Background and Aim: A growing body of evidence reveals not only the mu-opioid receptors but also the N-methyl-D-Aspartate (NMDA) receptors, as well as the dopamine receptors may play important roles in the induction of morphine addiction and tolerance. Methods Male Wistar rats were used, and morphine tolerance was induced with eight days injections of morphine ۱۰ mg/kg (i.p.) twice daily. A control group of rats received saline (۱ ml/kg) instead of morphine during the treatments. We performed a hotplate test of analgesia on days ۱ and ۸ of the schedule to examine morphine-induced analgesic tolerance. The gene expression study was performed using a real-time qPCR method in the intended tissues of saline or morphine-treated groups on day ۸ of the treatments. A two-way repeated measure ANOVA followed by Holm-Sidak post hoc test was used for analyzing the hotplate data. For analyzing the gene expression results, Iwe used the Ct values of real time q-PCR data to calculate ۲-AACT., and then we used an independent t-test for analyzing pairwise between group comparisons. The significant statistical level was set at P<۰.۰۵. Results: We aim to examine the mRNA levels of the mu-opioid receptor subunit ۱ (MORI), NMDA receptor subunit ۱ (GluN۱), dopamine D۱ receptor (DIR) and D۵ receptor (D۵R) in different brain areas, including the midbrain, striatum, hippocampus and prefrontal cortex of morphine-tolerant rats. Conclusion: The results of the hotplate test of analgesia revealed that eight days of the morphine treatments successfully developed morphine tolerance (P<۰.۰۰۱). The results of the real-time PCR indicated that the expressions of MORI significantly increased in the midbrain (P<۰.۰۵) and the
PFC (P<۰.۰۱) but remained without any significant changes in the hippocampus and striatum of morphine-tolerant rats compared to the control group. The results of the gene expression in morphine tolerant rats also revealed that the GluN۱ gene expression significantly decreased in the midbrain (P<۰.۰۱) and the striatum (P<۰.۰۱) but increased in the
PFC (P<۰.۰۵) while remained without any significant change in the hippocampus. The DIR gene expression significantly increased in the
PFC (P<۰.۰۰۱) but had no changes in the midbrain, striatum and hippocampus. The results also revealed no significant changes in the D۵R gene expression in no one of the examined brain areas. According to the results, we conclude that the midbrain and the
PFC are more susceptible areas for the changes in the gene expression, which may underlie morphine addiction and tolerance.
