SUMMARY Biased agonism has been proposed as a means to separate desirable and adverse drug responses downstream of G protein-coupled receptor (GPCR) targets. Herein we describe structural features of a series of mu opioid receptor (MOR)-selective agonists that preferentially activate receptor to couple to G proteins or to recruit βarrestin proteins. By comparing relative bias for MOR-mediated signaling in each pathway, we demonstrate a strong correlation between the respiratory suppression/antinociception therapeutic window in a series of compounds spanning a wide range of signaling bias. We find that βarrestin-biased compounds, such as fentanyl, are more likely to induce respiratory suppression at weak analgesic doses, while G protein signaling-bias broadens the therapeutic window, allowing for antinociception in the absence of respiratory suppression.

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Visual and auditory hallucinations accompany certain neuropsychiatric disorders, such as schizophrenia, and they also can be induced by the use or abuse of certain drugs. The heptahelical serotonin 2A receptors (5-HT2ARs) are molecular targets for druginduced hallucinations. However, the cellular mechanisms by which the 5-HT2AR mediates these effects are not well understood. Drugs acting at the 5-HT2AR can trigger diverse signaling pathways that may be directed by the chemical properties of the drug. ␤-arrestins are intracellular proteins that bind to heptahelical receptors and represent a point where such divergences in liganddirected functional signaling could occur. Here we compare the endogenous agonist, serotonin, to a synthetic 5-HT2AR hallucinogenic agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI), in mice lacking ␤-arrestin-2, as well as in cells lacking ␤-arrestins. In mice, we find that serotonin induces a head twitch response by a ␤-arrestin-2-dependent mechanism. However, DOI invokes the behavior independent of ␤-arrestin-2. The two structurally distinct agonists elicit different signal transduction and trafficking patterns upon activation of 5-HT2AR, which hinge on the presence of ␤-arrestins. Our study suggests that the 5-HT2AR-␤-arrestin interaction may be particularly important in receptor function in response to endogenous serotonin levels, which could have major implications in drug development for treating neuropsychiatric disorders such as depression and schizophrenia.5-HT2A receptor ͉ G protein-coupled receptor ͉ internalization ͉ MAP kinase ͉ schizophrenia

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Agonists targeting the kappa opioid receptor (KOR) have been promising therapeutic candidates because of their efficacy for treating intractable itch and relieving pain. Unlike typical opioid narcotics, KOR agonists do not produce euphoria or lead to respiratory suppression or overdose. However, they do produce dysphoria and sedation, side effects that have precluded their clinical development as therapeutics. KOR signaling can be fine-tuned to preferentially activate certain pathways over others, such that agonists can bias signaling so that the receptor signals through G proteins rather than other effectors such as βarrestin2. We evaluated a newly developed G protein signaling–biased KOR agonist in preclinical models of pain, pruritis, sedation, dopamine regulation, and dysphoria. We found that triazole 1.1 retained the antinociceptive and antipruritic efficacies of a conventional KOR agonist, yet it did not induce sedation or reductions in dopamine release in mice, nor did it produce dysphoria as determined by intracranial self-stimulation in rats. These data demonstrated that biased agonists may be used to segregate physiological responses downstream of the receptor. Moreover, the findings suggest that biased KOR agonists may present a means to treat pain and intractable itch without the side effects of dysphoria and sedation and with reduced abuse potential.