Introduction: Mesolimbic dopaminergic system as the brain reward circuity originates from midbrain and is projected to forebrain limbic structures. The chronic administration of morphine leads to a gradual tolerance to the analgesic and rewarding effects of morphine. The precise underlying mechanism of morphine tolerance remains still unresolved. According to recent reports, the immune cell signaling in the central nervous system contributes to the decreased efficacy of morphine. It has been reported that activated glial cells facilitate pain transmission and oppose morphine analgesia. Morphine affects glia by binding to the innate immune receptor toll-like receptor 4 (TLR4), leading to the release of proinflammatory cytokines such as interlukine-1 (IL-1) and interlukine-6 (IL-6). In this study, we aim to investigate changes in the gene expression of TLR1, TLR4, IL-1R and IL-6R in the midbrain of morphine-tolerant rats. Method: Two experimental groups of male Wistar rats received a regimen of 8 days treatments of saline (1 ml/kg) or morphine (10 mg/kg) twice daily. Induction of analgesic tolerance to the morphine was assessed with a hotplate apparatus on day 8 of the schedule. For gene expression study, each rat was sacrificed, the whole brain was removed, and the midbrain was dissected on day 8 of the schedule. The gene expression of TLR1, TLR4, IL-1R, and IL-6R were examined by using a real-time q-PCR method. A two-way repeated measure ANOVA was used for analyzing the hotplate data. The real time-PCR results were converted to 2-ΔΔCT data, and then an independent sample t-test was used for the pairwise between group comparisons. P<0.05 was set as a statistically significant level. Results: The results revealed that the repeated morphine treatment induced analgesic tolerance to the drug on day 8 of the schedule (P<0.001). The results of the q-PCR revealed significant decreases in the gene expression of TLR1, TLR4, IL-1R and IL-6R at mRNA levels in the midbrain in morphine-tolerant rats compared to the saline-treated control group. Conclusion: We conclude that chronic morphine treatment induces inflammation via increases in inflammatory cytokines in the midbrain, which may in turn alter the gene expression of toll-like receptors and proinflammatory cytokine receptors in the midbrain. We propose that changes in inflammatory cytokine receptors induces disturbances in the downstream signaling molecules, which may underlie morphine tolerance.
