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

Cannabidiol Acts at 5-HT1A Receptors in the Human Brain: Relevance for Treating Temporal Lobe Epilepsy

Experimental evidence indicates that cannabidiol (CBD) induces anxiolytic and antiepileptic effects through the activation of 5-HT_1A receptors. These receptors are coupled to G_i/o proteins and induce inhibitory effects. At present, the interaction of CBD with 5-HT_1A receptors in the human brain is unknown. The aim of this study focused on evaluating the interaction between CBD and 5-HT_1A receptors in cell membranes obtained from the hippocampus and temporal neocortex of autopsies and patients with drug-resistant mesial temporal lobe epilepsy (DR-MTLE). Cell membranes were isolated from the hippocampus and temporal neocortex of a group of patients with DR-MTLE who were submitted to epilepsy surgery (n = 11) and from a group of autopsies (n = 11). The [³H]-8-OH-DPAT binding assay was used to determine the pharmacological interaction of CBD with 5-HT_1A receptors. The [³⁵S]-GTPγS assay was used to investigate the CBD-induced activation of G_i/o proteins through its action on 5-HT_1A receptors.The CBD affinity (pK _i) for 5-HT_1A receptors was similar for autopsies and patients with DR-MTLE (hippocampus: 4.29 and 4.47, respectively; temporal neocortex: 4.67 and 4.74, respectively). Concerning the [³⁵S]-GTPγS assay, no statistically significant changes were observed for both hippocampal and neocortical tissue (p > 0.05) at low CBD concentrations (1 pM to 10 μM). In contrast, at high concentrations (100 μM), CBD reduced the constitutive activity of G_i/o proteins of autopsies and DR-MTLE patients (hippocampus: 39.2% and 39.6%, respectively; temporal neocortex: 35.2% and 24.4%, respectively). These changes were partially reversed in the presence of WAY-100635, an antagonist of 5-HT_1A receptors, in the autopsy group (hippocampus, 59.8%, p < 0.0001; temporal neocortex, 71.5%, p < 0.0001) and the group of patients with DR-MTLE (hippocampus, 53.7%, p < 0.0001; temporal neocortex, 68.5%, p < 0.001). Our results show that CBD interacts with human 5-HT_1A receptors of the hippocampus and temporal neocortex. At low concentrations, the effect of CBD upon G_i/o protein activation is limited. However, at high concentrations, CBD acts as an inverse agonist of 5-HT_1A receptors. This effect could modify neuronal excitation and epileptic seizures in patients with DR-MTLE.

Alterations of 5-HT1A receptor-induced G-protein functional activation and relationship to memory deficits in patients with pharmacoresistant temporal lobe epilepsy

The 5-hydroxytryptamine-1A (5-HT1A) receptors are known to be involved in the inhibition of seizures in epilepsy. Moreover, studies propose a role for the 5-HT1A receptor in memory function; it is believed that the higher density of this receptor in the hippocampus plays an important role in its regulation. Positron emission tomography (PET) studies in patients with mesial temporal lobe epilepsy (mTLE) have demonstrated that a decrease in 5-HT1A receptor binding in temporal regions may play a role in memory impairment. The evidences lead us to speculate whether this decrease in receptor binding is associated with a reduced receptor number or if the functionality of the 5-HT1A receptor-induced G-protein activation and/or the second messenger cascade is modified. The purpose of the present study is to determine 5-HT1A receptor-induced G-protein functional activation by 8-OH-DPAT-stimulated [(35)S]GTPγS binding assay in hippocampal tissue of surgical patients with mTLE. We correlate functional activity with epilepsy history and neuropsychological assessment of memory. We found that maximum functional activation stimulation values (Emax) of [(35)S]GTPγS binding were significantly increased in mTLE group when compared to autopsy samples. Furthermore, significant correlations were found: (1) positive coefficients between the Emax with the age of patient and frequency of seizures; (2) negative coefficients between the Emax and working memory, immediate recall and delayed recall memory tasks. Our data suggest that the epileptic hippocampus of patients with mTLE presents an increase in 5-HT1A receptor-induced G-protein functional activation, and that this altered activity is related to age and seizure frequency, as well as to memory consolidation deficit.

Opposite effects of neuropeptide FF on central antinociception induced by endomorphin-1 and endomorphin-2 in mice

Neuropeptide FF (NPFF) is known to be an endogenous opioid-modulating peptide. Nevertheless, very few researches focused on the interaction between NPFF and endogenous opioid peptides. In the present study, we have investigated the effects of NPFF system on the supraspinal antinociceptive effects induced by the endogenous µ-opioid receptor agonists, endomorphin-1 (EM-1) and endomorphin-2 (EM-2). In the mouse tail-flick assay, intracerebroventricular injection of EM-1 induced antinociception via µ-opioid receptor while the antinociception of intracerebroventricular injected EM-2 was mediated by both µ- and κ-opioid receptors. In addition, central administration of NPFF significantly reduced EM-1-induced central antinociception, but enhanced EM-2-induced central antinociception. The results using the selective NPFF1 and NPFF2 receptor agonists indicated that the EM-1-modulating action of NPFF was mainly mediated by NPFF2 receptor, while NPFF potentiated EM-2-induecd antinociception via both NPFF1 and NPFF2 receptors. To further investigate the roles of µ- and κ-opioid systems in the opposite effects of NPFF on central antinociception of endomprphins, the µ- and κ-opioid receptors selective agonists DAMGO and U69593, respectively, were used. Our results showed that NPFF could reduce the central antinociception of DAMGO via NPFF2 receptor and enhance the central antinociception of U69593 via both NPFF1 and NPFF2 receptors. Taken together, our data demonstrate that NPFF exerts opposite effects on central antinociception of endomorphins and provide the first evidence that NPFF potentiate antinociception of EM-2, which might result from the interaction between NPFF and κ-opioid systems.

Cyclic enkephalins with a diversely substituted guanidine bridge or a thiourea bridge: synthesis, biological and structural evaluations

Two series of 22 and 15 atom cyclic enkephalins incorporating a diversely substituted guanidine bridge have been prepared to assess the potential effect of the bridge substitutions on their opioid activity profile. The most notable results were obtained with the shortest cyclic analogues, which showed a significant variation of their binding affinity toward μ and δ opioid receptors in relation to bridge substitution. NMR studies were performed to rationalize these data. Some small analogues were found to exist as at least one major and one minor stable forms, which could be separated by chromatography. In particular, the compounds 13 and 14 with a cyclic substituent were separated in three isomers and the basis of this multiplicity was explored by 2D NMR spectroscopy. All compounds were agonists with slight selectivity for the μ opioid receptor. Compounds 7a (thiourea bridge) and 10a (N-Me-guanidine bridge) showed nanomolar affinity toward μ receptor, the latter being the more selective for this receptor (40-fold).

Synaptic connections between endomorphin 2-immunoreactive terminals and μ-opioid receptor-expressing neurons in the sacral parasympathetic nucleus of the rat

The urinary bladder is innervated by parasympathetic preganglionic neurons (PPNs) that express μ-opioid receptors (MOR) in the sacral parasympathetic nucleus (SPN) at lumbosacral segments L6-S1. The SPN also contains endomorphin 2 (EM2)-immunoreactive (IR) fibers and terminals. EM2 is the endogenous ligand of MOR. In the present study, retrograde tract-tracing with cholera toxin subunit b (CTb) or wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) via the pelvic nerve combined with immunohistochemical staining for EM2 and MOR to identify PPNs within the SPN as well as synaptic connections between the EM2-IR terminals and MOR-expressing PPNs in the SPN of the rat. After CTb was injected into the pelvic nerve, CTb retrogradely labeled neurons were almost exclusively located in the lateral part of the intermediolateral gray matter at L6-S1 of the lumbosacral spinal cord. All of the them also expressed MOR. EM2-IR terminals formed symmetric synapses with MOR-IR, WGA-HRP-labeled and WGA-HRP/MOR double-labeled neuronal cell bodies and dendrites within the SPN. These results provided morphological evidence that EM2-containing axon terminals formed symmetric synapses with MOR-expressing PPNs in the SPN. The present results also show that EM2 and MOR might be involved in both the homeostatic control and information transmission of micturition.

Opioid activity profiles of oversimplified peptides lacking in the protonable N-terminus

Recently, we described cyclopeptide opioid agonists containing the d-Trp-Phe sequence. To expand the scope of this atypical pharmacophore, we tested the activity profiles of the linear peptides Ac-Xaa-Phe-Yaa (Xaa = l/d-Trp, d-His/Lys/Arg; Yaa = H, GlyNH(2)). Ac-d-Trp-PheNH(2) appeared to be the minimal binding sequence, while Ac-d-Trp-Phe-GlyNH(2) emerged as the first noncationizable short peptide (partial) agonist with high μ-opioid receptor affinity and selectivity. Conformational analysis suggested that 5 adopts in solution a β-turn conformation.

Temporal lobe epilepsy causes selective changes in mu opioid and nociceptin receptor binding and functional coupling to G-proteins in human temporal neocortex

There is no information concerning signal transduction mechanisms downstream of the opioid/nociceptin receptors in the human epileptic brain. The aim of this work was to evaluate the level of G-proteins activation mediated by DAMGO (a mu receptor selective peptide) and nociceptin, and the binding to mu and nociceptin (NOP) receptors and adenylyl cyclase (AC) in neocortex of patients with pharmacoresistant temporal lobe epilepsy. Patients with temporal lobe epilepsy associated with mesial sclerosis (MTLE) or secondary to tumor or vascular lesion showed enhanced [3H]DAMGO and [3H]forskolin binding, lower DAMGO-stimulated [35S]GTPgammaS binding and no significant changes in nociceptin-stimulated G-protein. [3H]Nociceptin binding was lower in patients with MTLE. Age of seizure onset correlated positively with [3H]DAMGO binding and DAMGO-stimulated [35S]GTPgammaS binding, whereas epilepsy duration correlated negatively with [3H]DAMGO and [3H]nociceptin binding, and positively with [3H]forskolin binding. In conclusion, our present data obtained from neocortex of epileptic patients provide strong evidence that a) temporal lobe epilepsy is associated with alterations in mu opioid and NOP receptor binding and signal transduction mechanisms downstream of these receptors, and b) clinical aspects may play an important role on these receptor changes.

Enzymatic degradation of endomorphins

Centrally acting plant opiates, such as morphine, are the most frequently used analgesics for the relief of severe pain, even though their undesired side effects are serious limitation to their usefulness. The search for new therapeutics that could replace morphine has been mainly focused on the development of peptide analogs or peptidomimetics with high selectivity for one receptor type and high bioavailability, that is good blood-brain barrier permeability and enzymatic stability. Drugs, in order to be effective, must be able to reach the target tissue and to remain metabolically stable to produce the desired effects. The study of naturally occurring peptides provides a rational and powerful approach in the design of peptide therapeutics. Endogenous opioid peptides, endomorphin-1 and endomorphin-2, are two potent and highly selective mu-opioid receptor agonists, discovered only a decade ago, which display potent analgesic activity. However, extensive studies on the possible use of endomorphins as analgesics instead of morphine met with failure due to their instability. This review deals with the recent investigations that allowed determine degradation pathways of endomorphins in vitro and in vivo and propose modifications that will lead to more stable analogs.

Capsaicin inhibits the in vitro binding of peptides selective for mu- and kappa-opioid, and nociceptin-receptors

Capsaicin inhibited the equilibrium specific binding of endogenous opioid-like peptide ligands such as endomorphin-1, nociceptin, and dynorphin((1-17)) in rat brain membrane preparations. We studied the in vitro effect of capsaicin (1-10 microM) on homologous and heterologous competitive binding of opioid ligands, using unlabeled synthetic peptides and the following tritiated compounds: [(3)H]endomorphin-1, [(3)H]endomorphin-2, [(3)H]nociceptin((1-17)) and [(3)H]dynorphin((1-17)). Capsaicin-dependent inhibition was also observed in [(35)S]GTPgammaS stimulation assays in the presence of certain opioid peptides. The inhibition of opioid binding was further investigated using other synthetic and natural mu-opioid ligands such as [D-Ala(2),(NMe)Phe(4),Gly(5)-ol]enkephalin (DAMGO), morphine and naloxone. The decrease in opioid ligand affinity upon capsaicin treatments was most apparent with endomorphin-1, followed by nociceptin and dynorphin. The binding of other investigated opioids were not affected in the presence of capsaicin. In [(3)H]endomorphin-1 binding assays, capsazepine antagonized the inhibitory effect of capsaicin in rat brain membranes suggesting the involvement of TRPV1 receptors. In Chinese hamster ovary (CHO) cells stably expressing mu-opioid receptors, but lacking vanilloid receptors, the inhibition by capsaicin on the binding of [(3)H]endomorphin-1 was not present. It is concluded that the inhibitory effect of capsaicin on the receptor binding affinity of endogenous opioid peptides in brain membrane preparations seems not to be a direct effect, it is rather a negative feedback interaction with opioid receptors.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Novel diastereomeric opioid tetrapeptides exhibit differing pharmacological activity profiles

A novel opioid peptide antagonist analogue, [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe, derived from the potent, delta-receptor selective TIPP tetrapeptide (Tyr-Tic-Phe-Phe) series was synthesized and radiolabeled by catalytic tritiation of its iodinated precursor peptide. The purified radioprobe exhibited a specific activity of 2.15 TBq/mmol (58 Ci/mmol). The novelty of this compound is that it contains structurally modified tyrosine residue (2',6'-dimethyltyrosine, Dmt1) replacing tyrosine (Tyr1) at the N-terminus, and beta-methyl substituted phenylalanine (betaMePhe3) at the third position. As the configuration of betaMePhe3 side-chain might be different due to diastereomerism, and accordingly can alter the biological activity, both unlabeled threo (2S,3R and 2R,3S) diastereomeric analogues were also prepared and included in this study. The affinity and selectivity (delta-opioid versus mu-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potencies were determined in [35S]GTPgammaS binding experiments using Chinese Hamster Ovary (CHO) cells selectively expressing delta- or mu-opioid receptors. The equilibrium binding of the radiolabeled peptide derivative [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe to rat brain membranes was saturable and the Scatchard analysis indicated a single binding site with a Kd of 0.3 nM and a Bmax of 127 fmol/mg protein. A study of [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe binding displacement by various receptor-type specific opioid ligands showed the rank order of competitor's potency delta > mu > kappa, suggesting selective labeling of opioid delta-sites. In the functional tests, the (2S,3R) and (2R,3S) peptides exhibited partial agonist behaviour by weakly stimulating regulatory G-proteins in CHO cell membranes transfected with different receptors. Both isomers were quite weak partial agonists at the delta-receptor and reasonable partial agonists at the mu-receptor, with a prevalence of (2S,3R) over (2R,3S) for the mu-receptor. Consistent with these observations both stereomers competitively inhibited the stimulation of [35S]GTPgammaS binding induced by the prototype delta-agonist peptide (pClPhe4)-DPDPE in delta(m) CHO cell membranes, and still the (2S,3R) compound exerted more potent delta-antagonist effect. [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe represents a high affinity new radioligand and also constitute further example of the influence of beta-methyl substitution on the potency and selectivity of TIPP analogues, thus becoming a valuable biochemical and pharmacological tool in opioid research.

Synthesis, radiolabeling and receptor binding of [3H][(1S,2R)ACPC2]endomorphin-2

Previously, we have shown that substitution of Pro(2) for cis-2-aminocyclopentanecarboxylic acid, ACPC in endomorphin-2 results in an analogue with greatly augmented proteolytic stability, high mu-opioid receptor affinity and selectivity. We now report the synthesis and biochemical characterization of [(3)H][(1S,2R)ACPC(2)]endomorphin-2 with a specific activity of 1.41 TBq/mmol (38.17 Ci/mmol). Specific binding of [(3)H][(1S,2R)ACPC(2)]endomorphin-2 was saturable and of high affinity with an equilibrium dissociation constant, K(d)=1.80+/-0.21nM and receptor density, B(max)=345+/-27 fmol x mg protein(-1) at 25 degrees C in rat brain membranes. Similar affinity values were obtained in kinetic and displacement assays. Both Na(+) and Gpp(NH)p decreased the affinity proving the agonist character of the radioligand. [(3)H][(1S,2R)ACPC(2)]endomorphin-2 retained the mu-specificity of the parent peptide. The new radioligand will be a useful tool to map the topographical requirements of mu-opioid peptide binding due to its high affinity, selectivity and enzymatic stability.

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.

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.

Enzymatic degradation studies of endomorphin-2 and its analogs containing N-methylated amino acids

In this paper, we describe the synthesis of novel endomorphin-2 analogs, containing N-methylated amino acids, consecutively in each position. The receptor-binding profile of the new analogs and their stability against enzymatic cleavage by commercially available peptidases, carboxypeptidase Y and aminopeptidase M, and a rat brain homogenate are reported. The best analog of this series, [Sar2]endomorphin-2, was almost equipotent with the parent peptide in the mu-receptor-binding assay and was also highly resistant to enzymatic degradation. This analog may be a suitable candidate for the in vivo antinociceptive studies.

Synthesis and biological activity of N-methylated analogs of endomorphin-2

In this paper, we describe the synthesis of a series of endomorphin-2 analogs containing N-methylated amino acids, consecutively in each position. The mu-opioid receptor binding affinities of the new analogs were determined in the displacement experiments. Their in vivo antinociceptive activity was assessed in the hot-plate test in mice after central (icv) and peripheral (ip) administration. [Sar2]endomorphin-2, which had the highest mu-receptor affinity, also showed the strongest analgesic effect when administered centrally and was the only analog that retained activity after peripheral injection.

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.

Increased release of immunoreactive dynorphin A1–17 from the spinal cord after intrathecal treatment with endomorphin-2 in anesthetized rats

We previously demonstrated pretreatment with antiserum against dynorphin A1-17 attenuates endomorphin-2-induced analgesia and antianalgesia, suggesting that these endomorphin-2 effects are mediated by the release of dynorphin A1-17. Lumbar-cisternal spinal perfusion was used to measure the release of immunoreactive dynorphin A1-17 into spinal perfusates from urethane-anesthetized rats following endomorphin-2 or endomorphin-1 treatment within the perfusion solution. Treatment with endomorphin-2 (5-50 nmol) for 3 min caused a dose-dependent increase of immunoreactive dynorphin A1-17 in spinal perfusates, with a maximal increase detected between 24 and 48 min after endomorphin-2 treatment, while levels returned to baseline within 60 min. Endomorphin-2-induced release of immunoreactive dynorphin A1-17 was attenuated by pretreatment with mu-opioid receptor antagonist naloxone or 3-methoxynaltrexone. Endomorphin-1 induced a slight increase in immunoreactive dynorphin1-17 as well, but only at the highest dose used (50 nmol). Our results suggest that endomorphin-2 stimulated a specific subtype of mu-opioid receptor to induce the release of immunoreactive dynorphin A1-17 in spinal cords of rats.

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.

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.

Binding characteristics of [3H]14-methoxymetopon, a high affinity mu-opioid receptor agonist

The highly potent micro -opioid receptor agonist 14-methoxymetopon (4,5alpha-epoxy-3-hydroxy-14beta-methoxy-5beta,17-dimethylmorphinan-6-one) was prepared in tritium labelled form by a catalytic dehalogenation method resulting in a specific radioactivity of 15.9 Ci/mmol. Opioid binding characteristics of [3H]14-methoxymetopon were determined using radioligand binding assay in rat brain membranes. [3H]14-Methoxymetopon specifically labelled a single class of opioid sites with affinity in low subnanomolar range (Ki = 0.43 nm) and maximal number of binding sites of 314 fmol/mg protein. Binding of [3H]14-methoxymetopon was inhibited by ligands selective for the micro -opioid receptor with high potency, while selective kappa-opioids and delta-opioids were weaker inhibitors. 14-Methoxymetopon increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding with an EC50 of 70.9 nm, thus, providing evidence for the agonist character of this ligand. The increase of [35S]GTPgammaS binding was inhibited by naloxone and selective micro -opioid antagonists, indicating a micro -opioid receptor-mediated action. [3H]14-Methoxymetopon is one of the few nonpeptide mu-opioid receptor agonists available in radiolabelled form up to now. Due to its high affinity and selectivity, high stability and extremely low nonspecific binding (<10%), this radioligand would be an important and useful tool in probing mu-opioid receptor mechanisms, as well as to promote a further understanding of the opioid system at the cellular and molecular level.

Endomorphins and related opioid peptides

Opioid peptides and their G-protein-coupled receptors (delta, kappa, mu) are located in the central nervous system and peripheral tissues. The opioid system has been studied to determine the intrinsic mechanism of modulation of pain and to develop uniquely effective pain-control substances with minimal abuse potential and side effects. Two types of endogenous opioid peptides exist, one containing Try-Gly-Gly-Phe as the message domain (enkephalins, endorphins, dynorphins) and the other containing the Tyr-Pro-Phe/Trp sequence (endomorphins-1 and -2). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has high mu receptor affinity (Ki = 0.36 nM) and remarkable selectivity (4000- and 15,000-fold preference over the delta and kappa receptors, respectively), was isolated from bovine and human brain. In addition, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), isolated from the same sources, exhibited high mu receptor affinity (Ki = 0.69 nM) and very high selectivity (13,000- and 7500-fold preference relative to delta and kappa receptors, respectively). Both opioids bind to mu-opioid receptors, thereby activating G-proteins, resulting in regulation of gastrointestinal motility, manifestation of antinociception, and effects on the vascular systems and memory. To develop novel analgesics with less addictive properties, evaluation of the structure-activity relationships of the endomorphins led to the design of more potent and stable analgesics. Opioidmimetics and opioid peptides containing the amino acid sequence of the message domain of endomorphins, Tyr-Pro-Phe/Trp, could exhibit unique binding activity and lead to the development of new therapeutic drugs for controlling pain.

Evaluation of endomorphin-1 on the activity of Na(+),K(+)-ATPase using in vitro and in vivo studies

The goal of this study was to investigate the effects of endomorphin-1 on Na(+),K(+)-ATPase activity in mouse brain synaptosome in vitro, and its antinociceptive interaction with the Na(+),K(+)-ATPase inhibitor ouabain. Endomorphin-1 (0.1 nM-10 microM) produced a concentration-dependent (EC(50): 43.19 nM, CI: 23.38-65.71 nM, E(max): 25.86%, CI: 24.53-27.20%), naloxone-reversible increase of the synaptosomal Na(+),K(+)-ATPase activity. The intrathecally (i.t.) administered endomorphin-1 (2-20 microg) produced a dose-dependent short-lasting increase in the tail-flick latency. Ouabain itself (1-1000 ng, i.t.) did not cause antinociception. Treatment with 10 ng ouabain significantly decreased the antinociceptive effect of 2 microg endomorphin-1, but none of the other combinations did significantly differ from the endomorhin-1-treated groups. These data indicate that endomorphin-1 increases the activity of Na(+),K(+)-ATPase in vitro but this effect may play a weak role in the antinociception induced by intrathecal endomorphin-1.

In vitro quantitative study of the degradation of endomorphins

The catabolism of the endomorphins was investigated in detail. The endomorphins were degraded relatively slowly in the rat brain homogenate (t1/2(endomorphin-1)=4.94 min; t1/2(endomorphin-2)=3.81 min). The inhibition of metalloproteases and aminopeptidases stabilised the endomorphins to the greatest extent. The digestion of endomorphins tritiated specifically on Tyr(1), Pro(2) or Phe(3) established also that only the aminopeptidase pathways were essential for inactivation of the endomorphins, and that the tetrapeptides were degraded by cleavage of the Pro(2)-Trp(3) or Pro(2)-Phe(3) bond. The end-products of the catabolism were amino acids; the fragments Tyr-Pro-OH and Pro-Trp-Phe-NH2 were present as intermediates. Metabolites produced by brain carboxypeptidases were not detected.

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.

Effects of cannabinoids in Krox-24 targeted mice

Krox-24 is an immediate early gene encoding a zinc-finger transcription factor implicated in several adaptive responses, and its induction by cannabinoids has been reported. We used mice targeted in the Krox-24 gene to specifically dissect the role of this protein in the acute and chronic central actions of cannabinoids. We report here on the ability of cannabinoids to activate G-proteins and to inhibit adenylyl cyclase, and to elicit behavioral responses in wild-type and mutant mice. The behavioral parameters and the biochemical correlates of abstinence after delta9-THC withdrawal were evaluated. We show that Krox-24 is not involved in the acute analgesic effects of delta9-THC and in the SR precipitated delta9-THC withdrawal syndrome.

Endomorphin-1 and endomorphin-2: pharmacology of the selective endogenous mu-opioid receptor agonists

The recently discovered endogenous opioid peptides, endomorphins-1 and -2, appear to have properties consistent with neurotransmitter/neuromodulator actions in mammals. This review surveys the information gained so far from studies of different aspects of the endomorphins. Thus, the endomorphins have been found unequally in the brain; they are stored in neurons and axon terminals, with a heterogeneous distribution; they are released from synaptosomes by depolarization; they are enzymatically converted by endopeptidases; and they interact specifically and with high affinity with mu-opioid receptors. The most outstanding effect of the endomorphins is their antinociceptive action. This depends on both central and peripheral neurons. Additionally, the endomorphins cause vasodilatation by stimulating nitric oxide release from the endothelium. Their roles in different central and peripheral functions, however, have not been fully clarified yet. From a therapeutic perspective, therefore, they may be conceived at present as potent antinociceptive and vasodilator agents.

Specific activation of the mu opioid receptor (MOR) by endomorphin 1 and endomorphin 2

The recently discovered endomorphin 1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin 2 (Tyr-Pro-Phe-Phe-NH2) were investigated with respect to their direct receptor-binding properties, and to their ability to activate G proteins and to inhibit adenylyl cyclase in both cellular and animal models. Both tetrapeptides activated G proteins and inhibited adenylyl cyclase activity in membrane preparations from cells stably expressing the mu opioid receptor, an effect reversed by the mu receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2), but they had no influence on cells stably expressing the delta opioid receptor. To further establish the selectivity of these peptides for the mu opioid receptor, brain preparations of mice lacking the mu opioid receptor gene were used to study their binding and signalling properties. Endomorphin 2, tritiated by a dehalotritiation method resulting in a specific radioactivity of 1.98 TBq/mmol (53.4 Ci/mmol), labelled the brain membranes of wild-type mice with a Kd value of 1.77 nM and a Bmax of 63.33 fmol/mg protein. In membranes of mice lacking the mu receptor gene, no binding was observed, and both endomorphins failed to stimulate [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding and to inhibit adenylyl cyclase. These data show that endomorphins are capable of activating G proteins and inhibiting adenylyl cyclase activity, and all these effects are mediated by the mu opioid receptors.

Antinociceptive effects of intrathecal endomorphin-1 and -2 in rats

Endomorphin-1 and endomorphin-2 were recently postulated to be endogenous mu-opioid receptor agonists. We have investigated the antinociceptive and antihyperalgesic effects of intrathecally administered endomorphins in cumulative doses (0.1-100 microg) on acute and inflammatory pain sensations in awake rats. In the tail-flick test, both peptides caused a dose-dependent short-lasting antinociception, except at the highest dose, which caused motor impairment also. The dose-response curves revealed the development of acute tolerance (tachyphylaxis) to endomorphin. Similarly in the carrageenan-injected paw, the endomorphins (10 microg) exerted transient antinociceptive effects. These are the first data to demonstrate decreased responsivity in models of both acute and inflammatory pain after intrathecal administration of endomorphin-1 and -2 in awake rats.

Liquid chromatographic study of the enzymatic degradation of endomorphins, with identification by electrospray ionization mass spectrometry

The recently discovered native endomorphins play an important role in opioid analgesia, but their metabolic fate in the organism remains relatively little known. This paper describes the application of high-performance liquid chromatography combined with electrospray ionization mass spectrometry to identify the degradation products resulting from the incubation of endomorphins with proteolytic enzymes. The native endomorphin-1, H-Tyr-Pro-Trp-Phe-NH2 (1), and endomorphin-2, H-Tyr-Pro-Phe-Phe-NH2 (2), and an analog of endomorphin-2, H-Tyr-Pro-Phe-Phe-OH (3), were synthetized, and the levels of their resistance against carboxypeptidase A, carboxypeptidase Y, aminopeptidase M and proteinase A were determined. The patterns of peptide metabolites identified by this method indicated that carboxypeptidase Y first hydrolyzes the C-terminal amide group to a carboxy group, and then splits the peptides at the Trp3-Phe4 or Phe3-Phe4 bond. The remaining fragment peptides are stable against the enzymes investigated. Carboxypeptidase A degrades only analog 3 at the Phe3-Phe4 bond. Aminopeptidase M cleaves the peptides at the Pro2-Trp3 or Pro2-Phe3 bond. The C-terminal fragments hydrolyze further, giving amino acids and Phe-NH2-s while the N-terminal part displays a resistance to further aminopeptidase M digestion. Proteinase A exhibits a similar effect to carboxypeptidase Y: the C-terminal amide group is first converted to a carboxy group, and one amino acid is then split off from the C-terminal side.