Results of radioligand binding and transfected receptor studies indicate that mu-receptor down-regulation and phosphorylation may be critical to the expression of morphine tolerance. In this study, an animal model of morphine tolerance was used to correlate antinociception with changes in receptor number and phosphorylation state. mu-Opioid receptor protein was quantitated by Western immunoassay of brainstem tissue from morphine-treated mice. Degree of receptor phosphorylation was assessed using immunoprecipitation (IP) of the receptor followed by back-phosphorylation. Acutely administered morphine produced no changes in mu-receptor quantity. Chronic morphine administration resulted in a 50% reduction in receptor protein quantity over placebo-treated samples. Back-phosphorylation experiments showed a drop in cAMP-dependent protein kinase A (PKA)-induced receptor phosphorylation shortly after acute morphine administration, followed by a naloxone-reversible increase in phosphorylation of the receptor that correlated with the onset of antinociception. Chronic morphine administration resulted in a decrease in PKA-induced phosphorylation of the mu-receptor. Since it has been shown that PKA activity is enhanced in the brains of morphine-tolerant mice, this decrease in mu-receptor phosphorylation suggests that the mu-receptor may be structurally or conformationally altered in the morphine-tolerant state.
Copyright 1998 Elsevier Science B.V.