Glutamate Receptor Ligands

16 May

V. Neugebauer
Department of Neuroscience and Cell Biology, The University of Texas Medical Branch,
301 University Blvd., Galveston TX, 77555-1069, USA

Abstract Glutamate acts through a variety of receptors to modulate neurotransmission and neuronal excitability. Glutamate plays a critical role in neuroplasticity as well as in nervous system dysfunctions and disorders. Hyperfunction or dysfunction of glutamatergic neuro- transmission also represents a key mechanism of pain-related plastic changes in the central and peripheral  nervous  system. This chapter  will review the classification of glutamate receptors and their role in peripheral  and central nociceptive processing. Evidence from preclinical pain models and clinical studies for the therapeutic  value of certain glutamate receptor ligands will be discussed.

Keywords  Ionotropic glutamate receptors · NMDA and non-NMDA receptor antagonists · Metabotropic glutamate receptors · Neurotransmission · Nociception · Plasticity · Analgesia · Preclinical · Clinical

Glutamate Receptors

Glutamate is the major excitatory amino acid in the mammalian nervous sys- tem. A highly flexible molecule, glutamate binds to a number of diverse families of receptors and transporters. These include the ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels, and the metabotropic glutamate receptors (mGluRs), which couple through different G proteins to a variety of signal transduction pathways. Vesicular transporters (vGluT1 and 2) pack- age glutamate into vesicles for synaptic exocytosis whereas plasma membrane glutamate transporters (excitatory amino acid transporters EAAT1–5) remove glutamate from the synaptic space. Glutamate receptor classification, struc- ture, and function  have been reviewed. This chapter  will focus on the role of iGluRs and mGluRs in nociception and pain (for reviews of glutamate re- ceptor classification, structure, and function see Michaelis 1998; Anwyl 1999; Dingledine et al. 1999; Schoepp et al. 1999; Lerma et al. 2001; Wollmuth and Sobolevsky 2004; Mayer and Armstrong 2004; Swanson et al. 2005; Mayer 2005).

Ionotropic Glutamate Receptors

The iGluRs allow the flow of cations through the channel in the center of the re- ceptor complex upon binding of an extracellular ligand (glutamate). Typically, this results in the depolarization of the plasma membrane and the generation of an electrical signal, the action potential, that propagates along the axon and triggers the release of transmitter(s) from the synaptic terminal. The iGluRs are tetrameric complexes of subunits transcribed from separate genes (Michaelis

1998; Dingledine et al. 1999; Wollmuth and Sobolevsky 2004; Mayer and Arm- strong 2004; Mayer 2005). Each subunit has four hydrophobic domains but only three transmembrane segments because the second domain forms a pore loop at the inner opening of the ion channel and does not go through the membrane. Thus, different from other ionotropic receptors, iGluRs have an extracellular N-terminal and an intracellular C-terminal domain. The C-terminus contains

sites for phosphorylation and protein–protein interaction  and is subject to posttranscriptional processes such as RNA editing and alternative  splicing. Phosphorylation  of iGluRs by various kinases (including PKA, PKC, CaM ki- nase II, tyrosine kinase) can increase the ion channel function. Interaction with intracellular scaffolding and cytoskeletal proteins may be important for receptor trafficking, anchoring, and signaling.

The iGluRs are subdivided into three major groups based on their phar- macology, structural similarities, and sequence homology (Table 1; Michaelis

1998; Dingledine et al. 1999; Lerma et al. 2001; Wollmuth  and Sobolevsky

2004; Mayer and Armstrong 2004; Mayer 2005). Each group includes different subunits that are encoded by different genes: N -methyl-d-aspartate (NMDA) receptors (NR1, NR2A-D, NR3A, and NR3B subunits); α-amino-3-hydroxy-5- methyl-4-isoxazolepropionic acid (AMPA) receptors (GluR1–4); and kainate receptors (GluR5–7, KA1 and KA2). Two orphan receptor subtypes (δ1 and δ2) also have been cloned, which share sequence homology with other iGluRs, but their ligand-binding  and functional properties  remain to be determined.

AMPA and kainate receptors are also referred to as non-NMDA receptors.


NMDA Receptors

Functional NMDA receptors are obligate heteromeric assemblies of NR1 sub- units  with NR2A-D or,  less commonly,  with NR3A,B subunits  (Michaelis

1998; Dingledine  et al. 1999; Wollmuth  and  Sobolevsky 2004; Mayer and

Armstrong  2004; Mayer 2005). NMDA receptors  have binding sites for glu- tamate (formed by the NR2 subunits) and the co-agonist glycine (strychnine- insensitive glycineB site formed by NR1) as well as binding sites for polyamines (NR2B) and phencyclidine (PCP site inside the ion channel), which can modu- late NMDA receptor function. Ion currents through NR2B-containing NMDA receptor  heteromers  have a much slower decay time (i.e., longer duration) compared to NR2A-containing receptors. NMDA receptors are unique in that

occupation  of the glycine binding site and removal of the Mg2+ blockade of the ion channel are required for full activation. Furthermore, NMDA receptors

are tonically inhibited by protons, and Zn2+ (NR2A>NR2B) enhances whereas polyamines (NR2B) relieve proton  inhibition.  Compared  to non-NMDA re-

ceptors, NMDA receptors respond more slowly to glutamate due to the tonic and voltage-dependent  inhibition by Mg2+; they participate  in slow synaptic transmission,  desensitize only weakly, and have a high permeability to Ca2+ (tenfold greater than to Na+).

NMDA is the diagnostic ligand for these receptors. d-AP5 (d-2-amino-5- phosphonopentanoic acid) is the most commonly used competitive antagonist at the glutamate binding site. Noncompetitive antagonists that bind to the PCP site in the NMDA receptor channel include the high-affinity compound  MK-

801 (dizocilpine; 10,11-dihydro-5-methyldibenzocyclohepten-5,10-imine) and

Glutamate Receptor Ligands

AC, adenylylcyclase; AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; AP5, d-2-amino-

5-phosphonopentanoic acid; APDC, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate; CHPG, (RS)-2- chloro-5-hydroxyphenylglycine; CNQX, cyano-7-nitroquinoxaline-2,3-dione; CP-101606 (traxoprodil; (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol);  CPCCOEt, 7-(hydroxy- imino)  cyclopropa[b]chromen-1a-carboxylate  ethyl  ester;  DHPG, (S)-3,5-dihydroxyphenylglycine; DXM, dextromethorphan; EGLU, (2S)-alpha-ethylglutamic acid; GIRK, G protein-activated inwardly rectifying potassium  channels;  LAP4, L-(+)-2-amino-4-phosphonobutyric acid; LSOP, l-serine-O- phosphate;  LY293558, (3S,4aR,6R, 8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]   decahydroisoquinoline-3- carboxylic acid; LY341495, 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid; LY354740,  [1S, 2S,5R,6S]-2-aminobicyclo[3.1.0] hexane-2,6-dicarboxylic  acid; LY367385,  (S)-

(+)-α-amino-4-carboxy-2-methylbenzeneacetic  acid;  LY379268, (−)-2-oxa-4-aminobicylco hexane-

4,6-dicarboxylic acid; LY382884, (3S, 4aR, 6S, 8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a- decahydroisoquinoline-3-carboxylic  acid;  MPEP, 2-methyl-6-(phenylethynyl)pyridine;  NBQX, 2,3- dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f ]quinoxaline-7-sulphonamide;  NMDA, N-methyl-d-aspartic

acid;    Ro    25–6981,   (R-R*,S*)-alpha-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidine

propanol;  UBP1112, α-methyl-3-methyl-4-phosphonophenylglycine;  UBP302, (S)-1-(2-amino-2- carboxyethyl)-3-(2-carboxybenzyl)pyrimidine-2,4-dione; VGCC, voltage-gated  calcium channel Numbers  in parentheses  refer to receptor  subtype/subunit selectivity *Denotes drugs  tested/used

clinically +Also binds to other classes of receptors (α1, 5HT1α, and σ)

clinically used agents such as ketamine, memantine,  and dextromethorphan with its main metabolite dextrorphan. Noncompetitive  antagonists  selective for NR2B subunit-containing NMDA receptors include CP-101606 [traxoprodil; (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol)

and Ro 25–6981 (R-R*,S*)-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-

1-piperidine  propanol]  and  their  parent  compound  ifenprodil,  which also

binds to other receptors besides glutamate receptors (Table 1; Michaelis 1998; Dingledine et al. 1999; Parsons 2001; Wollmuth and Sobolevsky 2004; Mayer



Non-NMDA Receptors

AMPA and kainate receptors can be homo- or heteromeric tetramers.  AMPA

receptors (Michaelis 1998; Dingledine et al. 1999; Wollmuth and Sobolevsky

2004; Mayer 2005) are composed of subunits GluR1–4, which are encoded by separate genes. All AMPA receptor subunits  exist as two splice variants, flip and flop, which yield altered  kinetics such that  the flop forms show more rapid desensitization.  Each subunit  has a ligand binding site made up from the N-terminal  region; the C-terminus  plays a role in receptor  trafficking. Different from NMDA receptors, native AMPA receptors mediate fast synaptic transmission,  have fast gating kinetics, desensitize rapidly, and show a lower

permeability to Ca2+. Ca2+ permeability of AMPA receptors is determined by the GluR2 subunit,  which in its native form is impermeable  to Ca2+ due to posttranscriptional RNA editing that changes a single amino acid in the pore-

forming second membrane domain from glutamine (Q) to arginine (R). Most neurons express GluR2 in combination with one or more of the GluR1, 3, and

4 subunits, yielding AMPA receptors with low Ca2+ permeability.

Kainate receptors (Michaelis 1998; Dingledine et al. 1999; Chittajallu et al.

1999; Lerma 2003; Wollmuth and Sobolevsky 2004; Mayer 2005) are tetrameric assemblies of low-affinity GluR5–7 subunits and high-affinity KA1 and 2 sub- units. Homomeric expression of KA1 and KA2 does not form functional ion channels, but KA1 and KA2 contribute  to heteromeric  receptor  complexes when expressed with the other subunits. GluR5–7 can form homomeric and heteromeric  kainate  receptors.  Like the other  iGluRs, kainate  receptor  sub- units undergo alternate splicing and RNA editing, yielding a number of phar- macologically and functionally distinct receptors. Similar to AMPA receptors, kainate receptors with Q to R editing in the GluR5 or GluR6 subunits are im-

permeable to Ca2+; receptors with unedited subunits are calcium permeable. Interestingly, some actions of kainate may involve the interaction of ionotropic kainate receptors with a G protein. Kainate receptors contribute to postsynaptic excitatory responses and plasticity; presynaptic kainate receptors can increase

or decrease the release of glutamate and inhibit the release of the inhibitory transmitter γ-aminobutyric acid (GABA) (resulting in disinhibition).

The pharmacological  differentiation  of AMPA and kainate  receptors  has been difficult (see Table 1; Michaelis 1998; Dingledine et al. 1999; Chittajallu et al. 1999; Wollmuth  and Sobolevsky 2004). Naturally, AMPA and kainate show some selectivity for AMPA- and kainate-preferring receptors,  respec- tively. ATPA [(RS)-2-amino-3-(3-hydroxy-5-tert-butyl-4-isoxazolyl) propionic acid] is a more selective kainate receptor agonist, particularly for GluR5. The most commonly used non-NMDA receptor antagonists  are CNQX (cyano-7- nitroquinoxaline-2,3-dione) and NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydro- benzo[f ]quinoxaline-7-sulfonamide), which have limited selectivity for AMPA over  kainate  receptor  subunits.  GYKI53655, a 2,3-benzodiazepine,  is cur- rently the most selective AMPA receptor antagonist (non-competitive allosteric modulator).  The competitive  antagonist  LY293558  [(3S,4aR,6R, 8aR)-6-[2- (1(2)H-tetrazole-5-yl)ethyl] decahydroisoquinoline-3-carboxylic acid], which has been tested in phase II clinical trials, shows higher selectivity for AMPA re- ceptors over GluR6 and GluR7 kainate receptors, but it is also active at the GluR5 subunit (Sang et al. 2004). Antagonists that distinguish kainate from AMPA re- ceptors have only recently become available. LY382884 [(3S, 4aR, 6S, 8aR)-6-(4- carboxyphenyl)methyl-1, 2, 3, 4, 4a, 5, 6, 7, 8, 8a-decahydroisoquinoline-3-carb- oxylic acid] and UBP302 [(S)-1-(2-Amino-2-carboxyethyl)-3-(2-carboxyben- zyl)pyrimidine-2,4-dione]  are GluR5-selective antagonists  at concentrations that do not affect AMPA receptors (Bortolotto et al. 2003; More et al. 2004).

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