Morphine-induced decrease in mechanical allodynia is mediated by central, but not peripheral, opioid receptors in rats with inflammation.
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.
Li
Shao
S
Institute of Neurobiology, Fudan University, Shanghai 200433, China.
Zhao
Zhi-Qi
ZQ
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*
D011957:Receptors, Opioid; D009020:Morphine