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

The interaction of mu-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 mu-opioid receptor binding to midbrain P2 membranes from these animals revealed that the ability of 10 mumol/l Gpp(NH)p (guanyl-5'-yl imidodiphosphate) to decrease [3H]DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) binding affinity, i.e., the ratio Kd(+Gpp(NH)p)/Kd(-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 Km GTPase activity was also investigated. The EC50 significantly increased from 2.7 +/- 1.1 x 10(-8) mol/l in the control group to 10.8 +/- 1.5 x 10(-8) mol/l after 4 days of morphine treatment and was further increased to 13.5 +/- 2.1 x 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 mu-opioid receptors and G proteins had been altered after chronic morphine treatment.