Inhibition of morphine tolerance and dependence by diazepam and its relation to cyclic AMP levels in discrete rat brain regions and spinal cord

Diazepam inhibits morphine tolerance and dependence and reverses a decrease in the met-enkephalin level in brain induced by morphine. In this study, we investigated whether inhibition of morphine-induced tolerance and dependence by diazepam involved a change in cyclic AMP levels in discrete rat brain regions and spinal cord. Male Sprague-Dawley rats were made tolerant and dependent by subcutaneous (s.c.) implantation of six morphine pellets (two pellets on the first day, and four on the second day). Diazepam (0.25 mg/kg b.wt.) was injected once daily intraperitoneally (i.p.) for 5 days. Control rats were implanted with placebo pellets and injected once daily with saline or diazepam (i.p.). Tail-flick antinociception was measured 1 h after injections everyday. Animals were administered s.c. naloxone (10 mg/kg) to induce naloxone-precipitated withdrawal syndrome on the final day of the experiment (day 5), and the jumping behavior was observed for 30 min. Concomitant treatment with diazepam (0.25 mg/kg) significantly decreased the development of morphine tolerance and dependence. Diazepam (0.25 mg/kg) treated rats also showed a significant decrease in the jumping behavior compared to animals treated with morphine alone. Rats were sacrificed 2 h after the injection of saline or diazepam (0.25 mg/kg) on the fifth day. Cyclic AMP was estimated by RIA. In the control rats, the concentration of cyclic AMP in cortex was > hippocampus > cerebellum > hypothalamus > striatum > midbrain > pituitary > pons/medulla > spinal cord. There was no change in the concentration of cyclic AMP in any of the brain regions examined from morphine tolerant animals. Chronic treatment of diazepam also had no effect on cyclic AMP levels in the brain regions or spinal cord of rats implanted with placebo or morphine pellets. However, concentration of cyclic AMP increased 137% and 71% in the cortex and striatum of rats undergoing naloxone-induced abstinence syndrome. Rats implanted with morphine pellets and treated chronically with diazepam did not show this increase in cyclic AMP concentrations. These results suggest that the inhibition of morphine-induced tolerance and dependence by diazepam may, at least in part, be due to prevention of an increase in cyclic AMP levels in cortex and striatum exhibited by animals undergoing abstinence.