Inhibition of morphine tolerance and dependence by diazepam and its relation to μ-opioid receptors in the rat brain and spinal cord

We have recently observed that concomitant administration of diazepam to morphine pellet implanted rats results in the inhibition of the development of morphine tolerance and dependence. We have now analyzed μ-opioid receptors in rats treated with morphine and diazepam for 5 days by using -DAMGO for binding studies. 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.). Animals were administered s.c. naloxone (10 mg/kg) to induce naloxone-precipitated withdrawal syndrome on the final day of the experiment (day 5). There was an up-regulation of μ-receptor (B_max increased) in the spinal cord of morphine tolerant (+139%) and dependent (+155%) rats compared to saline treated animals. Diazepam treatment abolished the up-regulation of μ-receptors in spinal cord of morphine treated rats. In the cortex, B_max was not affected in morphine tolerant or dependent rats but it decreased by 38% in morphine tolerant and 65% in morphine dependent rats treated with diazepam. The K_d of μ-receptors increased in the cortex, striatum and hypothalamus of morphine dependent rats. Diazepam treatment decreased the K_d of μ-receptors in the cortex of morphine tolerant and hypothalamus of morphine-dependent rats. These results suggest that diazepam treatment antagonizes the up-regulation of CNS μ-receptors observed in morphine tolerant rats. In addition, morphine tolerance and dependence may be associated with conversion of μ-opioid receptors to μ*-constitutive opioid receptors that are less active, and this conversion is prevented in the brain of animals treated with diazepam.