Kinetics of recovery from opioids at wild-type and mutant mu opioid receptors expressed in Xenopus oocytes

To investigate a previous observation that classical antagonists behave as agonists at mutant H297N and H297Q mu opioid receptors, we compared the kin etics of recovery from opioids at wild-type and mutant mu receptors express ed in voltage-clamped Xenopus oocytes. The cDNA for the potassium channel G IRK1 was coinjected into the oocytes with that of the mu receptors to trans duce agonist binding into a coupled electrophysiological response. The kine tics of recovery were estimated by brief test pulses of the agonist normorp hine given at a frequency of 0.67 or 1 per min. After treatment with a vari ety of agonists, the receptors recovered from desensitization at rates that depended on the agonist, but there was little difference between mutant an d wild-type receptors. Antagonists, however, induced agonist-like currents and demonstrated faster recovery at the mutant receptors. These results sug gest that His-297 may comprise part of an antagonist subsite. This conclusi on, when coupled with the steric theory that intrinsic activity depends on independent binary equilibration of a drug between agonist and antagonist s ubsites, could unify the paired observations that antagonists become agonis ts and recover faster at the mutant than at the wild-type receptors. Publis hed 2000 Wiley-Liss, Inc.dagger