Agonist-Specific Regulation of -Opioid Receptor Desensitization and Recovery from Desensitization

Agonist-selective actions of opioids on the desensitization of -opioid receptors (MORs) have been well characterized, but few if any studies have examined agonist-dependent recovery from desensitization. The outward potassium current induced by several opioids was studied using whole-cell voltage-clamp recordings in locus ceruleus neurons. A brief application of the irreversible opioid antagonist -chlornaltrexamine ( -CNA) was applied immediately after treatment of slices with saturating concentrations of opioid agonists. This approach permitted the measurement of desensitization and recovery from desensitization using multiple opioid agonists, including [Met]enkephalin (ME), [D-Ala,N-Me-Phe,Gly-ol]-enkephalin (DAMGO), etorphine, fentanyl, methadone, morphine, morphine-6-glucuronide, oxycodone, and oxymorphone. The results indicate that desensitization protects receptors from irreversible antagonism with -CNA. The amount of desensitization was measured as the decrease in current during a 10-min application of a saturating agonist concentration and was a good predictor of the extent of receptor protection from irreversible inactivation with -CNA. After desensitization with ME or DAMGO and treatment with -CNA, there was an initial profound inhibition of MOR-induced current that recovered significantly after 45 min. There was, however, no recovery of MOR-mediated current with time after treatment with agonists that did not cause desensitization, such as oxycodone. These results demonstrate that desensitization prevents irreversible inactivation of receptors by -CNA. Opiates are the most effective analgesics known. Activation of the -opioid receptor (MOR) belies their therapeutic efficacy as well as the euphoria and rewarding properties that lead to their abuse. Agonist-bound MORs activate G proteins and signal through downstream effectors. They also become substrates for the regulatory machinery responsible for agonist-induced MOR desensitization, endocytosis, and recovery from desensitization (Williams et al., 2001; Connor et al., 2004; von Zastrow, 2004; Bailey and Connor, 2005). Different opioid agonists have widely varying signaling efficacies in each of these processes. For example, the highly efficacious endogenous peptide agonist [Met]enkephalin (ME) causes both profound desensitization and internalization. However, morphine and some other alkaloid opiates are regarded as partial agonists that induce desensitization and endocytosis to a lesser degree (Yu et al., 1997; Keith et al., 1998; Whistler and von Zastrow, 1998; Alvarez et al., 2002; Borgland et al., 2003; Celver et al., 2004; Schulz et al., 2004; Dang and Williams, 2005; Koch et al., 2005; Johnson et al.,