Abstract

Fentanyl, and its structural analogs lofentanil and sufentanil, are potent analgesics used clinically in the management of pain. However, the high analgesic potency of these compounds is limited by the development of tolerance after chronic use. To investigate whether their tolerance development may be related to mu receptor desensitization, the cloned mouse mu receptor as well as mutant forms of the receptor were stably expressed in HEK 293 cells and tested for their response to continuous opioid treatment. Fentanyl and its analogs potently bound to the mu receptor and effectively inhibited cAMP accumulation. Three-hour pretreatment ofmu receptors with fentanyl and its analogs desensitized themu receptor by uncoupling it from adenylyl cyclase. The fentanyl analogs caused a slight internalization of the mureceptor as accessed by antibody binding to the epitope-taggedmu receptor. Truncation of the mu receptor by removal of its carboxyl terminus at Glu³⁴¹ did not affect the ability of the fentanyl analogs to bind to and activate themu receptor nor did it prevent the fentanyl analogs from desensitizing the receptor. In a previous study we showed that morphine did not desensitize the cloned mu receptor even though it is a potent and effective agonist at the mu receptor. Mutagenesis studies revealed that morphine interacts differently with the mu receptor to activate it than do the fentanyl analogs which may explain its lack of desensitization of the mureceptor. These results indicate that desensitization of themu receptor may be a molecular basis for the development of tolerance to fentanyl and its analogs.