title: A putative relay circuit providing low-threshold mechanoreceptive input to lamina I projection neurons via vertical cells in lamina II of the rat dorsal horn.
abstract: BACKGROUND
  Lamina I projection neurons respond to painful stimuli, and some are also activated by touch or hair movement. Neuropathic pain resulting from peripheral nerve damage is often associated with tactile allodynia (touch-evoked pain), and this may result from increased responsiveness of lamina I projection neurons to non-noxious mechanical stimuli. It is thought that polysynaptic pathways involving excitatory interneurons can transmit tactile inputs to lamina I projection neurons, but that these are normally suppressed by inhibitory interneurons. Vertical cells in lamina II provide a potential route through which tactile stimuli can activate lamina I projection neurons, since their dendrites extend into the region where tactile afferents terminate, while their axons can innervate the projection cells. The aim of this study was to determine whether vertical cell dendrites were contacted by the central terminals of low-threshold mechanoreceptive primary afferents.
  
  
  RESULTS
  We initially demonstrated contacts between dendritic spines of vertical cells that had been recorded in spinal cord slices and axonal boutons containing the vesicular glutamate transporter 1 (VGLUT1), which is expressed by myelinated low-threshold mechanoreceptive afferents. To confirm that the VGLUT1 boutons included primary afferents, we then examined vertical cells recorded in rats that had received injections of cholera toxin B subunit (CTb) into the sciatic nerve. We found that over half of the VGLUT1 boutons contacting the vertical cells were CTb-immunoreactive, indicating that they were of primary afferent origin.
  
  
  CONCLUSIONS
  These results show that vertical cell dendritic spines are frequently contacted by the central terminals of myelinated low-threshold mechanoreceptive afferents. Since dendritic spines are associated with excitatory synapses, it is likely that most of these contacts were synaptic. Vertical cells in lamina II are therefore a potential route through which tactile afferents can activate lamina I projection neurons, and this pathway could play a role in tactile allodynia.
authors.affiliation: Institute of Neuroscience and Psychology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK. yasaka@cc.saga-u.ac.jp.
authors.forename: Toshiharu
authors.initials: T
authors.lastname: Yasaka
authors.forename: Sheena Yx
authors.initials: SY
authors.lastname: Tiong
authors.forename: Erika
authors.initials: E
authors.lastname: Polgár
authors.forename: Masahiko
authors.initials: M
authors.lastname: Watanabe
authors.forename: Eiichi
authors.initials: E
authors.lastname: Kumamoto
authors.forename: John S
authors.initials: JS
authors.lastname: Riddell
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Todd
mesh_terms: D000818:Animals; D002772:Cholera Toxin / Q000494:pharmacology; D049229:Dendritic Spines / Q000187:drug effects / Q000378:metabolism; D040542:Mechanotransduction, Cellular / Q000187:drug effects; D008959:Models, Neurological; D009186:Myelin Sheath / Q000378:metabolism; D020671:Posterior Horn Cells / Q000187:drug effects / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012584:Sciatic Nerve / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D050598:Vesicular Glutamate Transport Protein 1 / Q000378:metabolism; D050599:Vesicular Glutamate Transport Protein 2 / Q000378:metabolism
chemical_list: C493475:Slc17a6 protein, rat; C493466:Slc17a7 protein, rat; D050598:Vesicular Glutamate Transport Protein 1; D050599:Vesicular Glutamate Transport Protein 2; D002772:Cholera Toxin