Oregon Health & Science University
Opioids are the most effective analgesics known, but they are also widely abused due to prescription diversion and the availability of illicit formulations, such as heroin. A fundamental feature of opiate use is tolerance and this property both compromises clinical utility and escalates the degree of dependence in opiate abusers. The effects of opioids are mediated through the f...L-opioid receptor (MOR), and following agonist binding, MOR becomes the substrate for several regulatory events. These events - desensitization, internalization and re-sensitization - represent the cellular corollaries of tolerance. However, tolerance is a complex physiological adaptation and the specific cellular mechanisms contributing to its development remain elusive. In addition to peptide agonists, there are several alkyloid opiates that signal differently through MOR and induce different degrees of regulation. Despite their prevalent use, few studies have examined their potential for inducing rapid desensitization and subsequent MOR recovery, particularly in neurons. In this study, whole-cell patch clamp recordings were made in slices containing locus coeruleus neurons and G protein coupled inward rectifying potassium (GIRK) currents were measured to monitor MOR signaling. The results of this study indicate that efficacy for inducing GIRK currents was correlated with the degree of desensitization induced for any given agonist. Furthermore, for each agonist, the degree of desensitization measured correlated with the amount of recovery observed. Oxycodone and buprenorphine demonstrated unique effects. Oxycodone was the only agonist that failed to induce desensitization or any subsequent recovery. Buprenorphine was capable of causing a small membrane hyperpolarization, but did not induce any measurable current. While bound to MOR, however, it completely inhibited the otherwise robust desensitization characteristic of [Met] 5enkephalin (ME). Despite the attenuation of ME-induced desensitization, MOR signaling was facilitated in buprenorphine pre-treated cells exposed to repeat ME applications. These findings indicate that buprenorphine has a novel influence on MOR regulation. Chronic treatment with five different opioid agonists revealed different persistent adaptations specific to acute MOR regulation. Drugs were delivered via osmotic minipumps for 6-7 days before brain slices containing LC neurons were prepared for whole-cell voltage clamp recordings. Morphine- and methadone-treatment induced the most discrepant findings for MOR recovery following ME-induced desensitization: morphine treatment inhibited the recovery of signaling, whereas recovery following methadone-treatment was the same as observed in sham-treated animals. Oxycodone and fentanyl treatment yielded intermediate results on MOR recovery. Finally, buprenorphine treatment had several effects that were different from the other 4 agonists used for chronic treatment: ME-induced current was inhibited, ME-induced desensitization was eliminated, and MOR signaling was preserved after long exposures to supersaturing ME concentrations. Taken together, these results suggest that there is no single property of an opiate predictive of its potential to induce tolerance. An agonist's ability to induce acute MOR regulatory events and long-term, persistent cellular adaptations as well as pharmacokinetics and active metabolites must be considered in order to effectively manage pain and successfully treat opiate addiction.
Neuroscience Graduate Program
School of Medicine
Virk, Michael S., "Agonist-specific [mu]-opioid receptor regulation following acute and chronic treatment" (2007). Scholar Archive. 836.