• Michael A Emery, M L Shawn Bates, Paul J Wellman, and Shoshana Eitan.
• Department of Psychology, Behavioral and Cellular Neuroscience Program, and the Interdisciplinary Program in Neuroscience, Texas A&M Institute for Neuroscience (TAMIN), Texas A&M University, 4235 TAMU, College Station, TX, USA.
• Pain Med. 2016 May 1; 17 (5): 908-914.

BackgroundOpioids alter the responses of D2-like dopamine receptors (D2DRs), known to be involved in the pathology of addiction and other mental illnesses. Importantly, our recent results demonstrated that various opioids differentially modulate the behavioral responses of D2DRs.ObjectiveTo examine the effect of various opioids on striatal activation levels of Akt and ERK1/2, as well as the signaling responses of D2DRs following opioid exposure.MethodsMice were pre-treated with 20 mg/kg morphine, hydrocodone, oxycodone, or saline for 6 days. Twenty-four hours later, mice were injected with vehicle or a D2/D3 receptor agonist, quinpirole. Thirty minutes later, dorsal striatum was collected and analyzed using Western blot.ResultsIn morphine-pretreated animals, baseline Akt activation level was unchanged, but was reduced in response to quinpirole. In contrast, baseline Akt activation levels were reduced in mice pretreated with hydrocodone and oxycodone, but were unchanged in response to quinpirole. In mice pretreated with all opioids, baseline ERK2 activation levels were unchanged and increased in response to quinpirole. However, quinpirole-induced ERK2 activation was significantly higher than drug naïve animals only in the morphine-pretreated mice.ConclusionsVarious opioids differentially modulate the baseline activation levels of signaling molecules, which in turn results in ligand-selective effects on the responses to a D2/D3 dopamine receptor agonist. This demonstrates a complex interplay between opioid receptors and D2DRs, and supports the notion that various opioids carry differential risks to the dopamine reward system. This information should be considered when prescribing opioid pain medication, to balance effectiveness with minimal risk.Wiley Periodicals, Inc.

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