Exploiting Biased Agonism at the Ghrelin Receptor (GHSR1a) for Opioid Addiction

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Abstract In the U.S., the number of Opioid Use Disorder (OUD) diagnoses has exceeded 2 million. Although existing OUD medications (e.g., methadone, buprenorphine, naltrexone) ameliorate OUD symptoms and relapse rates, they are not without problematic side-effects and medication discontinuation remains an issue. Consequently, NIDA has prioritized the development of novel medications to prevent and/or treat OUD. Among NIDA's "10 most wanted" pharmacologic targets for OUD is the ghrelin receptor, growth hormone secretagogue receptor 1a (GHSR1a). GHSR1a is densely expressed within dopamine (DA) neurons of mesolimbic ("reward") system and its activation leads to increased DA release and enhanced motivation to acquire rewarding/reinforcing stimuli (e.g., food, drugs of abuse). Like other GPCRs, the physiological effects of GHSR1a depend whether G protein- or b-arrestin (barr)-dependent signaling pathways are preferentially engaged, a process known as functional selectivity (or biased agonism). Considerable prior evidence supports that leveraging GPCR functional selectivity may provide a means by which to enhance pharmacotherapeutic efficacy and/or diminish deleterious side- effects. The Caron group has previously shown that GHSR1a antagonism blocks hyperlocomotion in cocaine- sensitized mice via a barr-dependent mechanism in DA neurons. Additionally, the Caron group has also demonstrated that GHSR1a-induced actin stress fiber formation -- a molecular process integral to the neuroadaptive changes produced by reinforcing substances -- is barr-mediated. In a large-scale drug screen for novel GHSR1a ligands coupled with medicinal chemistry, the Caron group in collaboration with NCATS, has identified a lead compound, 8279, which acts as a GHSR1a-selective G protein-biased agonist and thus, attenuates GHSR1a-mediated barr signaling. Initial data shows that 8279 reduces basal hyperlocomotion in dopamine transporter knockout (DAT KO) mice (an animal model of hyperDAergia), but does not affect hedonic feeding. The objectives of this application are to delineate the mechanisms of G protein-biased ligands at GHSR1a and interrogate their therapeutic potential in mouse models of opioid abuse. The central hypothesis is that G protein-biased GHSR1a agonism attenuates opioid-induced neuroplasticity and self-administration via reduced barr recruitment and/or signaling following GHSR1a activation. This hypothesis will be tested by pursuing three specific aims using 8279, or its derivatives, as our primary tool compound: 1) Elucidate the biochemical mechanisms underlying GHSR1a biased signaling, 2) Identify the effects of GHSR1a biased signaling on neuroplasticity, and 3) Evaluate the effects of GHSR1a biased signaling on opioid-induced behavior. These three aims will be tested using established GPCR trafficking, protein-protein interaction, and signaling assays, viral vector-based measures of neuroplasticity in brain tissue, and mouse self-administration. These studies are significant and novel in that they will provide a thorough preclinical investigation into the potential therapeutic utility of exploiting GHSR1a functional selectivity for the prevention and/or treatment of OUD.