Prescription opioid narcotics, such as morphine, oxycodone, and fentanyl, produce analgesia and side effects through activation of the mu opioid receptor (MOR), a G protein coupled receptor (GPCR). Our long-standing goal is to understand how MOR signals to produce distinct biological effects and to ultimately inform the development of therapeutics that will take advantage of good receptor signaling (pain relief) and avoid bad receptor signaling (tolerance, dependence, constipation and other side effects). It has become increasingly evident that different drug structures can elicit different receptor signaling cascade at a single receptor, likely by changing the affinities for association with intracellular binding partners. Further, the intracellular binding partner profile differs between neuronal populations. Therefore, the nature of a drug response can be determined not only by the chemical properties of the drug, but also by the complement of signaling proteins found in residence with the receptor; making it critical to study receptor signaling in physiologically relevant systems. One particular intracellular protein that influences MOR function is betaarrestin2. betaArrestin2 is a scaffolding protein that can act as desensitizing element or as a signal transduction facilitator. Our studies have shown that morphine-induced analgesia is enhanced while tolerance is attenuated in mice lacking betaarrestin2, which implicates betaarrestin2 as a desensitizing factor in pain regulating brain regions. Our collective body of work shows that the severity of certain side effects, including physical dependence and constipation, are significantly reduced in mice lacking betaarrestin2 suggesting that in some organ systems and brain regions, betaarrestin2 facilitates MOR signaling. Since receptor responsiveness to a drug in vivo is ultimately dependent upon the cellular environment that encompasses the receptor, we hypothesize that betaarrestin2 dampens morphine responsiveness in analgesia pathways while it mediates morphine-associated side effects such as physical dependence and constipation. To this end, we propose to elucidate the mechanisms by which betaarrestins regulate MOR in brain regions and tissues that mediate morphine-induced antinociception and tolerance (brainstem), physical dependence (striatum) and constipation (colon). We will utilize new MOR agonists that are functionally selective for activating G protein signaling pathways (we hypothesize this will promote antinociception) and against recruiting betaarrestin2 (we hypothesize that recruiting betaarrestin2 leads to tolerance, dependence and constipation). Published and preliminary evidence suggests that the G protein biased agonists promote antinociception with fewer side effects. We will use these tools to gain a greater understanding of MOR regulation in the endogenous setting as it pertains to in vivo physiologies. These studies should provide guidance for developing therapeutics that preferentially enhance desired effects such as improving pain therapy while preventing adverse reactions.
Extended use of opioids, such as morphine and oxycodone, for pain management leads to tolerance and dependence as well as chronic constipation. Our research has shown that the mu opioid receptor is regulated by a protein called betaarrestin2 and the genetic deletion of betaarrestin2 in mice prevents morphine tolerance while attenuating morphine dependence and constipation. New opioid drugs have recently been developed that activate the receptor but do not lead to recruitment of betaarrestin2; these pharmacological tools will be used to gain both a greater understanding of how the receptor is regulated in different systems (pain relief centers vs. dependence centers vs the gut) and inform how therapies can be fine-tuned to improve pain relief without tolerance, physical dependence and constipation.
| Grim, Travis W; Park, Scarlet Jinhong; Schmid, Cullen L et al. (2018) The effect of quinine in two bottle choice procedures in C57BL6 mice: Opioid preference, somatic withdrawal, and pharmacokinetic outcomes. Drug Alcohol Depend 191:195-202 |
| Luttrell, Louis M; Maudsley, Stuart; Bohn, Laura M (2015) Fulfilling the Promise of ""Biased"" G Protein-Coupled Receptor Agonism. Mol Pharmacol 88:579-88 |
