In this study, we investigated changes in mu-opioid and NMDA receptors gene expression at mRNA levels after induction of morphine tolerance in the spinal cord and the midbrain. Male Wistar rats weighting 250-300 (g) were used. Groups of rats received daily injections of morphine (10 mg/kg) for 14 days, while a control group received saline (1 ml/kg). A hot plate test of analgesia was used to assess morphine analgesic tolerance after 5, 10 and 15 days of injections. One day after induction of morphine tolerance, rats were sacrificed, and their midbrain extracted. A semi-quantitative RT-PCR method was used for evaluating gene expressions. The results of the hot plate test showed that morphine tolerance was induced during 14 days of morphine injections, as revealed by a decrease of the analgesic effect of morphine (10 mg/kg) on one day after the last morphine injection (on day 15). The results of gene expression study showed that mRNA levels of mu-opioid receptors were not significantly altered in the spinal cord and the midbrain after morphine tolerance compared to the control group. The results also revealed that expression of NMDA receptor subunit 1 at mRNA levels in the spinal cord was not significantly altered, but its expression in the midbrain was significantly increased after morphine tolerance compared to the control group. It can be concluded that morphine tolerance dose not result in changes in mu-opioid receptors expression in the spinal cord and midbrain, so morphine analgesic tolerance may result from changes in expression of NMDA receptors and their modulatory effects on pain pathways mainly located in the midbrain.
