Thalidomide Promotes Morphine Efficacy and Prevents Morphine-Induced Tolerance in Rats with Diabetic Neuropathy

Opioid analgesics have less efficacy in diabetic neuropathy treatment, and tolerance often occurs after chronic usage. Given that thalidomide can potentiate the morphine efficacy in diabetic neuropathy treatment, we investigated the effects of intrathecal administrations of thalidomide on morphine tolerance during the treatment of diabetic neuropathy. We found that intrathecal administrations of thalidomide (25 mg/kg/ml) potentiated the analgesic effects of morphine on mechanical hyperalgesia and prevented the development of morphine tolerance. While this treatment regimen did not alter the protein levels of μ-opioid receptor (MOR) in the spinal cord of diabetic rats, chronic morphine treatment robustly increased MOR binding density in the synaptic plasma membranes fraction, but decreased it in the microsomal fraction. Furthermore, thalidomide was able to reverse the distribution of MOR altered by chronic morphine treatment. Finally, STZ-induced diabetes promoted PKC activation and enhanced TNFα level in the spinal cord, which were attenuated by intrathecal administrations of thalidomide. Taken together, these results suggested that thalidomide may potentiate morphine efficacy on diabetic neuropathy and prevent the development of morphine tolerance by suppressing PKC activation and TNFα level in the spinal cord.