title: Functional mu opioid receptors are reduced in the spinal cord dorsal horn of diabetic rats.
abstract: BACKGROUND
  The mechanisms of decreased spinal analgesic potency of morphine in neuropathic pain are not fully known. Agonist-stimulated [35S]GTPgammaS receptor autoradiography has been used to measure receptor activation of G proteins in vitro. Using this technique, we determined changes in the functional mu opioid receptors in the spinal dorsal horn in diabetic rats.
  
  
  METHODS
  Rats were rendered diabetic with an intraperitoneal injection of streptozotocin. The lumbar spinal cord was obtained from age-matched normal and diabetic rats 4 weeks after streptozotocin treatment. [D-Ala2,N-MePhe4,Gly5-ol]-enkephalin (DAMGO, 10 microm)-stimulated [35S]GTPgammaS binding was performed in both tissue sections and isolated membranes.
  
  
  RESULTS
  The DAMGO-stimulated [35S]GTPgammaS binding in the spinal dorsal horn was significantly reduced (approximately 37%) in diabetic rats compared with normal rats. However, [35S]GTPgammaS bindings in the spinal dorsal horn stimulated by other G protein-coupled receptor agonists, including [D-Pen2,D-Pen5]-enkephalin, R(-)N6-(2-phenylisopropyl)-adenosine, and WIN-55212, were not significantly altered in diabetic rats. The basal [35S]GTPgammaS binding in the spinal dorsal horn was slightly (approximately 13%) but significantly increased in diabetic rats. Western blot analysis revealed no significant difference in the expression of the alpha subunits of G(i) and G(o) proteins in the dorsal spinal cord between normal and diabetic rats.
  
  
  CONCLUSIONS
  These data suggest that the functional mu opioid receptors in the spinal cord dorsal horn of diabetic rats are reduced. The impaired functional mu opioid receptors in the spinal cord may constitute one of the mechanisms underlying the reduced spinal analgesic effect of mu opioids in diabetic neuropathic pain.
authors.affiliation: Department of Anesthesiology, The Pennsylvania State College of Medicine, Hershey, Pennsylvania, USA.
authors.forename: Shao-Rui
authors.initials: SR
authors.lastname: Chen
authors.forename: Kristi L
authors.initials: KL
authors.lastname: Sweigart
authors.forename: Joan M
authors.initials: JM
authors.lastname: Lakoski
authors.forename: Hui-Lin
authors.initials: HL
authors.lastname: Pan
mesh_terms: D000818:Animals; D001345:Autoradiography; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D008297:Male; D020671:Posterior Horn Cells / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000187:drug effects / Q000378:metabolism*
chemical_list: D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D019204:GTP-Binding Proteins