EFFECT OF ?1-ADRENOCEPTOR SUBTYPE-SELECTIVE INVERSE AGONISTS ON NON-PREGNANT AND LATE-PREGNANT CERVICAL RESISTANCE IN VITRO IN THE RAT

1. The aim of the present study was to compare and elucidate the effects of alpha(1)-adrenoceptor (alpha(1)-AR) subtype-selective inverse agonists on non-pregnant and late-pregnant rat cervical tone. 2. Cervical resistance was investigated in in vitro stretching tests in the absence or presence of alpha(1)-AR subtype-selective inverse agonists (WB 4101, AH 11110A and BMY 7378; all at 10(-6) mol/L), whereas the mRNA levels and density of the alpha(1)-AR subtypes and the G-protein-activating effects of the inverse agonists were determined by reverse transcription-polymerase chain reaction, western blot and [(35)S]-GTPgammaS binding techniques, respectively. 3. The inverse agonists did not cause any change in resistance in non-pregnant and 18-day-pregnant samples. WB 4101 increased cervical resistance from Day 20, whereas AH 11110A had no effect and BMY 7378 exhibited such an action only on Day 21. Phenylephrine (10(-4) mol/L) had no effect on cervical resistance on Day 22. The mRNA levels and density of all alpha(1)-AR subtypes were increased on Day 18, but no further changes were observed after that. The [(35)S]-GTPgammaS binding studies revealed increased G-protein activation of alpha(1A)-AR and a moderate G-protein activation of alpha(1B)- and alpha(1D)-AR. The effect of WB 4101 to increase [(35)S]-GTPgammaS binding was blocked by pertussis toxin (50 ng/mL). Phenylephrine caused a slight and significant decrease in the amount of activated G-protein on Day 22. 4. The effects of inverse agonists on the alpha(1A)-AR can enhance cervical resistance in the late-pregnant rat in vitro. This action is mediated, at least in part, by a pertussis toxin-sensitive G(i)-protein. This effect of the alpha(1A)-AR inverse agonist WB 4101 may offer a new therapeutic target in the prevention of premature labour.

β-Methyl Substitution of Cyclohexylalanine in Dmt-Tic-Cha-Phe Peptides Results in Highly Potent δ Opioid Antagonists

The opioid peptide TIPP (H-Tyr-Tic-Phe-Phe-OH, Tic:1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) was substituted with Dmt (2',6'-dimethyltyrosine) and a new unnatural amino acid, beta-MeCha (beta-methyl-cyclohexylalanine). This double substitution led to a new series of opioid peptides displaying subnanomolar delta antagonist activity and mu agonist or antagonist properties depending on the configuration of the beta-MeCha residue. The most promising analog, H-Dmt-Tic-(2S,3S)-beta-MeCha-Phe-OH was a very selective delta antagonist both in the mouse vas deferens (MVD) assay (Ke = 0.241 +/- 0.05 nM) and in radioligand binding assay (K i delta = 0.48 +/- 0.05 nM, K i mu/K i delta = 2800). The epimeric peptide H-Dmt-Tic-(2S,3R)-beta-MeCha-Phe-OH and the corresponding peptide amide turned out to be mixed partial mu agonist/delta antagonists in the guinea pig ileum and MVD assays. Our results constitute further examples of the influence of Dmt and beta-methyl substitution as well as C-terminal amidation on the potency, selectivity, and signal transduction properties of TIPP related peptides. Some of these compounds represent valuable pharmacological tools for opioid research.

Opioid receptor binding characteristics and structure-activity studies of novel tetrapeptides in the TIPP (Tyr-Tic-Phe-Phe) series

The development of the prototype synthetic delta-opioid receptor antagonist peptides TIPP [(H-Tyr-Tic-Phe- Phe-OH); Tic: tetrahydroisoquinoline-3-carboxylic acid] and TIPPpsi (H-Tyr-psiTic-Phe-Phe-OH) by Schiller and coworkers was followed by extensive structure-activity relationship studies, leading to the emergence of numerous analogs that are of pharmacological interest. Eight novel diastereomeric compounds in this peptide family were designed, prepared, and tested biologically to gain structure-activity relationship information. The new multisubstituted tetrapeptide analogs contain both a 2',6'-dimethyltyrosine residue at the N-terminus and beta-methyl-cyclohexylalanine at the third position as replacements for the original first tyrosine and the third phenylalanine, respectively. These derivatives wear either free acidic (-COOH) or amidated (-CONH2) C-terminal. The potency and delta- versus mu-opioid receptor selectivity were evaluated by in vitro radioreceptor-binding assays, while the intrinsic G-protein-activating efficacy of these analogs was tested in [35S]GTPgammaS-binding assays using rat brain membranes or Chinese hamster ovary cells stably expressing mu- or delta-opioid receptors. The analogs showed delta-antagonist selectivity with differences regarding their isomeric forms, and these analogs containing a C-terminal carboxamide group displayed a mixed mu-agonist/delta-antagonist profile, thus they are expected to be safer analgesics with a low propensity to produce tolerance and physical dependence. These results constitute further examples of the influence of beta-methyl substitution and C-terminal amidation on potency, selectivity, and signal transduction properties of TIPP-related peptides as well as they represent valuable pharmacological tools for opioid research.

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.

Synthesis of enzymatically resistant nociceptin-related peptides containing a carbamic acid residue

Nociceptin, a 17-amino acid peptide (FGGFTGARKSARKLANQ, N/OFQ), is the endogenous ligand of the nociceptin/orphanin FQ (NOP) receptor. This receptor-ligand system is involved in various physiological as well as pathophysiological mechanisms, but owing to the peptidic structure, it is rapidly degraded by enzymes. The enzymatic digestion of nociceptin involves mainly aminopeptidases and yields Noc(2-17)-OH and other smaller fragments. We aimed at increasing the enzymatic stability against aminopeptidases in the case of peptide Noc(1-13)-NH(2), which possesses the minimum sequence capable of interacting with the NOP receptor. Therefore we developed a new procedure for the synthesis of peptides with the carbamic acid residue [...-NH-CH(R)-CO-NH-CO-NH-CH(Q)-CO-.]. A set of four carbamic acid-nociceptin derivatives were produced. The carbamic acid residue was incorporated into the inner part of the peptides, building on solid phase, by using a suitable dipeptide fragment with carbamic acid residue produced by a simple and efficient three-step solution phase procedure. Enzymatic stability of carbamic acid peptides was studied in the presence of aminopeptidase M (AP-M) and in rat brain membrane homogenate. The receptor-binding properties were also studied by radioligand binding assay on crude rat brain membranes and the activity of the ligands were analyzed on isolated mouse vas deferens (MVD) tissues. We found that incorporation of the carbamic acid residue into the N-terminal part of nociceptin significantly increases the resistance against AP-M. We observed the decrease of binding affinities to the NOP receptor in case of the peptides modified in the N-terminal portion. Consequently, the incorporation of the carbamic acid residue into peptides can be proposed as a promising and reasonable tool for increasing enzymatic stability, where the native molecule is less sensitive for carbamic acid residue-related structural change.

In vitro and in vivo pharmacological characterization of the nociceptin/orphanin FQ receptor ligand Ac-RYYRIK-ol

It was recently reported that the hexapeptide Ac-RYYRIK-ol binds with high affinity nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors and competitively antagonizes N/OFQ actions in the mouse vas deferens assay. Here we further describe the in vitro and in vivo pharmacological features of this NOP receptor ligand. In mouse brain homogenate the degradation half life of Ac-RYYRIK-ol (2.48 min) was significantly higher than that of the parent compound Ac-RYYRIK-NH2 (1.20 min). In the electrically stimulated mouse vas deferens, Ac-RYYRIK-ol (10-1000 nM) competitively antagonized the inhibitory effect of N/OFQ (pA2=8.46), while in the isolated mouse colon the hexapeptide mimicked N/OFQ contractile effects thus behaving as a NOP receptor agonist (pEC50=9.09). This latter effect was no longer evident in colon tissues taken from mice knock out for the NOP receptor gene (NOP-/-). In vivo in mice, similarly to N/OFQ, Ac-RYYRIK-ol (dose range 0.001-1 nmol) produced: i) pronociceptive effects after intracerebroventricular (i.c.v.) administration and antinociceptive actions when given intrathecally (i.t.) in the tail withdrawal assay; ii) inhibition of locomotor activity and iii) stimulation of food intake after supraspinal administration. Finally, in the forced swimming test, Ac-RYYRIK-ol was inactive per se, but reversed the antidepressant-like effects elicited by the NOP receptor selective antagonist UFP-101 ([Nphe(1),Arg(14),Lys(15)]N/OFQ-NH2). Thus, in all these in vivo assays Ac-RYYRIK-ol mimicked the actions of N/OFQ showing however higher potency. In conclusion, Ac-RYYRIK-ol displayed a complex pharmacological profile which is likely due to the low efficacy agonist nature of this novel ligand of the NOP receptor. The high potency, selectivity of action, and in vivo effectiveness make Ac-RYYRIK-ol a useful pharmacological tool for future studies in the field of N/OFQ and its NOP receptor.

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.

Pregnancy-induced decrease in the relaxant effect of terbutaline in the late-pregnant rat myometrium: role of G-protein activation and progesterone

The effectiveness of beta2-agonists in preterm delivery is reduced by several factors. The aim of this study was to determine the influence of late pregnancy in the uterus-relaxing effect of terbutaline in the rat in vitro. Rat uterine tissues from late pregnancy (days 15, 18, 20 and 22) were used. In vitro electrical field-stimulation (EFS) was used to evoke contractions. The radioligand-binding technique, reverse transcription-polymerase chain reaction and radioimmunoassay technique were used to determine the beta-adrenergic receptor density and mRNA level and the plasma sex hormone level, respectively. The activated G-protein level of the beta-adrenergic receptors was investigated by a radiolabelled GTP binding assay.EFS-induced contractions were inhibited by terbutaline. This effect decreased towards term with respect to both the EC50 and maximal inhibition values. A drop in plasma progesterone level was also detected. Binding studies revealed an increase in beta-adrenergic receptor number on the last day of pregnancy, which correlated with the change in receptor mRNA level. The G-protein-activating effect of terbutaline decreased continuously between days 15 and 20. Surprisingly, terbutaline decreased the G-protein activation to below the basal level on day 22. However, progesterone pretreatment set back the uterine action of terbutaline, increased the density of the beta2-adrenergic receptors and their mRNA level and increased the G-protein-activating property of terbutaline. These data provide evidence of a pregnancy-induced decrease in activated G-protein level after beta2-agonist stimulation. The decrease in plasma progesterone level has a crucial role in this process. The effects of beta2-adrenergic receptor agonists in tocolytic therapy may possibly be potentiated with progesterone.

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.

Terbutaline increases the cervical resistance of the pregnant rat in vitro

Cervical ripening is a crucial process leading to delivery. Early dilation of the pregnant cervix can contribute to premature labour. The maturity of the cervix can be characterized by its resistance to mechanical stretching. Although a number of compounds are considered to increase cervical resistance (e.g., progesterone, nitric oxide synthase inhibitors and nonsteroidal anti-inflammatory drugs), none of them seem to be safe for clinical application. Other compounds, such as beta(2)-adrenergic receptor (beta(2)-AR) agonists, have been used for several decades to stop premature myometrium contractions, but their cervical action has never been investigated. The aim of this study was to detect the effects of the beta(2)-AR agonist terbutaline on nonpregnant and late-pregnant (day 18, 20, 21 or 22) cervices isolated from Sprague-Dawley rats. Cervical resistance was measured by means of a mechanical stretching test in vitro, the beta(2)-AR density was determined by Western blot analysis, the beta(2)-AR mRNA was determined by RT-PCR, while the G-protein activation following cervical beta(2)-AR stimulation with terbutaline was evaluated via a [(35)S]GTPgammaS binding assay. Terbutaline at 10(-6) M increased the cervical resistance of the late-pregnant samples in vitro from day 18 to day 22, but did not alter the resistance of the nonpregnant samples. This cervical resistance-increasing effect was concentration dependent and antagonized with propranolol on day 21. Terbutaline was ineffective on cervical samples when gradual stretching was omitted. RT-PCR and Western blot studies revealed increased beta(2)-AR mRNA and beta(2)-AR levels respectively on day 18 of pregnancy compared with the nonpregnant cervix, but no further changes were detected up to the end of pregnancy. The [(35)S]GTPgammaS binding assay demonstrated a decreased G-protein activation on the days of pregnancy investigated, but no activation was found in the nonpregnant samples. The degree of decrease in G-protein activation by terbutaline was in harmony with its cervical resistance-increasing action. On day 21, the G-protein activation-decreasing effect of terbutaline was antagonized with propranolol. We presume that the cervical resistance-increasing effect of terbutaline is a consequence of its G-protein activation-decreasing property via beta(2)-ARs, which finally leads to an increased muscle resistance against mechanical stretching. This action of terbutaline seems unique among the smooth muscles, and may open up a new perspective in the prevention of premature labour. Clinical experience indicates that beta(2)-AR agonists will not be sufficient to stop the overall process, but their combination with more potent inhibitors of uterine contractions may be of clinical benefit.

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.

Pharmacological and functional biochemical properties of d-Ala2-d-Nle5-enkephalin-Arg-Phe

Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN) a synthetic analogue of the endogenous Met-enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF), was investigated in radioligand binding assays, [(35)S]GTPgammaS stimulation experiments as well as in in vivo algesiometric tests. Binding properties of [(3)H]DADN were measured in crude membrane fractions of rat spinal cord tissues and in homogenates of Chinese hamster ovary (CHO) cells selectively expressing delta-, kappa-or micro-opioid receptors. The highest affinity for [(3)H]DADN binding was observed in membranes from CHO cells transfected with micro-opioid receptors confirming the micro-selectivity of the peptide. Unlabeled DADN was also investigated in functional biochemical experiments by measuring opioid receptor-mediated G-protein activation in rat brain membrane fractions. The peptide stimulated the activity of the regulatory G-proteins in a concentration dependent manner, and the stimulation was efficiently inhibited in the presence of micro-receptor specific antagonist ligands further supporting the selectivity profile of DADN. Intrathecally administered DADN produced a dose-related, naloxone-reversible antinociception in rat hot water tail-flick tests. Among the selective opioid antagonists tested, the delta-selective naltrindole (NTI) and the kappa-specific norbinaltorphimine (norBNI) showed only slight blocking effects compared with naloxone. The results obtained in the in vitro agonist-stimulated [(35)S]GTPgammaS binding assays are in good agreement with the opioid agonist effect seen in the in vivo pain test.

Non-opioid actions of opioid peptides

Beside the well known actions of opioid peptides on mu-, delta- and kappa-opioid receptors, increasing amount of pharmacological and biochemical evidence has recently been published about non-opioid actions of various opioid peptides. These effects are not abolished by naloxone treatments. Such non-opioid effects are observed both in nervous tissues and in the cellular elements of the immune system. Peptides exhibiting non-opioid effects include beta-endorphin, dynorphin A, nociceptin/OFQ, endomorphins, hemorphins and a number of Proenkephalin A derived peptides, such as Met-enkephalin, Met-enkephalin-Arg-Phe (MERF) and bovine adrenal medullary peptide (BAM22). Non-opioid actions are exerted through different neuronal receptors, e.g., dynorphin hyperalgesia through NMDA receptor, Met-enkephalin induced regulation of cell growth through zeta receptors, pain modulation by nociceptin through ORL-1 or NOP receptors, while BAM22 acts through sensory neuron specific G protein-coupled receptors (SNSR). We have investigated Met-enkephalin-Arg-Phe (MERF) and its analogues by the means of direct and indirect radioligand binding assays. It has been found that in addition to kappa(2) and delta-opioid receptors, MERF can act also through sigma(2)- or probably via FMRF-NH(2) receptors in rat cerebellum. A role of functionally assembling heterodimer receptors in mediating the non-conventional actions of these peptide ligands can not be excluded as well.

Structure−Activity Study on the Phe Side Chain Arrangement of Endomorphins Using Conformationally Constrained Analogues

Endomorphins-1 and -2 were substituted with all the beta-MePhe stereoisomers in their Phe residues to generate a conformationally constrained peptide set. This series of molecules was subjected to biological assays, and for beta-MePhe(4)-endomorphins-2, a conformational analysis was performed. Incorporation of (2S,3S)-beta-MePhe(4) resulted in the most potent analogues of both endomorphins with enhanced enzymatic stability. Their micro opioid affinities were 4-times higher than the parent peptides, they stimulated [(35)S]GTPgammaS binding, and they were found to be full agonists. NMR experiments revealed that C-terminal (2S,3S)-beta-MePhe in endomorphin-2 strongly favored the gauche (-) spatial orientation which implies the presence of the chi(1) = -60 degrees rotamer of Phe(4) in the binding conformer of endomorphins. Our results emphasize that the appropriate orientation of the C-terminal aromatic side chain of endomorphins is substantial for binding to the micro opioid receptor.

Synthesis and binding characteristics of a novel enkephalin analogue, [3H]Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe

The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.

Restraint stress and anti-tumor immune response in mice

Psychological stress modulates the immune system through the hypothalamic-pituitary-adrenal axis, the sympatho-adrenomedullary axis and the opioid system. According to literature data, restraint stress increases the immune cell apoptosis, decreases the spleen and thymus cell content, the natural killer (NK) activity in the spleen, and it compromises the anti-tumor immune response in mice. We immobilized mice in two consecutive nights, and then determined the cell number, apoptosis, NK cell content, NK activity and the level of cytokine mRNAs (TNF-beta, TNF-alpha, IL-4, IL-5, IL-1alpha, IFN-gamma, IL-2, IL-6, IL-1beta and IL-3) in the thymus and spleen. No consistent changes were detected in any of the immune parameters either in C57Bl/6 or in DBA/2 mice. Stressed or control B6 mice were injected with B16 melanoma cells immediately after the immobilization or one week later. No significant differences were found in the growth of primary tumors and lung metastases in stressed and control animals. Taken together, our mice, kept in a general-purpose non-SPF animal house, seemed to be refractory to the stress-induced immunomodulation. Our interpretation is that stress-induced immunomodulation can occur only in mice isolated from any background stressors, or rather natural stimuli, throughout their life.

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.

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.

Receptor constants for endomorphin-1 and endomorphin-1-ol indicate differences in efficacy and receptor occupancy

The opioid properties of endomorphin derivatives containing a C-terminal alcoholic(-ol) function were compared to the parent amidated compounds in isolated organs (longitudinal muscle strip of guinea-pig ileum and mouse vas deferens). Similar data were also generated for the mu-opioid receptor selective agonist synthetic peptide (D-Ala2, MePhe4, Gly5-ol)-enkephalin (DAMGO) and its Gly5-NH2 congener (DAMGA). Endomorphin-1-ol (Tyr-Pro-Trp-Phe-ol) had an IC50 of 80.6 nM in mouse vas deferens and 61.2 nM in guinea-pig ileum; the corresponding values for endomorphin-2-ol (Tyr-Pro-Phe-Phe-ol) were 49.6 and 48.2 nM, for DAMGO 59.8 and 29.2 nM, respectively. As it was indicated by the antagonism by naltrexone, the agonist actions were exerted exclusively at mu-opioid receptors in both organs. The -ol derivatives were slightly (2.3-4.3 times) less potent than the parent amides in the bioassays: all peptides had, apparently, full agonist properties in intact preparations. With the aim of revealing potential partial agonist properties among the investigated peptides, we partially inactivated the mu-opioid receptor pool in mouse vas deferens by 5x10(-7) M beta-funaltrexamine. The calculated receptor constants indicated a "high-affinity, low intrinsic efficacy" profile (i.e. a potential partial agonist property) for endomorphin-1, an intermediate character for endomorpin-1-ol and full agonism for DAMGA and DAMGO. Apparently, a higher receptor fraction remained accessible for endomorphin-1 (42.8%) than for the -ol congener (14.0%), DAMGO (20.2%) and DAMGA (14.1%) after partial inactivation.

Radioligand binding properties of VV-hemorphin 7, an atypical opioid peptide

Receptor binding properties of the hemoglobin-derived nonapeptide VV-hemorphin 7 (Val-Val-Tyr-Pro-Trp-Thr-Gln-Arg-Phe-OH) were studied using both the unlabelled form and tritium-labelled derivative of the peptide. In binding studies using selective opioid radioligands, VV-hemorphin 7 exhibited a rank order of potency of mu > kappa >> delta. VV-hemorphin 7 was tritiated resulting in a compound with 1.03 TBq/mmol (27.8 Ci/mmol) specific radioactivity. The maximal number of binding sites was found to be 66.5 pmol/mg protein with an affinity of 82.1 nM in rat brain membranes. In competition studies, marked similarity was observed to the binding profile of the naturally occurring opioid heptapeptide Met-enkephalin-Arg-Phe (MERF) and its analogues to their naloxone-insensitive binding site. The common -Arg-Phe sequence at the carboxyl terminal end, which is similar to those of other endogenous peptides (-Arg-Phe-NH(2) in neuropeptide FF and FMRF-NH(2)) brings attention to the C-terminal end of the molecule and points to the possible existence of a common nonopioid binding site in mammals.

Promitogenic effects of ethanol, methanol, and ethanolamine in insulin-treated fibroblasts

The zinc-dependent potentiating effect of ethanol (EtOH) on insulin-stimulated DNA synthesis was studied with a focus on the possible site of EtOH action and the ability of other alcohols to elicit similar promitogenic effects. In serum-starved (27 hr) NIH 3T3 fibroblasts, 200-300 mM methanol (MeOH) and 0.1-1.5 mM ethanolamine (Etn), but not 3- to 9-carbon normal alcohols, enhanced the effect of insulin on DNA synthesis to varying extents. The promitogenic effects of EtOH and MeOH, but not that of Etn, required the presence of 15-25 microM zinc. The potentiating effects of Etn were enhanced by 5 mM choline (Cho) and inhibited by 1-3 mM hemicholinium-3 (HC-3), an inhibitor of Cho transporter and Cho kinase. In the presence of 15 microM zinc, 40 mM EtOH, which had no effect on its own, inhibited the potentiating effects of Cho and enhanced the inhibitory effects of HC-3 on synergistic stimulation of DNA synthesis by Etn and insulin. On the other hand, both Cho and HC-3 partially inhibited the promitogenic effect of 80 mM EtOH in the presence of 25 microM zinc. After a 10-min incubation, EtOH decreased the amount of cell-associated [(14)C]Cho in the absence but not in the presence of HC-3. After a 40-min incubation, Cho (5 mM) partially inhibited the cellular uptake as well as the metabolism of [(14)C]Etn. Whereas after the 40-min incubation 80 mM EtOH had no effects on Etn metabolism, in the absence of Cho it decreased the amount of cell-associated [(14)C]Etn. However, EtOH had no detectable effects on cell association of [(14)C]Etn after the 10-min incubation. The results suggest that in NIH 3T3 fibroblasts EtOH is a remarkably specific promitogen, and that it may act via a cell membrane site(s), also regulated by Cho (agonist) and HC-3 (antagonist), which can influence membrane binding and the promitogenic activity of Etn.

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.

Comparison of the endogenous heptapeptide Met-enkephalin-Arg6-Phe7 binding in amphibian and mammalian brain

In previous communications [4, 38] we published that [3H]Met-enkephalin-Arg6-Phe7 (MERF) binds to opioid (kappa2 and delta) and sigma2 sites in frog and rat brain membrane preparations, however no binding to kappa1 sites could be established. In the present paper we compare the frog, rat and guinea pig brain membrane fractions with respect to their MERF binding data. No qualitative differences were found between the three species but specific binding of labelled MERF was maximal in frog brain and lowest in guinea pig brain, which corresponds to their kappa2 opioid receptor distribution. The naloxone resistant binding was also present in all investigated species and varied from 25% in frog and guinea pig cerebrum, to 50% in rat cerebrum and cerebellum, but no naloxone inhibition was found in guinea pig cerebellum where no kappa2 opioid receptors have been found. The presence of sigma2-like receptor was demonstrated in each investigated membrane fraction with displacement experiments using (-)N-allyl-normetazocine as competitor of tritiated MERF. It was shown that this site was responsible for 60-80% of [3H]MERF binding. The remaining part of the naloxone resistant labelled MERF binding could be displaced only with endogenous opioid peptides as met-enkephalin, dynorphin and beta-endorphin. The eventual physiological role of multiple MERF receptors is discussed.

Nociceptin binding sites in frog (Rana esculenta) brain membranes

The recently discovered natural heptadecapeptide nociceptin (orphanin FQ) shares some homology with the opioid peptides but it binds to a distinct receptor type, termed nociceptin receptor. This study demonstrates the presence of specific nociceptin recognition sites in brain membrane fractions of an amphibian, Rana esculenta. Para-iodo-Phe(1)-nociceptin-amide was radiolabelled by catalytic dehalotritiation, resulting in p[(3)H]Phe(1)-nociceptin-amide of 25 Ci/mmol specific radioactivity. Specific binding of [(3)H]nociceptin-amide to frog brain membranes was found to be saturable and of high affinity with equilibrium K(d) values in the low nanomolar range. A single set of binding sites with about 180 fmol/mg protein maximal binding capacity was obtained in saturation and competition experiments. [(3)H]Nociceptin-amide binding could easily be inhibited by synthetic nociceptin compounds but not by opioid ligands. Both sodium ions and 5'-guanylylimidodiphosphate decreased the binding of the radioligand by transferring the receptor to a lower affinity state. Nociceptin dose-dependently stimulated the binding of the nonhydrolysable, radiolabeled GTP-analogue guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) to G-proteins in frog brain membranes. Addition of 1 microM naloxone caused no significant change in the curves, indicating that nociceptin-mediated activation of G-proteins occurred through nonopioid mechanism.

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.

Characterization of [3H]Met-enkephalin-Arg6-Phe7 binding to multiple sites in rat and guinea pig cerebellum

[3H]Met-enkephalin-Arg6-Phe7 (MERF) has been shown to label opioid (kappa2 and delta) and sigma2 sites in rat and frog brain membrane preparations, and no specific binding to kappa1 opioid receptors could be established (refs. 6 and 8). In this study the binding was examined in rat cerebellar membranes which are relatively rich in kappa2-sites, and in guinea pig cerebellar preparations where kappa1 opioid receptors are almost exclusively present. In accordance with our previous results, [3H]MERF binding could not be displaced in guinea pig cerebellar membranes neither with U-69,593 nor with naloxone or levorphanol suggesting no interaction with opioid sites, nevertheless a Kd of 2.8 nM was calculated in cold saturation experiments. In rat cerebellar membrane fractions about the half of the specific [3H]MERF binding sites was inhibited by opiate alkaloids such as naloxone, ethylketocyclazocine, or bremazocine. This portion of the heptapeptide binding sites was stereoselective as demonstrated by the difference in the affinities of the enantiomeric compounds levorphanol and dextrorphan, therefore it would represent an opioid site. In both tissues (-)N-allyl-normetazocine (SKF-10,047), which is also considered as sigma2 ligand, displayed the highest affinities. Among opioid peptides beta-endorphin and dynorphin(1-13) showed the highest potencies, displacing [3H]MERF also from its non-opioid sites. It was concluded therefore that [3H]MERF does not bind to kappa1 sites, and besides kappa2-opioid sites substantial binding to peptide preferring non-opioid sites, and/or sigma2 receptors also occurs.

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.

Differential expression of the K-2 opioid receptor protein under the effect of opioid agonists in embryonic chick neurons in vitro

Neuronal cells cultured from 7-day-old chick embryos and differentiated under the chronic effect of opioid drugs were studied for kappa-opioid receptor expression. Plasma-membrane integrated receptors were measured by radioligand ([3H]-naloxone 1 nM, [3H]-ethylketocyclazocine, 4 nM) binding to intact neurons. These data were compared to the results of k-opioid receptor immunostaining (mAb KA8, Maderspach et al., 1991). The chronic presence of kappa-selective opioid agonist dynorphin1-13 (10(-6)-10(-7) M) or bremazocine (10(-7)-10(-8) M) between cultivation days 2-4 resulted in the significant (80%) down-regulation of the membrane integrated binding sites in concentration dependent fasion. Antagonists naloxone (10(-5) M) and norbinaltorphimine (10(-8) M) alone were ineffective, however, they essentially balanced the changes caused by the agonists. Mu ligand morphine was without effect while kappa-1 selective U 50488 H caused moderate decrease only at high concentrations. Interestingly, mAb KAB also caused down-regulation indicating that it has some agonist character. The down-regulation became measurable in short time (58% within 24 hours) and persisted during the three-day presence of the agonists. Kappa-receptor immunostaining of the neurons was not significantly influenced by the agonists. We suppose that the chronic opioid treatment may influence the intracellular traffic of the receptor protein.

Met5-enkephalin-Arg6-Phe7, an endogenous neuropeptide, binds to multiple opioid and nonopioid sites in rat brain

Receptor binding properties of the naturally occurring opioid heptapeptide MERF were studied in rat brain membrane preparations using tritium-labeled derivative of the peptide with 40 Ci/mmol specific radioactivity. Binding assays were performed in the presence of broad-spectrum peptidase inhibitors at 0 degree C. Under these conditions, the equilibrium binding was achieved in 30-40 min, and approximately 90% of the applied radioligand remained unchanged as determined by HPLC analysis. The apparent affinity (Kd value) of [3H]Met-enkephalin-Arg6-Phe7, calculated from saturation binding data, was 10.2 +/- 2.5 nM, and the maximal number (Bmax) of the heptapeptide binding sites was found to be 468 +/- 43 fmol/mg protein. About half the sites represent nonopioid sites because the Bmax was only 255 +/- 30 fmol/mg, when the nonspecific binding was measured with 1 microM naloxone. The rank order potencies of the examined compounds revealed that the opioid component of [3H]Met-enkephalin-Arg6-Phe7 recognition site are probably not mu and certainly not kappa 1 sites, whereas these sites are characterized by a kappa 2-like binding profile. Considering the discrepancies between rat and frog brain found in the affinity of some compounds, including naltrindole and norbinaltorphimine, the presence of a novel, MERF-selective "heptapeptide" binding site in rat brain membranes is also suggested. A number of the heterologous competition curves could be described by a high-affinity stereospecific component and a substantially lower-affinity binding element, which could completely be displaced with several peptide ligands such as Met5-enkephalin, dynorphin(1-13), and unlabeled MERF but not by other compounds such as [D-Ala2-(Me)Phe4-Gly5-ol]enkephalin, morphine, or naloxone. [3H]Met-enkephalin-Arg6-Phe7 binding can also be inhibited by FMRF-amide analogs and sigma receptor ligands, such as (+)N-allyl-normetazocine and haloperidol, although with moderate affinity. It is concluded that the stereospecific high-affinity binding is of opioid in character, whereas the residual sites characterized with their lower affinity are naloxone-insensitive nonopioid sites.

Affinity labelling of frog brain opioid receptors by dynorphin(1-10) chloromethyl ketone

It has been previously found that chloromethyl ketone derivatives of enkephalins bind irreversibly to the opioid receptors in vitro. Recently a novel affinity reagent, Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Gly chloromethyl ketone (Dynorphin(1-10)-Gly11 chloromethyl ketone, DynCMK) was synthesized, and its binding characteristics to frog (Rana esculenta) brain membranes were evaluated. In competition experiments, the product shows a relatively high affinity for the kappa-opioid binding sites labelled by [3H]ethylketocyclazocine (Ki is approximately equal to 200 nM), whereas its binding to the 1 ([3H]dihydromorphine) and to the delta sites ([3H]D-Ala2-Leu5]enkephalin) is weaker. Preincubation of the frog brain membranes with DynCMK at micromolar concentrations results in a washing-resistant and dose-dependent inhibition of the [3H]ethylketocyclazocine binding sites. Saturation binding analysis of the membranes preincubated with 50 microM DynCMK reveals a significant decrease in the number of specific binding sites for [3H]ethylketocyclazocine compared to the control values. The kappa-preferring binding properties of the compound suggest that it could serve as an affinity label for the kappa-type of opioid receptors.

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.

Opioid antagonist properties of the highly delta-receptor-selective BOC-Tyr-Pro-Gly-Phe-Leu-Thr (OtBu) peptide and of its Phe1 and Mel1 analogues

BOC-Tyr-Pro-Gly-Phe-Leu-Thr(OtBu) is a potent, highly delta-opioid receptor-selective competitive antagonist, the Ke values in the mouse vas deferens in vitro assay against [D-Ala2, D-Leu5]-enkephalin [D-Pen2, D-Pen5]enkephalin and deltorphin-II being 39.5, 38.7 and 27.3 nM, respectively, whereas those against [D-Ala2, MePhe4-Gly5-ol]enkephalin (DAMGO) and ethylketocyclazocine in the guinea-pig ileum are 368,000 and > 200,000 nM, giving a higher than 9000-fold delta- vs mu- and a higher than 5000-fold delta- vs kappa-selectivity ratio. The Ki values against various labeled delta-ligands in the rat brain receptor binding assay were in the 300-1000 nM range, whereas the Ki against [3H]-DAMGO was higher than 30,000 nM. The striking discrepancies between bioassay and receptor binding data show another aspect of already recognized differences of mouse vas deferens and rat brain delta-receptors. With the aim of producing a delta-selective affinity ligand, we synthesized the BOC-Mel1 derivative; however, there was a 175-fold loss of delta-receptor affinity in the bioassay and no indication of an irreversible interaction, but a delta-agonist effect appeared in spite of nonprotonated nitrogen. The corresponding BOC-Phe1 derivative had a 10 times higher affinity and, apparently, no agonist activity.

Characterization of [3H]Met-enkephalin-Arg6-Phe7 binding to opioid receptors in frog brain membrane preparations

A tritiated heptapeptide, [3H]Tyr-Gly-Gly-Phe-Met-Arg-Phe ([3H]Met-enkephalin-Arg6-Phe7), with high specific radioactivity has been synthesized in order to characterize its opioid binding activity to frog brain membrane fractions. The apparent KD value of the radioligand calculated from homologous displacement experiments was 3.4 nM, and the maximal number of specific binding sites was 630 fmol/mg of protein. The KD determined from equilibrium saturation binding studies was found to be 3.6 nM. However, the Hill coefficient was far below unity (nH = 0.43), which suggests the presence of a second, lower affinity binding site. The presence of this binding component is strengthened by the displacement experiments performed with levorphanol and some other ligands. It is assumed that the lower affinity site has no opiate character. The rank order of potency of the applied ligands in competing reversibly with [3H]Met-enkephalin-Arg6-Phe7 binding reflects a kappa 2- and/or delta-subtype specificity of the heptapeptide. Binding to a kappa 1 and/or mu site of opioid receptors is excluded, but the existence of a novel endogenous opiate receptor subtype for Met-enkephalin-Arg6-Phe7 in frogs cannot be ruled out. The [3H]-Met-enkephalin-Arg6-Phe7 binding was inhibited by both sodium ions and GppNHp, which suggests the opioid agonist character of the heptapeptide.

[3H]dynorphin1-8 binding sites in frog (Rana esculenta) brain membranes

Opioid binding sites specific for [3H]dynorphin1-8 were characterized in the particulate membrane fraction of frog (Rana esculenta) brain. The degradation of the radioligand during the assay was prevented by the use of a broad spectrum of peptidase inhibitors. The binding of [3H]dynorphin1-8 to frog brain membranes was stereoselective, reversible, saturable, and displaceable by a series of opioid ligands including dynorphin1-13, bremazocine, levorphanol and naloxone. The specific binding of [3H]dynorphin1-8 can be significantly inhibited by Na+ ions and/or guanine nucleotides confirming the agonist property of the ligand in vitro. A single set of high affinity opioid binding sites with a Kd approximately 7.5 nM is present in the membranes. The maximum density of binding sites (Bmax approximately 1.1 pmol [3H]dynorphin1-8 per mg protein) was considerably higher than such sites in guinea-pig brain. In addition, comparison with binding of tritiated opioid peptides selective for the mu- and delta-types of opioid receptor showed that in the frog brain most of the sites labelled by [3H]dynorphin1-8 are kappa-sites and that this is a rich source of such sites.

Morphine: new aspects in the study of an ancient compound

Morphine is the most widely used compound among narcotic analgesics and remains the gold standard when the effects of other analgetic drugs are compared. Apart from its presence in the poppy plant Papaver somniferum, morphine has been shown to be present in milk, cerebrospinal fluid and also in nervous tissue extracts. Recent evidence suggests that biosynthetic pathways for morphine exist in animal and even human tissues such as liver, blood and brain. The most characteristic effect of morphine is the modulation of pain perception resulting in an increase in the threshold of noxious stimuli. Antinociception induced by morphine is mediated via opioid receptors and therefore can be inhibited by opioid antagonists, e.g., naloxone. Nevertheless, consideration of morphine as endogenous ligand for opioid receptors seems to be speculative. Recently, the primary receptor for morphine-type drugs called the mu-opioid receptor has been cloned from rat brain. There is accumulating evidence that morphine actions are, at least partly, due to one of its major metabolite morphine-6-glucuronide in man. It is concluded that further investigations are necessary to elucidate the mechanisms, whereby multiple actions of morphine are expressed in the nervous system.

Novel, delta-opioid receptor-selective peptide antagonists: demonstration of a possible opioid interaction in the absence of a protonated nitrogen and attempts to locate the protonated nitrogen site

We have recently (Rónai et al., 1992) introduced a family of novel delta-opioid receptor-selective peptide antagonists, based on the Tyr-Pro-Gly-Phe-Leu-Thr structure, where the nitrogen accepts substituents that make protonation possible (e.g. diallyl) as well as substituents (e.g. t-Boc) where protonation cannot occur. In this paper, we present the details of a design strategy where the structurally closely related biologically active and inactive compounds are suggestive of conformational requirements of action. Furthermore, since even those derivatives of the antagonist peptides where the N-terminus was free were either devoid of opioid agonist activity or were extremely weak agonists, it is suggested that these antagonists do not interact with the conventional "opioid nitrogen site". To find this "conventional" site, a number of N-substituted (phenylglycyl-, alpha-Boc-lysyl-, alpha-Phe-beta-alanyl-) derivatives of Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide were synthesized and their biological activities were determined in the mouse vas deferens bioassay.

The kappa-opioid receptor: evidence for the different subtypes

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.

Species differences in the stereoselectivity of kappa opioid binding sites for [3H]U-69593 and [3H]ethylketocyclazocine

Stereoselectivity of the binding sites for the specific kappa-opioid agonist [3H]U-69593, a benzeneacetamido based ligand was investigated in membrane suspension prepared from frog and rat brain, as well as guinea pig cerebellum, using the pure chiral forms of different unlabelled opiates. The ligand binding sites showed stereospecificity with at least three orders of magnitude differences in the affinities (measured as Ki values) of the opioid stereoisomer pairs both in rat and guinea pig membrane fractions. However, in frog brain membranes there was no substantial difference in potencies of the (-) and (+) isomers competing for the [3H]U-69593 binding sites. Another type of the kappa-site preferring opioid ligand, [3H]ethylketocyclazocine, a benzomorphan derivative was able to discriminate between (-) and (+) forms of the same compounds even in frog brain membrane preparation. Our data concerning binding profile of [3H]U-69593 in frog brain membranes are consistent with the observation that kappa opioid binding sites in frog (Rana esculenta) brain differ from those kappa-sites found in mammalian brains.

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)

Opioid receptor labeling with the chloromethyl ketone derivative of [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol (DAMGO) II: Covalent labeling of mu opioid binding site by 3H-Tyr-D-Ala-Gly-(Me)Phe chloromethyl ketone

Opioid receptors of rat brain membranes were prelabeled with 3H-Tyr-D-Ala2-(Phe4)-Gly-CH2Cl, a chloromethyl ketone derivative of enkephalin, and solubilized in 1% digitonin. Hydrodynamic parameters of the receptor detergent complex derived from gel filtration and sucrose density gradient ultracentrifugation were found to be 51 A and 8.7 S, respectively, and the size was estimated to be about 200 kDa. Sodium dodecyl sulfate gel electrophoresis followed by fluorography revealed specific alkylation of a major protein at 58 kDa.

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.

Method for isolation of kappa-opioid binding sites by dynorphin affinity chromatography

A kappa-opioid receptor subtype was purified from a digitonin extract of frog brain membranes, using affinity chromatography. The affinity resin was prepared by coupling dynorphin (1-10) to AH Sepharose 4B. The purified receptor binds 4,750 pmol [3H]ethylketocyclazocine (EKC) per mg protein (5,600-fold purification over the membrane-bound receptor) with a Kd of 9.1 nM. The addition of cholesterol-phosphatidylethanolamine (2:1) enhanced 3.6-fold the binding activity of the purified material, which gives a purification very close to the theoretical. The purified receptor protein exhibits high affinity for kappa-selective ligands. The purified fraction shows one major band (65,000 Mr) in sodium dodecyl sulfate (SDS) gel electrophoresis.

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.