Predicted structures for kappa opioid G-protein coupled receptor bound to selective agonists.

Human kappa opioid receptor (κ-OR), a G protein-coupled receptor (GPCR), has been identified as a drug target for treatment of such human disorders as pain perception, neuroendocrine physiology, affective behavior, and cognition. In order to find more selective and active agonists, one would like to do structure based drug design. Indeed, there is an X-ray structure for an antagonist bound to κ-OR, but structures for activated GPCRs are quite different from those for the inactive GPCRs. Here we predict the ensemble of 24 low-energy structures of human kappa opioid receptor (κ-OR), obtained by application of the GEnSeMBLE (GPCR Ensemble of Structures in Membrane Bilayer Environment) complete sampling method, which evaluates 13 trillion combinations of tilt and rotation angles for κ-OR to select the best 24. To validate these structures, we used the DarwinDock complete sampling method to predict the binding sites for five known agonists (ethylketocyclazocine, bremazocine, pentazocine, nalorphine, and morphine) bound to all 24 κ-OR conformations. We find that some agonists bind selectively to receptor conformations that lack the salt bridge between transmembrane domains 3 and 6 as expected for active conformations. These 3D structures for κ-OR provide a structural basis for understanding ligand binding and activation of κ-OR, which should be useful for guiding subtype specific drug design.

Latent inhibition in an insect: the role of aminergic signaling.

Latent inhibition (LI) is a decrement in learning performance that results from the nonreinforced pre-exposure of the to-be-conditioned stimulus, in both vertebrates and invertebrates. In vertebrates, LI development involves dopamine and serotonin; in invertebrates there is yet no information. We studied differential olfactory conditioning of the proboscis extension response in the honeybee Apis mellifera, and we compared LI in individuals treated with antagonists of biogenic amines (dopamine, octopamine, and serotonin). An antagonist of octopamine receptors and two antagonists of serotonin receptors showed LI disruption. We thus provide evidence that serotonin would participate in the regulation of LI in honeybees.

Drug discrimination in pigeons trained to discriminate among morphine, U50488, a combination of these drugs, and saline.

This study compared fixed-ratio (FR) and fixed-interval (FI) schedules to investigate the discriminative stimulus properties of µ-opioid and/or κ-opioid receptor agonists. Pigeons were trained to discriminate among morphine (µ agonist), U50488 (κ agonist), the combination, and saline under FR 20-s or FI-300-s schedule. After training, correct-key responding averaged 94.4 (FR) and 66.5% (FI) after administration of training drugs. Dose-response curves were generally quantal under the FR and graded under the FI schedules, but highly variable among subjects under the FI. Under the FR schedule, the dose of naltrexone that blocked morphine's discriminative stimuli also blocked U50488. Combining high doses of morphine with low doses of U50488 produced responding on the morphine key and combining high doses of U50488 with low doses of morphine produced responding on the U50488 key. Combining high doses of both drugs produced responding on the drug-combination key. Increasing d,l-pentazocine doses shifted responding from the saline key to the U50488 key, then to the morphine key, and finally to the drug-combination key. Butorphanol and ethylketocyclazocine produced similar effects, except responding on the morphine key increased before responding on the U50488 key. The four-choice procedure under the FR schedule has the potential for determining the discriminative stimulus effects of mixed agonists.

The kappa-opiate receptor impacts the pathophysiology and behavior of substance use.

There is increasing evidence that the kappa-opiate receptor, in addition to the mu-opiate receptor, plays an important role in substance use pathophysiology and behavior. As dopamine activity is upregulated through chronic substance use, kappa receptor activity, mediated through the peptide dynorphin, is upregulated in parallel. Dynorphin causes dysphoria and decreased locomotion, and the upregulation of its activity on the kappa receptor likely dampens the excitation caused by increased dopaminergic activity. This feedback mechanism may have significant clinical implications for treating drug dependent patients in various stages of their pathology.

Differential effects of opioid agonists on G protein expression in CHO cells expressing cloned human opioid receptors.

Recent evidence indicates that agonist ligands of G protein coupled receptors (GPCR) can activate different signaling systems. Such "agonist-directed" signaling also occurs with opioid receptors. Previous work from our laboratory showed that chronic morphine, but not DAMGO, up-regulates the expression of Galpha12 and that both morphine and DAMGO decreased Galphai3 expression in CHO cells expressing the cloned human mu opioid receptor. In this study, we tested the hypothesis that chronic opioid regulation of G protein expression is agonist-directed. Following a 20h treatment of CHO cells expressing the cloned human mu (hMOR-CHO), delta (hDOR-CHO) or kappa (hKOR-CHO) opioid receptors with various opioid agonists, we determined the expression level of Galpha12 and Galphai3 by Western blots. Among five mu agonists (morphine, etorphine, DADLE, DAMGO, herkinorin) tested with hMOR-CHO cells, only chronic morphine and etorphine up-regulated Galpha12 expression. All five mu agonists decreased Galphai3 expression. Among six delta agonists (SNC80, DPDPE, deltorphin-1, morphine, DADLE, etorphine) tested with hDOR-CHO cells, all six agonists down-regulated Galphai3 expression or moderately up-regulated Galpha12 expression. Among five kappa agonists, ((-)-ethylketocyclazocine, salvinorin A, U69,593, etorphine, (-)-U50,488) tested with hKOR-CHO cells, only chronic (-)-U50,488 and (-)-EKC up-regulated Galpha12 expression. All kappa agonists decreased Galphai3 expression. These data demonstrate that chronic opioid agonist regulation of G protein expression depends not only on the agonist tested, but also on the type of opioid receptor expressed in a common cellular host, providing additional evidence for agonist-directed signaling.

Opioids modulate constitutive B-lymphocyte secretion.

The opioid system plays a major role in immunomodulation, while its action on cells of the immune system may be opioid receptor-mediated or not. Opioid effects on B-lymphocytes are considered as indirect, attributed to an interplay between distinct cell populations. The aim of the present study was to investigate whether opioid agonists (morphine, alpha(S1)-casomorphin and ethylketocyclazocine) may have a direct action on the secretion of antibodies and cytokines by multiple myeloma-derived cell lines and normal CD19+ B-lymphocytes. Our results show that opioids modulate antibody and cytokine secretion by multiple myeloma cells in a cell line-dependent and opioid receptor-independent manner, while they decrease antibody secretion by normal B-lymphocytes. Furthermore, they decrease the proliferation rate of multiple myeloma cells through opioid receptor activation. Our data suggest two different mechanisms of action of opioids, mediated by different signaling pathways: an early non-opioid receptor-related effect, modulating the constitutive immunoglobulin and cytokine secretion, and a long-term receptor-mediated action on cell growth. These data suggest a further opioid implication in the control of humoral immunity.

Effects of kappa opioid agonists alone and in combination with cocaine on heart rate and blood pressure in conscious squirrel monkeys.

As kappa agonists have been proposed as treatments for cocaine abuse, the cardiovascular effects of the kappa opioid receptor agonists ethylketocyclazocine (EKC) and enadoline were investigated in conscious squirrel monkeys. Both EKC and enadoline increased heart rate with little effect on blood pressure. This effect appeared to be specific for kappa receptors as the mu opioid agonist morphine did not mimic the effects of the kappa agonists. The opioid antagonist naltrexone, at a dose of 1.0 mg/kg, blocked the effect of EKC. An action at both central and peripheral receptors may be responsible for the heart rate increase following kappa agonist treatment. The ganglionic blocker chlorisondamine partially antagonized the effect of EKC on heart rate, suggesting central involvement, while the peripherally-acting agonist ICI 204,448 ((+/-)-1-[2,3- (Dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride) also increased heart rate, supporting a peripheral site of action. When given in combination with cocaine, EKC produced effects that were sub-additive, suggesting that the kappa agonists may be used safely as cocaine abuse treatments.

The inhibitory effect of opioids on HepG2 cells is mediated via interaction with somatostatin receptors.

Opioids, acting via G-protein coupled membrane receptors, induce analgesia. However their role is not limited to their anti-nociceptive action. They are found in several peripheral tissues acting as negative regulators of cellular processes. Even though that is not fully elucidated, it becomes obvious that opioids exert their effects in close relation to other neuropeptides such as somatostatin. Hepatocellular carcinoma is one tumor, among others, which secrete bioactive peptides while somatostatin analogs exert an inhibitory effect. We have used the human hepatocyte-derived cancer cell line HepG2, in order to examine the effect of opioids on cell growth and their possible mode of action. Our results show that the opioid ethylketocyclazocine (EKC) inhibits cell proliferation and induces apoptosis. This inhibitory effect is not exerted via opioids receptors since it was not reversed by the opioid antagonist diprenorphine and functional opioid receptors were not found on HepG2 cells. On the contrary, we show that EKC binds to somatostatin receptors, and activates a PTP signalling cascade. In this respect, the interaction of opioids with somatostatin receptors on hepatocellular carcinoma cells, and the fact that they are widely used for pain control, may provide some additional clues for the discrepancies during treatment with somatostatin analogues.

Redefining the structure-activity relationships of 2,6-methano-3-benzazocines. 4. Opioid receptor binding properties of 8-[N-(4'-phenyl)-phenethyl)carboxamido] analogues of cyclazocine and ethylketocycalzocine.

The synthesis and evaluation of a series of aryl-containing N-monosubstituted analogues of the lead compound 8-carboxamidocyclazocine were performed to probe a putative hydrophobic binding pocket of opioid receptors. High binding affinity to mu, kappa, and delta opioid receptors was observed for the 8-[N-(4'-phenyl)-phenethyl)carboxamido] analogue.

The opioid agonist ethylketocyclazocine reverts the rapid, non-genomic effects of membrane testosterone receptors in the human prostate LNCaP cell line.

Neuropeptides influence cancer cell replication and growth. Opioid peptides, and opiergic neurons are found in the prostate gland, and they are proposed to exert a role in tumor regulation, influencing cancer cell growth, as opioid agonists inhibit cell growth in several systems, including the human prostate cancer cell line LNCaP. In the same cell line, the existence of membrane testosterone receptors was recently reported, which increase, in a non-genomic manner, the secretion of PSA, and modify actin cytoskeleton dynamics, through the signaling cascade FAK-->PI-3 kinase-->Cdc42/Rac1. In the present work, we present data supporting that the general opioid agonist Ethylketocyclazocine (EKC) decreases testosterone-BSA (a non-internalizable testosterone analog) induced PSA secretion. Furthermore, we report that this opioid affects this non-genomic testosterone action, by modifying the distribution of the actin cytoskeleton in the cells, disrupting the above signaling cascade. In addition, after long (>24 h) incubation, opioids decrease the number of membrane testosterone receptors, and reverse their effect on the signaling molecules. In conclusion, our results provide some new insights of a possible action of opioids in prostate cancer control by interfering with the action and the expression of membrane testosterone receptors and signaling.

Diazabicyclononanones, a potent class of kappa opioid analgesics.

The 1,5-dimethyl 3,7-diaza-3,7-dimethyl-9-oxo-2,4-di-2-pyridine-bicyclo[3.3.1]nonane-1,5-dicarboxylate, HZ2, has a high and selective affinity for the kappa opioid receptor and an antinociceptive activity comparable to morphine. In addition, it is characterized by a long duration of action and a high oral bioavailability. QSAR studies within series of kappa agonists revealed a chair-boat conformation of a double protonated HZ2 characterized by an almost parallel orientation of the C9 carbonyl group and the N7-H group and at least one aromatic ring to be the pharmacophoric arrangement. Structural variations showed that the pyridine rings in 2 and 4 position can be replaced with p-methoxy-, m-hydroxy- and m-fluoro-substituted phenyl rings. However, all other substituents have to be kept the same for a high affinity to the kappa receptor.

Association of PI-3 kinase with PAK1 leads to actin phosphorylation and cytoskeletal reorganization.

The family of p21-activated kinases (PAKs) have been implicated in the rearrangement of actin cytoskeleton by acting downstream of the small GTPases Rac and Cdc42. Here we report that even though Cdc42/Rac1 or Akt are not activated, phosphatidylinositol-3 (PI-3) kinase activation induces PAK1 kinase activity. Indeed, we demonstrate that PI-3 kinase associates with the N-terminal regulatory domain of PAK1 (amino acids 67-150) leading to PAK1 activation. The association of the PI-3 kinase with the Cdc42/Rac1 binding-deficient PAK1(H83,86L) confirms that the small GTPases are not involved in the PI-3 kinase-PAK1 interaction. Furthermore, PAK1 was activated in cells expressing the dominant-negative forms of Cdc42 or Rac1. Additionally, we show that PAK1 phosphorylates actin, resulting in the dissolution of stress fibers and redistribution of microfilaments. The phosphorylation of actin was inhibited by the kinase-dead PAK1(K299R) or the PAK1 autoinhibitory domain (PAK1(83-149)), indicating that PAK1 was responsible for actin phosphorylation. We conclude that the association of PI-3 kinase with PAK1 regulates PAK1 kinase activity through a Cdc42/Rac1-independent mechanism leading to actin phosphorylation and cytoskeletal reorganization.

Endogenous morphine modulates acute thermonociception in mice.

The endogenous synthesis of morphine has been clearly demonstrated throughout the phylogenesis of the nervous system of mammals and lower animals. Endogenous morphine, serving as either a neurotransmitter or neurohormone, has been demonstrated in the nervous system of both vertebrates and invertebrates. As one of the effects of exogenous morphine is the modulation of pain perception, we investigated the effects that the depletion of endogenous morphine had on nociceptive transmission. The immunoneutralization of endogenous morphine from brain extracellular spaces was obtained through the intracerebroventricular administration of affinity purified anti-morphine IgG to mice, which then underwent the hot plate test. Endogenous morphine immunoneutralization decreased thermal response latency and attenuated the anti-nociceptive effect of the mu selective agonist DAMGO in hot plate test suggesting that endogenous morphine is involved in pain modulation.

GR89,696: a potent kappa-opioid agonist with subtype selectivity in rhesus monkeys.

GR89,696 is a synthetic kappa-opioid receptor agonist, recently reported to have an agonist profile consistent with selectivity at the proposed "kappa(2)" subtype. The present studies evaluated the effects of GR89,696 in vitro (i.e., in radioligand binding and [(35)S]guanosine-5'-O-(3-thio)triphosphate assays) and in vivo in rhesus monkeys, in assays used to study kappa-opioid agonists (i.e., thermal antinociception, sedation and muscle relaxation, diuresis, and increases in serum prolactin levels, as well as ethylketocyclazocine and U69,593 discrimination). Furthermore, the sensitivity of GR89,696 to naltrexone and nor-binaltorphimine (nor-BNI) antagonism was compared with that of U50,488 and U69,593, ligands selective for the proposed "kappa(1)" subtype. Overall, GR89,696 displayed the profile of a highly potent kappa-opioid agonist, following parenteral administration in rhesus monkeys. GR89,696 was less sensitive than U50,488 and U69,593 to naltrexone or nor-BNI antagonism, consistent with an action through the proposed kappa(2) receptor subtype.

Receptor selectivity of Met-enkephalin-Arg6-Phe7, an endogenous opioid peptide, in cerebral cortex of human and rat.

This study was undertaken to examine the receptor selectivity of Met-enkephalin-Arg6-Phe7 (MERF) employing radioreceptor binding assays in human cerebral cortex membranes, and to elucidate the responsible receptors that mediate the regulatory action of MERF on high (20 mM) K+-stimulated release of [3H]norepinephrine ([3H]-NE) in rat cortex slices. Specific binding of [3H]MERF was inhibited by DAMGO, Tyr-D-Arg-Phe-Sar(TAPS), bremazocine and ethylketocyclazocine (EKC), but not by U69,593 (U69) and DPDPE. MERF showed high affinity for specific binding sites of [3H]DAMGO. However, MERF had little influence on the specific binding of [3H]DPDPE, [3H]U69 and [3H]diprenorphine ([3H]DIP) in the presence of 1 microM each of DAMGO, DPDPE and U69. In [3H]NE release experiments using rat cortex slices, DAMGO, MERF and EKC, in order of their potency, inhibited K+-stimulated release of [3H]NE. The inhibitory effects of MERF and DAMGO were more sensitive than that of EKC to antagonism by CTAP, nor-binaltorphimine (nor-BNI) and naloxone. These results suggested that MERF possesses high affinity for mu-receptors, but not for delta-, kappa1-, and very low affinity for kappa2-receptors in human cerebral cortex membranes. Also, the inhibitory effect of MERF on the K+-stimulated release of [3H]NE appears to be mediated by mu-receptors in rat cerebral cortex slices.

Synthesis and opioid receptor affinity of morphinan and benzomorphan derivatives: mixed kappa agonists and mu agonists/antagonists as potential pharmacotherapeutics for cocaine dependence.

This report concerns the synthesis and preliminary pharmacological evaluation of a novel series of kappa agonists related to the morphinan (-)-cyclorphan (3a) and the benzomorphan (-)-cyclazocine (2) as potential agents for the pharmacotherapy of cocaine abuse. Recent evidence suggests that agonists acting at kappa opioid receptors may modulate the activity of dopaminergic neurons and alter the neurochemical and behavioral effects of cocaine. We describe the synthesis and chemical characterization of a series of morphinans 3a-c, structural analogues of cyclorphan [(-)-3-hydroxy-N-cyclopropylmethylmorphinan S(+)-mandelate, 3a], the 10-ketomorphinans 4a,b, and the 8-ketobenzomorphan 1b. Binding experiments demonstrated that the cyclobutyl analogue 3b [(-)-3-hydroxy-N-cyclobutylmethylmorphinan S(+)-mandelate, 3b, MCL-101] of cyclorphan (3a) had a high affinity for mu, delta, and kappa opioid receptors in guinea pig brain membranes. Both 3a,b were approximately 2-fold more selective for the kappa receptor than for the mu receptor. However 3b (the cyclobutyl analogue) was 18-fold more selective for the kappa receptor in comparison to the delta receptor, while cyclorphan (3a) had only 4-fold greater affinity for the kappa receptor in comparison to the delta receptor. These findings were confirmed in the antinociceptive tests (tail-flick and acetic acid writhing) in mice, which demonstrated that cyclorphan (3a) produced antinociception that was mediated by the delta receptor while 3b did not produce agonist or antagonist effects at the delta receptor. Both 3a,b had comparable kappa agonist properties. 3a,b had opposing effects at the mu receptor: 3b was a mu agonist whereas 3a was a mu antagonist.

Effects of kappa opioid agonists on the discriminative stimulus effects of cocaine in rhesus monkeys.

This study examined the effects of the kappa opioid agonists U50,488 and ethylketocyclazocine (EKC) on cocaine discrimination in rhesus monkeys trained to discriminate cocaine (0.4 mg/kg) from saline. Administration of U50,488 and EKC alone produced primarily saline-appropriate responding. Kappa agonist pretreatments produced variable effects on cocaine discrimination across monkeys, attenuating the discriminative stimulus effects of cocaine in some monkeys, but either having no effect on cocaine discrimination or enhancing the discriminative stimulus effects of cocaine in other monkeys. The effects of kappa agonists on cocaine discrimination were reversed by pretreatment with the opioid antagonist naloxone (1.0 mg/kg). These results indicate that kappa agonists do not consistently block the discriminative stimulus effects of cocaine in rhesus monkeys.

Differential antagonism of the rate-decreasing effects of kappa-opioid receptor agonists by naltrexone and norbinaltorphimine.

Eight kappa-opioid receptor agonists were examined for their effects in squirrel monkeys responding under a fixed interval 3-min schedule of stimulus termination. Six of these kappa-opioid receptor agonists decreased dose-dependently the total number of responses and with an order of potency consistent with kappa-opioid receptor interaction. Three of these kappa-opioid receptor agonists, bremazocine, U69,593 [[(5a,7a,8b)-(+)-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl)] benzeneacetamide] and enadoline, were evaluated following pretreatment with 1.0 mg/kg of naltrexone or 3.0 mg/kg of norbinaltorphimine. The effects of the three agonists were antagonized significantly by naltrexone, but only those of bremazocine and U69,593 were antagonized significantly by norbinaltorphimine. Statistical analysis of the data averaged over six monkeys revealed that naltrexone was significantly more potent than norbinaltorphimine at antagonizing enadoline and U69,593, but naltrexone and norbinaltorphimine were equipotent at antagonizing bremazocine. Moreover, naltrexone was 8-fold more potent at antagonizing U69,593 and enadoline than at antagonizing bremazocine. These results suggest that under these conditions the effects of U69,593 and enadoline may be mediated, in part, by a different receptor population, perhaps a subtype of kappa-opioid receptors, from the one that mediates the effects of bremazocine.

[3H]ethylketocyclazocine binding to brain opioid receptor subtypes in alcohol-preferring AA and alcohol-avoiding ANA rats.

We measured brain regional patterns of [3H]ethylketocyclazocine binding to brain opioid receptors in ethanol-naive alcohol-preferring Alko, Alcohol (AA) and alcohol-avoiding Alko, Non-Alcohol (ANA) rats, by using quantitative autoradiography. This agonist ligand labels all opioid receptor subtypes. The proportions of mu- and delta-opioid receptor binding were evaluated by displacing the mu- and delta-opioid receptor components by the peptides Tyr-D-Ala-Gly-N(Me)Phe-Gly-ol (DAMGO, 100 nM) and Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE, 100nM), respectively, the K-component being the naltrexone-sensitive binding left after removal of the above two components. The labeling patterns in the brains of the AA and ANA rats were consistent with the well-known distributions of the opioid receptor subtypes in nonselected rat strains and there was no major difference between the lines. The mu-opioid receptor binding was greater in the AA than ANA rats in several brain regions, most interestingly in the substantia nigra pars reticulata and striatal clusters with elevated shell/core ratios in the nucleus accumbens. The delta-opioid receptor binding did not differ between the lines, whereas the AA rats had more K-opioid receptors than the ANA rats in several brain regions, including limbic areas and basal ganglia. The observed results might indicate altered action of the opioidergic system on dopaminergic pathways in rats with differential alcohol preference.

kappa-Opioid receptor effects of butorphanol in rhesus monkeys.

Butorphanol and nalbuphine have substantial affinity for mu and kappa-opioid receptor sites, yet their behavioral effects in monkeys are largely consistent with a mu receptor mechanism of action. Using ethylketocyclazocine (EKC) discrimination and diuresis assays in rhesus monkeys (Macaca mulatta), the purpose of the current investigation was to characterize the in vivo kappa-opioid activity of these compounds through the use of an insurmountable mu-opioid receptor antagonist, clocinnamox. Alone, butorphanol (0.001-0.032 mg/kg i.m.) failed to generalize to EKC, and pretreatment with the competitive opioid receptor antagonist quadazocine (0.1 or 0.32 mg/kg i.m.) did not alter this generalization. At 24 h after clocinnamox (0.1 mg/kg i.m.) administration, butorphanol fully generalized to EKC, and this generalization was maintained in two of three monkeys at 72 h. Parallel results were observed in diuresis: butorphanol alone and in the presence of quadazocine (1 mg/kg i.m.) did not alter urine output, and a marked diuretic effect was demonstrated 24 h to 2 weeks after clocinnamox administration. Clocinnamox did not alter the discriminative stimulus or diuretic effects of nalbuphine or of the kappa-opioid receptor agonists EKC or U69593. These results are consistent with an in vivo agonist activity of butorphanol at kappa-opioid receptors that can only be demonstrated when an insurmountable antagonist has substantially eliminated the dominant receptor population through which it exerts its action.