Molecular organization of receptors. Efficacy, agonists, and antagonists.
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Amino Acid Sequence↗
Biomedical subjects
F Porreca
Publications and source records attributed to F Porreca.
Probes for narcotic receptor-mediated phenomena. 20. Alteration of opioid receptor subtype selectivity of the 5-(3-hydroxyphenyl)morphans by application of the message-address concept: preparation of delta-opioid receptor ligands.
Derivatives of racemic and optically active 5-(3-hydroxyphenyl)-2-methylmorphan (5-(3-hydroxyphenyl)-2-methyl-2-azabicyclo[3.3.1]nonane, 1) were synthesized containing additional aromatic moieties, as an application of the message-address concept targeted at producing delta-opioid receptor selective ligands. In vitro radioreceptor binding studies in rat brain revealed that both of the parent enantiomers, (-)- and (+)-1, had a high affinity for the mu-opioid receptor (21 nM), a slight affinity for kappa 1-opioid receptors (approximately 800-900 nM), and less than 1000 nM affinity for the delta-opioid receptor (mu/delta IC50 ratio of < 0.02 for both). A derivative of (-)-1 containing an indole moiety fused at the C6-C7 position of the phenylmorphan nucleus, (-)-11, displayed a > 180-fold increase in affinity for the delta-opioid receptor with an IC50 value of 6 nM. The parent compound (-)-1 had only 26% agonist activity at 30 microM in the mouse vas deferens (delta) bioassay, whereas compound (-)-11 had an IC50 of 393 nM in this preparation, indicating the importance of the indole moiety in imparting delta-opioid agonist activity to the phenylmorphan (-)-11. A structure-activity relationship (SAR) study of N-alkyl derivatives of the racemic nor 11 indicated similarities between the interaction of various derivatives with the mu- and delta- but not the kappa 1-opioid receptor. As studies on the molecular basis of the interaction of opioid ligands with their respective receptors continue to gain momentum, the SAR data described herein for the synthetic phenylmorphans will prove useful for further studies.
Animals↗
Increased expression of opioid delta receptors by deoxy conformation heme proteins in NG108-15 cells.
Adaptations to prolonged hypoxia include an increase in the expression of proteins that may facilitate survival. One mechanism by which hypoxia increases protein expression involves a change of heme proteins from oxygenated to deoxygenated conformations. In the present study, we tested the hypothesis that treatment of NG108-15 cells with metallic cations, which are known to induce a deoxygenated conformation of heme proteins, would increase delta opioid receptor (DOR) expression. Cells were treated with cobalt and nickel, which induce deoxygenated heme protein conformation, or zinc as a control for 48 h prior to quantifying DOR expression. Cobalt and nickel, but not zinc, significantly increased DOR expression. Heme synthesis inhibitors would block the synthesis of cobalt-substituted heme proteins which are locked in a deoxygenated conformation. The cobalt-induced increase in DOR expression was blocked by the heme synthesis inhibitor, 4,6-dioxoheptanoic acid. These experiments indicate that deoxygenated conformation heme proteins, which are thought to partially mimic hypoxia, increase DOR expression. The increase in DOR expression suggests that the DOR gene may be hypoxia-sensitive. Further, the increase in DOR expression suggests a potential adaptation strategy to hypoxia and may represent one of the first findings of physiological regulation of DOR expression.
Cell Line↗
Highly kappa receptor-selective dynorphin A analogues with modifications in position 3 of dynorphin A(1-11)-NH2.
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Animals↗
The increase in morphine antinociceptive potency produced by carrageenan-induced hindpaw inflammation is blocked by naltrindole, a selective delta-opioid antagonist.
Carrageenan-induced inflammation of the rat hindpaw has been used as a model for persistent pain of inflammatory origin. The induction of inflammation resulting from carrageenan injection in the rat hindpaw has been shown to elicit an increase in the antinociceptive potency of morphine, an effect postulated to be related to reduced levels of spinal cholecystokinin (CCK). Recent findings have related the anti-opioid effect of CCK to a decrease in activation of delta-opioid receptors. For this reason, we have examined the effects of the delta-opioid antagonist naltrindole (NTI) on the modulation of morphine antinociceptive potency resulting from carrageenan-induced inflammation. Rats with carrageenan-induced hindpaw inflammation received several doses of morphine in the absence or presence of NTI and were tested in the hot plate (HP) and tail flick (TF) tests. These results were compared to those of non-carrageenan injected rats. Morphine was significantly more potent in inflamed, than in control, rats in both tests. While NTI did not affect morphine antinociceptive potency in control rats in either test, this opioid delta antagonist blocked the increase in morphine potency resulting from carrageenan inflammation in nearly every case. The blockade of the enhancement of morphine potency was such that the effect of a given dose of morphine was similar in control rats and carrageenan-injected rats with NTI. We suggest that carrageenan-induced inflammation may alter endogenous enkephalin levels, perhaps by a decrease in CCK availability. The enhancement of morphine antinociceptive potency may result from the well-established synergism seen following the activation of opioid delta receptors by enkephalins.
Animals↗
The synthesis and opioid receptor binding affinities of analogues of dermorphin and its N-terminal tetrapeptide fragment with dibasic acids in position 2.
Analysis of possible mu opioid receptor active conformations for dermorphin suggested that the topographical location of the tyramine moiety of the N-terminal tyrosine can be simulated with the phenol of tyrosine or desamino-tyrosine (4-hydroxyphenylpropionic acid) and a basic group located on the side chain of a dibasic acid residue located in position 2. The biological properties of respective analogs with D- or L-arginine, and D- or L-lysine in the position 2 of dermorphin or desamino-dermorphin and their N-terminal tetrapeptide fragments, has provided evidence in support of this prediction, and questions the dogma that an N-terminal tyrosine is a necessary element for opioid agonist peptides.
Amino Acid Sequence↗
The sea anemone purine, caissarone: adenosine receptor antagonism.
Caissarone, a sea anemone iminopurine, produced an increase in the twitch response of the electrically stimulated guinea-pig ileum-myenteric plexus. In the same assay, caissarone reduced the inhibitory response to the endogenous neuromodulator, adenosine, the A1 adenosine receptor agonist, R-phenylisopropyladenosine (R-PIA), and the A2 agonist, 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA) in a dose-dependent manner. Schild plot analysis of antagonism by caissarone yielded slopes of near unity, indicating that caissarone acts as a simple competitive antagonist at the adenosine receptor. The dissociation constants (KB) for caissarone ranged from 0.53 mM to 0.78 mM. In functional nicotinic receptor assays in two human cell lines, caissarone failed either to potentiate or to reduce carbamylcholine-mediated 86Rb+ efflux. Thus, the enhancing activity of caissarone on the gut could not be attributed to activity at the ganglionic nicotinic receptor. Based on structure and pharmacological activity, caissarone appears to be the first marine product described as an adenosine receptor antagonist.
Animals↗
Opioid peptide receptor studies. 1. Identification of a novel delta-opioid receptor binding site in rat brain membranes.
Our laboratory was among the first to propose the existence of delta receptor subtypes: a delta site thought to be associated with a mu-delta-opioid receptor complex termed the delta cx binding site and delta site not associated with the mu-delta-opioid receptor complex, termed the delta ncx site. In previous studies, we assayed the delta cx site with [3H][D-Ala2,D-Leu5]enkephalin using rat brain membranes depleted of delta ncx sites by pretreatment with the site-directed acylating agent, (+)-trans-SUPERFIT. In the present study, we investigated, using (+)-trans-SUPERFIT-pretreated membranes, the possibility of heterogeneity of the delta cx binding site. Two sites were resolved: the delta cx-1 site at which mu ligands are potent noncompetitive inhibitors and delta ligands are weak competitive inhibitors, and the delta cx-2 site where delta ligands are potent and mu ligands are weak, mixed competitive-noncompetitive inhibitors. Although the delta cx-2 site has a delta-like ligand-selectivity profile, several experiments distinguished it from the delta ncx site. Two lines of evidence suggest that the delta ncx site corresponds to the cloned delta receptor. One, the delta receptor was cloned from the NG108-15 cell line, and this receptor, like the delta ncx binding site, irreversibly binds SUPERFIT and (+)-trans-SUPERFIT. Secondly, administration of delta-antisense DNA selectively decreases delta ncx binding. Viewed collectively, the major finding of this study is the discovery of a novel SUPERFIT-insensitive and delta-antisense-insensitive delta cx-2 binding site.
Analgesics↗
Long-lasting desensitization of bladder afferents following intravesical resiniferatoxin and capsaicin in the rat.
The present study was conducted to determine whether long-lasting desensitization of bladder afferents could be achieved using a single local application of the capsaicin (CAP)-like irritant resiniferatoxin (RTX), and to compare the effects of RTX and CAP on behavioral and histological endpoints. While rats were anesthetized, vehicle (VEH), RTX (10-100 nmol) or CAP (10-100 mumol) was instilled in the bladder (intravesical, i.ves.) via a cannula surgically implanted into the bladder dome. Beginning 1 week after treatment, once per week for 4 weeks, rats were tested behaviorally for desensitization to i.ves. RTX (10 nmol) using the abdominal lick test. Rats pretreated with low doses of RTX and CAP were partially desensitized at week 1; desensitization diminished over weeks 2-3. In contrast, rats pretreated with high doses of RTX or CAP were more completely desensitized at week 1, and desensitization did not diminish by week 4. Separate groups of rats tested 8 weeks after treatment showed substantial recovery. Rats pretreated with RTX but tested only with VEH for the first 3 weeks showed desensitization at week 4 approximately equivalent to that of RTX-treated rats tested with RTX every week. Sensitivity of corneal afferents to RTX (1.0 microgram/ml) at week 4 was not different between VEH- and RTX- or CAP-treated rats. Gross and histological examination of bladder tissue indicated that both RTX and CAP produced inflammation, which diminished in a dose- and time-dependent manner (1-8 weeks post-treatment).(ABSTRACT TRUNCATED AT 250 WORDS)
Administration, Intravesical↗
Novel met-enkephalin analogue: a potent systemic mu agonist antinociceptive agent.
A new met-enkephalin analogue (compound 82/205) was evaluated for its opioidergic activity in mice. The compound showed antinociception (warm water tail-flick test), tolerance, cross tolerance to morphine and physical dependence. The time course of antinociceptive effect of the compound was comparable to morphine. The antinociceptive ED50 (mumol kg-1, i.p.) values for the compound and morphine base were 5.31 and 7.59, respectively. Its antinociceptive effect was blocked by naloxone, beta-FNA (mu antagonist) and naloxonazine (mu1 antagonist) but not by ICI 174,864 (delta antagonist). Naloxone precipitated withdrawal jumpings were 2.6 times less in compound 82/205 treated mice than the morphine treated group. The new analogue compound 82/205 is a potent mu agonist antinociceptive with a possible weak dependence liability.
Analgesics↗
Retrovirus-mediated expression of an artificial beta-endorphin precursor in primary fibroblasts.
Peptides are of potential interest in the field of gene therapy but require modification by genetic engineering to facilitate their secretion. Amino terminal addition of a signal peptide is not always sufficient to achieve this goal, as found in this study for beta-endorphin. To overcome this problem, addition of the pre-pro-sequence of mouse nerve growth factor to beta-endorphin was tested. Retrovirus-mediated expression of a hybrid construct of the pre-pro-sequence of nerve growth factor and human beta-endorphin in primary fibroblasts resulted in the secretion of beta-endorphin immunoreactivity at a rate of 620 pg/h/10(6) cells. Analysis of the secreted beta-endorphin immunoreactivity with reverse-phase HPLC, immunoassays using three different antibodies, and an assay for the specific displacement of [3H][D-Ala2,N-MePhe4,Gly-ol5]enkephalin from mu-opioid receptors suggests that the pre-pro-sequence is cleaved off from the pre-pro-sequence/beta-endorphin construct prior to secretion, resulting in bona fide beta-endorphin. Transplantation of beta-endorphin-secreting cells into brain or spinal cord may provide a gene therapy approach for the treatment of chronic, opioid-sensitive pain states.
3T3 Cells↗
Tonic suppression of gastric acid secretion by endogenous peptides in neonatal rats.
Stimulation of gastric acid secretion by secretagogues was measured in developing rats by in vivo and in vitro techniques. Basal acid outputs in vivo were very low in 8- and 14-day-old rats compared with those in 20- and 30-day-old rats. In 20-day-old rats, all secretagogues increased acid output in vivo, whereas only carbachol, pentagastrin, and sulfated cholecystokinin octapeptide (CCK-8S) were active in 14-day-old rats. In contrast, basal acid output in vitro and stimulation by secretagogues did not differ significantly with age. CCK-8S-stimulated acid output in vitro in 14-day-old rats was blocked by L-365,260, L-364,718, tetrodotoxin, and atropine, but not by hexamethonium, whereas gastrin-stimulated acid output was blocked only by L-365,260. Furthermore, acid output in vivo was elevated three- to fourfold by subcutaneous naloxone-methiodide or L-364,718, but not by L-365,260, in 14-day-old rats; none of these antagonists produced an effect in 20-day-old rats. These studies show that low basal gastric acid output in neonatal rats is caused by tonic inhibitory regulation by endogenous regulatory peptides.
Animals↗
SNC 80, a selective, nonpeptidic and systemically active opioid delta agonist.
The present study has investigated the pharmacology of SNC 80, a nonpeptidic ligand proposed to be a selective delta agonist in vitro and in vivo. SNC 80 was potent in producing inhibition of electrically induced contractions of mouse vas deferens, but not in inhibiting contractions of the guinea pig isolated ileum (IC50 values of 2.73 nM and 5457 nM, respectively). The delta selective antagonist ICI 174,864 (1 microM) and the mu selective antagonist CTAP (1 microM) produced 236- and 1.9-fold increases, respectively, in the SNC 80 IC50 value in the mouse vas deferens. SNC 80 preferentially competed against sites labeled by [3H]naltrindole (delta receptors) rather than against those labeled by [3H]DAMGO (mu receptors) or [3H]U69, 593 kappa receptors) in mouse whole-brain assays. The ratios of the calculated Ki values for SNC 80 at mu/delta and kappa/delta sites were 495- and 248-fold, respectively, which indicates a significant degree of delta selectivity for this compound in radioligand binding assays. SNC 80 produced dose- and time-related antinociception in the mouse warm-water tail-flick test after i.c.v., i.th. and i.p. administration. The calculated A50 values (and 95% C.I.) for SNC 80 administered i.c.v., i.th. and i.p. were 104.9 (63.7-172.7) nmol, 69 (51.8-92.1) nmol and 57 (44.5-73.1) mg/kg, respectively. The i.c.v. administration of SNC 80 also produced dose- and time-related antinociception in the hot-plate test, with a calculated A50 value (and 95% C.I.) of 91.9 (60.3-140.0) nmol.(ABSTRACT TRUNCATED AT 250 WORDS)
Analgesics↗
Regulation of morphine antiallodynic efficacy by cholecystokinin in a model of neuropathic pain in rats.
Neuropathic pains have often been classified as opioid-resistant. Here, spinal (intrathecal) actions of morphine and nonmorphine opioids have been studied in a nerve ligation model of neuropathic pain in rats. Mechanical allodynia was evaluated using von Frey filaments. Nerve-injured animals exhibited allodynia that was stable for up to 6 weeks after the surgery. Morphine did not alter allodynia at doses up to 300 nmol (100 micrograms). In contrast, [D-Ala2, NMPhe4, Gly-ol]enkephalin (DAMGO), a high-efficacy mu opioid agonist, produced a significant, dose-related antiallodynic action. [D-Ala2, Glu4]deltorphin (delta agonist) produced a significant antiallodynic effect only at 300 nmol, reaching approximately 70% of the maximum. Coadministration of morphine with a dose of [D-Ala2, Glu4]deltorphin, which was inactive alone, produced a significant and long-lasting antiallodynic action that was antagonized by NTI (delta receptor antagonist); NTI alone had no effect. Although blockade of cholecystokinin-B (CCKB) receptors with L365,260 did not produce effects alone, a significant antiallodynic action was observed when coadministered with morphine; this elevation of nociceptive threshold was abolished by NTI. The finding that DAMGO, but not very large doses of morphine, produced antiallodynic actions suggests that the ability of mu opioids to alleviate the allodynia is related, in part, to efficacy at postsynaptic mu receptors. At an inactive dose, a delta agonist or a CCKB antagonist enhanced morphine antiallodynic efficacy in an NTI-sensitive fashion. CCKB receptor blockade may enhance endogenous enkephalin actions, resulting in enhancement of morphine efficacy through a mu-delta receptor interaction.
Analgesics, Opioid↗
Opioid antinociception in a rat model of visceral pain: systemic versus local drug administration.
Antinociceptive effects of systemically or locally administered opioid mu, kappa and delta agonists were evaluated in a rat model of visceral pain. Resiniferatoxin (RTX, 3 nmol), a capsaicin-like mutant, produced abdominally directed grooming behavior after direct administration into the urinary bladder (intravesical, Lves.) by indwelling cannula. Systemic (s.c. or i.p.) pretreatment with the mu agonists morphine or [D-Ala2, NMePhe4, Gly-ol]enkephalin (Damgo), the kappa agonists trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide (U50,488) or [5R-(5,7,8-beta)]-N-methyl-N-[7-(1-pyrrolidinyl)1-oxaspiro[4,5]dec - 8-yl]-4-benzofuranacetamide (CI-977), or the nonpeptidic delta agonist (+/-)-4-((alpha-R*)-alpha-((2S*,5R(*)-4-Allyl-2,5-dimethyl-1- piperazinyl)-3-hydroxybenzyl)-N, N-diethylbenzamide (BW373U86) dose-dependently decreased RTX-induced abdominal licking; such antinociception was selectively blocked by the appropriate receptor-selective antagonists beta-funaltrexamine (mu), nor-binaltorphimine (kappa) and naltrindole (delta). Local (i.ves.) BW373U86, [D-Ala2,Glu4]deltorphin (DELT II) and Cl-977 also significantly decreased RTX-induced licking. Intracerabroventricular quaternary naloxone partially blocked the effects of systemic morphine, but not that of CI-977 or BW373U86. Intraperitoneal quaternary naloxone blocked the effect of local and systemic BW373U86 but not that of local or systemic CI-977; systemic morphine was partially blocked. Thus, systemic mu, kappa and delta agonists all produced antinociception against a novel visceral chemical stimulus in the rat. Local CI-977 also produced antinociception, but the only compound clearly acting at peripheral opioid receptors was BW373U86, a delta agonist. This study suggests that opioid delta receptors may be present on bladder nociceptive afferents and may be activated for production of peripheral analgesia.
Analgesics, Opioid↗
Antisense oligodeoxynucleotide to the CCKB receptor produces naltrindole- and [Leu5]enkephalin antiserum-sensitive enhancement of morphine antinociception.
Cholecystokinin (CCK) has been shown to attenuate, while CCK antagonists enhance, the antinociceptive activity of morphine, suggesting that this peptide may act as an endogenous modulator of the opioid system. Here, we have investigated the effects of administration of a synthetic oligodeoxynucleotide (oligo) complementary to the 5' coding region of the cloned mouse CCKB receptor (antisense), or a mismatch oligo, on the antinociceptive effects of morphine. Intracerebroventricular (i.c.v.) treatment of mice with CCKB antisense, but not mismatch, oligo for 3 days resulted in an enhancement of the antinociceptive potency of i.c.v. morphine, as indicated by an approximately 6-fold leftward shift of the dose-effect curve. The antinociceptive effects of morphine in control and CCKB antisense-treated animals were investigated in the presence or absence of naltrindole, an opioid delta receptor antagonist, as well as in the presence or absence of antisera directed against either [Leu5]- or [Met5]enkephalin. The enhanced potency of morphine in mice pretreated with CCKB antisense oligo was blocked by a delta-selective dose of naltrindole and antisera to [Leu5]enkephalin, but not [Met5]enkephalin; naltrindole, or antisera towards [Leu5]enkephalin or [Met5]enkephalin did not produce antinociceptive effects when given alone and did not alter the antinociceptive actions of morphine in control mice. These data suggest that CCK may act via CCKB receptors to tonically inhibit the release of [Leu5]enkephalin, or a [Leu5]enkephalin-like peptide. The enhancement of morphine antinociception seen in the presence of blockade of the CCKB receptor may be the result of the well-known enhancement of morphine antinociception by opioid delta agonists.
Animals↗
Development of a model for the delta opioid receptor pharmacophore. 1. Conformationally restricted Tyr1 replacements in the cyclic delta receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]OH (JOM-13).
A series of analogues of the conformationally restricted delta opioid receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]OH (JOM 13) was prepared in which the conformationally labile Tyr residue was replaced with several less flexible tyrosine analogues. Among these tyrosine analogues were the bicyclic structures 1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (HO-Tic), 2-amino-6-hydroxytetralin-2-carboxylic acid (Hat), and 2-amino-5-hydroxyindan-2-carboxylic acid (Hai) in which rotations about the C alpha-C beta and C beta-C gamma bonds are restricted due to cyclization of the side chain to the backbone. Also examined were analogues in which tyrosine was replaced with either trans-3-(4'-hydroxyphenyl)proline (t-Hpp) or cis-3-(4'-hydroxyphenyl)proline (c-Hpp), residues in which rotations about C alpha-C beta, but not C beta-C gamma, are restricted. Both the t-Hpp1 and c-Hpp1 analogues displayed delta receptor binding affinity similar to the parent Tyr1-containing peptide, while the D-Hat1, L-Hat1, and L-Hai1 analogues exhibited somewhat lower affinity. The results observed for the t-Hpp1 and c-Hpp1 analogues are particularly significant since these two residues have little accessible conformational space in common. Since the binding conformation of residue 1 must be included in this limited conformational intersection, its elucidation is facilitated. Bioassay results from guinea pig ileum and mouse vas deferens preparations are in general agreement with the binding results; however some potency discrepancies are observed. These discrepancies may reflect different selectivities among delta receptor subtypes for the analogues or may represent differing efficacies among these conformationally restricted peptides. The conformational properties of the parent tetrapeptide and the residue 1-modified analogues were studied by molecular mechanics computations. All these peptides share a common rigid tripeptide cycle with a single energetically preferred backbone conformation and three different conformers of the D-Cys, D-Pen disulfide bridge, two of which are observed in the solid state and in aqueous solution, as previously determined from X-ray crystallography and 1H NMR spectroscopy data (Lomize, A; et al. J. Am. Chem. Soc. 1994, 116, 429-436). All the peptides have similar sets of low-energy conformations of their common flexible elements, the Phe3 side chain and the peptide group between the first residue and the rigid tripeptide cycle. However, possible conformations of the first residue differ and depend on the covalent constraints incorporated into the side chain.(ABSTRACT TRUNCATED AT 400 WORDS)
Amino Acid Sequence↗
Development of a model for the delta opioid receptor pharmacophore. 2. Conformationally restricted Phe3 replacements in the cyclic delta receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]OH (JOM-13).
The in vitro pharmacological properties and conformational features of analogs of the delta opioid receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]OH (JOM-13) in which the Phe3 residue was replaced by each of the four stereoisomers of beta-methylphenylalanine (beta-MePhe) were investigated. Both analogs in which the alpha carbon of the Phe3 replacement has L-stereochemistry display high affinity for delta receptors with the (2S,3S)-MePhe3 analog exhibiting approximately 8-fold higher affinity than the (2S,3R)-MePhe3 diastereomer. Surprisingly, one analog with D-stereochemistry in residue 3, the (2R,3R)-MePhe3 analog, also displays high affinity for the delta receptor and is extraordinarily selective for this receptor. All analogs were agonists in the mouse vas deferens (MVD) and guinea pig ileum (GPI) smooth muscle bioassays, displaying MVD and GPI potencies consistent with their delta and mu opioid receptor affinities, respectively. The use of beta-MePhe as a replacement for Phe3 was based upon the desire to reduce the conformational flexibility of the Phe3 side chain by imposing a steric rotational constraint in the form of the beta-methyl substituent and to thus deduce the residue 3 side chain orientation in the delta receptor-bound conformation from the correlation between delta receptor binding affinities and conformational preferences. Molecular mechanics computations revealed, however, that the conformational constraints imposed by the beta-methyl group in the (2S,3S)-MePhe3 and (2S,3R)-MePhe3 analogs were too modest to allow unequivocal determination of delta receptor-bound residue 3 side chain conformation. However, analysis of the high-affinity (2R,3R)-MePhe3 analog revealed a strong preference for a single side chain conformer (chi 1 approximately 60 degrees). Low-energy conformers of this analog could only be effectively superimposed with low-energy conformers of the parent peptide in which the Phe3 side chain conformation was limited to chi 1 approximately -60 degrees. This observation eliminates the last remaining uncertainty regarding conformational features of the pharmacophore elements in the delta receptor-bound state, allowing the proposal of a complete model.
Amino Acid Sequence↗