AMPA Receptor and Pain<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

姚允泰　龚志毅　黄宇光

中国医学科学院 中国协和医科大学 北京协和医院麻醉科， 北京 100730

Yun-tai Yao, Zhi-yi Gong, Yu-guang Huang

Department of Anesthesiology，Peking Union Medical College Hospital， Chinese Academy of Medical Sciences & Peking Union Medical College，Beijing 100730

ABSTRACT

The role of AMPA receptors in synaptic plasticity and development of central sensitization in the CNS is complex. The composition of AMPA receptors expressed at synapses in the pain pathway strongly influence the generation of hypersensitivity in welther inflammatory or neuropathic pain model. And pain states can change AMPA receptor composition. Alteration in the levels of AMPA receptor interacting proteins, as well as those of receptor subunits have been reported in chronic pain models. In addition, synaptic AMPA receptor function is dynamically regulated by activity-dependent delivery of synaptic AMPA receptors and via phosphorylation of existing receptors. It may be possible that selectively target AMPA receptor is involved in chronic pain states without altering the normal processing of acute nociceptive input.

Abbreviations: AMPA (alpha-amino- 3 -hydroxy- 5-methyl- 4- isoxazolepropionic acid); CNS (central nervous system); NMDA (N-methyl-D-aspartate); KA (kainite acid ); PAG (periaqueductal gray); RVM (rostral ventromedial medulla); PSD (postsynaptic density); GABA (gamma-aminobutyric acid); GRIP(glutamate receptor interacting protein; ABP (AMPA receptor binding protein) ; PICK1(protein interacting with C kinase 1); SAP97 (synapse-associated protein-97 kDa); NSF(N-ethylmaleimide-sensitive fusion protein ); PDZ (PSD-95/Dlg/ZO-1 domain); CaMKII (calcium/calmodulin dependent protein kinase); PKC(protein kinase C); PKA (protein kinase A).

Key words: AMPA receptor; Pain

Corresponding author: Department of Anesthesiology, PUMC Hospital, CAMS and PUMC, Beijing 100730, China; Tel: 010-85119116, E-mail: yuntaiyao@126.com.

Introduction<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Glutamate is the principal excitatory neurotransmitter in the CNS, involved in all kinds of CNS dysfunctions, ranging from stroke, epilepsy to pain (Miller, 1991; Barnes et al., 2003). Glutamate activates two types of receptors: ionotropic receptors(iGluRs) and metabotropic receptors(mGluRs), the former can be subdivided into 3 families according to their selective agonists as: NMDA, AMPA and KA receptors. AMPA receptor was initially believed simpler to understand as compared to NMDA receptor^[1]. Later, the four genes (GluR1, GluR2, GluR3 and GluR4, namely GluRA, GluRB, GluRC and GluRD) encoding subunits of the tetrameric receptor were identified. And it was known that absence of GluR2 unit could make the multisubunit receptor calcium-permeable. Replacement of glutamine by arginine at the mouth of ion pore, which is attributed to RNA editing, leads to exclusive calcium-impermeability property of GluR2 subunit^[2,3,4,5].

　　Structure of AMPA receptor

AMPA receptor subunits are synthesized by a route that involves alternative splicing of their mRNAs, giving rising to two versions of splicing variants named “flip” and “flop”^[6,7]. With the four receptor subunits and two splicing derivatives, there are a number of potential combinations of AMPA receptors possessing different pharmacological features and ion channel dynamics. Different types of neurons also have different subunit and splicing variant combinations: for example, native AMPA receptors in hippocampal neurons have been shown to be assembled from combination of mainly GluR1/GluR2 and GluR2/GluR3^[8]. Furthermore, AMPA receptor conformations reveal development or growth dependency, "flop" variant, GluR2 subunit and calcium-impermeable AMPA receptor are deemed “adult”^{[7,8,9 ]}.

AMPA receptor subunit consists of an extracellular N-terminal domain (NTD), an intracellular carboxy-terminal tail and 3 channel-forming transmembrane segments (M1, M3 and M4) ^[10,11]. M2 is a re-entrant loop connecting M1 and M3, contributing to specific ion permeability of receptor-channel complex^[12]. Extracellular S1 and S2 regions provide binding sites for glutamate, agonists and antagonists of AMPA receptor^[10,11]. Diversity of AMPA receptors, plus the complexed transmembrane domains, makes elucidation of the intact receptor structure challenging. Recently, X-ray diffraction and single-particle electron microscopy have been applied to the field^[11,13].

Calcium-permeable AMPA receptor

GluR2 subunit containing AMPA receptors are widely expressed, the overwhelming majority of AMPA receptors in the CNS has low calcium ion permeability (Monyer et al., 1991; Petralia et al., 1997). Only a subset of neurons, predominantly GABAergic interneurons and glia cells express AMPA receptors with high calcium permeability (Monyer et al., 1991; Geiger et al., 1995). AMPA receptor mediated calcium ion influx may play a role in triggering key developmental events^[9].

Calcium-permeable AMPA receptors function as key constituents of activity-induced sensitization in pain pathways^[14]. Activation of calcium-permeable AMPA receptors in spinal dorsal horn can strengthen synaptic transmission (Gu et al., 1996). Calcium permeability and ion conductance properties of synaptic AMPA receptors are not static, but modified dynamically by synaptic activity via rapid alterations in GluR1 and GluR2 subunits^[14]. Superficial spinal laminae express a mixture of calcium-permeable and impermeable AMPA receptors. GluR3 subunit also contributes to the formation of calcium-permeable AMPA receptors in the spinal laminae that process pain inputs^[14]. However, its significance is much less than GluR1 homer^[14]. The dorsal horn of the spinal cord shows a high density of calcium-permeable AMPA receptors, particularly in superficial spinal laminae where primary afferents carrying nociceptive inputs terminate and synapse on spinal second order neurons (Engelman et al., 1999).

Trafficking of AMPA receptor <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

GluRs are synthesized, folded and assembled in endoplasmic reticulum under rigorous quality controls to ensure that only correctly folded and assembled proteins exit to the Golgi for further decoration^[15]. Sheng and Lee presented a picture concerning the road map of AMPA receptor trafficking, which is composed of exocytosis, synaptic insertion/targeting, endocytosis and degradation/recycling. Every single step is under delicate regulation by different mechanisms. The GluR1 subunit governs the exocytosis of heteromeric GluR1/GluR2 into synapse, while GluR2 plays a dominant role in endocytosis. The different involvement of receptor subunits in exocytosis and endocytosis is determined by their intracellular C-terminals. The roles played by GluR3 and GluR4 subunits in AMPA receptor trafficking remain unknown. Different rules may be obeyed in different neuronal cell-types in receptor trafficking.

AMPA receptor turnover appears to be directly regulated by synaptic activity (Lissin et al., 1999; Ehlers et al., 2000). Changing the numbers of postsynaptic AMPA receptors, is a key mechanism of activity-dependent modulation of synaptic strength, a phenomenon thought to underlie aspects of learning, memory and chronic pain state^[15].

　　Expression of AMPA receptor

　　Distribution of AMPA receptors in adult nervous system have been extensively explored(Keinanen et al., 1990; Gallo et al., 1992; Lambolez et al., 1992; Bonet et al., 1994). In spinal cord: GluR1 subunit is preferentially associated with primary afferent terminals, most restricted in laminae I and II. GluR2 subunit is evenly expressed throughout all superficial laminae and abundant in motor nuclei. GluR3 and GluR4 subunits are highly expressed in ventral horn, with moderate levels in deep dorsal horn and limited expression in the superficial laminae (Keinanen et al., 1990; Craig et al.,1993; Bochet et al., 1994; Morrison et al., 1998; Spike et al., 1998; Engelman et al., 1999; Shibata et al., 1999).

Not only can AMPA receptor type influence the generation of pain states, but pain states can change AMPA receptor composition^[1]. Nociceptive input can also lead to changes of AMPA receptor in supraspinal centers that modulate nociception via descending pathways, such as the PAG and brainstem RVM.

AMPA receptor and central sensitization

AMPA receptor, as well as NMDA receptor, is involved in sensitized pain states. AMPA receptor mediates the first stage pain reaction of inflammation, and then sensitizes dorsal horn neurons. Intrathecal application of a number of AMPA receptor antagonists has been demonstrated to inhibit thermal hyperalgesia with less marked, but still clear effects on mechanical allodynia without affecting normal motor function. The fact that mechanical allodynia is resulted from co-activation of both AMPA and metabotropic receptors, can explain the relatively limited effects of AMPA receptor agents alone on mechanical allodynia. There was also seemingly contrary evidence supporting that spinal AMPA receptors play a major role in the initiation of secondary tactile allodynia induced by focal thermal injury. Calcium-permeable receptors in the dorsal horn are also presumed to be responsible for tactile allodynia after a noxious thermal stimulus or carrageenan-induced inflammation.

AMPA receptor interacting proteins and pain<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Postsynaptic AMPA receptors can interact with a host of structural and signaling proteins collectively referred to as the PSD. AMPA receptor interacting proteins fall into two main categories: those that bind AMPA receptor via PDZ domains such as GRIP, ABP, PICK1 and SAP97; and those that not via PDZ domains, such as NSF, stargazin and Narp. PDZ domains provide subunit-protein interaction sites, so AMPA receptor "gets handed" from one interacting protein to another in a sequential and probably hierarchical process, mediating receptors trafficking and directing kinases/phosphatases toward their substrates.

Alteration in the levels of receptor interacting proteins have been reported in chronic pain models, and blocking interactions between AMPA receptor subunits and interacting proteins has been demonstrated effective for neuropathic pain treatment.

AMPA receptor phosphorylation and pain

Membrane receptor phosphorylation is an important post-　translational mechanism underlying synaptic plasticity in neural systems in learning, memory and pain transmission. Phosphorylation is also involved in AMPA receptor trafficking and can alter ion channel profiles. AMPA receptor shows increased responsiveness through a phosphorylation step during central sensitization.

Serine 831 of GluR1 subunit can be phosphorylated by CaMKII or PKC, and serine 845 by PKA; tyrosine 876 of GluR2 subunit can be phosphorylated by Src family tyrosine kinases, and serine 880 by PKC. A rapid and prolonged upregulation of GluR1 serine 831 phosphorylation is associated with hyperalgesia. Primary noxious stimulation phosphorylate AMPA receptor subunits in descending pain modulation pathway, as well as ascending pain transmission pathway, and is correlated to an enhancement of AMPA-produced descending pain inhibition.

References

1. Torsney C, Macdermott AB. AMPA receptors bring on the pain. Neuron. 2004, 18;44(4):577-578.

2. Verdoorn TA, Burnashev N, Monyer H, et al. Structural determinants of ion flow through recombinant glutamate receptor channels.Science. 1991,21;252 (5013):1715-1718.

3. Burnashev N, Monyer H, Seeburg PH, et al. Divalent ion permeability of AMPA receptor channels is dominated by the edited form of a single subunit. Neuron. 1992, 8(1):189-198.

4. Hume RI, Dingledine R, Heinemann SF. Identification of a site in glutamate receptor subunits that controls calcium permeability. Science. 1991,30;253(5023): 1028-1031.

5. Sommer B, Kohler M, Sprengel R, et al. RNA editing in brain controls a determinant of ion flow in glutamate-gated channels.Cell. 1991, 4;67(1):11-19.

6. Sommer B, Keinanen K, Verdoorn TA,et al.  Flip and flop: a cell-specific functional switch in glutamate-operated channels of the CNS.Science. 1990, 28;249(4976):1580-1585.

7. Monyer H, Seeburg PH, Wisden W. Glutamate-operated channels: develop mentally early and mature forms arise by alternative splicing.Neuron. 1991,6 (5):799-810.

8. Wenthold RJ, Petralia RS, Blahos J II, et al. Evidence for multiple AMPA receptor complexes in hippocampal CA1/CA2 neurons.J Neurosci. 1996,15;16 (6):1982-1989.

9. Pellegrini-Giampietro DE, Bennett MV, Zukin RS. Are Ca(2+)-permeable kainate/AMPA receptors more abundant in immature brain? Neurosci Lett. 1992,14;144(1-2):65-69.

10. Bennett JA, Dingledine R. Topology profile for a glutamate receptor: three transmembrane domains and a channel-lining reentrant membrane loop.Neuron. 1995, 14(2):373-384.

11. Nakagawa T, Cheng Y, Ramm E, et al. Structure and different conformational states of native AMPA receptor complexes. Nature. 2005, 3;433(7025):545-549.

12. Wollmuth LP, Sobolevsky AI. Structure and gating of the glutamate receptor ion channel.Trends Neurosci. 2004, 27(6):321-328.

13. Gouaux E. Structure and function of AMPA receptors.J Physiol. 2004,15;554 (Pt 2):249-253.

14. Hartmann B, Ahmadi S, Heppenstall PA,et al. The AMPA receptor subunits GluR-A and GluR-B reciprocally modulate spinal synaptic plasticity and in flammatory pain. Neuron. 2004, 18;44(4):637-650. 

15. Vandenberghe W, Bredt DS. Early events in glutamate receptor trafficking. Curr Opin Cell Biol. 2004,16(2):134-139.