title: Chemokine signaling involving chemokine (C-C motif) ligand 2 plays a role in descending pain facilitation.
abstract: OBJECTIVE
  Despite accumulating evidence on a role of immune cells and their associated chemicals in mechanisms of pain, few studies have addressed the potential role of chemokines in the descending facilitation of persistent pain. The present study was undertaken to test the hypothesis that the chemokine (C-C motif) ligand 2 (CCL2) (commonly known as monocyte chemoattractant protein-1) signaling in the rostral ventromedial medulla (RVM), a pivotal structure in brainstem pain modulatory circuitry, is involved in descending pain facilitation in rats.
  
  
  METHODS
  An L5 spinal nerve ligation (SNL) was produced in rats under pentobarbital anesthesia. Western blot and immunohistochemistry were used to detect the expression levels of CCL2 and CCL2 receptor (CCR2), and examine their distributions compared with the neuronal marker NeuN as well as glial markers glial fibrillary acidic protein (GFAP, astroglial) and CD11b (microglial), respectively.
  
  
  RESULTS
  SNL induced an increase in CCL2 expression in the RVM, and this returned to the control level at 4 weeks after injury. The induced CCL2 colocalized with NeuN, but not with GFAP and CD11b. CCR2 was also upregulated by SNL in the RVM, and this increase lasted for at least 4 weeks. CCR2 was colocalized with CD11b but not GFAP. Few RVM neurons also exhibited CCR2 staining. Neutralizing CCL2 with an anti-CCL2 antibody (0.2-20 ng) or injecting RS-102895 (0.1-10 pmol), a CCR2b chemokine receptor antagonist, into the RVM on day 1 after SNL, significantly attenuated the established thermal and mechanical hypersensitivity. In addition, injection of recombinant rat CCL2 (0.03-3 pmol) into the RVM induced dose-dependent hyperalgesia, which was prevented by pretreatment with RS-102895 (10 pmol). Interleukin-1β (IL-1β), a potent inducer of neuronal CCL2, was also selectively upregulated in RVM reactive astrocytes. Injection of IL-1β (120 fmol) into the RVM induced behavioral hyperalgesia, which was blocked by RS-102895 (10 pmol). However, an IL-1 receptor antagonist (3 pmol) did not prevent CCL2 (3 pmol)-induced hyperalgesia. These results suggest that the effect of CCL2 is downstream to IL-1β signaling.
  
  
  CONCLUSIONS
  The IL-1β and CCL2-CCR2 signaling cascades play a role in neuron-glia-cytokine interactions and the descending facilitation of neuropathic pain.
authors.affiliation: Department of Neural and Pain Sciences, School of Dentistry & Program in Neuroscience, University of Maryland, Baltimore, Maryland 21201, USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Guo
authors.affiliation: Department of Neural and Pain Sciences, School of Dentistry & Program in Neuroscience, University of Maryland, Baltimore, Maryland 21201, USA.
authors.forename: Hu
authors.initials: H
authors.lastname: Wang
authors.affiliation: Department of Neural and Pain Sciences, School of Dentistry & Program in Neuroscience, University of Maryland, Baltimore, Maryland 21201, USA.
authors.forename: Shiping
authors.initials: S
authors.lastname: Zou
authors.affiliation: Department of Neural and Pain Sciences, School of Dentistry & Program in Neuroscience, University of Maryland, Baltimore, Maryland 21201, USA.
authors.forename: Ronald
authors.initials: R
authors.lastname: Dubner
authors.affiliation: Department of Neural and Pain Sciences, School of Dentistry & Program in Neuroscience, University of Maryland, Baltimore, Maryland 21201, USA.
authors.forename: Ke
authors.initials: K
authors.lastname: Ren
mesh_terms: D000818:Animals; D018932:Chemokine CCL2 / Q000502:physiology*; D059350:Chronic Pain / Q000139:chemically induced / Q000378:metabolism*; D004195:Disease Models, Animal; D004525:Efferent Pathways / Q000378:metabolism; D053583:Interleukin-1beta / Q000037:antagonists & inhibitors / Q000502:physiology; D008297:Male; D008526:Medulla Oblongata / Q000166:cytology / Q000378:metabolism; D009474:Neurons / Q000166:cytology / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D054390:Receptors, CCR2 / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*
chemical_list: C491766:Ccl2 protein, rat; C515336:Ccr2 protein, rat; D018932:Chemokine CCL2; D053583:Interleukin-1beta; D054390:Receptors, CCR2