Introduction: Transient receptor potential vanilloid 1 (TRPV1) channel is one type of cationic channels that is involved in pain perception. Considering the existence of TRPV1 channels in the pain pathway, it is logical to hypothesize that these channels are involved in morphine analgesic tolerance. In this study, we aimed to examine changes in TRPV1 gene expression in rat lumbosacral cord and midbrain during induction of morphine analgesic tolerance. Methods: Male Wistar rats weighing 280-300 g were used. First, two experimental groups received morphine (10 mg/kg, i.p.) or saline (1 ml/kg, i.p.) twice daily for 8 consecutive days and induction of morphine analgesic tolerance was examined with a hotplate test on days 1, 4 and 8 of the injections. Second, six groups of animals in three sets of two groups were received the same treatments and they were sacrificed on days 1, 4 or 8 of the injections. The rat lumbosacral cord and midbrain were immediately dissected to examine changes in gene expression of TRPV1 during induction of morphine analgesic tolerance. Results: The result of hotplate test during the 8 days of the injections showed that morphine induced significant analgesia on day 1 of the injections but its analgesic effect was significantly decreased on days 4 and 8 compared to the saline-treated group, suggesting induction of morphine analgesic tolerance. The results also revealed that gene expression of TRPV1 in the lumbosacral cord was increased on day 4 of morphine injections compared to the saline-treated control group but its expression was not significantly differ from control group on days 1 and 8 of the injections. However, the TRPV1 gene expression in the midbrain was significantly increased on day 1 of morphine injection but its expression returned to its expression level in the control group on days 4 and 8 of the injections. Conclusions: It can be concluded that the TRPV1 gene expression has a site specific pattern in the spinal cord and midbrain
