Email updates

Keep up to date with the latest news and content from Molecular Pain and BioMed Central.

Open Access Short report

Inhibitory role of supraspinal P2X3/P2X2/3 subtypes on nociception in rats

Masato Fukui1, Takayuki Nakagawa1*, Masabumi Minami2, Masamichi Satoh3 and Shuji Kaneko1

Author Affiliations

1 Department of Molecular Pharmacology, Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto 606-8501, Japan

2 Department of Pharmacology, Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan

3 Yasuda Women's University, Hiroshima 731-0153, Japan

For all author emails, please log on.

Molecular Pain 2006, 2:19  doi:10.1186/1744-8069-2-19

The electronic version of this article is the complete one and can be found online at: http://www.molecularpain.com/content/2/1/19


Received:28 April 2006
Accepted:5 June 2006
Published:5 June 2006

© 2006 Fukui et al; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Extracellular ATP is known to mediate synaptic transmission as a neurotransmitter or a neuromodulator via ionotropic P2X and metabotropic P2Y receptors. Several lines of evidence have suggested that ATP facilitates pain transmission at peripheral and spinal sites via the P2X receptors, in which the P2X3 subtype is considered as an important candidate for the effect. Conversely, we previously found that the activation of supraspinal P2X receptors evoked antinociception. However, the subtypes responsible for the antinociception via supraspinal P2X receptors remain unclear. In the present study, we showed that intracerebroventricular (i.c.v.) pretreatment with A-317491 (1 nmol), the novel non-nucleotide antagonist selective for P2X3 and P2X2/3 receptors, attenuated the antinociceptive effect produced by i.c.v. administered α,β-methylene-ATP (10 nmol), the P2X receptor agonist, in rats. Similarly, the abolishment of the P2X3 receptor mRNA in the brainstem by repeated i.c.v. pretreatments with antisense oligodeoxynucleotide for P2X3 gene once a day for 5 consecutive days diminished the antinociceptive effect of α,β-methylene-ATP. Furthermore, i.c.v. administration of A-317491 (1 and 10 nmol) significantly

    enhanced
the inflammatory nociceptive behaviors induced by the intraplantar injection of formalin and intraperitoneal injection of acetic acid. Taken together, these results suggest that supraspinal P2X3/P2X2/3 receptors play an inhibitory role in pain transmission.

Findings

Extracellular ATP has been established as a signaling molecule that mediates diverse biological effects via P2 purinoceptors (P2Xn and P2Yn) in both the peripheral and central nervous systems [1,2]. A body of evidence indicates that ionotropic P2X receptors are involved in both peripheral and spinal pain transmission [3-5]. Of the seven P2X receptors identified to date, the expression of the P2X3 receptors appear selective for a subpopulation of small-diameter dorsal root ganglion neurons, which are probably associated with nociception [6-8]. In vivo studies have provided evidence that the activation of P2X receptors, especially the P2X3 subtype and P2X2/3 heteromer, contributes to an acute nociceptive behavior, hyperalgesia and allodynia [9-12]. These observations support the idea that the P2X3 and P2X2/3 receptors play a crucial role in facilitating pain transmission at the peripheral and spinal sites. On the other hand, at the supraspinal level, we previously reported that the intracerebroventricular (i.c.v.) administration of ATP and P2X receptor agonists produced mechanical and thermal antinociception in rats, suggesting that supraspinal P2X receptors play an inhibitory role in nociceptive transmission [13-15]. However, it remains unclear what subtypes of supraspinal P2X receptors are involved in the antinociceptive effect. In the present study, to clarify the involvement of P2X3/P2X2/3 receptors, we investigated the effect of i.c.v. pretreatment with A-317491, the novel non-nucleotide antagonist selective for P2X3/P2X2/3 receptors [16,17] and down-regulation of P2X3 receptors in the brain by antisense oligodeoxynucleotide (A-ODN) in rats. Furthermore, to determine the roles of

    the endogenous purinergic system through supraspinal P2X3/P2X2/3 receptors in the exacerbated pain states
, we examined the effect of A-317491 administered i.c.v. on the inflammatory nociceptive behaviors induced by chemical agents.

Male Sprague-Dawley rats weighing 180–250 g were used. The experiments were conducted in accordance with the ethical guidelines of the Kyoto University animal experimentation committee, and the guidelines of the Japanese Pharmacological Society. The surgery for i.c.v. administration was conducted according to our previous report [13]. After surgery, the animals were returned to cages and housed individually, being allowed to recover for 5 to 7 days until the experiments started. The i.c.v. injection was carried out in a volume of 5 μl at a constant rate of 5 μl/30 sec.

The 20-base A-ODN endcapped with phosphorothioate linkages (Nisshinbo Inc, Japan) were designed according to the primary sequence of the rat P2X3 receptor cDNA (X91167) as previously described [9]. The sequence of A-ODN was 5'-T*G*AA*GAAGTCTGATATAC*A*G-3', while the sequence of sense oligodeoxynucleotide (S-ODN) served as a control was 5'-C*T*GTATATCAGACTTCTT*C*A-3' (* as phosphorothioate). A-ODN and S-ODN were dissolved in PBS. I.c.v. injection of PBS (5 μl), A-ODN (1 nmol/5 μl), or S-ODN (1 nmol/5 μl) was carried out once a day for 5 consecutive days, and the behavioral assessment was conducted on next day of the last injection. After the behavioral test, the brain was removed, and the brainstem sample was dissected. Then, the mRNA levels for P2X3 receptor and β-actin in the brainstem were analyzed using an RT-PCR method.

The antinociceptive effect of α,β-methylene-ATP administered i.c.v. was measured by the paw pressure test, as previously described [13,14]. Briefly, using an analgesimeter (Ugo Basile, Milan, Italy), the cuneate piston was put on the right hind paw and the pressure was loaded at a rate of 32 g/sec. The pressure that elicited paw withdrawal behavior, was determined as the nociceptive threshold. After several habituation procedures, α,β-methylene-ATP (Sigma, St. Louis, MO) dissolved in PBS was administered i.c.v. at a dose of 10 nmol, and the nociceptive threshold was measured 5 min after the i.c.v. administration according to our previous reports [13,14]. Nociceptive behaviors evoked by inflammatory stimuli using formalin and acetic acid were assessed. For the measurement of formalin-induced nociceptive behavior, scoring of nociceptive behavior started immediately after the intraplantar (i.pl.) injection of 2% formalin at a volume of 100 μl into the right hindpaw. The nociceptive behaviors were observed for 60 min and were quantified using a rating scale method by assigning weights to the following categories: the injected paw was elevated, and was not in contact with the floor (weight = 1); and the injected paw was licked, bitten, or shaken (weight = 2). The time an animal spent in each behavioral category was multiplied by the category weight (1 or 2), summed, and then divided by the total test session period (300 sec). For the measurement of acetic acid-induced nociceptive writhing behavior, counting the number of typical writhing behaviors, characterized by a wave of contraction of the abdominal musculature followed by extension of the hind limbs, started immediately after the intraperitoneal (i.p.) injection of 2% acetic acid at a volume of 1 ml. The writhing behavior was observed for 60 min and the number of occurrences every 5 min was evaluated.

The statistical significance was calculated using one-way or two-way analysis of variance (ANOVA), followed by the Bonferroni post hoc test. Differences at P < 0.05 were considered significant.

The effect of i.c.v. pretreatment with A-317491 (Sigma) on the antinociceptive effect produced by i.c.v. administered α,β-methylene-ATP was examined (Fig. 1A). I.c.v. administration of α,β-methylene-ATP (10 nmol) at 15 min after i.c.v. pretreatment with PBS produced a significant antinociceptive effect, compared with the PBS-administered group (P < 0.001). The antinociceptive effect was dose-dependently and significantly reduced by i.c.v. pretreatment with A-317491 (F5,29 = 9.43, P < 0.001). The significant inhibitory effect of A-317491 was observed at a dose of 1 nmol, compared with PBS-pretreated group (P < 0.05). A-317491 (1 nmol) itself had no effect on the nociceptive threshold.

thumbnailFigure 1. The involvement of supraspinal P2X3/P2X2/3 receptors in the antinociception produced by i.c.v. administration of α,β-methylene-ATP. The nociceptive thresholds at 5 min after the i.c.v. administration of α,β-methylene-ATP (10 nmol) were compared in the paw pressure test. The nociceptive threshold of each animal before the i.c.v. administration served as the control value (100%), and the values are presented as the means of the % of the control ± S.E.M. (A) The effect of i.c.v. pretreatment with A-317491. α,β-Methylene-ATP or PBS was administered i.c.v. 15 min after i.c.v. pretreatment with A-317491 (0.01–1 nmol) or PBS. ***P < 0.001 vs PBS-PBS group, #P < 0.05 vs PBS-α,β-methylene-ATP group (n = 5–8). (B) RT-PCR analyses of the mRNA expression levels for P2X3 receptor (upper) and β-actin (bottom) in the brainstem after repeated i.c.v. pretreatments with PBS, P2X3 A-ODN (1 nmol), and P2X3 S-ODN (1 nmol). (C) The effects of repeated i.c.v. pretreatments with PBS, P2X3 A-ODN and P2X3 S-ODN on the antinociception by i.c.v. administered α,β-methylene-ATP. *P < 0.05 (n = 5–6).

    In the rat i.c.v. pretreated
repeatedly with PBS and P2X3 S-ODN, the expression of P2X3 receptor mRNA in the brainstem was observed, while it was diminished by the repeated i.c.v. pretreatments with P2X3 A-ODN (Fig. 1B). In the rat pretreated with PBS and P2X3 S-ODN, i.c.v. administration of α,β-methylene-ATP (10 nmol) produced an antinociceptive effect, and there was no difference between them. The antinociceptive effect was significantly reduced by the repeated i.c.v. pretreatments with P2X3 A-ODN (F2,14 = 6.61, P < 0.01), compared with the PBS-pretreated (P < 0.05) and P2X3 S-ODN-pretreated (P < 0.05) groups (Fig. 1C).

To clarify the roles of

    supraspinal P2X3/P2X2/3 receptors
in inflammatory pain, we examined the effect of the i.c.v. administration of A-317491 on inflammatory nociceptive behaviors induced by i.pl. formalin and i.p. acetic acid (Fig. 2). The i.pl. injection of formalin produced characteristic biphasic nociceptive behaviors. The i.c.v. administration of A-317491 (1 and 10 nmol) 5 min before the formalin injection significantly increased the nociceptive behaviors (F2,168 = 6.07, P < 0.01). Similarly, the number of acetic acid-induced writhing behaviors was significantly increased by the i.c.v. administration of A-317491 (1 and 10 nmol) 5 min before the acetic acid injection (F2,168 = 6.73, P < 0.01).

thumbnailFigure 2. The effect of i.c.v. administration of A-317491 on the inflammatory nociceptive behaviors. (A) For the measurement of formalin-induced nociceptive behavior, the scoring of nociceptive behavior started immediately after the i.pl. injection of formalin into the right hindpaw. The nociceptive behaviors were observed for 60 min and were quantified using a rating scale method by assigning weights for each 5 min. (B) For the measurement of acetic acid-induced nociceptive writhing behavior, counting the number of typical writhing behaviors started immediately after the i.p. injection of acetic acid. The writhing behavior was observed for 60 min and the number of occurrences every 5 min was evaluated. The i.c.v. administration of A-317491 (1 and 10 nmol) or PBS was performed 5 min before the i.pl. injection of formalin or i.p. injection of acetic acid. The values are presented as the means ± S.E.M. *P < 0.05, ***P < 0.05 vs PBS group (n = 5–6).

The present study provides evidence that

    P2X3 and/or P2X2/3 receptors
are responsible for supraspinal P2X receptor-mediated antinociception. A-317491, a selective antagonist for P2X3 and P2X2/3 receptors, produces antinociception at peripheral and spinal sites
    in rat models of inflammatory and neuropathic pain
[16-19]. Conversely, we found that the acute i.c.v. injection of A-317491 blocked the antinociceptive effect of α,β-methylene-ATP,
    which is selective for P2X1, P2X3 and P2X2/3 receptors
, suggesting that the activation of supraspinal P2X3/P2X2/3 receptors exerted the antinociceptive effect [16,17]. The P2X3 subtype expressed in the brain is primarily distributed throughout the rat hindbrain, including the nucleus of the solitary tract, medial vestibular nucleus, medial and lateral parabrachial nuclei, rostral ventrolateral medulla and locus coeruleus (LC) [20,21], while the P2X2 subtype is widely distributed in the brain, including the hindbrain [22]. Therefore, it is assumed that the decreased effect of α,β-methylene-ATP-induced antinociception by repeated i.c.v. pretreatments with P2X3 A-ODN is attributed to the decreased expression of P2X3 receptors in the hindbrain.
    In the the nucleus of the solitary tract, the ultrastructural study showed that P2X3-immunoreactive boutons synapsed on dendrites and cell bodies, and had complex synaptic interaction with other axon terminals and vesiculated dendrites
[20]. It is also described that α,β-methylene-ATP excites the neurons arising from the hindbrain, such as the LC [23,24]. We previously found that the bilateral microinjection of α,β-methylene-ATP into the LC produced robust antinociception, which was inhibited
    by co-injection with the P2X receptor antagonist PPADS
[15], suggesting that the decreased level of P2X3 receptors in the LC by P2X3 A-ODN might have resulted in the blockade of α,β-methylene-ATP-induced antinociception.

There are several lines of evidence that endogenous ATP appears to play a facilitatory role in pain transmission at the peripheral and spinal levels, which is consistent with the effect of the exogenous injection of ATP and P2X receptor agonists. For instance, it has been reported that the intrathecal injection of PPADS attenuated the nociceptive behaviors induced by the i.pl. injection of formalin and capsaicin [25]. The intrathecal injection of P2X3 A-ODN also attenuated the hyperalgesia and allodynia after the nerve injury [9-11], however, the role of endogenous ATP in a supraspinal pain transmission has been unclear. Our present study found that i.c.v. administration of A-317491 deteriorated the inflammatory pain induced by formalin and acetic acid, suggesting that the endogenous ATP also plays an inhibitory role in pain transmission through P2X3/P2X2/3 receptors at the supraspinal level. The effect of A-317491 was rapidly normalized to the controlled level in both formalin- and acetic acid-induced inflammatory nociceptive behaviors. It is consistent with our previous findings that the antinociceptive effect by i.c.v. administered α,β-methylene-ATP was rapid and short lasting. The rapid effect of A-317491 is not probably due to the enzymatic degradation, as it is reported that A-317491 induces a long-lasting effect following systemic injection [16]. This might have resulted from the characteristic of the P2X3 receptor, which is known to be rapidly desensitized following its activation. Nevertheless, the i.c.v. injection of A-317491 did not affect the baseline nociceptive threshold in the paw pressure test (see Fig. 1A).

    These findings support the possibility that the inflammatory pain, but not acute pain, might stimulate the release of endogenous ATP into the extracellular space at the supraspinal level, as well as the spinal level
[18,26,27].

Taken together, our findings suggest that the activation of supraspinal P2X3/P2X2/3 receptors produces antinociception. Since the effect at the supraspinal site is opposed to that at peripheral and spinal sites, further study might be needed to understand their role at the supraspinal site to develop potential analgesics via these receptors. Moreover, our results indicate that endogenous ATP at the supraspinal site plays an inhibitory role in the early stage of inflammatory pain.

Abbreviations

ATP: adenosine 5'-triphosphate

A-317491: (5-({(3-phenoxybenzyl) [(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino}-carbonyl)-1,2,4-benzenetricarboxylic acid)

PBS: phosphate-buffered saline

PPADS: pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid

RT-PCR: reverse transcriptase-polymerase chain reaction

Competing interests

The author(s) declare that they have no competing interests.

Authors' contributions

MF carried out all the studies outlined in the manuscript, participated in the design of the studies and wrote the initial draft of the manuscript. TN conceived the study, participated in the design and the coordination of the study, performed statistical analysis, and wrote the final draft of the manuscript. MM participated in the design and coordination of the study. MS and SK supervised the experiments. All authors read and approved the final manuscript.

Acknowledgements

This work was partly supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

References

  1. Ralevic V, Burnstock G: Receptors for purines and pyrimidines.

    Pharmacol Rev 1998, 50:413-492. PubMed Abstract | Publisher Full Text OpenURL

  2. Franke H, Illes P: Involvement of P2 receptors in the growth and survival of neurons in the CNS.

    Pharmacol Ther 2006, 109:297-324. PubMed Abstract | Publisher Full Text OpenURL

  3. Burnstock G: A unifying purinergic hypothesis for the initiation of pain.

    Lancet 1996, 347:1604-1605. PubMed Abstract | Publisher Full Text OpenURL

  4. Chizh BA, Illes P: P2X receptors and nociception.

    Pharmacol Rev 2000, 53:553-568. OpenURL

  5. Tsuda M, Inoue K, Salter MW: Neuropathic pain and spinal microglia: a big problem from molecules in 'small' glia.

    Trends Neurosci 2005, 28:101-107. PubMed Abstract | Publisher Full Text OpenURL

  6. Chen CC, Akopian AN, Sivilotti L, Colquuhoun D, Burnstock G, Wood JN: A P2X purinoceptor expressed by a subset of sensory neurons.

    Nature 1995, 377:428-431. PubMed Abstract | Publisher Full Text OpenURL

  7. Lewis C, Neidhart S, Holy C, North RA, Buell G, Surprenant A: Coexpression of P2X2 and P2X3 receptor subunits can account for ATP-gated currents in sensory neurons.

    Nature 1995, 377:432-435. PubMed Abstract | Publisher Full Text OpenURL

  8. Vulchanova L, Riedl MS, Shuster SJ, Stone LS, Hargreaves KM, Buell G, Surprenant A, North RA, Elde R: P2X3 is expressed by DRG neurons that terminate in inner lamina II.

    Eur J Neurosci 1998, 10:3470-3478. PubMed Abstract | Publisher Full Text OpenURL

  9. Tsuda M, Shigemoto-Mogami Y, Ueno S, Koizumi S, Ueda H, Iwanaga T, Inoue K: Downregulation of P2X3 receptor-dependent sensory functions in A/J inbred mouse strain.

    Eur J Neurosci 2002, 15:1444-1450. PubMed Abstract | Publisher Full Text OpenURL

  10. Barclay J, Patel S, Dorn G, Wotherspoon G, Moffatt S, Eunson L, Abdel'al S, Natt F, Hall J, Winter J, Bevan S, Wishart W, Fox A, Ganju P: Functional downregulation of P2X3 receptor subunit in rat sensory neurons reveals a significant role in chronic neuropathic and inflammatory pain.

    J Neurosci 2002, 22:8139-8147. PubMed Abstract | Publisher Full Text OpenURL

  11. Honore P, Kage K, Mikusa J, Watt AT, Johnston JF, Wyatt JR, Faltynek CR, Jarvis MF, Lynch K: Analgesic profile of intrathecal P2X3 antisense oligonucleotide treatment in chronic inflammatory and neuropathic pain states in rats.

    Pain 2002, 99:11-19. PubMed Abstract | Publisher Full Text OpenURL

  12. Dorn G, Patel S, Wotherspoon G, Hemmings-Mieszczak M, Barclay J, Natt FJC, Martin P, Bevan S, Fox A, Ganju P, Wishart W, Hall J: siRNA relieves chronic neuropathic pain.

    Nucleic Acids Res 2004, 32:e49. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  13. Fukui M, Nakagawa T, Minami M, Satoh M: Antinociceptive effects of intracerebroventricularly administered P2 purinoceptor agonists in the rat.

    Eur J Pharmacol 2001, 419:25-31. PubMed Abstract | Publisher Full Text OpenURL

  14. Fukui M, Nakagawa T, Minami M, Satoh M: Involvement of β2-adrenergic and μ-opioid receptors in antinociception produced by intracerebroventricular administration of α,β-methylene-ATP.

    Jpn J Pharmacol 2001, 86:423-428. PubMed Abstract | Publisher Full Text OpenURL

  15. Fukui M, Takishita A, Zhang N, Nakagawa T, Minami M, Satoh M: Involvement of locus coeruleus noradrenergic neurons in supraspinal antinociception by α,β-methylene-ATP in rats.

    J Pharmacol Sci 2004, 94:153-160. PubMed Abstract | Publisher Full Text OpenURL

  16. Jarvis MF, Burgard EC, McGaraughty S, Honore P, Lynch K, Brennan TJ, Subieta A, van BiesenT, Cartmell J, Bianchi B, Niforatos W, Kage K, Yu H, Mikusa J, Wismer CT, Zhu CZ, Chu K, Lee CH, Stewart AO, Polakowski J, Cox BF, Kowaluk E, Williams M, Sullivan J, Faltynek C: A-317491, a novel potent and selective non-nucleotide antagonist of P2X3 and P2X2/3 receptors, reduces chronic inflammatory and neuropathic pain in the rat.

    Proc Natl Acad Sci USA 2002, 99:17179-17184. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  17. Jarvis MF, Bianchi B, Uchic JT, Cartmell J, Lee CH, Williams M, Faltynek C: [3H]A-317491, a novel high-affinity non-nucleotide antagonist that specifically labels human P2X2/3 and P2X3 receptors.

    J Pharmacol Exp Ther 2004, 310:407-416. PubMed Abstract | Publisher Full Text OpenURL

  18. McGaraughty S, Wismer CT, Zhu CZ, Mikusa J, Honore P, Chu KL, Lee CH, Faltynek CR, Jarvis MF: Effects of A-317491, a novel and selective P2X3/P2X2/3 receptor antagonist, on neuropathic, inflammatory and chemogenic nociception following intrathecal and intraplantar administration.

    Br J Pharmacol 2003, 140:1381-1388. PubMed Abstract | Publisher Full Text OpenURL

  19. Wu G, Whiteside GT, Lee G, Nolan S, Niosi M, Pearson MS, Ilyin VI: A-317491, a selective P2X3/P2X2/3 receptor antagonist, reverses inflammatory mechanical hyperalgesia through action at peripheral receptors in rats.

    Eur J Pharmacol 2004, 504:45-53. PubMed Abstract | Publisher Full Text OpenURL

  20. Llewellyn-Smith IJ, Burnstock G: Ultrastructural localization of P2X3 receptors in rat sensory neurons.

    Neuroreport 1998, 9:2545-2550. PubMed Abstract | Publisher Full Text OpenURL

  21. Yao ST, Barden JA, Finkelstein DI, Bennett MR, Lawrence AJ: Comparative study on the distribution patterns of P2X1-P2X6 receptor immunoreactivity in the brainstem of the rat and the common marmoset (Callithrix Jacchus): association with catecholamine cell groups.

    J Comp Neurol 2000, 427:485-507. PubMed Abstract | Publisher Full Text OpenURL

  22. Kanjhan R, Housley GD, Burton LD, Christie DL, Kippenberger A, Thorne PR, Luo L, Ryan AF: Distribution of the P2X2 receptor subunit of the ATP-gated ion channels in the rat central nervous system.

    J Comp Neurol 1999, 407:11-32. PubMed Abstract | Publisher Full Text OpenURL

  23. Harms L, Finta EP, Tschopl M, Illes P: Depolarization of rat locus coeruleus neurons by adenosine 5'-triphosphate.

    Neuroscience 1992, 48:941-952. PubMed Abstract | Publisher Full Text OpenURL

  24. Nieber K, Poelchen W, Illes P: Role of ATP in fast excitatory synaptic potentials in locus coeruleus neurons of the rat.

    Br J Pharmacol 1997, 122:423-430. PubMed Abstract | Publisher Full Text OpenURL

  25. Tsuda M, Ueno S, Inoue K: Evidence for the involvement of spinal endogenous ATP and P2X receptors in nociceptive responses caused by formalin and capsaicin in mice.

    Br J Pharmacol 1999, 128:1497-1504. PubMed Abstract | Publisher Full Text OpenURL

  26. Gourine AV, Llaudet E, Dale N, Spyer KM: ATP is a mediator of chemosensory transduction in the central nervous system.

    Nature 2005, 436:108-111. PubMed Abstract | Publisher Full Text OpenURL

  27. Sawynok J, Liu XJ: Adenosine in the spinal cord and periphery: release and regulation of pain.

    Prog Neurobiol 2003, 69:313-340. PubMed Abstract | Publisher Full Text OpenURL