Extracellular signal-regulated kinase (ERK), a
mitogen-activated protein kinase (MAPK), transduces a broad range of extracellular stimuli into diverse intracellular responses. It has been reported that ERK is involved in the modulation of nociceptive information and central sensitization produced by intense noxious stimuli or peripheral tissue
inflammation. Our previous studies showed that the spinal neurons sensitization was involved in
morphine withdrawal response. This study was to investigate the role of the spinal ERK in
morphine dependence and
naloxone-precipitated withdrawal response. To set up
morphine-dependent model, rats were subcutaneously injected with
morphine (twice a day, for 5 d). The dose of
morphine was 10 mg/kg on the first day and was increased by 10 mg/kg each day. On day 6, 4 h after the injection of
morphine (50 mg/kg),
morphine withdrawal syndrome was precipitated by an injection of
naloxone (4 mg/kg, i.p.). Using anti-phospho-ERK (pERK) antibody, the time course of pERK expression was detected by Western blot.
U0126, a
mitogen-activated protein kinase kinase (
MEK) inhibitor, or phosphorothioate-modified
antisense oligonucleotides (ODN) was intrathecally injected 30 min or 36, 24 and 12 h before
naloxone-precipitated withdrawal. The scores of
morphine withdrawal symptom and
morphine withdrawal-induced
allodynia were observed. One hour after
naloxone-precipitated withdrawal, pERK expression in the spinal dorsal horn was assessed by immunohistochemical analysis and Western blot was used to detect the expression of cytosolic and nuclear fraction of pERK in the rat spinal cord. The results showed that the expression of cytosolic and nuclear fraction of pERK, not non-phospho-ERK, in the spinal cord was gradually increased following the injection of
morphine. When
morphine withdrawal was precipitated with
naloxone, the expression of the spinal pERK further increased. Intrathecal administration of
U0126 or antisense ODN against ERK decreased the scores of
morphine withdrawal, attenuated
morphine withdrawal-induced
allodynia and also inhibited the increase of pERK expression in the spinal cord of
morphine withdrawal rats. These results suggest that activation of the spinal ERK is involved in
morphine-dependent and
naloxone-precipitated withdrawal response.