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 dec rease in the met-enkephalin level in brain induced by morphine. In thi s study, we investigated whether inhibition of morphine-induced tolera nce and dependence by diazepam involved a change in cyclic AMP levels in discrete rat brain regions and spinal cord. Male Sprague-Dawley rat s were made tolerant and dependent by subcutaneous (s.c.) implantation of six morphine pellets (two pellets on the first day, and four on th e second day). Diazepam (0.25 mg/kg b.wt.) was injected once daily int raperitoneally (i.p.) for 5 days. Control rats were implanted with pla cebo pellets and injected once daily with saline or diazepam (i.p.). T ail-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 (d ay 5), and the jumping behavior was observed for 30 min. Concomitant t reatment with diazepam (0.25 mg/kg) significantly decreased the develo pment of morphine tolerance and dependence. Diazepam (0.25 mg/kg) trea ted rats also showed a significant decrease in the jumping behavior co mpared 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 concent ration of cyclic AMP in cortex was > hippocampus > cerebellum > hypoth alamus > 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 treatme nt of diazepam also had no effect on cyclic AMP levels in the brain re gions or spinal cord of rats implanted with placebo or morphine pellet s. However, concentration of cyclic AMP increased 137% and 71% in the cortex and striatum of rats undergoing naloxone-induced abstinence syn drome. Rats implanted with morphine pellets and treated chronically wi th diazepam did not show this increase in cyclic AMP concentrations. T hese results suggest that the inhibition of morphine-induced tolerance and dependence by diazepam may, at least in part, be due to preventio n of an increase in cyclic AMP levels in cortex and striatum exhibited by animals undergoing abstinence.