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 mo rphine treatment was investigated in rats. Male Sprague-Dawley rats (2 00 - 260 g) were rendered tolerant to morphine by i.p. injections of i ncreasing doses of morphine twice daily for 4 or 6 days. During this p eriod, there was a time-dependent increase in the AD50 values for morp hine to inhibit the tail-flick response. In addition, in vitro mu-opio id receptor binding to midbrain P2 membranes from these animals reveal ed that the ability of 10 mumol/l Gpp(NH)p (guanyl-5'-yl imidodiphosph ate) to decrease [H-3]DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) binding affin ity, i.e., the ratio K(d)(+Gpp(NH)p)/K(d)(-Gpp(NH)p), decreased signif icantly 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 stim ulate low K(m) GTPase activity was also investigated. The EC50 signifi cantly increased from 2.7 +/- 1.1 x 10(-8) mol/l in the control group to 10.8 +/- 1.5 x 10(-8) mol/I after 4 days of morphine treatment and was further increased to 13.5 +/- 2.1 x 10(-8) mol/l after 6 days of m orphine treatment. The maximal stimulation by DAMGO decreased signific antly from 18.0 +/- 1.7% to 12.8 +/- 1.6% after 6 days of morphine tre atment. These results indicate that the interaction between mu-opioid receptors and G proteins had been altered after chronic morphine treat ment.