Opioid receptors (ORs) mediate the actions of endogenous and exogenous opioids for many essential physiological processes including regulation of pain, respiratory drive, mood, and, in the case of κ-opioid receptors (KOR), dysphoria and psychotomimesis. Here we report the crystal structure of the human KOR (hKOR) in complex with the selective antagonist JDTic, arranged in parallel-dimers, at 2.9 angstrom resolution. The structure reveals important features of the ligand binding pocket that contribute to JDTic’s high affinity and subtype-selectivity for hKOR. Modeling of other important KOR-selective ligands, including the morphinan-derived antagonists nor-BNI and GNTI, and the diterpene agonist salvinorin A analog RB-64, reveals both common and distinct features for binding these diverse chemotypes. Analysis of site-directed mutagenesis and ligand structure-activity relationships confirms the interactions observed in the crystal structure, thereby providing a molecular explanation for hKOR subtype-selectivity along with insight essential for the design of hKOR compounds with new pharmacological properties.

Salvia divinorum, whose main active ingredient is the neoclerodane diterpene Salvinorin A, is a hallucinogenic plant in the mint family that has been used in traditional spiritual practices for its psychoactive properties by the Mazatecs of Oaxaca, Mexico. More recently, S. divinorum extracts and Salvinorin A have become more widely used in the U.S. as legal hallucinogens. We discovered that Salvinorin A potently and selectively inhibited 3 H-bremazocine binding to cloned opioid receptors. Salvinorin A had no significant activity against a battery of 50 receptors, transporters, and ion channels and showed a distinctive profile compared with the prototypic hallucinogen lysergic acid diethylamide. Functional studies demonstrated that Salvinorin A is a potent opioid agonist at cloned opioid receptors expressed in human embryonic kidney-293 cells and at native opioid receptors expressed in guinea pig brain. Importantly, Salvinorin A had no actions at the 5-HT 2A serotonin receptor, the principal molecular target responsible for the actions of classical hallucinogens. Salvinorin A thus represents, to our knowledge, the first naturally occurring nonnitrogenous opioid-receptor subtype-selective agonist. Because Salvinorin A is a psychotomimetic selective for opioid receptors, opioid-selective antagonists may represent novel psychotherapeutic compounds for diseases manifested by perceptual distortions (e.g., schizophrenia, dementia, and bipolar disorders). Additionally, these results suggest that opioid receptors play a prominent role in the modulation of human perception.

Opioid receptor selective antagonists are important pharmacological probes in opioid receptor structural characterization and opioid agonist functional study. Thus far, a nonpeptidyl, highly selective and reversible μ opioid receptor (MOR) antagonist is unavailable. On the basis of our modeling studies, a series of novel naltrexamine derivatives have been designed and synthesized. Among them, two compounds were identified as leads based on the results of in vitro and in vivo assays. Both of them displayed high binding affinity for the MOR (K i = 0.37 and 0.55 nM). Compound 6 (NAP) showed over 700-fold selectivity for the MOR over the δ receptor (DOR) and more than 150-fold selectivity over the κ receptor (KOR). Compound 9 (NAQ) showed over 200-fold selectivity for the MOR over the DOR and approximately 50-fold selectivity over the KOR. Thus these two novel ligands will serve as leads to further develop more potent and selective antagonists for the MOR.

Background:The -opioid receptor can be activated by structurally diverse agonists. Results: Four structurally diverse agonists differentially bound to and activated wild type and mutant -opioid receptors. Conclusion:The structural features of the agonists dictate how they interact with and stabilize G i -signaling receptor conformations. Significance: The results provide insights into the structural basis of opioid receptor ligand recognition and activation.

Salvinorin A is a naturally occurring hallucinogenic diterpenoid from the plant Salvia divinorumthat selectively and potently activates kappa-opioid receptors (KORs). Salvinorin A is unique in that it is the only known lipid-like molecule that selectively and potently activates a G-protein coupled receptor (GPCR), which has as its endogenous agonist a peptide; salvinorin A is also the only known non-nitrogenous opioid receptor agonist. In this paper, we identify key residues in KORs responsible for the high binding affinity and agonist efficacy of salvinorin A. Surprisingly, we discovered that salvinorin A was stabilized in the binding pocket by interactions with tyrosine residues in helix 7 (Tyr313 and Tyr320) and helix 2 (Tyr119). Intriguingly, activation of KORs by salvinorin A required interactions with the helix 7 tyrosines Tyr312, Tyr313, and Tyr320 and with Tyr139 in helix 3. In contrast, the prototypical nitrogenous KOR agonist U69593 and the endogenous peptidergic agonist dynorphin A (1-13) showed differential requirements for these three residues for binding and activation. We also employed a novel approach, whereby we examined the effects of cysteine-substitution mutagenesis on the binding of salvinorin A and an analogue with a free sulfhydryl group, 2-thiosalvinorin B. We discovered that residues predicted to be in close proximity, especially Tyr313, to the free thiol of 2-thiosalvinorin B when mutated to Cys showed enhanced affinity for 2-thiosalvinorin B. When these findings are taken together, they imply that the diterpenoid salvinorin A utilizes unique residues within a commonly shared binding pocket to selectively activate KORs.

Salvinorin A, the most potent naturally occurring hallucinogen, has gained increasing attention since the κ-opioid receptor (KOR) was identified as its principal molecular target by us (Roth et al, PNAS, 2002). Here we report the design, synthesis and biochemical characterization of novel, irreversible, salvinorin A-derived ligands suitable as active state probes of the KOR. Based on prior substituted cysteine accessibility and molecular modeling studies, C315 7.38 was chosen as a potential anchoring point for covalent labeling of salvinorin A-derived ligands. Automated docking of a series of potential covalently-bound ligands suggested that either a haloacetate moiety or other similar electrophilic groups could irreversibly bind with C315 7.38 . 22-thiocyanatosalvinorin A (RB-64) and 22-chlorosalvinorin A (RB-48) were both found to be extraordinarily potent and selective KOR agonists in vitro and in vivo. As predicted based on molecular modeling studies, RB-64 induced wash-resistant inhibition of binding with a strict requirement for a free cysteine in or near the binding pocket. Mass spectrometry (MS) studies utilizing synthetic KOR peptides and RB-64 supported the hypothesis that the anchoring residue was C315 7.38 and suggested one biochemical mechanism for covalent binding. These studies provide direct evidence for the presence of a free cysteine in the agonist-bound state of KOR and provide novel insights into the mechanism by which salvinorin A binds to and activates KOR.Salvinorin A, the active ingredient of the hallucinogenic plant Salvia divinorum, is the most potent known naturally-occurring hallucinogen (1,2). In 2002, we discovered that the κ-opioid receptor (KOR) was the molecular target for the actions of salvinorin A in vitro (3). Studies with KOR knock-out mice (4) unequivocally demonstrated that the KOR was also the site of † This research was supported in part by NIH R01DA017204 (to B . NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2010 July 28. Published in final edited form as:Biochemistry. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript action of salvinorin A in vivo-a finding which has been widely replicated (see (5) and (6) for reviews). Subsequently, salvinorin A emerged as an attractive lead compound for drug discovery and during the past few years, hundreds of salvinorin A derivates have been synthesized (6). Some of these analogues present interesting pharmacological profiles, from full KOR agonist to partial δ-opioid receptor (DOR) or µ-opioid receptor (MOR) agonists and antagonists (7) (8) (9) (10) (11). However, most of the hundreds of analogues displayed decreased affinity (or even no affinity) to KOR. The present challenge now is to use the knowledge about salvinorin A-KOR interactions (12) (13) to design unique salvinorin A derivatives with novel pharmacological profiles and therapeutic potential. In recent years, covalently-bound ligands emerged as a new class of receptor ligands with unique pharmacologi...

Although ligand-induced conformational changes in G protein-coupled receptors (GPCRs) are well-documented, there is little direct evidence for G protein-induced changes in GPCR conformation. To investigate this possibility, the effects of overexpressing Galpha-subunits (Galpha16 or Galphai2) with the kappa-opioid receptor (KOR) were examined. The changes in KOR conformation were subequently examined via the substituted cysteine accessibility method (SCAM) in transmembrane domains 6 (TM6) and 7 (TM7) and extracellular loop 2 (EL2). Significant conformational changes were observed on TM7, the extracellular portion of TM6, and EL2. Seven SCAM-sensitive residues (S3107.33, F3147.37, and I3167.39 to Y3207.43) on TM7 presented a cluster pattern when the KOR was exposed to baseline amounts of G protein, and additional residues became sensitive upon overexpression of various G proteins. In TM7, S3117.34 and N3267.49 were found to be sensitive in Galpha16-overexpressed cells and Y3137.36, N3227.45, S3237.46, and L3297.52 in Galphai2-overexpressed cells. In addition, the degree of sensitivity for various TM7 residues was augmented, especially in Galphai2-overexpressed cells. A similar phenomenon was also observed for residues in TM6 and EL2. In addition to an enhanced sensitivity of certain residues, our findings also indicated that a slight rotation was predicted to occur in the upper part of TM7 upon G protein overexpression. These relatively modest conformational changes engendered by G protein overexpression had both profound and differential effects on the abilities of agonists to bind to KOR. These data are significant because they demonstrate that Galpha-subunits differentially modulate the conformation and agonist affinity of a prototypical GPCR.

Salvinorin A, the active component of the hallucinogenic sage Salvia divinorum, is an apparently selective and highly potent -opioid receptor (KOR) agonist. Salvinorin A is unique among ligands for peptidergic G protein-coupled receptors in being nonnitrogenous and lipid-like in character. To examine the molecular basis for the subtype-selective binding of salvinorin A, we utilized an integrated approach using chimeric opioid receptors, site-directed mutagenesis, the substituted cysteine accessibility method, and molecular modeling and dynamics studies. We discovered that helix 2 is required for salvinorin A binding to KOR and that two residues (Val-108(2.53) and Val-118(2.63)) confer subtype selectivity. Intriguingly, molecular modeling studies predicted that these loci exhibit an indirect effect on salvinorin A binding, presumably through rotation of helix 2. Significantly, and in agreement with our in silico predictions, substituted cysteine accessibility method analysis of helix 2 comparing KOR and the ␦-opioid receptor, which has negligible affinity for salvinorin A, revealed that residues known to be important for salvinorin A binding exhibit a differential pattern of water accessibility. These findings imply that differences in the helical orientation of helix 2 are critical for the selectivity of salvinorin A binding to KOR and provide a structurally novel basis for ligand selectivity.Salvia divinorum, a member of the sage family, is a hallucinogenic plant that has been used for traditional spiritual purposes by Mazatec shamans of Oaxaca, Mexico (1, 2). More recently, S. divinorum leaves and extracts have been used as legal hallucinogens in the United States. The active compound of S. divinorum is the neoclerodane diterpene salvinorin A, which is comparable in potency with the synthetic hallucinogen lysergic acid diethylamide (3, 4). Following extensive screening of the receptorome, we identified the -opioid receptor (KOR) 2 as the molecular target of salvinorin A (5). Salvinorin A has negligible affinity for all other tested G protein-coupled receptors (GPCRs), including the -and ␦-opioid receptors (MOR and DOR), as well as serotonin 5-HT 2A receptors (5, 6), which represent the molecular target for classical hallucinogens (7).Salvinorin A is both pharmacologically and chemically unique in that it represents the first nonnitrogenous, naturally occurring KOR-selective agonist and the only known nonalkaloidal hallucinogen. Given that salvinorin A lacks the basic amino group present in all other KOR-selective ligands (for a review, see Ref. 8), ionic interactions are unlikely to stabilize salvinorin A in the binding pocket of KOR (9, 10). This has led to the hypothesis that salvinorin A binding at KOR may involve novel ligand-receptor interactions that utilize distinct residues within a conserved ligand binding pocket (10).Site-directed mutagenesis and molecular modeling studies have been widely used to gain insight into the mechanisms of receptor-ligand binding interactions (11)(12)(13)(14). In fact, rece...