ABSTRACT

The pharmacological properties of opioid peptides and alkaloids are dependent on which Gα subunit is coupled to the opioid receptor. The opioid receptors are members of the Gi/o family of inhibitory G proteins, which include Gαi1, Gαi2, Gαi3, GαoA, GαoB, and Gαz. Using transfected HEK 293T cells, expressing one type of opioid receptor and one type of Gα subunit, BRET assays were used to determine the potency and efficacy of opioids signaling through different Gα subunits. For example, at the μ opioid receptor (MOR), β-endorphin and endomorphin-1 were full agonists regardless of which Gα subunit mediated MOR signaling. However, the EC50 value for β-endorphin activating the G protein ranged from 86 nM when the MOR was coupled to Gαz to 1700 nM when the MOR was coupled to Gαi1 and GαoB. A similar pattern was observed with all full agonists, showing Gα subunit bias regarding potency. Partial agonists displayed the greatest differences among the Gα subunits with changes in both potency and efficacy being influenced by the specific Gα subunit. Partial agonists, such as morphine, (-) pentazocine, and ethylketocyclazocine, were more potent and more efficacious when the MOR signaled through Gαz compared to the other Gαi/o subunits. Similar results were obtained with the κ (KOR) and δ (DOR) opioid receptors. Gαz, which is expressed throughout the brain, has a slower rate of GTP hydrolysis than the other Gα subunits. This slow rate of GTP hydrolysis results in sustained receptor activation leading to increased potency and efficacy. Mutation studies confirmed that the rate of GTP hydrolysis was responsible for influencing the pharmacological properties of opioid peptides and alkaloids. Experiments were performed to determine if positive allosteric modulators (PAMs) were biased depending on the Gα subunit. PAMs had the greatest effects on partial agonists when both the MOR and DOR were studied. The PAM BMS-986122 and BMS-986187 enhanced the potency and efficacy of partial agonists. Like the agonist itself, the PAMs showed bias toward certain Gα subunits. In conclusion, the specific Gα subunit coupled to the opioid receptor greatly influences potency and efficacy of opioid peptides and alkaloids.

BIO

Dr. Bidlack is a Professor and Associate Chair in the Department of Pharmacology and Physiology at the University of Rochester, School of Medicine and Dentistry. Dr. Bidlack received her B.A. in Biology-Chemistry from Skidmore College and a Ph.D. in Biophysics from the University of Rochester. She started studying opioid receptor pharmacology as a postdoctoral fellow a few years after the discovery of the opioid peptides. In 1982, Dr. Bidlack was appointed as an Assistant Professor in the Center for Brain Research at the University of Rochester. She joined the Department of Pharmacology as an Associate Professor in 1987 and was promoted to full Professor in 1997. Dr. Bidlack was Treasurer of the International Narcotics Research Conference from 2005 – 2012. From 2005-2006, she was President of the Society of NeuroImmune Pharmacology. Her lab was the first to show that cells from the immune system expressed a brain-type kappa opioid receptor. She has worked with several medicinal chemists. When her collaborator Dr. Mark Wentland licensed his opioid compounds to Alkermes, Inc., Dr. Bidlack started an 8-year consulting position with Alkermes. The Bidlack lab is interested in G protein bias and how this bias affects the pharmacological properties of opioid peptides and alkaloids.