Introduction: ATP-mediated neurotransmission is present in peripheral and central nervous systems. Purinergic receptors responsible for effects of ATP on neurons are divided into three main classes, including P1, P2Y, and P2X receptors. According to many published pharmacological studies, morphine tolerance and withdrawal are associated not only with changes in mu-opioid receptor signaling but also with the signaling pathway of some other receptors such as purine receptors. The involvement of two types of P2X receptors known as P2X4 and P2X7 in morphine tolerance and withdrawal has been proposed. In this study, we aimed to investigate changes in the p2rx4 and p2rx7 gene expression in the striatum and cerebellar cortex of rats after morphine tolerance and withdrawal in rats. Method: Four groups of male Wistar rats were used. Morphine tolerance was induced by repeated injections of morphine (10 mg/kg) twice daily for 10 days. A control group received saline (1 ml/kg) instead of morphine during the schedule. Induction of morphine tolerance was assessed using a hotplate test of analgesia on day 10. Two hours after the last repeated injections on day 10, each rat was anesthetized, decapitated, and the striatum and cerebellar cortex were dissected on an ice-chilled surface. Two other groups of rats subjected to 30 days withdrawal after 10 days of the repeated saline or morphine treatments, and the intended brain areas were extracted on day 30 of the withdrawal. Changes in gene expression were assessed using real-time PCR. A two-way repeated measure ANOVA was used to analyze the hotplate data. The real time-PCR data was converted to 2-ΔΔCT values. An unpaired t-test was used for comparisons of the experimental groups. P<0.05 was considered as statistically significant level throughout. Results: Real-time PCR results showed that expression of p2rx4 and p2rx7 in the cerebellar cortex significantly decreased in morphine-tolerant rats (P <0.001). After morphine withdrawal, no group difference was detected for p2rx4 expression, but p2rx7 expression significantly increased compared with the control group (P <0.001). The gene expression results in the striatum of morphine-tolerant rats revealed no group difference for the p2rx4 gene expression, but there was a significant decrease in expression of p2rx7 compared with saline-treated control group (P <0.01). However, expression of p2rx4 in the striatum of rats after withdrawal significantly increased compared with control group (P <0.001). No group difference was detected in expression of p2rx7 between the experimental groups. Conclusion: It can be concluded that morphine tolerance site-specifically affects the gene expression of P2X4 and P2X7 receptors in the cerebellar cortex and striatum, which are compensated after Morphine withdrawal. The present results suggest important functional interaction between the purinergic system and morphine tolerance and withdrawal.
