Inhibition of morphine tolerance and dependence by diazepam and its relation to the CNS Met-enkephalin levels

The effect of diazepam on the development of morphine tolerance and dependence was investigated. Male Sprague-Dawley rats were rendered tolerant and dependent by subcutaneous implantation of six morphine pellets. Diazepam (0.025, 0.25 or 2.5 mg/kg body weight) was once daily injected intraperitoneally into rats starting on the first day of implantation. Antinociception was measured by tail-flick (TF) and hot plate (HP) tests, and the extent of sedation determined by a rotarod test before and one hour after diazepam injections everyday for 5 days. Physical dependence on morphine was assessed by an antagonist-precipitated abstinence syndrome on the fifth day of treatment by injecting naloxone 10 mg/kg subcutaneously. Diazepam (0.025-2.5 mg/kg body weight) did not produce significant antinociception or sedation (sensorimotor impairment) in rats implanted with placebo pellets. Diazepam (0.25 and 2.5 mg/kg) inhibited tolerance to TF antinociception in rats implanted with morphine pellets. Sedation as evidenced by sensorimotor impairment induced by morphine pellet implantation was not influenced by diazepam (0.025-2.5 mg/kg). Diazepam administration (0.25 mg/kg) also decreased the degree of jumping behavior observed following naloxone injection in morphine pellet implanted rats. Serum morphine concentration in morphine-diazepam treated rats was not significantly different from that in morphine-saline treated rats. Finally, a decrease in the Met-enkephalin levels observed in the hypothalamus, hippocampus, cortex and spinal cord of morphine dependent rats was reversed by injecting diazepam along with morphine pellet implantation. These results suggest that diazepam inhibits morphine tolerance and dependence, and also prevents morphine-induced decrease in the CNS Met-enkephalin levels in morphine dependent rats.