KMID : 0363120090220010021
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Korean Journal of Pain 2009 Volume.22 No. 1 p.21 ~ p.27
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The Antiallodynic Effect and the Change of the ¥á2 Adrenergic Receptor Subtype mRNA Expression by Morphine Administration in a Spinal Nerve Ligation Rat Model
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Chung Kyu-Yeon
Shin Sang-Wook Kwon Su-Ah Kim Tae-Kyun Baek Seung-Hoon Baik Seong-Wan
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Abstract
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Background: The neuropathic pain arising from nerve injury is difficult to treat and the therapeutic effects of opioid drugs remain debatable. Agonists acting at the ¥á2 adrenergic and opioid receptors have analgesic properties and they act synergistically when co-administered in the spinal cord. The lack of subtype-selective pharmacological agents has previously impeded the synergistic effects that are mediated by the adrenergic receptor subtypes.
Methods: We created neuropathic pain model by ligating the L5 spinal nerve in Sprague-Dawley rats (n = 18). We divided the rats into three groups (n = 6 for each group), and we administered intraperitoneal morphine (1 mg/kg, 3 mg/kg, 5 mg/kg) and then we measured the mechanical allodynia with using von-Frey filaments for 8 hours. We then injected morphine (5 mg/kg) intraperitoneally, twice a day for 2 weeks. We measured the tactile and cold allodynia in the morphine group (n = 9) and the saline group (n = 9). After 2 weeks, we decapitated the rats and harvested the spinal cords at the level of lumbar enlargement. We compared the ¥á2 subtype mRNA expression with that of control group (n = 6) by performing real time polymerase chain reaction (RTPCR).
Results: Intraperitoneal morphine reduced the neuropathic pain behavior in the dose-dependent manner. Chronic morphine administration showed an antiallodynic effect on the neuropathic pain rat model. The rats did not display tolerance or hyperalgesia. The expression of the mRNAs of the ¥á2A, ¥á2B, ¥á2C subtypes decreased, and morphine attenuated this effect. But we could not get statistically proven results.
Conclusion: Systemic administration of morphine can attenuate allodynia during both the short-term and long-term time course. Morphine has an influence on the expression of ¥á2 receptor subtype mRNA. Yet we need more research to determine the precise effect of morphine on the ¥á2 subtype gene expression.
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KEYWORD
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allodynia, ¥á2 adrenergic receptor, mRNA, neuropathic pain, spinal nerve ligation
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