Selective labeling of κ2 opioid receptors in rat brain by [I]IOXY: Interaction of opioid peptides and other drugs with multiple κ2a binding sites

Recent studies from our laboratory resolved two subtypes of the κ₂ binding site, termed κ_2a and κ_2b, using guinea pig, rat, and human brain membranes depleted of μ and δ receptors by pretreatment with the site-directed acylating agents BIT (μ-selective) and FIT (δ-selective). 6β-Iodo-3,14-dihydroxy-17-cyclopropylmethyl-4,5-epoxymorphinan (IOXY), an opioid antagonist that has high affinity for κ₂ sites, was radioiodinated to maximum specific activity (2200 Ci/mmol) and purified by high pressure liquid chromotography and used to characterize multiple κ₂ binding sites. The results indicated that [¹²⁵I]IOXY, like [³H]bremazocine, selectively labels κ₂ binding sites in rat brain membranes pretreated with BIT and FIT. Using 100 nM [ -Ala²-MePhe⁴,Gly-ol⁵]enkephalin to block [¹²⁵I]IOXY binding to the κ_2b site, two subtypes of the κ_2a binding site were resolved, both in the absence and presence of 50 μM 5′-guanylyimidodiphosphate. Viewed collectively, these results provide further evidence for heterogeneity of the κ opioid receptor, which may provide new targets for drug design, synthesis, and therapeutics.     

Binding of [H][D-Ala, MePhe, Gly-ol] Enkephalin, [H][D-Pen, D-Pen]Enkephalin, and [H]U-69,593 to Airway and Pulmonary Tissues of Normal and Sensitized Rats

Bhargava, H. N., V. M. Villar, J. Cortijo and E. J. Morcillo. Binding of [³H][D-Ala², MePhe⁴, Gly-ol⁵]enkephalin, [³H][D-Pen², D-Pen⁵]enkephalin, and [³H]U-69,593 to airway and pulmonary tissues of normal and sensitized rats. Peptides 18(10) 1603–1608, 1997.—The role of endogenous opioid peptides in the regulation of bronchomotor tone, as well as in the pathophysiology of asthma is uncertain. We have studied the binding of highly selective [³H]labeled ligands of μ-([D-Ala², MePhe⁴, Gly-ol⁵]enkephalin; DAMGO), δ ([D-Pen², D-Pen⁵]enkephalin; DPDPE), and κ-(U-69,593) opioid receptors to membranes of trachea, main bronchus, lung parenchyma and pulmonary artery obtained from normal (unsensitized) and actively IgE-sensitized rats acutely challenged with the specific antigen. [³H]DAMGO, [³H]DPDPE and [³H]U-69,593 bound to membranes of normal and sensitized tissues at a saturable, single high-affinity site. The rank order of receptor densities in normal tissues was δ- ≥ κ- ≥ μ-, with lung parenchyma exhibiting the greatest binding capacity for δ- and μ- receptors compared to the other regions examined. The K_d values showed small differences between ligands and regions tested. The μ- and δ-opioid receptor densities were decreased in sensitized main bronchus and lung parenchyma, respectively, compared to normal tissues. By contrast, κ-opioid receptor density was augmented in sensitized lung parenchyma but an increase in K_d values was also observed. These differential changes in the density and affinity of opioid receptor types may be related to alterations in endogenous opioid peptides during the process of sensitization.     

Three-way Interaction between 14-3-3 Proteins, the N-terminal Region of Tyrosine Hydroxylase, and Negatively Charged Membranes

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines, is activated by phosphorylation-dependent binding to 14-3-3 proteins. The N-terminal domain of TH is also involved in interaction with lipid membranes. We investigated the binding of the N-terminal domain to its different partners, both in the unphosphorylated (TH-(1–43)) and Ser¹⁹-phosphorylated (THp-(1–43)) states by surface plasmon resonance. THp-(1–43) showed high affinity for 14-3-3 proteins (K_d ∼ 0.5 μm for 14-3-3γ and -ζ and 7 μm for 14-3-3η). The domains also bind to negatively charged membranes with intermediate affinity (concentration at half-maximal binding S_0.5 = 25–58 μm (TH-(1–43)) and S_0.5 = 135–475 μm (THp-(1–43)), depending on phospholipid composition) and concomitant formation of helical structure. 14-3-3γ showed a preferential binding to membranes, compared with 14-3-3ζ, both in chromaffin granules and with liposomes at neutral pH. The affinity of 14-3-3γ for negatively charged membranes (S_0.5 = 1–9 μm) is much higher than the affinity of TH for the same membranes, compatible with the formation of a ternary complex between Ser¹⁹-phosphorylated TH, 14-3-3γ, and membranes. Our results shed light on interaction mechanisms that might be relevant for the modulation of the distribution of TH in the cytoplasm and membrane fractions and regulation of l-DOPA and dopamine synthesis.     

Regional interactions of opioid peptides at μ and δ sites in rat brain

Opiate binding sites in five brain regions were labeled with the μ and δ markers, ³H-morphine and ³H-[D-Ala²,D-leu⁵]enkephalin, respectively. The highest densities of both ³H-morphine and ³H-DADLE labeled sites are found in striatum and frontal cortex. Hypothalamus and midbrain contain predominantly ³H-morphine labeled sites. The selectivity of the opioid peptides [D-Ala²,D-leu⁵]enkephalin, β-endorphin and dynorphin(1–13) for the two opiate sites was investigated by comparing the potency of these unlabeled compounds against the μ and δ markers in different brain regions. This determination has the effect of controlling for the breakdown of peptides within each region. While the enkephalin analogue shows a preference for the δ binding site and β-endorphin is more nearly equipotent towards the two binding sites, dynorphin(1–13) shows a high affinity and selective preference for the μ binding site over the δ site. The potency of the opioid peptides in displacing the μ and δ markers varies from region to region according to the relative densities of the two opiate binding site populations.     

Assessment of substitution in the second pharmacophore of Dmt-Tic analogues

The Dmt-Tic pharmacophore exhibits potent δ-opioid receptor antagonism. Analogues with substitutions in the second pharmacophore with (1, 1′) or without a COOH function (2–9) were synthesized: several had high δ affinity (1′, 2, 7, and 9), but exhibited low to non-selectivity toward μ receptors similar to H-Dmt-Tic-amide and H-Dmt-Tic-ol. Functional bioactivity indicated high δ antagonism (pA₂ 7.4–7.9) (1′, 2, and 9) and modest μ agonism, pEC₅₀ (6.1–6.3) (1′, 2, 8, and 9), but with E_max values analogous to dermorphin. These Dmt-Tic analogues with mixed δ antagonist/μ agonist properties would appear to be better candidates as analgesics than pure μ agonists.     

Probing the opioid receptor complex with (+)-trans-SUPERFIT. II. Evidence that mu ligands are noncompetitive inhibitors of the delta cx opioid peptide binding site.

Previous studies delineated two classes of δ binding sites; a δ binding site not associated with the opioid receptor complex, termed the δ_ncx site, and a δ site associated with the opioid receptor complex, termed the δ_cx site. The δ_ncx site has high affinity for [ -Pen², -Pen⁵]enkephalin, and is synonymous with what is now identified as the δ₁ binding site. Pretreatment of membranes with the δ-selective acylating agents FIT, or (+)-trans-SUPERFIT, deplete membranes of the δ_ncx binding site, which permits the selective labeling of the δ_cx binding site with [³H][ -Ala²,Leu⁵]enkephalin. The present study compared the properties of the δ_cx binding site present in brain membranes pretreated with (+)-trans-SUPERFIT with the properties of the δ_cx site present in untreated membranes. The major findings are: 1) pretreatment of membranes with (+)-trans-SUPERFIT decreased the IC₅₀ values of δ-preferring drugs, and increased the IC₅₀ values of μ-preferring drugs, for the δ_cx binding site; 2) the degree of δ selectivity was highly correlated with the magnitude of the (+)-trans-SUPERFIT-induced shift in the IC₅₀ values; 3) the ligand-selectivity patterns of the μ and δ_cx sites present in (+)-trans-SUPERFIT-pretreated membranes were poorly correlated; 4) whereas μ-preferring drugs were noncompetitive inhibitors of [³H][ -Ala²,Leu⁵]enkephalin binding to the δ_cx site, δ-preferring drugs were competitive inhibitors. Viewed collectively, these data support the hypothesis that the μ and δ_cx binding sites are distinct, provide additional evidence for δ receptor heterogeneity, and suggest that ( (+)-trans-SUPERFIT-pretreated membranes will provide a useful preparation for studying the δ_cx binding site.     

Effect of Chronic Administration of Morphine, U-50, 488H and [D-Pen, D-Pen]enkephalin on the Concentration of cGMP in Brain Regions and Spinal Cord of the Mouse

Bhargava, H. N. and Y. J. Cao. Effect of chronic administration of morphine, U-50,488H and [ -Pen², -Pen⁵]enkephalin on the concentration of cGMP in brain regions and spinal cord of the mouse. Peptides 18(10) 1629–1634, 1997.—The effects of chronic administration and subsequent withdrawal of μ-, κ- and δ-opioid receptor agonists on the levels of cyclic GMP in several brain regions and spinal cord of mice were determined in an attempt to further study the role of NO cascade in opioid actions. The agonists at μ-, κ- and δ-opioid receptor included morphine, U-50,488H and DPDPE, respectively. Tolerance to morphine was associated with highly significant increases in cGMP levels in corpus striatum (41%), cortex (36%), midbrain (73%) and cerebellum (51%) relative to controls. Abstinence caused increases in cGMP levels in corpus striatum (61%) and pons and medulla (45%). Tolerance to U-50,488H resulted in increases in cGMP levels in midbrain (52%) whereas abstinence from U-50,488H increased the cGMP levels in pons and medulla(76%). Tolerance to DPDPE was associated with increases in cGMP levels in hypothalamus (12%) and pons and medulla (33%) but decreases in cerebellum (66%) and spinal cord (58%). Abstinence from DPDPE produced increases in cGMP levels in pons and medulla (14%) but decreases in cerebellum (67%) and spinal cord (50%). Overall treatment with morphine and U-50,488H produced increases in cGMP levels in brain regions whereas DPDPE produced decreases in brain regions and spinal cord. Previous studies have shown that chronic administration of μ- and κ- opioid receptor agonists induce NO synthase (NOS) in certain brain regions and that the inhibitors of NO synthase attenuate tolerance to μ- and κ- but not to δ-opioid receptors agonists. Since activation of NO increases the production of cGMP, the present results demonstrating alterations of cGMP levels by μ-, κ- and δ-opioid receptor agonists are consistent with the behavioral results with NOS inhibitors on tolerance to μ-, κ- and δ-opioid receptor agonists.     

The possible involvement of endogenous ligands for mu-, delta- and kappa-opioid receptors in modulating morphine-induced CPP expression in rats

Previous studies suggested that electroacupuncture (EA) can suppress opioid dependence by the release of endogenous opioid peptides. To explore the site of action and the receptors involved, we tried to inject highly specific agonists for μ-, δ- and κ-opioid receptors into the CNS to test whether it can suppress morphine-induced conditioned place preference (CPP) in the rat. Male Sprague–Dawley rats were trained with 4 mg/kg morphine, i.p. for 4 days to establish the CPP model. This CPP can be prevented by (a) i.p. injection of 3 mg/kg dose of morphine, (b) intracerebroventricular (i.c.v.) injection of micrograms doses of the selective μ-opioid receptor agonist DAMGO, δ-agonist DPDPE or κ-agonist U-50,488H or (c) microinjection of DAMGO, DPDPE or U50488H into the shell of the nucleus accumbens (NAc). The results suggest that the release of endogenous μ-, δ- and κ-opioid agonists in the NAc shell may play a role for EA suppression of opiate addiction.     

Structure-activity relationships of cyclic β-casomorphin-5 analogues

Cyclic analogues of the β-casein-derived opioid peptide β-casomorphin-5 (H-Tyr-Pro-Phe-Pro-Gly-OH) were prepared through substitution of the Pro² residue with various ,ω-diamino acid residues (lysine, ornithine, 2,4-diaminobutyric acid) and cyclization of the ω-amino group to the C-terminal carboxyl function. Compounds of this type, with D-configuration at the 2-position residue, showed high opioid receptor affinity with some preference for μ receptors over δ receptors, high potency in the guinea pig ileum assay and considerable activity in the mouse vas deferens assay. Configurational inversion at the 4-position in these cyclic analogues resulted in enhanced affinity for both μ and δ receptors, whereas N-methylation of the Phe³ residue produced a potency decrease.     

Selective labeling of κ2 opioid receptors in rat brain by [I]IOXY: Interaction of opioid peptides and other drugs with multiple κ2a binding sites

Recent studies from our laboratory resolved two subtypes of the κ₂ binding site, termed κ_2a and κ_2b, using guinea pig, rat, and human brain membranes depleted of μ and δ receptors by pretreatment with the site-directed acylating agents BIT (μ-selective) and FIT (δ-selective). 6β-Iodo-3,14-dihydroxy-17-cyclopropylmethyl-4,5α-epoxymorphinan (IOXY), an opioid antagonist that has high affinity for κ₂ sites, was radioiodinated to maximum specific activity (2200 Ci/mmol) and purified by high pressure liquid chromotography and used to characterize multiple κ₂ binding sites. The results indicated that [¹²⁵I]IOXY, like [³H]bremazocine, selectively labels κ₂ binding sites in rat brain membranes pretreated with BIT and FIT. Using 100 nM [d-Ala²-MePhe⁴,Gly-ol⁵]enkephalin to block [¹²⁵I]IOXY binding to the κ_2b site, two subtypes of the κ_2a binding site were resolved, both in the absence and presence of 50 μM 5′-guanylyimidodiphosphate. Viewed collectively, these results provide further evidence for heterogeneity of the κ opioid receptor, which may provide new targets for drug design, synthesis, and therapeutics.     

Synthesis and opioid activity of N,N-dimethyl-Dmt-Tic-NH-CH(R)-R' analogues: acquisition of potent delta antagonism

N,N-Dimethylation of the H-Dmt-Tic-NH-CH(R)-R′ series of compounds produced no significant affect on the high δ-opioid receptor affinity (K_i=0.035–0.454 nM), but dramatically decreased that for the μ-opioid receptor. The effect of N-methylation was independent of the length of the linker (R); however, the bioactivities were affected by the chemical composition of the third aromatic group (R′): phenyl (Ph) (5′–8′) elicited a greater reduction in μ-affinity (40–70-fold) compared to analogues containing 1H-benzimidazole-2-yl (Bid) (9-fold). The major consequences of N,N-dimethylation on in vitro bioactivity were: (i) a loss of δ-agonism coupled with the appearance of potent δ antagonism (4′–7′) (pA₂=8.14–9.47), while 1 exhibited only a 160-fold decreased δ agonism (1′) and the δ antagonism of 8 enhanced >10-fold (pA₂=10.62, 8′); and (ii) a consistent loss of μ-affinity resulted in enhanced δ-opioid receptor selectivity. With the exception of compound 1′, the change in the hydrophobic environment at the N-terminus and formation of a tertiary amine by N,N-dimethylation in analogues of the Dmt-Tic pharmacophore produced potent δ-selective antagonists.     

Binding to δ and μ opioid receptors by deltorphin I/II analogues modified at the Phe and Asp/Glu side chains: a report of 32 new analogues and a QSAR study

The synthesis and binding affinities of 32 X³Gly⁴ dual-substitution analogues of the natural opioid heptapeptides deltorphin I and II are reported. A multiple regression QSAR analysis was performed using those results along with literature data for the X³Asp⁴ and Phe³X⁴ side chain analogues. Fitting to a three-term potential well model with hydrophobic and van der Waals attraction terms and a steric repulsion term indicates that the δ and μ receptor sites for binding the residue three side chain are similar, and that the binding interaction is primarily van der Waals and secondarily hydrophobic. Further analysis indicates that both sites are more constrained with respect to side chain length than width or thickness, and the μ site appears to be somewhat larger. A binding model consistent with these findings pictures the native third residues Phe ring laying on a step notched out of the receptor surface, pointing toward the back (riser) of the step, and sandwiched between the receptor and ligand. However, the binding sites for the residue four side chains are quite different on δ and μ receptors. Binding to the δ site appears to involve both electrostatic attraction (probably to a partial positive charge) and van der Waals attraction, but not necessarily hydrogen bonding, and more constraint with respect to side chain length than width or thickness. In contrast, there is no evidence for any kind of binding attraction between the side chain of residue four and the μ site, which acts more as steric repulsion site, as though the space that is a pocket on the δ receptor is filled in on the μ receptor. A regression model based only on steric repulsion by van der Waals bulk and/or the effective bulk of a hydration layer accounts for over 80% of the residue four related variation in μ affinity.

Abstract

Thirty-two new X³Gly⁴ analogues of deltrophin I/II opioid peptides are described. A QSAR study of the X³Gly⁴, X³Asp⁴, and Phe³X⁴ analogue series using a potential well model reveals the roles of hydrophobic, van der Waals, electrostatic, hydrogen bonding and steric interactions in δ and μ receptor binding of X³ and X⁴ side chains.     

Involvement of opioid receptors in the oxytocin-induced antinociception in the central nervous system of rats

Recent studies showed that oxytocin and opioid peptides play important roles in pain modulation at different levels in the central nervous system. The present study was performed to explore whether opioid system is involved in the oxytocin-induced antinociception in the brain of rats. The results showed that: (1) intracerebroventricular injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWL) to noxious thermal and mechanical stimulation in rats. (2) The antinociceptive effect of oxytocin was attenuated dose-dependently by intracerebroventricular injection of naloxone, indicating an involvement of opioid system in the oxytocin-induced antinociception. (3) It is interesting that the antinociceptive effect of oxytocin was attenuated by subsequent intracerebroventricular injection of the μ-opioid antagonist β-funaltrexamine (β-FNA) and the κ-opioid antagonist nor-binaltorphimine (nor-BNI), but not the δ-opioid antagonist naltrindole. The results indicate that oxytocin plays an antinociceptive role in the brain of rats; μ- and κ-opioid receptors, not δ-receptors, are involved in the oxytocin-induced antinociception in the central nervous system of rats.     

Antiinflammatory effects of the cyclopentenone isoprostane 15-A(2)-IsoP in human gestational tissues

Proinflammatory prostaglandins and cytokines are involved in the initiation of human labor and delivery. Although cyclopentenone prostaglandins regulate the formation of these prolabor mediators via nuclear factor-κB (NF-κB) and/or peroxisome proliferator-activated receptor-γ, recent evidence suggests that they do not exist in vivo. Cyclopentenone isoprostanes (IsoPs), which are highly reactive structural isomers of bioactive cyclopentenone prostaglandins, do exist physiologically and have been shown to inhibit the inflammatory response in macrophages. Therefore the aim of this study was to determine the effect of the synthetic cyclopentenone IosP 15-A₂-IsoP on the expression of prolabor mediators in human gestational tissues. Human placenta and gestational membranes (n = 5) were incubated in the absence or presence of 12.5, 25, and 50 μM 15-A₂-IsoP with 10 μg/ml lipopolysaccharide (LPS). Treatment of placenta and fetal membranes with 15-A₂-IsoP caused a dose-dependent decrease in LPS-stimulated release of the cytokines IL-1β, IL-6, IL-8, and TNF- and the prostaglandins PGE₂ and PGF₂. NF-κB p65 DNA binding activity was significantly inhibited by treatment with 50 μM 15-A₂-IsoP. Collectively, these data suggest that 15-A₂-IsoP exhibits antiinflammatory properties via antagonism of NF-κB activity. Cyclopentenone IsoPs may serve as negative feedback regulators of the inflammatory response in human gestational tissues.     

Hymenoic acid, a novel specific inhibitor of human DNA polymerase lambda from a fungus of Hymenochaetaceae sp

Hymenoic acid (1) is a natural compound isolated from cultures of a fungus, Hymenochaetaceae sp., and this structure was determined by spectroscopic analyses. Compound 1 is a novel sesquiterpene, trans-4-[(1′E,5′S)-5′-carboxy-1′-methyl-1′-hexenyl]cyclohexanecarboxylic acid. This compound selectively inhibited the activity of human DNA polymerase λ (pol λ) in vitro, and 50% inhibition was observed at a concentration of 91.7 μM. Compound 1 did not influence the activities of the other seven mammalian pols (i.e., pols , γ, δ, ε, η, ι, and κ), but also showed no effect even on the activity of pol β, which is thought to have a very similar three-dimensional structure to the pol β-like region of pol λ. This compound also did not inhibit the activities of prokaryotic pols and other DNA metabolic enzymes tested. These results suggested that compound 1 could be a selective inhibitor of eukaryotic pol λ. This compound had no inhibitory activities against two N-terminal truncated pol λ, del-1 pol λ (lacking nuclear localization signal (NLS), BRCA1 C-terminus (BRCT) domain [residues 133–575]), and del-2 pol λ (lacking NLS, BRCT, domain and proline-rich region [residues 245–575]). The compound 1-induced inhibition of intact pol λ activity was non-competitive with respect to both the DNA template-primer and the dNTP substrate. On the basis of these results, the pol λ inhibitory mechanism of compound 1 is discussed.     

Direct interactions of androgenic/anabolic steroids with the peripheral benzodiazepine receptor in rat brain: Implications for the psychological and physiological manifestations of androgenic/anabolic steroid abuse

The peripheral benzodiazepine receptor (PBR) is a mitochondrial protein involved in regulating steroid synthesis and transport. We report here the effects of androgenic/anabolic steroids (AAS) on the binding of the PBR-specific ligand [³H] PK11195 to male rat brain cortical synaptoneurosomes. Two synthetic AAS, stanozolol and 17β-testosterone cypionate (17β-cyp), significantly inhibited 1 nM [³H] PK11195 binding at concentrations greater than 5 and 25 μM, respectively. Stanozolol was the most effective inhibitor, reducing [³H] PK11195 binding by up to 75%, compared to only 40% inhibition by 17β-cyp, at 50 μM AAS concentration. Two other AAS, 17-methyltestosterone and nortestosterone decanoate, were incapable of inhibiting [³H] PK11195 binding at concentrations up to 50 μM. On the basis of Scatchard/Rosenthal analysis, [³H] PK11195 binds to two classes of binding sites, and the inhibition of [³H] PK11195 binding by stanozolol appears to be allosteric, primarily reducing binding to the higher affinity [³H] PK11195 binding site. These results, in combination with earlier studies indicating the direct effects of AAS on the function of additional central nervous system receptor complexes, suggest that the behavioral and psychological effects of AAS result from the interactions of AAS with multiple regulatory systems in the brain.     

Analogs of thyrotropin-releasing hormone using an aminoglycine-based template

Novel thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH₂) analogs, made by solid phase, were derived from the general scaffold pGlu-(D/L)Agl(X)-Pro-NH₂ where Agl = aminoglycine. Analogs ranged from X being a proton to an acylating agent derived from substituted (aromatic heterocyclic rings) formic or acetic acids or an aminotriazolyl moiety (3′-amino-1H-1′,2′,4′-triazolyl) built on N of aminoglycine or N^β of ,β-diaminoproprionic acid (Dpr). X was expected to mimic the electronic and structural characteristics of the imidazole ring of histidine. Analogs were purified by HPLC, characterized by mass spectrometry and isolated as either diastereoisomeric mixtures or pure isomers. Analogs, tested for their binding affinity to mouse pituitary TRH receptors, have apparent equilibrium inhibitory constants >1 μM.     

New series of potent delta-opioid antagonists containing the H-Dmt-Tic-NH-hexyl-NH-R motif

Heterodimeric compounds H-Dmt-Tic-NH-hexyl-NH-R (R = Dmt, Tic, and Phe) exhibited high affinity to δ- (K_iδ = 0.13–0.89 nM) and μ-opioid receptors (K_iμ = 0.38–2.81 nM) with extraordinary potent δ antagonism (pA₂ = 10.2–10.4). These compounds represent the prototype for a new class of structural homologues lacking μ-opioid receptor-associated agonism (IC₅₀ = 1.6–5.8 μM) based on the framework of bis-[H-Dmt-NH]-alkyl (Okada, Y.; Tsuda, Y.; Fujita, Y.; Yokoi, T.; Sasaki, Y.; Ambo, A.; Konishi, R.; Nagata, M.; Salvadori, S.; Jinsmaa, Y.; Bryant, S. D.; Lazarus, L. H. J. Med. Chem. 2003, 46, 3201), which exhibited both high μ affinity and bioactivity.