title: An anatomical and immunohistochemical characterization of afferents innervating the C6-C7 facet joint after painful joint loading in the rat.
abstract: METHODS
  This study used retrograde neuronal tracing and immunohistochemistry to identify neurons innervating the C6-C7 facet joint and those expressing calcitonin gene-related peptide (CGRP) in the dorsal root ganglion (DRG) of rats after painful cervical facet joint injury.
  
  
  OBJECTIVE
  The objective of this study was to characterize the innervation of the C6-C7 facet joint after painful joint injury in the rat.
  
  
  BACKGROUND
  The cervical facet joint is a source of neck pain, and its loading can initiate persistent pain. CGRP is a nociceptive neurotransmitter; peptidergic afferents have been identified in the facet joint's capsule. Although studies suggest that facet joint injury alters CGRP expression in joint afferents, the distribution of neurons innervating the C6-C7 facet joint and their expression of CGRP after a painful joint injury have not been investigated.
  
  
  METHODS
  Holtzman rats (Harlan Sprague-Dawley, Indianapolis, IN) received an intra-articular injection of cholera toxin subunit B in the C6-C7 facet joints. After injection, subgroups underwent either a painful joint distraction or sham procedure. Mechanical sensitivity was assessed, and immunohistochemical techniques were used to quantify CGRP expression and cholera toxin subunit B labeling in the C5-C8 DRGs.
  
  
  RESULTS
  Facet joint distraction-induced (P ≤ 0.0002) hypersensitivity. Neurons labeled by the joint injection were identified in the C5-C8 DRGs. Significantly, more (P ≤ 0.0001) cholera toxin subunit B-positive neurons were identified in the C7 DRG than any other level. At C7, 54.4% ± 15.3% of those neurons were also CGRP-positive, whereas only 41.5% ± 5.4% of all neurons were CGRP-positive; this difference was significant (P = 0.0084).
  
  
  CONCLUSIONS
  The greatest number of afferents from the C6-C7 facet joint has cell bodies in the C7 DRG, implicating this level as the most relevant for pain from this joint. In addition, peptidergic afferents seem to have an important role in facet joint-mediated pain.
authors: Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104-6392, USA.
authors: Jeffrey V
authors: JV
authors: Kras
authors: Kosuke
authors: K
authors: Tanaka
authors: Taylor M
authors: TM
authors: Gilliland
authors: Beth A
authors: BA
authors: Winkelstein
mesh_terms: D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000378:metabolism*; D002574:Cervical Vertebrae / Q000293:injuries / Q000294:innervation* / Q000601:surgery; D005552:Forelimb; D005727:Ganglia, Spinal / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology; D006930:Hyperalgesia / Q000209:etiology / Q000503:physiopathology; D007150:Immunohistochemistry; D008297:Male; D008856:Microscopy, Fluorescence; D018715:Microscopy, Video; D009475:Neurons, Afferent / Q000378:metabolism*; D019857:Osteogenesis, Distraction / Q000009:adverse effects; D010146:Pain / Q000209:etiology / Q000503:physiopathology; D051381:Rats; D017207:Rats, Sprague-Dawley; D021801:Zygapophyseal Joint / Q000293:injuries / Q000294:innervation* / Q000601:surgery
chemical_list: D015740:Calcitonin Gene-Related Peptide