Association of morphine-induced analgesic tolerance with changes in gene expression ofGluN1andMOR1in rat spinal cord and midbrain.
OBJECTIVE
We aimed to examine association of gene expression of MOR1 and GluN1 at mRNA level in the lumbosacral cord and midbrain with morphine tolerance in male Wistar rats.


METHODS
Analgesic effects of morphine administrated intraperitoneally at doses of 0.1, 1, 5 and 10 mg/kg were examined using a hot plate test in rats with and without a history of 15 days morphine (10 mg/kg) treatment. Morphine-induced analgesic tolerance was also assessed on days 1, 5, 10 and 15 of chronic morphine injections. Two groups with history of 15 days injections of saline or morphine (10 mg/kg) were decapitated on day 15 and their lumbosacral cord and midbrain were dissected for evaluatingMOR1andGluN1gene expression.


RESULTS
The results of the hot plate test showed that morphine (5 and 10 mg/kg) induced significant analgesia in naïve rats but its analgesic effects in rats receiving 15 days injections of morphine (10 mg/kg) was decreased, indicating tolerance to morphine analgesia. The results also showed that theGluN1gene expression in tolerant rats was decreased by 71% in the lumbosacral cord but increased by 110 % in the midbrain compared to the control group. However, no significant change was observed for theMOR1gene expression in both areas.


CONCLUSIONS
It can be concluded that tolerance following administration of morphine (10 mg/kg) for 15 days is associated with site specific changes in theGluN1gene expression in the spinal cord and midbrain but theMOR1gene expression is not affected.
Department of Biological Science, Faculty of Science, University of Kurdistan, Sanandaj, Iran.
Shamseddin
S
Ahmadi
Department of Biological Science, Faculty of Science, University of Kurdistan, Sanandaj, Iran.
Fatemeh
F
Miraki
Department of Biological Science, Faculty of Science, University of Kurdistan, Sanandaj, Iran.
Jalal
J
Rostamzadeh
Analgesic tolerance; Gene expression; Mu-opioid receptor; NMDA receptor; Rat