Decreased basal endogenous opioid levels in diabetic rodents: effects on morphine and delta-9-tetrahydrocannabinoid-induced antinociception

We have previously demonstrated synergy between morphine and Δ⁹-tetrahydrocannabinol (Δ⁹-THC) in the expression of antinociception in acute pain models and in arthritic models of chronic pain. Our data has been extended to include acute pain in both diabetic mice and rats. In diabetic mice, Δ⁹-THC p.o. was more active in the tail-flick test in the diabetic mouse than in the non-diabetic mouse. Morphine (s.c.) was less potent in diabetic than in non-diabetic mice 6.1 (5.1-7.2) versus 3.2 (2.4-4.1) mg/kg, respectively], an effect previously extensively documented in pre-clinical and clinical testing. In addition, the combination of Δ⁹-THC with morphine produced a greater-than-additive relief of acute pain in mice. In the rat, the induction of the diabetic state decreased the antinociceptive effect of morphine, an effect temporally related to a decreased release of specific endogenous opioids. Conversely, Δ⁹-THC retained the ability to release endogenous opioids in diabetic rats and maintained significant antinociception. Extrapolation of such studies to the clinical setting may indicate the potential for use of Δ⁹-THC-like drugs in the treatment of diabetic neuropathic pain, alone or in combination with very low doses of opioids.