Introduction: Dependence and analgesic tolerance developed after the repeated or continuous utility of morphine are major problems that hinder its use in pain management. A large number of studies at the molecular, cellular, and systems levels have been devoted to elucidating the underlying mechanisms of morphine dependence and tolerance but they still have remained elusive. According to research, there is potential crosstalk between toll-like receptor 4 (TLR4) and opioid receptor signaling pathways, which is implicated in opioid analgesia and tolerance. Chronic morphine treatment activates TLR4 in microglia, which enhances proinflammatory cytokines and resultantly facilitates the development and maintenance of analgesic tolerance. Besides, the involvement of non-coding RNAs, including microRNAs, long non-coding RNAs, and circular RNAs (circRNAs) as epigenetic regulators of morphine tolerance has also been revealed during recent years. MiR-181 family could bind to TLR4 mRNA and decrease its expression. It has also been shown that a circRNA known as circSerpini may sponge miR-181 family. In this study, we aimed to investigate the expression of circSerpini and miR-181 Family in the striatum after morphine tolerance and withdrawal in rat. Method: Morphine tolerance was induced by repeated injections of morphine (10 mg/kg) twice daily for 10 consecutive days. A control group received saline (1 ml/kg) instead of morphine during 10 days of the repeated injection. Two hours after the last repeated injection on day 10, each rat was anesthetized, decapitated, and the striatum was immediately dissected on an ice-chilled surface. Two other groups of rats after receiving 10 days of saline or morphine treatments subjected to additional 30 days of morphine withdrawal and their striatum was extracted on day 30 of the drug washout. Changes in the expression of circular RNA and microRNAs were assessed with real-time PCR. The real time-PCR raw data was converted to 2-ΔΔCT values and then analyzed with an independent t-test. P<0.05 was considered a statistically significant level throughout. Results: The results revealed that expression of circSerpini in the striatum significantly increased in morphine-tolerant rats compared to the control saline-treated group. In line with this result, the expression of miR-181 family, including miR-181b-3p, miR-181b-5p, miR-181c-3p, and miR-181c-5p significantly decreased in the striatum (P < 0.001) in morphine-tolerant rats compared to the control group. Interestingly, after 30 days of withdrawal, the circSerpini expression significantly increased in the morphine withdrawal group compared to the control group. The results also revealed a significant increase in expression of miR-181b-3p and miR-181c-5p (P < 0.001), a significant increase in miR-181b-5p expression (P < 0.05), but no group difference was detected for miR-181c-3p in the morphine withdrawal group compared to the control group. Conclusion: It can be concluded that the expression of circSerpini via affecting the level of miR-181 family is involved in morphine tolerance and withdrawal. It is possible that circSerpini via sponging miR-181 family relieves their suppression of TRL-4 expression, thus evokes neuroinflammation and promoting morphine tolerance finally.
