Introduction: Morphine tolerance limits the use of morphine as a potent painkiller. Mechanisms underlying morphine tolerance and dependence have remained unclear. Circular RNAs (circRNAs) are derived from circularization and covalent joining of back-spliced exons and/or introns transcribed from classic genes. CircRNAs might be a novel and promising target involved in changes at gene expression levels. However, very little is known about their potential involvement in epigenetic regulation of morphine tolerance and dependence. We aimed to examine the expression of circHomer1a, a neuronal-enriched circRNA abundantly expressed in the brain, derived from Homer protein homolog 1 (Homer1a), in the striatum. We also examine the effects of circHomer1a on some microRNA levels such as miR-124-5p, miR-141-5p, and miR-183-3p in the striatum after induction of morphine tolerance and also after withdrawal. Method: Male Wistar rats were used in the present experiments. A control group (n = 8) received saline (1 ml/kg) but another experimental group received morphine (10 mg/kg) twice a day for 10 days in order to induce morphine tolerance. On day 10, each rat was anesthetized, decapitated, and the striatum was dissected on an ice-chilled sterile surface. Two other experimental groups of rats after receiving 10 days of saline or morphine injections subjected to 30 days of drug-free, and then their striatum was extracted on day 30 of the withdrawal. The expression of Homer1a, circHomer1a, miR-124-5p, miR-141-5p, and miR-185-3p were assessed by using a real-time PCR method. Stem-loop primers were used for evaluating miRNAs expression. The gene expression data were analyzed with 2-ΔΔCT method. An independent t-test was used for pairwise between-groups comparisons. P<0.05 was considered a statistically significant level. Result: The results of real-time PCR indicated that the expression of Homer1a and cirHomer1a significantly increased in the striatum after induction of morphine tolerance compared to the control group. Expression of miR-124-5p significantly decreased, miR-185-3p significantly increased, but no group difference was detected for miR-141-5p expression in the morphine-tolerated group compared to the saline-treated control group. After 30 days of morphine withdrawal, the expression of Homer1a significantly decreased in the striatum compared to the respective control group but no group difference was detected for circHomer1a in comparison with the control group. Expression of miR-124-5p, miR-141-5p, and miR-185-3p significantly increased in the striatum of morphine withdrawal rats compared to the respective control group. Conclusion: The present results support this notion that Circular RNA circHomer1a in rat striatum acts as a miR-124-5p sponge in the induction of morphine tolerance but its effect on expression levels of miR-124-5p, miR-141-5p, and miR-185-3p during withdrawal is more complicated and needs more evaluations. It can be proposed that circHomer1a may have additional functional roles in morphine tolerance and withdrawal, which needs more investigation in the future.
