Received for publication May 11, 2006.
Revised August 9, 2006.
Accepted for publication August 9, 2006.

Effects of GSK3 and Cdk5 Kinase Inhibitors on Morphine-induced Analgesia and Tolerance in Rats

Abstract

Repeated administration of morphine is associated with the development of tolerance, yet the mechanism underlying this phenomenon is still poorly understood. Recent evidence implicating glycogen synthase kinase 3 (GSK3) in opioid receptor signaling pathways has prompted us to investigate its role in morphine tolerance. Administration of 10 mg/kg morphine i.p. to Wistar rats twice daily for 8 days resulted in complete tolerance to its analgesic effects as measured by the tail-flick test. When injections of morphine were preceded by intrathecal (i.t.) administration of either an inhibitor of GSK3 (SB216763 or 6-bromoindirubin-3'oxime), or an inhibitor of cyclin dependent kinases (Cdk) - roscovitine, development of tolerance to morphine analgesia was completely abolished. Additionally, a single i.t. injection of either kinase inhibitor was able to restore in a dose dependent manner the analgesic effect of morphine in morphine-tolerant rats. None of the inhibitors in doses used in the present study had analgesic effects of their own, nor had an effect on morphine's analgesic potency. Repeated i.t. administration of either inhibitor had caused an increase in abundance of GSK-3 phosphorylated at Ser9 in the dorsal lumbar part of the spinal cord of rats that were chronically treated with morphine. Furthermore, reversal of morphine tolerance by single injection of either inhibitor was always associated with increased abundance of phospho-GSK3. In conclusion, our data indicate that chronic morphine treatment activates a highly efficient pathway by means of which Cdk5 regulates GSK3 activity.

Key words: cyclin dependent kinase 5, glycogen synthase kinase 3, morphine, opioid analgesia, opioid tolerance, rats