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 kinetics of recovery from opioids at wild-type and mutant mu receptors expressed in voltage-clamped Xenopus oocytes. The cDNA for the potassium channel GIRK1 was coinjected into the oocytes with that of the mu receptors to transduce agonist binding into a coupled electrophysiological response. The kinetics of recovery were estimated by brief test pulses of the agonist normorphine given at a frequency of 0.67 or 1 per min. After treatment with a variety of agonists, the receptors recovered from desensitization at rates that depended on the agonist, but there was little difference between mutant and wild-type receptors. Antagonists, however, induced agonist-like currents and demonstrated faster recovery at the mutant receptors. These results suggest that His-297 may comprise part of an antagonist subsite. This conclusion, when coupled with the steric theory that intrinsic activity depends on independent binary equilibration of a drug between agonist and antagonist subsites, could unify the paired observations that antagonists become agonists and recover faster at the mutant than at the wild-type receptors. Synapse 38:254-260, 2000. Published 2000 Wiley-Liss, Inc.