title: An ultrastructural study of calcitonin gene-related peptide-immunoreactive nerve fibers innervating the rat posterior longitudinal ligament. A morphologic basis for their possible efferent actions.
abstract: METHODS
  The present study investigated ultrastructural characteristics of calcitonin gene-related peptide-immunoreactive nerve fibers in the posterior longitudinal ligament of the rat lumbar spine.
  
  
  OBJECTIVE
  To provide a morphologic basis for assessment of the afferent and, in particular, efferent functions of calcitonin gene-related peptide immunoreactive nerves in the posterior longitudinal ligament and their eventual role in degenerative spondylarthropathies and low back pain.
  
  
  BACKGROUND
  Previous studies using light-microscopic localization of sensory neuronal markers such as calcitonin gene-related peptide have reported the presence of sensory fibers in the supporting structures of the vertebral column. Meanwhile, accumulating research data have suggested efferent properties for calcitonin gene-related peptide, i.e., a trophic action that alters the intrinsic properties of target cells not through transient action of synaptic transmission, but through long-lasting signal transmission by the secreted neuropeptides. To verify such trophic, paracrine actions of the calcitonin gene-related peptide-containing fibers in the posterior longitudinal ligament, however, ultrastructural details of the terminals and their spatial relationship to their eventual target structures have to be elucidated.
  
  
  METHODS
  Rat posterior longitudinal ligaments were stained immunohistochemically for calcitonin gene-related peptide. Light-microscopic analysis of the semithin sections facilitated subsequent electron microscopy of specific sites of the posterior longitudinal ligament to determine ultrastructural details and nerve fiber-target relationships.
  
  
  RESULTS
  The rat lumbar posterior longitudinal ligament was found to be innervated by two distinctive calcitonin gene-related peptide immunoreactive nerve networks. In immunoelectronmicroscopy, the fibers of the deep network had numerous free nerve endings, whereas those of the superficial network showed spatial associations with other non-calcitonin gene-related peptide immunoreactive components of the network. In both systems, naked axons not covered by the Schwann cells made close spatial contact with smooth muscle cells: of blood vessels and resident posterior longitudinal ligament fibroblasts.
  
  
  CONCLUSIONS
  The ultrastructural characteristics of the innervation of the rat posterior longitudinal ligament would be compatible not only with a nociceptive function, but also with neuromodulatory, vasoregulatory, and trophic functions, as has already been established in some visceral organs.
authors.affiliation: Department of Anatomy, University of Helsinki, Finland.
authors.forename: S
authors.initials: S
authors.lastname: Imai
authors.forename: Y T
authors.initials: YT
authors.lastname: Konttinen
authors.forename: Y
authors.initials: Y
authors.lastname: Tokunaga
authors.forename: T
authors.initials: T
authors.lastname: Maeda
authors.forename: S
authors.initials: S
authors.lastname: Hukuda
authors.forename: S
authors.initials: S
authors.lastname: Santavirta
mesh_terms: D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000032:analysis* / Q000502:physiology; D007124:Immunoenzyme Techniques; D017846:Longitudinal Ligaments / Q000294:innervation*; D008297:Male; D016253:Microscopy, Immunoelectron; D009412:Nerve Fibers / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure*; D009476:Neurons, Efferent / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar
chemical_list: D015740:Calcitonin Gene-Related Peptide