Preparation OF [7, 8, 19, 20 - 3H] Naloxone of high specific activity

Synthesis and characterization of tritium-labelled substances

This article reviews the methods and strategies to radiolabel and characterise substances with tritium.

In vitro binding and functional studies of Ac-RYYRIK-ol and its derivatives, novel partial agonists of the nociceptin/orphanin F/Q receptor

Following the discovery of nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) and its endogenous ligand, an extensive search has started to find selective agonists and antagonists targeting this novel receptor-ligand system due to their therapeutic potentials. By the help of the combinatorial chemistry a series of hexapeptides with a general formula of Ac-RYY-R/K-W/I-R/K-NH(2) having high NOP receptor affinity and selectivity were identified. On the basis of this information we developed a number of novel compounds. The detailed structure-activity studies on the partial agonist Ac-RYYRIK-NH(2) are reported in this communication. Besides the modifications on N- and C-terminal, Arg-Cit exchange was performed on the template structure. The novel hexapeptides were analyzed in radioligand binding, functional biochemical [(35)S]GTPgammaS binding assays by using membranes from rat brains and Chinese hamster ovary cells expressing human NOP receptor. The agonist/antagonist properties were also tested on in the mouse vas deferens bioassay. C-terminal modification yielded a high affinity, selective and potent NOP ligand (Ac-RYYRIK-ol) with a partial agonist property. Several analogs of this compound were synthesized. The presence of the positively charged arginine residue at the first position turned out to be crucial for the biological activity of the hexapeptide. The N-terminal modifications with various acyl groups (ClAc, pivaloyl, formyl, benzoyl, mesyl) decreased the affinity of the ligand towards the receptor and the intrinsic activity for stimulating the G-protein activation was also decreased. The structure-activity studies on the hexapeptide derivatives provided some basic information on the structural requirements for receptor binding and activation.

Partial and full agonism in endomorphin derivatives: comparison by null and operational model

The partial mu-opioid receptor pool inactivation strategy in isolated mouse vas deferens was used to determine partial agonism of endomorphins and their analogs (endomorphin-1-ol, 2',6'-dimethyltyrosine (Dmt)-endomorphin-1, endomorphin-2-ol and (D-Met2)-endomorphin-2) using morphine, normorphine, morphiceptin, (D-Ala2,MePhe4,Gly5-ol)-enkephalin (DAMGO) and its amide (DAMGA) as reference opioid agonists. Agonist affinities (KA) and efficacies were assessed both by the "null" and the "operational" method. The KA values determined by the two methods correlated significantly with each other and also with the displacing potencies against 3H-naloxone in the receptor binding assay in the presence of Na+. DAMGO and DAMGA were full agonist prototypes, morphine, endomorphin-1, endomorphin-1-ol, Dmt-endomorphin-1, endomorphin-2-ol and (D-Met2)-endomorphin-2 were found by both methods to be partial agonists whereas the parameters for normorphine, morphiceptin and endomorphin-2 were intermediate.

Synthesis and biological studies of nociceptin derivatives containing the DTPA chelating group for further labeling with therapeutic radionuclides

Nociceptin is an endogenous anti-opiate heptadecapeptide primarily interacting with the nociceptin (NOP) receptor. This neuropeptide-receptor system is involved in pain regulation, tolerance to and dependence on opiates as well as many other physiological and pathophysiological events. The role and mechanisms of nociceptin in pathological conditions is not clearly known yet. In an attempt to have a radiopharmaceutical labeled either with 99mTc or (111)In, we incorporated diethylenetriaminepentaacetic acid (DTPA) as chelator into the structure of [Arg14,Lys15]nociceptin(1-17)-NH2 at the epsilon-amino group of Lys15. Such a radiopeptide may be useful in imaging for diagnostical purposes. Preparation of the peptide ligands was carried out by solid phase synthesis. Two peptides containing DTPA were obtained and purified. The products were [Arg14,Lys(DTPA)15]nociceptin(1-17)-NH2 and its cross-linked dimer on the basis of mass spectrometric analysis. In (115)In3+ binding experiments the conjugates exhibited preserved indium ion chelating properties, indicating the potential use of radiolabeled DTPA-nociceptin derivatives as radiopharmaceutical. Biological properties of these compounds were studied in rat brain membrane preparations by radioligand binding, functional biochemical [35S]GTPgammaS binding assays and mouse vas deferens (MVD) bioassay. Besides the similar in vitro binding characteristics to nociceptin receptor, both of the DTPA-chelated compounds were more potent and efficient than nociceptin in functional biochemical and mouse vas deferens bioassays. Our further aim is to radiolabel these compounds in order to get a radiopharmaceutical which can be used diagnostically.

Nociceptin antagonism: probing the receptor by N-acetyl oligopeptides

In search for effective antagonist structures for the nociceptin (NOP) receptor, a number of N-acylated oligopeptides, including N-acyl tetra- and pentapeptides selective for the kappa-opioid receptor, as well as N-acyl hexapeptides bearing the Ac-Arg-Tyr-Tyr-Arg-Ile-Lys (Ac-RYYRIK) core sequence originally isolated from combinatorial chemical libraries, were synthesized and studied in radioreceptor binding assays, [(35)S]GTPgammaS functional tests and in mouse vas deferens (MVD) bioassays. The properties of the novel antagonist candidates were compared to known antagonists. A new antagonist structure with a reduced, primer alcohol C-terminus, Ac-Arg-Tyr-Tyr-Arg-Ile-lysinol (Ac-RYYRIK-ol) was described in the mouse vas deferens tests, showing an equilibrium inhibitory constant value (K(e)) of 2.44 nM, and no agonist effect at 10 microM ligand concentration. Schild-analysis indicated a clearly competitive interaction at the NOP receptor, whereas the peptide did not affect the action of the delta-opioid receptor agonist [D-Ala(2),D-Leu(5)]enkephalin. Ac-RYYRIK-ol also exhibited a high affinity in [(3)H]nociceptin-NH(2) binding competition assays using rat brain membranes. Agonist-induced G-protein activation via NOP receptors was studied in [(35)S]GTPgammaS binding stimulation assays by the use of both native brain tissue preparations and membranes from cultured CHO cells expressing recombinant nociceptin receptors. Ac-RYYRIK-ol displayed only weak intrinsic agonist activity, whereas it effectively inhibited the stimulation generated by nociceptin. The results support the high potency and antagonist nature of Ac-RYYRIK-ol and reveal important roles for both the N- and the C-terminal region of the molecule.

Kappa-receptor selective binding of opioid ligands with a heterocyclic bicyclo[3.3.1]nonan-9-one structure

Previous pharmacological results have suggested that members of the heterocyclic bicyclo[3.3.1]nonan-9-one-like compounds are potent kappa-opioid receptor specific agonists. One lead molecule of this series. called compound 1 (dimethyl 7-methyl-2,4-di-2-pyridyl-3.7-diazabicyclo[3.3.1]nonan-9-one-1,5-dicarboxylate) exhibited high affinity for [3H]ethylketocyclazocine and [3H]U-69.593 binding sites in guinea pig cerebellar membranes which known to be a good source for kappa1 receptors. It was shown by molecular modelling that heterocyclic bicyclo[3.3.1]nonan-9-ones fit very well with the structure of ketazocine, a prototypic kappa-selective benzomorphan compound; when compared to the arylacetamide structure of U-69.593, a specific kappa1-receptor agonist, a similar geometry was found with a slightly different distribution of the charges. It is postulated, that the essential structural skeleton involved in the opioid activity is an aryl-propyl-amine element distributed along the N7-C6-C5-C4-aryl bonds.

Autoradiography of opioid and ORL1 ligands in opioid receptor triple knockout mice

Three genes for the opioid receptors ( micro, delta and kappa) and a gene coding for a related receptor (ORL1) have been cloned but pharmacological studies suggest that further subtypes exist that remain poorly understood. To determine if there are other classically defined opioid binding sites we have carried out homogenate binding and section autoradiography with [3H]naloxone in mice that lack all three opioid genes and are hyperalgesic in a thermal nociceptive test. We have also examined [3H]bremazocine labelling in triple knockout brain and spinal cord as this ligand has been proposed to label novel kappa-receptors. No receptor labelling for either ligand was detected in the brains or spinal cord of knockout mice demonstrating that all binding is the product of the three known receptors and that there is no cross-labelling of the ORL1 receptor. Nociceptin (1 micro m) caused marked displacement of [3H]bremazocine in wild-type brains indicating that nociceptin at high concentrations can displace classical opioid binding. As a number of studies have proposed a close association between the classical opioid receptors and the ORL1 system we also hypothesized that loss of all of the classical opioid receptors might lead to compensatory changes in ORL1 receptors. Labelling of the ORL1 receptor with [3H]nociceptin showed region-dependent quantitative increases in triple knockout brains indicating a close relationship between the two systems in specific brain areas.

Biochemical characterisation of newly developed beta-etorphine and beta-dihydroetorphine derivatives

The highly potent synthetic narcotic compound etorphine is known to cause strong analgesia, catatonia and blockade of conditioned reflexes in laboratory animals and is widely used for the immobilisation of game animals. In this study, a number of new structural analogues of etorphine, including C18-beta-structures, 3-O-methylether derivatives and saturated C7-C8 dihydro-compounds, were synthesised and examined in in vitro ligand binding experiments. Opiate receptor-mediated activation of G-proteins by these derivatives was also investigated using the [35S]GTPgammaS binding assay. The receptor binding affinity constant and G-protein stimulatory potency of the novel beta-etorphins were compared with those of the corresponding C18-alpha-derivatives. In rat brain membrane preparations, all the compounds tested displayed high affinity (K(i)'s ranging 0.4-22 nM) using [3H]naloxone in competition assays. The alpha-etorphines had somewhat higher affinity in comparison with the beta-structures. Methylether derivatives were consistently weaker than the corresponding phenolic compounds. Dihydroetorphine and beta-dihydroetorphine, which have a partially saturated ring structure, showed as good potency in the binding assays as did etorphine and beta-etorphine with C7-C8 double bonds. The etorphine derivatives were potent but naloxone-reversible activators of G-proteins in the [35S]GTPgammaS functional tests. It was also found that the C3 phenolic group is favourable for G-protein activation. On the basis of our experimental results, neither the configuration of C18 nor the saturation of the C7-C8 double bond appears to play a critical role in the biological activity of etorphines.

Side chain modifications change the binding and agonist properties of endomorphin 2

Side chain modifications were introduced to endomorphin 2 (E2) to improve its binding properties and biological activity. A number of C-terminal modifications decreased the binding affinity to the mu-opioid receptor and the intrinsic activity in rat brain membranes. The exception was E2-ol, which showed increased binding affinity to MOR and higher potency in stimulating [(35)S]GTPgammaS binding. N-methylation of Phe(3) (MePhe(3)) attenuated the binding affinity and produced a rightward shift of [(35)S]GTPgammaS binding curves. All derivatives had lower intrinsic activity than E2. Some of the modified peptides partially inhibited, while YPF-benzyl-allyl-amide fully inhibited, the E2 or [d-Ala(2),MePhe(4),Gly(5)ol]enkephalin stimulated [(35)S]GTPgammaS binding. Marked differences were found between the results obtained using tritiated E2, tritiated naloxone, and [(35)S]GTPgammaS binding, indicating the possible involvement of multiple binding sites. The data presented demonstrate that the C-terminal amide group has an essential role in the regulation of the binding and the agonist/antagonist properties of E2.

Influence of degradation on binding properties and biological activity of endomorphin 1

The recently-isolated endogenous peptide endomorphin 1 has high affinity for the mu opioid receptor and plays an important role in analgesia. Several of its degradation products have been isolated from the central nervous system. Degradation products present structural similarities and may influence the receptor binding properties and biological activity of the parent compound. Therefore, we investigated how degradation of endomorphin 1 might influence ligand binding to the mu opioid receptor, the consequent activation of G proteins and its antinociceptive effect. Both N- and C-terminal truncation of endomorphin 1 resulted in peptides presenting considerably lower opioid receptor binding potency. None of these peptides had an effect on GTP binding, nor was able to produce analgesia, suggesting that degradation destroys the biological activity of endomorphin 1.

Receptor binding and G-protein activation by new Met5-enkephalin-Arg6-Phe7 derived peptides

Met5-enkephalin-Arg6-Phe7 (Tyr-Gly-Gly-Phe-Met-Arg-Phe, MERF) is a naturally occurring heptapeptide that binds to opioid and non-opioid recognition sites in the central nervous system. Four synthetic analogs with single or double amino acid substitutions were prepared by solid phase peptide synthesis to achieve proteolytically more stable structures: Tyr-D-Ala-Gly-Phe-Met-Arg-Phe (I), Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (II), Tyr-D-Ala-Gly-Phe-L-Nle-Arg-Phe (III) and Tyr-Gly-Gly-Phe-L-Nle-Arg-Phe (IV). In this study receptor binding characteristics and G-protein activation of MERF and its derivatives were compared in crude membrane fractions of frog and rat brain. Synthetic MERF-derived peptides were potent competitors for [3H]MERF and [3H]naloxone binding sites with the exception of analog (II) which turned to be substantially less active. The presence of 100 mM NaCl or 100 microM 5'-guanylylimidodiphosphate, Gpp(NH)p, decreased the affinity of the peptides in [3H]naloxone binding assays, suggesting that these ligands might act as agonists at the opioid receptors. Some of the compounds were also used to stimulate guanosine-5'-O-(3-[gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding in rat and frog brain membranes at concentrations of 10(-9)-10(-5) M. The EC50 values of analog (II) were the highest in both tissues. Analog (I) was as effective as MERF in rat brain membranes, but showed lower maximal stimulation in frog brain preparation. Again, analog (II) seemed to be the least efficacious peptide that stimulated [35S]GTPgammaS binding only by 59%. Specificity of the peptides was further investigated by the inhibition of agonist-stimulated [35S]GTPgammaS binding in the presence of selective antagonists for the opioid receptor types. The mu-selective antagonist cyprodime displayed the lowest potency in inhibiting the effects of the peptides, whereas norbinaltorphimine (kappa-selective antagonist) and naltrindole (delta-selective antagonist) were quite potent in both tissues. We concluded that MERF and its derivatives are able to activate G-proteins mainly via kappa- and delta-opioid receptors.

New opioid affinity labels containing maleoyl moiety

Opioid receptor binding properties and pharmacological profiles of novel peptides containing maleoyl function were determined in order to develop new affinity labels. Based on the enkephalin structure peptide ligands were synthesized and tested. Both in in vitro receptor binding experiments and pharmacological studies, all ligands showed agonist character with relatively high affinity (Ki values in the nanomolar range) and good to moderate selectivity. Replacement of Gly2 in the enkephalin frame with D-Ala led to higher affinities with a small decrease in selectivity. The longer peptide chains resulted in compounds with high percentage (up to 86%) of irreversible binding. The selectivity pattern of the ligands is in good agreement with the data obtained from the pharmacological assays (guinea pig ileum and mouse vas deferens bioassays). The newly synthesized peptides could be used in further studies in order to determine more detailed characteristics of the ligand-receptor interaction.

Affinity labeling of delta opioid receptors by an enkephalin-derivative alkylating agent, DSLET-Mal

Opioid binding properties of Tyr-D-Ser-Gly-Phe-Leu-Thr-NH-NH-Gly-Mal (DSLET-Mal), a novel enkephalin-framed affinity label, was determined in rat brain membranes. In competition studies the ligand showed high affinity for the delta opioid sites, labelled by [(3)H][Ile(5,6)]deltorphin II (K(i) = 8 nM), whereas its binding to the mu ([(3)H]DAMGO) and kappa ([(3)H]EKC) sites was weaker. Preincubation of the rat brain membranes with DSLET-Mal at micromolar concentrations resulted in a wash-resistant and dose-dependent inhibition of the [(3)H][Ile(5,6)]deltorphin II binding sites (96% blocking at 10 microM concentration). Intracerebroventricular (ICV) administration of DSLET-Mal reduced the density of delta opioid receptors and had no effect on mu and kappa receptors, as determined by saturation binding studies. [Ile(5, 6)]deltorphin II-stimulated [(35)S]GTPgammaS binding was determined in membrane preparations of different brain areas of the ICV-treated animals. In both frontal cortex and hippocampus DSLET-Mal significantly decreased G protein activation by the delta agonist, having no effect on DAMGO stimulated [(35)S]GTPgammaS binding. DSLET-Mal had qualitatively similar effects on both receptor binding and G protein activation. These characteristics of the compound studied suggest that DSLET-Mal can serve as an affinity label for further studies of the delta-opioid receptors.

Mu-opioid receptor specific antagonist cyprodime: characterization by in vitro radioligand and [35S]GTPgammaS binding assays

The use of compounds with high selectivity for each opioid receptor (mu, delta and kappa) is crucial for understanding the mechanisms of opioid actions. Until recently non-peptide mu-opioid receptor selective antagonists were not available. However, N-cyclopropylmethyl-4,14-dimethoxy-morphinan-6-one (cyprodime) has shown a very high selectivity for mu-opioid receptor in in vivo bioassays. This compound also exhibited a higher affinity for mu-opioid receptor than for delta- and kappa-opioid receptors in binding assays in brain membranes, although the degree of selectivity was lower than in in vitro bioassays. Cyprodime has recently been radiolabelled with tritium resulting in high specific radioactivity (36.1 Ci/mmol). We found in in vitro binding experiments that this radioligand bound with high affinity (K(d) 3. 8+/-0.18 nM) to membranes of rat brain affording a B(max) of 87. 1+/-4.83 fmol/mg. Competition studies using mu, delta and kappa tritiated specific ligands confirmed the selective labelling of cyprodime to a mu-opioid receptor population. The mu-opioid receptor selective agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]enkephalin (DAMGO) was readily displaced by cyprodime (K(i) values in the low nanomolar range) while the competition for delta- ([D-Pen(2), D-Pen(5)]enkephalin (DPDPE)) and kappa- (5alpha,7alpha, 8beta-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4, 5)dec-8-yl]-benzene-acetamide (U69,593)) opioid receptor selective compounds was several orders of magnitude less. We also found that cyprodime inhibits morphine-stimulated [35S]GTPgammaS binding. The EC(50) value of morphine increased about 500-fold in the presence of 10 microM cyprodime. These findings clearly indicate that cyprodime is a useful selective antagonist for mu-opioid receptor characterization.

Opioid binding profiles of new hydrazone, oxime, carbazone and semicarbazone derivatives of 14-alkoxymorphinans

Several hydrazone, oxime, carbazone and semicarbazone derivatives of 14-alkoxycodeinones and 14-alkoxydihydrocodeinones were synthesised [1] and characterised in in vitro radioligand binding assays in rat brain membrane preparations. The tested compounds show the highest affinity for the mu opioid binding sites and most of them have agonist character. Subtype analysis of the binding shows mu2 specificity. However, some of these ligands are able to block partially (40-60%) the high affinity (putative mu1) opioid binding sites while all of them act as reversible ligands at the low affinity (putative mu2) sites.

Irreversible labelling of the opioid receptors by a melphalan-substituted [Met5]enkephalin-Arg-Phe derivative

[Met5]enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe) was modified with the methyl esther of melphalan (Mel; 4-bis(2-chloroethyl)amino-L-phenylalanine) and the resulting compounds were studied for their opioid binding properties in guinea pig and rat brain membranes. Three new peptides, with a substitution of a single amino acid, were synthesized (Mel-Gly-Gly-Phe-Met-Arg-Phe, Tyr-Gly-Gly-Mel-Met-Arg-Phe and Tyr-Gly-Gly-Phe-Met-Arg-Mel). In the rat brain, none of these ligands displayed any type specificity, whereas in guinea pig brain membranes the C-terminally modified peptide, Tyr-Gly-Gly-Phe-Met-Arg-Mel ([Mel7]peptide), displayed a kappa-binding profile and was a weak kappa-opioid-receptor agonist in isolated guinea pig ileum. The effect of sodium ions on [Mel7]peptide competition against [3H]naloxone binding indicated a weak agonist nature of the compound. When guinea pig brain membranes were preincubated with 1-10 microM of [Mel7]peptide, an apparently irreversible inhibition of [3H]naloxone ligand binding was observed. These results suggest that the heptapeptide containing melphalan at the C-terminus can be used as a relatively high-affinity irreversible label for the kappa-opioid receptor.

In vitro binding and signaling profile of the novel mu opioid receptor agonist endomorphin 2 in rat brain membranes

The recently discovered endogenous mu receptor selective endomorphin 2 was prepared in tritiated form by a catalytic dehalogenation method resulting in a specific radioactivity of 1.98 TBq/mmol (53.4 Ci/mmol), and used for in vitro labelling of rat brain membranes. The binding was saturable, stereospecific and of high affinity (Kd: 0.97 and 1.12 nM based on kinetic and equilibrium binding studies, respectively). The maximal number of binding sites (Bmax) was found to be 114.8 fmol/mg protein. [3H]Endomorphin 2 was displaced by mu-receptor selective specific peptides and heterocyclic compounds with high affinity, whereas kappa and delta receptor specific ligands were much less potent. The Ki values of endomorphin 1 and 2 in inhibiting [3H]naloxone binding increased by 15-fold in the presence of 100 mM NaCl which indicates the agonist property of these peptides. Endomorphins stimulated [35S]GTPgammaS binding and inhibited adenylyl cyclase activity which also provides evidence for the agonist character of endomorphins.

Opioid binding profile of morphiceptin, Tyr-MIF-1 and dynorphin-related peptides in rat brain membranes

Opioid properties of several morphiceptin- (Tyr-Pro-Phe-Pro-NH2), Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and dynorphin-derivatives were characterized in rat brain in vitro receptor binding assay and in electrically stimulated longitudinal muscle strip preparation of guinea pig ileum. In the case of morphiceptin-related peptides, an excellent correlation was found between the [3H]-naloxone binding displacement data and the agonist potencies determined in the bioassay. The "turning point' was the C-terminal amidation in the tri- and tetrapeptide pairs in both series. Tyr-MIF-1 derivatives showed weak affinity in the opioid receptor binding assay and none of them had any remarkable effect in the bioassay either as agonist or antagonist. The dynorphin A(1-10)-peptides modified at positions 5 and 8 retained their affinity with Pro5-, Pro8-, and Ala8-substituents, whereas some loss of affinity was observed in the case of Gly8-Dyn A(1-10).

Thermodynamic parameters of opioid binding in the presence and absence of G-protein coupling

We have investigated the thermodynamic parameters of various opioid ligands interacting with their receptors in rat brain membranes. Affinity constants (Ka), enthalpy and entropy values were obtained from homologous displacement experiments performed at 0, 24 and 33 degrees C. It was found that all the opioid agonists tested ([3H]dihydromorphine (DHM) mu alkaloid; [3H]DAMGO mu peptide; [3H]deltorphin-B delta peptide) display endothermic binding accompanied with a large entropy increase, regardless of their chemical structure (alkaloid or peptide), or of their mu or delta receptor selectivity. In contrast, binding of the antagonist naloxone is exothermic, mainly enthalpy driven. Na+ or Mg2+ results only in quantitative changes of the thermodynamic parameters. In the presence of the GTP-analog Gpp(NH)p; or Gpp(NH)p + Na+; or Gpp(NH)p + Na- + Mg2+ the affinity of DHM binding dramatically decreases which might reflect functional uncoupling of the receptor-ligand complex and G-proteins. This altered molecular interactions are also indicated by curvilinear van't Hoff plot and entropy increase. It is concluded that the thermodynamic analysis provides means of determining the underlying driving forces of ligand binding and helps to delineate its mechanism.

Immunocytochemical visualization of kappa-opioid receptors on chick embryonic neurons differentiating in vitro

The present paper is the first immunocytochemical demonstration of kappa-opioid receptors in neurons isolated from seven-day-old chick embryonic forebrains and cultivated for one to seven days. The monoclonal antibody KA8 (IgG1-k) [Maderspach et al. (1991) J. Neurochem. 56, 1897-1904] was raised against the frog brain kappa-opioid receptor as an antigen and recognizes an epitope in or near the ligand binding site. The KA8 immunostaining of the neurons displayed individual variations and changed with the in vitro differentiation. Receptors often appeared at the pole of the primary outgrowing process, later on in the whole soma and finally on the branched processes. Specific radioligand binding and KA8 immunocytochemistry both presented an increase in the receptor concentration with development. The equilibrium binding values that were measured at 1 nM [3H]naloxone concentration were 2.9 and 6.1 fmol/10(6) cells on the first and sixth cultivation days, respectively. Neurons were treated with 10(-7) M bremazocine or dynorphine (agonists with relative specificity to kappa-opioid receptors) on the second and third cultivation days. The agonist promoted the morphological differentiation which was already visible within 24 h. It also promoted the expression of the 200,000 mol. wt neurofilament protein, this became pronounced after two to three days. The changes provoked by the agonist were reduced by the opioid antagonist norbinaltorphimine (10(-7) M) or naloxone (10(-5) M) indicating that the effect was receptor-mediated. The hypothesis that kappa-opioid agonists through their receptors may function as regulatory signals in the early neuronal differentiation is discussed.

Highly potent novel opioid receptor agonist in the 14-alkoxymetopon series

The newly synthesized 14-alkoxymetopon derivatives, 14-methoxymetopon, 14-ethoxymetopon, 14-methoxy-5-methyl-morphinone, exhibit high affinity for the naloxone binding sites in rat brain. A substantial decrease in affinity was observed, in the presence of NaCl indicating a high degree of agonist activity. All three 14-alkoxymetopon derivatives displayed high affinity for [3H][D-Ala2,(Me)Phe4,Gly-ol5]enkephalin ([3H]DAMGO) binding sites, much less potency toward delta sites and were the least effective at kappa sites. Isolated tissue studies using the guinea pig ileum preparation confirmed their high agonist potency. Following administration the new compounds produced naloxone reversible antinociceptive effects and were 130-300 times more potent than morphine in the acetic acid induced abdominal constriction model in the mouse, and the hot plate and tail flick tests in the rat. The compounds also produced dose-dependent muscle rigidity, and potentiated barbiturate-induced narcosis. The in vivo apparent pA2 values for naloxone against 14-ethoxymetopon and morphine were similar in analgesia, suggesting an interaction with the same (mu) receptor site. The dependence liability of 14-alkoxymetopon derivatives in the withdrawal jumping test was less pronounced than that of morphine in either rats or mice, similar to tolerance to the their analgesic action. It is concluded that the 14-alkoxymetopon derivatives studied are selective and potent agonists at mu opioid receptors, with reduced dependence liability.

Differential coupling of opioid binding sites to guanosine triphosphate binding regulatory proteins in subcellular fractions of rat brain

In this study, we present evidence for the occurrence of mu, delta, and kappa opioid binding sites in synaptic plasma membranes (SPM) and microsomes of rat brain. Binding to all three opioid classes was inhibited by 5'-guanylylimidodiphosphate (Gpp[NH]p) in SPM, while microsomal sites proved to be insensitive to this GTP analog. Sensitivity was restored upon solubilization of microsomes with digitonin, suggesting that opioid receptors are physically separated from G proteins in this fraction. Modulation of microsomal binding by Na+ and Mn++ was greater than that of SPM. Pertussis toxin-catalyzed adenosine diphosphate (ADP) ribosylation revealed the presence of G proteins with alpha-subunit molecular weights of 40 kDa in both subcellular fractions. Basal low Km GTPase activity in SPM was greater than in microsomes. Etorphine elicited a concentration-dependent stimulation of guanosine triphosphatase (GTPase) activity in SPMs but not in microsomes, indicating functional coupling of opioid receptors to G protein in the former and an uncoupling in the latter. Microsomes from 3-day-old rat brain contained more mu opioid sites and they were more sensitive to Gpp(NH)p inhibition than those in adults. These results are consistent with the hypothesis that opioid binding sites in adult microsomes are internalized and G protein uncoupled, while those in neonates are newly synthesized, coupled receptors.

A monoclonal antibody recognizing kappa- but not mu- and delta-opioid receptors

A monoclonal antibody (mAb), KA8 that interacts with the kappa-opioid receptor binding site was generated. BALB/c female mice were immunized with a partially purified kappa-opioid receptor preparation from frog brain. Spleen cells were hybridized with SP2/0AG8 myeloma cells. The antibody-producing hybridomas were screened for competition with opioid ligands in a modified enzyme-linked immunosorbent assay. The cell line KA8 secretes an IgG1 (kappa-light chain) immunoglobulin. The mAb KA8 purified by affinity chromatography on protein A-Sepharose CL4B was able to precipitate the antigen from a solubilized and affinity-purified frog brain kappa-opioid receptor preparation. In competition studies, the mAb KA8 decreased specific [3H]ethylketocyclazocine ([3H]EKC) binding to the frog brain membrane fraction in a concentration-dependent manner to a maximum to 72%. The degree of the inhibition was increased to 86% when mu- and delta-opioid binding was suppressed by 100 nM [D-Ala2,NMe-Phe4,Gly-ol]-enkephalin (DAGO) and 100 nM [D-Ala2,L-Leu5]-enkephalin (DADLE), respectively, and to 100% when mu-, delta-, and kappa 2-sites were blocked by 5 microM DADLE. However, the mu-specific [3H]DAGO and the delta-preferring [3H]DADLE binding to frog brain membranes cannot be inhibited by mAb KA8. These data suggest that this mAb is recognizing the kappa- but not the mu- and delta-subtype of opioid receptors. The mAb KA8 also inhibits specific [3H]naloxone and [3H]EKC binding to chick brain cultured neurons and rat brain membranes, whereas it has only a slight effect on [3H]EKC binding to guinea pig cerebellar membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Irreversible blockade of the high and low affinity (3H) naloxone binding sites by C-6 derivatives of morphinane-6-ones

C-6 derivatives--hydrazones, phenylhydrazones, dinitrophenylhydrazones, oximes and semicarbazones--of morphinane-6-ones were synthesized and their binding characteristics were studied on rat brain membranes. The dihydromorphinone and oxymorphone derivatives compete for the (3H)naloxone binding sites with high affinity, while the dihydrocodeinone and oxycodone derivatives are less potent. The affinity of the new compounds is decreased for the delta sites as compared to the parent ligands. The ligands bearing bulky substituents also bind with low affinity to the kappa sites. The modification decreased the Na(+)-index of compounds indicating their mixed agonist-antagonist character. The dihydromorphinone derivatives are all capable to block irreversibly the high affinity binding site of (3H)naloxone, whereas the dihydrocodeionone derivatives block irreversibly the low affinity site. A possible mechanism for the inhibition is suggested.

Characterization of kappa 1 and kappa 2 opioid binding sites in frog (Rana esculenta) brain membrane preparation

The distribution and properties of frog brain kappa-opioid receptor subtypes differ not only from those of the guinea pig brain, but also from that of the rat brain. In guinea pig cerebellum the kappa 1 is the dominant receptor subtype, frog brain contains mainly the kappa 2 subtype, and the distribution of the rat brain subtypes is intermediate between the two others. In competition experiments it has been established that ethylketocyclazocine and N-cyclopropylmethyl-norazidomorphine, which are nonselective kappa-ligands, have relatively high affinities to frog brain membranes. The kappa 2 ligands (Met5)enkephalin-Arg6-Phe7 and etorphine also show high affinities to the frog brain. Kappa 1 binding sites measured in the presence of 5 microM/D-Ala2-Leu5/enkephalin represent 25-30% of [3H]ethylketocyclazocine binding in frog brain membranes. The kappa 2 subtype in frog brain resembles more to the mu subtype than the delta subtype of opioid receptors, but it differs from the mu subtype in displaying low affinity toward beta-endorphin and /D-Ala2-(Me)Phe4-Gly5-ol/enkephalin (DAGO). From our data it is evident that the opioid receptor subtypes are already present in the amphibian brain but the differences among them are less pronounced than in mammalian brain.

Effect of sodium on [3H]ethylketocyclazocine binding to opioid receptors in frog brain membranes

The specific binding of [3H]ethylketocyclazocine to frog brain membrane preparation was enhanced in the presence of sodium ions administered as NaCl, both at 0 degree C and at room temperature. The optimal NaCl concentration was 25 mM at 0 degree C and 50 mM at 24 degrees C. MgCl2 inhibited the [3H]ethylketocyclazocine binding. Two binding sites (high and low affinity) were established with [3H]ethylketocyclazocine as ligand by equilibrium binding studies. Addition of NaCl increased the Bmax of the low-affinity site more than that of the high-affinity site at both temperatures. Affinities were higher at 0 degree C than at 24 degrees C. The KD values were not significantly influenced by sodium ions. The dissimilarities between the rat and frog brain opioid receptors in [3H]ethylketocyclazocine binding are attributed to the different lipid composition of the two membranes.

Effects of oxymorphazone in frogs: long lasting antinociception in vivo, and apparently irreversible binding in vitro

Oxymorphazone (at doses of 50-200 mg/kg) was found to be a relatively weak antinociceptive drug in intact frog (Rana esculenta) when acetic acid was used as pain stimulus. Frogs remained analgesic for at least 48 hrs following oxymorphazone (200 mg/kg) administration. The ligand increased the latency of wiping reflex in spinal frogs too. These effects were blocked by naloxone. In equilibrium binding studies (3H)oxymorphazone had high affinity to the opioid receptors of frog brain and spinal cord as well (apparent Kd values were 8.9 and 10.6 nM, respectively). Kinetic experiments show that only 25% of the bound (3H)oxymorphazone is readily dissociable. Preincubation of the membranes with labeled oxymorphazone results in a washing resistant inhibition of the opioid binding sites. At least 70% of the (3H)oxymorphazone specific binding is apparently irreversible after reaction at 5 nM ligand concentration, and this can be enhanced by a higher concentration of tritiated ligand.

Enkephalin photoaffinity probes: synthesis and binding properties

The diazomethyl ketone derivative of D-Ala2-Leu-enkephalin and Leu-enkephalin were synthesized. Replacement of the C-terminal carboxyl group with CO-CHN2 resulted in a potency decrease, the new compounds display micromolar affinities to 3H-naloxone and 3H-DALE binding sites. Photolysis of the ligands bound to rat brain membranes resulted in an approximately 30% irreversible loss of the receptors. Photoinactivation was prevented by the opiate antagonist, naloxone, thus providing that the ligands are specific photoaffinity probes of the opioid receptors.

Glial and neuronal opioid receptors: apparent positive cooperativity observed in intact cultured cells

Opioid receptors were characterized in glial and neuronal homogeneous cultures of embryonic chick forebrain, using [3H]naloxone as a labelled ligand. Binding experiments were performed on intact cells. The specific binding of [3H]naloxone reached equilibrium after 1 min. The apparent dissociation constants were estimated as 0.51 nM for glial and 0.63 nM for neuronal cells. Equilibrium measurements indicated the apparent positive cooperativity of the binding, resulting in Hill coefficients of 2.61 for glial and 2.04 for neuronal cells. Competition of unlabelled naloxone for specific binding sites resulted in maximum-shape curves in glial cells if measured at low receptor occupancy. This supports the positive cooperativity of ligand binding. Opioid agonists, ethylketocyclazocine (EKC), morphine and [D-Ala2,L-Leu5]enkephalin (DALA), provoked biphasic competition curves in both cell types with a characteristic maximum at low competitor concentrations. The possible physiological role of glial opioid receptors in neuron-glia communication and the significance of cooperativity is discussed.

Detailed analysis of heterogeneity of [3H]naloxone binding sites in rat brain synaptosomes

The experiments reported in this paper address the question of heterogeneity of [3H]naloxone binding sites in rat brainstem synaptosomal preparations at 23 degrees C in the presence of 100 mM sodium chloride. Kinetic analysis in the presence of 0.4, 4 and 10 nM [3H]naloxone gave pseudo-first order association rate of 0.9 +/- 0.04, 1.23 +/- 0.08 and 1.06 +/- 0.08 min-1, respectively. The dissociation of a 1 nM [3H]naloxone receptor complex was biphasic with dissociation rate constants of 1.8 and 0.4 min-1. On the other hand, dissociation of 10 nM [3H]naloxone was monophasic with a kd of 1.1 min-1. Two subpopulations of binding sites were also observed by steady state binding studies, with Kd values of 0.5 and 3.4 nM and a ratio of high to low affinity sites of 1:9. Heterogeneity of [3H]naloxone binding sites could be seen by displacement studies performed with opioid peptides and alkaloids. We suggest that our data best fits a model with two independent naloxone binding sites wherein either one or both undergo a multi-step interaction with ligand.

Synthesis and binding of 3H-oxymorphazone to rat brain membranes

Oxymorphazone is a 14-hydroxydihydromorphinone derivative which contains a C-6 hydrazone group and hence could serve as an irreversible label for opioid receptors. 3H-oxymorphazone was synthesized by the reaction of 3H-oxymorphone with excess hydrazine. A specific radioactivity of 640 GBq/mmol (17,3 Ci/mmol) was achieved. Both the unlabelled compound and the tritiated ligand show high affinity to mu and kappa opiate receptor subtypes in rat brain membranes. Two binding sites were detected by equilibrium binding studies, with apparent Kd values of 0.62 nM and 28 nM. About 20% of the H-oxymorphazone specific binding is irreversible after reaction at 1 nM ligand concentration, and this can be enhanced by a higher concentration of tritiated ligand. No azine formation was detected. Preincubation of the membranes with unlabelled oxymorphazone resulted in an irreversible blockade of the high affinity 3H-naloxone binding sites.

Tyr-D-Ala-Gly-(Me)Phe-chloromethyl ketone: a mu specific affinity label for the opioid receptor

An alkylating tetrapeptide enkephalin derivative, Tyr-D-Ala-Gly-(Me)Phe-chloromethyl ketone (DAMK) was synthesized, and its binding characteristics on rat brain membranes were evaluated. In competition experiments, the product shows high affinity for the mu opioid binding site of the rat brain membranes, whereas its binding to the delta and kappa subtypes is weak. Micromolar concentrations of this ligand produce a dose-dependent, apparently irreversible inhibition of /3H/-naloxone binding, with apparent IC50 value of 1-5 uM. Neither reversibly binding opioids nor tosyl-amino acid chloromethyl ketones show these effects. Saturation binding analysis with /3H/-naloxone of membranes preincubated with Tyr-D-Ala-Gly-(Me)Phe-CH2Cl reveal a selective and irreversible inhibition of the high affinity /3H/-naloxone binding site. Irreversible blockade of mu-selective /3H/-ligand binding by Tyr-D-Ala-Gly-(Me)Phe-CH2Cl is much more effective than that of the binding of /3H/-enkephalin or /3H/-ethylketocyclazocine. The mu-selective binding properties of this new irreversible enkephalin analogue suggest that it could serve as an affinity label for the mu opioid receptor subtype.

Azidomorphine is an agonist of high-affinity opioid receptor binding sites

Azidomorphine at low concentration (10(-9) M) inhibits the high-affinity binding site of labeled naloxone in rat brain membrane preparations. In the presence of Na+ and guanine nucleotides the displacement curves of azidomorphine are increased toward high concentrations, whereas Mg2+ ions decrease the IC50 values; This demonstrates the agonist behavior of azidomorphine in binding experiments. When compared with morphine, azidomorphine displayed five-fold lower IC50 values. Based on the presented results, azidomorphine appears to be a good candidate for photoaffinity labeling of opiate receptors.

Irreversible labelling of rat brain opioid receptors by enkephalin chloromethyl ketones

Chloromethyl ketone derivatives of leucine enkephalin (LE), D-Ala2-Leu5-enkephalin (DALE) and D-Ala2-D-Leu5-enkephalin (DADLE) were synthesized. They all show high affinity for rat brain opioid binding sites. Preincubation of the membrane fraction with enkephalin chloromethyl ketones causes a significant inhibition of /3H/-naloxone binding which cannot be reversed by extensive washing. It was found that the irreversible inhibition is selective for the high affinity (KD less than 1 nM) /3H/-naloxone binding site (putative mu-1 site). The irreversible blockade of opioid binding was partially protected by opiate alkaloids and opioid peptides, suggesting that non-specific labelling also occurs. Affinity of enkephalin chloromethyl ketones toward the mu sites is greater than that of the parent compounds. It was also found that the covalent inhibition of mu sites (/3H/-dihydromorphine and /3H/-DAGO binding) is more effective than that of delta sites (/3H/-DALE binding). We conclude that these chloromethyl ketone derivatives can be used as affinity labels for the opioid receptors, allowing us to study the structure of the mu receptor subtype.

Microtubule disassembly increases the number of opioid receptor binding sites in rat cerebrum membranes

The role of microtubules in opioid receptor binding was studied by using microtubule assembly inhibitors. Preincubation of rat cerebrum membranes with podophyllotoxin or colchicine provoked a marked increase in the number of binding sites as judged by [3H]-naloxone, [3H]-morphine and [3H]-D-Ala2-Leu5-enkephalin binding experiments. These results indicate microtubule involvement in regulation of opioid receptor expression.

Kinetics and physical parameters of rat brain opioid receptors solubilized by digitonin and CHAPS

Rat brain opioid receptors were solubilized with digitonin and a zwitterionic detergent, 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS). The yield of solubilization was 70-75% with digitonin and 30-35% with CHAPS. Kinetic and equilibrium studies performed from digitonin extracts resulted in KD values comparable with those of the membrane fractions. Two [3H]naloxone binding sites were obtained in the extracts similarly to membrane fractions. The rank order potency of drugs used in the competition experiments did not change during solubilization. The distributions of mu, delta, and kappa opioid receptor binding sites were similar in membrane and digitonin-solubilized fractions (48-50% mu, 35-37% kappa, and 13-17% delta subtypes). The hydrodynamic properties of digitonin- and CHAPS-solubilized preparations were studied by sucrose density gradient centrifugation and Sepharose-6B chromatography. In all cases, two receptor populations were identified with the following parameters: sedimentation coefficients for the digitonin extracts were 9.2S and 13.2S and for CHAPS extract 8S and 15.6S; the Stokes radii were 45 A and 65A for the digitonin extract and 31A and 76A for the CHAPS-solubilized preparation.

Opiate receptor binding affinities of some D-amino acid substituted beta-casomorphin analogs

beta-Casomorphin-(5) and some analogs modified by the introduction of some D-amino acids and D-pipecolic acid as well as by C-terminal amidation were tested for their affinities to mu- and delta-binding sites in rat brain membranes. The binding affinities of these compounds are compared with the known activities in the guinea pig ileum (GPI) and mouse vas deferens (MVD) test and their antinociceptive potencies in rats. The substitution of D-proline for proline in position 4 in beta-casomorphin-(5) and beta-casomorphin-(4)amide (morphiceptin) results in derivatives with very high mu-binding affinity and mu-selectivity. These affinities correspond to the respective analgesic potencies. Both binding to mu-receptors and analgesic potency are also enhanced by the introduction of D-Phe in position 3. Testing D-Ala2 substituted derivatives with respect to their ability to compete for 3H-naloxone, we observed apparent differences between the pentapeptide amides (biphasic displacement curves) and the tetrapeptide amides (monophasic displacement curves). The substitution of L-Pro2 by D-pipecolic acid yields an analog with preferential delta-receptor affinity in the organ preparations (MVD) but preferential mu-receptor affinity in brain membranes. This finding suggests a possible difference between peripheral and central mu-binding sites.

Hydrodynamic parameters of opioid receptors from frog brain

Active opioid receptors were solubilized from frog (Rana esculenta) brain membranes using 1% digitonin. The solubilized preparation was sedimented in sucrose density gradient and applied to Sepharose-6B column. In the ultracentrifugation experiments, two distinct molecular forms of the opioid receptors were observed with apparent s20, w values of 15.7 and 10.8 S. The estimated molecular weights were 470 and 180 kD. The Stokes radii of the two separate forms were determined by gel filtration and found to be 71 and 42 A. The corresponding molecular weights were 500 and 140 kD indicating a good correlation with data obtained from the sedimentation experiments.

Melphalan potently substitutes the N-terminal Tyr of D-Ala2-Leu5-enkephalin methyl ester

In search of an affinity label of the opioid receptor, the nitrogen mustard melphalan, Mel, was built into the peptide chain of D-Ala2-Leu5-enkephalin (DALE) methyl ester in different positions. We report now that in contrast to the previous observations that an intact Tyr in position 1 is essential for opioid activity [(1980) Annu. Rev. Pharmacol. Toxicol. 20, 81-110], substitution of Tyr by Mel did not result in a loss of the binding affinity. Mel1, Leu5-enkephalin-OMe competed for the binding sites of [3H]naloxone as potently as DALE did; IC50 values for both compounds were 50 nM. Mel substitution has led to one order potency decrease in binding to the delta-sites. 0.5-1 microM of the compound irreversibly inactivates 50% of the binding sites of [3H]naloxone, and 5-10 microM of that of [3H]DALE. These results shed new light on the structural requirements established for opioid peptides. In addition, the new derivative can be used as an affinity label of the opioid receptor.

Solubilization and characterization of opioid binding sites from frog (Rana esculenta) brain

Active opioid receptors were solubilized from frog (Rana esculenta) brain membrane fractions by the use of 1% digitonin. It was found by kinetic as well as by equilibrium measurements that both the membrane and the solubilized fractions contain two binding sites. For the membrane preparations, KD values were 0.9 and 3.6 nM, and Bmax values were 293 and 734 fmol/mg protein. For the solubilized preparations, KD values were 0.4 and 2.6 nM, an Bmax values were 35 and 266 fmol/mg protein. The stereospecificity of the binding did not change during solubilization. Both the membrane-bound and the solubilized receptors showed weak binding of enkephalin and mu-specific drugs, suggesting that they are predominantly of the kappa-type. The membrane-bound and the soluble receptors showed the same distribution of subtypes, i.e., 70% kappa, 13% mu, and 17% delta for the membrane-bound and 71% kappa, 17% mu, and 12% delta for the soluble receptors.

Binding characteristics and analgesic activity of D-ALA2-Leu5-enkephalin chloromethyl ketone

The chloromethyl ketone derivative of D-Ala2-Leu5-enkephalin (DALECK) was synthesized and its potency was tested in competing for 3H-naloxone binding sites and inducing analgesia. It was established that the compound is a potent affinity reagent at alkaline pH, blocking selectively and irreversibly the high-affinity (KD less than 1 nM) binding site. Intracisternally given DALECK showed a long-lasting, dose-dependent antinociceptive effect in the rat tail-withdrawal test. This could be completely antagonized by naloxone administration showing the reversible nature of DALECK in this in vivo assay. It is suggested that DALECK binds reversibly to the morphine receptor which mediates analgesia but irreversibly to the enkephalin receptor, the function of which remains to be elucidated.

A new potential affinity label for the opiate receptor

D-Ala2-Leu5-enkephalin (DALA) was elongated with the methyl ester of melphalan (Mel), a nitrogen mustard on the C-terminus. The new derivative, DALA-Mel-OMe might be a potential affinity label of the opiate receptor. The compound shows high affinity in displacement experiments with an IC50 of 10 nM and 100 nM against 3H-D-Ala2-Leu5-enkephalin and 3H-naloxone, respectively. 10-100 microM of DALA-Mel-OMe causes a significant inhibition of 3H-naloxone binding, which effect can't be reversed by extensive washes. This irreversible blockade is significantly but only partially protected by high concentrations of naloxone and Leu-enkephalin. Our results suggest that DALA-Mel-OMe binds irreversibly to the opiate receptor, but nonspecific labelling also occurs.