Because nonopioid analgesics are much sought after, we computationally docked more than 301 million virtual molecules against a validated pain target, the α _2A -adrenergic receptor (α _2A AR), seeking new α _2A AR agonists chemotypes that lack the sedation conferred by known α _2A AR drugs, such as dexmedetomidine. We identified 17 ligands with potencies as low as 12 nanomolar, many with partial agonism and preferential G _i and G _o signaling. Experimental structures of α _2A AR complexed with two of these agonists confirmed the docking predictions and templated further optimization. Several compounds, including the initial docking hit ‘9087 [mean effective concentration (EC ₅₀ ) of 52 nanomolar] and two analogs, ‘7075 and PS75 (EC ₅₀ 4.1 and 4.8 nanomolar), exerted on-target analgesic activity in multiple in vivo pain models without sedation. These newly discovered agonists are interesting as therapeutic leads that lack the liabilities of opioids and the sedation of dexmedetomidine.

The α _2A adrenergic receptor (α _2A AR) is a G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor that mediates important physiological functions in response to the endogenous neurotransmitters norepinephrine and epinephrine, as well as numerous chemically distinct drugs. However, the molecular mechanisms of drug actions remain poorly understood. Here, we report the cryo–electron microscopy structures of the human α _2A AR-GoA complex bound to norepinephrine and three imidazoline derivatives (brimonidine, dexmedetomidine, and oxymetazoline). Together with mutagenesis and functional data, these structures provide important insights into the molecular basis of ligand recognition, activation, and signaling at the α _2A AR. Further structural analyses uncover different molecular determinants between α _2A AR and βARs for recognition of norepinephrine and key regions that determine the G protein coupling selectivity. Overall, our studies provide a framework for understanding the signal transduction of the adrenergic system at the atomic level, which will facilitate rational structure-based discovery of safer and more effective medications for α _2A AR.

The M2 muscarinic receptor (M2R) is a prototypical G-protein-coupled receptor (GPCR) that serves as a model system for understanding GPCR regulation by both orthosteric and allosteric ligands. Here, we investigate the mechanisms governing M2R signaling versatility using cryo-electron microscopy (cryo-EM) and NMR spectroscopy, focusing on the physiological agonist acetylcholine and a supra-physiological agonist iperoxo, as well as a positive allosteric modulator LY2119620. These studies reveal that acetylcholine stabilizes a more heterogeneous M2R-G-protein complex than iperoxo, where two conformers with distinctive G-protein orientations were determined. We find that LY2119620 increases the affinity for both agonists, but differentially modulates agonists efficacy in G-protein and β-arrestin pathways. Structural and spectroscopic analysis suggest that LY211620 stabilizes distinct intracellular conformational ensembles from agonist-bound M2R, which may enhance β-arrestin recruitment while impairing G-protein activation. These results highlight the role of conformational dynamics in the complex signaling behavior of GPCRs, and could facilitate design of better drugs.