MU-OPIOID RECEPTOR DOWN-REGULATION AND CAMP-DEPENDENT PROTEIN-KINASE PHOSPHORYLATION IN A MOUSE MODEL OF CHRONIC MORPHINE-TOLERANCE

Results of radioligand binding and transfected receptor studies indica te that mu-receptor down-regulation and phosphorylation may be critica l to the expression of morphine tolerance. In this study, an animal mo del of morphine tolerance was used to correlate antinociception with c hanges in receptor number and phosphorylation state. mu-Opioid recepto r protein was quantitated by Western immunoassay of brainstem tissue f rom morphine-treated mice. Degree of receptor phosphorylation was asse ssed using immunoprecipitation (IP) of the receptor followed by back-p hosphorylation. Acutely administered morphine produced no changes in m u-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 prote in kinase A (PKA)-induced receptor phosphorylation shortly after acute morphine administration, followed by a naloxone-reversible increase i n phosphorylation of the receptor that correlated with the onset of an tinociception. Chronic morphine administration resulted in a decrease in PKA-induced phosphorylation of the mu-receptor. Since it has been s hown that PKA activity is enhanced in the brains of morphine-tolerant mice, this decrease in mu-receptor phosphorylation suggests that the m u-receptor may be structurally or conformationally altered in the morp hine-tolerant state. (C) 1998 Elsevier Science B.V.