MU-OPIOID RECEPTOR INTERNALIZATION - OPIATE DRUGS HAVE DIFFERENTIAL-EFFECTS ON A CONSERVED ENDOCYTIC MECHANISM IN-VITRO AND IN THE MAMMALIAN BRAIN

mu-Opioid receptors are the pharmacological targets of endogenous opio id peptides and morphine-like alkaloid drugs. Previous studies of tran sfected cells and peripheral neurons indicate that opioid receptors ar e rapidly internalized after activation by the alkaloid agonist etorph ine but not after activation by morphine. To determine whether opioid receptors in the central nervous system are regulated by a similar pro cess of agonist-selective internalization, mu-opioid receptors were ex amined in rat brain neurons after treatment of animals with opioid dru gs. Internalized mu receptors were observed within 30 min after intrap eritoneal injection of the alkaloid agonist etorphine, and this proces s was blocked by the antagonist naloxone. Colocalization of internaliz ed opioid receptors with transferrin receptors in confocal optical sec tions indicated that receptor internalization observed in vivo is medi ated by a membrane trafficking pathway similar to that observed previo usly in vitro using transfected human embryonic kidney 293 cells. Morp hine failed to induce detectable rapid internalization of receptors, e ven when administered to animals at doses far in excess of those requi red to induce analgesia. To quantify these agonist-selective differenc es and to analyze an array of opioid ligands for their ability to trig ger internalization, we used flow cytometry on stably transfected 293 cells. These studies indicated that the different effects of individua l agonists are not correlated with their potencies for receptor activa tion and that a variety of clinically important agonists differ signif icantly in their relative abilities to stimulate the rapid internaliza tion of opioid receptors.