| Summary: | 博士 === 國防醫學院 === 醫學科學研究所 === 100 === Opioids are potent and effective analgesics; they are widely used in clinical pain management. Long-term exposure to morphine leads to analgesic tolerance. There are two possible mechanisms for drug tolerance; within-system and between-system adaptation. Recent investigations on the mechanisms of within-opioid system adaptation include downregulation of G-protein coupled receptors, G protein uncoupling, and β-arrestin binding and internalization of opioid receptors. Studies of between-system adaptation have focused on glutamatergic synaptic transmission via downregulation of glutamate transporters (GTs), increased glutamate level and activation of NMDA receptor function, and glia activation with increasing of proinflammatory cytokine expression; which reduced antinociceptive effect of morphine. Tumor necrosis factor-α (TNF-α) has been demonstrated to correlate with neuronal plasticity via activating spinal glia cells and enhancing glutamatergic transmission. The present study examined the mechanism of etanercept, a TNF-α inhibitor on morphine tolerance in male Wistar rats. Intrathecal morphine infusion (15 μg/h) for 5 days was used for tolerance induction. On day 5, either etanercept (5 μg, 25 μg and 50 μg/10 μl) or saline (10 μl) was injected after discontinued morphine infusion, and three hours later morphine (10 μg/10 μl, i.t) was given for tail-flick latency measurement. We found that pretreatment with etanercept (50 μg) caused a significant antinociceptive effect of morphine in morphine-tolerant rats. The real-time PCR data showed that the expression of TNF-α mRNA was increased by 2.5 fold, IL-1β mRNA increased by 13 fold and IL-6 mRNA by 111 fold in the dorsal horn of morphine-tolerant rat spinal cords. The increase in TNF-α, IL-1β and IL-6 mRNA expression was blocked by 50 μg etanercept pretreatment. The immunohistochemistry analysis revealed that 50 μg etanercept suppressed the proinflammatory cytokines expression and neuroinflammation in the microglia. In addition, Western blotting indicated that etanercept downregulated membrane glutamate transporters (GTs), EAAC1, GLT1 and GLAST in morphine tolerant rats. Etanercept also inhibited the upregulation of surface AMPA- and NMDA-receptor subunits, including GluR1/GluR2 and NR1/NR2A. In summary, blockade of TNF-α signaling attenuated morphine tolerance. This effect may be mediated by restoration of glial membrane GTs expression, inhibition of surface AMPA- and NMDA-receptor upregulation, and suppression of microglial activation and proinflammatory cytokine expression. The results suggest that etanercept could also be an adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.
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