Interleukin-1 beta-induced up-regulation of opioid receptors in the untreated and morphine-desensitized U87 MG human astrocytoma cells

Background: Interleukin-1beta (IL-1 beta) is a pro-inflammatory cytokine that can be produced in the central nervous system during inflammatory conditions. We have previously shown that IL-1 beta expression is altered in the rat brain during a morphine tolerant state, indicating that this cytokine may serve as a convergent point between the immune challenge and opiate mediated biological pathways. We hypothesized that IL-1 beta up-regulates opioid receptors in human astrocytes in both untreated and morphine-desensitized states. Methods: To test this hypothesis, we compared the basal expression of the mu (MOR), delta (DOR), and kappa (KOR) opioid receptors in the human U87 MG astrocytic cell line to SH-SY5Y neuronal and HL-60 immune cells using absolute quantitative real time RT-PCR (AQ-rt-RT-PCR). To demonstrate that IL-1 beta induced up-regulation of the MOR, DOR and KOR, U87 MG cells (2 x 10(5) cells/well) were treated with IL-1 beta (20 ng/mL or 40 ng/mL), followed by co-treatment with interleukin-1 receptor antagonist protein (IL-1RAP) (400 ng/mL or 400 ng/mL). The above experiment was repeated in the cells desensitized with morphine, where U87 MG cells were pre-treated with 100 nM morphine. The functionality of the MOR in U87 MG cells was then demonstrated using morphine inhibition of forksolin-induced intracellular cAMP, as determined by radioimmunoassay. Results: U87 MG cells treated with IL-1 beta for 12 h showed a significant up-regulation of MOR and KOR. DOR expression was also elevated, although not significantly. Treatment with IL-1 beta also showed a significant up-regulation of the MOR in U87 MG cells desensitized with morphine. Co-treatment with IL-1 beta and interleukin-1 receptor antagonist protein (IL-1RAP) resulted in a significant decrease in IL-1 beta-mediated MOR up-regulation. Conclusion: Our results indicate that the pro-inflammatory cytokine, IL-1 beta, affects opiate-dependent pathways by up-regulating the expression of the MOR in both untreated and morphine-desensitized U87 MG.