title: Morphine-induced decrease in mechanical allodynia is mediated by central, but not peripheral, opioid receptors in rats with inflammation.
abstract: The aim of this study was to investigate the mechanism underlying the effect of morphine on allodynia to complete Freund's adjuvant-induced inflammation in rats. Morphine (5 mg/kg, i.v.) markedly inhibited the mechanical stimulation-induced nociceptive reflex of the gastrocnemius muscle in the inflamed hind-limb, and the inhibition was blocked by naloxone (1 mg/kg). Teased fiber recordings were made from the tibial nerve innervating the inflamed hindpaw. Morphine at the same dose did not affect the spontaneous firing rate of A-type fibers, whereas it markedly decreased the spontaneous firing of C-type fibers. The present data suggested that the central, but not peripheral, plasticity triggered by inflammation-induced facilitation of A(beta) fibers plays an important role in morphine-induced alleviation of allodynia, whereas activation of opioid receptor expression on the peripheral terminals of C fibers may contribute to morphine-induced alleviation of persistent pain of inflammation.
authors.affiliation: Institute of Neurobiology, Fudan University, Shanghai 200433, China.
authors.forename: Shao
authors.initials: S
authors.lastname: Li
authors.forename: Zhi-Qi
authors.initials: ZQ
authors.lastname: Zhao
mesh_terms: D000818:Animals; D007249:Inflammation / Q000139:chemically induced / Q000188:drug therapy*; D008297:Male; D009020:Morphine / Q000494:pharmacology* / Q000627:therapeutic use; D009413:Nerve Fibers, Myelinated / Q000187:drug effects / Q000502:physiology; D036421:Nerve Fibers, Unmyelinated / Q000187:drug effects / Q000502:physiology; D010147:Pain Measurement / Q000187:drug effects* / Q000379:methods; D010812:Physical Stimulation / Q000379:methods; D051381:Rats; D017208:Rats, Wistar; D011957:Receptors, Opioid / Q000819:agonists* / Q000502:physiology*
chemical_list: D011957:Receptors, Opioid; D009020:Morphine