plasticity.[]

Receptor modulation

The NMDA receptor is a non-specific cation channel that can allow the passage of Ca2+

and Na+

into the cell and K+

out of the cell. The excitatory postsynaptic potential (EPSP) produced by activation of an NMDA receptor increases

the concentration of Ca2+

in the cell. The Ca2+

can in turn function as a second messenger in various signaling

pathways. However, the NMDA receptor cation channel is blocked by Mg2+

at resting membrane potential. To

unblock the channel, the postsynaptic cell must be depolarized.[]

Therefore, the NMDA receptor functions as a "molecular coincidence detector". Its ion channel opens only when the

following two conditions are met simultaneously: Glutamate is bound to the receptor, and the postsynaptic cell is

depolarized (which removes the Mg2+

blocking the channel). This property of the NMDA receptor explains many

aspects of long-term potentiation (LTP) and synaptic plasticity.[]

NMDA receptors are modulated by a number of endogenous and exogenous compounds and play a key role in a

wide range of physiological (e.g., memory) and pathological processes (e.g., excitotoxicity).

Clinical significance

Cochlear NMDARs are the target of intense research to find pharmacological solutions to treat tinnitus. Recently,

NMDARs were associated with a rare autoimmune disease, Anti-NMDAR encephalitis, that usually occurs due to

cross reactivity of antibodies produced by the immune system against ectopic brain tissues, such as those found in

teratoma.

Antagonizing the NMDA receptor with the Drug Memantine (Namenda(R)) has shown some benefit in treating

Alzheimer's Dementia.

Compared to dopaminergic stimulants, the NMDA receptor antagonist PCP can produce a wider range of symptoms

that resemble schizophrenia in healthy volunteers, in what has led to the glutamate hypothesis of schizophrenia.

Experiments in which rodents are treated with NMDA receptor antagonist are today the most common model when it

comes to testing of novel schizophrenia therapies or exploring the exact mechanism of drugs already approved for

treatment of schizophrenia.

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NMDA receptor 6

External links

Media related to NMDA receptor at Wikimedia Commons

NMDA receptor pharmacology[11]

Motor Discoordination Results from Combined Gene Disruption of the NMDA Receptor NR2A and NR2C

Subunits, But Not from Single Disruption of the NR2A or NR2C Subunit[12]

A schematic diagram summarizes three potential models for the switching of NR2A and NR2B subunits at

developing synapses.[13]

- a figure from Liu et al., 2004[]

DrosophilaNMDA receptor 1 - The Interactive Fly[14]

References

[1] Clinical Implications of Basic Research: Memory and the NMDA receptors (http://content.nejm. org/cgi/content/full/361/3/302), Fei Li

and Joe Z. Tsien, N Engl J Med, 361:302, July 16, 2009

[4] Ryan, T. J. & Grant, S. G. N. (2009) The origin and evolution of synapses (vol 10, pg 701, 2009).Nat Rev Neurosci 10, Doi 10.1038/Nrn2748

[8] http:/ /www.clinicalpharmacology-ip. com

[9][9] Toxicol. Sci. 2010 116: 249-263;

[10][10] >

[11] http://www.bris.ac. uk/Depts/Synaptic/info/pharmacology/NMDA.html

[12] http://www.jneurosci.org/cgi/content/full/16/24/7859

[13] http://www.jneurosci.org/cgi/content-nw/full/24/40/8885/FIG8

[14] http://www.sdbonline. org/fly/hjmuller/nmda1. htm

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Article Sources and Contributors 7

Article Sources and ContributorsNMDA receptor Source: http://en.wikipedia.org/w/index.php?oldid=567872222 Contributors: A. Rad, A314268, ABCD, AJVincelli, Abductive, Absg2011eur, Acdx, Alibobar,

Aloneyouaregeek, Amelvin, Aplested, Arcadian, ArionVII, Arseni, AxelBoldt, Axl, Bad2101, Bebebas, Benjah-bmm27, Bignoter, Biochemza, Boghog, Brandonazz, Brodyt66, CMBJ, Cacycle,

Cafeturco, Calvero JP, Ccevo2011, Chemgirl131, Clicketyclack, CopperKettle, Cyberfay, Cytocon, Dactyle, DarkLaguna, Dcirovic, Delldot, Delta G, Diberri, Dr. Vinzenz, Draicone,

Drphilharmonic, Ekretzmer, EmanWilm, Excirial, Forluvoft, Fuzzform, Gadfium, Gould363, Hieu nguyentrung12, Hokanomono, IlyaV, Informedbanker, Ippyy, Jab843, Jakaufman, Jasongallant,

JeremyA, Jesse V., John, Jolb, JonatasM, Karn, Kate, Kernsters, Lepidoptera, Marqueed, Meodipt, Mike.lifeguard, Millencolin, Mlbish, Nbauman, Neuro100, NeuronExMachina, Neuroscience

Research, Nmg20, NotWith, Nrets, Oda Mari, Odieiscool, OldakQuill, PhilipO, Piperh, Pjoef, Ramorum, Rich Farmbrough, Richwil, Rjwilmsi, Rob Hurt, SJFriedl, Sedmic, Selket, Shao, Shaun,

Shushruth, SilentWings, Skingski, Sournick3, Speshuldusty, Stepa, Steven J. Anderson, StockTrader, Subcellular, SuperiorCerebrum, Supermartin, TheOltimate, User931, Verpies, Viralmemesis,Wavelength, Wfseidel, William Avery, Wolfkeeper, Zigger, 142 anonymous edits

Image Sources, Licenses and ContributorsImage:Nmda.png Source: http://en.wikipedia.org/w/index.php?title=File:Nmda.png License: GNU Free Documentation License Contributors: Original uploader was Jarombouts at

nl.wikipedia

Image:L-glutamic-acid-skeletal.png Source: http://en.wikipedia.org/w/index.php?title=File:L-glutamic-acid-skeletal.png License: Public Domain Contributors: Arrowsmaster,

Benjah-bmm27, Edgar181

Image:Activated NMDAR.PNG Source: http://en.wikipedia.org/w/index.php?title=File:Activated_NMDAR.PNG License: Public Domain Contributors: en:User:Delldot

File:Model of NR2 Subunit of NMDA receptor (vertebrate and invertebrate).jpg Source:

http://en.wikipedia.org/w/index.php?title=File:Model_of_NR2_Subunit_of_NMDA_receptor_(vertebrate_and_invertebrate).jpg License: Creative Commons Attribution 2.0 Contributors: Ryan

TJ, Emes RD, Grant SG, Komiyama NH.

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NMDA Receptors

Source: http://www.ncbi.nlm.nih.gov/books/NBK11526/

NMDA receptors are highly permeant for Ca2+

, show slower gating kinetics and the

channel is blocked in a voltage-and use-dependent manner by physiological

concentrations of Mg2+

ions (Mcbain and Mayer, 1994). These properties make them

ideally suited for their role as a coincidence detector underlying Hebbian processes in

synaptic plasticity such as learning (see later), chronic pain, drug tolerance and

dependence (Collingridge and Singer, 1990; Bear and Malenka, 1994; Trujillo and Akil,

1995; Danysz and Parsons, 1995; Collingridge and Bliss, 1995; Dickenson, 1997).

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Glycine as a co-agonist

Glycine is a co-agonist at NMDA receptors at a strychnine-insensitive recognition site

(glycineB) and its presence at moderate nM concentrations is a prerequisite for channel

activation by glutamate or NMDA (Danysz and Parsons, 1998). Physiological

concentrations reduce one form of relatively rapid NMDA receptor desensitization.

Recently it has been suggested that D-Serine may be more important than glycine as

an endogenous co-agonist at NMDA receptors in the telencephalon and developing

cerebellum. There is still some debate as to whether the glycineB site is saturated in

vivo (Danysz and Parsons, 1998) but it seems likely that the degree of NMDA receptor

activation varies depending on regional differences in receptor subtype expression and

local glycine or D-serine concentrations. Moreover, glycine concentrations at synapticNMDA receptors could be finely modulated by local expression of specific glycine

transporters such as GLYT1 (Supplisson and Bergman, 1997).

Polyamines

The polyamines spermine and spermidine have multiple effects on the activity of NMDA

receptors (Johnson, 1996; Williams, 1997). These include an increase in the magnitude

of NMDA-induced whole-cell currents seen in the presence of saturating concentrationsof glycine, an increase in glycine affinity, a decrease in glutamate affinity, and voltage-

dependent inhibition at higher concentrations. Endogenous polyamines could act as a

bi-directional gain control of NMDA receptors, by dampening toxic chronic activation by

low concentrations of glutamate-through changes in glutamate affinity and voltage-

dependent blockade-but enhancing transient synaptic responses to mM concentrations

of glutamate (Williams, 1997; Zhang and Shi, 2001).

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Molecular Biology

Two major subunit families designated NR1, NR2 as well as a modulatory subunit

designated NR3 have been cloned. Most functional receptors in the mammalian CNS

are formed by combination of NR1 and NR2 subunits which express the glycine and

glutamate recognition sites respectively (Hirai et al., 1996; Laube et al., 1997).

NR1 Subunits

Alternative splicing generates eight isoforms for the NR1 subfamily (Zukin and Bennett,

1995). The variants arise from splicing at three exons one encodes a 21-amino acid

insert in the N-terminal domain (N1, exon 5), and two encode adjacent sequences of 37

and 38 amino acids in the C-terminal domain (C1, exon 21 and C2, exon 22). NR1

variants are sometimes denoted by the presence or absence of these three alternatively

spliced exons (from N to C1 to C2). NR1111 has all three exons, NR1000 has none, and

NR1100 has only the N-terminal exon. The variants from NR1000 to NR1111 are

alternatively denoted as NMDAR1E, C, D, A, G, F, H and B respectively or NMDAR1-

4a,-2a,-3a,-1a,-4b,-2b,-3b and-1b respectively, but the more frequent terminology using

non-capitalized suffices for the most common splice variants is NR1a (NR1011 or

NMDAR1A) and NR1b (NR1100 or NMDARIG). MRNA for double splice variants in theC1/C2 regions such as NR1011 (NR1a) show an almost complementary pattern to those

lacking both of these inserts such as NR1100 (NR1b); the former are more concentrated

in rostral structures such as cortex, caudate, and hippocampus, while the latter are

principally found in more caudal regions such as thalamus, colliculi, locus coeruleus and

cerebellum (Laurie et al., 1995).

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NR2 Subunits

The NR2 subfamily consists of four individual subunits, NR2A to NR2D. Various

heteromeric NMDA receptor channels formed by combinations of NR1 and NR2

subunits are known to differ in gating properties, Mg2+ sensitivity and pharmacological

profile (Sucher et al., 1996). The heteromeric assembly of NR1 and NR2C subunits for

instance, has a lower sensitivity to Mg2+

but increased sensitivity to glycine and a very

restricted distribution in the brain.In situ hybridization has revealed overlapping but

different expression for NR2 mRNA e.g. NR2A mRNA is distributed ubiquitously like

NR1 with highest densities occurring in hippocampal regions and NR2B is expressed

predominantly in forebrain but not in cerebellum where NR2C predominates. The spinal

cord expresses high levels of NR2C and NR2D (Tolle et al., 1993) and these may formheteroligomeric receptors with NR1 plus NR2A which would provide a basis for the

development of drugs selectively aimed at spinal cord disorders(Sundstrom et al.,

1997). NMDA receptors cloned from murine CNS have a different terminology to those

in the rat: z1 remains the terminology for the mouse equivalent of NR1 and e1 to e4

represent NR2A to 2D subunits respectively.

NR3 Subunits

NR3 (NRL or Chi-1) is expressed predominantly in the developing CNS and does not

seem to form functional homomeric glutamate-activated channels but co-expression of

NR3 with NR1 plus NR2 subunits decreases response magnitude (Sucher et al., 1995;

Kinsley et al., 1999; Matsuda et al., 2002). However, NR3A or NR3B do co-assemble

with NR1 alone inXenopus oocytes to form excitatory glycine receptors that are

unaffected by glutamate or NMDA, Ca2+

-impermeable, resistant to blockade by Mg2+

uncompetitive and competitive antagonists and actually inhibited by the glycine co-

agonist D-serine. (Chatterton et al., 2002)

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Uncompetitive NMDA receptor antagonists

Antagonists which completely block NMDA receptors cause numerous side effects such

as memory impairment, psychotomimetic effects, ataxia and motor dis-coordination as

they also impair normal synaptic transmission - a two edged sword. The challenge has

therefore been to develop NMDA receptor antagonists that prevent the pathological

activation of NMDA receptors but allow their physiological activation. It has been

suggested that uncompetitive NMDA receptor antagonists with rapid unblocking kinetics

but somewhat less pronounced voltage-dependency than Mg2+

should be able to

antagonise the pathological effects of the sustained, but relatively small increases in

extracellular glutamate concentration but, like Mg2+, leave the channel as a result of

strong depolarization following physiological activation by transient release of mMconcentrations of synaptic glutamate (Parsons et al., 1999; Jones et al., 2001). As such,

uncompetitive NMDA receptor antagonists with moderate, rather than high affinity may

be desirable. Memantine, ketamine, dextromethorphan and possibly felbamate and

budipine are clinically-used agents which belong to this category NB: for the last two it

is unsure if uncompetitive NMDA receptor antagonism really contributes to their

therapeutic efficacy. Others such as neramexane, remacemide, NPS-1506 and possibly

the cannabinoid dexanabinol are at different stages of clinical development. Several

promising agents have unfortunately been abandoned at late stages of development,

possibly due to the choice of the wrong, too ambious, clinical indications such as stroke

and trauma.

Glycine site antagonists

Most full glycineB antagonists (i.e. those without intrinsic partial agonist activity) show

very poor penetration to the CNS although some agents with improved, but by no

means optimal pharmacokinetic properties have now been developed. GlycineB

antagonists have been reported to lack many of the side effects classically associated

with NMDA receptor blockade such as no neurodegenerative changes in the cingulate /

retrosplenial cortex even after high doses (Hargreaves et al., 1993) and no

psychotomimetic-like or learning impairing effects at anticonvulsive doses (Murata and

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Kawasaki, 1993; Kretschmer et al., 1997; Baron et al., 1997; Danysz and Parsons,

1998). The MSD compound L-701,324 has even been proposed to have atypical

antipsychotic effects (Bristow et al., 1996). The improved neuroprotective therapeutic

profile of glycineB full antagonists could be due to their ability to reveal glycine-sensitive

desensitization (Parsons et al., 1993).

Kynurenic acid is an endogenous glycineB antagonist but it seems unlikely that

concentrations are sufficient to interact with NMDA receptors under normal conditions

(Danysz and Parsons, 1998; Stone, 2001). However, concentrations are raised under

certain pathological conditions (Danysz and Parsons, 1998; Stone, 2001) and

interactions with other receptors such as a7 neuronal nicotinic have been reported at

lower concentrations (Hilmas et al., 2001). Strategies aimed at increasing kynurenicacid concentrations by for example by giving its precursor 4-Cl-kynurenine, inhibiting

brain efflux with probenecid or inhibiting its metabolism have been proposed to be of

therapeutic potential (Danysz and Parsons, 1998; Stone, 2001).

D-cycloserine and (+R)-HA-966 are partial agonists at the glycineB site with different

levels of intrinsic activity: 57% and 14% respectively in cultured hippocampal neurones

(Karcz-Kubicha et al., 1997). Although these systemically-active partial agonists do not

induce receptor desensitization (Henderson et al., 1990; Kemp and Priestley, 1991;

Karcz-Kubicha et al., 1997) they have favourable therapeutic profiles in some in vivo

models (Lanthorn, 1994; Witkin et al., 1997). This may, in part, be due to their own

intrinsic activity as agonists at the glycineB site which would serve to preserve a certain

level of NMDA receptor function even at very high concentrations (Priestley and Kemp,

1994; Fossom et al., 1995; Krueger et al., 1997).

D-cycloserine shows agonist like features at low doses, while with increasing dosing

antagonistic effects predominate (Lanthorn, 1994). Such findings are often falsely

interpreted to be typical for partial agonists i.e. agonism at low and antagonism at high

doses. However, partial agonism actually means that an agent reaches a ceiling, non-

maximal effect at higher doses (intrinsic activity) i.e. will antagonise receptor activation

by high concentrations of a full agonist but facilitate at low concentrations of a full

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agonist (Henderson et al., 1990; Karcz-Kubicha et al., 1997). Recent data indicate that

the consistent biphasic effects of D-cycloserine seen in vivo may rather be related to

different affinities and intrinsic activities at NMDA receptor subtypes. D-cycloserine is a

partial agonist for the murine equivalents of NR1/2A and NR1/2B heteromers (38% and

56% intrinsic activity compared to glycine 10 M) but is more effective than glycine at

NR1/2C (130%) (O'Connor et al., 1996). This effect is accompanied by higher affinity at

NR1/2C receptors - NR1/2C > NR1/2D >> NR1/2B > NR1/2A (O'Connor et al., 1996).

Very similar data were published recently by a different group, except that the intrinsic

activity at NR1/2C was even higher (192%) (Sheinin et al., 2001). As such, it is likely

that the biphasic effects seen in vivo are due to agonistic actions at NR1/2C receptors

at lower doses and inhibition of NR1/2A and NR1/2B containing receptors at higher

doses. This receptor subtype selectivity and differential intrinsic activity could well

underlie its promising preclinical profile in some animals models.

Although ACPC has been reported to be a partial agonist with very high intrinsic activity,

it is probably really a full agonist at the glycineB site and actually behaves as an

antagonist in some in vivo models (neuroprotection, anticonvulsive effects) which are

likely to be mediated via competitive antagonistic properties at higher concentrations

{NahumLevy et al., 1999 #18977} (Skolnick et al., 1989). The consistent observation

that chronic treatment with ACPC is neuroprotective could be because it desensitizes or

uncouples NMDA receptors (Skolnick et al., 1992; Papp and Moryl, 1996) or may be

related to an increase in the relative levels of NR2C expression (Fossom et al., 1995).

NR2B selective antagonists

Ifenprodil and its analogue eliprodil block NMDA receptors in a spermine-sensitive

manner and were originally proposed to be polyamine antagonists. It is now clear that

both agents are selective for NR2B subunits (Legendre and Westbrook, 1991) and bind

to a site that is distinct from the polyamine recognition site, but interact allosterically with

this site and the glycineB site. NR2B selective agents may also offer a promising

approach to minimize side effects as agents would not produce maximal inhibition of

responses of neurons expressing heterogeneous receptors. Thus, cortical and

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hippocampal neurons express both NR2A and NR2B receptors in approximately similar

proportions, but very little NR2C or NR2D. NR2B selective agents therefore block

NMDA receptor mediated responses of such neurons to a maximal level of around 30-

50% of control. Several studies have shown that ifenprodil and eliprodil reduce seizures

and are effective neuroprotectants against focal and global ischaemia and trauma at

doses that do not cause ataxia or impair learning (Parsons et al., 1998). These

compounds are not devoid of side effects and some companies attempted to improve

the selectivity NR2B antagonists by reducing affects at other receptors such as a1 and

a2 adrenergic receptors - traxoprodil (CP-101,606) and CP-283,097 showed improved

selectivity and in vivo potency (Butler et al., 1997; Menniti et al., 1997; Chenard and

Menniti, 1999). However, an unfortunate new side effect has recently been reported, i.e.

that some of these agents may produce a prolongation of the QT interval in the cardiac

action potential due to blockade of human ether-a-go-go-related gene (hERG)

potassium channels (Gill et al., 1999). This would be less of a problem in acute

excitotoxicity and traxoprodil is still under development for stroke / TBI.

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Glutamate and Glutamate Receptors in the Vertebrate Retina

Victoria Connaughton

General Overview of Synaptic Transmission

Cells communicate with each other electrically, through gap junctions, and chemically, using

neurotransmitters. Chemical synaptic transmission allows nerve signals to be exchanged

between cells that are electrically isolated from each other. The chemical messenger, or

neurotransmitter, provides a way to send the signal across the extracellular space, from the

presynaptic neuron to the postsynaptic cell. The space is called a cleft and is typically more

than 10 nanometers across. Neurotransmitters are synthesized in the presynaptic cell and stored

in vesicles in presynaptic processes, such as the axon terminal. When the presynaptic neuron

is stimulated, calcium channels open, and the influx of calcium ions into the axon terminal

triggers a cascade of events leading to the release of neurotransmitter. Once released, the

neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic cell,allowing the signal to propagate. Neurotransmitter molecules can also bind onto presynaptic

autoreceptors and transporters, regulating subsequent release and clearing excess

neurotransmitter from the cleft. Compounds classified as neurotransmitters have several

characteristics in common (reviewed in Massey (1) and Erulkar (2)).

Briefly: 1) the neurotransmitter is synthesized, stored, and released from the presynaptic

terminal; 2) specific neurotransmitter receptors are localized on the postsynaptic cells; and 3)

there exists a mechanism to stop neurotransmitter release and clear molecules from the cleft.

Common neurotransmitters in the retina are glutamate, GABA, glycine, dopamine, and

acetylcholine. Neurotransmitter compounds can be small molecules, such as glutamate and

glycine, or large peptides, such as vasoactive intestinal peptide (VIP). Some neuroactive

compounds are amino acids, which also have metabolic functions in the presynaptic cell.

Glutamate (Fig. 1) is believed to be the major excitatory neurotransmitter in the retina. In

general, glutamate is synthesized from ammonium and -ketoglutarate (a component of the

Krebs cycle) and is used in the synthesis of proteins, other amino acids, and even other

neurotransmitters (such as GABA) (3). Although glutamate is present in all neurons, only a

few are glutamatergic, releasing glutamate as their neurotransmitter. Neuroactive glutamate is

stored in synaptic vesicles in presynaptic axon terminals (4). Glutamate is incorporated into

the vesicles by a glutamate transporter located in the vesicular membrane. This transporter

selectively accumulates glutamate through a sodium-independent, ATP-dependent process

(4-6), resulting in a high concentration of glutamate in each vesicle. Neuroactive glutamate is

classified as an excitatory amino acid (EAA), because glutamate binding onto postsynaptic

receptors typically stimulates, or depolarizes, the postsynaptic cells.

Histological Techniques Identify Glutamatergic NeuronsUsing immunocytochemical techniques, neurons containing glutamate are identified and

labeled with a glutamate antibody. In the retina, photoreceptors, bipolar cells, and ganglion

cells are glutamate immunoreactive (7-12) (Fig. 2). Some horizontal and/or amacrine cells can

also display weak labeling with glutamate antibodies (7,8,10,13). These neurons are believed to

release GABA, not glutamate, as their neurotransmitter (14), suggesting that the weak glutamate

labeling reflects the pool of metabolic glutamate used in the synthesis of GABA. This has been

supported by the results from double-labeling studies using antibodies to both GABA and

glutamate; glutamate-positive amacrine cells also label with the GABA antibodies (8,13).

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Photoreceptors, which contain glutamate, actively take up radiolabeled glutamate from the

extracellular space, as do Muller cells (Fig. 3) (15,16). Glutamate is incorporated into these cell

types through a high-affinity glutamate transporter located in the plasma membrane. Glutamate

transporters maintain the concentration of glutamate within the synaptic cleft at low levels,

preventing glutamate-induced cell death (17). Although Muller cells take up glutamate, they

do not label with glutamate antibodies (8). Glutamate incorporated into Muller cells is rapidly

broken down into glutamine, which is then exported from glial cells and incorporated into

surrounding neurons (18

). Neurons can then synthesize glutamate from glutamine (18

,19

).

Thus, histological techniques are used to identify potential glutamatergic neurons by labeling

neurons containing glutamate (through immunocytochemistry) and neurons that take up

glutamate (through autoradiography). To determine whether these cell types actually release

glutamate as their neurotransmitter, however, the receptors on postsynaptic cells have to be

examined.

Glutamate Receptors

Once released from the presynaptic terminal, glutamate diffuses across the cleft and binds onto

receptors located on the dendrites of the postsynaptic cell(s). Multiple glutamate receptor types

have been identified. Although glutamate will bind onto all glutamate receptors, each receptor

is characterized by its sensitivity to specific glutamate analogs and by the features of the

glutamate-elicited current. Glutamate receptor agonists and antagonists are structurally similar

to glutamate (Fig. 4), which allows them to bind onto glutamate receptors. These compounds

are highly specific and, even in intact tissue, can be used in very low concentrations because

they are poor substrates for glutamate uptake systems (20,21).

Two classes of glutamate receptors (Fig. 5) have been identified: 1) ionotropic glutamate

receptors, which directly gate ion channels; and 2) metabotropic glutamate receptors, which

may be coupled to an ion channel or other cellular functions via an intracellular second

messenger cascade. These receptor types are similar in that they both bind glutamate, and

glutamate binding can influence the permeability of ion channels. However, there are several

differences between the two classes.

Ionotropic Glutamate ReceptorsGlutamate binding onto an ionotropic receptor directly influences ion channel activity because

the receptor and the ion channel form one complex (Fig. 5a). These receptors mediate fast

synaptic transmission between neurons. Each ionotropic glutamate receptor, or iGluR, is

formed from the co-assembly of individual subunits. The assembled subunits may or may not

be homologous, with the different combinations of subunits resulting in channels with different

characteristics (22-26).

Two iGluR types (Fig. 6) have been identified: 1) NMDA receptors, which bind glutamate and

the glutamate analogN-methyl-D-aspartate (NMDA) and 2) non-NMDA receptors, which are

selectively agonized by kainate, AMPA, and quisqualate, but not NMDA.

Non-NMDA Receptors

Glutamate binding onto a non-NMDA receptor opens non-selective cation channels more

permeable to sodium (Na+) and potassium (K+) ions than calcium (Ca2+) (27). Glutamate

binding elicits a rapidly activating inward current at membrane potentials negative to 0 mV

and an outward current at potentials positive to 0 mV. Kainate, quisqualate, and AMPA (-

amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) are the specific agonists at these

receptors; CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), NBQX (1,2,3,4-tetrahydro-6-

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nitro-2,3-dione-benzo[f]quinoxaline-7-sulfonamide), and DNQX (6,7-dinitroquinoxaline-2,3-

dione) are the antagonists.

In retina, non-NMDA receptors have been identified on horizontal cells, OFF-bipolar cells,

amacrine cells, and ganglion cells (see below). Patch clamp recordings (28-32) indicate that

AMPA, quisqualate, and/or kainate application can evoke currents in these cells. However, the

kinetics of the ligand-gated currents differ. AMPA- and quisqualate-elicited currents rapidly

desensitize, whereas kainate-gated currents do not (Fig. 7a). The desensitization at AMPA/quisqualate receptors can be reduced (Fig. 7b) by adding cyclothiazide (33), which stabilizes

the receptor in an active (or non-desensitized) state (33,34).

Each non-NMDA receptor is formed from the co-assembly of several subunits (25,35,36). To

date, seven subunits (named GluR1 through GluR7) have been cloned (22,35-40). Expression

of subunit clones inXenopus oocytes revealed that GluR5, GluR6, and GluR7 (along with

subunits KA1 and KA2) co-assemble to form kainate(-preferring) receptors, whereas GluR1,

GluR2, GluR3, and GluR4 are assembled into AMPA(-preferring) receptors (25).

NMDA Receptors

Glutamate binding onto an NMDA receptor also opens non-selective cation channels, resulting

in a conductance increase. However, the high conductance channel associated with these

receptors is more permeable to Ca2+ than Na+ ions (27), and NMDA-gated currents typicallyhave slower kinetics than kainate- and AMPA-gated channels. As the name suggests, NMDA

is the selective agonist at these receptors. The compounds MK-801, AP-5 (2-amino-5-

phosphonopentanoic acid), and AP-7 (2-amino-7-phosphoheptanoic acid) are NMDA receptor

antagonists.

NMDA receptors are structurally complex, with separate binding sites for glutamate, glycine,

magnesium ions (Mg2+), zinc ions (Zn2+), and a polyamine recognition site (Fig. 6b). There

is also an antagonist binding site for PCP and MK-801 (41). The glutamate, glycine, and

magnesium binding sites are important for receptor activation and gating of the ion channel.

In contrast, the zinc and polyamine sites are not needed for receptor activation but affect the

efficacy of the channel. Zinc blocks the channel in a voltage-independent manner (42). The

polyamine site (43,44) binds compounds such as spermine or spermidine, either potentiating

(43,44) or inhibiting (44) the activity of the receptor, depending on the combination of subunitsforming each NMDA receptor (44).

To date, five subunits (NR1, NR2a, N2b, N2c, and N2d) of NMDA receptors have been cloned

(45-49). As with non-NMDA receptors, NMDA receptor subunits can co-assemble as

homomers (i.e., five NR1 subunits) (23,49) or heteromers (one NR1 + four NR2 subunits)

(23,46-48). However, all functional NMDA receptors express the NR1 subunit (23,25,46).

The glutamate, glycine, and Mg2+ binding sites confer both ligand-gated and voltage-gated

properties onto NMDA receptors. NMDA receptors are ligand gated because the binding of

glutamate (ligand) is required to activate the channel. In addition, micromolar concentrations

of glycine must also be present (Fig. 8) (50,51). The requirement for both glutamate and glycine

makes them co-agonists (51) at NMDA receptors.

Mg2+ ions provide a voltage-dependent block of NMDA-gated channels (52). This can be seen

in the current-voltage (I-V) relationship presented in Fig. 9 (from Nowak et al. (52)). I-V curves

plotted from currents recorded in the presence of Mg2+ have a characteristic J-shape (Fig. 9,

dotted line), whereas a linear relationship is calculated in Mg2+-free solutions (Fig. 9, solid

line). At negative membrane potentials, Mg2+ ions occupy the binding site, causing less current

to flow through the channel. As the membrane depolarizes, the Mg2+ block is removed (52).

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Retinal ganglion cells and some amacrine cell types express functional NMDA receptors in

addition to non-NMDA receptors (i.e., 29,53-57). The currents elicited through these different

iGluR types can be distinguished pharmacologically. Non-NMDA receptor antagonists block

a transient component of the ganglion cell light response, whereas NMDA receptor antagonists

block a more sustained component (29,53,57,58). These findings suggest that the currents elicited

through colocalized NMDA and non-NMDA receptors mediate differential contributions to

the ON- and OFF-light responses observed in ganglion cells (53).

Metabotropic Glutamate Receptors

Unlike ionotropic receptors, which are directly linked to an ion channel, metabotropic receptors

are coupled to their associated ion channel through a second messenger pathway. Ligand

(glutamate) binding activates a G-protein and initiates an intracellular cascade (59).

Metabotropic glutamate receptors (mGluRs) are not co-assembled from multiple subunits but

are one polypeptide (Fig. 5b). To date, eight mGluRs (mGluR1 through mGluR8) have been

cloned (60-66). These receptors are classified into three groups (I, II, and III) based on structural

homology, agonist selectivity, and their associated second messenger cascade (Table 1)

(reviewed in Nakanishi (67), Knopel et al. (68), Pin and Bockaert (69), and Pin and Duvoisin

(70)).

In brief, Group I mGluRs (mGluR1 and mGluR5) are coupled to the hydrolysis of fatty acids

and the release of calcium from internal stores. Quisqualate and trans-ACPD are Group I

agonists. Group II (mGluR2 and mGluR3) and Group III (mGluR4, mGluR6, mGluR7, and

mGluR8) receptors are considered inhibitory because they are coupled to the downregulation

of cyclic nucleotide synthesis (70). L-CCG-1 and trans-ACPD agonize Group II receptors; L-

AP4 (also called APB) selectively agonizes Group III receptors.In situ hybridization studies

have revealed that the mRNAs encoding Groups I, II, and III mGluRs are present in retina (see

below); however, with the exception of the APB receptor, the function of all of these receptor

types in retina has not been characterized.

APB Receptor

In contrast to non-NMDA and NMDA receptors, glutamate binding onto an APB receptor

elicits a conductance decrease (71-73) because of the closure of cGMP-gated, non-selective

cation channels (74) (Fig. 10).

APB application selectively blocks the ON-pathway in the retina (Fig. 11) (73), i.e., ON-bipolar

cell responses and the ON-responses in amacrine cells (75) and ganglion cells (29,76,77) are

eliminated by APB. Experimental evidence (73,78) suggests that the APB receptor is localized

to ON-bipolar cell dendrites. Inhibition of amacrine and ganglion cell light responses,

therefore, is due to a decrease in the input from ON-bipolar cells, not a direct effect on

postsynaptic receptors.

APB (2-amino-4-phosphobutyric acid, also called L-AP4) is the selective agonist for all Group

III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8). So, which is the APB receptor located

on ON-bipolar cell dendrites? MGluR4, mGluR7, and mGluR8 expression has been observed

in both the inner nuclear layer and the ganglion cell layer (61,79), suggesting that these mGluRs

are associated with more than one cell type. In contrast, mGluR6 expression has been localizedto the inner nuclearmlayer (INL) (64,79) and the outer plexiform layer (OPL) (80), where bipolar

cell somata and dendrites are located. Furthermore, ON-responses are abolished in mice lacking

mGluR6 expression (81). These mutants also display abnormal ERG b-waves, suggesting an

inhibition of the ON-retinal pathway at the level of bipolar cells (81). Taken together, these

findings suggest that the APB receptor on ON-bipolar cells is mGluR6.

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Glutamate Transporters and Transporter-like Receptors

Glutamate transporters have been identified on photoreceptors (15,21,82) and Muller cells

(15,16). From glutamate labeling studies, the average concentration of glutamate in

photoreceptors, bipolar cells, and ganglion cells is 5 mM (10). Physiological studies using

isolated cells indicate that only M levels of glutamate are required to activate glutamate

receptors (32,83,84). Thus, the amount of glutamate released into the synaptic cleft is several

orders of magnitude higher than the concentration required to activate most postsynapticreceptors. High-affinity glutamate transporters located on adjacent neurons and surrounding

glial cells rapidly remove glutamate from the synaptic cleft to prevent cell death (17). Five

glutamate transporters, EAAT-1 (or GLAST), EAAT-2 (or GLT-1), EAAT-3 (or EAAC-1),

EAAT-4, and EAAT-5, have been cloned (85-90).

Glutamate transporters are pharmacologically distinct from both iGluRs and mGluRs. L-

Glutamate, L-aspartate, and D-aspartate are substrates for the transporters (21,82,91); glutamate

receptor agonists (20,21,82,91) and antagonists (82,92) are not. Glutamate uptake can be blocked

by the transporter blockers dihydrokainate (DHKA) and DL-threo--hydroxyaspartate (HA)

(82,92).

Glutamate transporters incorporate glutamate into Muller cells along with the co-transport of

three Na+ ions (91,93) and the antiport of one K+ ion (93,94) and either one OH or one

HCO3- ion (94) (Fig. 12). The excess sodium ions generate a net positive inward current, which

drives the transporter (91,93). More recent findings indicate that a glutamate-elicited chloride

current is also associated with some transporters (85,95).

It should be noted that the glutamate transporters located in the plasma membrane of neuronal

and glial cells (discussed in this section) are different from the glutamate transporters located

on synaptic vesicles within presynaptic terminals (see General Overview of Synaptic

Transmission). The transporters in the plasma membrane transport glutamate in a Na+- and

voltage-dependent manner independent of chloride (17,91,93). L-Glutamate, L-aspartate, and D-

aspartate are substrates for these transporters (91). In contrast, the vesicular transporter

selectively concentrates glutamate into synaptic vesicles in a Na+-independent, ATP-dependent

manner (4-6) that requires chloride (4,6).

Glutamate receptors with transporter-like pharmacology have been described in photoreceptors

(96-98) and ON-bipolar cells (99,100). These receptors are coupled to a chloride current. The

pharmacology of these receptors is similar to that described for glutamate transporters, because

the glutamate-elicited current is: 1) dependent upon external Na+; 2) reduced by transporter

blockers; and 3) insensitive to glutamate agonists and antagonists. However, altering internal

Na+ concentration does not change the reversal potential (100) or the amplitude (96,99) of the

glutamate-elicited current, suggesting that the receptor is distinct from glutamate transporters.

At the photoreceptor terminals, the glutamate-elicited chloride current may regulate membrane

potential and subsequent voltage-gated channel activity (99). Postsynaptically, this receptor is

believed to mediate conductance changes underlying photoreceptor input to ON-cone bipolar

cells (99).

Localization of Glutamate Receptor Types in the RetinaPhotoreceptor, bipolar, and ganglion cells compose the vertical transduction pathway in the

retina. This pathway is modulated by lateral inputs from horizontal cells in the distal retina and

amacrine cells in the proximal retina (Fig. 13). As described in the previous sections,

photoreceptor, bipolar, and ganglion cells show glutamate immunoreactivity. Glutamate

responses have been electrically characterized in horizontal and bipolar cells, which are

postsynaptic to photoreceptors, and in amacrine and ganglion cells, which are postsynaptic to

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bipolar cells. Taken together, these results suggest that glutamate is the neurotransmitter

released by neurons in the vertical pathway. Recent in situ hybridization and

immunocytochemical studies have localized the expression of iGluR subunits, mGluRs, and

glutamate transporter proteins in the retina. These findings are summarized below.

Retinal Neurons Expressing Ionotropic Glutamate Receptors

In both higher and lower vertebrates, electrophysiological recording techniques have identified

ionotropic glutamate receptors on the neurons composing the OFF-pathway (Table 2). In the

distal retina, OFF-bipolar cells (Fig. 14) (84,101,102) and horizontal cells (Fig. 15) (32,103,104)

respond to kainate, AMPA, and quisqualate application, but not NMDA nor APB. (However,

NMDA receptors have been identified on catfish horizontal cells (105,106), and APB-induced

hyperpolarizations have been reported in some fish horizontal cells (107-109)).

Non-NMDA agonists also stimulate both amacrine cells (Fig. 16a) (28,54,55) and ganglion cells

(Fig. 16b) (29,31,53,57,58). Ganglion cells responses to NMDA have been observed

(29,53,55-57), whereas NMDA responses have been recorded in only some types of amacrine

cells (28,54,55) but see Hartveit and Veruki (110).

Consistent with this physiological data, antibodies to the different non-NMDA receptor

subunits differentially label all retinal layers (Table 3) (111-114), and mRNAs encoding the

different non-NMDA iGluR subunits are similarly expressed (115-117). In contrast, mRNAs

encoding NMDA subunits are expressed predominantly in the proximal retina, where amacrine

and ganglion cells are located (INL, IPL, GCL) (Table 3) (111,115), although mRNA encoding

the NR2a subunit (111) has been observed in the OPL and antibodies to the NR2d (118) and the

NR1 subunits (112) label rod bipolar cells.

Retinal Neurons Expressing Metabotropic Glutamate Receptors

All metabotropic glutamate receptors, except mGluR3, have been identified in retina either

through antibody staining (113,114,119,120) orin situ hybridization (61,64,79). MGluRs are

differentially expressed throughout the retina, specifically in the outer plexiform layer, inner

nuclear layer, inner plexiform layer, and the ganglion cell layer (Table 4). Although different

patterns of mGluR expression have been observed in the retina, only the APB receptor on ON-

bipolar cells has been physiologically examined.

Retinal Neurons Expressing Glutamate Transporters

The glutamate transporters GLAST, EAAC1, and GLT-1have been identified in retina (Table

5). GLAST (L-glutamate/L-aspartate transporter) immunoreactivity is found in all retinal layers

(121) but not in neuronal tissue. GLAST is localized to Muller cell membranes (121-124). In

contrast, EAAC-1 (excitatory amino acid carrier-1) antibodies do not label Muller cells or

photoreceptors. EAAC-1 immunoreactivity is observed in ganglion and amacrine cells in

chicken, rat, goldfish, and turtle retinas. In addition, bipolar cells positively labeled with

EAAC-1 antibody in lower vertebrates, and immunopositive horizontal cells were observed in

rat (90). GLT-1 (glutamate transporter-1) proteins have been identified in monkey (125), rat

(124), and rabbit (126) bipolar cells. In addition, a few amacrine cells were weakly labeled with

the GLT-1 antibody in rat (124), as were photoreceptor terminals in rabbit (126).

Summary and Conclusions

Histological analyses of presynaptic neurons and physiological recordings from postsynaptic

cells suggest that photoreceptor, bipolar, and ganglion cells release glutamate as their

neurotransmitter. Multiple glutamate receptor types are present in the retina. These receptors

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are pharmacologically distinct and differentially distributed. IGluRs directly gate ion channels

and mediate rapid synaptic transmission through either kainate/AMPA or NMDA receptors.

Glutamate binding onto iGluRs opens cation channels, depolarizing the postsynaptic cell

membrane. Neurons within the OFF-pathway (horizontal cells, OFF-bipolar cells, amacrine

cells, and ganglion cells) express functional iGluRs. mGluRs are coupled to G-proteins.

Glutamate binding onto mGluRs can have a variety of effects, depending on the second

messenger cascade to which the receptor is coupled. The APB receptor, found on ON-bipolar

cell dendrites, is coupled to the synthesis of cGMP. At these receptors, glutamate decreasescGMP formation, leading to the closure of ion channels. Glutamate transporters, found on glial

and photoreceptor cells, are also present at glutamatergic synapses (Fig. 17). Transporters

remove excess glutamate from the synaptic cleft to prevent neurotoxicity. Thus, postsynaptic

responses to glutamate are determined by the distribution of receptors and transporters at

glutamatergic synapses which, in retina, determine the conductance mechanisms underlying

visual information processing within the ON- and OFF-pathways.

Figure 1.Structure of the glutamate molecule.

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Figure 2.Glutamate immunoreactivity.

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Figure 3.Autoradiogram of glutamate uptake through glutamate transporters.

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Figure 4.Glutamate receptor agonists and antagonists.

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Figure 5.

Ionotropic and metabotropic glutamate receptors and channels. From Kandel et al. (127).

Figure 6.

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Comparison between NMDA and non-NMDA receptors. From Kandel et al. (127).

Figure 7.Whole-cell patch clamp to show quisqualate- and kainate-gated currents.

Figure 8.

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NMDA receptor activation.

Figure 9.

Mg2+ ions block NMDA receptor channels.

Figure 10.Whole-cell current traces to show kinetics of APB receptor-gated currents.

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Figure 11.Intracellular recordings to show that APB selectively antagonizes the ON-pathways.

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Figure 12.Glutamate transporters in Muller cells are electrogenic.

Figure 13.The types of neurons in the vertebrate retina.

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Figure 14.Whole-cell currents in OFF bipolar cells.

Figure 15.

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Whole-cell currents in horizontal cells.

Figure 16.Glutamate receptors on amacrine and ganglion cells.

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Figure 17.The ribbon glutamatergic synapse in the retina.

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Table 1

Metabotropic glutamate receptor groups (from Pin and Duvoisin (70)).

Group mGluR Agonist(s) Intracellular pathway

I mGluR1, mGluR5 quisqualate, ACPD Increase phospholipase C activity, increase cAMP levels, increase

protein kinase A activity

II mGluR2, mGluR3 L-CCG-1, ACPD Decrease cAMP levels

III mGluR4, mGluR6. mGluR7, mGluR8 L-AP4 (APB) Decrease cAMP or cGMP levels

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Table 2

Glutamate receptor types on retinal neurons, electrophysiological measurements

Retinal cell

type

Non-NMDA

receptor

NMDA

receptor

mGluR Glutamate

receptor with

transporter-

like

pharmacology

Species Reference

Photoreceptors ++ (cones) Salamander Eliasof & Werblin (82); Picaud et al (98).

++ (rods) Salamander Grant & Werblin (96)

OFF-bipolar

cells

++ Mudpuppy Slaughter & Miller (73,128)

++ Cat Sasaki & Kaneko (84)

++ Salamander Hensley et al. (58)

++ Rat Euler et al. (102)

++ Mudpuppy Slaughter & Miller (128)

ON-bipolar cells ++ ++

(APB)

Mudpuppy Slaughter & Miller (73,128)

++

(APB)

++ White perch Grant & Dowling (99,100)

++

(APB)

Salamander Hirano & MacLeish (129)

++ (L-

AP4)

Salamander Hensley et al. (58)

++

(AP-4)

Rat Euler et al. (101)

++ (APB

and

cGMP)

Salamander Nawy & Jahr (74)

++ (APB

and

cGMP)

Cat de la Villa et al. (130)

Horizontal cells ++ White perch Zhou et al. (32)

++ Mudpuppy Slaughter & Miller (128)

++ Salamander Yang & Wu (104)

++ ++ Catfish O'Dell & Christensen (106); Eliasof & Jahr

(105)

Amacrine cells ++ (AII) Rat Boos et al. (28)

++ ++ Mudpuppy Slaughter & Miller (128)

++ ++ Rabbit Massey & Miller (55

)

++ ++ Rat Harveit & Veruki (110)

++ (transient

& sustained

AC)

++

(transient

AC)

Salamander Dixon & Copenhagen (54)

Ganglion cells ++ ++ Salamander Diamond & Copenhagen (53); Mittman et al

(57); Hensley et al (58).

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Retinal cell

type

Non-NMDA

receptor

NMDA

receptor

mGluR Glutamate

receptor with

transporter-

like

pharmacology

Species Reference

++ ++ Primates Cohen & Miller (29)

++ ++ Rat Aizenman et al. (83)

++ ++ Mudpuppy Slaughter & Miller (128)

++ ++ Cat Cohen & Miller (29)

++ ++ Rabbit Massey & Miller (55,56)

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Table 3

Ionotropic glutamate receptor expression in retinal neurons and retinal layers, immunocytochemistry,

and in situ hybridization

Retinal cell type

or layer

Non-NMDA receptor subunits NMDA receptor subunits Species Reference

Photoreceptors GluR6/7 (single cone outer segments) Goldfish Peng et al. (113

)

GluR1 (cone pedicles) Cat Pourcho et al. (114)

OPL GluR2, GluR2/3, GluR6/7 Rat Peng et al. (113)

NR2A (punctate) Cat Harveit et al. (111)

GluR2, GluR2/3 (photoreceptors) Goldfish Peng et al. (113)

Bipolar cells GluR2 (Mb cells) Goldfish Peng et al. (113)

GluR2, GluR2/3 Rat Peng et al. (113)

NR2D (RBC) Rat Wenzel et al. (118)

GluR2 and/or GluR4 NR1 (RBC) Rat Hughes (112)

GluR2 (RBC) Rat Hughes et al. (117)

Horizontal cells GluR6/7 Goldfish Peng et al. (113)

GluR2/3 Cat Pourcho et al. (114)

INL GluR2/3, GluR6/7 Rat Peng et al. (113)

NR2A (inner) Rat Hartveit et al. (111)

GluR1, 2, 5 > GluR4 (outer third),

GluR1, 2, 5 (middle third), GluR1-5

(inner third)

Rat Hughes et al. (117)

GluR1-7 Rat, cat Hamassaki-Britto et al. (116)

KA2 (homogeneous), GluR6 (inner),

GluR7 (inner two-thirds)

NR1 (homogeneous), NR2A-

B (inner third, patchy), NR2C

(inner two-thirds)

Rat Brandstatter et al. (115)

IPL GluR1, GluR2/3, GluR6/7 Rat Peng et al. (113)

NR2A Rat, cat, rabbit,

monkey

Harveit et al. (111)

Amacrine cells GluR6 NR2A-C Rat Brandstatter et al. (115)

GluR2/3 Cat Pourcho et al. (114)

GluR1, GluR2/3 Rat Peng et al. (113)

Ganglion cells GluR1 Rat Peng et al. (113)

GCL GluR2/3, GluR6/7 Rat Peng et al. (113)

GluR1-5 Rat Hughes et al. (117

)

GluR1-7 Rat, cat Hamassaki-Britto et al. (115)

GluR6/7, KA2 NR1, NR2A-C Rat Brandstatter et al. (115)

Muller cells GluR4 Rat Peng et al. (113)

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Table 4

Metabotropic glutamate receptor expression in retinal neurons and retinal layers, immunocytochemistry,

and in situ hybridization

Retinal cell type or

layer

Group I Group II Group III Species Reference

OPL mGluR1alpha, mGluR5a (RBCdendrites)

Rat Koulen et al. (120

)

mGluR6 (RBC

dendrites)

Rat Nomura et al. (80)

INL mGluR8 Mouse Duvoisin et al. (61)

mGluR6 Rat Nakajima et al. (64)

mGluR5 (BC, HC), mGluR1 (AC) mGluR2 (AC) mGluR6 (RBC),

mGluR7 (BC),

mGluR4, 7 (AC)

Rat Hartveit et al. (79)

IPL mGluR1alpha Rat Peng et al. (113)

mGluR7 (CBC

terminals; AC

dendrites; few GC

dendrites)

Rat Brandstatter et al. (115)

mGluR1alpha, mGluR5a (AC

dendrites)

Rat Koulen et al. (120)

Amacrine cells mGluR1alpha Rat Peng et al. (113)

mGluR1alpha Cat Pourcho et al. (114)

Ganglion cells mGluR1alpha Rat Peng et al. (113)

GCL mGluR8 Mouse Duvoisin et al. (61)

mGluR1alpha mGluR2/3 Cat Pourcho et al. (114)

mGluR1 mGluR2 mGluR4, 7 Rat Hartveit et al. (79)

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Table 5

Glutamate transporters in retinal neurons and retinal layers, immunocytochemical localizations

Retinal cell type EAAC-1 GLAST GLT-1 Species Reference

Photoreceptors + (cone soma

to pedicles)

Rabbit Massey et al. (126)

OPL ++ Rat Rauen et al. (124)

++ (rod

spherules >

cone pedicles)

Rabbit Massey et al. (126)

Horizontal cells ++ Rat Schultz & Stell (90); Rauen et al (124).

Bipolar cells ++ (2 types of

CBCs)

Rabbit Massey et al. (126)

++ (faint) ++ Rat Rauen et al. (124)

++ Turtle, salamander Schultz & Stell (90)

++ (DB2, flat

midget bipolar

cells)

Monkey Grunert et al. (125)

IPL ++ (diffuse) Rabbit Massey et al. (126)

++ ++ Rat Rauen et al. (124)

++ Goldfish,

salamander, turtle,

chicken, rat

Schultz & Stell (90)

Amacrine cells ++ ++ Rat Rauen et al. (124)

++ Schultz & Stell (90)

Ganglion cells ++ Chicken, rat,

goldfish, turtle

Schultz & Stell (90)

++ Rat Rauen et al. (124)

Muller cells ++ Rat Rauen et al. (124); Lehre et al (123); Deroiche & Rauen

(122)

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