Intrathecal etanercept partially restores morphine's antinociception in morphine-tolerant rats via attenuation of the glutamatergic transmission.

BACKGROUND: Long-term exposure to morphine leads to analgesic tolerance. In addition to an opioid receptor conformational change, enhancing the glutamatergic signal transmission is also involved in morphine tolerance. Tumor necrosis factor-α has been demonstrated to correlate with neuronal plasticity via activation of glutamatergic transmission. We examined the effect of etanercept, a tumor necrosis factor-α inhibitor on morphine tolerance in rats. METHODS: Male Wistar rats were implanted with 2 intrathecal (IT) catheters, and 1 IT catheter was connected to a mini-osmotic pump, used for either morphine infusion (15 µg/h) or saline (1 µL/h) infusion for 5 days. On day 5, either etanercept (50 µg) or saline (10 µL) was injected after discontinued morphine infusion. Three hours later, acute morphine (15 µg/10 µL, IT) treatment was given and all rats received a nociceptive tail-flick test. RESULTS: The results showed that acute etanercept (50 µg) treatment caused a significant antinociceptive effect of morphine in morphine-tolerant rats. Western blotting indicated that etanercept attenuated the downregulation of membrane glutamate transporters GLT-1 and GLAST in morphine-tolerant rats. Etanercept also inhibited the upregulation of surface AMPA-receptor and N-methyl-d-aspartate-receptor subunits, including GluR1/GluR2 and NR1/NR2A. CONCLUSIONS: These results demonstrate that etanercept partially restores the antinociceptive effect of morphine in morphine tolerance after a morphine challenge. Etanercept has potential for use in the clinical management of pain, particularly in patients who require long-term opioid treatment, and the effectiveness of which can be hampered by tolerance.

Etanercept restores the antinociceptive effect of morphine and suppresses spinal neuroinflammation in morphine-tolerant rats.

BACKGROUND: In the present study we examined the effect of the tumor necrosis factor (TNF)-α antagonist etanercept on the antinociceptive effect of morphine in morphine-tolerant rats. METHODS: Male Wistar rats were implanted with 2 intrathecal catheters, and 1 was connected to a mini-osmotic pump for either morphine (15 µg/h) or saline (1 µL/h) infusion for 5 days. On day 5, either etanercept (5 µg, 25 µg, and 50 µg/10 µL) or saline (10 µL) was injected via the other catheter after morphine infusion was discontinued. Three hours later, morphine (15 µg/10 µL, intrathecally) was given and tail-flick latency was measured to evaluate the antinociceptive effect of morphine. Rats were then killed and their spinal cords were removed for quantitative real-time polymerase chain reaction and immunohistochemistry to measure proinflammatory cytokines expression. RESULTS: We found that acute etanercept (50 µg) treatment preserved a significant antinociceptive effect of morphine in morphine-tolerant rats. In addition, the expression of TNFα mRNA was increased by 2.5-fold, interleukin (IL)-1ß mRNA increased by 13-fold and IL-6 mRNA by 111-fold in the dorsal spinal cord of morphine-tolerant rats. The increase in TNFα, IL-1ß, and IL-6 mRNA expression was blocked by 50 µg etanercept pretreatment. The immunohistochemistry analysis revealed that 50 µg etanercept suppressed proinflammatory cytokines expression and neuroinflammation in the microglia. CONCLUSIONS: The present study demonstrates that etanercept restores the antinociceptive effect of morphine in morphine-tolerant rats by inhibition of proinflammatory cytokine TNF-α, IL-1ß, and IL-6 expression and spinal neuroinflammation. The results suggest that etanercept could also be an adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.