Investigating the Acute and Chronic Effects of Known and Novel Opioid Ligands

Abstract

Acute and chronic pain are widespread and debilitating diseases that, for a large population, cannot be adequately managed with current pain treatments. Opioid analgesics, such as morphine, have long been used to treat pain and exert their effects by activating the mu-opioid receptor (MOR). While effective, these MOR agonists produce on-target adverse effects such as tolerance, physical dependence, and euphoria. Overall, the experiments described in the current thesis evaluated ways of minimizing opioid tolerance by targeting multiple opioid receptor types simultaneously (chapters 1 and 2) or different populations of opioid receptors in vivo (chapter 3). Previous studies demonstrated that simultaneous modulation of both the MOR and the delta-opioid receptor (DOR) could improve the therapeutic profile of opioid ligands. These experiments sought to further characterize mixed-efficacy opioid ligands, specifically ligands binding to both MORs and DORs with nearly equal affinity. These studies utilized multiple pre-clinical pain models to determine antinociceptive properties of the mixed efficacy opioid ligands, AMB67 and AAH8, following acute and chronic administration. To evaluate the antinociceptive effects of AMB67 and AAH8, we used models of thermal, chemical, and mechanical nociception. In C57BL/6 mice, AMB67 produced dose-dependent antinociceptive effects in all three models of nociception. Mice chronically administered AMB67 failed to develop tolerance to the antinociceptive effects. Chronic administration of AMB67 produced physical dependence as mice exhibited naltrexone-precipitated withdrawal-like behaviors; however, these effects were significantly less than morphine. AMB67 was also less potent than morphine in multiple models assessing abuse potential. In contrast to AMB67, AAH8 significantly attenuated chemical-induced visceral pain following system administration but failed to produce antinociceptive effects in other pain models. Repeated administration of small doses of AAH8 failed to produce tolerance to the antinociceptive effects of AAH8; however, chronic treatment with more frequent, larger doses produced tolerance to the antinociceptive effects of AAH8. These data provide support that mixed-efficacy MOR-DOR ligands may offer improvement over current pharmacotherapies for pain management. Additionally, this study assessed the involvement of peripheral opioid receptors in the development of tolerance to the centrally-mediated antinociceptive effects of MOR agonists. This study used a model of acute, peripherally-mediated visceral pain, acetic acid stretch assay (AASA) and a centrally-mediated thermal reflex assay, warm water tail withdrawal (WWTW). Morphine and the peripherally restricted MOR agonist loperamide produced acute antinociceptive effects in the AASA, and NLX, a non-selective opioid receptor antagonist, blocked these effects. However, only morphine produced opioid-receptor mediated antinociceptive effects in the WWTW. Chronic administration of morphine (3x/day for five days) shifted the ED50 of the morphine dose-effect curve 2.5-fold to the right in the WWTW. Naloxone-methiodide pretreatments to chronic morphine prevented the rightward shift in the morphine dose-effect curve. Conversely, chronic administration of a peripherally active dose of loperamide (3.2 mg/kg, 4x per day for five days) shifted the morphine dose-effect (DE) curve to the right. Pretreatments with naloxone-methiodide completely reversed loperamide-induced cross-tolerance to the antinociceptive effects of morphine. Overall, these data suggest the involvement of peripheral opioid receptors in tolerance development to centrally acting opioids. Overall, the work presented in this thesis furthers our understanding of the in vivo mechanisms of opioid tolerance and potentially identifies novel, safer opioid analgesics. Minimizing the adverse effects of chronic opioid use would significantly improve opioid-based treatment, improve the lives of those who require opioids for everyday function, and help fight the current opioid epidemic.