PT - JOURNAL ARTICLE AU - John T. Williams TI - Desensitization of Functional μ-opioid Receptors Increases Agonist Off-rate AID - 10.1124/mol.114.092098 DP - 2014 Apr 18 TA - Molecular Pharmacology PG - mol.114.092098 4099 - http://molpharm.aspetjournals.org/content/early/2014/04/18/mol.114.092098.short 4100 - http://molpharm.aspetjournals.org/content/early/2014/04/18/mol.114.092098.full AB - Desensitization of μ-opioid receptors (MORs) develops over 5-15 min following application of some but not all opioid agonists and lasts for 10s of min following agonist removal. The decrease in function is receptor selective (homologous) and could result from (1) a reduction in receptor number or (2) a decrease in receptor coupling. The present investigation used photolysis of two caged opioid ligands in order to examine the kinetics of MOR-induced potassium conductance before and following MOR desensitization. Photolysis of a caged antagonist, caged-naloxone (CNV-NLX), blocked the current induced by a series of agonists and the time constant of decline was significantly decreased following desensitization. The increase in the rate of current decay was not observed following partial blockade of receptors with the irreversible antagonist, β-CNA. The time constant of current decay following desensitization was never more rapid than 1s, suggesting an increased agonist off rate rather than an increase in the rate of channel closure downstream of the receptor. The rate of GIRK current activation was examined using photolysis of a caged agonist, [Leu]5enkephalin (CYLE). Following acute desensitization or partial irreversible block of MORs with β-CNA there was an increase in the time it took to reach a peak current. The decrease in the rate of agonist-induced GIRK conductance was receptor selective and dependent on receptor number. The results indicate that opioid receptor desensitization reduced the number of functional receptors and the remaining active receptors have a reduced agonist affinity.