Pre-injury administration of morphine prevents development of neuropathic hyperalgesia through activation of descending monoaminergic mechanisms in the spinal cord in mice

Department of Molecular Pharmacology and Neuroscience, Nagasaki University, Nagasaki, Nagasaki, Japan
Molecular Pain (Impact Factor: 3.65). 02/2005; 1(1):19. DOI: 10.1186/1744-8069-1-19
Source: PubMed


The present study examined whether pre-injury administration of morphine can prevent partial sciatic nerve injury-induced neuropathic pain in mice. We observed that pre-injury administration of subcutaneous (s.c.) and intracerebroventricular (i.c.v.) morphine dose-dependently prevented the development of both thermal and mechanical hyperalgesia at 7 days following nerve injury in mice. The pre-injury morphine (s.c.)-induced analgesia was significantly blocked by pretreatment with naloxone injected s.c. or i.c.v., but not i.t., suggesting that systemic morphine produced the pre-emptying effects mainly by acting at the supra-spinal sites. Since it is believed that activation of descending monoaminergic mechanisms in spinal cord largely contributes to the supra-spinal analgesic effects of morphine, we investigated the involvement of serotonergic and noradrenergic mechanisms in spinal cord in the pre-injury morphine-induced analgesic effects. We found that pre-injury s.c. morphine-induced analgesic effect was significantly blocked by i.t. pretreatment with serotonergic antagonist, methysergide and noradrenergic antagonist, phentolamine. In addition, pre-injury i.t. injection of serotonin uptake inhibitor, fluoxetine and alpha2-adrenergic agonist, clonidine significantly prevented the neuropathic hyperalgesia. We next examined whether pre-injury morphine prevented the expression of neuronal hyperactivity markers such as c-Fos and protein kinase C gamma (PKCgamma) in the spinal dorsal horn. We found that pre-injury administration of s.c. morphine prevented increased expressions of both c-Fos and PKCgamma observed following nerve injury. Similar results were obtained with i.t. fluoxetine and clonidine. Altogether these results suggest that pre-injury administration of morphine might prevent the development of neuropathic pain through activation of descending monoaminergic pain inhibitory pathways.

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    • "The preemptive morphine analgesia in clinic is based on this concept (Richmond et al. 1993). Indeed, we have successfully reproduced similar pre-emptive analgesia in the experimental animal model, in which systemic and central pre-treatments with morphine abolished the nerve injury-induced neuropathic pain (Rashid and Ueda 2005). In this report, the administration of morphine 30 min prior to nerve injury completely abolished neuropathic pain and its underlying mechanisms such as long-lasting PKCc up-regulation in the dorsal horn. "
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