The opiate receptor: a model explaining structure-activity relationships of opiate agonists and antagonists

Molecular-dynamics simulations of [Met5]- and [D-Ala2,Met5]-enkephalins. Biological implication of monomeric folded and dimeric unfolded conformations

To investigate the biologically active conformation of enkephalin, molecular-dynamics simulations were applied to [Met5]- and [D-Ala2,Met5]-enkephalins. The dynamic trajectory of monomeric extended [Met5]-enkephalin was analysed in terms of relative mobility between respective torsions of backbone chain. After 10 ps of the dynamics simulation, the conformational transition was converged into a stationary state among the beta-bend folded forms, where they are stabilized by several intramolecular hydrogen-bond formations. Similar conformational transition was also observed in the dynamics simulation of [D-Ala2,Met5]enkephalin, which is a more mu-receptor-specific peptide than [Met5]enkephalin. The geometrical correspondence between the monomeric enkephalin conformation in the stationary state and morphine molecule (a mu-specific rigid opiate) was surveyed by virtue of the triangular substructures generated by choosing three functional atoms in each molecule, and good resemblances were observed. On the other hand, the dynamics simulation of the antiparallel extended [Met5]enkephalin dimer showed a trajectory different from that of the monomeric one. Two intermolecular hydrogen bonds at Tyr1 (NH3+)...Met5(CO2-) end residues were held throughout the 100 ps simulation, the dimeric structure being consequently kept. The conformational transition of the backbone chains from the antiparallel extended form to the twisted one took place via an intermediate state. Many conformations revealed during the dynamics simulation showed that the relative orientations of each two Tyr1, Gly3, Phe4 and Met5 residues in the dimer are nearly related by a pseudo-C2-symmetry respectively, and both halves of the dimer structure could be further fitted to the monomeric folded enkephalin conformation. The monomeric and dimeric conformations of enkephalin at their stationary states are discussed in relation to the substrate-specificity for mu- and delta-opioid receptors.

Conformation-activity relationships of opiate analgesics

Extensive conformational calculations were performed on the potent opiate analgesics etorphine, PET, R30490 and etonitazene to determine all of their many low-energy conformations. The results were used to characterize four possible models for binding of a simple pharmacophore, comprising two phenyl rings plus a protonated nitrogen, to opiate analgesic receptors. These four models may define the necessary three-dimensional features leading to particular opiate actions. The model favoured for mu receptor activity can accommodate a protonated nitrogen, an aromatic ring (which may be substituted with an electronegative group) and a second lipophilic group. These structural features must be presented in a precise three-dimensional arrangement. It appears likely that a hydrophilic substituent in a certain region of the analgesic pharmacophore may also interact with the receptor as a secondary binding group.

Differential regulation of two molecular forms of a mu-opioid receptor type by sodium ions, manganese ions and by guanyl-5'-yl imidodiphosphate

The rabbit cerebellum contains a very high proportion (up to 80%) of mu-opioid receptor sites (Meunier, J.C., Kouakou, Y., Puget, A. and Moisand, C., Mol. Pharmacol. 24, 23-29, 1983). A membrane fraction derived therefrom is labeled either with the opioid agonist, 3H-etorphine or with the opioid antagonist, 3H-diprenorphine, and solubilized with digitonin. Centrifugation of the soluble extracts in linear sucrose gradients reveals that bound 3H-etorphine sediments faster than does bound 3H-diprenorphine: 12S vs 10S. Pre-incubation of membranes and radioligand in the presence of 120 mM NaCl results in considerably decreased recovery of the 3H-agonist in 12S form while recovery of the 3H-antagonist in 10S form is substantially increased. The opposite situation is observed when the membranes have been prelabeled with radioligand in the presence of 1 mM MnCl2. Guanyl-5'-yl imidodiphosphate, a metabolically stable structural analog of GTP is found to selectively reduce recovery of labeled 12S receptors while it does not affect that of labeled 10S receptors. These data indicate that the mu-opioid receptor from rabbit cerebellum is capable of existing in two forms which differ in apparent molecular size: an "antagonist" (10S) form of apparent Mr approximately 230,000 which is stabilized in the presence of sodium ions and an "agonist" (12S) form of apparent Mr approximately 300,000 which, unlike the antagonist one, is sensitive to guanyl-5'-yl imidodiphosphate. It is thought that the form of larger apparent size represents the mu-opioid receptor associated with a guanine nucleotide regulatory protein.

Molecular structure of two conformationally restrained fentanyl analogues: cis- and trans-isomers of N-(3-methyl-1-[2-(1,2,3,4-tetrahydro)naphthyl] -4-piperidinyl)-N-phenylpropanamide

The X-ray crystallographic structures of two analogues of the potent analgetic fentanyl in which the N-phenethyl substituent is restrained through incorporation into a tetrahydronaphthyl ring system are reported. The tetrahydronaphthyl moiety exists in an equatorial conformation with respect to the piperidine ring which exists in the chair conformation. The propanilido group is also equatorial and antiperiplanar. The orientation of the N-phenyl group with respect to the N-acyl moiety is essentially invariant with an approximately 90 degrees dihedral angle. The implications of these conformations in the interaction of fentanyl-type analgetics with opiate receptors are discussed. The following data were obtained: cis-[C25H33N2O+]Cl- . 1/2 C2O4H2, triclinic, P-1; a = 7.005(7), b = 13.189(2), c = 14.312(4)A, alpha = 111.27(2), beta = 99.15(5), gamma = 93.52(4)0, V = 1205.9A, Z = 2, T = 110K, R = 0.0596, Rw = 0.0760; trans-[C25H33N2O+] C2O4H- . 1/2 C2O4H2 . 2 H2O, triclinic, P-1, a = 8.235(6), b = 10.546(8), c = 17.108(9)A, alpha = 107.72(5), beta = 95.73(5), gamma = 90.63(6)0, V = 1406.8A3, Z = 2, T = 110K, R = 0.0737, Rw = 0.0934.

Analgesic potencies of morphine 3- and 6-sulfates after intracerebroventricular administration in mice: relationship to structural characteristics defined by mass spectrometry and nuclear magnetic resonance

Morphine 3-sulfate, which carries a polar, acidic group at the 3-position much like morphine, does not differ greatly in analgesic potency from morphine following intracerebroventricular administration. This differs from the non-ionizable 3-methyl and 3-ethyl ethers, which are less potent analgesics than morphine. Morphine 6-sulfate, which differs from morphine by having an ionizable group at carbon-6 at physiological pH, is a more potent analgesic than morphine following intracerebroventricular administration. Variations in analgesic potency following modifications at the hydroxyl groups appear only to reflect alterations in point charges rather than structural alterations.

The interference of naloxone hydrochloride in the teratogenic activity of opiates

Diamorphine hydrochloride, methadone hydrochloride, and the synthetic enkephalin analogue FK 33-824 are potent teratogens for the central nervous system in mouse embryos. They induce the "neurotropic syndrome of malformations," which is restricted to the central nervous system if administered during the critical period of neural tube closure. Pretreatment with corresponding equimolecular doses of the antagonist naloxone hydrochloride applied 30 minutes before treatment with the opiate agonists abolishes the major severe malformations, i.e., exencephaly, craniorachischisis, and brachyury, and reduces the number of cases of kinking of the spinal cord. Dilation of the fourth brain ventricle remains unaffected. It is suggested that the mechanism of interference in the teratogenicity of the opiates by naloxone hydrochloride reported here is based on competition for opiate receptors. In general, these observations are regarded as evidence that the pharmacological affinity of opiate agonists to receptors in the central nervous system is responsible for the malformations caused by them in this system.

X-ray diffraction studies of enkephalins. Crystal structure of [(4'-bromo) Phe4,Leu5]enkephalin

In order to investigate the structure-activity relationship of [Leu5]- and [Met5]enkephalins, [(4'-bromo)Phe4, Leu5]-, [(4'-bromo)Phe4, Met5]- and [Met5] enkephalins were synthesized and crystallized. The crystal structure of [(4'-bromo) Phe4, Leu5]- enkephalin was determined by X-ray diffraction method using the heavy atom method and refined to R = 0.092 by the least-squares method. The molecule in this crystal took essentially the same type I' beta-turn conformation found in [Leu5]enkephalin [Smith & Griffin (1978) Science 199, 1214-1216). On the other hand, the preliminary three-dimensional Patterson analyses showed that the most probable conformations of [(4'-bromo)Phe4,Met5]- and [Met5]enkephalins are both the dimeric extended forms. Based on these insights, the biologically active conformation of enkephalin was discussed in relation to the mu- and delta-receptors.

Minimum structure opioids-dipeptide and tripeptide analogs of the enkephalins

Through a systematic reduction of peptide structure, a series of 25 tripeptide and 5 dipeptide amide and alcohol analogs of enkephalin were synthesized and assayed in vitro on the stimulated guinea pig ileum. Tyr-Pro-Phe-NH2, Tyr-D-Ala-Phe-NH2, Tyr-D-Ala-Phe-ol and Tyr-D-Phe-Phe-NH2 had 20-25% the potency of Met-enkephalin. Four aromatic alkylamides of the dipeptide Tyr-D-Ala were made with benzylamine, phenethylamine, phenylpropylamine and phenylbutylamine. All had full naloxone reversible enkephalin-like activity in the ileum assay. Tyr-D-Ala-phenylpropylamide has about 80% the potency of Met-enkephalin in vitro, and is equipotent with Tyr-D-Ala-Gly-Phe-Met-NH2 in producing analgesia in mice after intraventricular administration. Tyr-D-Phe-NH2 is the smallest peptide to show full intrinsic enkephalin-like activity in vitro, although its potency is very low.

Analgesic and other pharmacological activities of a new narcotic antagonist analgesic (-)-1-(3-methyl-2-butenyl)-4-[2-(3-hydroxyphenyl)-1-phenylethyl]-piperazine and its enantiomorph in experimental animals

Of 1-chclohexyl-4-[2-(3-hydroxyphenyl)-1-phenylethyl]piperazine (I) and its 1-(3-methyl-2-butenyl) derivative (II), the S(+)-isomers were analgesically more active than either their +(-)-isomers or their racemates, having 15 to 44 times the potency of morphine in mice and rats. R(-)-I had comparable analgesic activity to morphine R(-)-II to pentazocine in mice, rats and dogs and they were nearly equipotent with pentazocine in reversing some actions of morphine. The S(+)-isomers and racemates lacked this action. R(-)-II required about 10 times more naloxone to reverse its analgesic activity than was needed to antagonise the S(+)-isomers, morphine and pentazocine. The S(+)-isomers and racemates produce a typical Straub tail reaction and increased spontaneous locomotor activity in mice, but the R(-)-isomers did not. R(-)-II had no significant physical dependence liability in mice, rats and monkeys. From these results, it is suggested that the compounds show an uncommon steroselectivity in comparison with morphine and its surrogates, and that R(-)-II is worth investigating further as a narcotic antagonist analgesic.

Long-range substituent effects in morphine-type agonists and antagonists: a possible explanation for some opiate anomalies

Anomalous variations in the pKa values of variously substituted morphine-type agonists and antagonists are interpreted as a reflection of long-range substituent effects operating in these molecules. Based on the operation of long-range effects, a mechanism is proposed by which substitution into the N-normorphine portion of morphine-type agonists and antagonists changes the activity of the parent molecule. Thus, a remote substituent would distort the whole molecule via a conformational transmission effect and thereby (a) change the fit between the opiate and its receptor; (b) change the electron density distribution throughout the molecule and, therefore, at the nitrogen; (c) modify the directionality of the lone electron pair on the nitrogen; and (d) affect the pKa of the drug. The operation of long-range effects as proposed here could account for some of the anomalous changes in opiate activity effected by substitution into the parent molecule.

[On the physiology and pharmacology of endorphins (author's transl)]

Endorphins are peptides with opiate-like action synthesized in various tissue, e.g. in intestine and central nervous system. Exact characterization of opioid-specific receptors and sensitive biological test assays for opioids were prerequisites for the discovery of these substances. Met- and leu-enkephalin were the first endorphins discovered. Both are pentapeptides. One of them, namely met-enkephalin (H-Tyr-Gly-Gyl-Phe-Met-OH) is likely to be a fragment of the peptides alpha- and beta-endorphin, both showing opioid-like actions, as well as of beta-lipotropin, a polypeptide showing no opioid-like activity: all these peptides include the pentapeptide met-enkephalin within their molecules. beta-liportropin and ACTH are likely to be fragments of a common precursor. At least both enkephalins (which are studied better as yet than the other endorphins) are supposed to be formed in the soma of the neuron and transported to the nerve ending, where they are released. They seem to have the function of neuromodulator or even of neurotransmitters. The pharmacological actions of endorphins resemble those of "classical opiates", both having e.g. analgesic effects. Both enkephalins are, among various other brain and spinal cord areas, localized in those areas which seem to be of particular relevance for perception and transmission of pain. They might, under certain conditions, play some part in the regulation of pain perception. Furthermore, they seem to be relevant for some neuroendocrine processes. Their relevance in symptoms of schizophrenic psychoses seems to be more doubtful. In opiate dependence no significant alterations of endorphin concentrations could be observed as yet.

Adrenocorticotrophic hormone fragments mimic the effect of morphine in vitro

1 Fragments of the N terminal part of adrenocorticotrophic hormone (ACTH) inhibited the electrically evoked contractions of the mouse vas deferens. This inhibition could be antagonized by naloxone. 2 The same fragments displaced radiolabelled morphine from morphine antiserum. 3 Structure-activity relationship studies showed that in both assay systems the active core is located within the sequence ACTH 7--10. 4 It is postulated that the Trp9 residue and the peptide bond between Trp9 and Gly10 are particularly important for interaction of ACTH fragments with morphine receptors.

New opiate-receptor model

A new opiate-receptor model is proposed in which only one conformation of the receptor is needed for binding of both agonists and antagonists. There are two different spacially fixed amine-binding sites in this model: one agonist and one antagonist. The opiates undergo binding to their amine-binding sites via the lone electron pair on nitrogen. The role of the N-allyl or other such group in imparting antagonist properties is explained in terms of the steric requirements of this group. For this group to be accommodated without imposing severe steric interactions in the rest of the opiate molecule, the piperidine ring must assume a flexible (skew boat) conformation; in this conformation, the N-lone-pair electron lobe assumes the characteristic directionality of an antagonist toward its amine-binding site. If the N-lone-pair lobe is not rigorously maintained in this direction, the opiate molecule assumes both antagonist and agonist conformations and mixed antagonist-agonist activity is observed. The observed differences in the effect of sodium on the degree of binding of an agonist versus an antagonist can be explained in this model by the different effects of sodium on the two amine-binding sites. The antagonist activity of an N-methyl antagonist can be rationalized on the basis of the proposed model.

Conformation of [Leu5]enkephalin from X-ray diffraction: features important for recognition at opiate receptor

The conformation of [Leu5]enkephalin is produced by a Tyr-Gly-Gly-Phe beta bend stabilized by antiparallel hydrogen bonding between tyrosine and phenylalanine. On the basis of a comparison of the observed structure with the structure of known opiate agonists, three hydrophilic and two hydrophobic regions have been identified as contributing to the recognition of the molecule at the opiate receptor site.

Elucidation of the receptor-bound conformation of the enkephalins

The biologically relevant conformers of enkephalin predicted by solid state, solution state, and theoretical energy studies have been compared with the published structure-activity data on these compounds. No conformational technique proposes a model consistent with all the pharmacological data; the shortcomings of each approach are evaluated. An alternative approach, which correlates the structure-activity data of opiate compounds with that of the enkephalins, is described and shown to produce a model consistent with the available structure-activity data.

Proposal for the biologically active conformation of opiates and enkephalin

A model for the opiate receptor has been defined by using a computer-based molecular display and x-ray crystallographic input data. The model can explain the stereochemical fashion in which the morphine, morphinan, and oripavine classes of compounds interact with the receptor. The minimal structural unit of the enkephalins demonstrated to be pharmacologically active. Tyr-Gly-Gly-Phe, was also fitted to this model by using a systematic search of conformational space.