title: Molecular mechanism of changes in the morphine-induced pharmacological actions under chronic pain-like state: suppression of dopaminergic transmission in the brain.
abstract: In the present study, we demonstrated whether a neuropathic pain-like state induced by sciatic nerve ligation in rodents could cause a long-lasting change in intracellular signaling in both supraspinal and spinal cord related to the suppression of morphine's effect. Mice with sciatic nerve ligation exhibited a significant suppression of the morphine-induced antinociception. Under this condition, phosphorylated-conventional protein kinase C-like immunoreactivity (p-cPKC-IR) and phosphorylated-micro-opioid receptor (p-MOR)-IR were clearly increased on the ipsilateral side in the dorsal horn of the spinal cord of nerve-ligated mice. It is of interest to note that astroglial hypertrophy as well as its proliferation was also noted in this area of sciatic nerve-ligated mice. Like nerve injury, the increase in cPKC activities and astroglial hypertrophy/proliferation in this region was observed by repeated morphine treatment. These findings suggest that the phosphorylation of both cPKC and MOR in the dorsal horn of the spinal cord by sciatic nerve ligation may play a substantial role in the suppression of morphine-induced antinociception under a neuropathic pain-like state. Sciatic nerve injury also caused a significant inhibition of MOR-mediated G-protein activation onto GABAergic neurons and a dramatic reduction in ERK activities onto dopaminergic neurons in the ventral tegmental area (VTA) regulating the rewarding effect of opioids. Furthermore, we found that the inhibition of ERK cascade in the VTA by treatment with specific inhibitors suppressed the morphine-induced rewarding effect in normal mice. These findings provide evidence that the direct reduction in MOR function and the persistent decrease in ERK activity of dopaminergic neurons in the VTA may contribute to the suppression of the morphine-induced rewarding effect under a neuropathic pain-like state. Conclusively, our recent findings provide novel evidences for the mechanism underlying the less sensitivity to opioids under a neuropathic pain-like state.
authors.affiliation: Department of Toxicology, Hoshi University School of Pharmacy and Pharmaceutical Sciences, 2-4-41 Ebara, Shinagawa-ku, Tokyo, 142-8501, Japan. narita@hoshi.ac.jp
authors.forename: Minoru
authors.initials: M
authors.lastname: Narita
authors.forename: Masami
authors.initials: M
authors.lastname: Suzuki
authors.forename: Satoshi
authors.initials: S
authors.lastname: Imai
authors.forename: Michiko
authors.initials: M
authors.lastname: Narita
authors.forename: Satoru
authors.initials: S
authors.lastname: Ozaki
authors.forename: Yayoi
authors.initials: Y
authors.lastname: Kishimoto
authors.forename: Kousei
authors.initials: K
authors.lastname: Oe
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Yajima
authors.forename: Mitsuaki
authors.initials: M
authors.lastname: Yamazaki
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Suzuki
mesh_terms: D000701:Analgesics, Opioid / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D004298:Dopamine / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D051379:Mice; D009020:Morphine / Q000378:metabolism / Q000494:pharmacology*; D010146:Pain / Q000378:metabolism / Q000503:physiopathology*; D011493:Protein Kinase C / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000378:metabolism; D012584:Sciatic Nerve / Q000293:injuries*; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology; D017557:Ventral Tegmental Area / Q000378:metabolism
chemical_list: D000701:Analgesics, Opioid; D017450:Receptors, Opioid, mu; D009020:Morphine; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D004298:Dopamine