The interaction of the mu-opioid receptor and G protein is altered after chronic morphine treatment in rats

The interaction of μ-opioid receptors and G proteins after chronic morphine treatment was investigated in rats. Male Sprague-Dawley rats (200-260 g) were rendered tolerant to morphine by i.p. injections of increasing doses of morphine twice daily for 4 or 6 days. During this period, there was a time-dependent increase in the AD₅₀ values for morphine to inhibit the tail-flick response. In addition, in vitro μ-opioid receptor binding to midbrain P₂ membranes from these animals revealed that the ability of 10 μmol/l Gpp(NH)p (guanyl-5′-yl imidodiphosphate) to decrease [³H]DAMGO (TyrD-Ala-Gly-McPhe-Gly-ol) binding affinity, i.e., the ratio K_d(+Gpp(NH)p)/K_d(-Gpp(NH)p), decreased significantly from the control value of 3.68±0.40 to 2.37±0.35 after 6 days of morphine treatment (P<0.05). The ability of DAMGO to stimulate low K_m GTPase activity was also investigated. The EC₅₀ significantly increased from 2.7± 1.1 × 10^-8 mol/l in the control group to 10.8±1.5 × 10^-8 mol/l after 4 days of morphine treatment and was further increased to 13.5±2.1 × 10^-8 mol/l after 6 days of morphine treatment. The maximal stimulation by DAMGO decreased significantly from 18.0±1.7% to 12.8± 1.6% after 6 days of morphine treatment. These results indicate that the interaction between μ-opioid receptors and G proteins had been altered after chronic morphine treatment.