Anti-diarrheal activities of phytol along with its possible mechanism of action through in-vivo and in-silico models.

Phytol (PHY), a chlorophyll-derived diterpenoid, exhibits numerous pharmacological properties, including antioxidant, antimicrobial, and anticancer activities. This study evaluates the anti-diarrheal effect of phytol (PHY) along with its possible mechanism of action through in-vivo and in-silico models. The effect of PHY was investigated on castor oil-induced diarrhea in Swiss mice by using prazosin, propranolol, loperamide, and nifedipine as standards with or without PHY. PHY at 50 mg/kg (p.o.) and all other standards exhibit significant (p < 0.05) anti-diarrheal effect in mice. The effect was prominent in the loperamide and propranolol groups. PHY co-treated with prazosin and propranolol was found to increase in latent periods along with a significant reduction in diarrheal section during the observation period than other individual or combined groups. Furthermore, molecular docking studies also suggested that PHY showed better interactions with the α- and ß-adrenergic receptors, especially with α-ADR1a and ß-ADR1. In the former case, PHY showed interaction with hydroxyl group of Ser192 at a distance of 2.91Å, while in the latter it showed hydrogen bond interactions with Thr170 and Lys297 with a distance of 2.65 and 2.72Å, respectively. PHY exerted significant anti-diarrheal effect in Swiss mice, possibly through blocking α- and ß-adrenergic receptors.

Discovery of Kynurenines Containing Oligopeptides as Potent Opioid Receptor Agonists.

Kynurenine (kyn) and kynurenic acid (kyna) are well-defined metabolites of tryptophan catabolism collectively known as "kynurenines", which exert regulatory functions in host-microbiome signaling, immune cell response, and neuronal excitability. Kynurenine containing peptides endowed with opioid receptor activity have been isolated from natural organisms; thus, in this work, novel opioid peptide analogs incorporating L-kynurenine (L-kyn) and kynurenic acid (kyna) in place of native amino acids have been designed and synthesized with the aim to investigate the biological effect of these modifications. The kyna-containing peptide (KA1) binds selectively the m-opioid receptor with a Ki = 1.08 ± 0.26 (selectivity ratio m/d/k = 1:514:10000), while the L-kyn-containing peptide (K6) shows a mixed binding affinity for m, d, and k-opioid receptors, with efficacy and potency (Emax = 209.7 + 3.4%; LogEC50 = -5.984 + 0.054) higher than those of the reference compound DAMGO. This novel oligopeptide exhibits a strong antinociceptive effect after i.c.v. and s.c. administrations in in vivo tests, according to good stability in human plasma (t1/2 = 47 min).

Discovery, structure-activity relationship studies, and anti-nociceptive effects of N-(1,2,3,4-tetrahydro-1-isoquinolinylmethyl)benzamides as novel opioid receptor agonists.

µ-Opioid receptor (MOR) agonists are analgesics used clinically for the treatment of moderate to severe pain, but their use is associated with severe adverse effects such as respiratory depression, constipation, tolerance, dependence, and rewarding effects. In this study, we identified N-({2-[(4-bromo-2-trifluoromethoxyphenyl)sulfonyl]-1,2,3,4-tetrahydro-1-isoquinolinyl}methyl)cyclohexanecarboxamide (1) as a novel opioid receptor agonist by high-throughput screening. Structural modifications made to 1 to improve potency and blood-brain-barrier (BBB) penetration resulted in compounds 45 and 46. Compound 45 was a potent MOR/KOR (κ-opioid receptor) agonist, and compound 46 was a potent MOR and medium KOR agonist. Both 45 and 46 demonstrated a significant anti-nociceptive effect in a tail-flick test performed in wild type (WT) B6 mice. The ED50 value of 46 was 1.059 mg/kg, and the brain concentrations of 45 and 46 were 7424 and 11696 ng/g, respectively. Accordingly, compounds 45 and 46 are proposed for lead optimization and in vivo disease-related pain studies.

Discovery of 5-substituted tetrahydronaphthalen-2yl-methyl with N-phenyl-N-(piperidin-4-yl)propionamide derivatives as potent opioid receptor ligands.

A new series of novel opioid ligands have been designed and synthesized based on the 4-anilidopiperidine scaffold containing a 5-substituted tetrahydronaphthalen-2yl)methyl group with different N-phenyl-N-(piperidin-4-yl)propionamide derivatives to study the biological effects of these substituents on µ and δ opioid receptor interactions. Recently our group reported novel 4-anilidopiperidine analogues, in which several aromatic ring-contained amino acids were conjugated with N-phenyl-N-(piperidin-4-yl)propionamide and examined their biological activities at the µ and δ opioid receptors. In continuation of our efforts in these novel 4-anilidopiperidine analogues, we took a peptidomimetic approach in the present design, in which we substituted aromatic amino acids with tetrahydronaphthalen-2yl methyl moiety with amino, amide and hydroxyl substitutions at the 5th position. In in vitro assays these ligands, showed very good binding affinity and highly selective toward the µ opioid receptor. Among these, the lead ligand 20 showed excellent binding affinity (2 nM) and 5000 fold selectivity toward the µ opioid receptor, as well as functional selectivity in GPI assays (55.20 ± 4.30 nM) and weak or no agonist activities in MVD assays. Based on the in vitro bioassay results the lead compound 20 was chosen for in vivo assessment for efficacy in naïve rats after intrathecal administration. Compound 20 was not significantly effective in alleviating acute pain. This discrepancy between high in vitro binding affinity, moderate in vitro activity, and low in vivo activity may reflect differences in pharmacodynamics (i.e., engaging signaling pathways) or pharmacokinetics (i.e., metabolic stability). In sum, our data suggest that further optimization of this compound 20 is required to enhance in vivo activity.

Amide alkaloids from Scopolia tangutica.

Four new hydroxycinnamic acid amides, scotanamines A-D (1-4), and seven known alkaloids, including N (1),N (10)-di-dihydrocaffeoylspermidine (5), scopolamine (6), anisodamine (7), hyoscyamine (8), anisodine (9), caffeoylputrescine (10), and N (1)-caffeoyl-N (3)-dihydrocaffeoylspermidine (11), were obtained from the roots of Scopolia tangutica. The present study represents the first recognition of hydroxycinnamic acid amides containing putrescine or spermidine in S. tangutica. Compound 1, in particular, contains a moiety resulting from the condensation of nortropinone and putrescine. Compound 2 exhibited moderate agonist activity at the µ-opioid receptor (EC50=7.3 µM). Compound 2 was tested in vivo and induced analgesia in mice. The analgesic effect was recorded using the tail-flick assay and was reversed by naloxone.

Identification of selective agonists and positive allosteric modulators for µ- and δ-opioid receptors from a single high-throughput screen.

Hetero-oligomeric complexes of G protein-coupled receptors (GPCRs) may represent novel therapeutic targets exhibiting different pharmacology and tissue- or cell-specific site of action compared with receptor monomers or homo-oligomers. An ideal tool for validating this concept pharmacologically would be a hetero-oligomer selective ligand. We set out to develop and execute a 1536-well high-throughput screen of over 1 million compounds to detect potential hetero-oligomer selective ligands using a ß-arrestin recruitment assay in U2OS cells coexpressing recombinant µ- and δ-opioid receptors. Hetero-oligomer selective ligands may bind to orthosteric or allosteric sites, and we might anticipate that the formation of hetero-oligomers may provide novel allosteric binding pockets for ligand binding. Therefore, our goal was to execute the screen in such a way as to identify positive allosteric modulators (PAMs) as well as agonists for µ, δ, and hetero-oligomeric receptors. While no hetero-oligomer selective ligands were identified (based on our selection criteria), this single screen did identify numerous µ- and δ-selective agonists and PAMs as well as nonselective agonists and PAMs. To our knowledge, these are the first µ- and δ-opioid receptor PAMs described in the literature.

Functionalization of the 6,14-bridge of the orvinols. Part 3: preparation and pharmacological evaluation of 18- and 19-hydroxyl substituted orvinols.

The orvinols are a class of potent opioids which have been extensively studied, yet little is known about the effects of introducing substituents into the 18- and 19-positions. The etheno bridge of thevinone was hydroxylated to give both the 18- and 19-hydroxyl substituted thevinols. After 3-O-demethylation to the corresponding orvinols, binding and GTPgammaS functional assays indicated that hydroxyl substitution at the 18- and 19-positions differentially affects the mu opioid efficacy of orvinols.

Complex of an active mu-opioid receptor with a cyclic peptide agonist modeled from experimental constraints.

Site-directed mutagenesis and design of Zn(2+)-binding centers have been used to determine a set of specific tertiary interactions between the mu-opioid receptor, a rhodopsin-like G protein-coupled receptor (GPCR), and its cyclic peptide agonist ligand, Tyr(1)-c(S-Et-S)[d-Cys(2)-Phe(3)-d-Pen(4)]NH(2) (JOM6). The binding affinity of the tetrapeptide is strongly dependent on the nature of its first and third residues and on substitutions at positions 213, 216, 237, 300, 315, and 318 of the mu-opioid receptor. His(1) and His(3) analogues of the ligand were able to form metal-binding complexes with the V300C and G213C/T315C receptor mutants, respectively. Direct contact of the Phe(3) residue of JOM6 with Gly(213), Asp(216), Thr(315), and Trp(318) of the receptor was suggested by the binding affinities of His(3)-, Nle(3)-, Leu(3)-, Aci(3)-, Delta(E)Phe(3)-, and Delta(Z)Phe(3)-substituted peptides with the G213C/T315C, D216V, T315C, and W318L mutants. The improved binding affinity of the free carboxylate analogue of JOM6 for binding to the E229D mutant revealed an interaction between the C-terminal group of the peptide and Glu(229) of the receptor. The experimental constraints that were obtained were applied for distance geometry modeling of the mu-receptor in complex with the tetrapeptide agonist ligand, JOM6. The active conformation of the opioid receptor was calculated using the crystal structure of "inactive" rhodopsin and published engineered and intrinsic metal-binding sites and disulfide bonds that allow or facilitate activation of GPCRs. Interhelical H-bonds existing in the mu-receptor were applied as additional distance constraints. The calculated model of the receptor-ligand complex can serve as a prototype of the active state for all rhodopsin-like GPCRs. It displays a strongly shifted transmembrane helix 6 (TM6) and reorientation of the conserved Trp(293) residue in TM6 upon its interaction with the agonist. Importantly, the binding pockets of the active and inactive states are not identical, which implies distinct interaction modes of agonists and antagonists. In the active state, the binding pocket of the mu-receptor is complementary to the previously proposed receptor-bound conformation of JOM6.

Selective interactions between helix VIII of the human mu-opioid receptors and the C terminus of periplakin disrupt G protein activation.

Analysis of interactions between the C-terminal tail of the MOP-1 and MOP-1A variants of the human mu-opioid receptor with proteins derived from a human brain cDNA library resulted in identification of the actin and intermediate filament-binding protein periplakin. Mapping of this interaction indicated that the predicted fourth intracellular loop/helix VIII of the receptor interacts with the C-terminal rod and linker region of periplakin. Periplakin is widely expressed in the central nervous system of both man and rat and demonstrated an overlapping but not identical distribution with mu-opioid (MOP) receptors. Co-expression of periplakin with MOP-1 or a MOP-1-eYFP fusion construct in HEK293 cells did not interfere with agonist-mediated internalization of the receptor. When co-expressed with a MOP-1-Gi1 alpha fusion protein periplakin significantly reduced the capacity of the agonist to stimulate binding of 35S-labeled guanosine 5'-3-O-(thio)triphosphate ([35S]GTP gamma S) to the receptor-associated G protein. By contrast, periplakin did not interfere with agonist-stimulation of [35S]GTP gamma S binding to either an alpha 2A-adrenoreceptor-Gi1 alpha fusion protein or a beta2-adrenoreceptor-Gs alpha fusion protein, indicating its selectivity of function. This represents the first example of an opioid receptor-interacting protein that functions to disrupt agonist-mediated G protein activation.

Recursive Partitioning analysis of mu-opiate receptor high throughput screening results.

Recursive Partitioning (RP) was used to analyze a heterogeneous data set of mu receptor High Throughput Screening results of combinatorial libraries, lead optimization products and reference opiate ligands from literature. Different sets of molecular descriptors and various parameterization schemes have been systematically assessed in search for an optimal RP tree, best discriminating between mu receptor ligands and inactive molecules. This discriminating ability has been evaluated in terms of a quality criterion (representing an enrichment factor corrected by the retrieval rate of active compounds), for both the learning set--with and without performing a cross-validation test--and a distinct validation set. The non-linearity of the approach, as well as the very large number of degrees of freedom of the models, render the statistical analysis--in particular, the detection of overfitting--quite difficult. The advantages and disadvantages of the RP approach are discussed on hand of the comparative analysis of the performances of the models under the studied conditions. Eventually, the features highlighted by the RP model as essential sources for the activity of compounds with respect to the mu receptor are analyzed in the light of commonly accepted mu receptor binding hypotheses. It is shown that the RP model considered to be "optimal" due to its simultaneous success in the cross-validation and validation simulations has been able to "discover" the existence of the two main ("morphine-like" and "meperidine-like") mu ligand families, represented as the two main "active nodes" of this tree.

Conformational and electrostatic similarity between polyprotonated and Ca(2+)-bound mu-opioid peptides.

In a previous paper (Zhorov and Ananthanarayanan, J. Biomol, Struct. Dynam. 1995, 13:1-13) we had calculated the minimum-energy conformations of monoprotonated and zwitterionic mu-opioid peptides and demonstrated the remarkable similarity between Ca(2+)-bound morphine on the one hand and the Ca(2+)-bound forms of these peptides on the other. We postulated that the Ca(2+)-bound forms of mu-opioids would activate the mu-receptor. To assess further the involvement of multiple positive charges on some of the mu-opioid ligands in their interaction with the receptor, we have, in this work, studied the geometry of five mu-opioid peptides containing two to four protonated groups and having chemical structures essentially different from the endogenous mu-opioid peptide Met-enkephalin (EK). Conformational space was searched using the Monte Carlo-with-energy-minimization method. Ca(2+)-bound forms of the selected peptides were found to be energetically unfavourable implying that one of the protonated groups plays a role similar to that Ca2+ plays in EK-Ca2+ complex. Bioactive conformations of the polyprotonated peptides were then selected using the criteria formulated earlier for Ca(2+)-bound ligands as well as additional criteria requiring ligands to have an elongated conical overall shape complementary to the interface between the transmembrane segments of mu-receptor. Low-energy conformations meeting these criteria were found in all the peptides considered, the protonated groups being separated from each other by about 8 and 16 A. The possible role of the ligands' cationic groups in mu-receptor activation is discussed.

Functional coupling of the delta-, mu-, and kappa-opioid receptors to mitogen-activated protein kinase and arachidonate release in Chinese hamster ovary cells.

To examine whether the mitogen-activated protein kinase (MAPK) cascade and phospholipase A2 (PLA2) are involved in the signal transduction mechanism of the opioid receptor, the delta-, mu-, and kappa-opioid receptors were stably expressed from cDNA in Chinese hamster ovary cells. Activation of the delta-, mu-, and kappa-receptors by agonists induced a rapid and transient increase in MAPK activity accompanied by reduced electrophoretic mobility of the 42-kDa isoform of MAPK (p42), probably owing to phosphorylation. The opioid receptor-mediated increase in MAPK activity was suppressed not only by pretreatment with genistein, a tyrosine protein kinase inhibitor, but also by prolonged exposure to phorbol 12-myristate 13-acetate and pretreatment with GF 109203X, a selective protein kinase C (PKC) inhibitor, suggesting the involvement of PKC as well as tyrosine protein kinase. Furthermore, stimulation of the delta-, mu-, and kappa-receptors with opioid agonists in the presence of A23187, a calcium ionophore, resulted in an increase in arachidonate release, suggesting that PLA2 is activated by the opioid receptors when the intracellular Ca2+ concentration is elevated. Both MAPK activation and increase in arachidonate release mediated by the opioid receptors were abolished by pretreatment with pertussis toxin, suggesting that these responses are mediated by Gi or Go types of GTP-binding regulatory proteins.

Molecular cloning and in situ hybridization histochemistry for rat mu-opioid receptor.

We cloned a cDNA for the rat mu-opioid receptor from a rat thalamus cDNA library. The deduced amino-acid sequence of rat mu-opioid receptor consists of 398 residues with the features shared by the members of the G-protein coupled receptor family, and is 59% and 60% identical with those of rat kappa-opioid and mouse delta-opioid receptors, respectively. Northern blot analysis showed that expression of mu-opioid receptor mRNA was intensive in the thalamus, striatum, hypothalamus and pons-medulla, moderate in the hippocampus and midbrain, and slight in the cerebral cortex and cerebellum. More detailed distribution of the mRNA in the rat brain was examined using the in situ hybridization technique. Intense expression of mu-opioid receptor mRNA was observed in the internal granular and glomerular layers of the olfactory bulb, caudate putamen, nucleus accumbens, medial septum, diagonal band, medial preoptic area, several nuclei of thalamus, amygdala, interpeduncular nucleus, medial raphe nucleus, inferior colliculus, parabrachial nucleus, locus coeruleus, nucleus solitary tract and ambiguus nucleus. Furthermore, mu-opioid receptor mRNA was moderately expressed in the hippocampus, globus pallidus, ventral pallidus, arcuate hypothalamic nucleus, supramammillary nucleus, superior colliculus, periacqueductal gray, and several nuclei of lower brain stem, including raphe magnus nucleus, reticular gigantocellular nucleus and lateral paragigantocellular nucleus.

Purification and partial amino acid sequence of a mu opioid receptor from rat brain.

A rat brain opioid receptor protein was isolated by binding [epsilon-biotinyl-Lys32] beta-endorphin to membranes, solubilizing the receptor-ligand (R.L) complex with deoxycholate-lysophosphatidylcholine and purifying on immobilized streptavidin and wheat germ agglutinin. The purified glycoprotein had a molecular mass of 60-70 kDa. Recovery of this protein was blocked by the nonselective opioid antagonist naloxone and the highly mu-selective agonist [D-Ala2,N-methyl-Phe4,Glyol5]-enkephalin but not by the highly delta-selective agonist [D-Pen2,4'-Cl-Phe4,D-Pen5]enkephalin when these compounds were added as competitors at the binding step. The 60-70-kDa receptor protein co-purified through the streptavidin column with 40-kDa protein recognized by anti-Gi alpha antibodies. GTP and Na+ influenced dissociation of the solubilized R.125I-L complex and elution of the receptor and G protein from streptavidin in fashions consistent with the pharmacology of mu-opioid receptors. A 23-amino acid residue sequence from the purified receptor differs at 4 positions from a similar sequence in the murine delta-opioid receptor and is encoded within a novel rat brain cDNA isolated by polymerase chain reaction with oligonucleotide primers related to the murine delta-opioid receptor gene.