Ultra-low dose naloxone upregulates interleukin-10 expression and suppresses neuroinflammation in morphine-tolerant rat spinal cords

Co-infusion of ultra-low dose naloxone and morphine attenuates morphine tolerance through the prevention of mu opioid receptor-Gs protein coupling. We previously demonstrated that chronic intrathecal infusion of morphine leads to tolerance and spinal neuroinflammation. The aim of present study was to examine the possible mechanisms by which ultra-low dose naloxone modulates spinal neuroinflammation, particularly the role of anti-inflammatory cytokine interleukin 10 (IL-10). Morphine tolerance was induced in male Wistar rats by intrathecal infusion of morphine (15 μg/h) for 5 days, and co-infusion of naloxone (15 pg/h) was used to evaluate the impact on spinal cytokine expression. Recombinant rat IL-10 (rrIL-10) or anti-rat IL-10 antibody was injected to elucidate the effect of IL-10 on morphine tolerance. Our results showed that co-infusion of naloxone (15 pg/h) with morphine not only attenuated tolerance, shifting the AD50 from 89.2 to 11.7 μg but also inhibited the increased expression of pro-inflammatory cytokine (TNF-α, IL-1β, and IL-6) caused by chronic intrathecal morphine infusion. The increase of IL-10 protein and mRNA were 1.5- and 3-fold, respectively, compared to that in morphine-infused rat spinal cords. A combination of daily rrIL-10 (1 μg) injection with morphine infusion produced, in a less potent, preservative antinociception and inhibited pro-inflammatory cytokine production compared to ultra-low dose naloxone co-infusion, and the effect of ultra-low dose naloxone co-infusion was inhibited by daily intrathecal anti-rat IL-10 antibody injection. These results demonstrate that IL-10 contributes to the attenuation of pro-inflammatory cytokine expression caused by ultra-low dose naloxone/morphine co-infusion and thus the attenuation of morphine tolerance.