The kappa opioid receptor (KOR) helps mediate responses to stress, yet is also implicated in developing and/or maintaining health disorders of chronic pain, drug addiction, anxiety and depression. KOR agonists have shown promise in ameliorating these disorders, but are limited by dysphoric side effects. Beneficial effects of KOR agonists (e.g., analgesia) are considered predominantly mediated by G protein signaling, whereas b-arrestin signaling is considered central to their detrimental side effects (e.g., dysphoria). However, the mechanism(s) by which these signals downstream of KOR are regulated are still being elucidated. The Regulators of G protein Signaling are intracellular proteins that accelerate signal termination after G protein-coupled receptor activation. RGS12 is a complex member of this protein family, with at least five different domains that interact with components of both G protein-dependent and -independent signaling pathways. We recently reported that RGS12 is enriched in the ventral striatum (vSTR), and global Rgs12 ablation decreases locomotor responses to dopamine (DA)-modulating psychostimulants. Our data correlate with the augmented DA transporter (DAT) expression and function in the vSTR, but not dorsal striatum (dSTR), seen in RGS12-null mice. Loss of RGS12 (especially as a Gai/o-directed GTPase-accelerating protein) may indirectly increase DAT expression / function by removing negative regulation downstream of KOR, given that KOR agonists are known to increase DAT surface expression and uptake function. Supporting this notion, the augmented DAT function and reduced AMPH-stimulated locomotion caused by RGS12 loss are both reversed following KOR antagonism. We also found elevated KOR expression in the vSTR, but not dSTR, of RGS12-null and b-arrestin2-null mouse strains. RGS12 over-expression augments b-arrestin recruitment to activated KOR ? an effect preserved following pertussis toxin-mediated Gi/o inhibition or mutation to the Ga-interacting domains of RGS12, suggesting a G protein-independent mechanism. These findings are consistent with our newest data that RGS12-null mice exhibit attenuated KOR agonist-induced conditioned place aversion, considered b-arrestin-dependent behavior. Collectively, our data highlight a role for RGS12 as a novel, differential regulator of both G protein-dependent and -independent signaling downstream of KOR activation, a regulation that may be exploitable pharmacologically to help shift KOR-mediated signaling to beneficial outcomes and away from detrimental ones.
Our first aim i s to determine the specific neuronal populations within which RGS12 acts to modulate G protein- dependent and -independent KOR signaling, testing hypotheses that RGS12 operates to modulate KOR and DAT function specifically in KOR- and DAT- expressing CNS neurons, and also operates at the level of the spinal cord.
Our second aim i s to delineate the molecular determinants that engender selective functional interactions between RGS12 and KOR. Success in pursuit of these two aims will provide key pre-clinical data for considering RGS12 a valid target for future analgesic and anti-addiction therapeutics that engage KOR signal transduction.
(Public Health Relevance): Chronic pain, drug addiction, anxiety and depression each extract substantial health and socioeconomic costs from the American populace. Although drugs directed toward the kappa opioid receptor (KOR) have been investigated as treatments for these human ailments, they are currently hampered by poor side effect profiles. A deeper understanding of the mechanisms by which the proteins RGS12 and b-arrestin2 regulate the beneficial versus detrimental effects of KOR-directed drugs will enable new strategies to maximize the clinical utility of these (and future) drugs that provide pain relief by targeting opioid receptors, but without addiction or side effect liabilities.
